Coupling between the Dynamics of Water and Surfactants in Lyotropic Liquid Crystals

Abstract

Bilayers composed of lipid or surfactant molecules are central to biological membranes and lamellar lyotropic liquid crystalline (LLC) phases. Common to these systems are phases that exhibit either ordered or disordered packing of the hydrophobic tails. In this work, we study the impact of surfactant ordering, i.e., disordered L and ordered L LLC phases, on the dynamics of water and sodium ions in the lamellar phases of dicarboxylate gemini surfactants. We study the different phases at identical hydration levels by changing the length of the hydrophobic tails; surfactants with shorter tails form L phases and those with longer tails form L phases. We find that the L phases exhibit lower density and greater compressibility than the L phases, with a hydration-dependent headgroup surface area. These structural differences significantly affect the relative dynamic properties of the phases, primarily the mobility of the surfactant molecules tangential to the bilayer surface, as well as the rates of water and ion diffusion. We find ∼20-50% faster water diffusion in the L phases compared to the L phases, with the differences most pronounced at low hydration. This coupling between water dynamics and surfactant mobility is verified using additional simulations in which the surfactant tails are frozen. Our study indicates that gemini surfactant LLCs provide an important prototypical system for characterizing properties shared with more complex biological lipid membranes.

Proton Diffusion through Bilayer Pores

“…The CO 2 -15­(4) dicarboxylate Gemini surfactants (Figure ) self-assemble into L β lamellar phases at hydration levels below λ ∼ 18 water molecules per surfactant, at 300 K . The surfactant tails pack in an ordered arrangement within the lamellar bilayer (see Figure ).…”

“…We study LLCs self-assembled from dicarboxylate gemini surfactant molecules with sodium counterions in water. Such gemini surfactants consist of two single-tail surfactants that are covalently bound by a linker, resulting in a greater propensity for self-assembly into micelles and LLCs; ,,− specifically, the self-assembly of dicarboxylate gemini surfactants has been characterized both experimentally , and theoretically. ,,, In this work, we focus on dicarboxylate surfactants with 15 carbon atoms in the surfactant tails and 4 carbon atoms in the gemini linker (Figure ), termed “CO 2 -15­(4)”. We consider a system composed of 134 surfactant molecules, 268 sodium counterions, and 2010 water molecules, resulting in a simulation box of x / y dimensions of ∼7.6 nm and z dimension of ∼3.4 nm, with the periodic bilayer stacking along the z dimension.…”

“…The nanoconfinement length scale and topology in LLCs are readily determined by sharp X-ray scattering peaks, , and this confinement can be precisely tuned on the basis of the surfactant weight percent. LLCs may exhibit a variety of morphologies, including hexagonally packed cylinders, bicontinuous cubic networks, or lamellar structures, depending on the type of surfactant and water composition; ,, in this regard, lamellar phases are the closest analogue to biological membrane bilayers, whereas other morphologies may be more desirable for technological applications. − As with biological membranes, lamellar LLCs may exhibit either ordered or disordered packing of hydrophobic tails within the bilayers, resulting in either L β or L α phases, respectively. − Furthermore, the relative stability of L α and L β LLCs can be controlled through synthetic modification of the surfactants, − which allows systematic characterization of water and ion dynamics for these different bilayer phases over a range of nanoconfinement length scales.…”

Consequences of Convex Nanopore Chemistry on Confined Water Dynamics

“…In the context of the current study, we would anticipate that increased counterion dissociation in convex pores would lead to slower water diffusion. However, we are unable to compare our present results to benchmark studies of water diffusion in the concave nanopools of sodium bis­(2-ethylhexyl) sulfosuccinate ( Na-AOT ) reverse micelles. , Beyond the obvious differences in surfactant structure and counterion identity, direct comparisons of water diffusion in convex and concave nanopores may be inherently problematic due to coupling with the dynamics of the surrounding matrix. , Thus, future explorations of this dynamical coupling that isolate the effect of pore interfacial curvature on confined water dynamics are warranted.…”

“…60,61 Beyond the obvious differences in surfactant structure and counterion identity, direct comparisons of water diffusion in convex and concave nanopores may be inherently problematic due to coupling with the dynamics of the surrounding matrix. 79,80 Thus, future explorations of this dynamical coupling that isolate the effect of pore interfacial curvature on confined water dynamics are warranted.…”