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45241-GB9
Fundamental Studies on the Interfacial Properties of Individual Aqueous Microdroplets in Hydrocarbons

Sunghee Lee, Iona College

During the second and final year of the ACS-PRF grant funding, we have successfully accomplished the following tasks:

1.         In accordance with our proposed studies of dissolution kinetics for individual aqueous microdroplets in oil solvent, we have extended our data collection and analysis of the droplet radius/time profile as a function of the nature (cationic, anionic and neutral) and concentration of self-assembled surfactant monolayer at the water-oil interface. We employed micromanipulation, which provided a convenient method to test the dissolution of a single liquid microdroplet in an infinite medium solution. We observed that rate of droplet shrinkage was dependent on concentration of surfactant; in general, the higher the concentration of surfactant in oil phase, the more time was required for the dissolution to proceed to completion. With each type of surfactant studied, we observed a clear minimum concentration above which a significant droplet retardation effect could be seen. However, this effect was more pronounced for cationic surfactants than for neutral or anionic ones. Our intriguing results have suggested that differences in the rates of droplet shrinkage can be correlated as a convenient indicative measure for coverage of surfactant molecules at the interface, as it is expected that surfactant molecules would be interfacially concentrated owing to their amphiphilic nature. In turn, these rate differences have allowed us to map important parameters for aqueous droplet stability in water-in-oil emulsion systems, since emulsion droplet stability in part depends upon mass transport rates for water outflow.

2.         In order to further correlate the role of surfactant composition (nature) and concentration on droplet dissolution kinetics and stability, we have continued our studies of interfacial tension (IFT) at the micron-sized water/oil interface. Our interface was entirely constrained within a micropipette, and allowed for observation of dynamic IFT. As a technique to measure the equilibrium and dynamic interfacial tension, a tapered micropipette was used as a way to control the interfacial geometry, and at the same time apply pressure across the interface, as described in our proposal. By comparison of IFT changes to droplet dissolution kinetics for each surfactant studied in section (1) at a given concentration, we were able to confirm the validity of our proposed approach.  A direct correlation was found between the presence of interfacial surfactant monolayer, as evidenced by IFT lowering, and the dissolution retardation for aqueous microdroplets having various levels of interfacial surfactant coverage. With our system we have successfully correlated IFT, a direct measure of surfactant at the monolayer, with resistance against dissolution to the bulk oil, which is a necessary condition for stable water in oil emulsions.

3.         In continuation from our first year findings, we have further extended the study of the interplay of incipient crystal lattices of known structure with interfacial amphiphiles, as a way to probe surfactant headgroup packing at the liquid-liquid interface. In particular, we have deduced packing parameters for a partially cationic surfactant (octadecylamine) assembled at an isolated aqueous microdroplet phase containing a crystallizable polyanionic solute of known structure (potassium ferricyanide). Our studies demonstrating the interplay between nucleation of solutes and amphiphilic monolayer provided further support for our previously reported findings. Notable evidence includes:

• A fully cationic surfactant (viz., octadecylammonium chloride, ODA•HCl) exhibited no templating effect, and doping ODA•HCl into an ODA monolayer progressively destroying any templating ability initially shown by ODA monolayer alone.

• An anionic surfactant (stearic acid) similarly was unable to template crystal nucleation.

These findings suggest that the driving force for the facilitated nucleation of KFC at the water-decanol interface in the presence of ODA is not solely due to electrostatic interactions, but rather, nucleation is likely fostered due to a combination of both electrostatic interactions and structural matching.

4.         The grant from the ACS-PRF (including supplement for SUMR fellowship) during the second year has provided the opportunity for eight undergraduate students (including one SUMR scholar, three female students) to be supported in the form of: SUMR fellowship, conference attendance; and related research project expenses. Research involvement from all of these undergraduates has resulted in the completion of the work scheduled to be done during the grant period, with our results ready for submission for publication with two undergraduate students as co-authors. Another manuscript with three undergraduate coauthors is currently being prepared for peer-reviewed publication. The results this year alone (Sept. 2007-Aug. 2008) have been presented at five conference presentations including 236^th ACS National Meeting, Albany Undergraduate Research Exposition and Senior Honors Thesis Research Presentation at Iona College. Abstracts were published with eight undergraduate co-authors, and three undergraduates have been working on this project as part of their B.S. honors dissertation.

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