Molecular Origins of Mesoscale Ordering in a Metalloamphiphile Phase

Abstract

Controlling the assembly of soft and deformable molecular aggregates into mesoscale structures is essential for understanding and developing a broad range of processes including rare earth extraction and cleaning of water, as well as for developing materials with unique properties. By combined synchrotron small- and wide-angle X-ray scattering with large-scale atomistic molecular dynamics simulations we analyze here a metalloamphiphile-oil solution that organizes on multiple length scales. The molecules associate into aggregates, and aggregates flocculate into meso-ordered phases. Our study demonstrates that dipolar interactions, centered on the amphiphile headgroup, bridge ionic aggregate cores and drive aggregate flocculation. By identifying specific intermolecular interactions that drive mesoscale ordering in solution, we bridge two different length scales that are classically addressed separately. Our results highlight the importance of individual intermolecular interactions in driving mesoscale ordering.

Figure 1

Concept of the Baxter sticky sphere model as applied to metalloamphiphile solutions. The metalloamphiphile aggregate is treated as a solid nanoscale particle with the hydrophilic coordination complex (purple) inside a solid spherical core (red) surrounded by the lipophilic functionality endowed by the organic amphiphile ligands (headgroups in orange, tails in gray). Particle aggregates interact through a narrow attractive well U(r) (dashed line, eq 4 in the Supporting Information), which is an example of ICI that causes inelastic collisions between molecular aggregates that drive mesoscopic assembly.

Figure 2

Experimental (i) and simulated (ii) SWAXS for solutions (a)–(d) and pure n-heptane solution. In the experimental X-ray data, the background scattering from air and the sample holder has been subtracted. The simulated results are normalized with respect to the experimental scattering intensity of the pure n-heptane system at 1.4 Å^–1. The low-q limit is restricted by the size of the simulation box by q > 2π/(D/2) based on Bragg’s law. Note that D/2, instead of D, is used due to the minimum-image convention in computer simulations under the periodic boundary conditions, where each atom is interacting only with the closest image of the remaining atoms in the system. See Supporting Information for details on the calculation of the SWAXS from atomistic simulations.

Figure 3

Snapshots of the last simulation frames for solutions (a)–(d). Only H₂O, HNO₃, Eu³⁺, and NO₃^– are shown (heptane and DMDOHEMA omitted for the display). See Figure S9 for the complete figures.

Figure 4

PDDFs generated from the background subtracted SAXS data for the solutions (a)–(d) formed under conditions described in Table 1. Insets show morphologies of the aggregate polar cores that are suggested by the PDDF functions.

Figure 5

Morphologies of representative aggregates in the simulations of solutions (a)–(d). In the upper row, the structures of single aggregates are plotted. In the lower row, the accumulated distributions of the aggregate core (DMDOHEMA headgroup, H₂O, HNO₃, Eu³⁺, and NO₃^–) are included to highlight the morphology of the polar core of such aggregates that produces the background-subtracted experimental SAXS signal from which the PDDFs in Figure 4 were generated. Each of the accumulated distributions is based on a simulation duration of 5 ns in a given system, except 0.5 ns for (c).

Figure 6

(i) A histogram showing the relative number of mono-, di-, and trinuclear metalloamphiphile aggregates in solutions (b) (blue) and (d) (red). (ii) Shapes and lengths of the long axis and short axis of linear Eu-centered aggregates containing 1, 2, and 3 Eu³⁺ per aggregate for solutions (b) and (d), and their respective structures (iii).

Figure 7

(i) RDFs between the central carbon atoms on the DMDOHEMA headgroups. (ii) The orientations between neighbor DMDOHEMA headgroups contributing to the first (6 Å) and the second (8 Å) RDF peaks are illustrated in black-shaded and red-shaded snapshots, respectively. (iii) Structure of one aggregate highlighting the first (black line) and the second (red line) correlations between DMDOHEMA headgroups. These link the DMDOHEMA molecules that surround the polar core of the aggregate where the accumulative positions of water, HNO₃, Eu, and nitrate molecules are shown in orange.

Figure 8

(i) Two Eu-centered aggregates are touching, where the red shadow areas represent their polar core regions. (ii) RDF between Eu³⁺ ions in the solution (b). The typical correlation peaks are labeled with (1–6), with corresponding structures provided in (iii). (1–3) stand for dinuclear Eu³⁺ within the same aggregates. (4, 5) denote H-bonds (red dotted lines) bridging aggregates. Green dotted lines stand for Eu–oxygen dative bonds. (6) originates from the dipolar correlation (highlighted sticks) between NO₃^– coordinated to the left Eu³⁺ and the DMDOHEMA headgroup coordinating the right Eu³⁺ ion. In (6′) all the coordinated ligands are plotted, where the red shadow areas highlight the inner core region of the two aggregates.