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# Isotropic-nematic phase separation in asymmetrical rod-plate mixtures

The Journal of Chemical Physics (Impact Factor: 2.95). 10/2001; 115(15):7319-7329. DOI: 10.1063/1.1403686

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Rik Wensink, Available from: Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. The impact factor represents a rough estimation of the journal's impact factor and does not reflect the actual current impact factor. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.

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**ABSTRACT:**The phase behavior of a liquid-crystal forming binary mixture of generic hard rodlike and platelike particles is studied with the theory of Onsager [L. Onsager, Ann. N. Y. Acad. Sci. 51, 627 (1949)] for nematic ordering. The mixture is chosen to be symmetric at the level of the second virial theory, so that the phase behavior of the two pure components is identical. A parameter q is used to quantify the effect of the unlike rod-plate excluded volumes on the phase behavior; a value of q>1 indicates that the unlike excluded volume is greater than the like excluded volume between the rods or plates, and a value of q<1 corresponds to a smaller unlike excluded volume. Two methods are used to solve the excluded volume integrals: the approximate L2 model [A. Stroobants and H. N. W. Lekkerkerker, J. Phys. Chem. 88, 3669 (1984)], in which a second-order Legendre polynomial is used; and a numerical method where the integrals are solved exactly. By varying the unlike excluded volume interaction q, the isotropic phase is seen to be stabilized (small q) or destabilized (large q) with respect to the nematic phase for both models. Isotropic-isotropic demixing is also observed for the largest values of q due to the unfavorable contribution of the unlike excluded volume to the entropy of the system. A second-order nematic-biaxial nematic phase transition is observed for small values of q in the L2 approximation, and for all q in the exact calculation; in the latter case the stability of the biaxial phase is enhanced by increasing q, while in the L2 approximation nematic-nematic phase separation is favored. This result is the most striking difference between the two methods, and is in contrast with the results of previous studies. We show that the accuracy of the L2 expansion is particularly poor for parallel and perpendicular particle orientations.Physical Review E 08/2002; 66(1 Pt 1):011707. DOI:10.1103/PhysRevE.66.011707 · 2.29 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The molecular requirements for the stabilization of an isotropic demixing transition in mixtures of plate- and rodlike particles were considered. Based on a comparison of the critical pressure of the isotropic demixing transition and the lowest pressure of the isotropic-nematic bifurcation curve, a global phase diagram in terms of the molecular parameters were obtained which indicates the regions with and without isotropic demixing. The stability analysis proposed was very simple but it was also approximate, as the isotropic-nematic bifurcation pressure rarely coincides with the coexistence pressure in first-order transitions.The Journal of Chemical Physics 10/2002; 117(15):7207-7221. DOI:10.1063/1.1507112 · 2.95 Impact Factor -
##### Article: Enhancement by Polydispersity of the Biaxial Nematic Phase in a Mixture of Hard Rods and Plates

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**ABSTRACT:**The phase diagram of a polydisperse mixture of uniaxial rodlike and platelike hard parallelepipeds is determined for aspect ratios kappa=5 and 15. All particles have equal volume, and polydispersity is introduced in a highly symmetric way. The corresponding binary mixture is known to have a biaxial phase for kappa=15, but to be unstable against demixing into two uniaxial nematics for kappa=5. The phase diagram for kappa=15 is qualitatively similar to that of the binary mixture, regardless of the amount of polydispersity, while for kappa=5 a sufficient amount of polydispersity stabilizes the biaxial phase. This provides clues for designing an experiment to observe this long searched biaxial phase.Physical Review Letters 11/2002; 89(18):185701. DOI:10.1103/PhysRevLett.89.185701 · 7.51 Impact Factor