One of the ubiquitous examples of self-assembly is the formation of micelles by surfactants. These micelles can assume a variety of shapes, including as long, flexible cylindrical chains, called wormlike micelles (WLMs). WLMs have attracted great interest from both academia and industry because their solutions exhibit viscoelastic properties. However, most studies on WLMs concern their assembly in water. To our knowledge, there are no known examples of WLMs in polar organic solvents like as glycerol, formamide, ethylene glycol or propylene glycol. In this study, we have devised a strategy to induce WLMs of a long-tailed cationic surfactant in several of these solvents. To form WLMs in these systems, we show that the surfactant has to be combined with a “binding” salt, i.e., one with a large organic counterion that is known to bind to the micelles. Examples of such salts include sodium salicylate (NaSal) and sodium tosylate (NaTos). Interestingly, the addition of simple, inorganic salts such as sodium chloride (NaCl) to the same surfactant does not induce WLMs in these polar solvents (although it does so in water). Thus, the rules for inducing WLMs in polar solvents are quite distinct from those in water. <br/><br/>WLMs are known to display interesting rheological properties as a function of temperature, and aqueous WLMs have been studied from room temperature to much higher values. However, few studies have examined WLMs under cold conditions, i.e., well below room temperature. A chief reason for this is that typical surfactants crystallize out when the sample is cooled below their Krafft temperature (~ 20°C). Here, we have carefully selected a surfactant with a very low Krafft point and induced its self-assembly in solvent mixtures with low freezing points. Thereby, we have been able to extend the range for WLM formation to temperatures as low as –20°C. Rheological techniques as well as small-angle neutron scattering (SANS) have been used to characterize the WLMs under these conditions. The dynamic rheology of the WLMs is very interesting and different from the rheology of WLMs in water. In addition to academic interest, WLMs in polar solvents could also find applications in diverse areas including as anti-freezing fluids or as synthetic lubricants.

Can self-assembly into wormlike micelles occur in polar solvents at sub-zero temperatures?

2019 Spring National Meeting

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