Year: 2023
Type: article
Abstract: When water is added to an anise-flavored drink of choice (for example, ouzo, arak, pastis, or raki) the clear liquid turns cloudy. Scientists have tried to explain the mechanism of this phenomenon by numerous indirect techniques such as X-ray scattering and light-scattering. A team, including Maria Vratsanos and Nathan Gianneschi, used the recently developed technique of liquid-phase transmission electron microscopy (LPTEM) to study the phenomenon directly. The distinctive licorice flavor is due to the molecule trans-anethole, an extract of anise, an annual herb of the parsley family. When a drink containing this extract is diluted with an appropriate amount of water, it becomes opaque due to the precipitation of trans-anethole droplets, an oil that is not soluble in water. This effect is generalized for systems with three components, in which one component, solvent (A), is soluble in both of two other solvents (B and C), but B and C do not mix with each other. For example, when B is added to a mixture of A and C, B mixes with A, forcing C to precipitate. The precipitate of C thereby forms an opaque emulsion. Although this phenomenon has been observed for a long time, it was named the “ouzo effect” only 20 years ago. Since then, it has attracted scientific attention because it is a low-energy emulsification surfactant-free process that can be used for various industrial processes, such as encapsulating drugs. The ouzo effect results in small, homogeneous droplets that are nearly identical to each other. This effect has the potential to replace current methods of producing such droplets that require surfactants, which may cause harm to the environment. LPTEM is an imaging system that enables the visualization of processes occurring in a fluid environment in real time by sandwiching an ultrathin layer of the liquid specimen between electron-transparent thin films. This isolates the specimen from the high-vacuum environment inside a TEM. Vratsanos et al. used brightfield optical and fluorescence microscopy and compared results to those using LPTEM. They concluded that the dark droplets in the LPTEM images are trans-anethole-rich regions. Closer inspection of TEM micrographs revealed internal structuring in the droplets (Figure), which have not been seen in previous investigations of emulsion formulations. This structure within droplets may be significant. Structure of trans-anethole droplet formed by the ouzo effect visualized via LPTEM. A, raw micrograph. B, the same image with minimal processing and a false color filter applied to enhance visualization. Scale bars = 2 µm. After establishing the fact that these trans-anethole droplets are formed, Vratsanos et al. could investigate the formation of the phase separation, since the liquid cell holder used in LPTEM allows imaging of a flowing solution. After carefully adjusting the system so that the reaction would be accurately timed, they found the observed nucleation of the trans-anethole droplets occurred within 30 minutes, which is what they expected. After making many other observations, Vratsanos et al. concluded that LPTEM provides an important tool for examining soft, even liquid, systems that are undergoing dynamic processes and changes in state, such as emulsification. Such examination has the potential to improve systems for synthesizing compounds, such as drugs, that are becoming increasingly important in our daily lives.
Source: Microscopy Today
Authors: Stephen W Carmichael, Philip Oshel
Institutions: Mayo Clinic, Central Michigan University
Language: English
Subfield: Physical and Theoretical Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W4384938564