Nano-scale self-assembly of polyelectrolyte with surfactant into structured 'complex' fluids is at the heart of many contemporary scientific and commercial endeavors. It is a major emphasis of the specialty chemical industry, and enables advances in personal care products. The same chemistry and physics also dominates the non-specific interactions in biological systems and plays an important role in downstream purification and formulation of proteins and other pharmaceuticals. Despite the obvious importance these mixtures, there are significant gaps in the understanding of how polyelectrolytes and surfactants interact to form specific microstructures and to dictate colloidal-scale properties. The structures are typically on the nanometer-scale and are used to create systems that can be functionalized for specific tasks including drug delivery, materials templating, or catalysis; the colloidal interactions often define properties such as rheology and product stability. This project addresses the science and engineering needed to advance the formulation of polyelectrolytes and surfactants, and combines the resources of an academic laboratory with those of the dominant US industrial laboratory in this field.
The basic theme is experimental study of mixtures of polyelectrolyte with either same or oppositely charged surfactants, with two specific aims:
1. Investigate the microstructure and phase behavior of mixtures of mechanically-formed vesicles made from insoluble surfactants (R ~ 15 nm) and same-charged polyelectrolyte (Rg ~ 100 nm), in what is often referred to as the nano-particle limit (i.e., Rg >> R). Emphasis will be on understanding the role polymer characteristics such as flexibility and inter-polymer interactions (e.g., virial coefficients and specific surfactant-polymer interactions) play in the colloidal-scale structure of these mixtures. There is particular interest in the dilute and semidilute polymer concentration regime, which can be interpreted in the context of the Polymer Reference Interaction Site Model (PRISM), as well as in concentrated regimes that reflect practical product formulations. 2. Investigate in a methodical and systematic way the interactions of polyelectrolyte and soluble surfactants. With one exception, the project will focus on the study of weak polyelectrolyte (in which the charge groups are weak acids or bases). Features to be explored include binding isotherms, the molecular features of the polyelectrolyte (charge density, backbone rigidity, and molecular weight), configurations of polymer/surfactant aggregates, and the characteristics of resulting gel phases and aggregates. Of particular interest will be documentation of the competition between cationic polymer and cationic surfactant for association with anionic surfactant, and the role of hydrotropes. While these specific aims reflect specific industry needs, this work should also provide a set of rational design rules relevant to all nano-material formulations.
Intellectual Merit: The experimental results of the project will provide a motivation for extension and development of PRISM-like theories, while at the same time providing empirical rules for product design. The tandem of cryo-visualization and scattering will provide new insights into the binding and organization of soluble polyelectrolyte/surfactant complexes heretofore not available for weak polyelectrolytes, and the systematic phase and structure work will enable observation of new patterns.
Broader Technical Impact: The commercial value of this work is potentially enormous, with surfactant/polymer industries generating multi-billion dollars in revenue per annum. The scientific aspects of this proposed work will enable discovery of new patterns of colloidal stability and microstructure evolution. This will be significant for both Procter & Gamble as well as the broader colloid community.
Broader Social Impact: Undergraduate and graduate students involved in this work will be exposed to a range of characterization tools in preparation for either academic or industrial work. Visualization studies by electron microscopy are especially appealing to K-12 students, who will be exposed to concepts relevant to this work, including surfactant and polymer properties and elements of nanotechnology.
