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45241-GB9
Fundamental Studies on the Interfacial Properties of Individual Aqueous Microdroplets in Hydrocarbons
During the first 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 completed data collection and analysis of the droplet radius/time profile as a function of solvent composition and other factors including temperature and initial droplet size, for homologous series (C9-C14) of alkanes and alcohols, respectively. Clear trends apparent from these systematic studies indicate the expected continuous decrease in dissolution rate as a function of carbon number. With these key metrics in hand, subsequent studies of interdroplet contact stability and permeability are enabled.
2. We have commenced rate studies for surfactant monolayer formation at the water/hydrocarbon interface at the single droplet level, in order to correlate the role of surfactant composition and concentration on droplet dissolution kinetics and stability. Dynamic dissolution rate measurements for aqueous microdroplets having various levels of coverage by oil-soluble surfactant monolayers (in the same homologous series of alkanes and alcohols as above), has confirmed the validity of our proposed approach. Monitoring the dramatic manner in which dissolution rate changes upon the adsorption and desorption of surfactant molecules at the interface provides a dynamic in-situ probe for how surfactants spread from aggregates to interfaces.
3. Surfactant headgroup packing at the liquid-liquid interface has been probed via study of the interplay of incipient crystal lattices of known structure with interfacial amphiphiles. In particular, we have deduced packing parameters for a cationic surfactant (octadecylamine) assembled at an isolated aqueous microdroplet phase containing a crystallizable polyanionic solute of known crystal structure (potassium ferricyanide). The high charge-size ratio of the hexacyanoferrate anion was found to interact strongly with the cationic headgroups of the surfactant (pKa=10), as evidenced by the induction of euhedral crystal morphology in the presence of submillimolar quantities of surfactant in the oil phase surrounding the microdroplet. Only in the presence of a threshold minimum concentration of surfactant was this effect seen, indicative of the quantity of surfactant molecules necessary for an ordered monolayer. Traditional studies of surfactant packing have been via surface pressure studies on Langmuir monolayers at the air-water interface, but considerably fewer such studies at the oil-water interface.
Other notable evidence from these studies that indirectly support the nature of the surfactant monolayer at the level of single droplet includes:
• Variation of onset concentration for crystallization, as evidence of molecular recognition of monolayer arrangement at the interface
• Variation of water droplet dissolution kinetics
• Monolayer disruption upon added intercalant
Information pertaining to surfactant area/molecule (surface excess concentration) which is gained from these studies is critical in determining the concentration of surfactant necessary to form a condensed monolayer at the aqueous droplet interface.
4. The grant from the ACS-PRF has provided the opportunity for six undergraduate students (five female) to participate in our research by: supporting a partial research stipend; conference attendance; and related project expenses. All of these undergraduate research participants have experienced a positive and immediate impact upon their learning, problem solving, and active motivation, through research experience done hand-in-hand with the PI.
In summary, the first year of the funded research has resulted in the completion of the work scheduled to be done during this year, with our results giving us confidence that the entire work will be completed in time and under budget. The results so far have been presented at five conference presentations including ACS national and regional meetings, and at the ACS New York Section undergraduate research symposium. Abstracts were published with four undergraduate co-authors, and two undergraduates are working on this project as part of their B.S. honors dissertation.
With the support of ACS-PRF grant, PI has designed and established a systematic approach to studying droplet formation, dissolution, and interdroplet interactions using a highly controllable experimental platform which uses micropipette manipulation combined with light microscopy. With the continued support from ACS-PRF, we anticipate that the number of students impacted will increase greatly by the end of the grant period of two years. We fully expect successful accomplishment towards enhanced understanding of and ability to control the liquid-liquid interfaces, critical for designing novel functional droplets employed in the food, pharmaceutical and petroleum industries.
