The phenomenon of an increase in the hydrophilic properties of such a polyhydroxy compound as cellulose on a limited substitution of its hydroxy groups by other radicals is due to the existence of a system of strong hydrogen bonds blocking the interaction of cellulose with water but breaking down on partial substitution. However, hydrophilicity is connected not only with the" substitution itself but also with the properties of the substituting radical [1, 2]. It was therefore desirable to study the sorption properties and the thermodynamics of the interaction of mixed cellulose esters with water.We have investigated a water-soluble cellulose acetate (WSCA) with a degree of substitution Fac = 0.72; a watersoluble cellulose acetophthalate (WCAP) with Fac = 0.72, Fph t = 0.08, OH group content 2.2; a water-soluble acetomaleate (WCAM) with Fae = 0.71, Fma I = 0.06, OH group content 2.2, and a water-soluble cellulose aminoacetate (WCAA) with Fae = 0.65, Famin o = 0.51, OH group content 1.84. The degree of polymerization of all the specimens studied was -200.The sorption of water vapor was studied by means of a McBain spiral balance with a sensitivity of -1.3-10 -3 m/kg at 293 K and a residual pressure of 10-3-10 -4 Pa. When the equilibrium values of the amount of water sorbed had been reached, the equilibrium vapor pressure was measured and sorption isotherms were plotted. The heat of mixing was determined in a modernized DAK-I-IA calorimeter. Before the beginning of the experiments, the samples were dried to constant weight and were placed in the working cell of the microcalorimeter. After thermal equilibrium had become established the samples were brought into contact with water vapor. The heat effect was shown on the panel of the integrator and recorded in the form of curves in a diagram. Figure 1 shows isotherms of the sorption of water vapor for all the samples investigated. As has been shown in [3], the sigmoid form of the curves, with a convex initial section, is due to the simultaneous occurrence of two processes: physical adsorption in the pores of the polymer, and its swelling. It follows from Fig. I that mixed cellulose esters have a high sorption capacity in water and that the magnitude of the equilibrium sorption depends on the nature of the substituting radical, decreasing in the sequence WCAM > WCAP > WSCA > WCAA.The sorption isotherms were used to calculate the thermodynamic affinities of water for the mixed cellulose esters. The difference in the chemical potentials of 1 g of water in the phase of a swollen polymer, A/~I, and of pure water, A/~ ~ was calculated by means of the equation

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