50
3 Sonication in Neoteric Solvents. A Further Look …
them. Indeed, the greater variety of deep eutectic solvents used for the improvement
of the extraction techniques, compared to the organic synthesis, makes it possible
to highlight the will of the experimenters to adapt the extraction agents with respect
to the submitted matrices to the synergistic effect. Binary choline chloride associations can be made with urea, oxalic and lactic acids, glycerol, ethylene glycol or
glycine associated to with different dilution factor in water. These mixtures present
various hydrogen bonding abilities, electrostatic interactions and viscosities able to
improve the extraction efficiency. Thus, the natural deep eutectic solvent (NADES)
are particularly effective for the extraction of polyphenol derivatives in medicinal
substances (Bakirtzi et al. 2016) or solubilization of proteins from gluten (Lores
et al. 2017). Lately, two novel hydrophilic magnetic deep eutectic solvents, based
on trinary mixtures of ChCl/phenol or ethylene glycol/FeCl 3 , were efficiently used
for the microextraction of thiophene proving that extraction is an important field of
investigation for new DES (Khezeli and Daneshfar 2017).
3.5 Conclusion
The synergistic effect of the NS/ultrasound combination has been demonstrated many
times in recent years. It manifests itself by an improvement of the reaction yields
in drastically reduced times and sometimes selectively in milder conditions. Today,
apart from SCF where yet domains of investigation and property tunability are lesser
as compared to ILs and DES, the tunability of ILs and DES is an undeniable property for the development of innumerable applications. According to the operating
parameters of ultrasonic devices, the neoteric solvents are particularly suitable to
replace traditional solvents in a greener approach of chemistry whatever the topic.
Up to date, DES seem more suitable due to the fact that they are little or non-toxic
and biodegradable as compared to ILs although some bio-sourced ones have recently
emerged. In addition, DES are economically more affordable and technically more
accessible as they require a single synthetic step whereas most of ILs require several
synthetic steps with long and fastidious workups. They are also easily reusable in
a simple and effective way. However, hydrophobic ILs can be easily designed what
can be hardly conceived with DES, what further gives yet an edge to the former on
certain applications. Finally, both NS being highly viscous, local hot spots may occur
leading to unexpected decomposition of the medium and a fine-tuning and control of
operating conditions is required. The decomposition of ILs has been already spotted
out in the case of harsh ultrasonic conditions but not yet in the case of DES owing to
their youngness but being as well viscous, it is of probable sight that decomposition
may occur in similar operating conditions.
From a purely ultrasonic point of view, the literature suffers from an excessive use
of cleaning baths compared to sonotrodes and ultrasonic reactors. In this sense, we
urge the community to investigate more with more sophisticated devices to further
explore the tremendous possibilities of the NS/US combination.
3 Sonication in Neoteric Solvents. A Further Look …
them. Indeed, the greater variety of deep eutectic solvents used for the improvement
of the extraction techniques, compared to the organic synthesis, makes it possible
to highlight the will of the experimenters to adapt the extraction agents with respect
to the submitted matrices to the synergistic effect. Binary choline chloride associations can be made with urea, oxalic and lactic acids, glycerol, ethylene glycol or
glycine associated to with different dilution factor in water. These mixtures present
various hydrogen bonding abilities, electrostatic interactions and viscosities able to
improve the extraction efficiency. Thus, the natural deep eutectic solvent (NADES)
are particularly effective for the extraction of polyphenol derivatives in medicinal
substances (Bakirtzi et al. 2016) or solubilization of proteins from gluten (Lores
et al. 2017). Lately, two novel hydrophilic magnetic deep eutectic solvents, based
on trinary mixtures of ChCl/phenol or ethylene glycol/FeCl 3 , were efficiently used
for the microextraction of thiophene proving that extraction is an important field of
investigation for new DES (Khezeli and Daneshfar 2017).
3.5 Conclusion
The synergistic effect of the NS/ultrasound combination has been demonstrated many
times in recent years. It manifests itself by an improvement of the reaction yields
in drastically reduced times and sometimes selectively in milder conditions. Today,
apart from SCF where yet domains of investigation and property tunability are lesser
as compared to ILs and DES, the tunability of ILs and DES is an undeniable property for the development of innumerable applications. According to the operating
parameters of ultrasonic devices, the neoteric solvents are particularly suitable to
replace traditional solvents in a greener approach of chemistry whatever the topic.
Up to date, DES seem more suitable due to the fact that they are little or non-toxic
and biodegradable as compared to ILs although some bio-sourced ones have recently
emerged. In addition, DES are economically more affordable and technically more
accessible as they require a single synthetic step whereas most of ILs require several
synthetic steps with long and fastidious workups. They are also easily reusable in
a simple and effective way. However, hydrophobic ILs can be easily designed what
can be hardly conceived with DES, what further gives yet an edge to the former on
certain applications. Finally, both NS being highly viscous, local hot spots may occur
leading to unexpected decomposition of the medium and a fine-tuning and control of
operating conditions is required. The decomposition of ILs has been already spotted
out in the case of harsh ultrasonic conditions but not yet in the case of DES owing to
their youngness but being as well viscous, it is of probable sight that decomposition
may occur in similar operating conditions.
From a purely ultrasonic point of view, the literature suffers from an excessive use
of cleaning baths compared to sonotrodes and ultrasonic reactors. In this sense, we
urge the community to investigate more with more sophisticated devices to further
explore the tremendous possibilities of the NS/US combination.