Free-energy of adsorption of a cationic surfactant onto Na-bentonite (Iran): inspection of adsorption layer by X-ray spectroscopy M. Ghiaci a, * , R.J. Kalbasi a , H. Khani a , A. Abbaspur a , H. Shariatmadari b a Department of Chemistry, Isfahan University of Technology, Isfahan 84156, Iran b Department of Soil Science, Isfahan University of Technology, Isfahan 84156, Iran Received 27 January 2004; accepted 7 May 2004 Available online 9 June 2004 Abstract The adsorption behaviour of hexadecylpyridinium bromide (CPB) onto bentonite was studied as a function of temperature under optimized condition of shaking time, and concentration of the adsorbate. Thermodynamic parameters such as DH °, and DS ° were calculated from the slope and intercept of the linear plot of log K d against 1=T . Based on the calculation of the thermodynamics of adsorption, it is evident that the principal contribution to the DG° of negative value ()10 to )14 kJ  mol 1 ) is the large positive value of DS° (58–60 J  K 1  mol 1 ), whereas DH ° is positive and in the range of 4–8 kJ  mol 1 . Analysis of adsorption results obtained at temperatures (288, 297, 303, 307, 313) K showed that the adsorption pattern on bentonite followed the Langmuir isotherm. The Langmuir sorption maximum at different temperatures for CPB sorption are reported (1100 to 1534 mmol  kg 1 ). The adsorption of the cationic surfactant CPB on a hydrophilic smectite (bentonite) surface has also been investigated, especially in the range of low coverage ratios where surfactant ions are adsorbed through cation-exchange with the counterions of the clay. The intercalation of surfactant in the interlamellar space was followed by X-ray measurements. At low surfactant concentrations, there is a peak cor- responding to d 001 of 1.32 nm, and after full coverage at about 1.5 CEC (cation-exchange capacity) value, two peaks are seen corresponding to interlamellar distances of both (2.0 and 3.8) nm. Fluorescence spectroscopy was used to obtain information about the adsorption layer of HDTMA at the interface. Ó 2004 Published by Elsevier Ltd. Keywords: Adsorption; Thermodynamic data; Gibbs free energy; Cationic surfactant; Bentonite; Langmuir isotherms; n-Cetylpyridinum bromide; Hexadecyltrimethylammonium bromide 1. Introduction Organobentonites are produced by the exchange of organic cations for inorganic ions (e.g., K þ , Na þ , and Ca 2þ ) on the layer surfaces of bentonite [1]. The sorp- tion properties of modified bentonite surfaces may be significantly altered by this exchange reaction. The mineral surfaces of the resulting organobentonite may become organophilic because the organic functional groups of the quaternary ammonium cations are not strongly hydrated. As a result, organobentonites are powerful sorbents for non-ionic organic pollutants rel- ative to natural bentonite and other clays [2]. Jordan and coworkers [3,4] developed a series of or- ganobentonite complexes which he called ‘‘bentones’’. Because of their unique sorption capabilities, organo- bentonites have been investigated for a wide variety of environmental applications [5–12]. The feasibility of using bentonite exchanged with dodecylpyridinium bromide (DPB) and hexadecyltrimethylammonium (HDTMA) cations to retard the transport of non-ionic organic pollutants from solution has been investigated [13,14]. Several models have been proposed to account for surfactant adsorption on solid surfaces. These models are * Corresponding author. E-mail address: mghiaci@cc.iut.ac.ir (M. Ghiaci). 0021-9614/$ - see front matter Ó 2004 Published by Elsevier Ltd. doi:10.1016/j.jct.2004.05.007 J. Chem. Thermodynamics 36 (2004) 707–713 www.elsevier.com/locate/jct