A Theoretical and Experimental Analysis of the Capillary Rise Method for Measuring the Surface Tension of Solutions of Electrolytes

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Abstract
Jones and Ray made measurements of the capillary rise of solutions of electrolytes which they interpreted as showing that there is a minimum in the surface tension‐concentration curve at low concentration (about 0.001N). Langmuir has suggested that the liquid film above the meniscus may be several hundred angstroms thick and may be sufficiently different for water and for solutions of electrolytes to account for the observed capillary rises without drawing the conclusion that there is a minimum in the surface tension. Langmuir's theory is analyzed in detail and one of its premises is shown to be inconsistent with experimental evidence of several kinds which indicates that a solution‐air interface is electrically charged. Langmuir's theory gives the thickness of the wetting film as an explicit function of the concentration and temperature of the solution; and of the capillary rise (which depends on the radius of the tube and on the surface tension of the liquid); and of the zeta‐potential. But unfortunately zeta‐potentials are not known with sufficient precision to permit reliable calculations of the thickness of the wetting film. Some of the consequences of the theory are tested experimentally.