SWELLING OF EPIDERMAL MEMBRANE 31 alkyl chain length of twelve carbon atoms (29), and is consistent with the amount of binding of these same surfactants by callus (29), and with the explanation of binding offered by Breuer (30) in his review of the interaction between surfactants and keratinous tissues. This optimum in EM swelling at the chain length of 12 carbon atoms was produced at constant molarity (0.069 M), with sodium as counterion in all cases, and shows large differences in the EM response to alkyl chain length. This demonstrates the importance of hydrophobic interactions on EM swelling. As Breuer (30) explains, the total bond energy between surfactant and epidermal protein is a combination of the bond energy of the ionic bond and the hydrophobic attractions of the alkyl chain with epidermal protein hydrophobic bond energy increases with increasing chain length. However, the energy required to penetrate the epidermal protein matrix also increases with increasing chain length, and above an alkyl chain length of 12 carbon atoms, the penetration energy resulting from increased chain size becomes greater than the resultant increase in binding energy. Thus there is the optimum in the amount of binding and in swelling at 12 carbon atoms. As the experiments were run at neutral pH, the differences in swelling could possibly be explained on the basis of increased uptake of surfactant i.e., as more surfactant is taken up into the membrane, the latter exhibits increased hydration and swelling. However, pH effects between EM and the anionic surfactant interaction demonstrate that the amount of surfactant uptake is only part of the story. When the effect of water, pH adjusted with HCI or NaOH was examined, no significant change in CW swelling was found between pH 3 and 9. Table VI summarizes some effects of pH on the CW Table VI Effects of pH on Surfactant Swelling of EM* Surfactant (0.069 M) pH SLS LAS DTAB 3 3.09** 2.84** 2.77** 6 3.24 3.00** 2.70 9 3.27 2.95** 2.69 *Values are actual membrane lengths (cm.) after 1 hour reaction time (average of 5 membranes each) **Significantly different from other column responses at o• = 0.05 level. swelling of EM by three different surfactants. At acidic pH, the greatest amount of ionic bonding occurs between a keratin and an anionic surfactant (31) however, for both anionic surfactants, the least swelling is produced at acidic pH, indicating that ionic bonding does not increase the keratin water binding to the same extent as hydrophobic bonding of anionic surfactants at neutral pH. This same phenomenon occurs in hair for the cationic surfactant, dodecyl trimethyl ammonium bromide (DTAB), which binds in greater amount to keratin at alkaline pH (32), but it produces more swelling at acidic pH. Thus, when pH varies, the amount of swelling is not proportional to the amount of surfactant that binds to the keratin. In fact, anionic surfactants can produce EM shrinkage when the surfactant treatment is at both low pH and low concentration (see Table VII). This experiment gives evidence that ionic bonding, the principal bonding made at low pH, can even cause a decrease in

32 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table VII pH and Concentration Effects on EM Swelling by LAS Concentration pH 0.0035M 0.069M A. C IV Swelling (Treated/water length) * 3 0.95** 1.03 6 1.03 1.10'* pH Volume Ratio*** B. Volume Swelling (treated/dry volume) 3 2.16'* 6 3.19'* *Each value is an average of 6 replicas after 1 hour reaction time. **Significantly different from other values at the = 0.05 level. ***Calculated from weight gains assuming density of keratin = 1.55, and density of sorbed solution 1.00. Reaction time one hour at 0.0035 M LAS. the water binding which is observed as decreased CW lengths compared to the hydrated lengths, or shrinkageJ Yet, when anionic surfactant bonds to EM primarily via hydrophobic interactions, e.g., at neutral pH, water binding is enhanced showing increased CW lengths. We intend to investigate this area of research in greater detail to more fully test these conclusions. THIOGLYCOLIC ACID Human hair is more responsive than EM to thioglycolic acid (TGA), a disulfide bond breaking agent (Table VIII). Although hair length actually shrinks in going from water Table VIII Swelling of Hair and EM by TGA (1 - Hour)* CW Length** Thickness** EM + 4% + 22% Length** Diameter** Hair -- 1.4% + 38% *% Change from water swollen state at 1-hour. **Average of 20 replications. to 6% TGA at pH 9.2, the diameter increases by almost 40 percent. Once again the Menefee model (1), which views the matrix as the region that resists swelling, explains the experimental data. However, the fact that EM is more responsive to anionic surfactant (SLS) than to TGA might not be expected (Table V). The low disulfide content of the EM matrix components must be partly responsible for this difference, and as described earlier the reciprocal relationship of cystine and glycine (25) may also be important. These differences again emphasize the importance of hydrophobic and ionic bonding to the stability of EM, and the importance of the disulfide bond to hair fiber stability.