Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 27–34 Argon and nitrogen adsorption studies of changes in connectivity of ordered cage-like large mesopores during the hydrothermal treatment Michal Kruk, Jivaldo R. Matos 1 , Mietek Jaroniec ∗ Department of Chemistry, Kent State University, P.O. Box 5190, Kent, OH 44242-0001, USA Available online 24 May 2004 Abstract FDU-1 silicas with large cage-like mesopores were synthesized, using tetraethyl orthosilicate as a silica source and a poly(ethylene oxide)–poly(butylene oxide)–poly(ethylene oxide) triblock copolymer (EO 39 BO 47 EO 39 ) as a structure-directing agent. The evolution of the FDU-1 pore structure during a hydrothermal treatment at 373, 393, and 413 K was studied, using nitrogen and argon adsorption at 77 K. As the time of the hydrothermal treatment was extended, a moderate increase in the pore cage diameter and a significant enlargement of the pore entrances took place, as inferred from the analysis of adsorption and desorption branches of isotherms, respectively. The materials retain a monodisperse distribution of pore cage diameters. On the other hand, argon and nitrogen desorption data provided evidences that there are two major populations of pore entrances according to their diameter and that they both undergo a concomitant enlargement as the hydrothermal treatment is prolonged. Argon desorption was superior in providing information about pore connectivity in FDU-1 samples, except for those obtained after particularly severe hydrothermal treatments (long time or high temperature). The latter materials had a fraction of pores, which were so large that the capillary condensation of argon at 77K in them was not observed, and consequently, the information regarding the connectivity and volume of this fraction of pores cannot be extracted from the argon data. In the case of relatively large mesopores (above ∼15–20 nm), nitrogen that exhibits capillary condensation at 77 K in much wider range of pore sizes (up to 100–200 nm) is better as far as the pore connectivity assessment from desorption data is concerned. © 2004 Elsevier B.V. All rights reserved. Keywords: Nitrogen adsorption; Pore entrance size; Pore size distribution; Mesoporous silica; Ordered mesopores; Hydrothermal treatment 1. Introduction Ordered materials with cage-like mesopore structures (OMCMSs) have recently attracted much attention [1–9]. One of crucial aspects in the characterization of OMCMSs is the elucidation of their pore entrance sizes [2–6,8], be- cause the latter structural property has a strong influence on transport, molecular sieving, and templating properties and suitability as hosts or encapsulation media. The first successful elucidation of the OMCMS pore entrance size was achieved through electron crystallography [2], which allows one to solve the structure of three-dimensionally (3D) ordered mesoporous material, thus providing informa- tion about the pore shape, size, and connectivity. However, ∗ Corresponding author. Tel.: +1-330-672-3790; fax: +1-330-672-3816. E-mail address: jaroniec@kent.edu (M. Jaroniec). 1 Permanent address: Instituto de Qu´ ımica da Universidade de São Paulo, CP 26.077, 05599-970 São Paulo, SP, Brazil. the electron crystallography is currently applicable only for 3D ordered materials with sufficiently large sizes of ordered domains [7] and it requires extensive high resolution trans- mission electron microscopy imaging from different direc- tions. Consequently, the powerful electron crystallography method is not suitable for the analysis of weakly ordered materials and not convenient for routine pore entrance size determinations. Another method for the assessment of the pore entrance size is based on the monitoring of pore ac- cessibility after the modification of the pore surface with ligands of different sizes [3]. This approach is primarily suitable for the characterization of silicas and organosilicas, both ordered and disordered, which can readily be mod- ified using organochlorosilanes, and it is expected to be suitable for the evaluation of the pore entrance diameters in 1–5 nm range. The results of surface modifications for one of the siliceous OMCMSs indicated that surface-modified OMCMSs may exhibit molecular sieving properties related to their pore entrance diameters less than 1 nm [3]. Pore entrances of sizes in the subnanometer range can also be 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.04.009