CE457/527 Project Report

 

Colloidal and Surface Phenomena Aspects of Sunscreen Lotion

 

Matthew B. Boyle, William W. Gikandi

Zhiyong Gu, Kassim S. Salih

 

 

 

 

 

 

 

 

 

 

 

 


Department of Chemical Engineering

University at Buffalo, SUNY

April 8, 2002

 

1. Introduction and Background

1.1 Some basic information about skin type, skin care, skin cancer, and the necessity of sunscreen

 

The sun, 93 million miles from the surface of the Earth, is composed mainly of hydrogen, helium, calcium, sodium, magnesium and iron. The temperature at the sun ranges from 11,000 degrees F on the surface to about 36,000,000 degrees F at the center. Due to this high temperature, the gases emit a continuous spectrum of electromagnetic radiations that end up reaching the earth.

A large proportion of these rays are either absorbed or scattered by our atmosphere. Roughly 7% of the radiation incident on the atmosphere is transmitted to the surface of the Earth.

 

Effect of incident radiation on the skin:

It seems evident that excessive exposure of the human body to sunlight may not only precipitate a predisposition to cancer, but can in itself produce malignant growth. Below is an outline of the short-term effects of the sun on the skin in order of increasing exposure to radiation:

1.      Formation of an erythema: A superficial inflammation of the skin, showing itself chiefly in rose-colored patches. This is often seen as the reddening of the skin right before a tan forms.

2.      Tanning response: the skin responds to incident erythemal rays by producing melanin, a darkening pigment that absorbs and scatters radiation; acting as a barrier to more damage to skin.

3.      Pain and itching

4.      Edema and blistering, peeling.

5.      Chills, fever

 

Milder degrees of sunburn may produce various forms of solar dermatitis, and it is probable that eczema, rosacea, pruritis and lupus erythematosus are at least exacerbated. Other rare effects include irregular discoloration of the skin.

 

Gross, long-term epidermal findings of skin damage include thickening of the exposed skin, increased marking of the skin, areas of pigmentation and atrophy; superficial scaling and telangiectasis or timorous swelling of the peripheral capillaries and small blood vessels. These changes constituted the visible signs of skin again, and the damaged skin assumes a dry, course, leathery appearance, lax and wrinkled, rough and mottled. This skin condition is medically termed poikiloderma of Civatte, but in its early states, is seen as a major cosmetic problem.

 

As a rule of thumb, susceptibility of the above long-term and short-term effects of erythemal rays depends largely on skin pigmentation. Darker skins in general are less likely to suffer harm from the sun. For people with lighter skins, it is advisable to expose themselves to the sun gradually, allowing the skin enough time to produce melanin to protect itself. It is also highly recommended to use sunscreen regularly during periods of intense sunlight, such as summer.

 

At the same time, it should be remembered that tanning is not by any means a sign of good health, bus is actually part of the procedure by which nature is trying to minimize, to the best of her ability, the damaging effects that would otherwise be produced by exposure to sunlight.

 

Hence the necessity of Sunscreen has been outlined. The following sections elaborate on the production of sunscreen.

 

1.2  Brief outline of factors to be considered for the production of sunscreen

Companies producing sunscreen must adhere strictly to the following criteria outlined below. This is both for the satisfaction of the customer, and also, more importantly, for safety’s sake:

The properties necessary in a sunscreen are:

 

1.      It must absorb erythemogenic radiation to a satisfactory level:

·        Suntan preparations are often sold under the improper premise that they can prevent sunburn and yet allow tanning. Medically speaking, melanogenesis (tanning) cannot occur without preceding erythema to trigger the process. Therefore, a good sunscreen must maintain a balance between allowing sufficient radiation to the skin to cause a desired tan; and yet keep it at a level that is deemed as safe for the skin. Ideally, a good sunscreen should not give the user a ‘tan’ on the first day; but should rather protect the consumer and allow a safe tan to develop over several days.

2.      It must be non-toxic and it must not influence bodily metabolism.

·        Ingredients in sunscreen must be ‘inert’ with respect to normal functions of the body. So that any surface application or ingestion, via subcutaneous penetration or otherwise, will not cause harmful short-term or long-term effects to the body.

3.      It must be dermatologically innocuous, i.e. free from primary irritant effect and from any danger of sensitization.

·        There are few other cosmetic preparations which are used repeatedly over such a large area of the body and which therefore present any greater hazards for toxic, irritating and sensitizing reactions. Usually, irritation problems occur at or near the folds of the skin; or near the mucosae. Lips, eyelids, around the nostrils, and the skin between fingers and toes are all targets for sensitivity. Therefore, manufacturers should balance effectiveness of the ingredients, active agents and concentrations they use against the their potential irritability to the skin.

4.      It must not be photo labile.

·        In absorbing the erythemogenic radiation, it must not undergo any chemical change that would reduce its efficacy as a sunscreen. More importantly it should not be converted to any compound will be toxic or innocuous to the skin. Its structure must also not change in a way that will affect its physical properties; such as its look or feel, surface coverage, etc.

5.      It must be non-volatile, and possess suitable Solubility Characteristics.

·        Customary vehicles for sunscreens are hydro alcoholic lotions, water-in-oil or oil-in-water emulsions, and oily lotions. It is essential that the sun-screening compound be dissolved or dispersed easily and permanently in the vehicle. Once it is spread on the skin, it should remain in place as a continuous film, closely adhering to the surface. It should resist washing off either by perspiration or by immersion in fresh or salt water. At the same time, its user must be able to wash it off easily by scrubbing with soap and water at the end of the day.

·        Also, it must not crystallize during storage or shipping through cold areas, as this will cause patchy coverage and lower protection. So it’s self solubility or its ability to remain in an emulsion must be sufficiently high.

6.      It should not be absorbed through the skin.

·        Virtually all chemicals placed on the skin diffuse to some extent through the underlying tissues. If the percutaneous absorption is marked, the chemicals may be detected in the blood stream or in the urine. However, complete avoidance of penetration is not desirable. The fraction of the screen that can perfuse the upper stratum corneum increases protection against the sun; especially because it cannot be removed by bathing or abrasion due to clothing. However, a screen that diffuses into deeper layers and into the blood stream is not desirable.

7.      Its physical properties must be acceptable to the consumer.

·        Having fulfilled all of the above characteristics, the screen must finally please picky consumers. Producers of sunscreens are in heavy competition with each other, and are pressed to come about with a product that will not only be Safe and Effective, but one that will also be pleasant to the touch and smell, and also be aesthetically pleasing.

 

1.3  How Sunscreen Works.

There are two main ways in which a sunscreen can protect the skin against the sun:

·        Scattering/ Reflecting Radiation: this method can be likened to how a mirror works. Shining a concentrated beam of light to a shiny, uneven mirror will cause the incident beam to be reflected back in different directions; effectively being scattered instead of harming the surface. Opaque powders applied to the skin either in the dry state or incorporated into suitable vehicles ill serve to scatter the erythemal rays falling upon them. Zinc Oxide is the most effective of such powders, but there are others of similar efficiencies available. This method seems to be the less popular among sunscreen producers. The main reason being that the amount of coating required for sufficient scatter or reflection is quite high. Also, it would be much easier to wash off powders from the skin, and maintaining ‘enough’ powder for protection in an emulsion may prove to be difficult.

 

·        Absorbing Radiation:  all kinds of organic and inorganic materials absorb ultraviolet energy. When a photon of energy strikes a molecule of such a compound, the energy may be absorbed if the molecule may exist in two different states (isomers). The difference in the energy levels between the two molecular structures must correspond exactly to the energy of the absorbed photons. The energy contained in a photon is directly related to its wavelength.  Therefore, the active ingredient in sunscreens of this nature has a chemical isomer whose transition corresponds very closely to the energy of a photon in the erythemal wavelength range of light. On absorbing these photons, the active ingredient undergoes a change in structure to for an ‘excited’ isomer that exists at a higher energy level. This excited isomer is usually not stable, and so it slowly releases its absorbed energy into its surroundings, returning to its original state as the active ingredient. The slow rate of energy release results in a shift of the emitted photon to a longer wavelength, and the re-emitted energy often lies in the visible or infrared range; a wavelength that has no harmful erythemal effects.

Note: it is important that the excited isomer be unstable, so that it will return to its original state and absorb more radiation. Otherwise, a stable isomer will stop working and the efficacy of the screen will decrease over time.

It is also important that the excited isomer does not react with surrounding chemicals or the atmosphere to form a compound that is different to the original active ingredient. Ideally, the excited isomer should not be extremely reactive.

 

2. Main Components and Compositions of Sunscreen Lotion and their Functions

            2.1 Basic components of sunscreen preparations.

(1) Discussion of Possible Sunscreen Vehicles and their advantages and disadvantages:
 It is one thing to have a good active ingredient that will protect from the sun. It is another to package it in a way that is acceptable to the consumer and that will not hinder its effectiveness. The vehicle of the screen must have all of the qualities mentioned previously. Listed below are examples of different types of vehicles and formulations of sunscreen that are representative of various types currently on the market. Producers decide on the type of formulation depending on cost, packaging and degree of protection desired:

Hydro-alcoholic Lotions:
These lotions are composed mainly of a water-alcohol mixture that is easy-spreading, and evaporates quickly from the skin.
Emulsified Cream Lotions:
These types of lotions are thicker emulsions than are too thick to pour readily from a bottle, but not viscous enough to pack in tubes. However, squeeze bottles make excellent dispensers for emulsified creams.

Emulsified Suntan Creams:
These are thicker and heavier than Cream Lotions, and can be packed into tubes. The thick consistency of the lotion also gives the customer the ‘psychological assurance’ that the sunscreen is actually protecting him from the sun. They also have a shiny appearance once applied on the skin, and thus act as reflectors to scatter incident radiation as well as absorbing it.

Gels and Jellies:
These are the thickest of sunscreen formulations. They have a high water repellent structure and so are harder to wash away after a swim or due to sweating. They are also very glossy and so reflect/ scatter incident rays the best. However, they are harder to spread, and more must be used to attain a good surface coverage.

Although different in nature, neither of these different formulations offer a major advantage over the other. The decision of a company to produce a certain type will depend on cost and ease of transport. A customer will buy the sunscreen that is most appealing to him. One person may feel better protected using the sunscreen gels. Another may not like the ‘stickiness’ of the gels, and will thus be more inclined to buy a hydro-alcoholic formulation. An active customer looking to do a lot of swimming will purchase a gel, which will hold better to the skin; where-as a casual sunbather will simply use a lotion. Also, facially applied sunscreens are more likely to be creams, because gels will be too thick for the face, and hydro-alcoholic formulations may have a drying effect on the facial tissue.


(2) Discussion of Possible Active ingredients.
There really is no end to the list of compounds that can be used as screening agents. Potential sunscreen compounds are found amongst chemical compounds possessing widely different physical and chemical characteristics which make it extremely difficult to classify them into chemical families. However, it is enough to say that these compounds are organic in nature.
 
Below is a short list of some compounds that absorb UV radiation to form excited isomers.

Compound

State

Solvent

Ethyl p-dimethyl amino-benzoate

Solid

Alcohol

Ethyl p-amino-benzoate

Solid

Alcohol

Methyl anthranilate

Liquid

Alcohol

Homomenthyl slicylate

Liquid

Alcohol

Phenyl salicylate

Solid

Alcohol

Menthyl Salicylate

Liquid

Alcohol

2-Naphthol-6sulphonic acid

Solid

Water

2-Naphthol-8-Sulphonic acid

Solid

Water



An important property that all these chemicals must possess is isomerism. The degree of protection offered by each chemical will depend on its structure. But other factors such as cost, ease of production and toxicity also come into play while selecting a good active agent. The solubility of the compound will then determine the nature of its carrier.

2.2 Efficacy of the Active Ingredient. (The Sunscreen Index)
Kumler (J. Amer. Pharm. Ass., Sci. edn., Sept. 1952) came up with a simple and rapid method of evaluating the relative screening action of sun-screen compounds. In this, the optical density of a 0.1% solution in a 0.1mm fused silica cell is measured at 3080Å. The optical density obtained from this measurement is then divided by the percent concentration of the sunscreen. This value attained (called the Sunscreen Index) depicts the strength of the screening compound. Samples of these values are shown in the table below.

Compound

Sunscreen Index (S.I) at Optical Density 3,080Å (conc. 1%)

Ethyl p-dimethylaminobenzoate

14.8

Ethyl p-aminobenzoate

9.6

Isobutyl p-aminobenzoate

9.2

n-Butyl p-aminobenzoate

8

Dehydroacetic acid

7

Uviteix RBS

0.01

Uvitex RS

0.005

 

Compounds with a higher S.I. value will absorb radiation better. However, this does not mean that only compounds with the highest S.I. will be used. Cost, Toxicity, Solubility, etc will all determine which compound is to be used. Using a compound of lower S.I. will simply mean that it will need to be more concentrated than a compound of higher S. I.  Also, each compound may absorb only a specific wave-length in the spectrum of erythemogenic radiation. Sometimes, a company may wish to use more than one compound in order to attain desired protection over all wavelengths. The graph below shows an example of some compounds and the amount of absorbance attained for each wavelength.

 

Typical absorbance curves for some popular sunscreens

 

Sun Protection Factor (SPF)

Sometimes companies will use a different measurement of the efficacy of the sunscreens they produce. They use a standard measurement called the Sun Protection Factor (SPF). This is to make it easier for the consumer to understand the strength of protection offered. SPF is a measure of the screens ability to prevent sunburn over a period of time. SPF numbers tell a person how much longer they can stay in the sun without burning if they wear the product as opposed to not wearing any sun protection product.

Below is a table of a qualitative translation of SPF values:

Rating of Sun Protection Product

SPF Values

Protection Level

Minimal

2 to 4

Least protection; permits tanning. Recommended for people who rarely burn, but tan profusely.

Moderate

4 to 6

Moderate protection from sun burning; permits some sun tanning. Recommended for people who burn minimally and tan well.

Extra

6 to 8

Extra protection from sunburn; permits limited tanning. Recommended for people who burn moderately and tan gradually.

Maximal

8 to 15

High protection from sunburn; permits little or no tanning. Recommended for people who burn easily and tan minimally.

Ultra

15 or more

Most protection from sunburn; permits no tanning. Recommended for people who burn easily and rarely tan.

 

For example, if your skin usually burns in 10 minutes (without any type of sun protection) then a SPF of 15 means that you can stay in the sun 15 times longer (150 minutes) until you begin to burn

Another example; if you use a sunscreen with an SPF 30, your skin will have received one thirtieth (1/30th) of a sun burning dose in the same time that you would receive one (1) sun burning dose without the sunscreen.

2.3 Detailed discussion of a common active ingredient in sunscreen and its mode of operation.

As mentioned previously, suns screening active ingredients are organic in nature and have the ability to form excited isomers. Below are pictures of a few of common active ingredients:

Homosalate

2-ethylhexyl-p methoxycinnamate

Octocrylene

Oxybenzone

 

On examining the structures above, it can be seen that all of them have this in common: they all have an unsaturated, aromatic Benzene ring and a Carbon-Oxygen (Carbonyl) double bond.

Most sunscreens are aromatic compounds conjugated with a carbonyl group with an electron-releasing amine or methoxyl group substituted in the ortho- or para- position.

 

Photochemistry of sunscreens (How they work)

The nature of photon absorption

To begin at the beginning: a photon is a quantum or "packet" of electromagnetic energy with an energy equal to Planck's Constant (h) times its frequency (n). The absorption of a photon by an organic molecule causes the excitation of one of a pair of electrons in a low energy orbital to a higher energy unoccupied orbital

 

The excited states:

Before absorption, the orbital configuration of the electrons in the aromatic ring and carbonyl group are in their "ground" state. Upon absorption, two electronic states are possible. In one, the spins of the two electrons remain paired and, as in the ground state, the net spin of the pair is zero. This is called the "singlet" excited state. In the other, the spins of the two electrons are unpaired, and there is a net spin. This is called a "triplet" excited state. The energy of both excited states is eventually dissipated as heat or light.

 

Photochemical reactions

The singlet state is often short-lived, typically 10-9-10-8 seconds. Therefore, reactions that proceed from it must be quite rapid. Of more importance to the sunscreen formulator are reactions that proceed from the much longer-lived triplet state, which may lasts 10-4 seconds or longer. During the triplet state lifetime, the excited molecule looks and behaves as a diradical, from which many chemical reactions are possible. Usually, an isomerization will occur at this stage, in which the molecule will temporarily change in structure.

 

Energy transfer:

The excited molecule may react (to produce isomers or new products), or return to the ground state in its original form. Clearly, the latter is the preferred outcome because, among other reasons, the active ingredient is again available to absorb a photon.

 

Pathway of the Excited Isomer:

Many factors determine the pathway an excited molecule will take; including the triplet energy the triplet lifetime, the identity and concentration of the reactants, and the rates and activation energies of each competing reaction. Under certain conditions, the excited molecule may return to the ground state and its original form by transferring its energy by emitting electromagnetic energy.

 

3. Surfactants and Polymers in Sunscreen Lotion

3.1 Surfactants and polymers used in sunscreen lotion. Several standard/commercial sunscreens used in marketplace.

 

Surfactants and polymers are widely used in sunscreen lotions. They are essential ingredients to modify and/or improve the properties of sunscreen lotions. As we mentioned in the previous sections, there are many types of vehicles that can be chosen as the carrier for the sunscreens. The basic vehicles are: emulsions, gels, oils, sticks, mousses, aerosols, ointments, etc (Lowe & Shaath, 1990). Based on the type of the vehicles used, different surfactants and polymers can be used.

 

We will first give several formulations of the sunscreen that are available in the market, then we discuss the properties that enhanced/improved by surfactants and polymers (in section 3.2). Following is some of the formulations currently available in the marketplace, which illustrate how diversity the sunscreen is (Lowe & Shaath, 1990).

 

Formula type: Cream

Comments: Emollient skinfeel

Expected SPF: 25+

Ingredients:

Phase A                                                                      % w/w

                        Octyl dimethyl PABA                                       8.0

                        Benzophenone-3                                              5.0

                        Octyl methoxy cinnamate                                  6.0

                        Cyclomethicone                                                10.0

                        Glyceryl stearate SE                                         5.0

                        Phenyldimethicone                                            2.0

                        Cetearyl alcohol (and) ceteareth-20                  2.0

                        Cetyl alcohol                                                    1.0

                        Octyl palmitate                                     10.0

            Phase B

                        Water                                                               QS

                        Preservative                                                      QS

                        Glycerine                                                          5.0

                        Diethynolamine p-methoxycinnamate     8.0

                        Titanum dioxide                                                3.0

                        Xanthan                                                            0.2

                        Hydroxyethylcellulose                           0.1

            Phase C

                        Fragrance                                                         0.3

 

Formation type: Lotion

Comments: Inexpensive lotion with outstanding high temperature stability

Expected SPF: 15

Ingredients                                                                             % w/w

            Phase A

                        Octyl dimethyl PABA                                       7.0

                        Benzophenone-3                                              3.0

                        Strearic acid XXX                                            4.0

                        Mineral oil                                                        10.0

                        Myreth-3 myristate                                           4.0

            Phase B

                        Water                                                               QS

                        Preservative                                                      QS

                        Carboner 1342                                                 0.2

                        Propylene glycol                                               5.0

            Phase C

                        Triehtanolamine 99%                                        0.7

            Phase D

                        Fragrance                                                         0.3

                       

Formation type: Sunscreen oil

Comments: Luxurious skinfeel

Expected SPF: 15

Ingredients                                                                             % w/w

            Octayl salicylate                                                            5.0

            Menthyl anthranilate                                                      3.5

            Benzophenon-3                                                            4.0

            Octyl dimethyl PABA                                                   8.0

            Tridecyl neopentanoate                                     15.0

            Fragrance                                                                     1.0

            Cyclomethicone                                                            20.0

            Phenyldimethicone                                                        5.0

            Mineral oil                                                                    QS

 

Formation type: Sunscreen gel

Comments: Poloxamine aids in sunscreen solubilization

Expected SPF: 12

Ingredients                                                                             % w/w

            Phase A

                        Carboner 940                                                   1.5

                        Poloxamine 940                                                7.0

                        Ethanol                                                 QS

            Phase B

                        Dioctyl malate                                                   10.0

                        Octyl methoxy cinnamate                                  7.5

                        Benzophenon-3                                                4.0

            Phase C

                        Triehtanolamine 99%                                        0.7

            Phase D

                        PEG-15 cocamine                                            3.5

                        Water                                                               12.5

 

Formation type: Lip balm stick

Comments: Smooth application characteristics

Expected SPF: 8

Ingredients                                                                             % w/w

            Octyl dimethyl PABA                                                   7.0

            Benzophenon-3                                                            3.0

            Castor oil                                                                     QS

            Octyldodecanol                                                            5.0

            Beeswax                                                                      15.0

            Ozokerite                                                                     6.0

            Myristyl lactate                                                 4.0

            Candililla wax                                                               6.0

            Petrolatum                                                                    5.0

            Fragrance                                                                     0.5

 

Formation type: Sunscreen mousse

Comments: Excellent skinfeel

Expected SPF: 2-4

Ingredients                                                                             % w/w

            Water                                                                           QS

            Propylene glycol                                                           5.0

            Quaternium-25                                                 3.0

            Octyl methoxy cinnamate                                              3.0

            Cetearyl alcohol (and) ceteareth-20                              1.0

             Octyldodecanol                                                           5.0

            Preservative                                                                  QS

            Package

                        90% Concentrate

                        10% A46 propellant

 

Formation type: Nose protectant

Comments: Contains sunscreen and opacifying agents to achieve a very high degree of protection

Expected SPF: 25+

Ingredients                                                                             % w/w

            Phase A

                        Octyl dimethyl PABA                                       7.0

                        Benzophenon-3                                                3.5

                        Mineral oil (and) lanolin alcohol             5.0

                        C12-15 Alcohol benzoate                                 15.0

            Phase B

                        Water                                                               QS

                        Preservative                                                      QS

            Phase C

                        Zinc oxide                                                        25.0

                        Titanium                                                           30.0    

 

3.2 How does sunscreen emulsion stabilize? – Emulsion stability

Emulsion is by far the most popular of all sunscreens used for sunscreens, however, emulsion is the most difficult to stabilize. There are basically two types of emulsions: oil-in-water and water-in-oil, and two “styles”, cream and lotion. From a thermodynamic point of view, a stable emulsion is almost impossible.  Actually there are two phases in an emulsion, one is the small droplet (internal phase), another is the liquid media (outside phase). Both phases are immiscible. The droplets are always trying to come together and coalesce, and to form a large drop. So, the goal of the sunscreen producing is to keep the sunscreen stable for a relatively long time.

 

There is an equation describing the particle interaction: Stokes law (Lowe & Shaath, 1990):

 

 

where V is the velocity of sedimentation (interaction); d is the diameter of the particles of the dispersed phase; p1 is the specific gravity of the dispersed phase; p2 the specific gravity of the external phase, g = gravitational constant; n = viscosity of the external phase.

 

So there are two ways to reduce the sedimentation speed:

·        decrease the value of the numerator

·        increase the value of the denominator

 

It’s obvious that reducing the diameter of the dispersed droplets can increase the emulsion stability. Several ways can be used to reduce the particle size, in which the most important one is using emulsifier (as we discussed in the next section). Another way to reduce the size of the droplet is by mechanical stirring. When high shear is introduced to the sunscreen lotion, the size of the drop will be reduced.

 

Another way to decrease the V value is to decrease the (p1-p2) value. In other words, to make the specific densities of the external and internal phases as close as possible. Since it is not very easy to change the specific gravity of the internal (normally oil) phase, it is a feasible way to change the specific gravity of external phase (normally water), e.g., adding some alcohol to the water phase.

 

Instead of changing the numerator, we can also change the denominator, i.e., by changing n, the viscosity of the external phase. There are many ways to change the viscosity of the external phase: (a) add more external phase; (b) reduce the particle size; (c) add a fatty moiety to form liquid crystal; or (d) add a thickener (“gum”) to the external phase.

 

3.3 Attributes of Sunscreen Lotion Enhanced by Colloidal and Surfactant Aspects.

·        Emulsion Stability – Emulsifier

As we mentioned in the early section, emulsifier is a good way to reduce the dispersion droplet size, thus stabilize the emulsions. HLB (hydrophilic lipophilic balance) is the oldest and most widely used method to select emulsifiers (Lowe & Shaath, 1990). The HLB can be determined for each emulsifier. There are both nonionic and anionic emulsifiers.  Anionic emulsifier is more widely used than nonionic emulsifier, for example, triethanol-amine strearate.

 

·        Film Formers

Film formation is very important to increase the efficacy of sunscreens. One of the primary factors that affect the sunscreen to achieve a high SPF is the uniformity of the sunscreen film on the skin. SO it is crucially important that the film is thick and uniform. There are several materials that can be used to develops this type of films. Cellulosic gums are very good candidates, which actually can be incorporated in almost any kind of   sunscreens. Polyvinylpyrillidone (PVP) is another good candidate.

 

·        Emmollients

There are both oil-soluble and water-soluble emollients, but the oil-soluble ones are the most widely used. They can provide a silky skinfeel upon application, acting as the vehicles in which the oil-sunscreen is delivered. There are several types of emollients: esters (liquid and solid), waxes, fatty alcohols, and mineral oils, in which esters are widely used in the compounds of sunscreens.

 

·        Stabilizers/Protectants

Since emulsion is very delicate, it is subject to many directions. Thickeners (gum) are one of the sunscreen stabilizers. Antioxidants are another type of stabilizers. BHA, BHT, propyl gallate, and dilauryl thiodipropionate are four of the most popular antioxidatants (Lowe & Shaath, 1990). Preservatives are another type of stabilizers. They should be kept in the water phase so that it can protect the sunscreen emulsion.

 

There are many other properties that can be improved or enhanced by surfactant or polymers, like spreadibility, washability, sunscreen substantivity, sunscreen rheology, etc. Here we will not introduce each one of them.

 

4. Structure-Property Relations In Sunscreen Lotion

 

This section will address the chemical processes that underlie the mechanism of sun-blocking by sunscreens. To start, the reader is given the following equation which is critical in understanding the sun-blocking process. It is the equation relating the energy of the suns rays to the wavelength of these rays and is given below as follows;

 

(1)      E = hc/l

 

Where E = The energy of the wave; h = Planck’s Constant; c = The speed of light; l = The wavelength of the ray

 

It is observed from this equation that the higher the wavelength a ray possess, the lower in energy it will be. The higher a wavelength is, the more damage it inflicts upon the skin. In the electromagnetic spectrum, the most damaging rays from sunlight are the ultraviolet rays.

 

The ultraviolet spectrum can be broken up into three categories; the UVC region, the UVB region, and the UVA region. Of these three, the UVC region is the most damaging to human beings. Fortunately, most UVC radiation is filtered out by the stratospheric ozone layer (however the use of chlorofluorohydrocarbons(CVC’s), a pollutant,  is depleting this layer). The UVB and UVA regions are not filtered out by the ozone. These ultraviolet rays cause the most damage to skin and hair.

 

Suncreens have two main components which protect the skin. These components are; physical blockers, and chemical absorbers. Physical blockers are the first line of defense in skin protection. They are chemicals the reflect and scatter ultraviolet rays. Some examples of physical blockers are; zinc oxide, titanium oxide, and red petrolatum. However physical blockers alone are not a sufficient source of protection from UVB and UVA rays. The other component optimized in sunscreens is the chemical absorber. The function of the chemical absorber is to take the UV radiation react with it and release it back into the atmosphere in the form of heat. This way the radiation will not react with the skin and cause harm. There are many chemical absorbers, some that only absorb in the UVB region, and some which will only absorb in the UVA region. The UVA and UVB ranges are given below as follows;

 

UVA : 320 – 360 (nanometers)

UVB : 290 – 320 (nanometers)

 

Some examples of UVA chemical absorbers are; benzophones, anthranilates, and dibenzoyl methanes. Some examples of UVB chemical absorbers are; PABA derivatives, salicylates, cinnamates, and camphor derivatives.

 

It is important to understand the mechanism behind sunscreen action. Most sunscreen chemicals are aromatic compounds conjugated with a carbonyl group. While the ring itself is usually unreactive, the side chain will undergo what is known  as a “photochemical reaction”. Very often, the side chain has an electron – releasing group(usually an amine or a methoxyl group). This is important because in the UVA and UVB regions , the energy of the radiation quanta is in the same order of magnitude as the resonance energy of electron delocalization in aromatic compounds. This means that the energy absorbed from UV radiation corresponds to the energy required to cause a “photochemical excitation” in the sunscreen molecule (Lowe & Shaath, 1990). The sunscreen chemical gets excited to a higher state (p*) from its ground state (n) by absorbing the UV radiation. The sunscreen molecule then releases the high energy rays as low energy rays ( rays with a longer wavelength). After releasing these rays back into the atmosphere, the sunscreen molecule will then return to its ground state.

  Another important factor in sunscreen is the solvent that the chemical absorbers are in solution with. Depending on the polarity of the chemical, the range in which that chemical absorbs the radiation in will change. This is very important because if the absorber is too polar or too apolar, the range of the chemical could get shifted out of the UVA and/or UVB regions, rendering it useless. There two types of shifts a sunscreen chemical absorber could undergo. One type is a hypochromic shift, the other is a bathochromic shift.

 

A hypochromic shift occurs when the chemical absorber is a polar compound in a polar solvent. For a polar compound in a polar solvent, the ground state of the compound stabilizes. This means that it will require a greater amount of energy for this compound to reach its excited state (Lowe & Shaath, 1990). As a result, the wavelength reflected back by the sunscreen will be lower(this is the hypochromic shift).

 

For less polar compounds in polar solvents, the excited state will be more polar than the ground state. The polar solvent will stabilize the excited state of the compound (Lowe & Shaath, 1990). This will lower the requirement for an energy transition, so a higher maximum occurs for the wavelength(this is the bathochromic shift).

 

Finally, the last important consideration for sunscreen protection is the molar extinction coefficient. The extinction coefficient is the primary indicator for effectiveness of a sunscreen chemical. The chemicals that have a higher extinction coefficient are better at absorbing more of the energy from harmful UV radiation. Typically, a compound that is symmetric(and therefore has symmetry - allowed electron transitions) has a higher extinction coefficient than a compound that does not have symmetry(these compounds have symmetry - forbidden electron transitions). Another contributor to an increased extinction coefficient value are conjugated bonds within the molecule. This is due to the increased resonance delocalization in a conjugated double bond. A desirable value for the molar extinction coefficient is 20,000 or greater.

 

 

 

5. Processing & Suppliers.

 

This section give some sunscreens produced from vegetable oil and plant phenols, which contains the consumer and manufacture information, also the unit operations and other processing informations.

 

5.1  Major suppliers of sunscreen with a link to their website, and a brief description of their product and it’s efficacy

 

Lipase-catalyzed transesterification reactions yield novel ferulyl-substituted or coumaryl-substituted acylglycerols with properties suitable for use as sunscreen agents having broad spectrum UV protection. These agents have the advantage of being synthesized from natural materials, while providing a value-added use for vegetable oils. They are readily incorporated into standard sunscreen formulations.

        The ferulyl-substituted triacylglycerols of this invention are characterized by the properties of having the UV absorptivity of a cinnamate ester and the water-insoluble properties of a lipid, thereby rendering them useful as sunscreen agents for the skin that do not readily wash off with water. The UV absorbance of these products extends from about 280 nm to about 350 nm, and is particularly effective in the range of about 310-350 nm. This is predominantly in the UVA range, but also covers part of the UVB range. For additional UVB protection, the subject compounds may be formulated with other sunscreen agents as discussed, below.

          The sunscreen agents of the invention as defined by the general formula (I) may be formulated into any cosmetic preparations that are especially designed to be water-resistant. The total level of sunscreen agent in these preparations will typically be on the order of about 0.1 to 20%, by weight, and preferably within the range of about 1-10%, by weight. The amount of sunscreen agent currently approved in the United States for inclusion in a topical skin treatment formulation is 15%. It is contemplated that the agents of this invention will be incorporated into formulations that are both effective and safe. An effective amount (or photo protective amount) is that amount which is sufficient to significantly induce a positive effect of protection against UV sunlight as compared to a control. One measure of the effectiveness of the sunscreen agent is the Sun Protection Factor (SPF) of the composition. SPF is a commonly used measure of photo protection of a sunscreen against sunburn. The SPF is defined as the ratio of the UV energy required to produce minimal erythema on protected skin to that required to produce the same minimal erythema on unprotected skin in the same individual. See Federal Register, 43, No. 166, pp. 38206-38269, Aug. 25, 1978). A safe amount is that which does not produce serious side effects.

 

I-    Transesterification of Ethyl Ferulate with Triolein in Solvent.


Triolein (Nu-Chek-Prep.RTM. Elysian, Minn., 447 mg, 0.5 mmol) was dissolved in 5 mL of toluene in a 25 mL Schlenk tube at 60 0C. Ethyl ferulate (ethyl 4-hydroxy-3-methoxy cinnamate, γ-Aldrich, 111 mg, 0.5 mmol) was added to the reaction mixture followed by Novozym.RTM. 435 lipase [Candida antarctica, with a 1-2% (w/w) water content, 110 mg, combined mass of the enzyme and its support]. Thus the ethyl ferulate and triolein reactants were present in a 1:1 mol ratio and were catalyzed by 1 wt equivalent (with respect to ethyl ferulate) of lipase. The reaction was performed under a nitrogen atmosphere using standard Schlenk line techniques. The suspension was stirred at 60 0C., and 100 .mu.L. The reaction reached equilibrium after 72 h.

This reaction resulted in a 44% conversion of ethyl ferulate to the desired ferulyl monoolein and ferulyl diolein.


Sunscreen cream (W/O)

               Phase

              Name

                   %

                A

Compound from the above

3.00

 

Arlacel 581

7.00

 

Paraffin highly liquid (Item No. 7174)

 6.00

 

Arlamol S 7

2.00

 

Lunacera M

5.00

 

Dow Corning 344

4.00

 

Miglyol 812                                                          

2.00

 

Oxynex 2004 (Item No. 6940

0.05

    B                

Glycerol (Item No. 4093)                                      

2.00

 

 

Magnesium sulfate heptahydrate (Item No. 5882) 

             0.17

                                                           Preservative                   q.s.

                                                      Water, demineralized     to    100.00

 

Preparation:

Heat phase A to 75 0C. and phase B to 80 0C. Slowly stir phase B into phase A. Homogenize. Cool with stirring. If desired perfume at 40 0C.

Suppliers:
(1) E. Merck, Darmstadt
(2) ICI, Essen
(3) LW Fuller, Luneburg
(4) Dow Corning, Dusseldorf
(5) Huls Troisdorf AG, Witten
 
II-Transesterification of Ethyl Ferulate with Triolein without SolventUsing Recycled  Lipase

The catalytic stability of Novozym.RTM. 435 lipase was demonstrated by repeating the transesterification of ethyl ferulate and with neat TO with previously used lipase. After equilibrium was reached in the reaction as described from the above(I), the triolein was decanted, and fresh triolein was added to the reaction vessel. Residual quantities of ethyl ferulate and reaction products from the reaction above (I) were determined by HPLC and subtracted from the yields obtained from the second reaction. Ethyl ferulate was added to initiate the reaction, which stirred for an additional 120 h. HPLC analysis revealed that the second glycerolysis produced 31% ferulyl monolein and 43% ferulyl diolein, a combined yield of 74%. The residual amounts of ethyl ferulate and ferulyl glycerol after 120 h were 17% and 9%, respectively. The slightly higher quantity of unreacted ethyl ferulate present at equilibrium (17%) compared to the quantity of unreacted ethyl ferulate present at equilibrium in the original glycerolysis (13%) is attributed to the shorter reaction time. These results indicate that Novozym.RTM. 435 lipase remains active at 600C for weeks and is able to catalyze multiple glycerolysis reactions. 

Sunscreen milk (W/O)

         Phase
          Name
                 %
           A             
 Compound from the above
              3.00
   
   Pionier L-15              
             19.00 
 
Paraffin highly viscous (Item No. 7160 
              15.00            
           B
Glycerol (Item No. 4093)  
                5.00
 
Magnesium sulfate heptahydrate (Item No. 5882) 
                 0.50
                                                   Preservative                    q.s.
                                             Water, demineralized      to    100.00
 


Preparation:
Heat phase A to 75 0C. and phase B to 80 0C. Slowly stir phase B into phase A. Homogenize. Cool with stirring. If desired perfume at 40 0C.

Suppliers:
(1) E. Merck, Darmstadt
(2) Hansen & Rosenthal, Hamburg

Sunscreen oil

          Phase
                 Name  
                      %
              A
Compound from (I) 
                    3.00
 
 Arlatone T
                    2.00
 
Miglyol 812
                    14.00
 
Cetiol B
                    22.50
 
Isopropyl myristate   
                       7.50
 
Paraffin highly liquid (Item No. 4174)
              48.85
 
Oxynex 2004 (Item No. 6940)
                0.05
                 B 
Perfume oil
                0.10



Preparation:
Heat phase A to 70.degree. C. with stirring until all components are dissolved, stir until cold and add phase B at 40.degree. C.
Suppliers:
(1) E. Merck, Darmstadt
(2) ICI, Essen
(3) Huls Troisdorf AG, Witten
(4) Henkel, Dusseldorf
(5) Haarmann & Reimer, Holzminden

3.2 Processing (unit operations) involved in production of a basic sunscreen product. (Including purpose of each operation.)

 

Tanning involves the formation of melanin polymers in our skin.  Melanin monomers are already present in the outer layer of the skin, but in a reduced state.  When oxidized upon exposure to UV, the melanin polymer forms and absorbs light in the visible and ultraviolet region.  We notice the effect because of the absorption of visible light, thus the darkening of our skin.  Since the level of melanin monomers is regulated in the skin, prolonged exposure to UV leads to the production of additional melanin monomers, again a photochemical process using UV light energy.  The complete reaction is shown below:
 

 

Tyrosine is the enzyme that initiates the production of melanin using the amino acid tyrosine.  (A genetic deficiency of tyrosine leads to albino skin.)  While this process seems sufficient for ordinary circumstances, prolonged exposure to sunlight (and its UV component) can overwhelm the skin’s natural mechanisms and lead to damage, including skin cancer.  Sun blocks and sunscreens were invented to supplement the body’s natural defenses.
 

Sun blocks work on a fairly simple premise.  They work by absorbing all of the UV light before it reaches the skin and tend to reflect visible light, which is why they are often white.  Most sun blocks contain either ZnO or TiO2.  Sunscreens, however, are generally a colorless cream or oil made of one or more organic compounds that specifically absorb UV radiation.  They rely on UV-specific energy absorbance to use the energy from the UV light and thus keep it from causing photochemical reactions in human cells. 

             

The Food and Drug Administration codified Sun Protection Factor (SPF) in 1993 for labeling and marketing purposes. It is defined as the ratio of delayed sunburn on protected skin to unprotected skin, where the protected skin is covered by 2mg/cm2 of sunscreen. (Abney, 1998; Federal Register, 1999)  It is defined as the ratios of the minimal erythema doses (MED) of UV radiation leading to delayed sunburn on protected versus unprotected skin.  Thus, it is based on the physiological response in the wearer; SPF 34 should protect you from burning for thirty-four times the time of unprotected skin. 

 

 

In starting to prepare for this laboratory, was choose a range of materials that included sunscreens in the SPF 30 region, face lotions that claimed SPF 15. 

The procedure used by Abney et al (1998), the experimental procedure by Abney et al was based on extracting the organic compound active in the sunscreen. The Abney procedure would be more comparable to the sunscreens.  Abney et al also claimed less than two percent error on repeated trials.

 Sunscreens contain the following active ingredients:

 

 

 

These compounds, and other sunscreens, are structurally characterized as highly conjugated compounds containing aromatic rings and oxygen containing functional groups. (Kimbrough, 1997)  Note that these compounds tend to have polar functional groups and short hydrocarbon chains.  This could provide evidence for why sunscreens tend to lose their effectiveness when exposed to water – they are soluble in (polar) water. Insoluble compounds tend not to have polar functional groups or to have long hydrocarbon chains.
 

While there is a limit to the number as of 1999 permitted active ingredients, since the federal Food and Drug Administration limits the maximum concentration of any one ingredient by mass. The maximum concentrations for sunscreens and sun blocks are listed below:
 

Amino benzoic acid

15%

 

Octyl salicylate

5%

Avobenzone

3%

 

Oxybenzone

6%

Cinoxate

3%

 

Padimate O

8%

Dioxybenzone

3%

 

Phenylbenzimidazole sulfonic acid

4%

Homosalate

15%

 

Sulisobenzone

10%

Methyl anthranilate

5%

 

Titanium dioxide

25%

Octocrylene

10%

 

Trolamine salicylate

12%

Octyl methoxycinnamate

5%

 

Zinc oxide

25%

                                    (FDA, 1999, p27687)

Generally, a consumer product will contain a variety of these FDA approved compounds, each of which much (per FDA regulations) be of sufficient concentration to increase the finished product’s SPF by 2 or more.
To test the products, dissolve 0.1g of sunscreen in 10mL of isopropyl alcohol, for a solution containing 10g/L of sunscreen.  Heat the solutions in a water bath at 45-50ºC for one minute with mild agitation, than allow the solution to return room temperature.  When at room temperature, shake vigorously for one minute, and allow to settle.  The supernatant can be stored until ready for use – in this case, the prepared solution was prepared one week prior to testing.
 

Before testing for absorbance, dilute the supernatant 1:100 with isopropyl alcohol.  More concentrated solutions of the supernatant were tested, but resulted in absorbance values above the range measured accurately by the laboratory instrument available. In the 325 to 400 nm range that we were interested in, the Spec 20 did not provide absorbance values similar to those of the automated UV/V is spectrometer, and had no additional lower range even without a UV bulb in the spectrometer. Most importantly, because the Spec20 is not designed to measure absorbance in the high 300nm range, the absorbance values were not always reproducible in the same machine in the same sample.

 

 

 

 

 

Brand Name

SPF

Mass(g)

 

Brand Name

SPF

Mass (g)

Lubriderm (Face)

15

0.1025

Nexus (Hair)

N/A

0.1934

Neutrogena (Face)

15

0.1186

V05 (Hair)

N/A

0.1583

L’Oreal (Face)

15

0.1844

L’Oreal (Hair)

N/A

0.2061

PreSun (General)

28

0.1029

Face refers to products marketed as face creams; General to those marketed as sunscreens; Hair to hair products.

Banana Boat (General)

30

0.1000

BioSun (General)

30

0.1000

 

 

Comparison of UV-A Protection

 

 

 

 

 

 

Final Tip: 10 Rules for Sensitive Sun Protection (from http://www.sunprotection.org/)

 

  1. Know your skin type and protect yourself accordingly. And everyone, no matter what their phototype, should take it slow for the first few days of exposure. Let your skin get used to the sunshine progressively.
  2. Don't stay out in the sun between 11am and 3pm. The sun's rays are less intense early in the morning and late in the afternoon.
  3. Use a sunscreen that's appropriate to your phototype and to the level of sun you're exposed to. The lighter your complexion, the more slowly you should increase your exposure time.
  4. Take extra-special care with your kids. Make sure they wear caps, light-colored T-shirts and sunglasses. Little ones especially need frequent applications of sunscreen on all parts of their bodies exposed to the sun. And babies should be protected from the sun entirely - their skin is ultrasensitive!
  5. Remember to apply and re-apply your sunscreen when exercising or playing sports in the sun. Perspiration causes sunscreen run-off!
  6. Stay away from surfaces that reflect sunlight back at you - or else increase your sun protection. Snow, ice, sand, white cement all increase the effects of UV rays. And did you know that most UV rays can "dive" down into the water? It's a fact to keep in mind when you take a dip.
  7. Self-tanners are a great cosmetic aid. But they offer no sun protection at all! The "tan" they produce won't block out any UV rays.
  8. Keep out of the sun altogether if you are taking medicine that contains photo-sensitizing compounds (ask your doctor to be sure).
  9. If you should experience a severe sunburn, especially a blistering burn, consult a doctor without delay. A bad sunburn always requires medical attention.
  10. Be sure that your sunscreen contains both UVB and UVA filters. Although UVA's don't "burn" the skin, they penetrate deep into the skin's lower layers where they do considerable damage. UVB's, which are most intense when the sun is at its zenith, are chiefly responsible for sunburn.

 

References

 

Balsam, M. S.; Sagarin, E. (Editors) Cosmetics: Science and Technology, Interscience Publishers, New York, 1957.

Hany, R. G.; Wilkinson, J. B. (Editors) Principles and Practice of Modern Cosmetics, Chemical Pub. Co., New York, 1962.

Lowe, N. J. Physician's guide to sunscreens, Dekker, New York, 1991.

Lowe, N. J.; Shaath, N. A. Sunscreens : development, evaluation, and regulatory aspects,

Marcel Dekker, New York, 1990.

Rieger, M. M. Surfactants in Cosmetics, Marcel Dekker, New York, 1985.

Urbach, F. “The Historical Aspects of Sunscreens” Journal of Photochemistry and Photobiology B: Biology, 2001, 64, 99-104.

Whelan, J. M. CFTA Cosmetic Ingredients Dictionary, third edition, Cosmetic, Toiletry, and Fragrance Association, Washington, D.C., 1985.

http://www.sunprotection.org/

http://www.sunshinesoapworks.com/basics.html

http://www.lorealparisusa.com/skincare/suncare/index_frame.asp

http://www.atforyou.com/sbb.asp

http://www.ivillage.com/topics/beauty/skincare/

http://www.birchtrees.com/inin.html, http://www.birchtrees.com/

http://chemistry.org/portal/Chemistry?PID=feature_tea.html&id=c39c5b30cd7e11d5e6d64fd8fe800100

http://www.surfactants.net/formulary/happi/happi.htm#pc

http://www.millenniumskin.com/ingredients.html

http://www.patft.uspto.gov

http://www.dictionary.com