Amphiphile Conformation Impacts Aggregate Morphology and Solution Structure Across Multiple Lengthscales [post]

Michael Servis, Biswajit Sadhu, Lynda Soderholm, Aurora Clark
2021 unpublished
2 Although the self-assembly of amphiphiles is well-studied in aqueous solutions, 3 much less is understood about the fundamental driving forces and structure property 4 relationships in non-polar media. In recent work [Journal of Physical Chemistry B, 5 2020, 124, 10822.] the authors have studied a series of malonomide-based amphiphiles 6 that are relevant to liquid-liquid extraction. That work demonstrated that aggregation 7 is largely driven by local dipole-dipole interactions between
more » ... es. Here, we build 8 upon this observation to develop a more detailed understanding of how the balance 9 of dipole-dipole interactions (controlled by conformation) and molecular architecture 10 influences the morphology of the aggregates across lengthscale. Using constrained 11 molecular dynamics about key degrees of freedom, we demonstrate that the conforma-12 tion of N,N-dimethyl,N,N-dioctylhexylethoxy malonamide (DMDOHEMA) and N,N-13 dimethyl,N,N-dibutyltetradecyl malonamide (DMDBTDMA) has a significant impact 14 upon self-association -where appropriate conformational sampling is essential. To 15 quantify the aggregate morphology, several graph theoretic and persistent homology 16 based properties are determined. The former examines the patterns of intermolecu-17 lar interactions within clusters, while the latter examines the 3-dimensional spatial 18 distribution across lengthscales. Based upon these analyses, we find that the mor-19 phology of aggregates, particularly at higher malonamide concentration, depends on a 20 balance of dipole alignment and alkyl tail sterics. Dipole alignment encourages linear 21 patterns of the intermolecular interactions within aggregates, while the the alkyl tail 22 steric interactions between the malonamide result in noticeably less linear aggregates 23 for DMDOHEMA than DMDBTDMA. This is reflected in the spatial distribution, 24 where more holes or voids exist between extractants within the DMDOHEMA that 25 distribute within the solution in more of a "swiss cheese" arrangement as opposed 26 to the more filamentous distribution of DMBDTDMA. This study links conformation 27 and molecular structure to the morphology of amphiphile assemblies, and serves as a 28 basis for ongoing study of multicomponent amphiphile solutions with polar and other 29 solutes, and how these impact aggregation phenomena. 30 2 Supramolecular assembly of amphiphilic molecules supports a breadth of soft matter appli-32 cations 1 -from catalysis 2 to drug delivery 3 to nano-devices. 4 Aqueous assembly has been 33 the subject of significant study, where many of the fundamental driving forces and struc-34 ture property relationships have been identified. 5,6 In comparison, organic phase amphiphile 35 assembly is less understood, despite important consequences to several technologies -includ-36 ing liquid-liquid extraction. Liquid-liquid extraction (LLE) is an industrial and analytical 37 process for the selective partitioning of solutes between immiscible liquid phases. 7 Solutes 38 are distributed between low and high dielectric phases by their relative solubilities. This 39 free energy-driven process is controlled, therefore, by solute speciation: for example, target 40 aqueous solutes complex with amphiphilic "extractant" molecules to solubilize the resulting 41 complexes in the low dielectric organic phase. The free energy differences which drive ef-42 fective separations are often small, including on the order of thermal energy. As a result, 43 relatively minor free energy contributions are essential to understand and model LLE. Among 44 these small free energy contributions is the organic phase aggregation of the extractant and 45 extracted solutes, which imparts mesoscale structure to that phase. 8 46 Organic phase aggregation is driven by intermolecular interactions across different energy 47 and length scales. The nanoscopic lengths over which the organization is manifested evades 48 characterization by many experimental techniques: it is too large to probe with techniques in-49 cluding extended X-ray adsorption fine structure (EXAFS), IR or Raman spectroscopy which 50 are sensitive only to local environments while also being too small to effectively interpret 51 using NMR diffusion 9 or small angle scattering data 10-19 fitted with colloidal models. For 52 this reason, a common approach to understanding organic phase aggregation is to combine 53 molecular dynamics (MD) simulation with experimental techniques including small angle X-54 ray scattering (SAXS). 18,20,21 This provides the benefit of validating the simulation organic 55 phase structure with the experimental data while not relying on ill-suited colloidal models 56 to interpret that data. 22 Instead, validated simulation structure can then be investigated in 57 3 detail to bridge atomic and mesoscopic length scales. 58 In this study, we consider two malonamide extractants commonly applied to f -element 59 separations: 23 N,N'-dimethyl,N,N'-dioctylhexylethoxy malonamide (DMDOHEMA) and N,N'-60 dimethyl,N,N'-dibutyltetradecyl malonamide (DMDBTDMA), illustrated in Figure 1. To 61 isolate the contributions to organic phase organization from the differences in molecular 62 structure between DMDOHEMA and DMDBTDMA, we consider a simple organic phase: 63 the extractant/solvent mixture in the absence of extracted polar solutes. This system will 64 also serve a baseline from which the impact of extracted solutes can be understood. First, 65 we investigate the impact of extractant conformation and alkyl tail molecular structure 66 on extractant self-association. Then, we apply a graph theoretic and persistent homol-67 ogy approach to quantify both the malonamide aggregate morphology and their associated 68 conformations. The former method provides a detailed understanding of the patterns of 69 intermolecular interactions within and between aggregates, and has been used with much 70 success to characterize complex solutions. The latter provides new and additional insight 71 into the resultant spatial arrangement across lengthscales and represents a powerful emerging 72 tool to connect intermolecular forces and geometric structure. In combination these analyt-73 ical tools clearly demonstrate that two predominant forces impact aggregate structure for 74 malonomide systems -namely dipole alignment and alkyl tail sterics. Modulation of either 75 of these features influence the interconnectdness of intermolecular interactions within ag-76 gregates, inter-aggregate interactions and the resulting geometric arrangement at local and 77 extended lengthscales. This study provides insight into the fundamental drivers of organic 78 phase aggregation and serves as a framework to interpret the effects of chemical structure 79 and composition upon self-assembly and solution organization across lengthscales. 80 157 Chemistry Letters 2014, 5, 1440-1444.
doi:10.33774/chemrxiv-2021-6zndd fatcat:r2tjegtyivfrnl74k5tmda6rl4