Chemistry Reference
In-Depth Information
O-alkyl glycoside hydrotropes are solids and are usually considered
with a C
8
alkyl-chain, because in the past, shorter and longer homologues
have proven to be less ecient.
9
The isotropic region extends up to high
concentrations for octyl glucoside (ca. 60% wt.) but liquid crystalline
phases do form at higher concentrations, just as conventional sur-
factants.
94
Shorter homologues might not form liquid crystalline phases
at all but may be too hydrophilic because of the high anity of the sugar
polar head for water (look at the MHC of butyl glucoside). However, amyl
xyloside
93
and more recently heptyl glucosides
45
have been presented as
e
cient hydrotropes.
3.3 Interaction with surfactants
Another important property of hydrotropes is their ability to interact with
surfactants and modify their association in water. Particularly, hydro-
tropes increase the cloud point of ethoxylated surfactants, i.e. the tem-
perature at which the surfactant solution separates into two immiscible
phases. Hydrotropes are also able to destabilise and eventually break
down the liquid crystalline phases formed by the surfactants at higher
concentrations and thus promote fluid formulations.
79
The effects of ethanol, monopropylene glycol (MPG), ethylene glycol
monobutyl ether (C
4
E
1
) and dimethylisosorbide (DMI) on the aqueous
behaviour of a commercial polyethoxylated surfactant (Synperonic A7,
C
13-15
E
7
) was studied.
61
Figure 12 shows the modification of the extent of
the liquid crystalline phases formed by the surfactant upon addition of
the various hydrotropes. In pure water, the polyethoxylated surfactant
forms a hexagonal phase (H
1
) from approximately 30% to 58 % wt., up to
37 1C, and a large lamellar phase (La) from 58 % to 85% wt. that is stable
up to more than 80 1C. C
4
E
1
and Ethanol proved to be the most ecient
in destructuring the liquid crystalline phases formed by the poly-
ethoxylated surfactant, since 10% and 12% wt. solutions respectively
prevent the formation of any liquid crystal, whereas higher amounts of
DMI or MPG are required to achieve an isotropic solution over the whole
concentration range (20% and 22% wt. respectively).
Fig. 12 Destructuration of the liquid crystal regions in the partial ternary diagram Syn-
peronic A7 (C
13-15
E
7
)/Water/Hydrotrope at 25
1
C. L
a
=
Lamellar phase, H
1
=
Hexagonal
phase. DMI
=
dimethylisosorbide, MPG
=
mono propylene glycol, C
4
E
1
=
ethylene glycol
monobutyl ether.
61
Reprinted with permission of J. Surfactants Deterg., a publication of
AOCS press.
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