of the solvent play important roles in the rate of evaporation. The completion of the
evaporation can be monitored by gas chromatography [43] or NMR spectroscopy
[44] and is usually realized within a few hours [45]. The role of the evaporation on
the hardening kinetics of the particles plays an important role, provided that the
continuous phase is saturated with the solvent mainly present in the dispersed phase
and that the diffusion rate of the solvent of the dispersed phase in the continuous
phase is fast compared with the solvent evaporation kinetics. In an experimental
study performed with dichloromethane, ethyl acetate, and acetonitrile as solvents,
Wang and Schwendeman demonstrated that the rate-limiting step for mass transport
of solvent depends on the properties of the solvent [43]. Dichloromethane at room
temperature is found to be liquid-side transport limited whereas ethyl acetate and
acetonitrile were gas-side transport limited. As expected, the evaporation rate was
largely affected by the diameter of the impeller, its rotational speed, and the
temperature. The particle’s hardening profile could be determined and predicted
without needing to measure the concentration of polymer in the solvent in time, but
by measuring the concentration of the solvent and by knowing the permeability
coefficient of the solvent at the liquid–air interface [43]. After evaporation of the
solvent, the dispersions can be dialyzed to remove unwanted or low molecular
weight polymer and can be freeze-dried.
One of the most critical properties of nanoparticles is size, hence its control is of
outmost importance. Because the particles are formed from droplets, their size is
largely dependent on the droplet size. In the case of miniemulsions, the size of the
droplets is controlled by the concentration of surfactant [40]. Other parameters such
as the nature of the solvent [46], the stirring rate, or ultrasonication time [47] also
influence the particle size and particle size distribution. Longer and/or stronger
emulsification usually leads to smaller and more narrowly distributed particle size
to a certain extent [47]. However, it is difficult to ascribe an observed effect upon
changing one parameter to this sole parameter, because most of the parameters are
not independent.
Physical processes responsible for the destabilization of emulsions such as
Ostwald ripening and coalescence are of crucial importance for the determination
of the final particle size and size distribution. It is known that the addition of a small
amount of a chemical that is preferentially soluble in the dispersed phase can hinder
the Ostwald ripening process [48]. This chemical, sometimes called the osmotic
pressure agent because it allows the building of an osmotic pressure upon possible
change of chemical composition of the droplets upon Ostwald ripening, is usually a
low molecular weight substance that is insoluble in the continuous phase. Thereby,
it counteracts the Laplace pressure of the droplets and stabilizes the emulsion
droplets. In the solvent evaporation process, no osmotic pressure agent is normally
added as the polymer itself can act as osmotic pressure agent because it is insoluble
in the continuous phase. However, the concentration of polymer must be above a
threshold value to effectively hinder Ostwald ripening [49]. Loxley and Vincent
have supposed that the relatively broad size distribution of the obtained particles is
caused by coalescence [50]. Dynamic light scattering (DLS) was employed to
measure the size of emulsion droplets and the obtained nanoparticles [45]. Based
Recent Advances in the Emulsion Solvent Evaporation Technique for the. . .
333
evaporation can be monitored by gas chromatography [43] or NMR spectroscopy
[44] and is usually realized within a few hours [45]. The role of the evaporation on
the hardening kinetics of the particles plays an important role, provided that the
continuous phase is saturated with the solvent mainly present in the dispersed phase
and that the diffusion rate of the solvent of the dispersed phase in the continuous
phase is fast compared with the solvent evaporation kinetics. In an experimental
study performed with dichloromethane, ethyl acetate, and acetonitrile as solvents,
Wang and Schwendeman demonstrated that the rate-limiting step for mass transport
of solvent depends on the properties of the solvent [43]. Dichloromethane at room
temperature is found to be liquid-side transport limited whereas ethyl acetate and
acetonitrile were gas-side transport limited. As expected, the evaporation rate was
largely affected by the diameter of the impeller, its rotational speed, and the
temperature. The particle’s hardening profile could be determined and predicted
without needing to measure the concentration of polymer in the solvent in time, but
by measuring the concentration of the solvent and by knowing the permeability
coefficient of the solvent at the liquid–air interface [43]. After evaporation of the
solvent, the dispersions can be dialyzed to remove unwanted or low molecular
weight polymer and can be freeze-dried.
One of the most critical properties of nanoparticles is size, hence its control is of
outmost importance. Because the particles are formed from droplets, their size is
largely dependent on the droplet size. In the case of miniemulsions, the size of the
droplets is controlled by the concentration of surfactant [40]. Other parameters such
as the nature of the solvent [46], the stirring rate, or ultrasonication time [47] also
influence the particle size and particle size distribution. Longer and/or stronger
emulsification usually leads to smaller and more narrowly distributed particle size
to a certain extent [47]. However, it is difficult to ascribe an observed effect upon
changing one parameter to this sole parameter, because most of the parameters are
not independent.
Physical processes responsible for the destabilization of emulsions such as
Ostwald ripening and coalescence are of crucial importance for the determination
of the final particle size and size distribution. It is known that the addition of a small
amount of a chemical that is preferentially soluble in the dispersed phase can hinder
the Ostwald ripening process [48]. This chemical, sometimes called the osmotic
pressure agent because it allows the building of an osmotic pressure upon possible
change of chemical composition of the droplets upon Ostwald ripening, is usually a
low molecular weight substance that is insoluble in the continuous phase. Thereby,
it counteracts the Laplace pressure of the droplets and stabilizes the emulsion
droplets. In the solvent evaporation process, no osmotic pressure agent is normally
added as the polymer itself can act as osmotic pressure agent because it is insoluble
in the continuous phase. However, the concentration of polymer must be above a
threshold value to effectively hinder Ostwald ripening [49]. Loxley and Vincent
have supposed that the relatively broad size distribution of the obtained particles is
caused by coalescence [50]. Dynamic light scattering (DLS) was employed to
measure the size of emulsion droplets and the obtained nanoparticles [45]. Based
Recent Advances in the Emulsion Solvent Evaporation Technique for the. . .
333