Sugar-based hydrotropes: preparation, properties and applications - Carbohydrate Chemistry: Chemical and Biological Approaches - page 54

Finally, solvo-surfactants are slow-evaporating solvents that are widely
used in paints and coatings 35,36 in which they act as co-solvents, diluents
or coalescing agents.
2 Sugar-based hydrotropes
2.1 Available carbohydrates and polyols for the design of hydrotropes
The amphiphilic nature of hydrotropes supposes to combine a hydro-
phobic moiety to a building-block that brings hydrophilicity and ensures
water-solubility. As will be developed in paragraph 3, the e ciency is
directly linked to the volume of the hydrophobic part, therefore when
designing a hydrotrope, one should look for the maximum hydro-
phobicity that the chosen polar group can balance.
The currently available hydrophilic building-blocks coming from
renewable resources are well known, and are basically those that are
currently used to prepare bio-based surfactants. A recent review by Foley
et al. on this topic is available. 37 Indeed, even if a large number of natural
or synthetic carbohydrates and polyols can be considered, only a few are
currently available in sucient quantity and at reasonable prices to stand
as starting materials for the synthesis of large-scale commodity
chemicals such as surfactants and hydrotropes. As for now, the main
molecules that fulfil these criteria are D -glucose, sorbitol and its
dehydration products (sorbitans and isosorbide), sucrose and glycerol.
This latter is obtained as a by-product of the vegetable oil processing in
biodiesel. However, it should be kept in mind that the development of
biorefineries currently concentrates enormous research efforts com-
bining chemistry, catalysis and biotechnology, both in the academic and
industrial communities, which should lead rapidly to the emergence of
novel hydrophilic building blocks available at the industrial scale.
Figure 2 presents a schematic route from biomass to well-defined
building blocks. The main chemical biomass feedstocks are poly-
saccharides, since cellulose and hemicellulose (ligno-cellulosic biomass)
account respectively for 50% and 24% of the total biomass world-
wide. The processing of this ligno-cellulosic material to well-defined
molecules is at the heart of the development of biorefineries. Pentoses
( D -xylose, L -arabinose) can already be obtained by hydrolysis of the
hemicellulose contained in wheat or corn by-products and are used for
the manufacture of surfactants but the processes are still costly and
energy demanding. 38 On the contrary, even if it represents only 1% of
the total biomass on earth, starch is easily hydrolyzed - chemically or
preferably enzymatically - to D -glucose that is then processed to a range of
well-defined chemicals. Sucrose and vegetable oils are also used as starting
materials for chemistry even if they each represent only 0.1% of the total
biomass worldwide.
As the main processed biomass feedstocks are polysaccharides, the
available building blocks are currently sugars and sugar derivatives
(polyols, organic acids obtained by fermentation), as indicated by solid
frames in Fig. 2. These building-blocks can be further transformed into
chemicals of interest 39 and the polyfunctional hydrophilic ones could act