Structure–interfacial properties relationship and quantification of the amphiphilicity of well-defined ionic and non-ionic surfactants using the PIT-slope method

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Abstract

The Phase Inversion Temperature of a reference C10E4/n-Octane/Water system exhibits a quasi-linear variation versus the mole fraction of a second surfactant S2 added in the mixture. This variation was recently proposed as a classification tool to quantify the Hydrophilic–Lipophilic Balance (HLB) of commercial surfactants. The feasibility of the so-called PIT-slope method for a wide range of well-defined non-ionic and ionic surfactants is investigated. The comparison of various surfactants having the same dodecyl chain tail allows to rank the polar head hydrophilicity as: SO3Na  SO4Na  NMe3Br > E2SO3Na  CO2Na  E1SO3Na  PhSO3Na > IsosorbideexoSO4Na  IsosorbideendoSO4Na  E8  NMe2O > E7 > E6  Glucosyl > E5  Diglyceryl  E4 > E3 > E2  Isosorbideexo > Glyceryl > Isosorbideendo. The influence on the surfactant HLB of other structural parameters, i.e. hydrophobic chain length, unsaturation, replacement of Na+ by K+ counterion, and isomerism is also investigated. Finally, the method is successfully used to predict the optimal formulation of a new bio-based surfactant, 1-O-dodecyldiglycerol, when performing an oil scan at 25 °C.

Introduction

The Phase Inversion Temperature (PIT) introduced by Shinoda and Arai [1] corresponds to the temperature at which the affinity of a nonionic ethoxylated surfactant “S” for water and oil switches by dehydration of the polyoxyethylene units during heating, leading to the emulsion phase inversion from O/W to W/O. Shinoda and Arai [1] studied the effect of surfactant polyoxyethylene chain length and hydrocarbon chain length on the PIT and found that the emulsion Phase Inversion Temperature was closely correlated with the cloud point. As defined by these authors, THLB is the temperature at which an ethoxylated surfactant presents the same affinity for oil and water in a SOW equilibrated system, while the PIT corresponds to the change of affinity of a surfactant in an emulsified system [2], [3]. For a well-defined non-ionic surfactant, the PIT, the THLB and the fish-tail temperature T of the so-called “fish diagram” are essentially equivalent at fw = 0.5 [4] but differ significantly when systems are studied at different water/oil ratios, especially at low surfactant concentrations [5].

Recently, a simple and fast method to classify surfactants was proposed, using the variation of the PIT of the 3% C10E4/Octane/Water reference system at fw = 0.5 when a second surfactant S2 is added [6]. The PIT was found to vary linearly with the second surfactant concentration, and the dPIT/dC parameter was found to allow the estimation of the Hydrophilic Lipophilic Balance of the added surfactant. Well-defined CiEj surfactants were studied to validate the classification method and to calibrate an HLB-scale. Then, various technical grade surfactants (Tweens, Spans, lecithin, sucrose esters) were located in this scale.

In the present work, well-defined ionic and nonionic surfactants are investigated in order to extend and complete the precedent scale, to assess the hydrophilicity of a wide variety of polar heads bound to a dodecyl hydrophobic tail, to establish the influence of the alkyl chain length on dPIT/dx2 (i.e., the PIT variation versus S2 molar fraction) and to compare the effects of other structural parameters like the counter-ion nature or the isomerism. The described method is also applied to characterize new glycerol and isosorbide-based ionic and non-ionic surfactants. Extending the scale to ionic surfactants permits also to evaluate the robustness of the method when the evaluated and reference surfactants are quite different. Finally, it is shown that dPIT/dx2 can be used as a guide to determine the “optimal oil” of a given surfactant, which leads to a Winsor III system at 25 °C when mixed with water and the surfactant under study.

Section snippets

Chemicals

Pure tetraethyleneglycol monodecyl ether (C10E4) reference surfactant (S1) was synthesized according to a method described elsewhere [7], [8]. Its purity was assessed by NMR and GC analyses (>99%) and by comparing its cloud point temperature [9] (20.4 °C at 2.6 wt.%) with the reference value (20.56 °C at 2.6 wt.%). n-Octane (99%) was obtained from Sigma–Aldrich. Sodium chloride NaCl (⩾99.5%) was supplied by Acros Organics. The commercial surfactants studied in this work (named S2 in what follows)

Results

The conductivity vs. temperature profile of stirred 3%C10E4/S2/n-Octane/10−2 M NaCl emulsions with increasing molar fraction x2 defined in Eq. (1) exhibits the usual shape. At low temperature, the high conductivity slightly increasing with temperature evidences an O/W emulsion up to a temperature range in which the conductivity drastically falls, indicating the inversion to a W/O emulsion.

The addition of a second surfactant S2 results in a small or greater increase or decrease in the emulsion

Discussion

As already presented in our previous paper [6] the slope parameter is a comparative criterion that indicates how surfactant S2 impacts the PIT of the reference emulsion at fw = 0.5. A positive value indicates that surfactant S2 is more hydrophilic than the reference C10E4 whereas a negative value evidences that it is more lipophilic in the conditions used. However, it is clear that some parameters such as the surfactant concentration, the water/oil ratio, the amount of electrolyte and the nature

Conclusion

Sixty-five years after the introduction of the empirical HLB concept by Griffin [39], the HLB value is still the most commonly used scale to characterize the relative affinity of a surfactant for the oil and the water phases. Indeed, newer and more scientifically founded concepts such as the « critical packing parameter » [40], and the « optimal formulation of SOW systems » [27], did not lead to a general classification of surfactants. The semi-empirical (α-EON) and σ parameters appearing in

Acknowledgments

The authors thank the University of The Andes Scholarship Program for financing J.F.O. doctoral studies and the Postgraduate Cooperation Program PCP n°20100000305 Green emulsions (FONACIT Venezuela and MAE-France) for sponsoring professor and graduate student exchanges.

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