Experimental Study On Thermal Diffusion Effect In Two-component Solution

In the past decade,the non-equilibrium effect has become a hot topic in the basic research of soft condensed matter which refers to matter between solid and ideal fluid.In an isotropic fluid system of an enclosed medium with no external forces,concentration gradient builds up due to the driving force of applied temperature gradient until system reaches into the steady state condition.This phenomenon is called the Soret effect or thermal diffusion effect,and the similar phenomenon produced in suspended particles in a carrier liquid or gas is called thermophoresis.The Soret effect describes the coupling between the temperature gradient and the mass flux produced in a multi-component system.The Soret coefficient quantifies the tendency of the mixture to separate under thermal gradients.Although this phenomenon has been discovered at an early stage,there is no complete microscopic explanation of it,so there is no complete theory that can successfully predict the size and sign of the Soret coefficients in a wide range of situations.In practical applications,the thermal diffusion effect mainly occurs in aqueous solutions.Therefore,different organic and inorganic solutes are used to explore the role of water in the thermal diffusion effect of different solutions in this study.This study builds a Soret unit device with thermal diffusion effect,and the refractive index of the solutions are measured to obtain the quantitative experimental analysis of the Soret effect and diffusion effect of three alkali metal halide solutions(NaCl,KCl and LiCl)and four alcohol solutions(ethylene glycol,iso-propanol,n-propanol and glycerol).The variation of the Soret effect and diffusion effect with temperature,concentration and interaction between components are obtained.1.High-salt concentrations and low temperatures in the LiCl solution are favorable for the thermophilic state,whereas high temperatures and low-salt concentrations are favorable for the thermophilic state.The low-salt concentration and low temperature of the NaCl solution are favorable for the heat-sensitive state,whereas the high temperature and high-salt concentration are favorable for the heat-dissipating state.Under experimental conditions,the Soret coefficient of the KCl solution is positive.Salt accumulates on the cold side,and the solution is hydrophobic on the whole.Low-salt concentrations and low temperatures in the KCl solution are conducive to thermal state,whereas high temperatures and high-salt concentrations are conducive to the thermal state.Cationic diameter plays an important role in the Soret coefficient.The larger the cationic size,the lower the thermotaxis of the solution and the stronger the hydrophobicity.2.An inversion effect exists in the Soret coefficient of alkali halide solutions,and the sign inversion temperature depends on the solution’s composition.The inversion temperatures of the Soret coefficient of the LiCl and NaCl solutions increase significantly with increasing salt concentration.No sign inversion is observed for the KCl solution at room temperature.However,the Soret coefficient also produces a sign change at sufficiently low temperatures.3.The diffusion coefficient,D,is related to both temperature and concentration.High temperatures and high concentrations are conducive to the diffusion speed,and the change of D with temperature is more obvious than that with concentration.The diffusion coefficient,D,of LiCl is the largest among three solutions,and the diffusion coefficient of KCl is the smallest.On the one hand,the increase of relative atomic mass affects the diffusion rate of particles.On the other hand,the increase of diffusion resistance leads to the decrease of the diffusion coefficient.4.At the same average temperature of the n-propanol and iso-propanol solutions,with the increase of concentration,the thermophilicity of the solution gradually increases,the thermophilicity gradually decreases,and the value of ST gradually decreases.There is sign inversion in the curve of the Soret coefficient change with concentration,and the concentration inversion point of the Soret coefficient,ST,decreases with the increase of average temperature.At high concentration,the solution is thermophilic,with the increase of average temperature,the value of the Soret coefficient,ST,gradually decreases.In the case of low concentration,the solution shows thermotropism,and the Soret coefficient increases with the increase of average temperature.5.The Soret coefficient of ethylene glycol and glycerol solution decreases continuously with the increase of concentration,and the hydrophobicity of solution gradually weakens.At lower concentrations,the magnitude of ST declines more severe.As the concentration increases,the trend of ST decline gradually becomes flat.At the same concentration,the ST values of the two solutions increases as the average temperature increased.When the temperature and concentration are low enough,the symbol reversal phenomenon still occurs in the ST.6.Under the same average temperature,Tmean,the diffusion coefficient,D,of the four organic solutions is both related to temperature and concentration.The relationship between diffusion coefficient,D,and concentration and average temperature is approximately linear.The diffusion rate increases with increasing temperature and decreases with increasing concentration.As the speed increases,the change in D with concentration is more pronounced than the change in the average temperature Tmean.