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The Physical Chemistry of Cosmetic Formulations

February 29, 2008

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.

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)
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