Journal of Colloid and Interface Science 222, 125–136 (2000) doi:10.1006/jcis.1999.6550, available online at http://www.idealibrary.com on Adsorption Mechanism and Structure of the Montmorillonite Complexes with (CH 3 ) 2 XO(X = C, and S), (CH 3 O) 3 PO, and CH 3 –CN Molecules G. Dios-Cancela, 1 L. Alfonso-M´ endez, F. J. Huertas, E. Romero-Taboada, C. I. Sainz-D´ ıaz, and A. Hern´ andez-Laguna Estaci´ on Experimental del Zaid´ ın (CSIC), C/ Prof. Albareda no. 1, 18008 Granada, Spain Received June 8, 1999; accepted September 15, 1999 The formation of complexes with different ligands in the in- terlayer space of montmorillonite saturated in Na + , Mg 2+ , Ca 2+ , Co 2+ , Cu 2+ , Ni 2+ , Fe 3+ , and Cr 3+ was studied. Acetone, acetoni- trile, dimethyl sulfoxide, and trimethylphosphate were used as ligands. The nature of the complexes was studied by means of X-ray diffraction, infrared spectroscopy, thermogravimetric analy- sis, microcalorimetry, and ab initio quantum mechanical methods. In all cases, the organic ligands penetrate into the interlayerspace at room temperature, forming complexes stable in vacuum with the interlayercations. The ligand–cation ratio depends on the valence of the saturating cation. The cation–ligand interaction in these com- plexes has an ion–dipole electrostatic nature. The complexes are formed by the direct interaction of the oxygen ornitrogen atom of ligand and the interlayer cation. Using the quantum mechanical ap- proach, allow us to determine the disposition of the ligand in these complexes. In all cases, only one layer of ligands is present in the stable complexes. C  2000 Academic Press Key Words: interaction; dimethylsulfoxide; trimethylphosphate; acetone; acetonitrile; montmorillonite; heats of adsorption; struc- ture; ab initio calculations. INTRODUCTION Montmorillonites are present in most soils and sediments. They are very important minerals in chemistry, the environment, agriculture, and industry. They play a relevant role in soil pollu- tion because of their important adsorption capacity and low per- meability. Adsorption of polar organic molecules is a top subject in clay research and is being studied by many authors; an exten- sive review cannot be given in this Introduction. The study of the physical–chemical properties related to the organic–inorganic interface can be very useful for understanding the macroscopic behavior of organic molecules in soils (mobility, diffusion, etc.) and in industrial applications, as catalysts and paints. Many organic molecules with polar functional groups can be adsorbed in the interlayer space of montmorillonite, producing the swelling. McEwan (1) reported that these polar groups me- diate with the exchange cations and layer effective charges lib- 1 To whom correspondence should be addressed. erating energy. This energy is able to overcome the cation-layer attraction forces and produce the interlayer swelling. Nonpolar adsorbates, wherein only van der Waals forces mediate, do not produce enough energy to surmount the cation-layer attraction forces in montmorillonites. Other scientists studied the swelling of montmorillonite by means of X-Ray diffraction. These works confirm the McEwan’s hypothesis and furthermore they show that the complex formation and the basal spaces are a function of the drying level of the sample (2) and the phase (gas or liquid) of the organic molecules (3–5). Barshad (3) reported that the complexation capacity of mont- morillonite with polar organic molecules depends on the di- electric constant of the molecule. This capacity decreases when the dielectric constant decreases for molecules with comparable dipole moment. Recently, van Asche et al. (4) discussed this fact, finding that the greater the dielectric constant value of the or- ganic molecule, the greater is the work to expand the layers of the silicate. Bissada and Johns (6) determined the adsorption heat of acetone and methanol in montmorillonite by means of gas phase chromatography, they proposed that the ion–dipole interaction is the specific mechanism for the adsorption in montmorillonite. Van Asche et al. (4) discussed once again those conclusions, because the adsorption of amines by Na + -montmorillonites is more effective than that of alcohols,in spite of that amines have lower dipolar moment than alcohols. In the 1960s, infrared (IR) spectroscopy studies of organic molecules adsorption in montmorillonite revealed that distinct mechanisms are possible. The most probable adsorption mech- anisms of polar molecules are due to ion–dipole forces or form- ing coordination complexes (7). Probability of complex forma- tion and the interaction type depend basically on the drying level of the sample and the crystal-chemistry properties of the exchange cations. To describe quantitatively the adsorption mechanism, we must know the interaction energy between exchange cations and lay- ers, the energy developed during the adsorption process, and a suitable hypothesis of the complex nature. For these thermo- dynamic requirements, heats of adsorption must be determined in a reliable way. In order to study the adsorption mechanisms, we determine the adsorption heats of polar neutral molecules by 125 0021-9797/00 $35.00 Copyright C  2000 by Academic Press All rights of reproduction in any form reserved.