Figure 5
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Setup. The experimental setup is shown in Figure 5. An aqueous continuous phase containing known amounts of l-tyrosine, potassium iodide, phosphate buffers, sodium hydroxide, and glycerin was charged to the column. The dispersed organic
phase consisting of a known concentration of iodine in methylene chloride was charged to a syringe. A PTFE needle was attached
to the syringe and the dispersed phase was fed into the continuous phase subsurface at a constant rate using a syringe pump.
The dispersed phase (i.e., the droplet) travelled down the continuous phase and was removed from the bottom of the column by a very small effluent rate.
Hydrodynamics around the droplet mixed the reactants. The volume of the continuous phase in the column remained fairly constant
during a given experiment. The column was 18 in. in effective length with a 3-in. internal diameter. The column was designed
with five discharge ports to allow top and bottom removal of a volume of the liquid phase. Only the middle portion of the
continuous phase was used for sampling.
We developed an HPLC method to quantify and analyze the results. The HPLC method uses a linear ramp of the mobile phase to
ensure peak separation and to shorten the run time of the method. All samples were analyzed with an HPLC instrument ("series
1100," Hewlett Packard) and software (ChemStation Rev. A.10.01, Agilent Technologies). The column was a 250 × 4.6 mm i.d.,
5 μm C8 column (ACE, Advanced Chromatography Technologies). All samples were analyzed at a wavelength of 210 nm. The mobile
phase consisted of acetonitrile (solvent A) and water containing 0.05 v% of perchloric acid and 0.1 v% of phosphoric acid
(solvent B). The wavelength and solvents were chosen to minimize the run time while providing good separation and peak formation
of each reaction component. (For more details, see Raffensberger [34]).
Table I: l-Tyrosine solution components.
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Procedure. The aqueous l-tyrosine solution was prepared using the recipe listed in Table I. The water was saturated with methylene chloride to prevent
solvent mass transfer from the dispersed phase. The resulting concentration of l-tyrosine solution used in the experiments was 0.084 ± 0.001 mg l-tyrosine/mL water.
The organic iodine solution was prepared using the recipe listed in Table II. The resulting concentration of iodine in methylene
chloride was 31.7 ± 0.1 mg iodine/mL methylene chloride. The solution was aged for several hours to ensure all of the iodine
had dissolved.
Table II: Iodine solution components.
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Once both solutions were prepared, the continuous phase for a given experiment was made. The continuous phase was 1250 mL
in volume and contained 75 mL of the l-tyrosine solution described previously. The remainder of the continuous phase consisted of water saturated with methylene
chloride, glycerin, and 1.2 g each of sodium phosphate dibasic and sodium phosphate tribasic dodecahydrate. For example, a
30% glycerin solution would contain 75 mL l-tyrosine solution, 375 mL glycerin, 800 mL of deionized water saturated with methylene chloride, 1.2 g of sodium phosphate
dibasic, and 1.2 g of sodium phosphate tribasic dodecahydrate. The solution was mixed for at least 5 min, and the pH and temperature
of the continuous phase were checked. The pH of the continuous phase at the start was between 10 and 11. For all of the experiments
reported, the starting temperature varied between 22 and 25 °C.
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