Rheological studies of thermotropic phase transitions in cationic vesicle suspensions: Instantaneous “jamming” and aging behavior

Abstract

Cationic double-tailed surfactants exhibit a rich thermotropic phase behavior. Here, we examine the effect of thermal gradients and processing history on the rheology and microstructure of concentrated multilamellar vesicle (MLV) suspensions made from a double-tailed cationic surfactant (diC18:0 DEEDMAC), whose bilayers are in the crystalline (solid) state at room temperature. The rheological properties of the MLV suspensions are found to be strongly dependent on the thermal behavior of the constituent bilayer with the visco-elastic moduli exhibiting a nonmonotonic variation with temperature, initially increasing by 1–2 orders of magnitude at an intermediate temperature, before rapidly decreasing at higher temperatures. Furthermore, when subject to a contraction flow through an extruder, above the main phase transition temperature of the bilayers, the suspensions instantaneously transform into a “jammed,” glassy-state at the extruder outlet. The glassy dispersions behave like stiff gel-like materials, having visco-elastic moduli that are several orders of magnitude higher than their unextruded counterparts. We probe mechanisms leading to the formation and subsequent aging of the jammed suspensions and show that the microstructural evolution of the extruded “gels” on aging is qualitatively different from that of the unextruded suspensions.