388 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS bility is greatest in such a case, be- coming less when the volume of the continuous phase is increased (13). It is interesting that if we take as the hydrocarbon one which is be- tween benzene and nujol in the drophobic series, two transparent systems can be prepared. This has been done, for example, in the case of decalin and of a petroleum frac- tion of the approximate formula C•sHa•. containing a high proportion of naphthenes. These give first very viscous, almost gel-like sys- tems, which are water-continuous, with conductivities, however, less than that of a liquid dispersion. They show pronounced streaming birefringence. When more para- methyl cyclohexanol is added, they break and, on adding still more, liquid dispersions like those pre- viously described are formed. If a hydrocarbon such as decalin or hexane is used, the final dispersion is oil-continuous, but if a mixture containing higher paraffins is used, the system remains water-contin- uous, but has a lower conductivity than if no gel had formed (14). Formation of the two types of dis- persion is consistent with the views outlined above. As more alcohol is added, above the amount required for an equimolecular mixed film with the soap, it will pass into the oil phase, rendering it less hydrophobic by virtue of the hydroxyl groups so introduced. This will weaken the interaction between the oil and the paraffin chains of the soap relative ß to that between the polar groups and water. This will favor an inversion of the dispersion from water- to oil-continuity, but whether the in- version actually takes place or not depends on the degree of hydro- phobic character possessed by the oil. Thus, nujol is sufficiently drophobic to prevent it, but decalin is not. These phenomena can also be expressed in terms of the wetta- bility of the interface by the oil phase and the water phase. The phase which has the lower contact angle with the interfacial film will be continuous, and as the alcohol is added to the oil, it causes its con- tact angle to approach that of water. If the angle is lowered sufficiently by this means, inversion to an oil- continuous system occurs in the case of nujol the lowering is not sufficient to bring this about. STRUCTURE OF THE TRANSPARENT DISPERSIONS In view of their properties, it is clear that the structure of these isotropic dispersions must be dif- ferent from that of concentrated solutions of soaps and of so-called "solubilized oils." It has been shown by the x-ray studies of Hess (15), McBain (16), Harkins (17), and their collaborators, that a soap micelle can swell with oil, and a soap crystallite with water, only within certain definite limits. In the present instance, much more of the disperse phase is incorporated, and it is thought that the alcohol molecules penetrate between the soap molecules in the crystallites introducing disorder into the lattice,

!'ii•iii!•..:Which then breaks up and forms a •:.•"i!:,'. .'•spherical shell in order to enclose )•'•i: ia given volume of water or oil in a •iii:!:iSurface of minimum area (13, 19). :.,? :: ß: •'.• .: :. : PENETRATION AND COMPLEX-FORMATION IN MONOLAYERS 389 the soap alcohol molecules, several hundred Angstrom units in diameter separated by the continuous phase. Confirmation of the postulated WATER SWOLLEN CRYSTALLIT E OIL AND WATER SWOLLEN wATER SWOLLEN LAMELLAR MICELLE MIXED CRYSTALLITE LAMELLAR MICELLE WATER WATER HYDROPHILIC OLEOM ICELLE SPHERICAL IMfC. ELLES WA' SOAP ALCOHO OLEO•HILIC HYDROMICEM. E Figure 4.--The phenomena of sofubilization of soap crystal lamellae by non-polar oils and penetration of the monolayer lattice by polar oils in the formation of micelies and emul- SLOBS. Thus, Schulman and Hoar (18) sug- gested that these transparent, fluid dispersions consisted of droplets, sur- rounded by a mixed monolayer of structure of the fluid dispersions was sought in three ways. In the first place, the area occupied at the air- water interface by a molecule of po-