The formation of water-repellent films on solids by treatment in aqueous solutions of heteropolar substances involves some relatively liktle known surface chemistry of considerable theoretical interest. The most important industrial use of the phenomenon is in the separation of minerals by flotation. Heteropolar substances useful in this application, k n o w as "collectors," differ widely both in their solution characteristics (solubility, degree of ionization, surface activity) and in their modes of reaction at the solid surface. We believe, in fact, that water-repellent films on solids form not only by chemisorption or chemical reaction at the solid surface but also by physical adsorption, the latter type occurring commonly in the extremely capillary-active heteropolar compounds (long-chain paraffin salts, acids, and bases) (4). 17Tater-repellent film-forming compounds may be divided into three main groups: (a) anionic substances, Le., substances in which the active part of the molecule is the anion, (b) cationic substances, and ( c ) nonelectrolytes, listed in order of their industrial importance. The soluble nonelectrolytic collectors are also somewhat heteropolar and undoubtedly adsorb as whole molecules (examples are dixanthogen, aliphatic and aromatic sulfides and disulfides, and triphenylphosphine) ( 7 ) . We shall not be interested here in water-insoluble types. Under optimum conditions for selective conditioning of solids in flotation, anionic and cationic types exist in the (bulk) solution largely as ions. Typical anionic collectors are the xant,hates; these are k n o m to be nearly completely ionized in the (generally) basic flotation circuits. Alkylamine hydrochlorides are typical cationic collectors, and these are also largely dissociated into ions in the flotation pulp. Because of the strong electrolytic properties of these substances, proposed mechanisms for the formation of hydrophobic films by type (a) and type ( b ) collectors have generally been ionic in character. The various mechanisms that have been suggested for the "ionic" collectors are discussed comprehensively by Wark ( 2 3 ) .Recently, nn interesting controversy of neutral molecule adsorption us. ion exchange has arisen in the theory of anion-exchange resins. Bishop (2) accounted for the "ion-exchange" properties of Amberlite IR-3 on the basis of extraction of free acids of even such strong acids as hydrochloric rather than the exchange of anions at the surface. Schwartz, Edwards, and Bourdeaux (20), in studying ' Research Fellow. Utsh Engineering Esperinient Station, Salt Lake City, Utsli. S o w Senior Lecturer,
