posted on 2012-01-17, 00:00authored byAshish
V. Sangwai, Radhakrishna Sureshkumar
A direct estimation of salt-mediated potential of mean
force (PMF)
between spherical micelles of cationic surfactants is obtained for
the first time using molecular dynamics (MD) simulations. Coarse-grained
(CG) potentials benchmarked in an earlier study [Langmuir, 2011, 27(11), 6628–6638] are
used to model a binary system of cetyltrimethylammonium chloride (CTAC)
surfactant micelles at varying concentrations of sodium chloride (NaCl)
or sodium salicylate (NaSal). The shape and structure of micelles
are not subject to external constraints. NaSal is significantly more
efficient in screening the intermicelle repulsive interactions shown
by the PMF compared to NaCl due to a stronger binding of salicylate
counterions to the micelle corona. Upon contact with each other, the
micelles coalesce in the presence of NaSal to form a cylindrical structure
which is stabilized by the adsorbed salicylate anions. Comparison
of the PMF with Derjaguin–Landau–Verwey–Overbeek
(DLVO) potentials shows qualitative agreement, while the magnitude
of PMF is significantly greater than that of the DLVO potentials.
To understand this discrepancy, PMF is evaluated by turning off (a)
long-ranged electrostatic interactions and (b) solvent polarizability.
The above effects are shown to play an important role in determining
the solvent-mediated and ion-correlated interactions between the two
micelles, which are not explicitly captured by mean-field double layer
theories such as DLVO.