Theoretical and Experimental Researches of the Phase Transitions in Small Molecule Liquids

Author:Wang Li Na

Supervisor:qi ming huang yi neng


Degree Year:2019





Phase transition is a phenomenon universally existing in nature,and is applied widely in modern industry and daily life,the measurement of which also provides the effective way to obtain microscopic structure and interaction of materials.In general liquids,it is believed that there is only one phase and that no liquid-liquid phase transition occurs.However,the recent researches show that monohydroxy alcohols have the transition of hydrogen-bonded molecular clusters,moreover,mesoscopic heterogeneity or microscopic phase separation has also been shown in the solution.Traditionally,the crystal melting is considered as the first order transition and only occurs at the melting point,but the pre-melting of crystals has been presented in researches.There are still many unsolved problems about the crystal melting mechanism.In addition,glass transition,a bizarre phase transition,between liquid and amorphous solid(i.e.,glass),is related to the continuous slowing down of the dynamics in the time scale of 16 orders of magnitude and no theory has been accepted universally yet.In this paper,the researches of the theory and experiment are carried out to investigate the above problems by focusing on the phase transitions of small molecule liquids.To research the Debye relaxation in monohydroxy alcohols,an Ising model constituted by the free-rotating pseudospin chains,is proposed based on the former studies of hydrogen bonded molecular chains.Moreover,the exact solution of the dielectric strength vs temperature in the model is gained and indicates a crossover from the low to the high temperature Curie-Weiss law,qualitatively consistent with the experimental results and superior to the other dielectric strength theories.In addition,the expressions of relaxation time and complex permittivity of the model are given in combination with Glauber dynamics.Quantitative comparison between the model results and the experimental data shows that the model can well describe the dielectric strength,relaxation time and relaxation peak of Debye relaxation in monohydroxy alcohols in a wide temperature range.Meanwhile,the values of the model parameters are reasonable.In the term of experimental studies,the melting processes of propylene carbonate and 1,3-propylene glycol are studied by the reed vibration mechanical spectroscopy for liquids which has highly sensitivity.The results show that both the real and the imaginary parts of the complex Young’s modulus can give some characteristic temperatures in the melting process of crystals.In the initial stage,the melting of crystals is a thermal activation behavior of the molecule groups and occurs at the grain boundaries after the nucleation saturation first,and then at the grain boundaries and edges after the nucleation saturation.To explore the micro-distribution of sodium chloride in its aqueous solution,the phase transition of the solutions with a series of molar ratios of sodium chloride is measured and analyzed by differential scanning calorimeter(DSC)in the temperature range of 223 K-323 K,and the results show that all measured solutions appear two endothermic peaks in melting.For the low temperature peak,the intension increases and peak position moves to the high temperature with the increase of the molar ratio,while those of the high temperature peak are opposed,at last,they become one peak.After the analyses,the changes of the characteristic parameters are obtained,which are absorbed heat of the low and the high temperature melting process,initial temperature of the low temperature melting process,termination temperature of the high temperature melting process,as well as peak temperature and half-width of the low and the high temperature peak.Combining with the microscopic phase separation model,the images are gained that the poor and rich water phase coexist in sodium chloride aqueous solution,and the spatial distributions of sodium chloride concentration are obtained in the poor and rich water phase.In addition,for glass transition,a series of the binary mixed liquids is measured and analyzed by DSC and dielectric spectroscopy,as well as the values of glass transition temperature(Tg)and dynamics parameters(A、τ0 and T0)are gained,which are between those of the two pure components.The comparisons between Tg and dynamics parameters show that the variation tendency of Tg vs the ratio of component is similar to that of A,while the change of Tg and that of lgτ0 have a strong correlation,which will provide some references to the studies of the correlation between thermodynamics and dynamics related to the glass transition.This paper investigates the Debye dielectric relaxation in monohydroxy alcohols,the melting of crystals,the microscopic distribution of sodium chloride in its aqueous solution,as well as the correlation between thermodynamics and dynamics related to the glass transition.The above works will lay the foundations and provide the methods as well as ideas for further researches of the related issues.