Steric and barrier eects for a Langmuirian adsorption process at an irregular surface M.M. GoÂmez a, *, J.M. Vara a, 1 , J.C. HernaÂndez a , R.C. Salvarezza b , A.J. Arvia b a Departamento de QuõÂmica-FõÂsica Aplicada, Facultad de Ciencias, Universidad Auto Ânoma, Madrid, Spain b Instituto de Investigaciones FisicoquõÂmicas Teo Âricas y Aplicadas (INIFTA), Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina Received 5 November 1997; received in revised form 22 December 1997 Abstract Two-dimensional (2-D) Monte Carlo simulations for a simple Langmuirian adsorption process on columnar- structured solid substrates (csss) were made to establish the in¯uence of pore width and depth on the adsorption parameters. The geometry of the substrate was adjusted using the cross section of the adsorbable particle (a) as the yardstick. The dierence in the adsorption isotherm parameters from columnar-structured substrates as compared to a smooth substrate (ss) was expressed by a correction factor ( f ) de®ned as the apparent adsorption free energy ratio resulting from csss and ss. The value of f diminishes with the roughness factor (R) of the substrate according to f AR  n . For this model the value of n changes from 1/5 to 1 as the pore mouth diameter approaches the yardstick size. The value of f re¯ects the in¯uence of various eects such as the excluded volume, the barrier at pore entrances, the plugged bottleneck eect and tortuosity at pores, as concluded from the analysis of the Monte Carlo snapshots. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Adsorption modeling; Irregular surfaces; Monte Carlo simulation 1. Introduction The in¯uence of irregularities in the surface topogra- phy on molecular adsorption on solids is sucient to induce adsorption/desorption hysteresis loops of dier- ent shapes [1±6]. The surface heterogeneity at solids may result from various concurrent reasons, namely, the speci®c crystallography of the surface exhibiting sites of a dierent coordination number for a single adsorbate, the existence of either a weak surface dis- order caused by defects such as steps and facets or a strong surface disorder due to the presence of voids, overhangs, crevices and other defects of this type [6]. For an irregular surface, the adsorbate-substrate in- teraction can be constrained to surface domains with an average size exceeding the size of the adsorbate cross section. In this case, adsorption involves a steric eect which depends on the adsorbate cross section usually appearing as an excluded area or volume for molecular adsorption. The excluded volume can be expressed by the dierence between the contour of the real substrate and the contour de®ned by the centers of adsorbed species on the substrate [2]. The excluded volume eect introduces uncertainties in both the evaluation of the true substrate active area by physical adsorption and the interpretation of physico-chemical magnitudes derived from adsorption isotherms. However, for an irregular surface with void or pore average sizes of the order of the adsorbate cross sec- tion, the problem of surface accessibility by adsorbates Electrochimica Acta 44 (1998) 1255±1262 0013-4686/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S0013-4686(98)00229-1 PERGAMON * Corresponding author.