Nuclear Magnetic Resonance Studies of Surfactant Mesophases

Abstract

Nuclear magnetic resonance (n.m.r.) spectroscopy has been used to study smectic and waxy mesophases occurring in surfactant systems. The measurement of n.m.r. line widths as a function of temperature and surfactant concentration in a system containing crystalline, mesomorphic and isotropic phases produces information about the phase diagram of the system. The different kinds of phases are easily distinguished by their line widths but it is not always possible to distinguish different phases of a given type. Line widths of waxy type mesophases in saturated soaps have been found to be sufficiently characteristic of the phase to allow transition temperatures to be determined. Transition temperatures determined by n.m.r. agree well with those determined by other methods. Studies of the neat and middle phases in several systems indicate that the phase structures are probably identical for members of a homologous series of compounds. Neat phase lines are in general wider than middle phase lines and are more sensitive to changes in molecular structure. The length of the hydrocarbon portion of the surfactant molecule appears to be the primary factor determining the widths of the lines from middle phase. Mesophase line widths are found to be only slightly temperature and concentration dependent. N.m.r. lines from mesophases appear to have a characteristic shape. The shape can perhaps best be described as “super-Lorentzian”. The widths of n.m.r. lines from mesomorphic phases appear to be almost independent of the bulk viscosity of the systems.