1.3 Homogeneous Nucleation of Solutes from Supersaturated Solutions
21
When water is added to a Greek liquor, Ouzo, a milky emulsion spontaneously
forms. This phenomenon is known as the “Ouzo effect” and is an example of spontaneous emulsification. Ouzo can be described as a dilute oil solution in ethanol as
a first approximation (there are myriad of other minor components that add flavor).
The physical mechanism of spontaneous emulsification is fairly straightforward: for
three components of oil, ethanol, and water, ethanol is fully miscible with each of
oil and water, whereas oil and water are sparingly soluble with each other. Thus,
when a small volume of oil-in-ethanol solution is added to a large volume of water,
ethanol diffuses through water and the solubility of the oil in the increasingly diluted
ethanol-in-water solution progressively falls. Eventually, the solubility limit of the oil
is reached and the oil precipitates out of the supersaturated solution [13]. Essentially,
the same mechanism is at work when a small volume of water-in-ethanol solution is
added to a large volume of oil: ethanol diffuses through oil and the solubility of the
water in the increasingly diluted ethanol-in-oil solution progressively falls. Eventually, the solubility limit of water is reached and the water precipitates out of the
supersaturated solution. Thus, spontaneous emulsification offers a convenient way
of dispersing oil in water, or dispersing water in oil, without the use of surfactants
or vigorous stirring [13].
These oil droplets in water (or water droplets in oil), once formed, can be somewhat
stabilized due to the presence of ethanol. Presumably, ethanol, being amphiphilic
(ethanol is known to act as co-surfactants), adsorbs to the oil–water interface. Like
all kinetically stabilized emulsions, they will eventually phase separate. Below we
only consider the nucleation involved in the formation of oil-in-water emulsions.
It has traditionally been considered that only three components are in play in
spontaneous emulsification. However, this turned out to be not quite true. Ubiquitous dissolved atmospheric gases, mainly nitrogen, are usually present during the
spontaneous emulsification. This is a fact that has largely been overlooked but there
is a good reason to believe that the presence of such ubiquitous dissolved atmospheric gases can affect the spontaneous emulsification. First of all, the amount of
ubiquitous dissolved atmospheric gases is substantial under atmospheric pressure,
as might be expected from Henry’s law—their concentration in water at the standard
temperature and pressure is of the order of 1 mM [14]. This concentration becomes
higher in the liquids that are involved in the spontaneous emulsification process: oil,
ethanol, and their solutions. Then, for a low concentration of oil, for example, 10 μM,
there would be 100 times more atmospheric gas molecules than the oil molecules. A
typical mechanical vacuum pump can reduce the amount of dissolved atmospheric
gases from about 1 mM to about 100 nM. This reduction corresponds to, in relative
terms, from about 10000% to about 1% of nitrogen molecules per oil molecule. If
such large amounts of any (unintended) substance were present in a sample, they
would surely have been treated as impurities. However, no such due diligence has
been exercised for ubiquitous dissolved atmospheric gases.
Against this backdrop, Sowa et al. investigated the effect of such ubiquitous atmospheric gases on the homogeneous nucleation in spontaneous emulsification [15].
It turned out, perhaps surprisingly, there was a significant difference between the
21
When water is added to a Greek liquor, Ouzo, a milky emulsion spontaneously
forms. This phenomenon is known as the “Ouzo effect” and is an example of spontaneous emulsification. Ouzo can be described as a dilute oil solution in ethanol as
a first approximation (there are myriad of other minor components that add flavor).
The physical mechanism of spontaneous emulsification is fairly straightforward: for
three components of oil, ethanol, and water, ethanol is fully miscible with each of
oil and water, whereas oil and water are sparingly soluble with each other. Thus,
when a small volume of oil-in-ethanol solution is added to a large volume of water,
ethanol diffuses through water and the solubility of the oil in the increasingly diluted
ethanol-in-water solution progressively falls. Eventually, the solubility limit of the oil
is reached and the oil precipitates out of the supersaturated solution [13]. Essentially,
the same mechanism is at work when a small volume of water-in-ethanol solution is
added to a large volume of oil: ethanol diffuses through oil and the solubility of the
water in the increasingly diluted ethanol-in-oil solution progressively falls. Eventually, the solubility limit of water is reached and the water precipitates out of the
supersaturated solution. Thus, spontaneous emulsification offers a convenient way
of dispersing oil in water, or dispersing water in oil, without the use of surfactants
or vigorous stirring [13].
These oil droplets in water (or water droplets in oil), once formed, can be somewhat
stabilized due to the presence of ethanol. Presumably, ethanol, being amphiphilic
(ethanol is known to act as co-surfactants), adsorbs to the oil–water interface. Like
all kinetically stabilized emulsions, they will eventually phase separate. Below we
only consider the nucleation involved in the formation of oil-in-water emulsions.
It has traditionally been considered that only three components are in play in
spontaneous emulsification. However, this turned out to be not quite true. Ubiquitous dissolved atmospheric gases, mainly nitrogen, are usually present during the
spontaneous emulsification. This is a fact that has largely been overlooked but there
is a good reason to believe that the presence of such ubiquitous dissolved atmospheric gases can affect the spontaneous emulsification. First of all, the amount of
ubiquitous dissolved atmospheric gases is substantial under atmospheric pressure,
as might be expected from Henry’s law—their concentration in water at the standard
temperature and pressure is of the order of 1 mM [14]. This concentration becomes
higher in the liquids that are involved in the spontaneous emulsification process: oil,
ethanol, and their solutions. Then, for a low concentration of oil, for example, 10 μM,
there would be 100 times more atmospheric gas molecules than the oil molecules. A
typical mechanical vacuum pump can reduce the amount of dissolved atmospheric
gases from about 1 mM to about 100 nM. This reduction corresponds to, in relative
terms, from about 10000% to about 1% of nitrogen molecules per oil molecule. If
such large amounts of any (unintended) substance were present in a sample, they
would surely have been treated as impurities. However, no such due diligence has
been exercised for ubiquitous dissolved atmospheric gases.
Against this backdrop, Sowa et al. investigated the effect of such ubiquitous atmospheric gases on the homogeneous nucleation in spontaneous emulsification [15].
It turned out, perhaps surprisingly, there was a significant difference between the