Manipulating Single Microdroplets of NaCl Solutions: Solvent Dissolution, Microcrystallization, and Crystal Morphology

Anders Utoft, Koji Kinoshita, Deborah Bitterfield, David Needham

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A new “three-micropipette manipulation technique”
for forming, dehydrating, crystallizing, and resolvating
nanograms of salt material has been developed to study
supersaturated single microdroplets and microcrystals. This is
the first report of studies that have measured in situ both
supersaturation (as homogeneous nucleation) and saturation
(as microcrystal redissolution) for single microdroplets of
NaCl solution using the micropipette technique. This work
reports a measure of the critical supersaturation concentration
for homogeneous nucleation of NaCl (10.3 ± 0.3 M) at a
supersaturation fraction of S = 1.9, the saturation concentration
of NaCl in aqueous solution as measured with nanograms of material (5.5 ± 0.1 M), the diffusion coefficient for water in
octanol, D = (1.96 ± 0.10) × 10−6 cm2/s, and the effect of the solvent’s activity on dissolution kinetics. It is further shown that
the same Epstein−Plesset (EP) model, which was originally developed for diffusion-controlled dissolution and uptake of gas, and
successfully applied to liquid-in-liquid dissolution, can now also be applied to describe the diffusion-controlled uptake of water
from a water-saturated environment using the extended activity-based model of Bitterfield et al. This aspect of the EP model has
not previously been tested using single microdroplets. Finally, it is also reported how the water dissolution rate, rate of NaCl
concentration change, resulting crystal structure, and the time frame of initial crystal growth are affected by changing the bathing
medium from octanol to decane. A much slower loss of water-solvent and concomitant slower up-concentration of the NaCl
solute resulted in a lower tendency to nucleate and slower crystal growth because much less excess material was available at the
onset of nucleation in the decane system as compared to the octanol system. Thus, the crystal structure is reported to be
dendritic for NaCl solution microdroplets dissolving rapidly and nucleating violently in octanol, while they are formed as single
cubic crystals in a gentler way for solution-dissolution in decane. These new techniques and analyses can now also be used for
any other system where all relevant parameters are known. An example of this is control of drug/hydrogel/emulsion particle size
change due to solvent uptake.
Udgave nummer12
Sider (fra-til)3626-3641
StatusUdgivet - 2018

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