Intercalation of ethylene glycol in smectites (glycolation) is widely used to discriminate smectites and vermiculites from other clays and among themselves. During this process, ethylene glycol molecules enter into the interlayer spaces of the swelling clays, leading to the formation of two-layer structure (~17 Å) in the case of smectites, or one-layer structure (~14 Å) in the case of vermiculites. In spite of the relatively broad literature on the understanding/characterization of ethylene glycol/water-clays complexes, the simplified structure of this complex presented by Reynolds (1965) is still used in the contemporary X-ray diffraction computer programs, which simulate structures of smectite and illite-smectite. The monolayer structure is only approximated using the assumption of the interlayer cation and ethylene glycol molecules lying in the middle of interlayer spaces. This study was therefore undertaken to investigate the structure of ethylene glycol/water-clays complex in more detail using molecular dynamics simulation. The structural models of smectites were built on the basis of pyrophyllite crystal structure (Lee and Guggenheim, 1981), with substitution of particular atoms. In most of simulations, the structural model assumed the following composition, considered as the most common in the mixed layer illite-smectites (Środoń et al. 2009): EXCH0.4(Si3.96Al0.04)(Al1.46Fe0.17Mg0.37)O10(OH)2 Atoms of the smectites were described with CLAYFF force field (Cygan et al., 2004), while atoms of water and ethylene glycol with flexible SPC (Berendsen et al., 1981) and OPLS (Jorgensen et al., 1996) force fields, respectively. Ewald summation was used to calculate long range Coulombic interactions and the cutoff was set at 8.5 Å. Results of the simulations show that in the two-layer glycolate the content of water is relatively small: up to 0.8 H2O per half of the smectite unit cell (thereafter phuc). Clear thermodynamic preference of mono- or two-layer structure of the complex is observed for typical smectite. Based on the calculated radial distribution functions, it was confirmed that water and ethylene glycol molecules compete for the coordination sites of the calcium ions in the clay interlayers. It was also found that the differences in the smectite layer charge, charge location, and the type of the interlayer cation affect the ethylene glycol and water packing in the interlayer space and as result have strong influence on the basal spacing and on the structure of complex. Varying amounts and ratio of both ethylene glycol and water are, however, the most important factor influencing the extent of the smectite expansion. Comparison of two-layer structure obtained from molecular dynamics simulations with previous models leads to the conclusion that the arrangement of ethylene glycol molecules in the interlayers, used in simulations of X-ray diffractograms of clays, should be modified. In contrast to the Reynolds (1965) model, the main difference is that, for different location of the clay charge, interlayer ions tend to change their positions. In the case of montmorillonite, calcium ions are located in the middle of the interlayer space, while for beidellite they are located much closer to the clay surface. Water in these structures does not form distinct layers but is distributed rather broadly with a tendency to be concentrated close to the smectite surface. One-layer structure of ethylene glycol/water-smectite complex, characteristic of vermiculite was also proposed. References Berendsen, H.J.C., Postma, J.P.M., van Gunsteren, W.F., Hermans, J. (1981) Interaction models for water in relation to protein hydration. In Intermolecular Forces; Pullman, B., Ed.; D. Reidel: Amsterdam, pp 331. Cygan, R. T., Liang, J. J., and Kalinichev, A. G. (2004) Molecular models of hydroxide, oxyhydroxide, and clay phases and the development of a general force field. Journal of Physical Chemistry B, 108, 1255-1266. Jorgensen,W.L., Maxwell, D.S., Tirado-Rives, J. (1996) Development and testing of the OPLS all-atom force field on conformational energetics and properties of organic liquids. J. Am. Chem. Soc., 118, 11225-11236. Lee J.H., Guggenheim S. (1981) Single crystal X-ray refinement of pyrophyllite-1Tc. American Mineralogist, 66, 350-357 Reynolds R. C. (1965) An X-ray study of an ethylene glycol-montmorillonite complex. American Mineralogist, 50, 990-1001 Środoń J., Zeelmaekers E., Derkowski A. (2009) The charge of component layers of illite-smectite in bentonites and the nature of end-member illite. Clays and Clay Minerals, 57, 650-672