Boundary lines and linactants: Structure, function, and dynamics

Kent State University, Kent OH

Investigators

Abstract

NON-TECHNICAL SUMMARY: This project develops general principles for the design of molecules that adsorb efficiently to a "line": "linactants", in analogy to "surfactants" (soaps and lipids, for example), which adsorb efficiently to surfaces. Surfaces are ubiquitous: cell membranes divide the inside from the outside of the cell; oil/water mixtures push through rock and sand. Any two surfaces meet at a line. Thus, effective linactants have potential for enhanced oil recovery and understanding linactant principles will enable a deeper understanding of the structure, dynamics and function of cell membranes. This project and Kent State University (KSU) provide a rich interdisciplinary environment for training high school, undergraduate and graduate students for a diversity of opportunities in today's economy. They receive personal, professional mentoring, and interact closely with Scattering Solutions Inc., a startup company, and with industrial partners of the KSU Liquid Crystal Institute. Further, the principle investigator, a woman, has a deep history of involving women in research: 65% of the graduate and 50% of the undergraduate students in her lab have been women, far above the norm in physics. All aspects of this project both fuel and are fueled by the principle investigator's active outreach to middle school, high school, and community college students and their teachers. TECHNICAL SUMMARY: This project uses Langmuir films, which consist of a layer of molecules confined at the air/water interface by a delicate balance of molecule/molecule and molecule/water attractive interactions, as a model system to explore the structural origin of line tension (the energy per unit length of a line) and line activity. The relationship between molecular structure and line activity is unclear. The analogy with surfactants suggests a hybrid molecule: two moieties with different affinities for the two surface phases. In practice, such molecules form a third phase, with moderate affinity to the line boundary. The contrasts between the structure of line and surface boundaries suggests a dramatically different model of line activity that would transform the search for line-active molecular structures. A surface boundary, away from a critical point, is about one molecule thick. A surfactant molecule can easily span the boundary between phases. In contrast, indirect evidence suggests that the line boundary is ten or more molecules thick: in that case, either a linactant should be much larger than a typical surfactant, or it should act nonlocally. This project is three-pronged, in order to develop the connection between molecular, mesoscopic, and macroscopic scales: (a) To test the width of the boundary both indirectly, through measurements not just of line energy but of line entropy, and directly, through cryoTEM and other methods. These tests concentrate on a model system that forms liquid crystalline multilayers on the surface, simplifying these challenging experiments and providing layer number contrast as an additional control variable. (b) To test as linactants molecules expected to act non-locally, through spontaneous curvature and curvature elasticities, both in the model system and in biologically-relevant mixed lipid layers. The project initially focuses on line-active molecules suggested by preliminary results, followed by liquid crystalline mesogens with different shapes and lipid molecules that have similar but opposite effective spontaneous curvatures. (c) To explore one case where line tension is critical, nucleation dynamics within lipid layers, with a unique combination of complementary techniques. Effective linactant design will enable the development of self-assembled films with domains of controllable scale as well as chemical and physical structure. Such films are applicable to e.g. sensors. Linactants are also relevant to cell membranes, thought to contain dynamic nano-scale domains that control communication between the inside and outside of the cell: what controls these domains remains a critical open question. This interdisciplinary project trains a diverse group of high school, undergraduate and graduate students and fuels the principle investigator's active outreach to middle school, high school, and community college students and their teachers.