Investigation of the shapes and sizes of soft molecular aggregates in solution via partial molar volume and dynamic light scattering measurements

Abstract

Soft molecular aggregates are naturally dynamic and deformable nanoscale objects of various shapes and sizes. However, soft nanoparticle shape can be difficult to measure and often requires advanced instrumentation or methods. Yet, foreknowledge of soft aggregate overall shape is often critically important in many experiments that measure the aggregate’s internal structure and properties. Small-angle neutron and x-ray scattering are often used to measure soft nanoparticle shape. Molecular dynamics simulations can also give an indication of soft molecular shape but, like small-angle scattering methods, requires considerable expense and training. To complement these approaches a more straightforward experimental method of determining soft molecular aggregate shape using a combination of partial molar volume and dynamic light scattering measurements (PMV-DLS) is presented. PMV-DLS is inexpensive, requires modest instrumentation, and can be performed in any laboratory. In this article, the PMV-DLS technique is demonstrated on two separate soft molecular aggregates: AOT reverse micelles and Triton X-100 micelles. The shapes of these aggregates were investigated over a range of solution properties. In both cases, the penetration of continuous phase solvent molecules into the aggregate was found to play an important role in shape determination. AOT reverse micelles were observed to transition from ellipsoidal to spherical as the solution water content increased. Triton X-100 micelle shape was found to be critically dependent upon the amount of water that partitions into the surfactant polyethylene oxide head group region.