Micro-structure And Ions Aggregation In Salts Aqueous Solutions

The properties of solutions are dependent on the micro-structure of solutions,especially the micro-structure of the hydration-shell.Hydration shell has been widely studied through vibrational spectra in OH stretching region.This is because the intermolecular interactions between solutes and water molecules strongly affect the OH stretching vibration.In this dissertation,we employ spontaneous Raman spectroscopy to study the micro-structure of mixtures.This dissertation maily include three aspects:① A novel Raman ratio spectra,which is obtained through dividing the Raman spectra of aqueous solutions from the spectrum of water,was applied to deduce the spectra of hydration shell of organic and inorganic compounds.These hydration spectra was applied to obtain the value of Raman scattering cross section ratio and hydration number of solutes.② We report a systematic study on the Raman spectra of series of alkali halide aqueous solutions.The hydration spectra and hydration number of anions were obtained through using Raman ratio spectra.These hydration number data were utilized to analysis the interactions between cations and anions and discuss the aggregation of ions in dilute and concentrated solutions.③The hydration spectra and hydration number of acetone,THF,aether in their aqueous solutions were obtained through using Raman Global Fitting methoed.We discussed the compatibility of acetone aqueous solutions according to the change trend of hydration number of acetone with the concentrations.Besides,pump-probe detection can be used for Time-resolved technique.This technique need three beam laser:excitation pulse,pump pulse and probe pulse.The spectral resolution is determined by convolution of the natural linewidth of the pump light and the probe light.In order to get a better spectral resolution,we tried cw laser,whose natural linewidth is 0.0008cm-1,act as pump laser.In this dissertation,we explore different Raman detection method with pulsed light and cw light—inverse Raman methoed and Raman induced Kerr effect.(1)Spectra of hydration-shell obtained though using the Raman Ratio SpectraThe Raman ratio spectrum is defined as the ratio of the spectrum of solution to that of pure water.It was applied to deduce the spectra of hydration shell of organic(acetone-D6)and inorganic compounds(NaNO3,NaSCN,NaClO4,Na2SO4,NaCl)in water.Spectra of hydration-shell of inorganic compounds were attributed to the hydration spectra of anions,which supported by previous studies.Those spectra of the hydration shell were employed to study the micro-structures of the first hydration shells of anions,the number of water molecules in the first hydration shell of free anions and acetone-D6.The number of water molecules in the hydration shell was supported by our molecular dynamic simulations.Employing the Raman ratio spectra,a single spectrum of the hydration shell is obtained in each concentration.The independent spectra reflect the particular micro-structure of the hydration shell in each concentration.Hence,the Raman ratio spectra was employed to obtain the hydration spectra and hydration number of NO3-with different concentrations and analyse the micro-structures in the aqueous solutions with different concentrations.(2)Aggregation in strong electrolyte aqueous solutions revealed by Raman SpectroscopyThe structures and dynamics of hydration-shell water molecules around ions is key issue of strong electrolyte aqueous solutions.We obtained the spectra of hydration shell and the hydration number of anion in alkali-halide solutions through using ratio spectra.Our results imply that there are 8.4,8.9,8.0 water molecules around Cl-,Br-,I-,respectively.In sufficient low concentration(0.1M),the hydration number of Cl-and Br-of solutions with different cations are failing to be a unique value.This result suggests that the existence of some degree of aggregation of ions in these solutions at 0.1 M,that is,anions and cations have strong interactions to form ion pairs.In the Cl-and Br-solutions with different concentrations,the hydration number of anions decrease with increasing anion size except lithium ion.For this phenomenon may have two explanations.One is that the hydration-shell of anions would lose more water molecules when form ion pairs with cations with bigger size.The other one is that the bigger cations lead to higher degree of ion pairing and clustering.While lithium salt solutions are special case.May be the electric field of Li+ can affect the hydration-shell and anions more effectively due to its small size.Many studies support that lithium has a distinct character compared to other alkali ions.Furthermore,the hydration number rapidly decreases with the increasing concentration,through which demonstrates that a lot of ion pairs formed in these strong electrolyte aqueous solution in the low concentration region(<1M)and lots of contact ion pairs and clusters formed in the near saturating concentrations.(3)micro-structre of hydration shell of organic compounds in waterWe obtained the spectra of hydration shell and the hydration number of anion in acetone,THF,aether aqueous solutions through using spectral Global Fitting methoed.Our results imply that there are 1.9±0.2,1.5±0.2 and 1.3±0.2 water molecules around acetone,THF,aether in their aqueous solutions with mole fraction of 0.01,respectively.The number of water molecules in the hydration shell was supported by our molecular dynamic simulations.We obtained the binding energy that acetone,THF,aether with one water and two water molecules through using Gaussian Calculation.The result suggests that acetone with water molecules have the strongest stability while aether have the weakest stability,which explains why the hydration number decrease with the sequence acetone>THF>aether.Furthermore,the hydration number increase with different extent when the mole fraction of acetone decrease from 0.5 to 0.1 and from 0.1 to 0.01.This demonstrates that acetone molecules coupling with each other at high concentration.(4)Inverse Raman and Raman induced Kerr effect with the pulsed laser and cw laserInverse Raman is a kind of stimulated Raman process,we detected the lose signal of the probe light(a cw laser).While RIKES is a kind of Four-wave mixing process,we detected the signal of photons which have polarization changed.We built and optimized the system depending on the different requirements of both methoed.Thermal Blooming signal is the pointer that can help to get the Inverse Raman signal and RIKES signal,while it is also a kind of noise.Therefore,in order to improve the signal-to-noise ratio,we explored the methods for reducing the impact of Thermal Blooming.We compared and interpreted the shape of spectra obtained from Inverse Raman and RIKES.Furthermore,We found that RIKES methoed has more advantageous in detecting high concentration solutions while Inverse Raman methoed has more advantageous in detecting low concentration solutions...