| Ion exchange is a very important physicochemical process and strongly influences surface properties of charged particles, interactions of particles, stability of aggregation as also structures of electric double layer (EDL), and then affects microprocess and macrophenomena. Ion exchange has been significant research contents of many subjects (including soil science, physics, chemistries and biologies), and thus was widely concerned. Nano-colloidal charged particles usually exist in system of above subjects. Strong electric field (108V/m magnitudes) was yielded near charged surfaces, which results from surface charges of particles. The adsorbed counterions in EDL are immersed in the strong electric field, the quantum fluctuation of outer shell electrons of ions are strongly induced by the electric field and then polarized; the polarized ions reversely also can weaken electric field. Conceptually, these interactions are called coupling effects between quantum fluctuation of ionic outer shell electrons and electric field from surface charges. Ionic polarization was different with difference of outer shell electrons conformation of ions, which must result in different ion exchange adsorption ability in solid/liquid interface. Unfortunately, current theories explain the difference of interface behaviors of ions merely focus on ionic size, hydration, dispersion force et al, but they neglect an important fact, i.e. strong electric field in EDL. Thus these theories predictions heavily deviate from many physical and chemical experimental phenomena. Existing ion exchange models, e.g. chemical reaction model, Donnan equilibrium model and classical EDL model, cannot correctly reflect ionic interface behaviors and explain many experimental results of ion exchange equilibrium. Although classical EDL model is a relative good theory for study ionic interfacial behaviors, it cannot explain the difference of exchange ability between ions, and is only applied in system of very low particle density. In this study, a new theory of ion exchange equilibrium taking into account the coupling effects between quantum fluctuation of ionic outer shell electrons and electric field from surface charges was established, which not only overcomes the defects of classical EDL model, but also poses a challenge to all relative theories of ion exchange equilibrium. Especially, our theory may provide theoretical guidance for developments and applications of EDL and Derjaguin-Landau-Verwey-Overbeek (DLVO) theories.In this study, the new theory of ion exchange equilibrium based on the coupling of ionic quantum fluctuation and electric field near surface includes following primary coverage:firstly, the applied conditions (including charge density of particles, ion-ion interactions in bulk solution and ionic size) of nonlinear Poisson-Boltzmann (PB) equation were investigated, the modified PB equation considering ionic steric effects and ion-ion interactions in bulk solution was established (Chapter3). Secondly, the analytical solutions of nonlinear PB equation, i.e. potential distributions in EDL, in mixed electrolyte with monovalent and bivalent ions were solved (Chapter4). Thirdly, the analytical expressions of surface potential determination based on the potential distributions (Chapter5). Fourthly, a new theory of ion exchange equilibrium was established (Chapter6), and effects of the coupling of mentioned above on ion exchange equilibrium in different ion group were investigated (Chapter7). Last but not least, the important application of the new theory in combined determination of surface properties of charged particles (Chapter8). The main results as follows:(1) Generalized Poisson-Boltzmann equation which takes into account both ionic interaction in bulk solution and steric effects of adsorbed ions has been suggested. We found that, for inorganic cations adsorption on negatively charged surface, the steric effect is not significant for surface charge density<0.32C/m2, while the ionic interaction is an important effect for electrolyte concentration>0.15mol/L in bulk solution. We conclude that for most actual cases the original PB equation can give reliable result in describing inorganic cation adsorption.(2) The Poisson-Boltzmann theory is still considered as an exact theory under low or even moderate electrolyte concentrations. As of today, however, just the analytical solutions in a single electrolyte were obtained, while people often meet mixed electrolytes in practical applications. Thus the analytical solutions of Poisson-Boltzmann equation (PBE) in different mixed electrolytes are still an important issue. The analytical solutions of PBE in mixed electrolytes with monovalent and bivalent counterions were firstly derived in the present study. The potential distribution and concentration profiles curves in EDL were obtained with one dimensional space by the analytical solutions of PBE. The potential distribution curves of different mixture showed that the power of screen surface charge for bivalent counterion was stronger than that for monovalent counterion, thus concentration of bivalent counterion in EDL was larger than that of monovalent counterion (Coulomb force).(3) Theoretical expression of surface potential was derived based on the analytical solutions of PB equation. Surface potential is a critical parameter in many interfacial phenomena. Up to present, however, it is difficult to determine surface potential in mixed electrolyte solution. The results showed that only the ion-surface Coulomb force cannot correctly describe the ion exchange equilibrium through Na/Ca exchange on illite surface, because the strong force that coupling between quantum fluctuation of ionic outer shell electrons and electric field from surface charges is unfortunately neglected. As considering these coupling effects, surface potential decrease with increase of difference of dipole moment between ions, the contribution of ionic polarization to surface potential drop increase with increase of ion-surface adsorption energy.(4) We observed strong Hofmeister effects in Ca/Na exchange adsorption on a permanently charged surface in a wide range of ionic strength, which cannot be explained by dispersion force, classical induction force, ionic size and hydration effects. We have concretely demonstrated that, another strong force was present in ion-surface interactions, and it would come from the coupling of electric field near surface and quantum fluctuations of ionic electrons in the field, which would be a general origin of Hofmeister effects. We have successfully evaluated the strength of this force and found that it was up to10000times stronger than the classical induction force, but can be comparable with Coulomb force. The observed strong force in ion-surface interactions will pose a challenge to all the related theories.(5) The relations of the strong coupling effects and selectivity of ion exchange equilibrium in different ion group on montmorillonite surface were quantified. The parameter β, reflects the strong coupling effects, obeys the following quantificational sequence:Ca (βca/βNa=1.699)> K (βK/βNa=1.646)> Mg (βMg/βNa=1.208)> Na (βNa/βNa=1)> Li (βLi/βNa=0.901); and the adsorption energy difference series:Ca> Mg (Ca/Mg=1.407)> K (Mg/K=1.467)> Na (K/Na=1.646)> Li (Na/Li=1.110). It provided a new theory for quantificational description ionic exchange adsorption selectivity for any ion pairs. The experimental data on various materials with very different charge densities agree with well each other. Thus we presume that the new model may apply to all charged surfaces.(6) Five parameters, including specific surface area, surface potential, charge density, electrostatic field strength and charge number on the solid/liquid interface, are important surface properties of charged nano-colloidal-particles. Currently, although the specific surface area (outer surface area) can be instrumentally determined through inert gas adsorption, there is not an instrument that could determine other four surface properties, let alone to give a combined measurement. A new method of combined measurements for the five surface properties by Na+-and Ca2+-selective electrodes with potentiometer was developed based on the new theory of ion exchange equilibrium, which can give precise results with good repeatability from the measurements. Besides, this new method bears three other significant advantages:(1) for the first time, the surface potential and total surface area (both inner and outer surface areas) of particles can be easily determined with potentiometer,(2) the enhanced polarization of counterions in surface electrostatic field from charged particles were considered and (3) a combined measurement for the five surface properties can be done with potentiometer. The combined measurement means the five surface properties of particles can be determined under the identical conditions, which is very important to study interface reactions in aqueous solutions. Based on the method, a new apparatus for combined measurement of the five surface properties of nano-colloidal-particles can be developed.In summary, the following original works in this study were performed: the analytical solutions of nonlinear PB equation in electrolyte mixture, the potential distribution and concentrations distribution were derived, which provide a theoretical direction for developments of EDL and DLVO theories; the selectivity of ion exchange could be theoretically calculated, and the ion-surface interaction energies and Hofmeister effects of ion exchange equilibrium on soil were quantified, thus the new theory could explain a series of experimental phenomena, such as the stability of aggregation, water translocation, soil transport and so on; Based on the new theory, a new apparatus for combined measurement of the five surface properties of nano-colloidal-particles can be developed, which also verify our ion exchange theory. |
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