Final Report Summary - MINE (Molecular Interfacial structure and dynamics of Nanoscopic droplets in Emulsions (MINE))
Emulsions consist of one liquid dispersed as nanoscopic droplets in another liquid, such as milk, and butter. The understanding of the structure and stability of emulsions is commonly obtained from empirical studies in which a macroscopic parameter is varied and properties pertaining to the droplets are measured (such as size, turbidity or electrokinetic mobility).
Since the work of Irving Langmuir and others (published in 1917) it is well established that the stability and properties of these nanoscopic droplets are strongly influenced by the state of the droplet interface. However, despite the abundance and importance of emulsions in our daily lives, the molecular structures and interactions that dictate the stability and properties of emulsions are still unknown. This lack of insight is caused by the system itself: the surrounding medium forms an impenetrable barrier to most molecular probes.
MINE was used to bring surface molecular understanding to the world of emulsions. By further developing sum frequency scattering we have been able to device a technique that can generate vibrational spectra and perform vibrational dynamics measurements of all compounds on the 1-2 molecular thick interfacial layer between the bulk oil droplet and the bulk water, with great accuracy. Subsequently we found, among many other things, a few very surprising results:
Charged amphiphiles structure differently and at much lower density on nanoscopic hydrophobic/aqueous interfaces than on identically composed planar interfaces.
Positive and negatively charged surfactants and salts have different influences on the interfacial structure: The negatively charged ones enhance water ordering on the water side of the interface and do not leave any measurable distortions in the oil phase. In contrast, positively charged surfactants reduce the water ordering at the interface and increase the disorder in the oil phase. These effects appear to be general for ions as well, indicating that cations are more hydrophobic than anions. The stabilization mechanism is thus different for both classes of surfactants and what is more, only a small amount of charged surfactant is present on the surface.
In addition, neat oil water droplet interface carry a net negative charge that is most commonly attributed to the strong and favorable adsorption of hydroxide ions (although the presence of hydroxide ions on the surface has not been confirmed experimentally). Despite our detection limit of ~ 0.02 charges/nm2 for adsorbed anions, we have not been able to detect surface hydroxide ions. Although this is in agreement with recent other spectroscopic evidence the question remains why the interface is negatively charged. We propose that charge transfer effects in the water phase may be responsible and could be consistent with the data.
Since the work of Irving Langmuir and others (published in 1917) it is well established that the stability and properties of these nanoscopic droplets are strongly influenced by the state of the droplet interface. However, despite the abundance and importance of emulsions in our daily lives, the molecular structures and interactions that dictate the stability and properties of emulsions are still unknown. This lack of insight is caused by the system itself: the surrounding medium forms an impenetrable barrier to most molecular probes.
MINE was used to bring surface molecular understanding to the world of emulsions. By further developing sum frequency scattering we have been able to device a technique that can generate vibrational spectra and perform vibrational dynamics measurements of all compounds on the 1-2 molecular thick interfacial layer between the bulk oil droplet and the bulk water, with great accuracy. Subsequently we found, among many other things, a few very surprising results:
Charged amphiphiles structure differently and at much lower density on nanoscopic hydrophobic/aqueous interfaces than on identically composed planar interfaces.
Positive and negatively charged surfactants and salts have different influences on the interfacial structure: The negatively charged ones enhance water ordering on the water side of the interface and do not leave any measurable distortions in the oil phase. In contrast, positively charged surfactants reduce the water ordering at the interface and increase the disorder in the oil phase. These effects appear to be general for ions as well, indicating that cations are more hydrophobic than anions. The stabilization mechanism is thus different for both classes of surfactants and what is more, only a small amount of charged surfactant is present on the surface.
In addition, neat oil water droplet interface carry a net negative charge that is most commonly attributed to the strong and favorable adsorption of hydroxide ions (although the presence of hydroxide ions on the surface has not been confirmed experimentally). Despite our detection limit of ~ 0.02 charges/nm2 for adsorbed anions, we have not been able to detect surface hydroxide ions. Although this is in agreement with recent other spectroscopic evidence the question remains why the interface is negatively charged. We propose that charge transfer effects in the water phase may be responsible and could be consistent with the data.