Orientational dynamics in a lyotropic room temperature ionic liquid

In a previous study of room temperature ionic liquid/water mixtures, the first clearly observed biexponential decays in optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments on a liquid were reported, (Sturlaugson, A. L.; Fruchey, K. S.; Fayer, M. D. J. Phys. Chem. B 2012, 116, 1777), and it was suggested that the biexponential behavior is indicative of the approach to gelation. Here, new OHD-OKE experiments on mixtures of the room temperature ionic liquid 1-methyl-3-octylimidazolium chloride (OmimCl) with water are presented. The OmimCl/water system is shown to gel over the water mole fraction range of 0.69-0.81. In the OHD-OKE decays, the biexponential behavior becomes more distinct as the gelling concentration range is approached from either high or low water concentrations. The biexponential decays are analyzed in terms of the wobbling-in-a-cone model, and the resulting diffusion constants and "relative" order parameters and cone angles are reported. Comparison of the OmimCl/water data with the previously reported room temperature ionic liquid/water OHD-OKE decays supports the previous hypothesis that the biexponential dynamics are due to the approach to the liquid-gel transition and suggests that the order of the concentration-dependent phase transition can be tuned by the choice of anion.