Thermodynamic features associated with intercalation of some n-alkylmonoamines into barium phosphate Margarete O. Machado, Ange´lica M. Lazarin, Claudio Airoldi * Instituto de Quı´mica, Universidade Estadual de Campinas, Caixa Postal 6154, 13084-971 Campinas, SP, Brazil Received 18 February 2005; received in revised form 7 April 2005; accepted 8 April 2005 Available online 9 June 2005 Abstract Elemental analysis for the synthesized crystalline lamellar compound conforms to the formula Ba(H 2 PO 4 ) 2 and the X-ray diffrac- tion patterns is in agreement with the lamellar structure for this compound. The precursor host was intercalated with a series of n- alkylmonoamines of the general formula H 3 C(CH 2 ) n -NH 2 (n = 1 to 4) in aqueous solution. The lamellar host was calorimetrically titrated with an aqueous amine solution at T = (298.15 ± 0.02) K and the enthalpy, Gibbs free energy and entropy were calculated. The enthalpic values increased, although not uniformly, with the number of carbon atoms is the amine chain, to give (13.96 ± 0.12, 14.00 ± 0.48, 15.75 ± 0.23, 16.05 ± 0.11) kJ mol 1 , from n = 1 to 4. The exothermic enthalpy, the negative Gibbs free energy and positive entropic values are in agreement with the favourable energetic process of intercalation for this system. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Barium phosphate; Intercalation; n-Alkylmonoamines; Calorimetrically titrated; Enthalpy 1. Introduction Intercalation means insertion of a guest molecule or charged species into a suitable crystalline structure with- out major rearrangement of the solid host structure [1]. Intercalation requires that the host structure is formed through a strong covalent network of atoms that re- mains unchanged during the intercalation reaction, which occurs due to available vacant sites in the struc- ture. Layered crystal structures satisfy very well these requirements and also are able to accommodate large guest molecules in the interlayer space by the free adjust- ment of interlayer separations [1–3]. Intercalation provides new routes for the synthesis of materials with controlled changes in their chemical and physical properties, which can be tuned by the proper choice of the (host + guest) combination, by the guest concentration and by co-intercalation of fur- ther guest species. This process permits creating a large variety of structures for a wide range of practical uses, such as adsorbents, catalysts, pharmaceutical products, chemical sensors, ionic conductors and var- ious kinds of electrochemical and optoelectronic de- vices [1–6]. Intercalates obey the rules of (host + guest) comple- mentary like all other supramolecular systems [7]. This behaviour, in the case of intercalated layered structures, can be characterized by a series of chemical and geomet- rical factors, describing the character and arrangement of the active sites in the host layers, guest species and geometry of guest molecules and host layers [8,9]. In the present publication, energetic studies evaluate the reaction of n-alkylmonoamines of the general for- mula CH 3 (CH 2 ) n NH 2 (n = 1 to 4) with the lamellar crys- talline structure of barium phosphate. These divalent metal phosphates are normally synthesized as water insoluble crystalline compounds of the general formula M(H 2 PO 4 ) 2 (M = Mg, Ca, Sr, Ba, etc.), a class of com- pounds that have a lamellar structure, enabling them 0021-9614/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2005.04.008 * Corresponding author. E-mail address: airoldi@iqm.unicamp.br (C. Airoldi). www.elsevier.com/locate/jct J. Chem. Thermodynamics 38 (2006) 130–135