Interface Molecular Dynamics Simulation And Second Harmonic Generation Study

As a powerful tool for surface and interface study, SHG has recently gained much interest. In the early1960s, surface SHG was firstly generated in calcite. Then based on the experiment results, the basic of surface nonlinear generation was explored and a comprehensive theory of surface nonlinear optics was established by Bloembergen et al. In the1980s, the microscopic mechanism of surface interaction with the optical beam was studied. Then the surface SHG was used to detect the formation of individual monolayers and also as a surface probe of molecular adsorption and orientation. Many physical and chemical interactions on the surface of different media were traced by this method, e.g. electron-transfer reactions occurring at interfaces, electrostatic properties of interfaces, dynamics of dye molecules adsorbed to liposomes, charge-transfer spectrums of a semiconductor-dye surface complex, etc. The study of the vibrational properties of adsorbed molecules in either frequency or time domain was realizable by surface SHG laser beam. After2000, many nonlinear crystals had been introduced for the study of different interface mechanisms, such as solid/solid, liquid/liquid and liquid/gas interfaces. Surfaces of many different materials including metal, semiconductor and nonconductor were used to generate surface SHG radiation.Apart from the advantage of being simple and sensitive, SHG study is surface specific. In the electric dipole approximation, SHG is forbidden in media with inversion symmetry. However, it can arise from the surface of bulk material with inversion symmetry or the interface of two media, since inversion symmetry is absent at the surface or the interface. SHG study of surfactant solution can provide a lot of information about the properties of molecules in the interface, for example, the molecular second-order polarizability and information of the molecular ordering, symmetry, and orientation.Due to the wide application of computer, a series of simulation methods, such as all-atom molecular dynamics simulation, have been widely used to study physics, chemistry, biology and materials science, and have achieved many significant results. According to two parameters of the time and length scales simulation can be divided into molecular mechanics molecular modeling, Monte Carlo, quantum mechanics, Brownian dynamics, referral concept simulation and molecular dynamics and other types. Currently the most widely used method is the molecular dynamics simulation which is an important way to solve the many-body problem at the atomic and molecular level. Molecular dynamics simulation for the study of surfactant solution is through the establishment of the gas-liquid interface model, to analysis model to achieve self-balancing results after implementation. By molecular dynamics simulations, the adsorption behavior of the interface can be obtained solvent molecules, including molecular density interface, molecular orientation ordering, molecular orientation angle and other parameters of the molecular chain length changes to achieve a combination of theoretical and experimental purposes. Molecular dynamics simulations to study the solution used in the model is Sandwich model.The paper is divided into three parts:In the first part, the molecular dynamics simulation of SDBS solution is mainly to simulate the function of the orientation angle and tail chain length and other parameters.In the second part, calculating the molecular orientation angle of the interface is divided into several second part of the second harmonic intensity solution interface signal detection, and application of monitoring data.The third part is an exploratory experimental study, mainly to detect the second harmonic signal on liquid-liquid interface, and make a simple analysis.In this paper a detailed analysis of the SHG data of Rhodamine B, SDBS and SDS solutions is presented. The SHG intensities with different concentration of SDBS and SDS are obtained and the interface molecular orientation angle of Rhodamine B is also calculated. In conclusion, it can be seen that the SHG intensities with different concentrations of SDBS and SDS change non-monotonously nearby CMC. The results of MD simulation indicate that the molecular orientational order increases with the increasing of the concentration, then tends to be a jumping and stabilizing gradually as the concentration reaches to the CMC. On the other hand, the simulated result of length of surfactant tail shows the same rule. These MD simulation results correspond with experimental results. The combination of SHG and MD simulation will be useful for the interface molecular configuration study of surfactant solution.