Assessment of droplet self-shaping and crystallization during temperature fluctuations exceeding the melting temperature of the dispersed phase

Abstract

Oil-in-water emulsions with crystalline dispersed phase are widely used formulations in life sciences. Ideally, during cooling each droplet crystallizes in its original size and shape set by chosen process parameters. Recently it was shown, that droplets may spontaneously break their shape symmetry crystallizing in non-spherical shape, which strongly depends on the surfactant molecular structure and cooling profile. In this contribution, we focus on the assessment of droplet self-shaping, crystallization and melting behavior during repeated cooling and heating cycles, and the influence of small changes in the temperature-time profile on these processes. These temperature cycles are typical conditions to which crystalline dispersions are subjected during storage and transport. Shape changes and crystallization of n-hexadecane droplets stabilized with the surfactants Brij® S20 and Tween® 60 were investigated by means of a thermo-optical method. In both model systems, self-shaping was observed during the first cooling cycle. Subsequent melting of particles led to the breakup of droplets into smaller, sub-micrometer droplets due to self-emulsification. When a second cooling step followed directly after melting at 20 and 22 °C, spontaneous self-shaping was again observed for n-hexadecane droplets stabilized with Brij® S20, while droplets stabilized with Tween® 60 crystallized in spherical shape. However, these samples showed self-shaping of droplets, when given longer times at 22 °C (incubation of 1 h) or being heated to the bulk Tween® 60 melting temperature (∼ 24 °C) before the second cooling step. During cooling of pure 1 wt% Tween® 60 solutions, crystal precipitation was observed to a great extent. Thus, the concentration of free surfactant molecules drops and may be too low to induce self-shaping of droplets. In 1 wt% Brij® S20 solutions, crystal precipitation was much less pronounced, probably leaving enough free molecules to induce self-shaping. In the case of Tween® 60, enough time after reheating and/or a high enough temperature is needed to melt the surfactant crystal precipitates and re-disperse the molecules to provide enough surfactants at the oil-water-interface for self-shaping during the second cooling scan. These results show, that the droplet crystallization and self-shaping behavior can greatly vary between different formulations, even when the surfactants are very similar in their molecular structure. Additionally, small changes in the temperature-time profile in the formulations’ lifetime have a distinctive influence on resulting particle size and morphology. The melting behavior of the dispersed phase, and crystallization and melting events of surfactants may have a distinctive influence on the particle size and morphology, and thus dispersion characteristics.