(694b) Structure of Poy(methacrylic acid) and Poly(acrylic acid) Adsorbed at Oil-Water Interface: A Molecular Dynamics Simulation Study

The structure and interactions of polyelectrolytes adsorbed at oil-water interface determine the stability of emulsion droplets. The molecular level structural details of polyelectrolytes at oil-water interface is important to applications related to emulsion stability and for the design of separation processes based on selectivity of adsorbed films at such interfaces. The dissociation and deprotonation of Poly(acrylic acid) PAA and Poly(methacrylic acid) PMA results in anionic polyelectrolytes that are industrially important. The effect of stereo-chemistry (i.e. tacticity) of the chain on the conformations, orientations and intermolecular structure of PMA adsorbed at CCl₄-water interface was studied by atomistic molecular dynamics (MD) simulation. The effect of the interface concentration on the structure of the adsorbed layer was studied for syndiotactic chains (s-PAA, s-PMA) by all-atom explicit-solvent MD simulations. 30 repeat unit oligomeric polyelectrolyte chains in un-dissociated state (i.e. uncharged) were studied in all systems. PMA adsorbs in a planar conformation at CCl₄-water interface irrespective of its chain tacticity. The radius-of-gyration of adsorbed PMA showed that isotactic chains are more coiled as compared to syndiotactic chains at the interface. The density distribution of different chemical groups of the PMA chain along the direction normal to the interface shows that PMA adsorbs in an ordered conformation in agreement with experimental results based on sum frequency spectroscopy of PMA [1]. The orientation of different types of bonds with respect to the interface-normal was obtained in terms of its orientation distribution curves. Methyl groups (C-CH₃) and carbonyl groups (C=O) show different orientation. The majority of the C-CH₃ bonds orient towards CCl₄ phase whereas the C=O bonds orient towards aqueous phase. The tacticity of PMA affects the number of groups that orient towards each solvent phase and there is evidence of a greater degree of hydrophobicity of isotactic PMA in agreement with experimental results [2]. The hydration of COOH obtained by the calculation of the hydrogen bonding of COOH with water showed that the majority of the COOH groups form H-bonds with water via the involvement of the carbonyl oxygen and the hydroxyl hydrogen. The variation of the concentration of s-PMA and s-PAA up to monolayer coverage showed that both polymers undergo conformational transition with increase in interface concentration. The radius-of-gyration, backbone torsion angle distribution, and the orientation distribution of different groups of the polymer chain indicate a change in the conformational characteristics of polymer chain, in going from a planar-ordered to a non-planar disordered structure with increase in concentration for both polymers. The variation of the properties of adsorbed layer such interface coverage and thickness of adsorbed layer is similar for s-PAA and s-PMA. However, the interface coverage values of s-PAA are greater than that of s-PMA due to its relatively extended conformation at the interface. While MD simulations studies of surfactant layers at liquid interfaces are available in the literature, such studies on polymer films are very limited. Our study provides molecular level structural details of polar polymers adsorbed at oil-water interface which are otherwise difficult to determine from experiments.

References:

1. Robertson, E. J.; Richmond, G. L. J. Phys. Chem. C 2014, 118(49), 28331-28343.

2. Valley, N. A.; Robertson, E. J.; Richmond, G. L. Langmuir 2014, 30(47), 14226-14233.