Tunable Nanostructures from Inverse Surfactants

TL;DR

The paper addresses how tunable molecular parameters control inverse-surfactant self-assembly morphologies and the mesoscale-to-macroscale transition. It integrates SANS experiments, coarse-grained MD simulations, and a mean-field aggregation theory in which the per-molecule free energy $F_m(p)=F_m^{\text{core}}(p)+F_m^{\text{corona}}(p)$ is governed by dimensionless groups $\beta\epsilon$, $\alpha$, and $r_h/r_t$. The combined results reveal that head-group size and the head-to-tail size ratio bias structures toward spheres, fibers, or planar morphologies and define a well-depth criterion $\Delta F_m(p)/p$ that marks mesoscale-to-macroscale transitions. The work provides design principles for de novo self-assembled nanostructures and demonstrates a quantitative path to predict and tune morphology via three key dimensionless parameters.

Abstract

Hierarchical materials in the natural world are often made through the self-assembly of amphiphilic molecules. Achieving similar structural complexity in synthetic materials requires understanding how various molecular parameters affect assembly behavior. In recent years, inverse surfactants -- molecules with hydrophobic head groups and hydrophilic macromolecular tails -- have been shown to self-assemble into supramolecular assemblies in aqueous solutions that show promise for a number of applications, including drug delivery. Here, we build an understanding of the morphological phase diagram of inverse surfactants using insights from scattering experiments, computer simulations, and statistical mechanics. The scattering and simulation results reveal that changing the head-group size is an important molecular knob in controlling morphological transitions. The molecular size ratio of the hydrophobic group to the hydrophilic emerges as a crucial dimensionless quantity in our theory and plays a determining role in setting the micelle structure and the transition from mesoscale to macroscale aggregates. Our minimal theory is able to qualitatively explain the key features of the morphological phase diagram, including the prevalence of fiber-like structures in comparison to spherical and planar micelles. Together, these findings provide a more complete picture for the molecular dependencies of assemblies of inverse surfactants, which we hope may aid in the de novo design of supramolecular structures.

Figures (8)

• Figure 1: Chemical structures and coarse-grained representations of the inverse surfactants explored in this work. The coarse-grained surfactants represent each $250$ Da DPCA molecule as a single attractive bead (red) and the $1140$ Da PEG chain as three linearly connected repulsive beads (blue).
• Figure 2: SANS profiles at room temperature ($25^\circ$C). The polymer chain is contrast-matched to the solvent so that scattering signal only comes from the hydrophobic aggregate core. The solid lines represent fits that are detailed in the SI. Error bars represent uncertainty after azimuthal detector averaging.
• Figure 3: Morphological phase diagrams from coarse-grained molecular dynamics simulations of (A) PD1, (B) PD2, and (C) PD3. Equilibrated simulations display spheres, fibers, and planar aggregates, as shown in the representative snapshots at the top of the figure, as well as regions where multiple morphologies coexist. Phase boundaries used to shade the regions are drawn to guide the eye.
• Figure 4: Per molecule free energies of formation for three representative values of $\beta \epsilon$ of (A) $5.0$, (B) $25.0$, and (C) $200.0$ with stars indicating the per-molecule minima for each morphology. All curves are shown for $\alpha = 1.1$ and $r_h/r_t = 0.06$. Stars denote the location of local minima.
• Figure 5: Dependence of the fiber and planar well depth on (A) $\beta\epsilon$ (for ${r_h/r_t = 0.06}$) and (B) $r_h/r_t$ (for ${\beta \epsilon = 7.0}$). Panels (C) and (D) show the minimizing aggregate sizes corresponding to panels (A) and (B), respectively. All plots are made using $\alpha = 1.1$. The dashed vertical orange and green lines represent the location of the mesoscopic-macroscopic transition for planar and fiber structures, respectively. The space between the dotted and dashed green lines marks the region for which the well depth is negative and the finite-sized fibers are metastable. At conditions in which local minima are absent, Eq. \ref{['eq: cutoff']} is ill defined and we report no value for it.