In binary or ternary surfactant systems, if a complete segregation at an interface between the hydrophilic and lipophilic domains is assumed, the microstructure is related to the interfacial geometry. This is determined by two factors: the local interfacial curvatures set by the balance of molecular forces at the interface, and the interfacial topology and degree of connectivity that is imposed by the need to satisfy global packing constraints. The local constraint is characterized by the packing parameter of the surfactant, upsilon/al. For lipids and membrane mimetic systems this is close to unity. Several microemulsions and liquid-crystalline phases formed from surfactants with surfactant parameter close to unity are shown to exhibit peculiar structural transitions. This is demonstrated by NMR and conductivity experiments. This feature, upsilon/al = 1, shared by lipids, seems to allow quite diverse flexibility in microstructure. This is despite the very different molecular structures of the surfactants studied (DDAB, AOT, MO, PFPE). Microemulsions and liquid-crystalline phases exhibit drastic structural changes on addition of a very small amount of a further new component, or with minimal variation in the composition of the system. Other properties exhibited by such systems are shared and quite general: on increasing the volume fraction of the hydrophobic domain in binary surfactant/water system, or upon water and oil dilution, for microemulsions, an evolution of microstructure from a continuous water network towards closed water domains is observed.
On microstructural transitions of lamellar phase forming surfactants
CABOI, FRANCESCA;MONDUZZI, MAURA
1998-01-01
Abstract
In binary or ternary surfactant systems, if a complete segregation at an interface between the hydrophilic and lipophilic domains is assumed, the microstructure is related to the interfacial geometry. This is determined by two factors: the local interfacial curvatures set by the balance of molecular forces at the interface, and the interfacial topology and degree of connectivity that is imposed by the need to satisfy global packing constraints. The local constraint is characterized by the packing parameter of the surfactant, upsilon/al. For lipids and membrane mimetic systems this is close to unity. Several microemulsions and liquid-crystalline phases formed from surfactants with surfactant parameter close to unity are shown to exhibit peculiar structural transitions. This is demonstrated by NMR and conductivity experiments. This feature, upsilon/al = 1, shared by lipids, seems to allow quite diverse flexibility in microstructure. This is despite the very different molecular structures of the surfactants studied (DDAB, AOT, MO, PFPE). Microemulsions and liquid-crystalline phases exhibit drastic structural changes on addition of a very small amount of a further new component, or with minimal variation in the composition of the system. Other properties exhibited by such systems are shared and quite general: on increasing the volume fraction of the hydrophobic domain in binary surfactant/water system, or upon water and oil dilution, for microemulsions, an evolution of microstructure from a continuous water network towards closed water domains is observed.
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