The Surface Activity of Tetra- n -alkylammonium Halides in Aqueous Solutions. The Effect of Hydrophobic Hydration

Abstract

Surface tensions are reported for aqueous solutions of a series of homologous tetra-n-alkylammonium chlorides, bromides, and iodides (R4NX) at 25 °C, and the experimental results are discussed with particular reference to the bulk properties of these salts in water. It is found that Me4NX salts are surface inactive in aqueous solutions and that they may be classified as simple salts. On the other hand, it is found that Et4NX and higher homologous salts are surface active in aqueous solutions and that the surface activity increases with the number of carbon atoms in the alkyl chains; thus, they are hydrophobic structure-makers in the water phase. The surface tensions for aqueous solutions of Me4NX salts are only slightly dependent on the kind of counter-ion, such as Cl−, Br−, and I−. In the cases of Et4NX, Pr4NX, and Bu4NX salts, the effectiveness of the anions for the same cation on the surface activity increases progressively in this order: I−>Br−>Cl−.