SORPTION BY STRATUM CORNEUM 551 EFFECT OF SURFACTANTS ON SORPTION OF POLYMER JR-125 BY STRATUM CORNEUM OF NEONATAL RAT 2O E 15 5 . 10•15-S-9 • 1% Na •URYL SULFATE 0.01% BARQUAT MB-50 8 16 24 TIME IN HOURS Figure 7. Effect of surfactants on the sorption of Polymer JR-125 mula (12), it can be estimated from this experiment that the diffusion con- stant of the polymer through the membrane must be of the order of 10 -14 cm2sec or less. The picture which emerges, therefore, is one of hindered diffusion due, largely, to sorption of the charged macromolecule along the diffusion routes. The driving force for this sorption must involve coulombic attraction be- tween the cationic polymer and the negatively charged stratum corneum. It has been shown already in l•ig. 3 that an uncharged polymer, of essentially the same basic structure as Polymer JR, shows little more than monolayer ad- sorption. If the above deduction is correct, the uptake of Polymer JR should be sensitive to the addition of ionic substances. This is, in fact, the case, as is demonstrated in Fig. 5 for addition of simple salts. There is a strong valence effect in the inhibition of sorption: La +++ Ca + + Na +. This can be ex- plained as a charge density effect. The higher valence ions compete more ef- fectively for available sorption sites since they are more tightly held by the substrate. It is, of course, true that the polymer solution properties are changed by the presence of salt. However, significant depression of sorption can be observed at salt levels of 0.001 M, which is too low to have much effect on the polymer solubility or configuration in solution.

552 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Another consequence of the coulombic attraction theory is that the addition of organic solvents should cause a decrease in sorption, due to lowering of the dielectric constant of the solution with attendant decrease in dissociation of the cationic polymer and hence lessened attraction to the substrate. Figure 6 shows this effect for ethanol-water. There is a limit to how far one can go in this direction, since Polymer JR is insoluble in pure liquids of low dielectric constant. Polymer JR finds commercial use in lotions and shampoos. In these vehicles it is mixed with many additiw•s, some of which are surface active. Therefore, it ,:,,as of interest to determine the effect of typical materials of this type on polymer uptake. Figure 7 shows some of the different possibilities. From the above discussion, it is not surprising to see that a nonionic surfactant, such as Tergitol© 15-S-9, has only a slight inhibitory effect even at a high level of con- centration relative to the polymer. However, the anionic sodium lauryl sulfate causes a large decrease in polymer sorption. This is probably due to a com- bination of two effects. One is the competition for sites by the sodium ions, and the other is the known interaction of surfactant and polymer (13). Some- what surprising is the great effect of the cationic surfactant, Barquat MB-50. The explanation for this may be that the free spaces in the outer layers of substrate become blocked by sorption of the rather large surfactant cations, or that the latter ions merely preempt the adsorption sites. CONCLUSION The data shown here support the hypothesis that a cationic polymer is capable of slowly diffusing into the outer layers of stratum corneum, rather than forming multilayers on i•. Although the conditions of sorption used in these studies are extreme and unlikely to be even approached in any prac- tical application, the uptake observed emphasizes the highly substantive na- ture of the polymer. The driving force for this diffusion is charge attraction between the cationic polymer and the negative sites within the stratum cor- neum. Added electrolytes and low dielectric constant solvents tend greatly to decrease the sorpfion by competition for sites or by decreasing the effective charge of the polymer. Ionic surfactants also reduce the sorption. ACKNOWLEI)GMENT l The procedure utilized for separating the neonatal rat stratum corneum was outlined for us by Dr. E. J. Singer and Mr. E. K. Boisits of Lever Brothers Company.