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Effect of Membrane Pore Size on the Measurement of Water Tension in Bentonite Suspensions¹

ABSTRACT

A pressure-transducer tensiometer and a dynamic osmometer were used to measure water tensions and osmotic pressures, respectively, in clay-water systems. The two methods of measurement produced the same results. Different membranes, having pore sizes ranging from 0.004µ to 0.8µ were used with the tensiometer. It was found that the measured water tension of a given system depended on the pore size of the membrane; as the pore size increased, the tension decreased. However, experience indicates that little or no turbidity appears in a solution when it is separated from a clay suspension by a membrane having pores as small as those of the present experiments. Therefore, the relationship between pore size and tension was not due to differences in membrane permeability to the clay particles. Nor was it due to differences in membrane permeability to dissolved solutes. Tests showed that no lasting tension developed when a NaCl solution or an extract of the clay were utilized in place of the clay suspensions. Two possible explanations were proposed, namely: (i) entrance of particles into the pores was all that was required to lower the reflection coefficient, and (ii) the gelation of the clay suspensions within the pores produced gels having particle densities and arrangements and, hence, permeabilities to dissolved solutes, that were dependent on the dimensions of these pores. It was pointed out that, regardless of the explanation, the existence of such a relationship between membrane pore size and measured water tension is noteworthy.

¹ Journal Paper no. 2913, Purdue University Agr. Exp. Sta., Lafayette, Ind. Contribution from the Department of Agronomy. Presented before Div. S-1, Soil Science Society of America, Aug. 22, 1966. This research was supported by the Div. of Agri. Develop., Tennessee Valley Authority and by the National Aeronautics & Space Administration.

² Formerly Post-Doctoral Research Associates and Professor of Soil Chemistry, respectively. The first-named author is presently at the Department of Soil Science, Hebrew University, Rehovot, Israel. The second-named author is presently at the Department of Soil Science, University of Sydney, Sydney, Australia.

Received for publication September 30, 1966. Accepted for publication November 15, 1967.