(14h) Effect of Interfacial Solvent Structures on the Formation of Worm-like Micelles: A Molecular Dynamics Study

Self-assembly of surfactants which are amphiphilic molecules with a polar headgroup and a nonpolar tail can lead to the formation of micelles. A particularly interesting class of micelles is the worm-like micelles (WLMs), which are long flexible cylindrical chains, with diameters around 5 nm and contour (end-to-length) lengths ranging from 100 to 5000 nm (Langmuir 2019, 35 (39): 12782-12791). WLM chains become entangled in solution, thereby rendering high viscosity and viscoelasticity behaviors. WLMs have numerous applications including oil recovery, heating and cooling fluid, drag-reducing agents, and cosmetic products, etc. 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, has been widely used to describe the shape of micelles (Soft Matter 2007, 3, 956-970). When CPP is less than 1/3, it is predicted that the surfactants would assemble into spherical micelles; when CPP is between 1/3 and 1/2, WLMs are expected; when CPP is larger than 1/2, lamellar structures should form spontaneously. Salt type and concentration, temperature, and PH, etc. can affect the value of CPP. In particular, simple inorganic salts (such as NaCl) can screen the charges of surfactant headgroups to reduce a_s, while binding salts (such as NaSal) can effectively bind with the surfactant headgroups via electrostatic, cationic-Ï€, and hydrophobic interactions. While there have been an extensive number of studies on WLM formation in water solution, the studies on WLMs in pure solvents of intermediate polarity has been rarely reported.

Recently, Agrawal et al. (Langmuir 2019, 35 (39): 12782-12791) observed the formation of WLMs when adding cationic surfactants (EHAC) to water in the presence of simple inorganic salt NaCl, whereas only spherical micelles appear in pure glycerol (Gly) solution with the addition of NaCl. In fact, Gly has a lower dielectric constant than water, in which the electrostatic screening is more significant. They hypothesized that NaCl does not completely dissociate into ions in Gly due to its low dielectric constant. On the other hand, Koya et al. (Journal of Molecular Liquids 2016, 219, 505-512) proposed that the lower dielectric constant can increase repulsive force between headgroups, which leave more space for the possible penetration of solvents into the micelles leading to the increase in the first critical micelle concentration (CMC). Therefore, interfacial solvent structures might greatly affect WLMs formation, while they are difficult to be explored via experimental measurements. To the best of our knowledge, there is no study reporting the effect of interfacial solvent structures on the WLMs formation.

Therefore, in this work, we use molecular dynamic (MD) simulations to study the effect of interfacial solvent structures on the WLMs formation in Gly and water with EHAC and NaCl at the ambient condition. We find that WLMs are formed in water while only spherical micelles are observed in Gly, which is in line with the experimental observations (Langmuir 2019, 35 (39): 12782-12791). Both water and Gly molecules can penetrate into the micelles, and in Gly, more counterions (Cl^-) can penetrate into the micelles, which is expected to effectively screen the charges of headgroups. However, the separation distance between the neighboring EHAC headgroups is larger in Gly than that in water as Gly molecules are much bigger rendering stronger excluded volume effect. The hydrophobic interaction between the C atoms in Gly and EHAC tails and the polar-polar interaction between Gly and EHAC headgroups might be responsible for Gly penetration into the micelles. As a result, the solvent (Gly) penetration induced a_s enlargement might be the reason that EHAC cannot form WLMs in Gly solution with NaCl.

Collectively, our study from molecular perspectives emphasizes the importance of individual molecular characteristics and interfacial solvent structures on the formation of WLMs which are largely ignored in the previous literatures. Our work can shed important insights into the design and optimization of the formation of WLMs for practical applications.