Diffusion Structure Of Aqueous Rubidium And Cesium Borate Solutions

The diffusion structures of aqueous rubidium borate and cesium borate solutions are far from clear until now,in which rubidium ion and cesium ion have a relative large coordination distance and coordination number.The diffusion structures of aqueous metaborate?MBO₂?,tetraborate?M₂B₄O₇?and pentaborate?MB₅O₈??M=Rb/Cs?)solutions were studied using X-ray and neutron scattering,assisted by other methods such as DFT?Density functional theory?calculation and Raman spectroscopy to explore the hydration structures of rubidium ion,cesium ion,borate ions and their association in the present work.The main contents and conclusions are as follows.1.X-ray scattering experiments were performed using Ag target?K?=0.056087nm?with free surface reflection method,and Mo target?K?=0.070926 nm?with capillary transmission method,respectively.The distribution of borate ions in solution was calculated by species distribution as the basis of data analysis.For aqueous MBO₂?M=Rb/Cs?solutions,model calculations of radial distribution functions and DFT calculations are on the basis of B?OH?₄^-,Rb⁺or Cs⁺.The results show that the coordination distance of the first hydration layer of Rb⁺?Rb-O?I??in Rb BO₂solutions?1.628-4.009 mol/L?at 303 K is about 0.293 nm,corresponding coordination number is about 7.7;The coordination number of contact ion pairs Rb-B is about 0.8,and the coordination distance is about 0.372 nm,in addition,Rb⁺and B?OH?₄^-are coordinated in bidentate structure;The effect of concentration on the hydration degree and hydration structure of the ion were discussed at the same time:The larger the concentration,the more contact ion pairs;On the contrary,the larger degree of ion hydration.In Cs BO₂ solutions?1.526-2.101 mol/L,303 K?,the distance of B-O?W?is?0.371 nm,and the hydration number around B is?7.8;The distance of contact ion pairs Cs-B is?0.458 nm and the interaction number is?0.77;The interaction distance and number of the first coordination layer of Cs⁺?Cs-O?I??are?0.325 nm and?8.0,eight-hydrated stable clusters of Cs⁺are also obtained by DFT calculation which agree with the experiment results.Model calculation of M₂B₄O₇?M=Rb/Cs?solutions are rely on four kind borate ions:B?OH?₄^-,B₃O₃?OH?₄^-,B₃O₃?OH?₅^2-and B₄O₅?OH?₄^2-;In Cs₂B₄O₇ solutions?1.405-2.308 mol/L,303 K?,the results show that the distance of the first coordination shell of Cs⁺?Cs-O?I??is?0.336 nm with the coordination number?7.8.The distance of the second hydration shell Cs-O?II?is?0.526 nm,and the coordination number is?8.4;The distance of Cs-O?ring?is about 0.488 nm in which O is the epoxy in borate,and the average coordination distance of Cs-B is 0.458 nm;In Rb₂B₄O₇solutions?0.688-1.509 mol/L,303 K?,the hydration distance of Rb-O?I?is about 0.292nm,and the hydration number is about 7.0;The average coordination distance of Rb-O?II?in the second hydration layer of Rb⁺is 0.467 nm,and the coordination number is about 8.0;The distance of hydrogen bond O-O is about 0.278 nm,and the number of the interaction is about 2.22;The distance of the contact ion pairs Rb-B is about 0.377nm,and the number is about 0.9;DFT calculations show that Rb⁺and B₄O₅?OH?₄^2-are bidentate coordinated with two adjacent epoxy or terminal oxygen with epoxy,Cs⁺also bidentate coordinated with two adjacent epoxy of B₄O₅?OH?₄^2-.B?OD?₃,B₃O₃?OD?₄^-and B₃O₃?OD?₅^2-as the main borate species in cesium pentaborate(Cs¹¹B₅O₈+D₂O,0.099 mol/L,353 K),the characteristic B₅O₆?OD?₄^-hydrolyzes in the solution,Cs-O?I?distance is?0.332 nm,and the coordination number is?8.4 in model calculation;The average coordination distance of Cs-B is?0.490 nm,and the coordination number is?0.6.2.Neutron scattering experiments were conducted at China Mianyang Research Reactor?CMRR?-reactor neutron source and Japan Proton Accelerator Research Complex,Materials and Life Science Experimental Facility?J-PARC,MLF?-spallation neutron source,respectively.In order to achieve the neutron scattering experiments and data processing of rubidium/cesium borate solutions,neutron scattering experiments of aqueous rubidium chloride?Rb Cl?and cesium chloride?Cs Cl?deuterium solutions at high temperature?298 K-523 K?and high pressure?0.1 MPa-4 GPa?were performed as a preliminary study to get familiar with the neutron scattering experiment and data processing.The results show that the hydration intensity of Rb⁺/Cs⁺and Cl^-changes with temperature and pressure.The specific features are that the hydration distance increases with the increasing temperature,and the hydration number becomes smaller at the same time;The hydration number increases and the coordination distance decreases with pressure increasing;Structural characteristics are observed such as four linear interactions of O-D…O in H-bonding,Rb/Cs…O-D in Rb⁺/Cs⁺hydration clusters,Cl…D-O in Cl^-hydration clusters and Cl…Rb/Cs…O in contact ion pairs by angle analysis;The second and the third hydration layer of Rb⁺/Cs⁺/Cl^-/H₂O are observed at extreme conditions of high temperature and high pressure;The deuterium atoms?D?are more biased towards the central metal ion and the interaction peak of Rb⁺/Cs⁺/Cl^--D split,and also contact ion pairs tend to form larger clusters at high temperature and high pressure.The O-O interaction in deuterated water does not follow the laws of compression by pressure and expansion by heating,different interactions with temperature and pressure do not follow the same rules.3.The neutron scattering experiments with ¹¹B substituted and deuterated rubidium/cesium borate solutions were performed on J-PARC BL21,the total scattering spectrometer NOVA.Since ¹¹B replace B and D replace H,neutron scattering experiments obtain more accurate scattering intensity,structure functions,and radial distribution functions.Among them,the point position of the first peak of g _Cs-O?r?obtained by EPSR in Cs BO₂ solution?1.823 mol/L,343 K?is?0.328 nm,and the coordination number obtained by integrating the area of the first peak is 5.86±2.51;The corresponding distance of Cs-B is about 0.407 nm,and the coordination number is1.77±0.94;The coordination distance of B-OW is?0.356 nm,and the coordination number is 12.13±4.83,meaning that there are about twelve water molecules around the B?OH?₄^-.There are small differences in the range of Q=0.33-0.38 nm^-1 in the structure functions among samples of M₂B₄O₇?M=Rb/Cs??298 K?solutions,the sample curve of the smaller concentration is higher,qualitatively reflecting the stronger hydrogen bond and the hydration intensity of rubidium/cesium ion.B₅O₆?OD?₄^-ions in the MB₅O₈?M=Rb/Cs??298 K/373 K?solutions can be considered in the data analysis,the structure functions of the same kind samples are similar;The structure functions of Rb B₅O₈ solutions are higher than that of Cs B₅O₈,and the curves of the lower concentration samples are higher,the curves of the higher temperature samples are slightly lower,which reflect the structure changes among the solutions.