Molecular Mobility and Order of Didodecyldimethylammonium Chloride Adsorbed on Silica Particles from 2H Nuclear Spin Relaxation

Didodecyldimethylammonium chloride (DDAC) was adsorbed onto two different silicas with different particle radii at pH 10. Bulk adsorption studies for Cab-O-Sil M5 suggest that the surfactant forms a complete bilayer on the solid surface, as judged from the observed plateau adsorption level (Γ = 1.25 mmol/g) and the calculated surface density. In order to further elucidate the structure and dynamics of the adsorbed surfactant, 2 H NMR studies on selectively labeled DDAC (DDAC-d 6 , deuterium labeled in the nitrogen bound methyls) were undertaken. Spectra and spin relaxation parameters were recorded for the surfactant adsorbed on silica, in reference to those for DDAC-d 6 dispersed in water and in lamellar liquid crystals. These studies reveal that the exchange between the solid surface and solution is slow on the pertinent NMR time scale. For DDAC-d 6 adsorbed on large particles a quadrupole splitting corresponding to an order parameter of S DF = 0.0156 is clearly visible. Longitudinal (T 1 ) spin relaxation measurements at different temperatures are consistent with a higher degree of motional constraints in the adsorbed state, as compared to the dissolved state. Transverse (T 2 ) spin relaxation measurements and the observed deuterium bandshapes point to the existence of modes of slow molecular motions. An attempt was made to characterize these slow motions further, by studying the dependence of T 2θ on the pulse dispersion in the quadrupolar CPMG (Q-CPMG) pulse sequence. The correlation time for slow molecular motions so-obtained suggests that additional modes of slow motions are more important for adsorbed DDAC-d 6 than for aggregates in solution, the most likely candidate being related to motions caused by a mismatch of the preferred structure of the surfactant bilayer and the silica surface.