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

Organogels composed of 12-hydroxystearic acid (12-HSA) and low viscosity so lvents such as decane, decalin and ethyl acetate have been used as model sy stems for lubricating greases. The fluidity of perdeuterated probe molecule s in the above mentioned model systems was studied using two NMR spectrosco py methods. First, translational diffusion was studied for the perdeuterate d probes benzene, toluene, decane and dodecane by pulsed-field gradient NMR . The probes were introduced into a commercial lubricating grease and the m odel systems mentioned above. Second, the rotational diffusion tensor for p erdeuterated trans-decalin was studied by deuterium spin relaxation measure ments. 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 tha t no strong interactions occur between 12-HSA, and it also indicates that t he obstruction effect is responsible for the decrease in diffusion rate com pared to the neat solvents in 12-HSA-thickened systems. (C) 1999 Elsevier S cience B.V. All rights reserved.