title: Salt effects on the air/solution interfacial properties of PEO-containing copolymers: equilibrium, adsorption kinetics and surface rheological behavior.
abstract: 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.
authors.affiliation: Departamento de Química Física I, Facultad de Química, Universidad Complutense de Madrid, 28040 Madrid, Spain.
authors.forename: Sara
authors.initials: S
authors.lastname: Llamas
authors.forename: Alma J
authors.initials: AJ
authors.lastname: Mendoza
authors.forename: Eduardo
authors.initials: E
authors.lastname: Guzmán
authors.forename: Francisco
authors.initials: F
authors.lastname: Ortega
authors.forename: Ramón G
authors.initials: RG
authors.lastname: Rubio
mesh_terms: D000327:Adsorption; D008094:Lithium / Q000737:chemistry*; D011092:Polyethylene Glycols / Q000737:chemistry*; D011137:Polystyrenes / Q000737:chemistry*; D011409:Propylene Glycols / Q000737:chemistry*; D012212:Rheology; D012492:Salts / Q000737:chemistry*; D013500:Surface Tension
chemical_list: C116176:PEO-PPO-PEO; D011137:Polystyrenes; D011409:Propylene Glycols; D012492:Salts; C541965:polystyrene-b-poly(ethylene oxide); D011092:Polyethylene Glycols; D008094:Lithium