The interactions between two macroscopic walls immersed in an isotropic symmetric sponge phase (L3) at different volume fractions, Φ, were studied with a surface force apparatus. At temperatures well below the lamellar (Lα)/L3 bulk transition, the force-distance profile is weakly oscillatory with a period that is twice the correlation length (ξ) of the L3 phase measured from SAXS. The oscillations are superimposed on an exponentially attractive background with an order correlation length of 2-4 cell sizes of the sponge structure. When the temperature is raised, a first-order phase transition to a lamellar structure can be induced for separations below a threshold. Another oscillatory interaction results which has a period that is twice the reticular distance for an Lα phase of similar Φ. In addition, the thickness of the induced lamellar film increases (reversibly) with temperature. The compressibility modulus extracted from the oscillatory interaction is consistent with that expected for lamellae stabilized by undulation forces
The hypernetted chain (HNC) theory is used to compare charge titration and zeta potential data for several colloids. It is found that the HNC diffuse layer potential is closer to the zeta potential than that calculated with the traditional Poisson−Boltzmann (PB) approximation, and hence the amount of counterion binding required to bring the two into conformity is reduced. Further improvement is obtained by using an effective PB diffuse layer potential, calculated from both the numerical HNC theory and the analytic extended PB approximation. The rationale for using the effective PB potential is that the conversion from the measured electrophoretic mobility to the zeta potential invokes the PB approximation. In the case of silver iodide quantitative agreement is obtained without any fitting parameters, which confirms that no counterion binding occurs. For the metal oxides analyzed (titanium dioxide, aluminum oxide, and silicon dioxide) the two measurements can only be reconciled by postulating counterion binding or surface charge mobility.
The behavior of an isotropic fluid near its first-order phase transition to an ordered phase is investigated when confined between two macroscopic walls. The sponge (L3) phase of the system sodium bis(2-ethylhexyl)-sulfosuccinate/brine was used as the isotropic phase since it displays a bulk first-order transformation to the lamellar (LR) phase upon reaching the transition temperature TLR. When confined, measurement of the interactions between the two confining walls with a surface force apparatus showed that over a limited temperature range below TLR the confined L3 phase transformed to the LR phase at a specific wall separation. This confinement-induced transformation was signaled by the existence of two distinct oscillatory force regimes. Structural characteristics of either the L3 or the LR phase were observed in the oscillatory profiles including the characteristic length or reticular spacing, the nature and topology of structural defects, the order parameter correlation length (for the L3 phase), and the elastic compressibility modulus (for the LR phase). The force-distance profiles also allowed the interfacial tension of the L3/LR interface to be evaluated.
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