The Confinement-Induced Sponge to Lamellar Phase Transition

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 (Lα) phase upon reaching the transition temperature T L α. When confined, measurement of the interactions between the two confining walls with a surface force apparatus showed that over a limited temperature range below T L α the confined L3 phase transformed to the Lα 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 Lα 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 Lα phase). The force−distance profiles also allowed the interfacial tension of the L3/Lα interface to be evaluated.