INTERACTION OF SURFACTANTS AND KERATINS 43 I I •ot 26'0øc ,2 _ o I I ! 0 O.OS 0'10 0'1,5 0-20 f, m01e 1-1 Figure 2. Binding isotherms of sodium naphthalene sulphonic acid to human hair f and r denote the equilibrium concentrations and the uptakes, respectively. (Reproduced with permission from reference 3.) characterizes the absorption processes and, thus, measures the strength of the binding. Two types of experimental approaches have been used for the determination of these quantities in the past. In the 1940s Steinhardt, Fuggit and Harris measured the uptake of many acids and bases by wool using potentiometric titration (2). From the plateau regions of the titration curves at extreme pH values, these authors determined the maximum binding capacity of wool towards the various acids (Figure 1). The mid-points of the titration curves, i.e., the pH values at which half saturation occurred, served as the basis for calculating the affinities of the acids to the wool. More recently (3,4), a different experimental approach was pursued for measuring uptakes of materials by hair and other keratins. In this technique the levels of binding were determined as a function of the concentration of the absorbate in solution at a constant pH value. Generally, reciprocal plots were also prepared from the binding data as these allowed a more accurate extrapolation to infinite concentrations and, therefore, a more precise determination of the binding capacities of the absorbants (3). (For typical isotherms, see Figures 2 and 3.) The binding free energies could be obtained from the absorption isotherms by a variety of mathematical techniques (4,5). The experimental data from these types of studies led to a number of important conclusions (3). Firstly, it appeared that, at given pH and molar concentration, charged compounds were taken up by keratins to a much lesser extent than their uncharged

44 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS .OOc o o o o x o I I I I 0 20 •,0 /• 60 80 100 l/r L rn0t½ -1 Figure 3. Reciprocal plots of the binding isotherms of sodium naphthalene sulphonic acid f and r denote the equilibrium concentrations and the uptakes, respectively. (Reproduced with permission from reference 3.) counterparts, i.e., molecules of similar sizes and shapes. In fact, it did not seem to matter whether the charges on the molecules were only of one type (i.e., either positive .or negative) or whether the molecules carried both types of charges attached to their structures (i.e., zwitterions). In either case, strongly diminished uptakes were observed compared to those obtained with uncharged molecules of similar sizes (Table I). Table I Uptake of Various Organic Compounds by Virgin Hair Uptake mol/g Phenol 2.8 X 10 -4 Resorcinol 3.0 ) 10-4 Catechol 2.9 X 10 -4 Hydroquinone 1.8 X 10 -4 1,3 Dihydroxy-naphthalene 8.0 ) 10 -4 Sodium benzene sulphonate Sodium naphthalene sulphonate Sodium picrate Trimethylamine-benzimide Trimethylamine-naphthimide 0.05 X 10 •4 0.80 X 10 -4 0.18 X 10 -4 0.05 X 10 -4 0.05 X 10 -4 All uptake values were taken at pH 6, 25øC and at equilibrium concentration 0.04 mol/l. (Data reproduced from reference 3.)