Adsorption of poly(ethylene oxide) at the air/water interface: A dynamic and static surface tension study

The adsorption of polyethylene oxide (PEO) homologues in a wide range of molecular weight (from MPEO=200MPEO=200 to 106) at the air/aqueous solution interface was investigated by dynamic and static surface tension measurements. An approximate estimate for the lower limit of PEO concentration was given at which reliable equilibrium surface tension can be determined from static surface tension measurements. It was shown that the observed jump in the earlier published σ–lg(cPEO)σ–lg(cPEO) curves is attributable to the nonequilibrium surface tension values at low PEO concentrations. The adsorption behavior of short chain PEO molecules (MPEO⩽1000MPEO⩽1000) is similar to that of the ordinary surfactants. The estimated standard free energy of PEO adsorption, ΔG0ΔG0, increases linearly with the PEO molecular weight until MPEO=1000MPEO=1000. In this molecular weight range, ΔG0ΔG0 was found to be approximately the fifth of the hydrophobic driving force related to the adsorption of a surfactant with the same number of methylene groups. In the case of the longer chain PEOs the driving force of adsorption is so high that the adsorption isotherm is near saturation in the experimentally available polymer concentration range. Above a critical molecular weight the PEO adsorption reveals universal features, e.g., the surface tension and the surface density of segments do not depend on the polymer molecular weight.

The observed upturn in the earlier published σ–lg(cPEO)σ–lg(cPEO) curves is shown to be attributable to the nonequilibrium surface tension values at low PEO concentrations.Figure optionsDownload as PowerPoint slide