Thermodynamic Investigation On The Aggregation Behavior Of Cationic Surfactants

Surfactant is a ubiquitous substance in aquatic environment, because it has the characteristic of forming colloids, it can enwrap insoluble substance into water. To solve this problem of direct or indirect effect of surfactant on wastewater, it is necessary to study the thermodynamic behavior of microscopic aggregation in environmental aqueous phase. CTAC,CTAB,NH₄Cl,(NH₄)₂SO₄ and ethylene glycol are selected as model substance, surface tension,conductivity,viscosity and Zeta potential are used to simulate the complex aqueous process.The aggregation behavior of CTAC and CTAB in aqueous water has been studied by surface tension at 298.15K. Critical micelle concentration(CMC),degree of counter ion dissociation(β), aggregation number (n) and the surface excess concentration of surfactant ions (Γ_s) are reported. The response of conductivity with temperature and concentration ofCTAC and CTAB aqueous solution,CTAC/NH₄Cl,CTAC/(NH₄)₂SO₄ and CTAC/EG-H₂O mixtures are traced from 293.15K to 323.15K. Critical micelle concentration and degree of counter ion dissociation are obtained from conductivity curve. The equilibrium model of micelle formation was applied to obtain the thermodynamics parameters of micellization such an standard Gibbs free energy of micellization(Δ_micG°),standard enthalpies of micelleformation (Δ_micH°) and standard entropies of micellization (Δ_micS°). The response ofviscosity with temperature and concentration of CTAC and CTAB are traced from 293.15K to 323.15K. Based on the data ,the values of viscosity activation energy are obtained. Results shows that,(1) In the temperature range investigated CMC increases with temperature andβis slightly temperature dependent. In the present of salt CMC reduces obviously, CMC decreasing in the order SO₄^2->C1^-. In ethylene glycol-water binary mixtures CMC increases with the weight percentage of EG.(2)Δ_micG°is always negative indicating the micellization process can occurspontaneously.Δ_micH°is always negative showing that the formation of micelles is an exothermic process in the temperature range investigated.Δ_micS°is always positive meaning the entropy increasing in the process of micelle formation. -TΔ_micS°is larger thanΔ_micH°in the CTAC and CTAB pure aqueous solution,CTAC/NH₄Cl and CTAC/(NH₄)₂SO₄ mixtures indicating that the process of micellization is entropy driven In the CTAC/EG-H₂O binary mixtures, -TΔ_micS°is smaller thanΔ_micH°, showing the processof micellization is enthalpy driven. Enthalpy-entropy compensation is observed in the CTAC and CTAB pure aqueous solution and CTAC/EG-H₂O binary mixtures.(3) Conductivity and temperature can be well described by In (?), based on theequation ,the values of conductivity activation energy are obtained. Viscosity and temperaturecan be well described by In (?), based on the equation ,the values of viscosityactivation energy are obtained.Thermodynamic characteristics and its effectors are discussed in this study .It is supposed to be as basic data and reference to academic research.