Abstract: This paper reports the effect of seven salts, namely sodium formate (C_1), sodium acetate(C_2), sodium propionate(C_3), sodium butyrate(C_4), sodium pentanoate(C_5), sodium caproate (C_6) and sodium heptylate(C_7), on the activity coefficients of three solutes, namely ethanol, acetone and acetonitrile, in aqueous solution at 298.1 K. Adopted method is to measure the variation of partial pressure of solute in the equilibrium vapor phase with the concentration of the salt at fixed composition of aqueous solution of solute by gas chromatography, thus to calculate the salt effect activity coefficient of the nonaqueous component in solution.
A design of vapor-liquid equilibrium apparatus for sequential sampling is described in detail.
According to the experimental result, it can be considered that the mechenism of salt effect changes dramatically as the num ber of carbon in the chain of anion increases. Electrostatic in teraction contributes to salt effect mainly for C_1 and C_2 salts. In addition to electrostatic, interaction between the hydrophobic group of C_3 and C_4 anion and that of solute raises a considerable contribution to it. The salt effects for C_5, C_6 and C_7 salts characterize the hydrophobic ions that the hydrophobic hydration, the hydrophobic interaction and the special interaction between the aggregation of hydrophobic ions and the various functional group cause the differentiation of salt effect o three solutes.