Abstract:

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(ΔG_m⁰) 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 cmc), the principal contribution to the ΔG_m⁰ of negative value(-25—-30 kJ/mol) is the maximum positive value of molar standard adsorption entropy(ΔS_m⁰ is about 110 J·K^-1·mol^-1), whereas the molar standard adsorption enthalpy(ΔH_m⁰) is negative(-7.92 kJ/mol). At high loadings, the main contribution to the ΔG_m⁰ of negative value(-30—-20 kJ/mol) is the maximun negative value of ΔH_m⁰(-34.41 kJ/mol), whereas the ΔS_m⁰ 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 ΔH_m⁰=-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(ΔS_m⁰>0) to an enthalpy-driven process(ΔH_m⁰<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.

Key words: Myristylpyridinium bromide; Bentonite; Sorption thermodynamics; Interaction mechanism; Organized structure