Orientational dynamics in water/cosolvent mixtures [electronic resource]
Keywords: Chemistry.

The design and characterization of new solvent systems is an ever growing field of research. As such, understanding how macroscopic solvent properties emerge from molecular structure and dynamics is foundational for rational solvent system design. Of particular interest are water/cosolvent systems since they can display a wide variety of desirable characteristics. Water is inexpensive, safe, and well understood, and a large volume of research is devoted to the dynamics in water/cosolvent systems, with recent focus on the influence of cosolvents on the dynamics of water. In this thesis, optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments on two water/cosolvent systems are presented. The OHD-OKE experiment is a well-established pump-probe pulsed laser technique that can measure orientational relaxation. The experimental setup described here has been improved substantially over previous versions and is demonstrated capable of measuring dynamics over seven decades in time and eight decades in signal amplitude. The broad time window is obtained through the use of regeneratively amplified pulses and continuous wave probing, while the large signal range is accessed by incorporating balanced detection, digital lock-in amplification, polarization modulation phase cycling, and beam geometry optimization. The signal from water in the OHD-OKE is small; thus, the dynamics primarily track the motions of the cosolvent. The first water/cosolvent system studied here is a model water/polyether binary mixture. Polyethers are a large class of compounds with applications including medicine, cosmetics, and electrochemistry. Based on previous studies, some have suggested that there are significant polyether conformational changes as the water content is increased. However, rotational diffusion measurements from the OHD-OKE, translational diffusion measurements from NMR, and detailed hydrodynamic calculations, both as a function water concentration and temperature, show that such claims are unwarranted. The rotational diffusion of the polyether, which is sensitive to molecular shape, is in good agreement with the hydrodynamic calculations, both as a function of water concentration and temperature. In contrast, the translational dynamics are only hydrodynamic at high water concentrations. The water poor mixtures are anomalously fast, which we hypothesize is due to the free volume of the system. Careful analysis of the hydrodynamic calculations support this hypothesis, showing that voids in the liquid can impact the translation of the anisotropic polyether molecules more than they impact the rotational diffusion, as measured by OHD-OKE. The second system studied consists of a series of water/ionic liquid (IL) mixtures. Research in and application of ILs has exploded in the past decade, but their use as pure solvents is often limited by their high viscosities and extremely hygroscopic nature. The influence of water on the orientational dynamics of alkylmethylimidazolium tetrafluoroborate ILs is studied as a function of water concentration and cation alkyl chain length. For low water concentrations or short-chain cations, the dynamics are single exponential and follow Debye-Stokes-Einstein (DSE) behavior. However, the long-chain ILs at high water concentrations show clearly biexponential behavior, with the slow component of the dynamics showing anti-DSE behavior. We attribute the slow component to overall cation reorientation due to alkyl tail aggregates and the onset of water-induced gelation (as seen in similar water/IL systems) and the fast component to the wobbling-in-a-cone motions of the charged headgroup of the cation. We hypothesize that this specific water/tetrafluoroborate IL system studied does not gel because it phase separates before the enough water can be added to induce gelation. These results should have implications for rational water/IL cosolvent system design.