The polymerization of water in benzene and toluene

Abstract

A model is described for the polymerization of water in benzene and toluene as solvents, which treats the water molecule like an ordinary tetrafunctional monomer. As in the quasi-chemical model of regular solutions, the law of mass action is applied to contacts between water molecules (hydrogen bonds), while the solvent is treated as an inert continuum. The size distribution of water clusters becomes identified with the classical molecular size-distribution law for tetrafunctional polymerization derived by Flory. In accordance with an idea suggested by Stockmayer, the critical saturation point in these solution systems corresponds to the gel point in polyfunctional polymerization. The model is tested in three ways. First, specific volume measurements of water solutions in benzene and toluene are successfully fitted. The results show a marked expansion resulting from the addition of monomeric water to hydrocarbons (i. e. in extremely dilute solutions). As the water concentration is raised, the partial specific volume of the water falls owing to hydrogen bond formation. The amount of contraction at various concentrations agrees well with the law of mass action. The amount of contraction at the saturation point agrees well with prediction. This is based on the critical conversion ( = 33·3 %) of hydrogen bonds to be expected at the gel point and on the known specific volume of ice (corresponding formally to 100% conversion of hydrogen bonds). Secondly, the model receives some support from relative viscosity measurements, though these effects are rather near the limit of experimental detectability. It does appear that water clusters raise the viscosity more strongly than loose clusters occurring in other binary systems in the absence of hydrogen bonding. On the other hand, the viscosity increments are much smaller than expected for a truly covalent polymer. Thirdly, the heat of solution of water in benzene and toluene can be simply interpreted in terms of the heat of hydrogen-bond formation. The failure of previous workers to find any polymerization of water in benzene by cryoscopic and spectroscopic measurements is discussed. An instrument is described which measures simultaneously the density and viscosity of 3 ml. of a liquid. The range of densities for a given instrument is about 0·001, and differences in density can be rapidly measured to about 0·000002 g/ml. The viscosity is measured to about 0·25 %. The principle employed is that of the usual falling sphere viscometer, but use of two (Pyrex glass) spheres of slightly different density permits the density of the liquid to be calculated as well as the viscosity. The instrument is useful in polymer chemistry generally, e. g. for intrinsic viscosity measurements at extremely low rates of shear.