Abstract
From the pH dependence of oscillation frequencies of one-electrode-separated piezoelectric quartz crystals (ESPQC’s) modified with chitosan and N-(1,3-dicarboxypropyl) chitosan (DCPC) in buffer solutions, it was deduced that the chitosan and DCPC films on an ESPQC contained water and that the water content depended on pH. Contrary to the expectation based on the Sauerbrey equation, the binding of ionic surfactants to chitosan and DCPC films increased the frequencies. This apparently abnormal phenomenon was explained by the release of water in swollen films. The frequency of the DCPC-modified ESPQC increased on addition of sodium dodecyl sulfate and hexadecyltrimethylammonium chloride at pH lower and higher, respectively, than 7.8. A linear relationship between surfactant concentration and frequency shift, whose slope depended on only the chain length of the surfactant alkyl group, was obtained in the range 5–50 µmol dm−3 hexadecyltrimethylammonium chloride. The DCPC-modified ESPQC can be used for a quantitative determination of cationic surfactants in this concentration range.
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