Application of an innovative mercury intrusion technique and relative permeability to examine the thin layer pores of sol–gel and CVD post-treated membranes

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 γ-alumina and silica membranes, supported on macroporous α-alumina tubes. Relative permeability measurements of the system He/H2O 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 SiO2 under several 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 intrusion 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.