Theoretical And Experimental Study On Electric Double Layer Structure Near Solid-liquid Interface

Property of the solid-liquid interface plays a critical role in many interfacial phenomena, such as surface forces, ion exchange between surfaces, chemical and physical properties of surfaces and so on. In recent years, interfacial phenomenon has become one of the research focus in physical, chemical and life sciences areas; these fundamental questions are significant to the progress of the mechanical, petrochemical engineering and biotechnology.Besides the inherent physical and chemical properties of the material itself, the most important factor that affects the prosperity of the solid-liquid interface is the counter ion and coion distribution. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory that has been extensively testified and reviewed on the notion that two independent types of forces governing the long-range interaction between similar colloidal particles immersed in polar (especially aqueous) solution, attractive van der Waals forces and repulsive double layer forces. As the most important part of the DLVO theory, Poisson-Boltzmann (PB) theory is used to predict the ion and potential distribution nearby the charged colloid surface in aqueous solutions when the ions are presented under the combined action of the Brownian motion (thermal energy, ksT) and the electrostatic interaction. With further research, it is found that not all colloidal behaviors can be readily accounted for the DLVO theory.The main work in this paper is to investigate the electric doulble layer structures near solid-liquid interface by theoretical and experimental methods, including surface force apparatus (SFA), atomic force microscopy (AFM) and scanning electron microscope (SEM). In this dissertation, based on the classical DLVO theory, ion-ion correlation theory is proposed to describe the electrical double layer structure near solid-liquid interface immersed in multivalent salt solutions. The reasons for the failure of the PB theory are given. In order to verify the correlation theory, the force curve as a function of the separation between two mica surfaces in LaCl3 solutions is sketched. It is confirmed that the La3+ions distribution meets the correlation theory under the condition of classical thermodynamic formula and the balance of the electrochemical potential in solution. From our measurements, it is found that the correlation theory can only be used when the following conditions are satisfied, i.e. a) Strongly charged surface in aqueous solutions; b) Conductive surface or semi conductive surface; c) Multivalent counterions (Z≥3) in aqueous solutions.The SFA system is improved in our laboratory. In our setup, the system is added with a homemade attachment for controlling the relative humidity (RH) in the SFA box, which makes it feasible for investigating the adsorption behavior of the gaseous molecule (water vapor, methanol vapor, ethanol vapor) adsorbed on solid surface; With the help of the modified SFA, the structure and properties of the molecularly thin liquid films adsorbed on the mica surface are investigated.The dielectric constant of the mica surface increases to about 14 (semiconductor surface) from the previous value of 7 (insulator surface) after the molecularly thin water film adsorbs on it.Finally, the work focuses on the distribution of the La3+which is expected to be governed by the correlation effect around the mica surface with the molecularly thin water film adsorbed. The experimental results of the double layer force, and the surface potential indicate the distribution of the multivalent counterions under the action of the correlation effect near solid-liquid interface.Our work firstly verifies experimentally the correlation theory, which extends the insight into the microstructure of the EDL beyond the DLVO theory. Moreover, the structure and properties of the molecularly thin liquid films adsorbed on a solid surface have been intensively studied with the help of the modified SFA system.