7.3.2 HLB systemIn spite of many ad

7.3.2 HLB system
In spite of many advances in the theory of
stability of lyophobic colloids, resort has still
to be made to an empirical approach to the
choice of emulsifier, devised in 1949 by Griffin.
In this system we calculate the hydrophile–
lipophile balance (HLB) of surfactants, which
is a measure of the relative contributions of
the hydrophilic and lipophilic regions of the
molecule. Values of the effective HLB of
surfactant mixtures can be calculated.
The HLB number of a surfactant is calculated according to an empirical formula. For
nonionic surfactants the values range from 0
to 20 on an arbitrary scale (see Fig. 7.12). At
the higher end of the scale the surfactants are
hydrophilic and act as solubilising agents,
detergents and ow emulsifiers. To maintain
stability, an excess of surfactant is required in
the continuous phase; hence, in general,
water-soluble surfactants stabilise ow emulsions and water-insoluble surfactants stabilise
wo emulsions. Oil-soluble surfactants with a
low HLB act as wo emulsifiers. In the stabilisation of oil globules it is essential that there
is a degree of surfactant hydrophilicity to
confer an enthalpic stabilising force and a
degree of hydrophobicity to secure adsorption
at the ow interface. The balance between the
two will depend on the nature of the oil and
the mixture of surfactants; hence the need to
apply the HLB system. The HLB of polyhydric
alcohol fatty acid esters such as glyceryl
monostearate may be obtained from equation
(7.14)
(7.14)
where S is the saponification number of the
ester and A is the acid number of the fatty
acid. The HLB of polysorbate 20 (Tween 20)
calculated using this formula is 16.7, with
S # 45.5 and A # 276.
Typically, the polysorbate (Tween) surfactants have HLB values in the range 9.6–16.7;
the sorbitan ester (Span) surfactants have
HLBs in the lower range of 1.8–8.6.
For those materials for which it is not possible to obtain saponification numbers, for
example beeswax and lanolin derivatives, the
HLB is calculated from
(7.15)
where E is the percentage by weight of oxyethylene chains, and P is the percentage by
weight of polyhydric alcohol groups (glycerol
or sorbitol) in the molecule.
Figure 7.12 The HLB scale and the approximate ranges into which solubilising agents, detergents, emulsifiers and
antifoaming agents fall.
Calculation of HLB of a polysorbate
Polysorbate 20 has a molecular weight of
approximately 1300 and contains 20 oxyethylene groups and two sorbitan rings. Thus,
Hence,
If the hydrophile consists only of oxyethylene
groups (CH 2CH 2O, mol. wt. # 44), a simpler
version of the equation is
(7.16)
giving the upper end of the scale (20) for
polyoxyethelene glycol itself.
Some HLB values of typical surfactants used
in pharmacy are given in Table 7.2. A more
detailed list is given in Tables 6.7 and 6.8.
Group contribution
The HLB system has been put on a more quantitative basis by Davies, who calculated group
contributions (group numbers) to the HLB
number such that the HLB was obtained from
(7.17)
Some group numbers are given in Table 7.3.
Choice of emulsifier or emulsifier mixture
The appropriate choice of emulsifier or emulsifier mixture can be made by preparing a
series of emulsions with a range of surfactants
of varying HLB. It is assumed that the HLB of a
mixture of two surfactants containing fraction
f of A and (l 0 f) of B is the algebraic mean of
the two HLB numbers:
(7.18)
For reasons not explained by the HLB
system, but from other approaches, mixtures
of high HLB and low HLB give more stable
emulsions than do single surfactants. Apart
from the possibility of complex formation
at the interface, the solubility of surfactant
components in both the disperse and the
continuous phase maintains the stability of
the surfactant film at the interface from the
reservoirs created in each phase. In the experimental determination of optimum HLB,
creaming of the emulsion is observed and is
taken as an index of stability. The system with
the minimum creaming or separation of
phases is deemed to have an optimal HLB. It is
therefore possible to determine optimum HLB
numbers required to produce stable emulsions
of a variety of oils. Table 7.4 shows the
required HLB of surfactants to achieve stability
of five oils. A more sensitive method would be
to determine the mean globule size in emulsions using modern techniques such as laser
diffraction methods to produce data such as
those in Fig. 7.13. For the mineral oil-in-water
emulsion stabilized by a mixture of two
Figure 7.13 Variation of mean globule size in a mineral
oil-in-water emulsion as a function of the HLB of the
surfactant mixtures present at a level of 2.5%. Surfactants:
Brij 92–Brij 96 mixtures.
Source: P. Depraetre, M. Seiller, A. T. Florence and F. Puisieux (unpublished)
nonionic surfactants, an optimal HLB of
between 7.5 and 8 is identified.
At the optimum HLB the mean particle size
of the emulsion is at a minimum (Fig. 7.13)
and this factor would explain to a large extent
the stability of the system (see equations 7.1
and 7.2, for example).
Although the optimum HLB values for
forming ow emulsions are obtained in this
way, it is possible to formulate stable systems
with mixtures of surfactants well below the
optimum. This is sometimes because of the
formation of a viscous network of surfactant
in the continuous phase. The high viscosity of
the medium surrounding the droplets prevents their collision and this overrides the
influence of the interfacial layer and barrier
forces due to the presence of the adsorbed
layer.
The HLB system has several drawbacks. The
calculated HLB, of course, cannot take account
of the effect of temperature or that of additives. The presence in emulsions of agents
which salt-in or salt-out surfactants will.
respectively increase and decrease the effective
(as opposed to the calculated) HLB values.
Salting-out the surfactant (for example, with
NaCl) will make the molecules less hydrophilic and one can thus expect a higher
optimal calculated HLB value for the stabilising surfactant for ow emulsions containing
sodium chloride. Examples are shown in
Fig. 7.14 in which the effects of NaCl and NaI
are compared.