NMR Study Of The Solid-liquid Interfacial Liquid Film

Solid-liquid(S/L) interface is ubiquitous in our lives. More in-depth studying of the molecular behavior in S/L interface phase is the key to understanding and improving many chemical and biological systems. At the same time, which is one of the most challenging research directions in surface/interface science field.Due to the lack of suitable experimental methods, experimental study on the behavior of molecules in the S/L interface area is relatively less.In this dissertation, by using of NMR spectorscopy, the interfacial liquid film of organic solvents forming on the neutral glass surface were studied in terms of structure and dynamics. Some of the conclusions were virified by molecular dynamics simulations(MD) and density functional theory(DFT) calculations. The main contents and results are as follows:A new set of resonances, that different from the one of bulk liquid, in1H NMR spectrum of the liquid film on S/L interface was observed. Chemical shifts and integrations of the new resonances reveal that those belong to the confined layer molecules of S/L interface liquid film. Changes in the coupling constants and the chemical shifts suggest the different aggregation of the confined layer molecules from bulk ones. The molecules formed more hydrogen bonds(HB) between each other when confined in S/L interface phase, the increasing HB indicated the increasing order of those molecules. According to the fact that the chemical shift of acetone shifts to downfield when turned into gas, and DFT study of electron density over different size of methanol clusters and the conclusions of the previous studies, change in proton chemical shift of confined liquid layer due to the change of intermolecular distance. The change of chemical shift(Δδδ) is proportional to that of liquid density(Δρ) as follows:Δδd=4πΔρ/3MχmThe S/L interfacial liquid film multi-layered model was proposed, the liquid film was mainly divided into two parts, from the solid surface, named the confined layer and the bulk layer. The bulk layer adjacent to the gaseous layer and the confined layer can be divided into many small layers with differetn molecular aggregation states. The signals "distortion" of the confined layer can be well explained by using this model. In addition, more information, such as integration and FWHM (full width at half maximum) and the thickness of each layer of the liquid film are obtained through nonlinear fitting and quantitative NMR.Methanol, ethanol, acetone, tetrahydrofuran, benzene liquid film on the glass surface were investigated by NMR to study their forming process. The change of density over time was somewhat different for different liquid films, this change may be related to the forming mechanism of liquid film. Since adjacent to the gaseous layer, bulk layer chemical shifts increase due to the increase of gaseous components throughout the forming. The total S/L interfacial liquid film thickness at around9-18μm, and the confined layer thickness of about6-15μn. During the process of interfacial liquid film forming, confined layer thickness increases over time, the bulk layer thickness decreases over time, the opposite trend of the thicknesses of confined layer and bulk layer cause the different total liquid film thickness change over time for different liquid films. The total liquid film thickness always tend to reach a fixed value finally, which indicates that the liquid film formation reaches a steady state.Chemical shifts change over time of the interfacial liquid film confined layer protons conform to allometric model. For the liquid that can form intermolecular HB, such as methanol and ethanol, the change rate of hydroxyl protons chemical shift was smaller than that of alkyl protons in liquid film confined layer. During the initial period of process, the protons chemical shift change rate was significantly different for different liquid film, finally, they reached a fixed value and closely for each liquid film.Preliminary study on the influence of temperature on the liquid film proton chemical shift changes. The aggregation states of liquid film bulk layer change significantly at different temperature, this change in turn affect the resonances of the liquid film layers. Experiment showed that greater impact on the liquid with higher saturation vapor pressure by temperature.The dynamic properties of the S/L interfacial liquid film molecules were investigated by NMR in terms of longitudinal relaxation time (T1) and self-diffusion coefficient(Dself) of the various parts of the liquid films. Shorter T1of both confined layer and bulk layer molecules comparing with bulk liquid shows that they are restricted by long-range force of solid surface to some degree. Dself of methanol molecules in liquid film confined layer obtained by NMR is smaller than the theoretical value by MD when the molecules studied in same density. These findings confirm the existence of highly restricted liquid film layers adsorbed onto solid surfaces. MD results also showed that the intermolecular hydrogen bond strength of methanol has not changed when density decreases, more molecules participate in the formation of molecular hydrogen bond clusters, but the average distance between molecules increase. The possible explanation is that the distance between the clusters increases, and slightly decreases the average size of the clusters formed. Thus, S/L interfacial liquid film on the order of micrometers are structurally and dynamically different from the bulk liquid.