Time evolution of the thermotropic behavior of spontaneous liposomes and disks of the DMPC-DTAC aqueous system.

In this work, solubilization of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by the cationic detergent n-dodecyltrimethylammonium chloride (DTAC) was studied in aqueous solution, at a fixed DMPC concentration and variable detergent:lipid (D:L) molar ratios. The colloidal nanostructures present in different stages of the solubilization process were characterized using micro-differential scanning calorimetry (DSC) and dynamic light scattering (DLS) techniques. For total (analytical) D:L molar ratios below approximately 1, DTAC monomers incorporate into the DMPC liposome bilayers, forming smaller and more fluid vesicles than pure DMPC liposomes. At D:L approximately 1-2, vesicles begin to rupture, coexisting with intact vesicles and bilayer fragments. At D:L approximately 2-12.5, discoidal and spherical micelles are formed and coexist with vesicles; a slow structural rearrangement of the system, monitored in successive DSC heating/cooling cycles, was observed, and is reported for the first time. Finally, for D:L above approximately 15-20, the bilayers are completely dissolved, and the main aggregates in solution become spherical micelles, which slowly evolve to cylindrical (threadlike) micelles. Based on the dependence of the temperature and enthalpy of transitions on the total D:L molar ratio, at constant DMPC concentration, a schematic model, showing the different colloidal nanostructures present in the solubilization process, is proposed.

Phase behaviour of oleanolic acid, pure and mixed with stearic acid: Interactions and crystallinity.

The phase behaviour of pure oleanolic acid (OLA) and in mixtures with stearic acid (SA) was characterized by differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and nuclear magnetic resonance (NMR). The crystalline OLA as received (OLA(ar)) becomes amorphous after being dissolved in chloroform and vacuum-dried at 50 degrees C (OLA(50)). Upon heating, both forms transform to the needle shape crystalline form (OLA(220)). Dimerization through H-bonding between COOH groups was detected both in OLA(ar) and OLA(220). Dimers are stronger in OLA(220), where H-bonding also involves the alcohol groups and plays a role in the crystalline organization. A eutectic type phase diagram was established for mixtures, with the eutectic composition close to pure SA. Mixtures rich in SA are miscible in the liquid and in the amorphous solid states, where the presence of SA-OLA co-dimers, formed through H-bonding between carboxyl groups, was detected. Miscibility and SA crystallinity decrease drastically with the OLA content.

Phase behaviour of oleanolic acid/stearyl stearate binary mixtures in bulk and at the air-water interface.

The solid-liquid phase behaviour of oleanolic acid (OLA)/stearyl stearate (SS) was investigated by differential scanning calorimetry and polarizing optical microscopy. A eutectic type diagram, with the eutectic composition close to pure SS was obtained. Complementary studies by NMR, X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy were performed. A mutual influence was detected in mixtures: the low melting form of SS is favoured at low OLA molar fractions (X(OLA)) and spherulitic structures appear at high X(OLA) and high temperature. Additionally, H-bonding between OLA carbonyl groups increases in the presence of SS. The study of OLA/SS by the Langmuir method and Brewster angle microscopy revealed the organization at the air-water interface: OLA interacts with water in the first layer, while SS is completely segregated to the upper layer for X(OLA)>0.3, and it distributes in the first and upper layers for X(OLA)<0.3.

Phase behaviour of binary mixtures involving tristearin, stearyl stearate and stearic acid: thermodynamic study and BAM observation at the air-water interface and AFM analysis of LB films.

The binary mixtures involving tristearin (TS), stearyl stearate (SS) and stearic acid (SA) were studied by surface pressure-area (pi-A) measurements and by Brewster angle microscopy (BAM), at the air-water interface, and the Langmuir-Blodgett (LB) monolayers, transferred onto mica substrates, were analysed by AFM. The thermodynamic analysis indicated miscibility in the whole composition range for the system SA/TS, and partial miscibility for systems SA/SS and TS/SS. This behaviour was further confirmed by BAM observation and AFM analysis of LB films. The AFM imaging of collapsed monolayers revealed domains with a multilayered structure varying with system and composition. The layers thickness determined by cross section analysis are consistent with estimated molecular lengths and conformations proposed for the molecules, assuming nearly perpendicular or tilted orientations of the hydrocarbon chains to the interface.

Microdomains in mixed monolayers of oleanolic and stearic acids: thermodynamic study and BAM observation at the air-water interface and AFM and FTIR analysis of LB monolayers.

Monolayers of oleanolic acid (OLA) mixed with stearic acid (SA) were studied at the air-water interface. The surface pressure-area (pi-A) isotherms, measured over the whole composition range, and BAM observations were used to investigate the phase behaviour and self-organization of these components in a two-dimensional structure. Pure OLA forms a very compressible monolayer, and BAM observation revealed the coexistence of large and irregular solid domains of different thickness dispersed in a gas matrix, compatible with the two most probable orientations of the OLA molecule at the interface. Mixtures of OLA/SA form condensed monolayers from low surface pressures and the thermodynamic analysis indicates that OLA molecules, in the presence of the long-chain SA, orient with the major axis almost perpendicular to the interface. Langmuir-Blodgett (LB) monolayers of pure SA and mixtures were further characterized by atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR). AFM images of LB mixed monolayers evidenced microphase separation, not observable by BAM. The SA rich domains are 4-6A thicker than those rich in OLA. The FTIR spectra of mixed LB films on CaF2 substrates showed that OLA does not perturb the all-trans conformation of the SA long alkyl chains, up to a mole fraction of 0.4. The carbonyl-stretching band of OLA suggests that the carboxylic groups of neighbour OLA molecules are involved in hydrogen bonds, forming dimers, as in pure solid phase OLA. These interactions seem to prevail over the OLA-water hydrogen bonds.

Thermotropic mesomorphism of a model system for the plant epicuticular wax layer.

As a model for the epicuticular wax layer of plant cuticular membranes, we have studied the phase behavior of 1-tetradecanol and 1-octadecanol and their binary mixtures between 5 and 70 degrees C, using differential scanning calorimetry and Fourier transform infrared spectroscopy (FTIR). Both pure compounds show two exothermic phase transitions corresponding to a transformation from a liquid phase to a hexagonally packed solid phase (S(HEX)), which at lower temperatures transforms to an orthorhombically-packed solid phase (S(ORT)). On heating the S(ORT) solid a single endothermic transition with a transition enthalpy corresponding to the sum of the exothermic transition enthalpies is obtained. These transitions were also followed using FTIR spectroscopy in the CH(2)-stretching (symmetric and asymmetric) and CH(2)-rocking vibration modes. The FTIR spectra of the pure compounds in the liquid, S(HEX), and S(ORT) phases were used to simulate experimental spectra in the phase transition regions. The simulations allowed us to estimate the molar fractions of each phase in the transition regions of the pure compounds. A phase diagram for the binary mixture of 1-tetradecanol and 1-octadecanol was obtained using differential scanning calorimetry and FTIR. FTIR studies on binary mixtures prepared from one perdeuterated component and the other nondeuterated permitted studying the thermotropic behavior of each component in the mixture independently. The mixture shows an eutectic behavior with an eutectic point between a molar fraction of octadecanol (X(18)) of 0.12 and 0.18 and a temperature of approximately 32 degrees C. Below 32 degrees C, a binary mixture of solid phases, one an S(ORT) phase and the other an S(HEX) phase, coexist up to approximately 25 degrees C, below which both solid phases are S(ORT) phases. We discuss the possible relevance of this complex phase behavior in a simple binary mixture of two long-chain alkanols in the context of the far more complex phase behavior expected for the plant epicuticular wax layer.