Estimation of the film thickness from the cohesive pressure at the air water interface |
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Authors: | Dr A K Chatterjee Dr D K Chattoraj |
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Institution: | (1) Chemistry Department, Jadavpur University, Calcutta 32, India;(2) Present address: Chemistry Department, Clarkson College of Technology, Potsdam, New York, USA;(3) Present address: Department of Biochemistry, University of Iowa, Iowa City, Iowa, USA |
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Abstract: | Summary The pressure-area curves for neutral monolayers of butyric, valeric, caproic and caprylic acids at the airwater interface
were obtained from the surface tension concentration data. Similar curves for the charged monolayers of sodium octanoate and
sodium lauryl sulphate in the presence and absence of excess neutral salts were constructed on the basis of the results on
surface tension. From the comparison of the pressure area curves of each of these monolayers at air-water and oil-water interfaces
under identical conditions, the cohesive pressures of the monolayer at the air-water interface were calculated for different
areas per adsorbed molecule using the equation ofDavies. The cohesive pressure was observed to vary linearly with the square of the reciprocal area per adsorbed molecule or ion.
From the slope of such linear plot, magnitude of the two-dimensionalvan der Waals constant in each case was estimated. The thickness of the interfacial film was estimated from the comparison of the values
of two- and three-dimensionalvan der Waals constants. This is, however, found to be considerably greater than the length of the stretched molecules adsorbed at the
interface. When correction for entropy, as suggested byGershfeld is introduced, the film thickness becomes identical with the length of the molecule.
Based in part upon the thesis submitted byA. K. Chatterjee for the Ph. D. degree of the Jadavpur University (1966). |
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