Investigating nano-structured domains within ionic liquids: the effect of cation change on thermal equilibrium and relaxation of spiropyran and spirooxazine.

The established belief that ionic liquids (ILs) behave as homogenous solvents such as that observed in molecular solvents has been challenged.1, 2 Previous use of solvatochromic probe dyes has allowed for the traditional parameter of ‘polarity’ to be determined.3 These values were compared to the kinetics of the photochromic spirocyclic compounds spirooxazine (SO) and spiropyran (SP). The nature of SO substituents limit the ability of hydrogen bond formation and so relies primarily on electrostatic interactions with the solvent system. Such an increase in solute freedom would be expected to increase the ability of the molecules to dissociate and migrate within the solvent system. A polarity-kinetic relationship for spirocyclic compounds has been established in molecular solvents with increasing polarity exhibiting decreased rates of thermal relaxation from the coloured merocyanine (MC) form to the spiro (SO/SP) form.4 However, thermal relaxation of SO in ionic liquids fails to present a correlation between polarity and kinetics. Kinetic studies were further enhanced by analysis of the relaxation process using thermodynamic parameters of activation. Previous studies have (primarily molecular modelling) proposed that ionic liquids possess a structured in-homogenous structure containing distinct areas of polar and non-polar regions.5 The probe dyes used to examine parameters such as hydrogen bonding (Kamlet-Taft) and polarity (ET30) may only examine a particular region in the solvent. This means that the probe molecules may solvate in one region while compounds such as SO may interact in another region completely and therefore not allow for correlation of polarity to thermal relaxation rates observed. The closed form is a neutral compound exhibiting non-polar characteristics. MC, due to its zwitterionic nature, is in contrast highly polar. We believe that the size and ratio of polar to non-polar regions may be a critical factor in the process of SO thermal relaxation. Increasing non-polar regions may encourage the SO form by shifting equilibrium and encouraging migration and facilitate enhanced closure of MC within the solvent system. Thermal relaxation of SO may therefore allow for confirmation of the theory of IL structuring. Due to the large range of ILs possible, a correlation in structural effects and a quantifiable change may aid in a more detailed understanding of ILs and facilitate customisability of the liquids to meet specific polarity/solvation requirements

Physico-chemical properties of ionic-liquid water mixtures

In order for Ionic Liquids (ILs) to be utilized to their full potential, it is necessary to have a complete understanding of their physical properties, including phase transitions temperatures1. We have previously reported into the extent of structuring of ILs using photochromic molecular probes, and investigated the appropriate IL water content to yield hydrated IL systems for analysis of polarity and to create environments suitable for effective enzyme activity 1,2,3. In this study we investigated interactions in hydrated ILs containing variable hydrophobic and hydrophilic regions through optical and thermal analysis. The enthalpies and phase transitions of the systems were compared, between the temperature range -50°C to +30°C for the ILs with varying degrees of hydration. Reichardtʼs dye was used as a molecular probe to monitor changes in interactions in the ILs as a function of temperature. Comparisons were made between ILs and for ILs with varying degrees of hydration. Spectroscopic studies were performed using Perkin Elmer UV-Visible Spectrometer and phase transitions monitored using a Perkin Elmer Differential Scanning Calorimeter. The ILs examined are; Trihexyltetradecylphosphonium Chloride [P6,6,6,14 Cl] Tributyl-tetradecylphosphonium Chloride, [P4,4,4,14 Cl], 1-Ethyl-methyl-3-imidazolium-ethyl Sulfate [Emim][EtSO4] and Trihexyltetradecylphosphonium Dicyanamide [P6,6,6,14 DCA]. 1. Robert Byrne, Simon Coleman, Simon Gallagher, and Dermot Diamond. Designer Molecular Probes for Phosphonium Ionic Liquids. Physical Chemistry Chemical Physics, 2010. 2. Kyoko Fujita, Douglas R. MacFarlane, Maria Forsyth, Masahiro Yoshizawa-Fujita, Kenichi Murata,† Nobuhumi Nakamura, and Hiroyuki Ohno*,Solubility and Stability of Cytochrome c in Hydrated Ionic Liquids: Effect of Oxo Acid Residues and Kosmotropicity, 2007. 3. SergeiV.DzyubaandRichardA.Bartsch,Expandingthepolarityrange of ionic liquids, Tetrehedron Letters, 2002

Hyper and structural Markov laws for graphical models

My thesis focuses on the parameterisation and estimation of graphical models, based on the concept of hyper and meta Markov properties. These state that the parameters should exhibit conditional independencies, similar to those on the sample space. When these properties are satisfied, parameter estimation may be performed locally, i.e. the estimators for certain subsets of the graph are determined entirely by the data corresponding to the subset. Firstly, I discuss the applications of these properties to the analysis of case-control studies. It has long been established that the maximum likelihood estimates for the odds-ratio may be found by logistic regression, in other words, the "incorrect" prospective model is equivalent to the correct retrospective model. I use a generalisation of the hyper Markov properties to identify necessary and sufficient conditions for the corresponding result in a Bayesian analysis, that is, the posterior distribution for the odds-ratio is the same under both the prospective and retrospective likelihoods. These conditions can be used to derive a parametric family of prior laws that may be used for such an analysis. The second part focuses on the problem of inferring the structure of the underlying graph. I propose an extension of the meta and hyper Markov properties, which I term structural Markov properties, for both undirected decomposable graphs and directed acyclic graphs. Roughly speaking, it requires that the structure of distinct components of the graph are conditionally independent given the existence of a separating component. This allows the analysis and comparison of multiple graphical structures, while being able to take advantage of the common conditional independence constraints. Moreover, I show that these properties characterise exponential families, which form conjugate priors under sampling from compatible Markov distributions

Investigating nano-structuring within imidazolium ionic liquids: A thermodynamic study using photochromic molecular probes

Following previous studies involving the thermal relaxation of spirocyclic compounds we extended our studies to investigate the formation of nano-structured domains in ionic liquids (ILs). Two compounds, spiropyran (BSP) and spirooxazine (SO) were added to imidazolium cation based ionic liquids with increasing chain lenghts (C2 –C12). Increasing side-chain length was found to have only minor effects on the rate of thermal relaxation of BSP and SO. BSP was found to be a suitable probe molecule as linear correlations in parameters were observed for this compound. This is believed to be due to the fact that BSP-IL interactions were based on hydrogen bonding between MCBSP and the cation compared to MCSO which is limited to electrostatic interactions thus enhancing the sensitivity of MCBSP to the charged polar regions. Increasing the side-chain of the cation resulted in slight increases in MC-BSP activation energy from 96.93 kJ.mol-1 in [C4mIm][NTf2] to 105.27 kJ.mol-1 in [C12mIm][NTf2]. MC-BSP S‡ and H‡ values also increased with increasing side-chain. Expansion and dispersion of polar regions due to increasing non-polar interactions may be enhanced by introduction of the bulky probe molecule. The resulting reorganisation of the system produced positive entropies of activation, 13.79 J.K- 1.mol-1 for C4mIm to 46.15 J.K-1.mol-1 for C12mIm, following an increase in disorder due to probe dye closure from MC to BSP and migration of dye to regions of preferential solvation. The ability for spirocyclic compounds to form both polar and non-polar isomers resulted in the ability to analyse both solvent regions using a single probe dye. Ground state equilibrium, Ke, examined non-polar regions of the IL while equilibrium of activation, K‡, examined the polar regions. A linear response to side chain length to equilibrium of activation was believed to be due to the fact that polar regions were possibly expanding due to increasing influence of non-polar side chain interactions upon the over solvent structure. The result of such reordering and dispersion of polar regions reduces solvent-solute interactions which increases rate of MC-BSP relaxation