Strong non-classical induction forces in ion-surface interactions: general origin of Hofmeister effects

Abstract

Hofmeister effects continue to defy all-encompassing theories, and their origin is still a matter of debate. We observed strong Hofmeister effects in Ca2+/Na+ exchange on a permanently charged surface over a wide range of ionic strengths. They could not be attributed to dispersion forces, classical induction forces, ionic size, or hydration effects. We demonstrated that another stronger force was active in the ion-surface interactions, and which would create Hofmeister effects in general. The strength of this force was up to 10(4) times that of the classical induction force, and could be comparable to the Coulomb force. Coulomb, dispersion and hydration effects appeared to be interwined to affect the force. The presence of the observed strong non-classical induction force implied that energies of non-valence electrons of ions/atoms at the interface might be heavily underestimated in current theories, and possibly just those underestimated energies of non-valence electrons determined Hofmeister effects.

Figure 1

Comparison between calculated () and experimental data (). K_E is the experimental selectivity coefficient; K_C is the calculated selectivity coefficient based on the classical theory.

Figure 2

K_E/K_Cvs. φ₀ (V).

Figure 3

Comparison between theoretical curves of K_C+I () and K_E ().

Figure 4

Comparison between theoretical curves of K_C+NI () and K_E ().

Figure 5

vs. φ₀ (V) under different a⁰_Na/a⁰_Ca ratios (numbers aside dots are the values of a⁰_Na/a⁰_Ca).

Figure 6

The schematic diagram of a cation that is strongly polarized in an external electric field and the field is weakened by the polarization.