Activity Coefficients and Partial Molar Volumes of Guanidinium Salts in Aqueous Solutions at 25°C

Abstract

The. effects of guanidinium ion and its salts (chloride, bromide, thiocyanate, nitrate, acetate, sulfate and carbonate) on the structure of water were studied by the measurements of the mean activity coefficient, conductivity and density at 25° C. The values of mean activity coefficients, of all guanidinium salts were smaller than those of alkali metal salts with the same counter ion except for the case of cesium chloride at low concentration. These low values of r were interpreted in terms of structure breaking actions of these salts. As shown in Table 3, the values of r for 1: 1 valence type lied in the order OAc- >Cl-> Br^- > SCN >NO, - and for 1: 2 type C082- >SO₄2-, which were the reverse orders of structure breaking abilities of these ions. These orders of r can be well interpreted by regarding the guanidinium ion as a structure breaker and using the Desnoyers' model of ion-ion interaction in which the overlapping effect of hydrated cosphere was taken into account.
Partial molar volumes of guanidinium ion at infinite dilution were 44 ml/mol and 49 mi/niol for 1: 1 and 1: 2 type respekively. These values are larger than the molar volume of guanidinium ion in crystal, 40 ml/mol.
As shown in Fig.2, the equivalent conductance of guanidinium chloride obeys the OnsagerFuoss' equation at low concentration, and the values of 129.7 and 53.3 were obtained for the equivalent conductances of guanidinium chloride and guanidinium ion at infinite dilution respectively. Application of, modified Stokes' law gave a value of 75.1 ml/mol for the molax volume of the hydrated guanidinium ion. From these data it was found that guanidiniuni ion was hydrated with about 2 molecules of water. This hydration number is considerablly small as compared to the results of Li+ and Na + which have the hydration numbers of 7 and respectively. From the above results, guanidinium ion was concluded to be a structure breaking ion.