Why Don't Alcohol and Water Mix Very Well?

Bartenders who make cocktails shouldn't worry about trying to get alcohol and water to mix completely. Nature prevents even the most patient drink-makers from fully blending the two. Studying methanol, a simple non-drinkable alcohol that nonetheless can provide insights into ethanol, or drinking alcohol, a US-Swedish collaboration (Jinghua Guo, LBL, 510-495-2230) has obtained new molecular-level details of why water and alcohol don't mix very well. Using LBL's Advanced Light Source, the researchers performed x-ray emission (XE) and x-ray absorption (XA) spectroscopy, which allowed them to study such things as the chemical bonds that form between molecules in the liquid over timescales of picoseconds to femtoseconds. Looking first at a liquid of pure methanol, the researchers observed the presence of rings and chains made of 6-8 methanol molecules. When they mixed methanol and water, they found that the 6-8 molecule chains connected with water molecules to form larger water/methanol clusters (see image). These clusters are very stable, because of the (hydrogen) chemical bonds that hold them together. But the water/methanol clusters also have a high amount of order, thereby reducing the liquid's overall disorder (entropy). Yet entropy must stay the same or increase in the liquid. So nature discourages the formation of more clusters in the liquid, and this can explain why alcohol and water don't like to mix completely. In addition, the research sheds light on a 40-year controversy over the molecular structure of pure methanol liquid, and the structures that are formed when water and methanol combine. For example, other researchers had suggested that water surrounded methanol in a static, ice-like structure. (Guo et al., Physical Review Letters, 10 October 2003).