We have characterized the lipid chain freezing in dilute aqueous vesicle dispersions of the cationic lipid dioctadecyldimethylammonium bromide (DODAB) using wide and small angle X-ray scattering, solid state NMR, DSC, turbidity and density measurements. The lipids freeze in two steps. Above 40 C the chains are fluid and the lipids are in a so-called liquid-crystalline state. When cooling below 40 C, the lipids form a gel phase where the chains stretch, the molecules are more densely packed and most molecular degrees of freedom are frozen, or at least dramatically slowed down. In the gel phase, the chain packing is still disordered, while the chain mobility is significantly reduced. From NMR data we further conclude that also the molecular rotational diffusion around the molecular long axis is quenched. Slow chain reorientation may occur, but then as individual reorientations of the separate chains. When cooling further below 36 C, crystalline ordering of the chains is obtained, resulting in a further increased packing density. We refer to this state as the subgel phase. The transitions are reversible. However, the formation of the ordered subgel is very slow for temperatures near the melting point. In fact, the gel phase can be supercooled by almost 20 C for considerable time. From analyzing this transition in terms of classical nucleation we obtain an estimate of the intra-bilayer interfacial tension between the gel phase and the growing subgel domains of 2 mN m À1 . Materials and methods MaterialsDioctadecyldimethylammonium bromide (DODAB, Fig. 1) with purity greater than 99% was used as purchased from Acros Organics (Belgium). Dioctadecyldimethylammonium chloride (DODAC) with purity greater than 97% was used as purchased from Alfa Aesar (Germany). Deuterium oxide (D 2 O) with purity

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. T relaxation time maps were used to identify the distribution of vesicles, with vesicle-poor regions exhibiting higher T 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 solubilization of the poorly water soluble anti-inflammatory drug flurbiprofen in non-ionic Tween 20 surfactant micellar solutions was studied by both (19)F and (1)H NMR spectroscopy in an acidic environment. These non-destructive techniques allowed us to investigate the effect of temperature cycling in situ. Using (19)F NMR, an increased solubilisation capacity was observed as the temperature increased. This effect became more pronounced above the cloud point, which was reduced by more than 30 degrees C in the presence of an excess of flurbiprofen. Upon clouding, peak splitting was observed in the (19)F spectrum, which indicates that two pools of solubilised flurbiprofen exist that are in slow exchange on the NMR frequency timescale. The clouding and solubilization processes were found to be reversible, albeit with slow kinetics. Based on chemical shift differences of both Tween 20 and flurbiprofen, as well as NOESY experiments, the flurbiprofen was found to be accumulated within the palisade layer of the Tween 20 micelles.

Lipid freezing in dilute sonicated vesicular dispersions was studied using differential scanning calorimetry (DSC) and 1H NMR. For charged, anionic, or cationic lipids, approximately half of the lipids remain in a fluid state when cooled 20 degrees C below the main chain melting temperature. With a zwitterionic phospholipid, on the other hand, essentially no supercooling of the liquid state was observed. The observations are analyzed in terms of the nucleation and growth of flat solid domains in originally fluid spherical vesicles. As the solid domains grow, the remaining fluid domain is deformed, resulting in a curvature stress. Depending on the vesicle size and the bilayer bending rigidity, the solid domain growth may terminate as the gain in cohesive free energy is balanced by the curvature stress of the remaining fluid domain. It is argued that high bending rigidities are required for having a significant supercooling, which is why it is only observed for charged lipids.