AbstractAn equation to estimate the thickness of ultrathin membranes obtained by Levich, assuming constant physical properties, was compared with the measured thickness in the range of 0.3–40 μ. The proportionality constant was found to be 0.4, and the equation is expressed as follows:documentclass{article}pagestyle{empty}begin{document} $$ bar delta _S = 0.4left({frac{{{rm rho L}}}{{{rm rho}_{rm s}}}} right){rm omega}_s sqrt {frac{{mu u_0}}{{{rm rho L}g}}} $$ end{document}The nitrogen gas adsorption isotherms at −195°C on the modified ultrathin membranes were measured and the surface area of the pores was determined by the B.E.T. equation. Distribution of pore volume was calculated by the method of Cranston and Inkley. On the assumption of straight cylindrical pores in the membrane, the crosssectional area of pores and the mean pore radius were calculated. Symmetric structure of the modified ultrathin membranes was confirmed by reverse osmosis tests with both the air and glass sides of the membrane facing the pressure sol
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