Macromolecular conformation changes at oil-water interface in the presence of cations

We probed the interaction forces between di-block co-polymer covered oil-in-water nanoemulsion droplets in the presence of monovalent, divalent and trivalent chloride salts using equilibrium force distance measurements to understand the effect of hydrated ionic environment on the adsorbed macromolecular conformation. Without added cations, the force-distance curve decays exponentially with a characteristic decay length comparable to the polymer radius of gyration (R-g). With 5 mM concentrations of Na+, Ca2+ and Fe3+ cations, the decay lengths were found to be similar to 0.9, 0.8 and 0.6 R-g, respectively. The interdroplet spacing at the minimum measurable repulsive force (approximate to 10(-13) N) in the presence of Na+, Ca2+ and Fe3+ cations at 0.24 mM were 76.6, 70.5 and 54 nm, respectively for PVA-Vac (molecular weight 40000) stabilized emulsion. On increasing the cation concentration and valency, a dramatic decrease in the onset repulsion was observed. The observed change in the force parameters with increasing cation concentrations and valency suggests a collapsed conformation of polymers at O/W interface in the presence of hydrated ionic surface structures due to the strong coulombic attraction between cations and oxygen, leading to bridging of PVA chain segments. However, the hydrated ion environment has not resulted in the total displacement of adsorbed polymers. Similar experimental results were observed with Ni2+, Mn2+ cations and PVA-Vac of higher molecular weight. Finally, the correlation between the onset of repulsion and the hydration thermodynamic properties of cations are compared and the possible mechanisms for cation-polymer interactions are discussed. Our results provide several new insights into ion-macromolecular interactions that may find useful applications in the design of multi-scale biological systems and industrial formulations. (C) 2016 Elsevier B.V. All rights reserved.