Structural, volumetric, and thermodynamic characterization of a micellar sphere-to-rod transition

The thermotropic sphere-to-rod transition of nonionic surfactants was characterized in terms of a large set of parameters: the transition temp. and width, the partial vol., coeff. of thermal vol. expansion, enthalpy, isobaric heat capacity, and structural parameters, such as radius of gyration and hydrodynamic radius. Data were recorded as a function of concn. of surfactants in H2O and in D2O. To this end, pressure perturbation calorimetry (PPC), small angle neutron scattering (SANS), dynamic light scattering (DLS), differential scanning calorimetry (DSC), and isothermal titrn. calorimetry (ITC) were applied in a study of aq. solns. contg. myristyl, tridecyl, and lauryl maltoside and heptaethyleneglycoltetradecyl ether (C14EO7). Small changes in the thermodn. and volumetric parameters (e.g., the partial vol. change is .apprx.+2.permill.) are discussed in detail as the result of three effects governing the transition. (i) Redn. of the water accessible hydrophobic surface area (ASAap) drives the transition. (ii) Shrinking in headgroup size by thermal dehydration triggers the transition. (iii) Hypothesized gradual ordering of the chains may control the effect of chain length on the transition. [on SciFinder (R)]