Apparent Molal Volumes of Some Organic Electrolytes in a Dilute Aqueous Solution at 5, 25, and 45°C

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Abstract The apparent molal volumes, φv, of five series of homologous organic salts of the H(CH2)nCOONa (n=0–6), (CH2)n(COONa)2 (n=0,2,3,4,6,8), H(CH2)nNH3Cl (n=0,2,6), (CH2)n(NH3Cl)2 (n=2,3,6), and H(CH2)n-SO3Na (n=1,2) types in a dilute aqueous solution have been measured by means of a float method as a function of the concentration and the temperature. The coefficients of the deviation of φv from the Debye-Hückel limiting law, bv, are negative for most organic electrolytes. The large positive bv values observed for the sodium oxalate and sodium succinate solutions at 5°C have been attributed to ion-pair formation. The partial molal expansibility of organic ions has been evaluated and interpreted on the basis of the multilayer hydration model proposed by Frank and Wen. The estimation of volume increments per CH2 group suggests that the influences of ammonium ions on the hydrophobic hydration around CH2 groups differ significantly from those of carboxylate ions. The volume change accompanying the substitution of a terminal hydrogen atom with an ionic group has been calculated, and discussed in connection with the overlapping effect of electrostriction in bolaform electrolytes.