Influence of Lanthanum on Stern Layer Conductance in the Nanochannel

Abstract

In this report, high-frequency electric impedance spectroscopy was performed to investigate ionic transport through nanochannels. Special attention was focused on (i) conductance behaviors depending on the role of cation valence in three background electrolytes (XCln): monovalent 1-1 (K + and Cl − ), divalent 2-1 (Mg 2+ and 2Cl − ), and trivalent 3-1 (La 3+ and 3Cl − ), (ii) the effects of proton and bicarbonate ions on bulk and surface conductance, and (iii) the connected microchannel dimension (surface/height ratio aspect) within the nanochannel apparent conductance. The results highlight a net quantitative increase in surface silanol density and a strong decrease in surface ionization degree when lanthanum cations are employed. The results also demonstrate that La 3+ strongly interacts with the silica surface, leading to negative values of standard free energy for ion-site interactions and chemical potential for ion−ion correlations in the Stern layer of −0.8 and −10.2 kT, respectively. We ascribed the evolution of surface charge density to the balance between the mole ratios of water molecules and adsorbed cations at equilibrium. We found that La 3+ behaves as an acidic cation (Lewis conceptualization) that neutralizes the negative silica surface accompanying water molecule expulsion due to steric hindrance. This study constitutes a new contribution to ion-site interactions and to ion−ion correlation phenomena on the planar silica surface to explain charge inversion observation in micro-nanofluidic devices.