The fluidity of hydrocarbon regions in organo-gels, studied by NMR - Basic translational and rotational diffusion measurements

Organogels composed of 12-hydroxystearic acid (12-HSA) and low viscosity solvents such as decane, decalin and ethyl acetate have been used as model systems for lubricating greases. The fluidity of perdeuterated probe molecules in the above mentioned model systems was studied using two NMR spectroscopy methods. First, translational diffusion was studied for the perdeuterated probes benzene, toluene, decane and dodecane by pulsed-field gradient NMR. The probes were introduced into a commercial lubricating grease and the model systems mentioned above. Second, the rotational diffusion tensor for perdeuterated trans-decalin was studied by deuterium spin relaxation measurements. All probed samples contained 5% probe (by total mass). The measurements show that the microviscosity in the gels is quite similar to the bulk viscosity in the corresponding neat solvent. This indicates that no strong interactions occur between 12-HSA, and it also indicates that the obstruction effect is responsible for the decrease in diffusion rate compared to the neat solvents in 12-HSA-thickened systems. (C) 1999 Elsevier Science B.V. All rights reserved.