Abstract
The present study aims to investigate the behavior of a series of cationic Gemini surfactants with a hydrophilic spacer at liquid–gas and solid–liquid interfaces, with particular emphasis on the effect of spacer length. Gemini surfactants containing two quaternary ammonium groups bound by an ethylene oxide spacer chain, referred to as 12-EOx-12 with x=1,3,7 and 12 were synthesized. Surface tension measurements were used to show that the hydrophilic spacer with oxyethylene moieties was not fully extended at the air–water interface. With increasing the spacer group size, it became sufficiently flexible to adopt a particular conformation with a loop at the water side of the interface. A combined study by adsorption isotherm measurements and 1H NMR spectroscopy allowed a detailed description of the adsorption mechanism of these investigated 12-EOx-12 surfactants, with NMR providing more precise information on the conformation of hydrophilic spacer at the solid–liquid interface. Binding to the silica surface involved one cationic headgroup for the surfactants with a short spacer and the two headgroups for the ones with a long spacer. The number of charged surface sites was estimated by considering the dimeric surfactant as a “molecular ruler.” The small density of adsorption sites gave rise to the formation of pinned surface micelles.