Dynamic surface tension detection: Novel applications to continuous flow analysis and interfacial analysis

Surface-active agents are important in many industrial and biological processes, and in the formulation of many products. This work involves the evaluation, and application of a dynamic surface tension detector (DSTD) that rapidly measures the interfacial properties of surface-active agents in real time. The DSTD is a continuous-flow analyzer that measures the differential pressure, as a function of time, of repeatedly growing and detaching drops. The drop pressure data, related to the dynamic surface tension, is converted in real time into dynamic surface pressure of the sample (a measure of surface activity). Continuous surface tension measurement is achieved for each eluting drop, providing insight into the kinetic behavior of molecular diffusion and orientation processes at the air/liquid interface. Dynamic surface pressure data are obtained for each drop and plotted vs. elution time, resulting in a three-dimensional plot for each sample.; Applications of DSTD to analysis of samples in viscous media, utilization of the kinetic signal for molecular sizing, and rapid analysis of samples blended with gradient mobile phase are presented herein. DSTD was used to examine the diffusion and surface adsorption characteristics of surface-active analytes as a function of solution viscosity. The dynamic surface pressures reported for a surfactant and polymer in viscous solution are correlated with analyte diffusion coefficient via the Stokes-Einstein equation. The DSTD was also applied to the determination of the molar mass of denatured proteins. In this method, the denatured protein is detected by surface-active properties and molar mass selectivity is contained in the dynamic DSTD signal. Additionally, a method is presented that combines DSTD with gradient mobile phase introduction and on-line sample blending, allowing rapid screening of analytes reducing the need for cumbersome batch sample preparation. Finally, development of a dynamic interfacial pressure detector (DIPD), which allows for analysis of analytes dissolved in organic mobile phase at the liquid/liquid interface, is presented and discussed with respect to applications. Thus, it is shown that the multi-dimensional DSTD and DIPD provide information on dynamic surface activity at either an air/liquid interface or liquid/liquid interface. Some future directions of these detection techniques are also discussed.