The effect of added co-solvent on the phase behaviour of the micellar liquid crystal system caesium pentadecafluorooctanoate/water : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Chemistry at Massey University

Abstract

The effect of the addition of the co-solvents formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA) on the phase behaviour of the micellar liquid crystal system caesium pentadecafluorooctanoate (CsPFO)/D2O (weight fraction CsPFO=0.5) has been determined. In all cases the effect of the addition of co-solvent is to decrease the isotropic (!)-to-nematic (N ) and the nematic - to - lamellar (Lo) phase transition temperatures. At a co-solvent mole fraction of 0.01 the observed decreases in the I-to-N phase transition temperatures (TNI) were 5.3 K, 15.5 K, 15.7 K, and 18.8 K for the co-solvents FA, NMF, DMF, and DMA respectively, whilst the corresponding decrease for the N -to-Lo phase transition temperatures (TNL) were 5.5 K, 16.1 K, 16.3 K, and 19.1 K. Thus, at low co-solvent concentrations, the main effect is simply to displace the phase transition lines to lower temperatures and there is no significant change in the overall phase behaviour other than a small increase in the temperature range of the N phase. 2H NMR measurements reveal that, in all cases, increasing the co-solvent concentration at constant temperature results in concomitant decreases in micelle aggregation numbers but the micelle sizes at TNI and those at TLN are essentially the same, i.e. the phase transitions are determined by hard particle interactions. Thus, the effect of co-solvent on the phase behaviour can be understood in terms of changes in micelle self-assembly, with addition of co-solvent shifting the distribution of micelle sizes to a lower mean aggregation number and decrease in temperature shifting the distribution of micelle sizes to a higher one. NMF, DMF, and DMA have a much greater effect on micelle size distribution than does FA.