Quantitative Description Of Surface Adsorption In Aqueous Solutions

Surface tension is one of the most important properties of liquids,and the surface adsorption behavior on the surface of liquids led by surface tension is an important physical process.Surface tension which is one of the most important properties of liquids and solutions plays an important role in the adsorption process.Gibss adsorption theory has always been widely used in explaining the surface adsorption phenomenon of solutions.However,in recent years,it has been found that the Gibbs adsorption theory cannot correctly describe the adsorption behavior of surfactant solutions and area-per-molecule.Based on the thermaldynamics framework,our team has proposed a new thermal state function and developed a new theory to explain the surface adsorption problems.In this article,we deduced new equations and applied them to pure liquids,strong electrolyte solutions and surfactant solutions to quantitatively explain the issues related to surface tension and surface adsoption.Not only for solutions,we found that there also exists surface adsorption in pure liquids' surface which corresponds to the the phase transition of pure liquid from bulk to surface,and we deduced the phase transition equation which can quantitatively describe the relationship between the surface tension of liquids and temperature.The phase transition of pure liquids from bulk to surface at constant pressure is exothermic.We applied the phase transition equation to 46 pure liquid systems and calculated their heat of phase transition per unit area.We totally investigated 16 strong electrolyte solutions,in which the surface tension values of NaCl,Na2SO4,MgCl2 and MgSO4 are determined by our experiments while other experimental values come from literatures.We analyzed the experimental data and quantitatively discussed the relationships between surface tension and the variation of concentration and temperature of strong electrolyte solutions.We calculated the minimum thickness of surface phase of these strong electrolyte solutions and gave theoretical explanations of factors which can influence the thickness of surface phase of strong electrolyte solutions.For surfactant solutions,we deduced a new formula and proposed a "three-step" procedure to calculate the thickness of surface phase,the solute concentration in surface phase and the area-per-molecule of surface phase in saturated solutions.Based on their surface tension data from literature,the area-per-molucule of hexyl,octyl and decyl trimethyl ammonium bromide in aqueous solutions have been calculated by our formula and density functional theory.The results from our formula are in agreement with the computational results of density functional theory.Gibbs theory has also been carried out for these three surfactants mentioned above,but the results are out of the range estimated with density functional theory which show the limitations of the Gibbs adsorption equation.