(4bv) Tailoring Structure, Thermodynamics, and Rheology in Surfactant-Colloid Mixtures for Soft Material Design

Helgeson, M. E. - Presenter, Massachusetts Institute of Technology
Kaler, E. W. - Presenter, Stony Brook University
Wagner, N. J. - Presenter, University of Delaware
Doyle, P. S. - Presenter, Massachusetts Institute of Technology

Surfactants and other amphiphilic molecules can self-assemble into a wide array of micellar structures in solution that can be used as templates and building blocks in more complex materials for nanotechnology, biology, and energy. The integration and interaction of these self-assembled structures with other colloidal entities such as nanoparticles, emulsions, macromolecules and proteins is particularly critical to the structure and functionality of materials in these emerging applications. However, gaps in understanding of mesoscale surfactant-colloid interactions and how they are manifest in macroscopic thermodynamic, rheological, and transport behavior is a significant roadblock in de novo engineering of new materials from these building blocks.

My research aims to advance the fundamental understanding of complex surfactant-colloid mixtures to enable new technologies and processes for the engineering of soft materials such as polymers, gels, and functional colloids. A platform that combines both experiment and theory, including microscopy, scattering, thermodynamics and rheology allows us to probe structure and dynamics at scales ranging from the molecular to the continuum level. Applying this approach to the toolkit of amphiphilic self-assembly results in new, flexible methodologies for material formulation and synthesis. This presentation summarizes two separate projects in this area from my doctoral research at the University of Delaware and postdoctoral work at MIT.

Controlling the structure and dynamics of wormlike micelles using colloidal particles (and vice versa)

Wormlike micelles (WLMs) are long, thread-like surfactant aggregates that exhibit dynamics and rheology similar to polymer solutions. The interactions and resulting structuring between colloids and WLMs is of great importance to applications ranging from consumer products to bioseparations to enhanced oil recovery. Here, we show that controlling interactions at the surfactant-colloid interface in mixtures of model WLMs and nanoparticles leads to unique structuring of the resulting fluid. Specifically, the fusion of WLMs with an equilibrium adsorbed surfactant layer at the nanoparticle surface gives rise to WLM-particle junctions. These junctions act as physical cross-links between micelles, resulting in significant gel-like viscoelasticity in an otherwise Newtonian fluid. This rheological enhancement is understood by the formation of a so-called ?double network?, where the viscoelasticity can be tuned through two energetic scales, the micellar end cap energy and micelle-nanoparticle junction energy, which are readily manipulated by adjusting solution conditions. This achieves sensitive control over the rheology of the WLM fluid, including the ability to suppress instabilities shear-induced phase separation and shear banding.

Conversely, the formation of micelle-nanoparticle junctions gives rise to new colloidal interactions between nanoparticles which can be used to tune the stability of the colloidal dispersion. A statistical mechanical model has been developed based on micellar bridging to describe these interactions in terms of experimentally measurable properties, allowing a priori predictions of the interaction potential between particles mediated by WLMs. Predictions of both the suspension microstructure and colloidal phase behavior are in excellent agreement with experimental measurements, and provide a firm basis for the model. These results show that the colloidal interactions can be easily tuned through small changes in temperature and composition via these WLM-mediated interactions, and provides an entirely new route to manipulating dispersion and stability of nanoparticles, emulsions, and proteins.

Formulation and properties of polymerizable nanoemulsions for the synthesis of polymer hydrogels with controlled porosity

Particulate hydrogels are finding a number of applications of biological importance, including tissue engineering, drug encapsulation and delivery, complex bioassays, and molecular separations. A commonality of all of these applications is the ability to control diffusion of molecular and colloidal species into and out of the hydrogel interior. As such, the ability to control the porosity and pore size distribution of the cross-linked polymer network is critical. Ideally, a method to control hydrogel porosity would enable simultaneous control over the volume fraction and size distribution of pores for average pore sizes at biologically relevant length scales. However, current methods for tuning the porosity of hydrogels represent tradeoffs between control over these various aspects of hydrogel porosity.

We have developed a synthesis method for particulate hydrogels with controlled porosity based on microfluidic preparation and polymerization of nanoscale polymerizable high internal phase emulsions (nano-polyHIPEs). In this method, nano-polyHIPEs are prepared by high-pressure homogenization, which yields nearly monodisperse droplets with controlled average size ranging from 50-500 nm at volume fractions exceeding 50%, properties that are unattainable using other methods. Subsequent photopolymerization of the continuous phase results in polymer microstructures with porous features templated by the nano-polyHIPE. Combining this polymerization method with the recently developed stop flow lithography (SFL) technique enables production of particulate hydrogels with exquisitely controlled size, shape, and porosity. This method is flexible toward a number of different polymer chemistries and microstructural motifs, including closed-cell and open-cell pores, as well as polymer composite structures. The resulting multi-functional hydrogel particles are capable of encapsulating both hydrophilic and hydrophobic species, and show promise as novel materials in pharmaceutical development and formulation.