Salt effects on the air/solution interfacial properties of PEO-containing copolymers: Equilibrium, adsorption kinetics and surface rheological behavior

• Li+ alters interfacial properties of layers of copolymers with PEO and/or PPO..
• Changes in monolayers behaviour are due to the formation of copolymer – Li+ complexes.
• Typical Hofmeister series effects cannot explain the behaviour observed.
• Contrary to the usual findings, added cations have a strong effect on the system.
• Major importance of Li+ effect on elasticity of Langmuir layers as compared with adsorption layers.

Lithium cations are known to form complexes with the oxygen atoms of poly(oxyethylene) chains. The effect of Li+ on the surface properties of three block-copolymers containing poly(oxyethylene) (PEO) have been studied. Two types of copolymers have been studied, a water soluble one of the pluronic family, PEO–b-PPO–b-PEO, PPO being poly(propyleneoxyde), and two water insoluble ones: PEO–b-PS and PEO–b-PS–b-PEO, PS being polystyrene. In the case of the pluronic the adsorption kinetics, the equilibrium surface tension isotherm and the aqueous/air surface rheology have been measured, while for the two insoluble copolymers only the surface pressure and the surface rheology have been studied. In all the cases two different Li+ concentrations have been used. As in the absence of lithium ions, the adsorption kinetics of pluronic solutions shows two processes, and becomes faster as [Li+] increases. The kinetics is not diffusion controlled. For a given pluronic concentration the equilibrium surface pressure increases with [Li+], and the isotherms show two surface phase transitions, though less marked than for [Li+] = 0. A similar behavior was found for the equilibrium isotherms of PEO–b-PS and PEO–b-PS–b-PEO. The surface elasticity of these two copolymers was found to increase with [Li+] over the whole surface concentration and frequency ranges studied. A smaller effect was found in the case of the pluronic solutions. The results of the pluronic solutions were modeled using a recent theory that takes into account that the molecules can be adsorbed at the surface in two different states. The theory gives a good fit for the adsorption kinetics and a reasonably good prediction of the equilibrium isotherms for low and intermediate concentrations of pluronic. However, the theory is not able to reproduce the isotherm for [Li+] = 0. Only a semi-quantitative prediction of the surface elasticity is obtained for [pluronic] ⩽ 1 × 10−3 mM.

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