Water and alcohol have been found to mix less readily than most people think (perhaps lending support to anecdotal evidence that "artificial vodka", made out of reagent-grade ethanol and fresh water, needs to sit quite a while before it is good to drink).
US and Swedish researchers using the Advanced Light Source at Lawrence Berkeley National Laboratory have used X-ray emission (XE) and X-ray absorption (XA) spectroscopy to study the bonds between molecules of methanol over picosecond and femtosecond timescales. The results not only reveal the incomplete mixing of water and methanol at the molecular level, but also clarify the molecular structure of methanol and cast light on the mixture's thermodynamic properties.
The team has found that liquid methanol consists of both chains and rings of between six and eight molecules. This has reconciled two conflicting views about its structure, both of which had found support in neutron-diffraction studies. When water is added to form an equal mixture, the resulting spectrum is very similar to the added spectra for the two separate liquids.
However, the spectrum of the mixture indicates fewer methanol chains, and further analysis indicates that water molecules bridge the methanol chains to form rings. This, the researchers say, helps to explain why the increase in entropy is smaller than expected when water and alcohol mix. Certainly there is more to mixing the liquids at the molecular level than is evident in the popular alcohol-water mixtures at the macroscopic level.
Further reading
J-H Guo et al. 2003 Phys. Rev. Lett. 91 157401.