HLB Concept: A Way to Never Forget It
Abstract
This mini-review discusses the physicochemical parameters related to
the formation of emulsified systems and the role of surfactants on this
process. To this regard, several considerations about the chemical
properties of surfactants (such as hydrophobicity and lipophilicity) and
their
hydrophilic-lipophilic balance (HLB) were discussed. The main idea of
this expert opinion is to show that surfactants with high HLB value are
predominantly hydrophilic, whereas low HLB scores indicate the
lipophilic net of the surfactants. Thus, based on the most relevant
aspects, a simple
and easy way to understand the HLB concept is suggested.
Text Body
Emulsions are dispersed systems made of the mixture of two
immiscible liquid phases stabilized by surfactants. They have been
used in several areas (specially in the food and pharmaceutical
industries) due to their advantages, such as the ability to deliver
drug molecules (commonly lipophilic drugs) as well as to
improve organoleptic and biopharmaceutical characteristics of
drugs and other bioactive compounds [1,2]. Although several
theories have been hypothesized to explain the chemical and
physicochemical mechanisms and interactions involved in their
formation, no universal emulsification theory has been reported
[2]. Furthermore, different types of emulsifying agents can be
used in emulsions to improve their stability according to several
physicochemical principles [3]. However, despite the knowledge
about the organization of oil droplets and surfactants in the
emulsified systems and considering the mechanisms involved on
the interfacial tension decrease, several emulsification theories are
reported, which makes it difficult for a scientific agreement about
the overall phenomena involved in the emulsion formation [2].
In this context, the interfacial tension can be understand as
the resistance of immiscible liquids in mixing themselves with
one another, preventing one to self-disperse in another in the
form of droplets. Moreover, surfactants are amphiphilic molecules
that can be used to reduce the interfacial tension, enabling the
dispersion of such liquids in themselves in the form of droplets,
providing a physicochemical stability to the emulsion and reducing
the occurrence of instability phenomena such as the flocculation
and coalescence of droplets [4]. Concerning the physicochemical
characteristics and chemical composition of the emulsions, these
dispersed systems stand out as an important technological strategy
on the development of new medicines in the biomedical area,
allowing the improvement of drug bioavailability and the decrease
of their side effects. However, due to the immiscibility of the oily
and aqueous phases, the correct choice of surfactants remains as a
critical step in the emulsion development [5-7].
Indeed, the knowledge about hydrophilic-lipophilic balance
(HLB) is an essential parameter related to the emulsion production
because it allows the selection of a suitable surfactant or even a
surfactant blend required to produce a stable emulsified system.
Historically, the HLB concept proposed by Griffin in 1949 showed
itself as one of the most important tools to develop and stabilize
emulsified systems [4,8], especially when before 1950, the
surfactant choice was carried out empirically, through trial-anderror
steps. In fact, Griffin’s HLB System was an attempt to systemize
the choice of a suitable surfactant, and as a result, the methodology
to prepare stable emulsions became more rational [4]. In Griffin’s
theory, surfactants and oily phases are classified in terms of HLB
values required for an emulsification process, which would produce
a stable emulsified system [6,7]. Thus, each surfactant presents one
intrinsic number, its HLB value, which represents its hydrophiliclipophilic
inherent property, which can range from 1 up to 40 [6,7].
Accordingly, 10 is the key number that divides the lipophilicity
and hydrophobicity character of each surfactant. Therefore, surfactants
with HLB value between 1 and 10 are considered
lipophilic and the ones with HLB ranging between 10 and 40 are
considered hydrophilic. In addition, the use of a blend of two or more
surfactants with different HLB values is a common practice required
to prepare more stable emulsified systems [6,7,9,10]. Indeed, based
on Griffin’s HLB concept, a mixture of immiscible liquids (such as
oil and water) will only result in a stable emulsion with fluidity and
milky characteristics if a surfactant or a blend of surfactants HLB
value reaches an optimal score, which is called critical HLB (cHLB)
and depends on the physicochemical properties of the oily phase
[6-8]. By way of explanation, the surfactant´s HLB scores and the
cHLB of the oily phase of the emulsions are highly correlated, since
only surfactant systems with HLB values approximate to the oily
phase cHLB are able to be located at the oil-in-water interface of the
emulsified systems and, by reaching this requirement, they allow
the formation of a metastable emulsion [4].
On the other hand, a surfactant or a blend at lower or higher
HLB scores than the oil cHLB will be dispersed in one of the
emulsion phases rather than at the interface, allowing, at the
first moment, the production of a system with a milky aspect,
but with poor physicochemical stability and high susceptibility to
instability phenomena. Currently, other parameters, such as zeta
potential, droplet size, presence of physical stabilizers (responsible
for increasing viscosity), temperature, surfactant concentration
and the surfactant electrical charge have been described and
widely studied in the emulsion stabilization process, showing
that although highly relevant, the HLB and cHLB are not the only
parameters involved in this process. However, the correct use of
the HLB and cHLB parameters reduces time of preformulation and
the amount of pharmaceutical excipients once further stabilization
adjustments might be unnecessary. Thus, it is important to highlight
that HLB and cHLB are still the first and most common parameters
to be studied on the development of emulsified systems, allowing
the obtaining of stable formulations suitable for manufacturing by
pharmaceutical companies
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