J. Soc. Cosmet. Chem., 45, 85-94 (March/April 1994) Adsorption to keratin surfaces: A continuum between a charge-driven and a hydrophobically driven process C. R. ROBBINS, C. REICH, and A. PATEL, Colgate Palmolive Company, 909 River Road, Piscataway, NJ 08854. Received September 3, 1993. Presented in part at the 8th International Hair Science Symposium, Kiel, Germany, September 1992. Synopsis A hypothesis is presented that explains and clarifies a wide variety of experimental observations regarding the adsorption to keratin fiber surfaces. It explains that the mechanism of the adsorption of conditioning agents, dyes, and other substances to hair fibers can be considered as a continuum between a charge-driven process and a hydrophobically driven process and that the exact nature of the reaction depends primarily on the structure of the adsorbing species and the pH of the system. Several examples are provided that illustrate and provide support to this hypothesis. INTRODUCTION The adsorption of conditioning agents to human hair has been studied for decades (1-3), and although it is generally recognized that several variables contribute to deposition, the mechanism of action is generally explained on the basis of a charged interaction providing the primary driving force (1,5). However, with the relatively recent advent of 2-in-1 conditioning shampoos that employ highly water-insoluble dispersed silicones as conditioning agents, hydrophobic factors, which lead to the separation of a water- insoluble conditioning phase during rinsing, and Van der Waals forces, which cause large particles to adhere to surfaces, are clearly becoming more important to mechanistic discussions (4,6). A hydrophobic role in the adsorption process has been appreciated for decades since the early demonstration of increased adsorption to keratins with increasing carbon chain- length for both anionic surfactants (6) and for cationic surfactants (1,3). Thus it occurred to us that some systems might adsorb to hair surfaces via a process that is largely charge-driven and others by a hydrophobically driven process, but that as structural modifications are made to the adsorbing species or as the pH of the adsorption medium is changed, the resultant altered nature of the adsorbing species or of the keratin itself might affect the mechanism to make it less charge-driven or more hydrophobically driven or vice versa. As a result, we hypothesized that adsorption to hair might be considered a continuum (see Figure 1) between a charge-driven process and a hydro- 85

86 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Mechanisms for Adsorption to Keratins Charge Driven • Adsorption Continuum Hydrophobic • Driven Adsorption • Add Lipid • PH Figure 1. Schematic of a mechanism for adsorption to keratins. phobically driven process. This paper describes some experimental studies and literature for several different systems that support this charge to hydrophobic continuum mech- anism for the adsorption to keratin substrates. EXPERIMENTAL MATERIALS AND METHODS The surfactants used in the adsorption studies of the cationic surfactants were hexade- cyltrimethyl ammonium bromide (CTAB), technical grade from Eastman Organic Chemicals dodecyltrimethyl ammonium bromide (DTAB), prepared in our labora- tories (% Br found 26.01 calculated 25.95) ditallowdimethyl ammonium chloride (DTDMAC) from Sherex Chemical Co. and Culversoft S-75 (CS) (2-heptadecyl-1- methyl-1 [(2stearoyl amide) ethyl] imidazolinium methyl sulfate), originally sold by what was formerly Culver Chemical Co. Stock solutions/emulsions were prepared from the above quats, and sorption studies were conducted as described by Scott et al. (1). For the adsorption studies with cationics and lipids, cetyltrimethyl ammonium chloride (CTAC), from Henkel Chemical Co., and cetyl alcohol, from Aldrich Chemical Co., were used, as well as stearalkonium chloride (Ammonyx 4002), from Onyx Chemical. These were radiochemical sorption studies, and the C 14-1abeled surfactants and alcohols were synthesized internally by Andrew Charig. The amodimethicones used were Q2-8220, x2-8107, and Type II-X2-8107 from Dow Corning. These varied by amine content and molecular weight. The amine content was provided by Dow Corning however, the supplier would not provide the molecular weights of these species, except that the amine content decreased with the molecular weight. These were incorporated into a shampoo base containing 19% ammonium lauryl sulfate and 4% unilin 425 (linear primary alcohols with a 20 to 40 carbon chain length) according to the making procedure described for examples 1-4 of U.S. Patent 5,106,613. Hair treatment was similar to that in the adsorption studies described above, and the hair was analyzed for silicone content according to the procedure of Kohl and Gooch (9). Petrolatum (Protopet white 1S•-Witco/Sonneborn Chemical) was incorporated into the shampoos by the same making procedure described in the above U.S. patent. Wool swatches (Testfabrics Inc., Middlesex, N.J.) were treated and analyzed by radiotracer analysis for C•4-1abeled octacosane (C 28 hydrocarbon, Sigma Chemical Co.). The ex- perimental procedure is described by Robbins et al. (8).