358 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS also determined that complex forma- tion was reversible, washing the protein with water resulting in the removal of the anion. Steinhardt also concluded from these and other experiments (7) that the at•nity of anions for protein increased with molecular weight. Aicken (8) stud- ied the adsorption of primary and secondary alcohol sulfates (molecu- lar weight about 280) by wool. Ad- sorption was found to increase with increasing acidity to a maximum comparable to the acid binding ca- pacity of the woolmabout 80 to 100 milliequivalents per 100 gin. of wool. At neutral pH's per cent adsorption increased with increasing concentra- tion of the solution and was also en- hanced by the addition of electro- lyte. Attempts to desorb the alco- hol sulfate by soaking in water for 48 hours proved unsuccessful, in marked contrast to Steinhardt's re- suits. Finally, reference might be made to the work of King (9) and Neville and Harris (10). The latter confirmed earlier work that wool im- mersed in neutral soap solution ad- sorbs sodium ions (cations) faster than it does fatty acid this was evidenced by a rapid drop in pH of the soap solution. King earlier had analyzed soap solutions in which wool was immersed and had found tl•at sodium ions were adsorbed preferentially and that adsorption of oleate ions or oleic acid increased as the concentration of the solution decreased. This behavior is in marked contrast to the behavior of the highly dissociated neutral .syn- thetic surface-active agents. The evidence would indicate, therefore, that conditioned as we are to the use of soap, the normal feel of skin is simply that produced by high sodium ion concentration and fatty acid adsorption on the skin surface. The truth, as some one said, is rarely pure and never simple the data just reviewed must be consid- ered critically. These studies were all made under conditions approach- ing equilibrium and with relatively dilute solutions. Equilibrium con- ditions are possibly not attained in the short shampooing operation and dilute solutions are obtained only during rinsing. The 'variations in complexity of detergent solutions as a function of concentration are not yet fully appreciated there is still considerable argument as to whether protein adsorbs large miceliar aggre- gates, surface-active ions, or mole- cules. The viewpoint just discussed ap- pears to be, however, of some value in explaining and perhaps indicating the possibility of attacking some troublesome shampoo problems. (1) The rinsing properties of synthetics are undoubtedly affected by their adsorption properties. Anionic agents should rinse better at slightly alkaline pH's cationics should rinse better at slightly acid pH's. (2) Harshening of the hair, unman- ageability, and di•culty in combing after shampooing may very well be the result of increases in friction such as that noted as the result of adsorption of surface-active ions on the'skin adjustment of pH should overcome these di•culties. (3)

SHAMPOO FORMULATION 359 Surface-active agents differ in the ease with which they lather on the skin or hair often several lather- ings are required. While defoaming . by the fatty acids present on the hair and skin is undoubtedly re- sponsible for some of this poor lathering, adsorption of the surface- ß •: : ': active agent on the skin would, of course, render this material unavail- able for lathering. (4:) Salts of sur- face-active anions and slow rinsing organic bases might be useful in decreasing or eliminating the scroop" effect. (5) If it is desir- able to duplicate the effects pro- duced on the skin and hair by soap, synthetic surface-active agents with carboxyli'c • acid groupings, but with better hard water properties, could be utilized. Materials of this sort have been: described and have actu- ally been found to be excellent skin detergents. Experiments devised to determine the role of conditioning agents, un- fortun&rely} have not been very successful. These materials--added to shampoo detergents to improve softness--are generally non-ionic agents, a typical example being lan- olin. Addition of this and other ma- terials to alkyl aryl sulfonates did not appreciably affect the feel pro- duced. There is evidence (11), however, that lecithin (a phospholip- oid) and fatty esters of diglyce•øl do modify the adsorption properties of anionic' and cationic agents on cell membranes as they inhibit germicidal activity. Incidentally• it is possible' to demonstrate visually the adsorpi•'i'on of quaternary atom0- nium compounds on glass surfaces (12) by development of rainbow color effects when steam is con- densed on the surface the addition of a wide variety of non-ionic agents did not appreciably affect the de- velopment of rainbow co)or, al- though the effect of pH was con- firmed as in the case of protein ad- sorption. The apparent contradiction in- volved in producing "scroop" on the skin by adsorption of surface- active agents of the type used as lathering agents, and the producing of softness by other surface-active agents, may be resolved as follows: These latter materials are all straight chain compounds. The theory of lubrication as developed years ago indicates that such ma- terials are much better lubricants than the shorter chain compounds of the type used for shampooing. Hydrophilic groups also affect lubri- cating properties. Aromatic char- acter is also known 'to affect lubrica- tion adversely. This all• corre- sponds with the observation that alkyl aryl sulfonates and. alkyl naphthalene sulfonates produce much harsher feel than straight chain alkyl sulfonates.. It was thought that perhaps the feel and frictional properties of the skin might be due to the presence of hy- drophilic groupings on the outer side of the surface-active agent film. There is some evidence that this type of arrangement does occur in some cases. However, it has been found that regardless of the type of film which may be on the skin'sur-