Simple models are developed for the first-order (main) gel–fluid phase transition in lipid mono-and bilayers. The lipid chains are assumed to lie on a triangular lattice in both phases and to interact via anisotropic van der Waals forces. The article begins with a review of previous experimental and theoretical work. Next a two-state Ising-like model is developed and shown to give a good qualitative description of the main phase transition of both mono- and bilayers and of statistical fluctuations about this transition. The article continues with a description of a more quantitative 10-state model for the main bilayer phase transition due to Pink. The polar head interactions are now explicitly taken into account. It is shown that this model is able to account for a variety of experimental results, e.g., Raman scattering data for pure lipid bilayers, phase separations for mixed lipid systems, and ionic permeability data for bilayers. The 10-state model is then extended to include the presence of intrinsic molecules (i.e., cholesterol, anaesthetics, and proteins) dissolved in lipid bilayers. The phase diagrams with their corresponding phase separation regions, the heat of transition, and in some cases the correlations are calculated for such mixed systems using several values of the parameters for the interactions. The penultimate section contains an analysis of the partitioning of anaesthetics between bilayers and the surrounding aqueous medium and its effect on the main phase transition. The article concludes with a discussion of the theoretical results and a summary of recent work.

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The interaction between a linear polymer and an interface is studied for three types of surfaces: reflective, attractive, and repulsive. The freely jointed chain model is assumed in the equation of diffusion approach, with volume exclusion effects neglected. In this case, the distribution function of the polymer is derived with the assumption that one or both ends of the polymer are bound to the interface. We evaluate the mean thickness of the polymer layer, the average number of adsorbed monomers, and the mean distance of the monomers from the surface. When one end of the polymer is bound to the interface, the results are qualitatively compatible with those of the previous lattice models. When both ends are bound, the polymer stays nearer to the interface but the macroscopic mean values stay qualitatively the same.

Critical properties of the Ising model on a stacked triangular lattice, with antiferromagnetic first and second-neighbor in-plane interactions, are studied by extensive histogram Monte Carlo simulations. The results, in conjunction with the recently determined phase diagram, strongly suggest that the transition from the period-3 ordered state to the paramagnetic phase remains in the xy universality class. This conclusion is in contrast with a previous suggestion of mean-field tricritical behavior. 75.40.Mg, 75.40.Cx, 75.10.-b Typeset using REVT E X regarding the criticality. ACKNOWLEDGMENTS We thank A. Bunker, A. Chubukov, A. Ferrenberg, and O. Heinonen for useful discussions and D. Loison for technical assistance. M.L.P. is grateful to the Université de Cergy-Pontoise for the hospitality which promoted this study. This work was also supported by NSERC of Canada and FCAR du Québec. 9

A simple model for first order phase transitions in monomolecular systems is developed and applied to ( i ) the condensed mesophaseliquid mesophase chain melting transition of phospholipid mono-and bilayers and ( i i ) a cooperative chain lifting phase transition found in monolayers of diesters with two polar heads. The excluded volume interactions are included in the model by requiring the polar heads to lie on a two-dimensional hexagonal lattice. The attractive van der Waals interactions are assumed to act between averaged molecular configurations of different surface area per polar head. Both the attractive intermolecular interaction and the description of the neighbourhood of the molecules are analyzed in the molecular field approximation. It is shown that the neighbourhood effects are negligible for the chain melting transition, which can then be described by a spin-; Ising model. They are, however, shown to be important for the chain lifting transition. The last section contains applications to expeiimental data and comments on the possibility of extending the model to the description of phospholipid mono-and bilayers with impurities.Un modele simple P O L I~ les transitions de phase de preniiei-ordre dans les systenies monomolkculaires est developpe et applique ( i ) B transition de fusion de chaine de la mesophase condensee i~ la mesophase liquide des mono-et bicouches de phospholipides et (ii) i la transition d'elevation coope~xtive de chaine trouvee dans les nionoco~~ches de diesters avec d e~~x tetes polaires. Les interactions de volume exclues sont incorporeesd>lns le niodeleen imposant que les tttes polaires soient sur Ll n reseiru bidimensionnel hexagonal. Les intelxctions attlxctives de van der Waals sont supposCes agir entre des configurations molecul;~ires nioyennes compoi-rant differentes aires superficielles par tSte polaire. A lafois I'intelxction internioleculaire attlxctive et la description du voisinage des niolecules sont analysees selon I'approximation du champ moleculaire. On montre que les effets de voisinage sont negligeables pour la transition de fusion ile la chaine, qui peut Ptre decrite par un modkle de Ising B spin A. Ils se revelent toutefois importants POLIS l i~ transition d'elevation. La derniere section contient des applications aux donnees experimentales et des commentaires sur la possibilite d'etendre le modele ?I la description des mono-et bicouches de phospholipides avec impuretks.[T~xduit par le journal] Can.