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Three experiments involving molecular self-assembly at liquid/liquid interfaces have been developed as part of this project.  The first of these involves assembly of small molecules at the interface between immiscible liquids.  The second is related, and involves the adsorption of gradient copolymers at similar immiscible liquid/liquid interfaces.  The third involves molecular assembly at the interface between miscible polymer solutions with different compositions.  In each case we are interested in the molecular origins of the mechanical response of the structure that forms at the interface.  The details of each of these three sub-projects are described below.

1)  Small-molecule assembly at immiscible interfaces. 

The basic approach utilized in these experiments the measurement of the chloroform/water interfacial tension in the presence of various molecules that are able to form well-defined structures at the interface between these two liquids.  A chloroform drop is suspended in water, and the interfacial tension is measured by an automated shape analysis of the pendant drop.  The interfacial tension drops substantially when amphiphilic molecules are added to the chloroform phase.  Several different types of amphiphilic molecules have been investigated.  Some of these amphiphiles are designed to present biotin to the aqueous phase in order to interact with streptavidin, while at the same time eliminating non-specific adsorption of streptavidin to the oil/water interface.  The lateral interfacial pressure due to the presence of these adsorbed molecules is measured by decreasing the volume of the chloroform droplet and measuring the corresponding decrease in the interfacial tension.  In the absence of bound streptavidin, the interfacial tension is independent of changes in the interfacial area that occur when fluid is added to or removed from the chloroform drop.  The behavior is changed dramatically when the streptavidin is added to the aqueous phase.  In this case a substantial interfacial pressure develops when the bound layer is compressed by shrinking the chloroform droplet.  The compression and expansion curves are fully reversible and are very sensitive to the structure of the bound layer.  In order to more effectively extract information from these curves, we initiated a collaboration with Prof. Igal Szleifer, currently in the biomedical engineering department at Northwestern.  His molecular models of protein adsorption have been compared quantitatively with measured interfacial pressure curves, confirming that our assumed model of the interfacial layer is correct.

The ability of our drop shape analysis technique to pressure/area isotherms, similar to those obtained from a traditional Langmuir trough experiment, has two significant advantages over more conventional experiments in that the length scales are very small, and that a variety of oil/water interfaces can easily be utilized.  The small length scale enables much more rapid diffusive transport of molecules to the interface.  In addition, small volumes are an important advantage for the application of this technique to studies of specific interactions in biological systems, where the relevant molecules are not available in large quantities. 

2)  Mechanics of Gradient Copolymer Layers at the Oil/Water Interface

The behavior of styrene/acrylic acid gradient and diblock copolymers at liquid/liquid interfaces was investigated by using drop shape analysis to measure the interfacial tension.  Copolymers were dissolved in chloroform, and pendant drops of these solutions were created in water.  Molecular conformations at the interface were inferred by measuring changes in the interfacial tension as the interface was contracted and expanded through control of the drop volume.  In this way we were able to independently determine the interfacial pressure and area modulus of the adsorbed layer.  Gradient copolymers showed the largest interfacial pressure, a result that is attributed to kinetic factors associated with the nature of the micellar aggregates that form in the chloroform phase.  The area modulus of the adsorbed layer depended on the processing history and was not directly related to the interfacial pressure.  This result is attributed to a local segmental desorption process where portions of the molecules reversibly desorb while the number of copolymer molecules at the interface remains fixed.

3)  Mechanics of Self Assembled forme at Liquid/Liquid Interfaces

The mechanical properties and water permeability of hierarchical self-assembling membranes and sacs formed from oppositely charged high molecular weight hyaluronic acid (HA) and small molecule peptide amphiphiles (PA) were studied. Techniques to make reproducible 2D planar membranes and 3D spherical sacs from these materials were developed while membrane inflation and osmotic swelling were used to quantify the mechanical properties and water permeability of these structures. It was found that incubation time and concentration of HA used had an effect on the area modulus and water permeability of the membranes. These factors also affected the kinetics of membrane growth as evidenced in SEM micrographs, which showed differences in the structure. Area modulus of membranes changed from about 6 N/m for the lower weight percent HA system at the shortest incubation time of 3 minutes, up to 12 N/m for the higher weight percent HA system at the longest incubation time of 60 minutes. Water permeability decreased with incubation time, but the lower weight percent HA system showed a lower water permeability when compared to the higher weight percent HA system at the same incubation time. This type of characterization and understanding of the structure-property relationships in self-assembling systems is a necessary step in both using these structures for specific applications and applying this knowledge to design new and better materials in the future.