Effect of pressure on clathrate formation in a water-ethanol system

The water-ethanol (E) system was studied by differential thermal analysis (DTA) at atmospheric pressure and pressures up to 10 kbar. At atmospheric pressure, the system was shown to contain an incongruently melting hydrate of E . 2H(2)O composition (t(decomp) = -69.0degreesC). In addition, the system involves two metastable hydrates: E . 3H(2)O (t(decomp) = -69.0degreesC) and E . 4.75H(2)O (t(decomp) = -74.5degreesC). The hydrate compositions were determined by Tamman's method for the corresponding temperature invariants. Studies at pressures below 10 kbar showed that E . 4.75H(2)O metastable hydrate becomes stable at pressures above 0.25 kbar. At pressures from 1.0 to 7.3 kbar, this hydrate melts congruently, which allowed its composition to be determined more accurately. Presumably, this hydrate is of a semiclathrate nature (i.e. the ethanol molecule is involved in constructing the water framework) and has cubic structure I. In experiments at high pressures, E . 3H(2)O was not found. Concerning E . 2H(2)O hydrate, its peritectic melting was observed up to similar to5.0 kbar. Further extrapolation extends the incongruent melting curve to 7.5 kbar.