Mary Heppenstall-Butler

Phase separation in gelatin/dextran and gelatin/maltodextrin mixtures

Abstract Phase separation mechanisms and kinetics in quiescent and shear conditions were studied using small-angle light scattering, optical polarimetry and confocal laser scanning microscopy in the gelatin/maltodextrin and gelatin/dextran systems. In the former system the temperature quench caused phase separation, which was studied in the gelled and liquid states, whereas in the latter system phase separation was triggered by the conformational ordering of the gelatin molecules and could only be studied when the system gelled. In both systems the different phase separation mechanisms of nucleation and growth and spinodal decomposition were identified from the different behaviour of structure function measured by light scattering. In the liquid state coarsening of the microstructure occurred by droplet coalescence that was accelerated by hydrodynamic effects when the droplets reached a certain size. Gelation hindered, but did not prevent coarsening. Reduced coarsening rates were measured in the gelled systems. In most cases the phase separation kinetics were faster than the gelation kinetics, and the system rapidly evolved into the late stages of phase separation that were characterised by a well-defined morphology with sharp interfaces. For sufficiently rapid ordering kinetics, corresponding to deep quenches, in the gelatin/dextran systems, however, it was possible to trap the microstructure in the early stages of phase separation while the interfaces were still diffuse. When the phase-separated liquid gelatin/maltodextrin system was sheared, coarsening was accelerated at low shear rates due to increased rates of droplet coalescence. At higher shear rates, stable elongated structures were formed. At one particular shear rate (approximately 1 s−1), the rates of break-up and coalescence were balanced and a monodisperse size distribution of elongated droplets was formed.

Nonequilibrium behavior in the three-component system stearic acid-sodium stearate-water

The state behavior of stearic acid−sodium stearate−water, which provides a model of a fatty acid structuring mechanism, was studied by differential scanning calorimetry (DSC), optical microscopy, and X-ray scattering. DSC mapped the state diagram of aged and quenched samples as a function of temperature. Optical microscopy identified a liquid crystal transition. Simultaneous small- and wide-angle X-ray scattering identified the structures present within different regions of the state diagram. Two acid/soaps differing in melting temperature, unit cell, lamellar spacing, and acid to soap chain ratio existed at room temperature. In half-neutralized samples, a lamellar hydrated gel phase with an hexagonal unit cell persisted to room temperature on quenching. Nonequilibrium behavior persisted to different degrees depending on the extent of neutralization of the system.