Phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D₂O system as determined by ²H and ¹⁴N NMR

Abstract

The high resolution phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D₂O system has been mapped out using ²H and ¹⁴NNMR spectroscopy. The ¹⁴N quadruple splittings are more than an order of magnitude larger than corresponding ²H splittings, while the line widths are only two to three times larger. Thus, ¹⁴NNMR offers an order of magnitude improvement over ²H NMR in the resolution of the spectra from coexisting phases. The ²H spectra of samples in biphasic regions are often complicated by chemical exchange of D₂O molecules between coexisting phases, particularly at low TMAHFN concentrations. Analysis of the ²H line shapes of a TMAHFN/D₂O sample with a weight fraction of TMAHFN of 0.230 obtained at various times following cooling of the sample into the isotropic/nematic biphasic region shows that the mean diameter for the dispersed nematic droplet grows from about 7 to about 26 μm over a period of 2 h. At a mean droplet size of 7 μm the exchange of TMA⁺ ions between the coexisting phases is slow on the NMR time-scale and exchange effects are not observed in ¹⁴N spectra. The TMAHFN/D₂O phase diagram exhibits the generic form of those of the CsPFO/water and APFO/D₂O systems, which are the only other systems composed of stable discotic micelles for which high resolution phase diagrams are currently available, but the nematic phase is displaced to smaller TMAHFN concentrations. Specifically, a discotic nematic phase N_D ⁺, intermediate between an isotropic micellar phase I and a lamellar phase L, exists for weight fractions of TMAHFN between 0.149 (φ = 0.105) and 0.420 (φa = 0.325) and temperatures between 277.3 and 327.6 K.