Hypothesis: Vesicle-polymer dispersions are found in drug-delivery systems and consumer products but undergo phase separation. Previous studies of phase separation have focussed on systems with high density differences between continuous and vesicular phases. In this study, we investigate phase separation in vesicle-polymer mixtures with very small density differences, in the presence and absence of air bubbles. Experiments: Magnetic resonance (MR) imaging, X-ray Computed Tomography and rheological measurements are reported which characterise the properties and stability of vesicle suspensions composed of the cationic surfactant, diethylesterdimethyl ammonium chloride, mixed with non-adsorbing polymer. H-1 T-2 MR relaxation images are employed to observe phase separation, for a range of vesicle-polymer mixtures, which are analysed using Moran's I spatial autocorrelation to quantify the extent and rate of phase separation. Findings: It was found that in presence of air bubbles, phase separation follows a compression/collapse mechanism, typical of colloidal gels with large density differences between the phases. Without air bubbles, phase separation develops through the formation of tiny cracks and fractures in the samples. MRI enabled visualisation of the evolution of phase separation inside highly turbid samples. The rate of phase separation was found to generally increase with increasing polymer concentration and decrease with increasing vesicle volume fraction. (C) 2020 Elsevier Inc. All rights reserved.

In complex colloidal systems, particle-poor regions can develop within particle-rich phases during sedimentation or creaming. These particle-poor regions are overlooked by 1D profiles, which are typically used to assess particle distributions in a sample. Alternative methods to visualise and quantify these regions are required to better understand phase separation, which is the focus of this paper. Magnetic resonance imaging has been used to monitor the development of compositional heterogeneity in a vesicle-polymer mixture undergoing creaming. T2 relaxation time maps were used to identify the distribution of vesicles, with vesicle-poor regions exhibiting higher T2 relaxation times than regions richer in vesicles. Phase separated structures displayed a range of different morphologies and a variety of image analysis methods, including first-order statistics, Fourier transformation, grey level co-occurrence matrices and Moran's I spatial autocorrelation, were used to characterise these structures, and quantify their heterogeneity. Of the image analysis techniques used, Moran's I was found to be the most effective at quantifying the degree and morphology of phase separation, providing a robust, quantitative measure by which comparisons can be made between a diverse range of systems undergoing phase separation. The sensitivity of Moran's I can be enhanced by the choice of weight matrices used.

The interaction mechanism of cationic diethylester dimethylammonium chloride (DEEDMAC) vesicles and the anionic surfactant sodium dodecyl sulphate (SDS) was investigated. Electrophoretic mobility measurements showed that the outer monolayer of the DEEDMAC vesicles became negatively charged immediately after addition due to sorption of an excess SDS. The subsequent increase of the vesicle surface charge density reflected the entering of SDS into the complete vesicle. Turbidity measurements showed that the flocculation behaviour was more pronounced than expected based only on the electrostatic interactions. The dynamic surface tension obtained immediately after mixing an SDS solution and a DEEDMAC dispersion was lower than the value of an SDS solution of the same concentration without any DEEDMAC, although DEEDMAC alone did not lower the surface tension. Based on the turbidity and surface tension observations, it is postulated that sorption of SDS leads not only to neutralisation of surface charges but also to a more hydrophobic vesicle surface due to the non-immediate penetration of the SDS dodecyl-chains into the DEEDMAC alkyl chain region. After waiting for a longer time, the surface tension of the DEEDMAC–SDS mixtures reached the value of pure water, indicating that all SDS is incorporated within the vesicles.