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 H-1 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. (C) 2008 Elsevier Inc. All rights reserved.