Low-angle X-ray diffraction has been used to characterize the two lyotropic mesomorphic phases (neat phase and middle phase) which are formed in the dodecylhexaoxyethylene glycol monoether+ water system. The occurrence of these two anisotropic mesomorphic phases does not affect the behaviour of the thermodynamic excess functions which have been interpreted in terms of solutewater interactions. Careful measurements of water vapour pressure have been necessary to establish the limited extent of the two phase co-existence regions which separate homogeneous mesomorphic phase regions from isotropic solution in this system. Volume and heat capacity measurements as a function of temperature across mesomorphic phase boundaries have confirmed that the neat phase/ isotropic solution and the middle phase/isotropic solution transformations are first-order phase transitions with very small volume and enthalpy changes.
A light-scattering study has been made of the effect of concentration and temperature on the micellar size in a series of homogeneous non-ionic detergents in aqueous solution. It has been found that above the critical micelle concentration, as determined by surface tension measurements, an increase in micellar size takes place until a critical concentration is reached. Above this value, constant micellar molecular weights are observed for a finite concentration increase.All the compounds examined show an approximately exponential dependence of micellar molecular weight on temperature. The logarithmic temperature coefficients are similar for the higher homologues examined.
A study has been made of the effect of temperature on the size of xnicelles formed by a pure non-ionic detergent in aqueous solution. It has been shown by light-scattering measurements that the micelle molecular weight increases exponentially with increasing temperature, reaching a value of nearly two million close to the lower consolute temperature. Increase in size of the micelles has little influence on their solubilization properties, but viscosity studies indicate that the particle asymmetry increases as the molecular weight increases.
Measurements of water vapour pressure and heat of mixing have been made at 25°C over a range of composition for the 'octyl-and decylhexaoxyethylene glycol monoether + water systems. The condensed binary phase diagrams show that only the system given by the higher homologue forms a lyotropic mesomorphic phase (middle phase) at 25°C. In both systems, the thermodynamic properties of the solutions are determined by the interaction of the hydrophilic groups with water. The water activity and heat of mixing are smooth functions with respect to composition with no measurable discontinuities at the mesomorphic phase boundaries. The consequences of this conclusion and its relationship to the structure of middle phase are discussed.
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