Biomedical Engineering Reference
In-Depth Information
Positive or normal curvature
p
<1
, H
o
>0
o/w microemulsions
Zero or planar curvature
p
»
1
, H
o
»
0
Bicontinuous microemulsions
Negative or reverse curvature
p
>1
, H
o
<0
w/o microemulsions
Figure 5.3
Formation of w/o, bicontinuous and o/w microemulsions, and their relationship to
p
and
H
0
(adapted from: Israelachvili, 1994; Lawrence and Rees, 2000).
is greater than that of its hydrophobic tail, o/w microemulsions will form. When the head
group has a smaller volume than its hydrophobic tail, w/o microemulsions are preferentially
formed. Bicontinuous structures will prevail when these volumes are similar, resulting in
p
≈
1 (Babak and Stebe, 2002 ; Malmsten, 2002 ) (Figure 5.3 ).
5.9.3 Hydrophile-lipophile balance
The hydrophile-lipophile balance (HLB) of a surfactant typically ranges from 1 to 20 and is
an indication of the relative affinity of a surfactant molecule for the oil or aqueous phase.
A widely used semi-empirical method of calculating the HLB value is:
∑
∑
HLB
=+
7
(hydrophilic group numbers)
−
(hydrophobic group numbers)
(5.4)
Each surfactant has a specific HLB value, which depends on its chemical structure, with a
higher HLB number indicating a higher ratio of hydrophilic to hydrophobic groups and vice
versa (Strey, 1994; McClements, 2004). Surfactants with a low HLB values (about 3-8)
promote w/o microemulsion formation whereas o/w microemulsions will be formed by
surfactants with higher HLB values (about 8-18).
5.9.4 Ingredient compatibility
There is a strong relationship between microemulsion monophasic total area, molecular
complementarity and the interface's flexibility. Compatibility between the alkyl chains of
the surfactant and oil will influence microemulsion formation by strongly affecting the
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