MACROION-MACROION CORRELATIONS IN THE PRESENCE OF DIVALENT COUNTERIONS. EFFECTS OF A SIMPLE ELECTROLYTE

B. Hribar and V. Vlachy
Faculty of Chemistry and Chemical Technology, University of Ljubljana,
Aškerčeva 5, 1000 Ljubljana, Slovenia.

ABSTRACT

The Monte Carlo and integral equation results are presented for a primitive
model of a micellar solution. The macroions and small ions are modeled as
charged hard spheres embedded in a continuous dielectric. The ratio of di­
ameters of the macroions and small ions (counterions and co­ions) is 3.0 nm
: 0.4 nm. The macroions are charged with 20 negative charges, the co­ions
have one negative charge, and the counterions may carry one or two positive
charges. In a previous study of solutions containing macroions and counterions
(B. Hribar and V. Vlachy, J. Phys. Chem. B, 1997, 101, 3457­3459) it was
found that in solutions with monovalent counterions (­20:+1) the macroions
were distributed at large distances from each other, while in solutions with
divalent counterions (­20:+2) the macroions shared the layer of counterions.
In the present paper the effects of addition of a simple electrolyte to these
solutions are explored. For systems with monovalent counterions, addition
of a low­molecular weight electrolyte causes the macroions to approach each
other; the first peak of the pair­distribution function is shifted toward smaller
distances. On the other hand, the solutions with divalent counterions seem to
be less affected by addition of a simple electrolyte. The results for the excess
internal energy, and osmotic coefficient for two different macroion concentra­
tions are presented in addition to the structural information.