The Adsorption of C 4 Unsaturated Hydrocarbons on Highly Dehydrated Silica. An IR-Spectroscopic and Thermodynamic Study Giuliana Magnacca* and Claudio Morterra Department of Chemistry IFM, University of Turin, and Consortium INSTM, Research Unit of Turin University, Via P. Giuria 7, 10125 Torino, Italy Received October 25, 2004. In Final Form: February 1, 2005 The adsorptive interaction of 1-butyne and 1-butene with a highly dehydrated pyrogenic silica system has been studied to understand the thermodynamic behavior of the adsorption process by the application of the Langmuir model and of the Van’t Hoff equation. In situ FTIR spectroscopy allowed the characterization of the adsorption phenomenon in terms of involved surface sites, gas-volumetric determinations yielded quantitative information relative to the adsorption process, and microcalorimetric results allowed the comparison between calculated and experimental data. Keq and ΔadsG° were obtained from Langmuir’s model application; ΔadsH data were obtained from the Van’t Hoff equation and by the isosteric heats method and were compared with experimental values. The virtual constancy of ΔadsH with equilibrium pressure and surface coverage (Langmuir model) allowed us to obtain the ΔadsH° values and, consequently, the ΔadsS° values for the systems of interest. Introduction It is known that adsorption phenomena are hard to interpret in terms of thermodynamic behavior. One of the few ways through which it is possible to obtain thermo- dynamic information on an adsorption process is to examine it in terms of a simple mechanism. The most convenient of such mechanisms is that described by Langmuir’s theory, but very seldom do the experimental data obtained with real systems fit in a sufficiently wide range of experimental conditions that the oversimplified Langmuir model actually predicts. In view of its convenience, Langmuir’s model has been applied in various fields. Most often the studies concern the kinetics of adsorption of complex molecules (sucrose, amino acids) or metal ions on solids from a liquid phase (e.g., see ref 1), because the average homogeneity of the solid/liquid interface guarantees a better applicability of the model; sometimes reasonable results have been obtained also with gas-solid adsorption (e.g., see ref 2). The Langmuir model for nondissociative adsorption 3 concerns the one-to-one interaction between a gas molecule (A) and a surface adsorbing site (M) to form the surface complex (A-M), according to the equilibrium reported in the following scheme: where k a is the rate constant for adsorption and k d is that for desorption. Other conditions imposed by the Langmuir model are: (i) the adsorption process is reversible and cannot proceed beyond the monolayer coverage; (ii) the surface is uniform and all sites are equivalent (i.e., there is no intrinsic heterogeneity); and (iii) adsorbed molecules do not interact with one another (i.e., there is no induced heterogeneity). The first form of the Langmuir isotherm (i.e., the form first proposed by Langmuir) is: where p is the equilibrium gas pressure, θ is the surface coverage, and b (the so-called Langmuir constant) is b ) k a /k d ) K eq , that is, the thermodynamic equilibrium constant. On rearranging the first θ versus p equation, the second form of the Langmuir isotherm is obtained: which is the form currently used to confirm the ap- plicability of Langmuir’s model to the experimental data. Although the conditions imposed by the starting Lang- muir hypotheses are quite strict and not very realistic on a physicochemical ground, some adsorbent/adsorbate systems have been reported that do agree with the Langmuir equations. In particular, as was recently demonstrated by Garrone et al., 4,5 the adsorption of several alkenes and alkynes on highly dehydrated nonporous silica is actually describable in terms of Langmuir’s equation because: (i) uptake is largely limited to a monolayer (the interaction implies a specific and reversible H-bonding to surface OH groups); (ii) all adsorption sites are of the same type (only isolated surface silanols are present); and * Corresponding author. Phone: +39 011 670 7543. Fax: +39 011 670 7855. E-mail: giuliana.magnacca@unito.it. (1) Singh, K.; Mohan, S. Appl. Surf. Sci. 2004, 221, 308. Titus, E.; Kalkar, A. K.; Gaikar, V. G. Colloids Surf., A 2003, 223, 55. Sekar, M.; Sakthi, V.; Rengaraj, S. J. Colloid Interface Sci. 2004, 279, 307. (2) Pakseresht, S.; Kazemeini, M.; Akbarnejad, M. M. Sep. Purif. Technol. 2002, 28, 53. Choudhary, V. R.; Mayadevi, S. Zeolites 1996, 17, 501. (3) Atkins, P. W. Physical Chemistry, V ed.; Oxford University Press: Oxford, 1994; p 987. (4) Garrone, E.; Barbaglia, A.; Onida, B.; Civalleri, B.; Ugliengo, P. Phys. Chem. Chem. Phys. 1999, 1, 4649. (5) Onida, B.; Allian, M.; Borello, E.; Ugliengo, P.; Garrone, E. Langmuir 1997, 13, 5107. A gas + M surf y \ z k a k d [A-M] surf θ ) bp 1 + bp (1) θ 1 - θ ) K eq p (2) 3933 Langmuir 2005, 21, 3933-3939 10.1021/la0473761 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/30/2005