Acta Chimica 125. (1988)
3. szám - Pizio, Orest Aleksandrovytch–Holovko, Miroslav Ferorovytch–Trokhimchuk, Andrij Dmytrovytch: The pair distribution functions and thermodynamics properties of the ion-dipole model of electrolyte solutions
2 PIZIO et al.: ION-DIPOLE MODEL 399 is the contribution of the short-range interactions and are the contributions of the electrostatic interactions. The dependence on x of the electrostatic terms in the ion activity coefficients is shown in Fig. 13. At low x, the dependence is linear as is the Debye limiting law. At higher x the significant departures from the linear dependence are evident, in particular at low values of y. Within the ion approach this nonlinearity is usually accounted for by introducing the concentration dependence of the dielectric constant. According to eqs (4.17) and (4.18), the activity coefficients consist of two terms with opposite signs. The contribution of short-range forces is positive, while the one of electrostatic forces is negative. The curves of ion activity coefficients depending on the values of different parameters and the comparison with the conventional ion model are presented in Fig. 14. In the framework of the ion model the corresponding expressions equal to: where 2Г, = (1 + 2*£~1/2)1/2 — 1. (4.23) In all cases considered, the curve for ln/Jj1 within the ion-dipole model lies lower than the corresponding one for the ion model, and the difference is (4.22) inЛ' = -J-ft,*2 - &,*(3y)1/2) + ßt-^=-^ • -f-24% £ ßi2 In ff = -7—(^i^(3y)1/2 + 6jfc2) + 24% 3 (4.20) (4.21) Fig. 13. Electrostatic part of the ionic activity coefficients ln/o = ln/f In/* = - 9r>° + ^ , In/j* = -ßS I (l-%)3 £(1+Г.) + ln/„el Acta Chim. Hung. 125, 1988