阳离子表面活性剂在膨润土上的有序化作用及吸附热力学

Organized Assemblies of Cationic Surfactant on Bentonites in Water and Their Thermodynamics

绘制了不同温度下溴化十四烷基吡啶(MPB)在膨润土上的等温吸附曲线,探讨了MPB的吸附热力学及作用机理;并用XRD,FTIR和UV固体漫反射表征了MPB在固相上的有序化作用.研究结果表明,MPB的吸附热力学、作用机制及有序化过程取决于其在膨润土上的负载量.不同负载量下标准吉布斯自由能ΔG—m0<0,表明为自发吸附过程,但存在3个不同阶段.在低负载量(<0.8CEC)时为吸热吸附[焓变ΔH—m0=7.92kJ/mol,熵变ΔSm—0≈110J/(K.mol)],以阳离子交换作用吸附为主;在高负载量阶段为放热吸附[ΔH—m0=-34.41kJ/mol,ΔS—m0≈-50J/(K.mol)],以脂肪链间的疏水性相互作用为主;最大吸附量随温度的升高(278K→328K),从2.8CEC降低为1.5CEC,相应的ΔH—0m=-9.74kJ/mol.在负载量由低到高的吸附过程中,吸附态MPB的状态从无序"液态"逐渐演变为有序"固相",吸附过程从熵驱动(ΔSm—0>0)过渡为焓驱动(ΔH—m0<0).

Sorption of myristylpyridinium bromide （MPB） on a model mineral, bentonite, was investigated under different temperatures （278, 283, 298, 308, 318 and 328 K）. The sorption thermodynamics and interaction mechanisms are discussed. The resultant complexes of MPB-mineral were characterized with XRD, FTIR and UV-Vis diffuse reflectance spectra to probe the organized assemblies of adsorbed-MPB. The thermodynamics, mechanism and organized process are dominated by the MPB-loadings. For the whole isotherms, the molar standard Gibbs free energy of adsorption（ △A m ） is negative, suggesting that adsorption is a spontaneous process, but the assembled process divided into three stages： sorption was driven by cationic exchanged firstly, then controlled by hydrophobic interaction after cationic exchanged reach maximum level. At low MPB- loadings（ 〈0. 8 emc）, the principal contribution to the △G m of negative value（ -25-- 30 kJ/mol） is the maximum positive value of molar standard adsorption entropy（ △S m is about 110 J· K^-1·mol^-1） , whereas the molar standard adsorption enthalpy（△H m） is negative（ -7.92 kJ/mol）. At high loadings, the main contribution to the △A m of negative value （ - 30--20 kJ/mol ） is the maximun negative value of △H m （ -34.41 kJ/mol）, whereas the △S m is negative（about -50 J·K^-1·mol^-1 ）. The saturated amount of MPB decreased from about 2.8CEC at 278 K to about 1.5CEC at 328 K, and the corresponding △H m = -9.74 kJ/mol. With the increase of loading, the adsorbed-MPB transited from a disordered liquid-like state to an ordered solid-like state, and the spontaneous adsorption evolved from an entropy-driven process（ △S m 〉 0） to an enthalpy-driven process （△H m 〈 0）. These observations provide a theoretical base to further understanding the adsorption behavior of cationic surfactant to mineral and an engineered reference to synthesize surfactant-mineral complexes for environmental applications.