Cosmetics physical stability

Surfactants are useful in formulating a wide variety of disperse systems. They are required not only during manufacture but also for maintaining an acceptable physical stability of these thermodynamically unstable systems. Besides the stabilizing efficiency, the criteria influencing the selection of surfactants for pharmaceutical or cosmetic products include safety, odor, color, and purity. 

Moisturizers and other similar stay-on products have often pH between 4 and 6. That pH-range is similar to skin surface pH and is often suitable for good physical stability of the cosmetic product. However, there are several moisturizing creams with world-wide acceptance, which have pH of about 7 or even 8, for example, those with stearic acid as the main emulsifier. Also skin protectants based upon zinc oxide often have an alkaline pH. 

As a rule, therefore, the demands of chemical and physical stability are similar for drugs and cosmetics. Under certain circumstances, the demands on physical stability may be especially critical as, for example, in hand and body lotions that have to perform under tropical conditions after having been exposed to the heat of the sun on the beach or after storage under arctic conditions in a ski hut. 

All of the above processes are influenced by the nature of the two emulsifiers used to prepare the multiple emulsion. Most reports on multiple emulsions are based on conventional nonionic surfactants, but unfortunately most of these surfactant systems produce multiple emulsions with Hmited shelf-Uves, particularly if the system is subjected to large temperature variations. During the past few years, multiple emulsions have been formulated using polymeric surfactants for both the primary and multiple emulsion preparation. These polymeric surfactants proved to be superior over conventional nonionic surfactants in maintaining the physical stability of the multiple emulsion, such that today they may be applied successfully to the formulation of agrochemical multiple emulsions. The results obtained using these polymeric surfactants offer several potential applications in formulations. The key in the latter cases is to use polymeric surfactants that are approved by the FDA for pharmacy and food, by the CTA for cosmetics, and by the EPA for agrochemicals. 

JL Zatz. Physical stability of suspen.sions. J. Soc. Cosmet. Chem. 36 393-411. 1985. 

Zatz JL. Physical stability of suspensions. J Soc Cosmet Chem 1985 36 393-411. 

M Gasperlin. J Smid-Korbar. J Kristi. Formulation and physical stability evaluation of creams comaining silicone surfactants. Farm Vestn (Ljubljana) 43 3- lO. 1992. JL Zalz. RY Lue. Effect of polyols on physical stability of suspensions containing nrmionic surfactant. J, Soc, Cosmet, Cheni. 33 149-155. 1982. 

Chapters 12 to 15 deal with some specific applications of surfactants in the following industries personal care and cosmetics, pharmaceuticals, agrochemicals and the food industry. These chapters have been written to illustrate the applications of surfactants, but in some cases the basic phenomena involved are briefly described with reference to the more fundamental chapters. This applied part of the book demonstrates that an understanding of the basic principles should enable the formulation scientist to arrive at the optimum composition using a rational approach. It should also accelerate the development of the formulation and in some cases enable a prediction of the long-term physical stability. 

From the above discussion, the formulation of effective sunscreen agents that meet the following requirements is clearly necessary (1) Maximum absorption in the UV-B and/or UV-A. (2) High effectiveness at low dosage. (3) Non-volatile agents with chemical and physical stability. (4) Compatibility with other ingredients in the formulation. (5) Sufficient solubility or dispersibility in cosmetic oils, emollients or in the water phase. (6) Absence of dermato-toxological effects with minimum skin penetration. (7) Resistance to removal by perspiration. 

Poly(dimethylsiloxane) was the backbone of this growing industry. The unprecedented inertness of this polymer under thermal, chemical, and biological environments, coupled with its unique physical behavior, led to a myriad of commercial applications. Its uses include mechanical applications, in which stability in hostile environments was the desired attribute surface treatments, in which low surface energy was the important feature and cosmetic and biomedical applications, in which the biological inertness of the material was exploited. During this period, this new and healthy industry was expanding in all directions. 

Emulsions are used extensively in the food, pharmaceutical, and cosmetics industries, where their most important properties include stability (both physical and qualitative), rheology, reactivity, shelf life, texture, appearance, and flavor. All of these properties are affected by droplet size and/or size distribution [107,108], which in turn are functions of the method of production. Because emulsions are thermodynamically unstable, they require energy input for production and usually rely on a stabilizing agent (emulsifier, surfactant) to remain stable over long periods [109]. 

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