Pretransitional behavior in a water-DDAB-5CB microemulsion close to the demixing transition. Evidence for intermicellar attraction mediated by paranematic fluctuations

TL;DR

This study investigates how nanodroplets in a water-in-oil microemulsion influence the phase transition of liquid crystal 5CB, revealing pretransitional fluctuations and intermicellar attraction mediated by paranematic fluctuations.

Contribution

It provides experimental evidence and a theoretical model for intermicellar attraction caused by paranematic fluctuations near the demixing transition.

Findings

Pretransitional fluctuations increase as temperature approaches transition.

Nanodroplets act as impurities disrupting local order.

Phase behavior aligns with a diluted Lebwohl-Lasher model.

Abstract

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 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.