Osmotic pressure and salt exclusion in electrostatically swollen lamellar phases

Abstract

Measurements of osmotic pressure, mainly of electrostatic origin, are reported in the diluted regime of charged bilayers. The interlamellar distances (100–1000 Å) are measured using small-angle neutron scattering. The electrostatic repulsive pressure is evaluated within the framework of the Poisson–Boltzmann model, taking into account the conditions of ionic conservation and electrochemical equilibrium, and checked by Monte Carlo simulations in the grand canonical ensemble. The predicted phenomenon of salt ejection from the bilayers is evidenced by direct chemical analysis. Implications concerning the phase behavior of double-tailed surfactants are discussed.

Coexistences of lamellar phases in ternary surfactant solutions

“…2(a) resembles that obtained for DDAB/water binary mixtures [4][5][6][7] if we assume that the interaction parameter χ is inversely proportional to the temperature. In these experiments, the existence of a critical point and associated critical phenomena were experimentally evidenced by various scattering methods [5].…”

“…The transition from the bound lamellar phase to the unbound phase is generally called the "unbinding transition" [3]. Although it is very rare, the coexistence of two lamellar phases in thermodynamic equilibrium has been also reported for binary surfactant/water solutions typically containing DDAB (didodecyldimethylammonium bromide) [4][5][6][7]. In this case, a higher-density condensed lamellar phase is in equilibrium with a lower-density swollen lamellar phase.…”

Nonlinear Poisson-Boltzmann theory of a Wigner-Seitz model for swollen clays

“…[15] that, at least within LPB theory, the results are very insensitive to the precise shape of the WS cell. Compared to previous studies of stacked clay platelets, assumed to be infinite planes [5], the present study takes into account edge (or finite platelet size) effects.…”

“…In this case the chemical potential of co and counterions must be identical in the suspension and in the reservoir, leading to the condition [5] …”

“…The non-linear PB equation for the local electrostatic potential may be solved analytically for a single planar double-layer, and two interacting double-layers in the salt-free case (i.e. in the absence of coions) [1,2,4]; in the presence of salt, the one-dimensional problem of interacting charged planes is easily solved numerically [5]. However, PB theory becomes increasingly difficult to handle for more complicated geometries, and must generally be linearised to become tractable.…”