Gas permeability of Langmuir-Blodgett (LB) films: characterisation and application

The present work deals with the quantitative mass transport of gases (N-2, He, O-2, CO2, H-2, CH4 and CO) through Langmuir-Blodgett (LB) systems depos ited onto thin pore-free polydimethylsiloxane (PDMS) membranes as support, forming a composite system. For gas permeation experiments the number of LB layers of this composite system was systematically varied. The polyglutama te LB films OM30 and OM10 yield a perfect permeation behaviour. This can be proved by applying transport equations, properly taking into account the s tate and number of layers necessary for flux limitation by the deposited fi lms. In this case permeation coefficients and selectivities can be calculat ed. The remarkable high gas selectivities of OM10/OM30 films (He/N-2, 22.7/ 8.3; O-2/N-2, 4.8/3.0; CO2/N-2, 43.5/27.8; H-2/N-2, 23.5/12.3; CH4/N-2, 1.8 /2.6; CO/N-2, 1.3/1.6) are a consequence of the homogeneous and pore-free s tate of both polyglutamate films. The difference in permeation coefficient and selectivity of OM30 and OM10 is related to their difference in film den sity which is indicated by different film thicknesses as measured by ellips ometry. In contrast to polyglutamate layers, Cd-arachidate LB films perform as porous systems with a complete lack of selectivity. The microscopic ima ging technique PSIM (phase-shift-interference microscopy) indicates a 3D-cr ystallisation (Ostwald-ripening) for Cd-arachidate LB films which are sever al days old. For the case of polyglutamate layers the reported gas selectiv ities are very promising, especially from a technological point of view. (C ) 2000 Elsevier Science B.V. All rights reserved.