Surfactants with the proper balance of hydrophilic and lipophilic affinities are effective emulsifying agents, since they concentrate at the oil–water inter-face, while being present in the two phases (oil and water) in different concentrations.
Hydrophile–lipophile
balance system
In
1949, Griffin devised an arbitrary scale of values to serve as a measure of
relative contributions of the hydrophilic and lipophilic regions of a
surfac-tant to its overall hydrophilic/lipophilic character, which could be
used to select emulsifying agents for a given application. This system is now
widely known as the hydrophile–lipophile
balance (HLB) system. The higher the HLB value of an emulsifying agent, the
more hydrophilic it is. The emul-sifying agents with lower HLB values are less
polar and more lipophilic.
The
HLB values of some commonly used surfactants are listed in Table 10.2. The Spans, that is, sorbitan esters, are
lipophilic and have low HLB val-ues (1.8–8.6); the Tweens, poly(oxyethylene)
derivatives of the Spans, are hydrophilic and have high HLB values (9.6–16.7). Figure 10.2 illustrates
Table 10.2 HLB values of commonly used surfactants
Figure 10.2 A scale showing surfactant function on the basis of HLB values.
Surfactants
with the proper balance of hydrophilic and lipophilic affinities are effective
emulsifying agents, since they concentrate at the oil–water inter-face, while
being present in the two phases (oil and water) in different con-centrations.
Thus, a lipophilic surfactant would have higher concentration in oil, while a
hydrophilic surfactant would have higher concentration in water. The phase with
higher surfactant concentration tends to become the external phase in an
emulsion. Thus, the HLB of a surfactant, or a combi-nation of surfactants,
determines whether an o/w or w/o emulsion results. An emulsifying agent with
high HLB is preferentially soluble in water and results in the formation of an
o/w emulsion. The reverse situation is true with surfactants of low HLB value,
which tend to form w/o emulsions. In general, o/w emulsions are formed when the
HLB of the emulsifier is ~9–12, and w/o emulsions are formed when the HLB is
~3–6.
The
lipid phase used in an emulsion can be assigned a required HLB (or RHLB)
value. The RHLB for the lipid phase
of an emulsion is the HLB value of
the surfactant that provides the lowest interfacial tension between the two
phases to form an o/w or a w/o emulsion. The RHLB, thus, provides guidance to
surfactant selection for a specified lipid phase for the formation of a stable
emulsion. The RHLB may be experimen-tally determined by preparing a series of
emulsions with surfactants of different HLB values and selecting the HLB value
that resulted in the physically most stable emulsion, as assessed, for example,
by the separa-tion of phases on undisturbed storage of the emulsion. A list of
RHLB values for common emulsifying agents is usually available in the
litera-ture (Table 10.3).
Table 10.3 Required HLB for some oil-phase ingredients for making o/w and w/o
emulsions
This
value is utilized in the HLB concept to prepare an emulsion by selecting an
emulsifier that has the same, or nearly the same, HLB value as the RHLB of the
oleaginous phase of the intended emulsion. For example, mineral oil has an
assigned RHLB value of 4 if a w/o emulsion is desired, and it has a value of
10.5 if an o/w emulsion is desired.
The
HLB values are additive. Therefore, calculation of the required HLB of a
formulation is done by weighting the RHLB of each oil-phase ingredient
(excluding any emulsifiers) as a weight percentage of the total oil-phase
ingredients. For example, if the oil-phase ingredients of an o/w emulsion
consist of 10% mineral oil, 3% capric/caprylic triglyceride, 2.5% isopropyl
myristate, 4% cetyl alcohol, and the remaining emulsifiers, water,
preser-vative, sweeteners, flavors, and colorants, the percentage oil-phase
ingredi-ents in the formulation would be calculated as 10 + 3 + 2.5 + 4 =
19.5%. The RHLB of the oil phase for a desired o/w emulsion would be calculated
as follows:
Thus,
the RHLB of the oil phase is 5.4 + 0.8 + 1.5 + 3.2 = 10.9. Hence, this
formulation would require the use of an emulsifier, or a combination of emulsifiers,
which should have the HLB of 10.9 to make an optimum physically stable o/w
emulsion.
The
HLB value of a surfactant reflects a one-fifth fraction of the hydro-philic
portion of the surfactant on a molecular-weight basis. For example, for
calculating the HLB value of 22 moles of ethoxylate of oleyl alcohol, the
molecular weight of 22 moles of ethylene oxide [−CH2−O−CH2−],
with the repeating unit molecular weight of 44, is calculated to represent the
hydro-philic portion of a surfactant. Thus, 22 × 44 = 968. This mass is added
to the molecular weight of oleyl alcohol, 270, to get the total molecular
weight of the surfactant. Thus, 968 + 270 = 1238. The percentage molecular
weight of the surfactant that is hydrophilic is, therefore, 968/1238 × 100 =
77%.
Taking
a one-fifth fraction, the HLB of this surfactant would be 77/5 = 15.4.
The
HLB values are assigned only to nonionic surfactants. Thus, the HLB values are
generally in the range of 0.5–19.5. Nevertheless, the HLB values of ionic
surfactants are provided in the literature as an indication of their relative
hydrophilicity. Some HLB values are listed in Table
10.2. Thus, an ionic surfactant with an HLB value of 40 simply indicates
that it is highly hydrophilic.
Frequently,
a combination of two or more surfactants, usually with different molecular
weight/size, is used instead of just one surfactant. A combination of
surfactants ensures better packing at the interface and greater physical
stability of the emulsion. In using a combination of surfactants, their HLB
values are additive. Thus, the HLB value of a combination of surfactants is the
weighted average of the HLB of each surfactant. For example, if 50% each of
Span 20 and Span 80 were mixed together, the HLB of their com-bination would be
50/100 × 8.6 + 50/100 × 4.3 = 6.45. Similarly, the use of 90% Span 80 and 10 %
Span 20 would give combined HLB value of 4.7, which is the same HLB value as
that of Span 60. However, the use of Span 20 + Span 80 is expected to give a
more stable emulsion than Span 60 in the same quantities.
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