Physicochemical Properties Of Polymeric Surfactants At Air/Water Interface

In recent years, the static and dynamic behavior of polymeric surfactants at air / water interface is a point of study on "soft matter and soft interface", which is of significance in understanding the physical phenomena of condensed phase in quasi-two-dimensional space. In the paper, for polymeric surfactants, the adsorption kinetics from the solution to the surface and their conformation at the surface were investigated. Also in the opposite process, the desorption kinetics of polymeric surfactants from the surface monolayer to the bulk was studied.As surfactant samples in the experiment, perfluoroclecanoyl modified poly(ethylene oxide)s, perfluorooctanoyl modified poly(vinyl alcohol)s, poly(acrylic acid / butyl acrylate)s, long-chain aliphatic alcoholic / esteric modified poly(ethylene oxide)s were used. The surface tension and surface pressure of the solutions, or monolayers were measured by ether drop-volume (drop-weight) method or ring method. With Word-Tordai adsorption rate equation, a plot of dynamic interfacial pressure ? versus adsorption time t-1/2 should be linear, from which the diffusion coefficients of adsorption may be evaluated. Using Gibbs adsorption equation to calculate interfacial concentrations of the solution, together with theoretical analyses for the conformation entropies, the conformation of the adsorption molecules may be predicted. In the desorption study, for monolayers made by the spreading technique, the interfacial concentration C0 was obtained by means of extrapolating the ? ~t curve to zero, and by using a thermodynamic method with Gibbs adsorption equation. Then the plot of C0~t could be drawn. So, the diffusion coefficients and energy barrier to desorption were calculated.It was found that the diffusion coefficients for adsorption were in the range 10-8~10-5 cm2s-2, on which the largest influent factor was the initial surfactant concentrations in the bulk, and there was a "mixed diffusion kinetic controlled adsorption" in the process. Oppositely for desorption, the diffusion coefficients were found to be the range of 10-5~10-4 cm2s-1, and the higher the initial interfacial concentrations, the larger the diffusion coefficients. The energy barrier of the surface pressure was of the order of 10-20 (J/molecule) magnitude that depended on the surface pressure. The conformation of comb shaped polymeric surfactants at air / water interface appeared mainly to be at an adsorption side of the interface with only one hydrophobic branched chain, and two hydrophilic tail chains in water. For a block copolymer, the conformation of molecules adsorpted at the surface showed principally a situation that several hydrophobic side chains were adsorpted at the surface side by side, with two tail chains in water.