290 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Cf(mMIKg) 160 ¸ 140 _ 120 100 80 /o / SAL 60 z.13 o...._.•o• •'• E] 0•, 3 4 5 6 7 8 9 pH Figure 2. Effect of pH on the equilibrium saturation uptake (Cf) of 1,12-diaminododecane on wool at 40 ø in the absence and presence of added sodium chloride. The dependence of fiber uptake on the pH of the amine solution is an indication of the contribution made by electrostatic interactions in the adsorption process. According to an electrophoretic study (19), the components which comprise the inner surface as defined above (prepared by mechanical abrasion of the fiber) were all found to be isoelectric at pH 4.5 (0.005 acetate buffer). Thus when fibers are immersed in a solution of this pH value it is assumed that the inner sorptive surface is electrically neutral [as a point of interest, the point of zero charge of the external surface of the wool fiber is found from streaming potential measurements to be atpH 3.5 (20) as compared to a value of 6.3 for the isoinoic point of the bulk fiber] at higher pH values the surface will become increasingly negative, thus inducing the cationic amine species to bind by coulombic attraction. The large difference observed in Cf between a mono- and diamine of the same number of carbon atoms, particularly at pH values greater than 7 [e.g., 240 mM/Kg for dodecylamine and 70 mM/Kg for diaminododecane at pH 8 (40 ø) in the absence of salt], could be explained in part by the manner of initial electrostatic binding to the surface as shown in Figure 3. If the diamine is bound to the surface by both terminal

AMINE ADSORPTION ON KERATIN 291 Adsorption of Lonc]-choin Monoomines •fion / oeou[ombic offroof ion • l•1t•1111i•111,t• - Fiber Surfoce I I !•111•111GI Mono[oyer I•1111•11 lel l l f•l Hemi-miceile Hydrophobic offroof ion Adsorpfion of Long-choin o(.,(u-Diomines Figure 3. Schematic representation of possible modes of adsorption of long-chain monoamines and c•, o0-diamines at a keratin fiber surface. charged groups as opposed to the vertical arrangement in the case of the monoamine, then subsequent aggregate formation by way of hydrophobic interactions (discussed later) would result in a comparatively lower uptake per unit surface area. ANALYSIS OF ADSORPTION ISOTHERMS AT CONSTANT pH Figure 4 shows the adsorption isotherms (at 40 ø) on wool of dodecylamine and 1,12-diaminododecane at pH 8 where each point represents the equilibrium distribution of sotbate between the fiber (Cf, mM/Kg dry fiber) and the solution in contact with the fiber (Cs mole/liter). Although it is more usual to represent concentration as a linear function, the isotherms shown in this study display concentrations of adsorbate as a negative logarithm. This mode of presentation was adopted in order to detect small differences in adsorptive behavior at low concentrations. When the isotherms were plotted with Cf as a function of Cs (mole/l), the shape of the curve could be classified according to the phenomenological scheme of Giles (21) as Class L, Subgroup 2. To test whether the isotherms shown in Figures 4 and 5 conform to ideal behavior in which monolayer formation results from adsorption at specific sites of equal energy, an attempt was made to fit the isotherm to the Langmuir equation. Two graphical plots were made: 1/Cf against 1/C s (Klotz plot) and Cf/Cs against Cf [known as the Scatchard plot, the form most frequently used in theoretical analyses of binding interactions in biopolymer systems (22)]. In neither case was linearity observed, which leads to the conclusion that the conditions mentioned above regarding monolayer adsorption are