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ORIGINAL PAPERFirst-principles study of ammonium ions and their hydrationin montmorillonitesJing Shi & Houbin Liu & Yingfeng Meng &Zhaoyang Lou & Qun Zeng & Mingli YangReceived: 21 November 2012 /Accepted: 21 December 2012 /Published online: 17 January 2013#Springer-Verlag Berlin Heidelberg 2013Abstract Density functional theory calculations were per-formed to investigate the adsorption and hydration of anammonium ion (NH4+) confined in the interlayer space ofmontmorillonites (MMT). NH4+is trapped in the six-oxygen-ring on the internal surface and forms a strongbinding with the surface O atoms. The hydration of NH4+is affected significantly by the surface. Water moleculesprefer the surface sites, and do not bind with the NH4+unless enough water molecules are supplied. Moreover,the water molecules involved in NH4+hydration tend tobind with the surface simultaneously. The hydration energyincreases with the intercalated water molecules, in contrastto that in gas phase. In addition, the hydration leads to theextension of MMT basal spacing.Keywords Adsorption.Ammoniumions.Densityfunctionaltheory.Hydration.MontmorilloniteIntroductionMontmorillonites (MMT) are characterized by their laminarstructures consisting of alternative octahedral aluminas andtetrahedral silicas. In a typical MMT structure, isomorphicsubstitutions of low-valence cations in the silica or thealumina sheet leave extra negative charge in the layer. Inorder to balance the negative charge, counterions includingNa+, K+, NH4+, Mg2+, Ca2+, Fe3+, etc. intercalate into theinterlayer space, resulting in various specific properties likecation exchange, swelling, nano-sheets, etc. [1–10]. Thestudy of NH4+intercalation has particular importance inpetroleum engineering, environment protection, and materi-al research. Ammonium ions are one of the main compo-nents of drilling fluid, effective in filtration reduction,swelling inhibition, and well-bore stabilization [11–13].Ammonium ions also exist in waste water, depositing be-neath stratum together with other wastes [11, 14–16]. SinceMMT layers are about 1 nm in width, they are naturaladditives in the preparation of nano-composites [17–23].Ammonium ions and their hydration are often used to splitMMT into separate sheets in nano size. In all these process-es, the NH4+-MMT interaction plays a key role and hasattracted much interest in past years. X-ray diffraction(XRD), infrared (IR) spectrum, and scanning electron mi-croscope (SEM) techniques have been employed to investi-gate the structures of ammonium-clay systems, confirmingthat NH4+and its hydrates are confined in the interlayerspace and interact with the internal surface [24–30]. To ourknowledge, however, neither the adsorption nor the hydra-tion of NH4+on the internal surface of clays has beentheoretically studied although first-principles calculationshave been proven an effective way in the study of molecularadsorption on surfaces [31–35]. In this work, we study thestructures of NH4+and its hydrates inside MMT, as well astheir interaction with the atoms on the internal surfaces bymeans of density functional theory (DFT) calculations.Our calculations aim to reveal how an NH4+ion adsorbson MMT surface, how water molecules affect the NH4+adsorption, and how the adsorption affects MMT struc-tures, all of which are interesting for understanding am-moniumions’diffusion,deposition,transportation,andactivityinside MMT.J. Shi:H. Liu:Y. Meng (*)State Key Laboratory of Oil and Gas Reservoir Geologyand Exploration, School of Petroleum Engineering,Southwest Petroleum University, Chengdu 610500, Chinae-mail: cwctmyf@swpu.edu.cnZ. Lou:Q. Zeng:M. Yang (*)Institute of Atomic and Molecular Physics, Sichuan University,Chengdu 610065, Chinae-mail: myang@scu.edu.cnJ Mol Model (2013) 19:1875–1881DOI 10.1007/s00894-012-1748-x