A fresh perspective on HLB and HLD concepts for selecting the "best" surfactant (J.M. Aubry)

Griffin's HLB is a great idea but it should not be used for non-ethoxylated surfactants. On the other hand, Salager's HLD equation well reflects the hydrophilic/lipophilic ratio of surfactants in complex environments. However, the current equation contains many parameters that are hard to measure and the numerical value of the HLD is difficult to apprehend when HLD ≠ 0. 

Recently, a simplified form of the HLD equation - denoted HLDN - has been proposed [1] so that its numerical value makes more sense for the formulator:

HLDN = PACN – ACN + τ.(T - 25) +δ.S

Where: PACN is the “preferred” ACN of the surfactant, that is its optimal oil at 25°C without salt. ACN reflects the oil hydrophobicity, T in °C, S in wt.% NaCl whereas τ and δ are two constants that reflect the surfactant sensitivity towards temperature and salinity respectively.

Two methods are available to determine PACN, τ and δ for a given surfactant:

● The standard method is based on the lengthy (≈ 2 weeks) identification of the optimal formulation in a series of tubes containing Surfactant/n-Alkane/Water (SOW) systems at equilibrium.

● An alternative method, much faster (≈ 30 min) and more precise, is based on the phase inversion of SOW systems under stirring, the HLD value of which is gradually modified by changing the temperature [2] or the salinity [3]. This rejuvenated toolbox has enabled us to solve various formulation problems such as the emulsification of highly viscous paint resins, the preparation of nanoemulsions using a novel bio-sourced surfactant and the design of formulations for enhanced oil recovery (EOR) of crude oil using technical surfactants.

[1] J.-M. Aubry et al., Adv. Colloid Interface Sci. (2020) 276, 102099.

[2] Determining the Preferred Alkane Carbon Number (PACN) of nonionic surfactants using the PIT-slope method J.F. ONTIVEROS, C. PIERLOT, M. CATTE, J.L. SALAGER, J.M. AUBRY ; Coll. Surf. A (2018) 536, 30-37

[3] The Salinity-Phase-Inversion method (SPI-slope): A straightforward experimental approach to assess the hydrophilic-lipophilic-ratio and the salt-sensitivity of surfactants; G. LEMAHIEU, J.F. ONTIVEROS, T. GAUDIN, V. MOLINIER, J.M. AUBRY; J. Coll. Interf. Sci, Accepted for publication