Double layer capacitance of the polarized nitrobenzene–water interface has been measured in the presence of the adsorption of l-α-dilauroylphosphatidylcholine (DLPC). The double layer capacitance was lowered with the increase of the DLPC concentration in the negative branch of the potential range where the aqueous phase was in negative potential with respect to the nitrobenzene phase. A saturated monolayer of DLPC was formed at a concentration of DLPC above 20 μmol dm−3. The area occupied by a DLPC molecule at the saturated monolayer was 0.73 nm2 which was estimated by comparing the capacitance data with those obtained from a spread monolayer of DLPC at the interface. In the negative branch the monolayer was stable over 100 mV of the applied potential across the monolayer. The adsorption was described by the Frumkin adsorption isotherm. The interaction between the adsorbed DLPC molecule was weak, corresponding to the penetration of nitrobenzene molecules and, possibly, ions between the hydrocarbon chains of DLPC. This penetration also explains the high capacitance value of 11 μF cm−2 for the saturation monolayer. In the potential range where the aqueous phase is positive, the capacitance increased with the DLPC concentration.

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The analysis of lower aliphatic aldehydes in water following derivatization to 2,4-dinitrophenylhydrazones, was investigated by micellar electrokinetic chromatography (MEKC). The effects of the buffer pH and acetonitrile addition on the separation were investigated. Under optimized conditions the 2,4-dinitrophenylhydrazine (DNPH) reagent was completely separated from four DNPH derivatized aldehydes with high separation efficiency. DNPH-formaldehyde and DNPH-acetaldehyde were also completely separated from one another. Their relative standard deviation of migration times and peak areas were within 1.0% and 3.3%, respectively. The reproducibility of peak areas in this MEKC analysis was as good as that of high-performance liquid chromatography (HPLC) analysis with the same derivatization method. The recovery values from tap and river water were in the range of 97-102%. The detection limits of formaldehyde and acetaldehyde were 0.05 mg/L and 0.08 mg/L respectively.

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