ADSORPTION TO KERATIN SURFACES 87 The wettability studies were conducted by first washing wool swatches with 1% sodium lauryl sulfate and then treating them with the appropriate dye or detergent solution for 60 seconds using an 8/1 solution to wool ratio (by weight). The fabric was then rinsed for 10 seconds under running tap water (80-ppm hardness), the excess water was squeezed out, and the fabric was allowed to dry overnight before testing. For the pH 1 experiments, solutions were brought to pH 1 with concentrated HC1 and to pH 7 with 0.4% phosphate buffer. After drying for 24 hours, single drops of tap water were placed on the swatches, and the time necessary for the drop to disappear into the fabric was observed with a stopwatch and recorded. Times for at least five drops (wetting time) was recorded per treatment. DISCUSSION To test this hypothesis of a mechanism for adsorption involving a continuum between a charge-driven process and a hydrophobically driven process, we explored existing literature and designed several experiments to test it. The variables we considered most relevant were: ß the structure of the adsorbing species ß changes in the charge nature of the keratin through pH variation STRUCTURE OF THE ADSORBING SPECIES: THE ADSORPTION OF CATIONIC SURFACTANTS TO HAIR The existing keratin literature shows that shorter-chain-length cationic surfactants have a lower affinity for both wool and human hair. For example, Steinhardt and Zaiser (2), decades ago demonstrated by measuring cation affinities to wool fiber that the affinities increase with increasing molecular weight or increasing hydrophobicity in the structure. More recently, Robbins et al. (1) demonstrated similar effects for hair by showing that cetyl trimethyl ammonium bromide (CTAB see structure 1 below) adsorbs to a greater extent onto human hair than dodecyl trimethyl ammonium bromide (DTAB see struc- ture 2 below) under a variety of test conditions. The data of Table I show this same effect for four different cationic surfactants at acid pH. H3+ CH 3- (CH2)15-N-CH 3 I CH 3 CTAB Br- (1) H + CH 3- (CH2)l• -N I - CH 3 CH 3 DTAB Br- (2)

88 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Cationic Adsorption Onto Human Hair mg/gm Adsorbed at 10-min. reaction time Cationic species pH 3.6 pH 6.9 Cation mol. wt. DTAB 2 8 228 CTAB 2 8 284 DTDMAC a 4 8 536 CS b 8 8 629 a Ditallow dimethyl ammonium chloride (see structure 3 below). b Culversoft S-75. The structure originally provided by Culver Chemical Co. for this high-molecular- weight quat corresponds to (2-heptadecyl-l-methyl-1 [(2stearoyl amide) ethyl] imidazolinium methyl sulfate)]. The structures originally assigned to many imidazolinium compounds have been found to be incorrect, and more recently open-structure compounds have been found to be the correct structures. The important point here is not the exact structure, but that this is a very high-molecular- weight quaternary ammonium compound and that its behavior is consistent with the trend found for the other quaternaries of this table. •3 + [CH 3- (CH2)17] 2 -N- CH 3 CI- (3) DTDMAC The data of Table I show two different effects as a function of pH. At the more acid pH (3.6), we see increasing adsorption with increasing molecular weight, i.e., increasing adsorption to hair as the hydrophobic character of the quaternary ammonium moiety increases. Therefore, at the lower pH, we see the hydrophobic effect however, at the higher pH (6.9), the hydrophobic effect is not apparent. At pH 3.6, the hair is close to its isoelectric point (7), and so the surface of the fiber is essentially neutral. As the pH of the system is increased, more carboxylate groups are formed in the hair. This ion- ization effect makes the hair more negatively charged and more receptive to the charge- driven process for a positively charged cationic adsorbing species. As a result, the lower-molecular-weight and less hydrophobic quaternary ammonium compounds adsorb more to the hair at neutral or higher pH. This result is consistent with the charge to hydrophobic continuum hypothesis. ADSORPTION OF QUATS IN THE PRESENCE OF LIPIDS Most hair conditioner formulations in the marketplace consist of cationic surfactants in combination with lipids such as fatty alcohols rather than as just plain cationic surfac- tants. The preferred pH for these products is also acidic, near 4.5, a pH more conducive to hydrophobic adsorption than to charged adsorption of the cationic surfactant. Fur- thermore, in these systems, there is evidence to suggest that aggregates of cationic and lipid adsorb, in contrast to adsorption by individual cationic surfactant molecules (8). In addition, data of Table II show that as the ratio of lipid to cationic in the formula increases, the ratio of lipid to cationic adsorbed onto the hair also increases. Therefore, as the adsorbing species develops more hydrophobic character (more lipid/cationic), it is not repelled by the hair, but continues to adsorb to the keratin. Thus, in conditioner formulations, by using combinations of cationic surfactant with lipids and by increasing