Application of an innovative mercury intrusion technique and relative permeability to examine the thin layer pores of sol–gel and CVD post-treated membranes A. Lambropoulos, G. Romanos * , Th. Steriotis, J. Nolan, F. Katsaros, E. Kouvelos, G. Charalambopoulou, N. Kanellopoulos National Center for Scientific Research, ‘‘Demokritos’’, Terma Patriarchou Grigoriou & Neapoleos, Aghia Paraskevi, Attica 15310, Greece Received 1 May 2006; received in revised form 25 August 2006; accepted 26 August 2006 Available online 31 October 2006 Abstract Two types of composite nanofiltration (NF) membranes were examined for their ultra thin separation layer integrity by initially applying gas permeability measurements. The examined materials were sol–gel derived c-alumina and silica membranes, supported on macroporous a-alumina tubes. Relative permeability measurements of the system He/H 2 O indicated the existence of defects with size greater than 25 nm, as Helium permeation was not completely blocked even at high water relative pressures. The existence of defects was also confirmed by application of a recently developed mercury intrusion technique, which additionally enabled the accurate definition of the defects size. Furthermore segments of the defect bearing membranes were modified by chemical vapor deposition of SiO 2 under sev- eral reaction conditions, to examine the possibility of simultaneously reducing the NF pores size and mending the defects. The similarity of water vapor differential permeability results before and after modification constitutes a proof that NF pores remained unaffected under the specific CVD conditions applied. However significant plugging of the defects has been observed with the novel mercury intru- sion technique and a good agreement with the outcome of relative permeability measurements was noted, mainly in the estimation of the largest pores size. SEM micrographs of the CVD treated membranes revealed that differential pressure across the reactor sides leads to the formation of an extra silica deposit on either the thin layer or the support surface of the membrane. This was especially the case in the presence of large pores (defects) and was further verified by the novel mercury intrusion method. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Thin separation layer; CVD; Defects size; Mercury intrusion; Relative permeability 1. Introduction The determination of the largest pore sizes and pore size distribution (PSD) of thin or ultra thin layers of composite membranes is of great importance as these factors control the performance characteristics of the entire membrane. From the methods available for PSD definition, Nitrogen porosimetry (77 K) [1] and Mercury intrusion [2,3] are very insensitive for the examination of sol–gel and CVD post-treated membranes because of the small mass of the separating layer compared to the mass of the (usually mac- roporous) support. Contrast matching adsorption in con- junction with small angle scattering of neutrons (SANS) or X-rays (SAXS) is another recently developed static method for the characterization of porous media [4,5] with pores of size between 1 and 200 nm. Nevertheless, the simultaneous existence of two different oxides with different electron (scattering length) densities and pore sizes, that is usually the case for composite membranes (i.e. SiO 2 , ZrO 2 , TiO 2 , c-Al 2 O 3 on a-Al 2 O 3 ), renders the method compli- cated. Moreover the limited availability of the advanced equipment required makes such methods unattractive for composite membranes characterization. 1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.08.038 * Corresponding author. Tel.: +30 2106503981; fax: +30 2106535294. E-mail address: groman@chem.demokritos.gr (G. Romanos). www.elsevier.com/locate/micromeso Microporous and Mesoporous Materials 99 (2007) 206–215