Abstract
The effects of surfactant molecules involved in macro-, mini-, nano-, and microemulsions used in cosmetics and pharmaceuticals are related to their amphiphilic interactions with oil and water phases. Basic ideas on their behavior when they are put together in a system have resulted in the energy balance concept labeled the hydrophilic-lipophilic deviation (HLD) from optimum formulation. This semiempirical equation integrates in a simple linear relationship the effects of six to eight variables including surfactant head and tail, sometimes a cosurfactant, oil-phase nature, aqueous-phase salinity, temperature, and pressure. This is undoubtedly much more efficient than the hydrophilic-lipophilic balance (HLB) which has been used since 1950. The new HLD is quite important because it allows researchers to model and somehow predict the phase behavior, the interfacial tension between oil and water phases, their solubilization in single-phase microemulsion, as well as the corresponding properties for various kinds of macroemulsions. However, the HLD correlation, which has been developed and used in petroleum applications, is sometimes difficult to apply accurately in real cases involving ionic–nonionic surfactant mixtures and natural polar oils, as it is the case in cosmetics and pharmaceuticals. This review shows the confusion resulting from the multiple definitions of HLD and of the surfactant parameter, and proposes a “normalized” Hydrophilic-Lipophilic Deviation (HLDN) equation with a surfactant contribution parameter (SCP), to handle more exactly the effects of formulation variables on the phase behavior and the micro/macroemulsion properties.
Highlights
Surfactants are amphiphilic molecules going to surfaces and interfaces to produce specific effects
It may be said that the hydrophilic-lipophilic balance (HLB) proposal from Griffin was helpful to a certain extent to select a surfactant for a given application, related to emulsion type and stability, but that it was quite inaccurate, because it depended on many other properties such as the surfactant molecular weight [8] and on many of its features such as its chemical formula, the nature of the hydrophilic group and the branching of the lipophilic tail, or some other specificities like the aromaticity of the oil, the presence of electrolyte in water, the usual addition of alcohol and other cosurfactants, the temperature, and the pressure
The definition of Hydrophilic-Lipophilic Deviation (HLDN) indicates that this increase in preferred alkane carbon number (PACN) by one unit can be compensated to keep the optimum with an alcohol ethoxylate system by an increase of one unit in the oil alkane carbon number (ACN), a decrease of ~0.5 carbon atom in the n-alkyl tail of the surfactant (SAT), a reduction of salinity by 1.2 wt% NaCl or a reduction of temperature by 3 ◦C
Summary
Surfactants are amphiphilic molecules going to surfaces and interfaces to produce specific effects. In 1950, the first trial with a semiquantitative classification was proposed by Griffin, who introduced the hydrophilic-lipophilic balance number (HLB) by mixing two surfactants to attain a high stability emulsion result. It may be said that the HLB proposal from Griffin was helpful to a certain extent to select a surfactant for a given application, related to emulsion type and stability, but that it was quite inaccurate, because it depended on many other properties such as the surfactant molecular weight [8] and on many of its features such as its chemical formula, the nature of the hydrophilic group and the branching of the lipophilic tail, or some other specificities like the aromaticity of the oil, the presence of electrolyte in water, the usual addition of alcohol and other cosurfactants, the temperature, and the pressure. This was performed according to the surfactant’s general behavior as far as a property of interest was concerned
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