In addition to their properties as wetting, suspending, emulsifying, and colloidstabilizing agents, dilute aqueous and non-aqueous solutions of soaps and other soap-like detergents-non-electrolytic as well as anionand cation-active materials-possess the remarkable property of being able actually to dissolve otherwise insoluble substances (4,15,16,19,25). Not only do such solutions form spontaneously, but the resulting colloidal solutions are thermodynamically stable, as shown by the fact that they enter into true reversible equilibria with non-aqueous solutions where the solute exists as simple molecules, such as a water-insoluble dye in toluene. They likewise form true equilibria with solid crystals (14).The formation of such solubilized systems is accompanied by a lowering of free energy. For example, the vapor pressure of a volatile insoluble liquid taken into colloidal solution by a solubilizer is far less than that of the volatile liquid alone (17). The vapor pressure of a solution of a soluble volatile hydrocarbon in water is reduced by adding a detergent, and the solubility is correspondingly increased. Clearly this phenomenon is quite distinct from peptization of preexisting particles or droplets, which do not exist in the cases just cited.The present communication includes experiments which help to clarify detergent processes, and provides systematic measurements of the "solubilization" of dyes. The formation of such thermodynamically stable colloidal solutions,-"solubilization,"-must be distinguished from other phenomena such as (a) emulsification, where the vapor pressure of the emulsified liquid is equal to that in bulk or even greater due to the curvature of the droplets; (b) change in solvent power due to the addition of a large proportion of a second liquid; (c) peptizing, protective, and suspending action of preexisting particles, or those formed by mechanical means or by condensation from a supersaturated solution; (d) "sequestration," for example, of calcium soaps in water by substances such as Calgon (sodium hexametaphosphate) for the action of which we may tentatively, in the absence of more definitive evidence, accept the explanation of Hall (5) as being due to the formation of complexes with the cation, especially since Reitemeier and Buehrer (22) found no colloidal particles visible in the ultramicroscope;and (e) "solutizing" of mercaptans (28), in part due to salt formation, by strongly alkaline concentrated solutions of substances which are not effective in dilute solution and where colloids are probably not involved.In general, the above actions are possessed in varying degrees by most commercial detergents and protective colloids. For example, some excellent proi Presented at the Eighteenth Colloid Symposium, which was held at
