How to Use the Normalized Hydrophilic-Lipophilic Deviation (HLDN) Concept for the Formulation of Equilibrated and Emulsified Surfactant-Oil-Water Systems for Cosmetics and Pharmaceutical Products

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.

Formulation Improvements in the Applications of Surfactant–Oil–Water Systems Using the HLDN Approach with Extended Surfactant Structure

“…Thus, this more precise approach resulted in a new definition of the interaction ratio as R = (A CO − A OO − A LL )/(A CW − A WW − A HH ), which is not discussed here because it is out of the scope of this review. Nevertheless, and as has been explained elsewhere [ 11 , 53 , 54 , 55 ], it must be noted that the effective interaction between the surfactant and the oil molecules, i.e., the numerator of R, tends to decrease when the oil ACN increases. This happens because A OO (between two n -alkane molecules) increases, in general, faster than A CO (between the surfactant tail and an oil molecule), while the other terms are unaltered.…”

“…The proper variable to make a comparison between different cases is to have the same change, which has been recently taken as a unit change in ACN, which is the same in all the cases [ 9 , 11 ]. Consequently, the previous list of equations on ΔHLD = 0 will be divided by the coefficient before ACN, so that in the “normalized” expression, it would become −1 as in Table 3 ΔHLD Ni Equations (1)–(3).…”

“…Transforming expression (4) to a differential equation level and taking into account that the first partial derivatives are constant (over some range) [ 10 , 11 ] dHLD N = 0 = −dACN + ∑ ± K i ΔdV i with i = 2, 3, 4 etc. and integrating it HLD N = 0 = −(ACN−ACNref) + ∑ ± Ki (Vi − Vi ref) with i = 2, 3, 4 etc.…”

“…The SCP N abbreviation means the normalized surfactant contribution term [ 9 , 11 ], which, for several reasons explained below, has been a source of confusion over the past 40 years. First of all, it was referred to with names depending on the experiments.…”

“…This was a significant advancement in the simplification of the system, using an innovative solution, i.e., including a PO-EO intramolecular extension to generate the polarity transition inside the molecule that produced more interactions with the oil and aqueous phases. A summary and classification of the sulfate head extended surfactants developed since 1992 with the reported surfactant classification parameter (SCP N ) [ 9 , 10 , 11 ] is presented in Table 1 . Nevertheless, other molecules have been developed with different types of polar heads (carboxylate, xylitol, glucose derived, ethoxylated nonionic), as summarized in the 2019 review [ 2 ].…”