Contribution of 1H NMR to the investigation of the adsorption of cationic Gemini surfactants with oligooxyethylene spacer group onto silica.
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-EO(x)-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 (1)H NMR spectroscopy allowed a detailed description of the adsorption mechanism of these investigated 12-EO(x)-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.
Bendjeriou-Sedjerari
Anissa
A
Institut Charles Gerhardt, Equipe Agrégats, Interfaces et Matériaux Pour l'Energie, CNRS UMR 5253, Université Montpellier 2, C.C. 1502, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France.
Derrien
Gaelle
G
Charnay
Clarence
C
Zajac
Jerzy
J
De Menorval
Louis Charles
LC
Lindheimer
Marc
M