Molecular interactions and binding are one of the most important and fundamental properties in the study of biochemical and biomedical systems. The understanding of such interactions and binding among biomolecules forms the basis for the design and processing of many biotechnological applications, such as bioseparation and immunoadsorption. In this study, we present a novel method to probe molecular interactions and binding based on surface tension measurement. This method complements conventional techniques, which are largely based on optical, spectroscopic, fluorescence polarization, chromatographic or atomic force microscopy measurements, by being definite in determining molecular binding ratio and flexible in sample preparation. Both dynamic and equilibrium (or quasi-equilibrium) information on molecular binding can be obtained through dynamic and equilibrium surface tension measurements. For an important pair of biological ligand and ligate, Protein A and immunoglobulin G (IgG), the existence of molecular interactions and the binding ratio of 1:2 have been determined unequivocally with the proposed surface tension method. These results are confirmed/supported by a mass balance calculation and spectrophotometry experiment. In addition, adsorption isotherms for Protein A and IgG separately at the air/water interface have been established with the dynamic surface tension measurements. The results show that the Langmuir isotherm equation can describe the adsorption data satisfactorily for both Protein A and IgG solutions.

Time-resolved fluorescence decays of three asymmetrically substituted phenylethynyl molecules of different size show a sizable solvent dependence. Nonradiative rate constants estimated from quantum yield and lifetime data are consistent with classical electron-transfer theory. The electronic coupling elements, H RP , derived from fits of theory to the data do not follow the trend of donor-acceptor distance (or molecular size). It is found to be largest for the molecule with a butadiyne linker which is intermediate in size. Our results suggest that the bridge is directly involved in excited-state charge recombination in these conjugated molecules.

The surface tension and adsorption kinetics of aqueous solutions of slightly volatile, organic amphiphiles are influenced by both liquid-and vapor-phase surfactant concentrations. Here we derive a new kinetic transfer equation, based on the classic Langmuir analysis, which can account for adsorption and desorption from both sides of the vapor/liquid interface during surface equilibration. The new transfer equation was tested against dynamic surface tension data from two normal alcohols (1-butanol and 1-hexanol) in aqueous solutions. The experimental data was collected at conditions where the dynamic surface tension is controlled by a combination liquid-and vapor-phase surfactant adsorption. The validity of the transfer equation was assessed based on its ability to model the experimental data accurately and generate suitable values for the kinetic rate constants. The theoretical predictions from the transfer equation fit well with the experimental data for both systems. However, variability was observed in the least-squares estimates of the rate constants. The variability is attributed to the limitations of empirical models that utilize adjustable fitting parameters to optimize the model predictions and the wide range of surfactant concentrations studied. Specific concentration regions were identified where the variability in the rate constants was minimal and, thus, where the model is most appropriate. The new transfer equation can be applied to volatile surfactant systems where the dynamic surface tension is influenced by surfactant adsorption and desorption from both sides of the vapor/liquid interface.

Tension at the surface is a most fundamental physicochemical property of a liquid surface. The concept of surface tension has widespread implications in numerous natural, engineering and biomedical processes. Research to date has been largely focused on the liquid side; little attention has been paid to the vapor—the other side of the surface, despite over 100 years of study. However, the question remains as to whether the vapor plays any role, and to what extent it affects the surface tension of the liquid. Here we show a systematic study of the effect of vapor on the surface tension and in particular, a surprising observation that the vapor, not the liquid, plays a dominant role in determining the surface tension of a range of common volatile organic solutions. This is in stark contrast to results of common surfactants where the concentration in the liquid plays the major role. We further confirmed our results with a modified adsorption isotherm and molecular dynamics simulations, where highly structured, hydrogen bonded networks, and in particular a solute depletion layer just beneath the Gibbs dividing surface, were revealed.