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The Physical Chemistry of Cosmetic Formulations

Formulation is the meticulous balance of all ingredients in a single physical entity. The author provides some guidance on how to create effective and esthetically-pleasing cosmetic products.

Toufique Choudhury, Ph.D.
Vice-President of Research & Development
Lifetech Resources
Chatsworth, CA

Cosmetic products are composed of multiple ingredients of different physico-chemical properties. In the process of formulation, all ingredients lose their intrinsic identity. In one sense, the basic philosophy of cosmetics formulation is as same as that of cooking gourmet foods. Formulation is the meticulous balance of all ingredients in a single physical entity. In order for all ingredients to take shape into a specific physical form, there plays an invisible physical chemistry. Cosmetics formulation is more than just simple mixing of all ingredients. There must be chemistry behind putting together all ingredients.^1,2 Without chemistry it would be called a “mixture” rather than a formulation.

All product development starts from an idea or dream that are converted into a physical form by combining multiple ingredients. Product ideas must match the chemistry of selected ingredients in order to obtain an expected physical form of a product. Formulation of any cosmetic product is mainly composed of two main parts:
• Structural and
• Functional.

There must be a strong affinity among the structural ingredients to produce a desired physical form.³ Structural ingredients have to be selected based on the purpose and packaging of the product. For a lipstick formula, structural ingredients are mainly composed of castor oil, carnauba wax, candelilla wax, bees wax, paraffin wax, ozokerite, isostearyl alcohol, isopropyl palmitate, butyl stearate, cetyl ricinooleate, etc. Adding too much of waxes may render the sticks brittle and with poor pay-off. Wax efficiency is an important criteria for the desired visco-elastic properties of lipsticks. Chemical attraction between waxes and oils determines the visco-elastic characteristics of lipsticks. If there is a stable structure of lipstick base, then that will be a good start to combine the pigments uniformly for the desired application properties.

In many instances the pH of the finished product determines the choice of structural ingredients. For example, the pH of a cream containing 10% alpha hydroxy acids (AHA) is likely to be between 3.5 and 4.5 after making an adjustment with the addition of an alkaline solution. In this case, carbomer and soap-based anionic emulsifiers are not good choices for structural ingredients. Acidic pH stable gum and emulsifiers would be better choices to obtain a stable emulsion system for an AHA cream. If there is no chemistry of affinity between structural and functional ingredients, then it would be ineffective to use even a high shear mixer or homogeneizer.

Chemistry of Product Forms

Each ingredient has its own specific physico-chemical parameters including solubility/miscibility, melting point, specific gravity, viscosity etc. In a mixture, all individual ingredients lose their own identity to produce a new physical form depending on the equilibrium of physico-chemical characteristics of all combined ingredients. For example, xanthan gum is a free flowing powder and water is a colorless liquid. If 1% of xanthan gum is dispersed in water, then both ingredients will lose their own identity producing a viscous aqueous solution. However, if xanthan gum is mixed with sunflower oil, then no viscous solution will be obtained due to their physico-chemical properties. Therefore, in a mixture, physico-chemical properties among different ingredients determine the texture, appearance and other characteristics of the final product.

The chemistry of individual ingredients determines the texture of finished goods. It is the mystery of chemistry how the same molecular weight can give products completely different texture and properties. For example, both stearic acid and isostearic acid have the same empirical formula and same molecular weight, but they have dissimilar physico-chemical properties. Stearic acid is a solid waxy material as opposed to isostearic acid which is a liquid. Stearic acid is one of the most widely used ingredients in cosmetics. A basic thick cream can be formulated using stearic acid, but similar texture can’t be obtained using isostearic acid.
    • Empirical formula: C₁₈H₃₆O₂ (stearic acid): Waxy
    • Empirical formula: C₁₈H₃₆O₂ (isostearic acid): Liquid
    • Empirical Formula: C₁₈H₃₈O (isostearyl alcohol): Liquid
    • Empirical Formula: C₁₈H₃₈O (stearyl alcohol): Waxy.

For example, the basic structural ingredients used in most deodorant sticks on the market are sodium stearate and propylene glycol. This combination, however, does not work to formulate antiperspirant stick, as there is no chemistry to hold the stick structure in the presence of aluminum chlorhydrate or other similar active ingredients.

Physical Forms and Efficacy of Products

Physical forms of finished products are mainly determined by the structural ingredients. Therefore, it is very important to fully understand the physical chemistry of structural ingredients in order to optimize the formulation of cosmetic products. Emulsion is the mostly widely used system in cosmetics. So, the fundamental basis for emulsion technology is the interactions among water, surfactant and oil.⁴

The Importance of HLB

Hydrophilic-Lipophilic Balance (HLB) of surfactants determines the outcome of the mixture of water-surfactant-oil in a ternary phase diagram.⁵ Point “P” of the phase diagram (below) represents 65% water, 25% oil and 10% surfactant. Physical characteristics of “P” will vary depending on the HLB of emulsifiers and the nature of the oil phase. Most of the emulsions are located around this area of the phase diagram. Increasing surfactants/emulsifiers does not necessarily increase the stability of emulsions. HLB of emulsifiers may not appear to play a significant rule in a complex formula, but its basic understanding would help to select optimum emulsifiers.

The ternary phase diagram was mainly originated from research works on soap technology.⁶ Formation of liquid crystalline neat (lamellar) phase is very important for the stability of emulsions.⁷ After applying creams/lotions almost as high as 85% of water evaporates leaving a surface film composed of non-volatile parts. Emulsifiers greatly determine the degree of continuous residual film on the skin surface. For a basic formula, the combination of surfactants that forms the larger area of isotropic oily phase in a phase diagram will also provide more continuous surface film.⁸ The formation of this continuous surface film is very crucial for the efficacy of physical sun filter based SPF products, makeup products, moisturizing creams, etc.

Conclusion

The conversion process from conception to marketable items involves multiple steps fraught with many challenging tasks. Every single ingredient has its own inherent chemistry that greatly influences the physico-chemical properties of finished products including tactile sensation. A clear understanding of physical chemistry forms the basis of optimizing cosmetics formulation for both performance and stability of products.

Cosmetic products are composed of multiple ingredients of different physico-chemical properties. In the process of formulation all ingredients lose their intrinsic identity. In one sense, the basic philosophy of cosmetic formulation is the same as that of cooking gourmet foods. Formulation is the meticulous balance of all ingredients in a single physical entity. In order for all ingredients to take shape into a specific physical form, there plays an invisible physical chemistry. Cosmetic formulation is more than just simple mixing of all ingredients. There must be chemistry behind them.

References

1. Levine I.N.;Solutions, Phase Equilibrium, Surface chemistry and solids & liquid in “ Physical Chemistry” McGraw-Hill (1995).
2. Brummer Rudiger; “Rheology Essentials of Cosmetics & Food Emulsions”. C.H.I.P.S.(2006).
3. Mollet Hans, Arnold Grubenmann ; “ Formulation Technology “ Wiley-VCH ( 2000).
4. Tharward F. Tadros; “Applied Surfactants: Principles and Applications “Wiley ( 2005).
5. Martin M. Rieger and Linda D. Rhein; “Surfactants in Cosmetics” CRC Press (1997).
6. James W. McBain and Williams C.Sierichs ;“ The Solubility of Sodium and Potassium Soaps and the phase diagrams od aqueous potassium soaps. “in Journal of the American Oil Chemists Society Vol. 25, No 6/June , 1948, pp 221-225.
7. Friberg Stig E and Concepcions Solans ; “ Surfactant Association Structures and the Stability of Emulsions and Foams “ in The ACS Journal of Langmuir Surfaces and Colloids. Vol.2 No 2, March/April 1986.
8. Choudhury T.H., Marty J.P., Orecchioni A.M. , Seiller M. and Wepierre J.;“ Factors on the occlusivity of aqueous emulsions. Influence of Humectants “ J. Soc. Cosmet. Chem.,36,255-269 (July/August 1985)/J.Soc.Cosmet.Chem,36, 255-269 ( July/August 1985)