Why don’t alcohol and water mix

Very well? A US–Swedish collaboration has obtained new molecular-level details of mixtures of water and methanol, the simplest alcohol. At Lawrence Berkeley National Laboratory’s Advanced Light Source, the researchers used x-ray emission and x-ray absorption spectroscopy to study, for example, the chemical bonds that form between molecules in the liquid over timescales of picoseconds and femtoseconds. In pure methanol, they observed rings and chains made of both six and eight methanol molecules. When the methanol and water were mixed, the molecular rings remained intact. The chains, however, connected with water molecules to form large, stable water–methanol clusters with a high degree of order, thereby reducing the liquid’s overall entropy, which explains the incomplete mixing. To preserve the second law of thermodynamics, only some of the chains are bridged. (J. -H. Guo et al., Phys. Rev. Lett. 91 , 157401, 2003 http://dx.doi.org/10.1103/PhysRevLett.91.157401 .)