Using Small-Angle Scattering and Contrast Matching to Understand Molecular Packing in Low Molecular Weight Gels

Emily R Draper,Bart Dietrich,Kate Mcaulay,Christopher Brasnett,Haleh Abdizadeh,Ilias Patmanidis,Siewert J Marrink,Hao Su,Honggang Cui,Ralf Schweins,Annela Seddon,Dave J Adams

doi:10.1016/j.matt.2019.12.028

Abstract

Small molecules can self-assemble into one-dimensional structures to give self-supporting gels. Such gels have a wide range of uses, including tissue engineering and drug delivery catalysis. It is difficult to understand how the molecules are packed in these structures, but this is hugely important if we are going to be able to learn from and design such materials. Here, we use a combination of small-angle X-ray and small-angle neutron scattering with selectively deuterated molecules to understand the packing in the pre-gelled aggregates and in the gel state. We also use kinetic measurements to understand the transition between these aggregates. Our data show that there is a lack of order in the gel state, correlating with the limited predictive design rules in this field and with the importance of kinetics in forming the gel state. This approach allows us to understand our specific systems but represents a general approach that could be taken with different classes of gelator.

Highlights

We have previously described the self-assembly of 2NapFF in detail. 2NapFF self-assembles at high pH into long anisotropic structures.[50]
The small-angle X-ray scattering (SAXS) data[52] for 2NapFF at high pH can be fitted to a cylinder model
We have shown that selective deuteration and contrast-matching experiments are a powerful approach to understanding the packing in aggregates formed by functionalized dipeptides at high pH

Summary

Introduction

Low molecular weight gels are formed by the self-assembly of small molecules into anisotropic structures.[1,2,3,4,5] These gels are widely used in numerous applications,[6,7] including tissue engineering,[8] drug delivery,[9] optoelectronics,[10,11] structuring,[12] remediation,[13] and catalysis,[14] among others.The small-molecule gelators self-assemble into structures such as fibers and nanotubes that are typically a few nanometers in diameter, but often micrometers in length. We combine small-angle X-ray scattering (SAXS) with small-angle neutron scattering (SANS) contrast-matching experiments to access information about the molecular packing; this approach is widely used in the surfactant literature[42,43,44,45] as well as (for example) structure determination in protein and polymer systems.[46,47,48,49] We start by describing a single well-studied and robust LMWG, 2NapFF (Figure 1). The scattering is very different (Figure 5H), and the data are best fit to a hollow cylinder, with a wall thickness of 2.5 nm, and a core radius of 1.7 nm, close to that found for 2NapFF in D2O as expected.

Results

Conclusion