Water and Solvents on Surfaces Bristling with Alkyl Chains

Abstract

Surfaces bristling with alkyl chains behave hydrophobic when the coverage exceeds a critical packing density which depends on the alkyl chain length. Suitable model systems are swelling-type layered materials such as mica-type clay minerals which on external and internal surfaces carry cationic surfactants.

Water and polar organic solvents can be sorbed on the internal surfaces under expansion of the layer distance. The interaction with water is governed by the alkyl chain length, the chain packing density and the charge distribution within the silicate layers. The uptake of water between the internal surfaces (and also on the external surfaces) is determined by formation of distinct water clusters between the alkyl chains. Under certain conditions interlamellar sorption of water can completely separate the layers, and the propyl- or butylammonium compounds delaminate into colloidal dispersions.

Addition of salts influences the sorption of water on the internal surfaces and can largely increase the layer distance. Evidently, the anions of the salts excert the dominant effect. In organic solvents, both cations and anions have decisive influence so that a straight-forward explanation of the salt effects is often difficult.