Study Of Liquid-solid Transition And Glass Transition Of Aqueous Solutions

Water is the most common matter on earth,and widely ackonwleged as the matrix of life.Water is very simple in structure but that is in no sence means water is straightforward and easy to understand.In contrary,because of the presence of hydrogen bonds breaking and forming in time scale of picosecond among water molecules,almost all properties of water are different from normal liquid and water is always regarded as the most extraordinary liquid.Water is ivolved in a variety of physical,chemical and life processes which is crucial for revealing the mysteries of nature.It is very difficult to investigate pure water directely and water seems refuse to be understood.However,aqueous solutions are relatively easy to scrutinize.Although adding solute makes water more complex,many properties of aqueous solutions depend closely on the solute and concentration,which often contains and conveys lots of physical features.Crystallization and vitrification of aqueous solutions is one of the hottest research topics and difficulties in the field of water science.In this dissertation,liquid-solid transition and glass transition behaviors of binary aqueous solutions of common electrolytes and organic molecules,ternary aqueous solutions for electrolytes and organic molecules and confined solutions were investigated systematically with Differential Scanning Calorimeter,Broadband Dielectric Spectrometer and Raman Spectra.The main research contents are addressed as follows:（1）By measuring the glass transition behavior of several solutions,a universal feature of water-content,X_aqu,dependence of glass transition temperature,T_g,is recognized,and according to that relationship aqueous solutions were divided into three regions.Solutions with X_aqu>X_aqu^cr,region I,vitrify at a constant temperature,T_g^’,referring to freeze-concentrated phase with X_aqu^’ left behind ice crystallization.Solutions with X_aqu<X_aqu^cr,in regionⅡ X_aqu^cr<X_aqu<X_aqu^’,and region Ⅲ,X_aqu<X_aqu^’,vitrfy totally in the cooling process,and the T_g of those solutions decrease monotonically with increasing X_aqu.Basing on the behaviors of glass transition and liquid-solid transition in those three regions,hydration number,nh,and critical hydration number,ncr,defined here referring the molar ratio of water to solute for solutions with concentration X_aqu^’ and X_aqu^cr respectively.According the unambiguous definition of hydration number above,the amount of free water can then be determined for water-rich solutions,whose mass fraction,X——f,is found to serve as a universal relevant parameter for characterizing the homogeneous ice nucleation temperature,T_H,the melting temperature of primary ice,T_m,and water activity,a_w,of solutions of electrolytes and smaller organic molecules.Moreover,the relations between X_f and T_H,Tm and aw are also applicable in pure water and solutions under pressure.（2）Liquid-liquid transition of water is a controversial and important hot topic in water science.Recently,it was claimed to have been confirmed in some aqueous solutions with special concentration based on annealing-induced upshift of glass transition temperature,T_g.According to the universal water-content.X_aqu,dependence of T_g for aqueous solutions,we found all the claimed special concentrations fall in our Ⅱ concentration region.And the upshift of T_g by annealing treatment is attributed to the increase of solutions concentration induced recrystallization of ice instead of liquid—liquid transition.（3）For aqueous solutions with freezable bound water,vitrification and recrystallization are mingled,which brings difficulty to application and misleads the interpretation of relevant experiments.Here we report a quantification scheme for the freezable bound water based on the water-content dependence of glass transition temperature,by which also the concentration range for the solutions that may undergo recrystallization finds a clear definition.Furthermore,we find that depending on the amount of the freezable bound water,different temperature protocols should be devised to achieve a complete recrystallization.（4）The hydration number,nh,and critical hydration number,ncr of several binary and mixed solute ternary aqueous solutions are determined.The ratio of n_cr/n_h measures 1.7 for electrolytes solutions,and larger values depending on types of solute for organic solutions were found.The ratio of n_cr^mix/n_h^mix for mixtures of glycerol with all the chlorides except ZnCl₂ falls within these two extremes.By measuring the change of hydration number of solutes with concentrations,the interactions between glycerol,ZnCl₂ and water were discussed.The interaction between glycerol and ZnCl₂ was found to depend both on the contents of glycerol and water.In addition,our results can help establish a pathway for the investigation of multi-body interactions in mixed solute aqueous solutions.（5）Bulk aqueous solution of NaCl is a poor glass former,it vitrifies only under high pressure.The effect of confinement conditions and the wettability of the pore surface on the glass forming ability of NaCl aqueous solution were investigated by measuring the glass transition behavior of the confined NaCl aqueous solutions with different nanopores size.It is the confinement results in the vitrification of the NaCl aqueous solution,and a model is proposed to explain the dependence glass transition behavior of confined NaCl aqueous solution on critical concentrations.