We investigate the bilayer pre-transition exhibited by some lipids at temperatures below their main phase transition, and which is generally associated to the formation of periodic ripples in the membrane. Experimentally we focus on the anionic lipid dipalmytoylphosphatidylglycerol (DPPG) at different ionic strengths, and on the neutral lipid dipalmytoylphosphatidylcholine (DPPC). From the analysis of differential scanning calorimetry traces of the two lipids we find that both pre- and main transitions are part of the same melting process. Electron spin resonance of spin labels and excitation generalized polarization of Laurdan reveal the coexistence of gel and fluid domains at temperatures between the pre- and main transitions of both lipids, reinforcing the first finding. Also, the melting process of DPPG at low ionic strength is found to be less cooperative than that of DPPC. From the theoretical side, we introduce a statistical model in which a next-nearest-neighbor competing interaction is added to the usual two-state model. For the first time, modulated phases (ordered and disordered lipids periodically aligned) emerge between the gel and fluid phases as a natural consequence of the competition between lipid-lipid interactions.

We present a simple model for an associating liquid in which polymorphism and density anomaly are connected. Our model combines a two dimensional lattice gas with particles interacting through a soft core potential and orientational degrees of freedom represented through thermal "ice variables." The competition between the directional attractive forces and the soft core potential leads to a phase diagram in which two liquid phases and a density anomaly are present. The coexistence line between the low density liquid and the high density liquid has a positive slope contradicting the surmise that the presence of a density anomaly implies that the high density liquid is more entropic than the low density liquid.

Dispersions of saturated anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG) have been extensively studied regarding their peculiar thermostructural behavior. At low ionic strength, the gel-fluid transition is spread along nearly 17 degrees C, displaying several thermal events in the calorimetric profile that is quite different from the single sharp peak around 23 degrees C found for higher ionic strength DMPG dispersions. To investigate the role of charge in the bilayer transition, we carefully examine the temperature dependence of the electrical conductivity of DMPG dispersions at different concentrations, correlating the data with the corresponding differential scanning calorimetry (DSC) traces. Electrical conductivity together with electrophoretic mobility measurements allowed the calculation of the dependence of the degree of ionization of DMPG vesicles on lipid concentration and temperature. It was shown that there is a decrease in vesicle charge as the lipid concentration increases, which is probably correlated with the increase in the concentration of bulk Na(+). Apart from the known increase in the electrical conductivity along the DMPG temperature transition region, a sharp rise was observed at the bilayer pretransition for all lipid concentrations studied, possibly indicating that the beginning of the chain melting process is associated with an increase in bilayer ionization. It is confirmed here that the gel-fluid transition of DMPG at low ionic strength is accompanied by a huge increase in the dispersion viscosity. However, it is shown that this measured macroviscosity is distinct from the local viscosity felt by either charged ions or DMPG charged aggregates in measurements of electrical conductivity or electrophoretic mobility. Data presented here give support to the idea that DMPG vesicles, at low ionic strength, get more ionized along the temperature transition region and could be perforated and/or deformed vesicle structures.

Abstract.We have investigated the effects of either distorting hydrogen bonds or removing proton degeneracy on the thermodynamic properties of a minimal model for associating liquids. The presence of two liquid phases and a density anomaly is unaffected in both cases. Increasing the degeneracy of bonded structures leads to lower temperature critical points and a steeper liquid-liquid coexistence line, implying a low density liquid of larger entropy.Analysis of the hydrogen bond net accross the phase diagram indicates that the density anomaly is accompanied by a steep reduction of hydrogen bond density, which introduces a restriction on a correlation which has been preconized long ago. This feature is present independent of bond distortion or of the presence of proton entropy. § To whom correspondence should be addressed (marcia.barbosa@ufrgs.br) Liquid polymorphism, density anomaly and H-bond disruption in an associating lattice gases 2

We investigate the phase diagram of a three-dimensional associating gas (ALG) model. This model combines orientational ice-like interactions and "van der Waals" that might be repulsive, representing, in this case, a penalty for distortion of hydrogen bonds. These interactions can be interpreted as two competing distances making the connection between this model and continuous isotropic soft-core potentials. We present Monte Carlo studies of the ALG model showing the presence of two liquid phase, two critical points and A density anomaly.

We have investigated the aggregation into micelles of nonionic amphiphiles in water through Monte Carlo simulations of a very simple model system. Amphiphiles and water are modeled, respectively, as three-site and one-site molecules on a square lattice. The model presents characteristic properties of experimental micellization, namely, monomer vs amphiphile concentration with a plateau above a ‘‘critical’’ concentration (cmc) and a distribution of micelle sizes (polydispersity). Results of the numerical experiment were compared with predictions from statistical thermodynamic theory. Sample size and relaxation properties of the simulation have also been analyzed.

We have reconsidered the Bell-Lavis model of liquid water and investigated its relation to its isotropic version, the antiferromagnetic Blume-Emery-Griffiths model on the triangular lattice. Our study was carried out by means of an exact solution on the sequential Husimi cactus. We show that the ground states of both models share the same topology and that fluid phases (gas and low- and high-density liquids) can be mapped onto magnetic phases (paramagnetic, antiferromagnetic, and dense paramagnetic, respectively). Both models present liquid-liquid coexistence and several thermodynamic anomalies. This result suggests that anisotropy introduced through orientational variables play no specific role in producing the density anomaly, in agreement with a similar conclusion discussed previously following results for continuous soft core models. We propose that the presence of liquid anomalies may be related to energetic frustration, a feature common to both models.

Two interacting particles in a spherical pore J. Chem. Phys. 134, 064508 (2011) Thermodynamical approach to sympathetic cooling of neutral particles J. Chem. Phys. 134, 044109 (2011) We investigate the thermodynamic and dynamic properties of a three dimensional associating lattice gas ͑ALG͒ model through Monte Carlo simulations. The ALG model combines a soft core potential and orientational degrees of freedom. The competition of directional attractive forces and the soft core potential results in two coexisting liquid phases which are also connected through order-disorder critical transitions. The model presents structural order, density, and diffusion anomalies. Our study suggests that the dynamic fragile-to-strong transitions are associated to changes in structural order.