(620c) Effect of Interfacial Structures on the Formation of Worm-like Micelles from Molecular Perspectives

Worm-like Micelles (WLMs) are elongated self-assembly structures formed by amphiphilic molecules with a polar headgroup and a nonpolar tail. WLM chains become entangled in solution, thereby rendering high viscosity and viscoelasticity behaviors. Under shear, WLMs transform into a constantly breaking and reforming network, so-called ‘living’ polymers. Therefore, WLMs have numerous applications including enhanced oil recovery, heating and cooling fluids, drag-reducing agents, etc. In water solution with either "simple" salts like sodium chloride (NaCl) or "binding" salts like sodium salicylate (NaSal), cationic surfactants can readily form WLMs. Unfortunately, the study of WLMs in solvent with relatively low polarity has been rarely reported.

Recently, the first formation of WLMs in glycerol (Gly), a less polar solvent than water, was reported (Langmuir 2019, 35 (39): 12782-12791), in which a long C22-tailed erucyl bis(hydroxyethyl)methyl-ammonium chloride (EHAC) as well as a binding salt, i.e., NaSal, are both necessary. Although EHAC/NaCl could form WLMs in water, only spherical micelles exist in Gly. Besides, C16-tailed surfactant cetylpyridinium chloride (CPyCl) could not form WLMs in Gly. Critical packing parameter (CPP), a geometrical quantity defined as v⁄(l_c a_s), where v is the volume of the lipophilic chain having maximum effective length l_c, and a_s is the effective area per molecule at the surfactant/solvent interface, serves as the "golden" criteria to characterize micelle formation. However, solvent and salt molecules are treated implicitly in CPP theory. Several questions cannot be readily answered by CPP theory, i.e. why surfactants readily form WLMs in water, but not in Gly; why binding salt NaSal can help form WLMs in Gly but NaCl cannot; why CPyCl cannot form WLMs in Gly even with NaSal. To address these questions, an explicit consideration of molecular characteristics is necessary.

We use molecular dynamics (MD) simulations to investigate the interfacial structures and their effect on the formation of EHAC and CPyCl micelles in the presence of NaCl and NaSal in water and Gly solutions. In Gly solution, the binding between counterions (Cl^-) and EHA⁺ headgroups is more significant, which is expected to effectively screen headgroup charges. However, the separation distance between the neighboring EHA⁺ headgroups is larger in Gly solution than that in water solution as Gly molecules are bigger, rendering stronger excluded volume effect. As a result, the solvent (Gly) penetration induced enlargement might be the reason that EHAC cannot form WLMs in Gly solution with NaCl. In the presence of NaSal, Sal^- can bind strongly to the micelle, which squeezes Gly out of the headgroup zone, leading to smaller a_s and WLMs formation. Due to the benzene ring headgroup structure, Gly molecules could still stay in the headgroup region of CPyCl even though the strong binding of Sal^-. However, due to the presence of arm-like structures in EHAC headgroup, the strong steric hinderance prevents the penetration of Gly molecules.

In summary, the explicit consideration of solvent and salt molecules at the micelle-solvent interface can help explain origin of the morphology difference of micelle. This work provides fundamental understanding about the role of interfacial structures in the formation of WLMs from molecular perspectives which is largely overlooked in previous studies. The knowledge from our work can help guide the rational design and optimization of WLM formation for practical applications.