INTERACTIONS BETWEEN CHARGED SPHERICAL MACROIONS

Monte Carlo (MC) simulations were used to study the screened interacti ons between charged spherical macroions surrounded by discrete counter ions, and to test previous theories of screening. The simulations were performed in the primitive cell of the bcc lattice, and in the spheri cal Wigner-Seitz cell that is commonly used in approximate calculation s. We found that the Wigner-Seitz approximation is valid even at high volume fractions phi and large macroion charges Z, because the macroio n charge becomes strongly screened. Pressures calculated from Poisson- Boltzmann theory and local density functional theory deviate from MC v alues as phi and Z increase, but continue to provide upper and lower b ounds for the MC results. While Debye-Huckel (DH) theory fails badly w hen the bare charge is used, MC pressures can be fit with an effective DH charge, Z(DH), that is nearly independent of volume fraction. As Z diverges, Z(DH) saturates at z psi(max)R(m)/lambda, where z is the co unterion charge, R(m) is the macroion radius, lambda is the Bjerrum le ngth, and psi(max) is a constant of order 10. (C) 1996 American Instit ute of Physics.