42
3 Sonication in Neoteric Solvents. A Further Look …
molecules remain as insuperable obstacles. In addition, its recycling is a highly
energy process. In parallel, the birth of Green Chemistry concept has allowed the
emergence of new types of eco-friendly solvents. This new generation of solvents is
often called neoteric solvents (neoteric comes from Greek ‘ne¯ oterikos’ for newer).
These latter, also named as second-generation solvents, can possibly offer flexible
physical–chemical properties to chemists and engineers to develop specific reaction
processes with optimized conditions. The simultaneous existence of several classes
of neoteric solvents underlines that there cannot be a single alternative to replace so
many organic molecular solvents. Indeed, these latter display different physical—
chemical properties in so many cases of different organic synthetic reactivity, and
the solution lies rather in a set of flexible alternatives.
As, for example, ionic liquids and DES exhibit some remarkable physical–chemical properties originating their success. However, both also display much higher
viscosity as compared to molecular organic solvents which can be a major obstacle
for R&D development since viscosity is a brake for chemical reactivity, molecular
diffusion, kinetics, etc. To overcome this, ultrasound, in particular low-frequency one
with enhanced mechanical effects, has been quickly approached to be coupled with
the use of these NS. The addition of a co-solvent to decrease viscosity values has been
approached but subsequent changes in the intrinsic properties of the resulting medium
lead to inevitable differences in chemical behaviour. Similarly, SCF possess some
remarkable intrinsic properties and are subsequently highly investigated notably in
the field of extraction. However, the absence of any kinds of mechanical effects rises
several problems when target compounds are trapped in a complex matrix. The usage
of ultrasound helps to improve greatly extraction efficiency notably through the harsh
mechanical effects generated by low-frequency ultrasound. In truth and in practical,
NS have become interdisciplinary fields utilized by chemists, chemical engineers,
physicists and specialized scientists (food, agronomy, biotechnologies, etc.), and it
is certain that the use of these new media will be intensified in a near future in both
science and engineering laboratories.
3.2 The Three Most Promising Classes of Neoteric Solvents
3.2.1 Supercritical Fluids (SCF)
Supercritical fluids (SCF) are considered as the ‘fourth’ state of the matter without
having a definite phase. A French Scientist, Baron Charles Cagniard de la Tour, has
discovered this state in 1822 but the term of SCF is from Thomas Andrew, an Irish
chemist. The first ever experiment with SCF was performed in 1879 when Hannay
and Hogarth studied the solubility of several halide-based metallic salts (CoCl 3 ,
FeCl 3 , KBr, etc.) in SC ethanol to probe solvating ability of these fluids. The state
of supercritical is reached when a fluid is heated and compressed above his critical
point. SCF have a particular behaviour at midway from the liquid and gas states
3 Sonication in Neoteric Solvents. A Further Look …
molecules remain as insuperable obstacles. In addition, its recycling is a highly
energy process. In parallel, the birth of Green Chemistry concept has allowed the
emergence of new types of eco-friendly solvents. This new generation of solvents is
often called neoteric solvents (neoteric comes from Greek ‘ne¯ oterikos’ for newer).
These latter, also named as second-generation solvents, can possibly offer flexible
physical–chemical properties to chemists and engineers to develop specific reaction
processes with optimized conditions. The simultaneous existence of several classes
of neoteric solvents underlines that there cannot be a single alternative to replace so
many organic molecular solvents. Indeed, these latter display different physical—
chemical properties in so many cases of different organic synthetic reactivity, and
the solution lies rather in a set of flexible alternatives.
As, for example, ionic liquids and DES exhibit some remarkable physical–chemical properties originating their success. However, both also display much higher
viscosity as compared to molecular organic solvents which can be a major obstacle
for R&D development since viscosity is a brake for chemical reactivity, molecular
diffusion, kinetics, etc. To overcome this, ultrasound, in particular low-frequency one
with enhanced mechanical effects, has been quickly approached to be coupled with
the use of these NS. The addition of a co-solvent to decrease viscosity values has been
approached but subsequent changes in the intrinsic properties of the resulting medium
lead to inevitable differences in chemical behaviour. Similarly, SCF possess some
remarkable intrinsic properties and are subsequently highly investigated notably in
the field of extraction. However, the absence of any kinds of mechanical effects rises
several problems when target compounds are trapped in a complex matrix. The usage
of ultrasound helps to improve greatly extraction efficiency notably through the harsh
mechanical effects generated by low-frequency ultrasound. In truth and in practical,
NS have become interdisciplinary fields utilized by chemists, chemical engineers,
physicists and specialized scientists (food, agronomy, biotechnologies, etc.), and it
is certain that the use of these new media will be intensified in a near future in both
science and engineering laboratories.
3.2 The Three Most Promising Classes of Neoteric Solvents
3.2.1 Supercritical Fluids (SCF)
Supercritical fluids (SCF) are considered as the ‘fourth’ state of the matter without
having a definite phase. A French Scientist, Baron Charles Cagniard de la Tour, has
discovered this state in 1822 but the term of SCF is from Thomas Andrew, an Irish
chemist. The first ever experiment with SCF was performed in 1879 when Hannay
and Hogarth studied the solubility of several halide-based metallic salts (CoCl 3 ,
FeCl 3 , KBr, etc.) in SC ethanol to probe solvating ability of these fluids. The state
of supercritical is reached when a fluid is heated and compressed above his critical
point. SCF have a particular behaviour at midway from the liquid and gas states