AMINE ADSORPTION ON KERATIN 293 not met. Even the Freundlich plot was not strictly linear, and hence interpretations of other isotherms in this work will be made only on a phenomenological basis. From a study by the Fuerstenau group (23) of the adsorptive interaction of an anionic surfactant with alumina, it was postulated that the adsorption process proceeds over three successive stages. The first (region I), characterised by a low isotherm slope [log (adsorption density) plotted as a function of log Cs], is operative at very low surfactant concentrations and is thought to be typical of uptake on a charged surface by ion exchange of individual molecules. The second (region II) is denoted by an abrupt increase in the isotherm slope. In this region surfactant ions are adsorbed by electrostatic attraction and hemi-micelle association of hydrocarbon chains. The third and final stage (region III) is identified by a reduction in the isotherm slope in this region of relatively high surfactant concentration, opposition to further adsorption is exerted by electrostatic repulsion between ions adsorbed on, and in the vicinity of, the surface. Fuerstenau has speculated that surfactant molecules adsorbed under these conditions may be oriented with the polar groups away from the surface. Rosen (24) has adopted this mechanism to explain the manner in which ionic surfactants are adsorbed to textiles such as wool and nylon that possess charged surfaces. It is possible that the change of slope shown by the isotherms in Figures 5 through 9 is indicative of a transition from region II to region III. At this point (log Cs ca. -2.5) the uptake has reached saturation level (i.e., Cf is constant with increasing Cs) unlike the above mentioned study with alumina where adsorption continued to a lesser extent in region III but was not halted. The concentration range over which region I predominates is, however, less clear, particularly in the absence of added electrolyte. The isotherms for mono- and diaminododecane shown in Figure 4 clearly display appreciable slopes at the lower limits of Cs (log C = ca. -4.5), and it is therefore likely that region II is still operative at this point (to extend the concentration to lower values would require radiochemical analytical techniques). It is possible, however, that the isotherm shown in Figure 5 for the diamine exhibits region I over the concentration range -log Cs = 4 to 3. The critical micelie concentrations (CMC) of the monoamine in the absence (Figure 4) and presence (Figure 5) of added salt are shown on the respective isotherms. It has been suggested (24) that monolayer coverage of the surface is complete in the neighborhood of the CMC. If this is the case for wool-amine systems, then higher orders of aggregation would need to be postulated to account for additional fiber sorption. Evidence that the first region of the isotherm represents a mode of adsorption influenced by electrostatic interactions can be gained by comparing the isotherms without and in the presence of added salt. This can be seen by comparing the isotherms for the C12 mono- and diamines in Figure 4 with the isotherms shown in Figure 5 which were determined in the presence of 0.1 M NaC1. It is readily apparent that over the concentration range log C from -4.0 to -3.5 the amount of amine taken up by the fiber is substantially lower when salt is present. By contrast, the amount taken up by the fiber at saturation uptake [termed the effectiveness of adsorption (23)] is approximately doubled when salt is added to the adsorbate solution. These two contrasting effects suggest that ionic interactions exert opposing influences. Thus at low concentrations of amine, adsorption is initiated largely by an electrostatic attraction to the fiber surface in which case high electrolyte concentrations effectively

294 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS inhibit amine uptake by contracting the electrostatic field around charged groups. On the other hand, at high amine concentrations the contraction of electrostatic fields surrounding the positive head-groups of the adsorbate molecules permit a higher density of packing. Consequently, the barrier to bilayer formation or other modes of aggregation is diminished. INFLUENCE OF SIZE OF HYDROPHOBIC CHAIN ON SATURATION UPTAKE Information concerning the arrangement of adsorbate molecules with respect to each other at the liquid/surface interface can be gained from a comparison of adsorption isotherms pertaining to amines of different hydrocarbon chain-lengths. If the adsorbed amines are arranged with their carbon chains perpendicular to the substrate surface, then variations in chain length will have little or no effect on the amount of adsorbate bound by the fiber (23). This is because the area occupied by each molecule is determined only by the size of the hydrophilic head-group. However, if the layer of adsorbate molecules is not close-packed or at an angle to the surface, then an increase in chain-length of the hydrocarbon-chain will favor more effective van der Waals' contacts, thereby creating a higher packing density and promoting hemi-micellar aggregation. The saturation uptakes at 40 ø and at two pH values for a series of mono- and c•,c0-diamines are listed in Table I. As already noted for the C•2 mono- and diamine TABLE I Saturation Uptake (mMole/Kg dry fiber) at 40øC of Straight-chain Mono- and Diamines on Wool in the Presence of 0.1 ]H NaCl Monoamines Diamines C n- NH 2 NH 2- Cn- NH2 pH (a) n = 8 10 12 n = 8 10 12 3.6 30 50 75 __(b) __ 40 7.8 65 115 385 10 (•) 120 (a) The pH value of the sorbate solution was that recorded at equilibrium for those samples which had attained maximum saturation sorption levels. (b) Uptake amount too small to be measured with certainty. shown in Figures 1 and 2, the saturation uptakes of the monoamines at pH 7.$ compared to pH 3.6 is considerably greater. For the C, and C•0 amines the increase is about double, while for the C,2 amine the increase is 5-fold. In the case of the diamines, a comparison can be made only for the C•2 where the increase in uptake at pH 7.8 compared to pH 3.6 is approximately 3-fold. In both classes of amine the effectiveness of adsorption increases significantly with the addition of two methylene groups in the hydrocarbon-chain. In the monoamine series the increase in uptake comparing C•2 to C, is 6-fold for the corresponding diamines a 12-fold increase is observed. From these results it could be concluded that hydrophobic interactions between adsorbate molecules serve to increase the extent of miceliar aggregation, and that the structure of these aggregates is such that the hydrocarbon chains are randomly oriented with respect to the fiber surface.