Phase transitions of n-hexadecane in nanoencapsulated binary solutions of n-hexadecane and 1-octanol.

Binary solutions of n-hexadecane and 1-octanol were encapsulated within poly(tert-butyl methacrylate) nanocapsules and the phase transition behavior of the n-hexadecane studied. A sigmoidal correlation was found between the binary composition of the encapsulated oil and the oil formulation prior to encapsulation. At low 1-octanol fractions in the synthesis, almost no alcohol was encapsulated, but at high 1-octanol fractions, the capsules were enriched in alcohol. The freezing point of the encapsulated n-hexadecane was considerably lower than in the bulk phase. This freezing point was further reduced as the radius of the particle was reduced, which is expected from the Gibbs-Thompson equation. The extracted value at infinite radii was, however, 14 °C below the bulk freezing point. The colligative effect of 1-octanol on the freezing point of n-hexadecane, i.e., dT(f)/dx(m), where x(m) is the mass fraction of 1-octanol, was nearly identical to that measured in the bulk system. The specific enthalpy of freezing for the encapsulated system was considerably different from the bulk system. In the bulk system, the freezing enthalpy for n-hexadecane was nearly constant up to 60% mass of 1-octanol. The freezing enthalpy for the encapsulated n-hexadecane showed greater variation with % mass 1-octanol, and had a considerably lower absolute value than the bulk system.

Phase transitions of hexadecane in poly(alkyl methacrylate) core-shell microcapsules.

Microcapsules containing subfemtoliter volumes of n-hexadecane (HD) within a 4-40 nm thick shell of poly(alkyl methacrylates) were prepared. The size of the HD drop was varied between 50 and 140 nm. The alkyl substituents on the methacrylate monomer were varied to alter the surface tension between the HD and the polymer shell in order to investigate the effects of surface tension on the freezing point of the HD. The size dependence of the supercooling as predicted by the G-T equation was not observed in our systems. An effect on the magnitude of supercooling with variation in the side chains was observed, where freezing the HD in capsules with bulkier side chains requires a greater magnitude of supercooling. This is in agreement with the increased hydrophobic character of the polymers and also correlates with the decrease in glass transition temperature of the polymer. We also observed aging of the capsules, which could be accelerated by heating.

Crystallization and melting transitions of hexadecane droplets in polystyrene nanocapsules.

Shifts to lower transition temperatures are observed for the freezing and melting of submicron-sized hexadecane droplets encapsulated within thin polystyrene shells. Supercooling of approximately 14 K is observed in the first-time cooling scans for the oil. We attribute this lowering predominantly to nucleation of the phase change originating from the oil/polymer/water interface. We obtain a rough estimate of the interfacial tension between the hexadecane oil droplet and the polystyrene of 14 mN/m assuming the Gibbs-Thomson relationship. Melting points for the hexadecane are 1-2 K below the bulk transition temperatures. This effect is connected with the surface/volume ratio of the capsules. Both the supercooling and the melting point depression approach the bulk-phase transition temperature when the sample is taken through multiple cool/heat cycles. The heating and cooling rates affects the number of cycles required before bulk-like behavior is observed. The thickness of the capsule wall is also observed to be critical to how many cooling cycles are required. Two hypotheses to explain this behavior are presented.

Solvent-filled matrix polyelectrolyte capsules: preparation, structure and dynamics.

Stable surfactant-free, water-dispersed, micron-sized organic solvent colloids have been a challenging subject of major interest both experimentally and theoretically in recent years. In this article, novel matrix capsules are introduced to carry an organic solvent ( toluene) into water and form a stable solvent dispersion in the aqueous phase without the addition of a surfactant. The structure and dynamics of the dispersion are investigated by confocal Raman microscopy, surface force microscopy, and pulsed field gradient nuclear magnetic resonance (PFG-NMR). The matrix capsules are fabricated according to a literature method using alternating layer-by-layer adsorption of oppositely charged polyelectrolytes onto porous calcium carbonate (CaCO) particles, followed by core removal. The highly rough surface and the inner cavities of CaCO particles result in a heavy matrix capsule, which can achieve a high solvent encapsulation efficiency and form a micron-sized carrier for the solvent in water that is stable for long times ( at least one week.) Two distinct diffusion coefficients are evidenced by PFG-NMR, which may indicate two distinct diffusion environments in the sample. This suggests that the toluene undergoes a partial exchange between environments within the 100 ms time frame of the NMR experiment.

Correlations among morphology, beta-sheet stability, and molecular structure in prion peptide aggregates.

The misfolding of proteins into beta-sheets and the subsequent aggregation of these sheets into fibrous networks underlies many diseases. In this paper, the role of peptide structure in determining the ordering of beta-sheet aggregates and the morphology of fibrils and protofibrils is dissected. Using a series of peptides based on residues 109-122 of the Syrian hamster prion protein (H1) with a range of substitutions at position 117, the link between side chain interactions and beta-sheet thermal stability has been investigated. The thermal stability of beta-sheets is associated with the peptides' ability to adopt the same alignment as wild-type H1, with residue 117 in register across all beta-strands [Silva, R. A. G. D., Barber-Armstrong, W., and Decatur, S. M. (2003) J. Am. Chem. Soc. 125, 13674-13675]. These aligned strands are capable of forming long, rigid, and twisted fibrils (as visualized by atomic force microscopy) which are thermostable. Peptides which do not adopt this strand alignment aggregate to form thin, flexible, and smooth protofibrils. The ability to form ordered aggregates, and thus to form twisted fibrils, is modulated by the structure of the side chain of residue 117.