ENHANCED SURFACTANT DETERMINATION BY ION-PAIR FORMATION USING FLOW-INJECTION ANALYSIS AND DYNAMIC SURFACE-TENSION DETECTION

Chemical analysis of surface active species (surfactants) is of intere st for many applications, such as in process monitoring, biomedical ap plications, environmental monitoring and surface science investigation s. Recently, we reported a dynamic surface tension detector (DSTD) bas ed upon optically probing the size of a repeating drop resulting from constant flow of an aqueous solvent out of the end of a capillary, Pre sence of a surfactant in a growing drop reduces the surface tension at the air-solvent interface, causing the drop to detach at a smaller vo lume, which is detected. The DSTD has a kinetic dependence, and with i ncreasing flow rate the sensitivity decreases due to diffusional and a dsorption effects. We report that for the sodium salt of dodecylsulfat e (DS), the DSTD performs significantly better with a stainless steel (S.S.) capillary dropper than with a fused silica dropper because the S.S. dropper exhibits a smaller adsorption effect as a function of tim e. Flow-injection analysis with the DSTD of DS was found to enhance se nsitivity 50-fold by in-situ reaction with the ion-pair reagent tetrab utylammonium hydroxide (TBA) in water, even though the TBA alone was n ot very surface active. The TBA-DS system serves as a model for a sele ctive detection method in which surface activity is exploited and enha nced. The detection limit for DS, as TBA-DS, was 400 ppb. Additionally , weakly surface active species such as TEA could be analyzed ''indire ctly'' by ion-pair formation with DS. The enhanced sensitivity is due to increased packing of the ion-pairs at the air-aqueous solvent inter face. The flow rate dependence on the sensitivity of detecting the TBA -DS ion-pair was examined. Two limiting conditions were observed as a function of ion-pair concentration: sensitivity decreases linearly wit h inverse flow rate at high flow rates and approaches a steady stale a t slower flow rates.