Experimental Study On Ion-Ion Correlations And Electric Double Layer At Solid/Liquid Interface

Electric double layer (EDL) is ubiquitous and paramount at solid-liquid interface. A thoroughly understanding of its structures and properties is of fundamental importance in flow mechanism of fluids in micro/nanochannels, biochip, micro/nano lubrication, adhesion, etc. However, a counterintuitive phenomenon called charge inversion emerges due to ion-ion correlation, which cannot be described properly by classic Possion-Boltzmann theory. The structures and properties of EDL under different ion-ion correlation strength conditions are investigated using a surface forces apparatus (SEA) in this dissertation.The EDL structures and forces are theoretically analyzed under weak ion-ion correlations, and the ions and surface potentials are calculated. The profiles of surface forces as a function of the distance between two mica surfaces are measured in monovalent and bivalent electrolyte solutions using an SFA, respectively. The results show that PB theory can reasonably predict the ion and surface potential distributions and EDL forces at solid-liquid interface in low concentration of monovalent and bivalent electrolyte solutions. In contrast, the PB theory fails.Considering the strong ion-ion correlations, surface forces between two mica surfaces in LaCl3 electrolyte solutions are measured using an SFA. The influences of ion-ion correlations on charge inversion and surface adhesion are investigated. Charge inversion and strong adhesion on mica surfaces are observed once the LaCl3 concentration reaches a critical value. It is proposed that the ion-ion correlation between lanthanum ions determines the formation of charge inversion and strong adhesion. The measured results are predicted and analyzed by an ion-ion correlation model. Moreover, the effect of pH value on ion-ion correlation is discussed.The interaction mechanism between polyelectrolyte chitosan nanofilms with different salt ions is investigated. The surface forces and adhesions between chitosan nanofilms are measured in both monovalent and trivalent buffer solutions. The effects of ionic strength, ion valence, relaxed time and loading on the surface forces between chitosan nanofilms are investigated. In addition, the molecularly interaction mechanism between chitosan nanofilms under different conditions is analyzed.