EFFECT OF ETHANOL ADDITION UPON THE STRUCTURE AND THE COOPERATIVITY OF THE WATER H BOND NETWORK

Density and water self-diffusion in ethanol-water mixtures are measure d as a function of the temperature and for several alcohol concentrati ons in the water rich region. An extension of the five species model f or liquid water, in which H bond cooperativity is taken into account, is proposed to interpret the temperature and concentration trends of t he mixture density, solvent self-diffusion and near infrared spectra. This approach allowed us to extract some significant parameters charac teristic of the solvent water H bond network such as the probability o f forming H bonds, the degree of cooperativity, the supercooling and s uperheating limit temperatures and the distribution of water molecules among the five species. The emerging picture suggests that alcohol ac ts upon the water H bond network as a structure maker in the normal te mperature region and, probably, as a structure breaker in the supercoo led one. Moreover, a progressive disappearance of the H bond network c ooperativity is observed as the alcohol concentration is increased.