Pretransitional behavior of a water in liquid crystal microemulsion close to the demixing transition: evidence for intermicellar attraction mediated by paranematic fluctuations.
We present a study of a water-in-oil microemulsion in which surfactant coated water nanodroplets are dispersed in the isotropic phase of the thermotropic liquid-crystal penthyl-cyanobiphenyl (5CB). As the temperature is lowered below the isotropic to nematic phase transition of pure 5CB, the system displays a demixing transition leading to a coexistence of a droplet-rich isotropic phase with a droplet-poor nematic. The transition is anticipated, in the high T side, by increasing pretransitional fluctuations in 5CB molecular orientation and in the nanodroplet concentration. The observed phase behavior supports the notion that the nanosized droplets, while large enough for their statistical behavior to be probed via light scattering, are also small enough to act as impurities, disturbing the local orientational ordering of the liquid crystal and thus experiencing pretransitional attractive interaction mediated by paranematic fluctuations. The pretransitional behavior, together with the topology of the phase diagram, can be understood on the basis of a diluted Lebwohl-Lasher model which describes the nanodroplets simply as holes in the liquid crystal.
INFM, Dipartimento di Chimica Biochimica e Biotecnologie per la Medicina, Università di Milano, 20090 Segrate, Milano, Italy. caggioni@deas.harvard.edu
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D001713:Biphenyl Compounds / Q000737:chemistry*; D004655:Emulsions; D050866:Liquid Crystals / Q000737:chemistry*; D008832:Microchemistry; D008962:Models, Theoretical; D009570:Nitriles / Q000737:chemistry*; D009821:Oils / Q000737:chemistry; D044367:Phase Transition; D013501:Surface-Active Agents / Q000737:chemistry*; D013696:Temperature; D014867:Water / Q000737:chemistry*
D001713:Biphenyl Compounds; D004655:Emulsions; D009570:Nitriles; D009821:Oils; D013501:Surface-Active Agents; D014867:Water; C433919:4-cyano-4'-pentylbiphenyl