The MD calculations allow us to interrogate the geometry of the water clusters in the reverse micelles to see if they are modified from pure water. We analyze the three-body angular distributions of water molecules in the MD ensemble. These angular distributions are calculated by averaging the angle formed by each water molecule with two of its four nearest neighbors. The difference between this angular distribution and that of pure water, plotted in Figure , demonstrates that there is a small deviation of the structure of water in DIPA-water mixtures compared to that in pure water. Water in the DIPA-water mixtures shows a decreased chance of tetrahedral packing. Surprisingly, as temperature increases, the tetrahedral character of water in the mixture increases: apparently, water molecules in the mixture adopt a structure more like that pure water. This is a larger effect, and an opposite trend, to that reported in previous simulations of the UCST 1-butanol-water mixture. The SAXS from the sample with xA = 0.1 was not well-fit by the spherical particle model. It shows a very strong small angle scattering signal that does not saturate at low-Q but continues to climb in intensity with decreasing Q (Figure ). This is characteristic of small angle scattering from a system without a welldefined size-scale, for example, as would be produced by critical fluctuations approaching a phase transition. Indeed, the xA = 0.1 mixture is very close to the critical point of the mixture, (xA = 0.036, T = 26 °C). In this region, thermal fluctuations can be expected to result in large compositional fluctuations on many different length-scales. The fact that this scale-invariant SAXS signal is not seen at other compositions further from the phase line suggests that, although some critical scattering is contributing to the signal in the xA = 0.1 case, the other SAXS signals are not a result of critical scattering since they are qualitatively different in shape. This is strong experimental evidence for the presence of well-defined nano-scale structures in the liquid as we hypothesize.