Histogram-reweighting Monte Carlo simulations were used to obtain polymer / solvent phase diagrams for lattice homopolymers of chain lengths up to r=1000 monomers.The simulation technique was based on performing a series of grand canonical Monte Carlo calculations for a small number of state points and combining the results to obtain the phase behavior of a system over a range of temperatures and densities. Critical parameters were determined from mixed-field finite-size scaling concepts by matching the order parameter distribution near the critical point to the distribution for the threedimensional Ising universality class.Calculations for the simple cubic lattice (coordination number z=6) and for a high coordination number version of the same lattice

The phase behavior and micellization of several model lattice diblock and triblock surfactants have been investigated by histogram-reweighting grand canonical Monte Carlo simulations. By studying the systemsize dependence of the calculated phase diagrams, it has been found that for the cases studied (for which interactions are short ranged and temperature independent) each surfactant system either micellizes or phase separates, but never both. These results suggest that the experimentally observed behavior, where the same aqueous surfactant solution shows both phase separation and micellization under different conditions, is a consequence of the unusual solvation properties of water. The tendency to self-assemble is responsible for appreciable deviations from quasichemical theory even in systems that do not form micellar aggregates but are close to the boundary of macroscopic phase separation. For the micelle-forming systems, the surfactant volume fraction at the critical micellar concentration, φcmc, has been calculated from the point where a change of slope in the osmotic pressure versus surfactant volume fraction plots is observed. In all cases investigated, φcmc was found to increase with increasing temperature. As a consequence, positive values of the heat of micellization were obtained. For surfactant architectures close to macroscopic phase separation, the cluster size distributions are broad and extend to very large aggregation numbers indicating the presence of elongated micellar aggregates. This was also confirmed by an examination of typical configurations. Triblock systems, with symmetric architecture, behave in a similar manner, and architectures where the solvent-insoluble block is on the outside tend to phase separate over a broader range of parameter space than the triblock where the middle block is solvophobic. These results provide a growing understanding of the role of interactions and chain architecture on the self-assembly of surfactant systems and can be employed to benchmark existing theories in this area.

No abstract

Small-angle neutron scattering measurements on sodium dodecyl sulfate aqueous solutions have been performed in the presence of n-alcohols, from methanol to octanol, at different alcohol concentrations. By modeling the experimental intensities, it was possible to obtain structural information and to derive simultaneously the distribution of the alcohols between the aqueous and the micellar phases. It was found that short chain alcohols tend to remain in the aqueous phase and, by altering the solvent properties, induce a decrease in the aggregation number of sodium dodecyl sulfate micelles. On the other hand, alcohols with longer hydrocarbon chains were found to be present in both phases though favoring the micellar phase the longer the alkyl chain and the larger the concentration; this could be rationalized by assuming that the insertion of alcohol molecules in the micelle produced weaker repulsive interactions between the charged head groups of the surfactant molecules. For long chain alcohols, appreciably localized in the micellar phase, screening of the interaction among head groups leads to bigger micelles than those observed in the absence of alcohol: in these cases the alcohol/surfactant molar ratio reaches the value of 0.86, and hence the aggregates can be considered as mixed micelles. Sodium dodecyl sulfate micelles, at the examined concentration, were found to deviate from spherical symmetry and, when added with heptanol or octanol, assumed an ellipsoidal shape growing preferentially along the rotation axis.

We examine one of a number of possible classes of exceptions to the usual rule that non-Arrhenius behavior in supercooled liquids is accompanied by a departure from exponential relaxation kinetics. The exceptions we study are the dielectric relaxations of monohydric alcohols and supercooled water, in which also the dielectric relaxation times may greatly exceed their mechanical relaxation counterparts. This paper gives evidence that the exceptional behavior is due to a clustering or self-micellization phenomenon by showing how both the relaxation time ratio and Debye relaxation anomalies can be removed by small additions of ionic solutes. These compete with the hydrogen bonding interactions responsible for the clustering. The study, which uses the electrical modulus formalism for data analysis, is restricted by the rapid merging of conductivity and dielectric loss peaks at salt content increases. The relaxation times extracted from a given data set depend on the formalism employed in the data analysis, and the importance of consistency in this respect when comparing mechanical and electrical pheonomena is emphasized. We conclude that comparisons between different responses are most appropriately made in the susceptibility formalism, that the dielectric response in n-propanol is ∼160 times slower than the mechanical response at ∼130 °C, and that the difference is due to the fact that slowly relaxing hydrogen bonded molecular clusters dominate the dielectric susceptibility, hence also the dielectric relaxation. Using the susceptibility formalism for comparsions, we then infer that in water the dielectric relaxation process is considerably slower than the mechanical relaxation process, and that this fact, as well as the fact that the dielectric relaxation in supercooled water remains exponential while uniquely non-Arrhenius in temperature dependence, is to be explained by the dominance of the dielectric relaxation process by ‘‘network clusters.’’