Colloids and Surfaces A: Physicochemical and Engineering Aspects
Revisiting the influence of carboxylic acids on emulsions and equilibrated SOW systems using the PIT-slope method
Graphical abstract
Introduction
The world production of fatty acids (FAs) from natural fats and oils represents about 4 million metric tons per year. Most fatty acids of vegetable origin have a straight-chain with an even number of carbon atoms while odd-numbered fatty acids are most commonly found in bacteria, some plants and animals [1]. Chain lengths typically range commonly from 12 to 24. FAs are simple in structure and can be saturated or unsaturated, named MUFA for the monounsaturated and PUFA for the polyunsaturated ones. In some animals, fatty acids may be more complex with branched chains or other functional groups.
Fatty acids are ultimately consumed in a wide variety of end-use industries (rubber, plastics, detergents…). They are used to modify polyesters providing alkyd resins for coatings. They are also widely introduced in pharmaceutical, food and cosmetic formulations as additives. The alkaline salts of fatty acids provide surfactant molecules which have been used in soap formulations since Roman civilization [2]. The presence of carboxylic acid groups in oil, as naphthenic and resins molecules, is at the origin of the so-called alkaline flooding process for enhanced oil recovery [3], [4]. Description of aqueous phase behavior and pH dependence of these compounds has largely been described in the literature [5], [6], [7], [8]. Because of the pH dependence of the carboxylic function, their influence on Surfactant/Oil/Water (SOW) systems is important. Indeed, under alkaline conditions (i.e. –COO−), they can play the role of a surfactant with a more or less important effect at the interface while under acidic conditions (i.e. –COOH), they are more lipophilic, changing thus the polarity of the oil depending on their concentration. However, as weak acids, both forms generally coexist over a certain range of pH.
Several studies dealing with S/O/W ternary systems and fatty acids and/or their salts at equilibrium have also been reported in the literature. According to the Winsor nomenclature, the behavior of microemulsions [9] can include systems with two phases (Winsor I and II), three phases (Winsor III) or one single phase behavior (Winsor IV). For the two-phase behavior, when the microemulsion is in equilibrium with an excess oil phase, the system is called Winsor I. When the microemulsion is in equilibrium with an excess water phase, the system is Winsor II. In the early studies of S/O/W equilibrated systems, Schulman et al. [10] studied oleic acid and its salt with different cations using alkanes, benzene or kerosene as oily phases. Ekwall et al. [11], [12] published detailed phase diagrams of sodium octanoate/oil/water systems, using alcohol or fatty acids as third component. Mendez et al.[3] have shown that some fatty acid (FA)/O/W systems exhibit an unusual retrograde transition, i.e. WII → WIII → WI → WIII → WII when NaOH is added. Indeed, pH raises first increases the proportion of fatty acid sodium salt (i.e. hydrophilic surfactant) at the interface thus promotes WII → WIII → WI transitions. Then, a further pH increase does not change the acid/carboxylate equilibrium and the rise of Na+ concentration results in the classical salinity driven WI → WIII → WII transitions. Bravo et al. [13] have studied the influence of pH and alkyl chain length on the phase behavior of FA/oil/alcohol/NaClaq systems. They have shown that it is possible to obtain a three-phase behavior, maintaining a constant quantity of NaOH in the system, by changing the concentration of the fatty acid. For each fatty acid, it is possible to determine the pH to achieve the so-called optimal formulation, i.e. the formulation at which the minimum interfacial tension and the maximal solubilization occurs. The longer the alkyl chain of the acid is, the higher the pH of the aqueous phase must be to obtain the optimal formulation. In addition, in the case of a triphasic system, i.e. Winsor III, for an equal amount of FA, the microemulsion middle phase increases in volume when the alkyl chain increases. Thus, the longer the alkyl chain, the more the FA solubilizes both water and oil. Researches concerning carboxylic acids have not been limited to equilibrated systems and several studies have also been published on emulsions [14], [15], [16], [17], [18], most of them related to formulations for pharmaceutical, cosmetics or food applications.
Recently, Ontiveros et al. have proposed a simple and convenient method for the classification of surfactants [19], [20]. The method is based on the perturbation of the Phase Inversion Temperature (PIT) through addition of a second surfactant. The concept of PIT, introduced by Shinoda et al. [21], [22], characterizes the change of the affinity of polyethoxylated fatty alcohol surfactants, noted CiEj, towards water, oil and interface when they are heated or cooled. Starting from the C10E4/n-octane/water reference system, with a PIT close to 25 °C, the linear decrease or increase evolution of the PIT through addition of a second surfactant is quantified through the so-called PIT-slope which affords a robust classification of surfactants compared to the reference C10E4.
Based on these previous studies, the influence of the alkyl chain length of a series of saturated carboxylic acids from C2 to C18 and oleic acid C18:1 on the PIT-slope, dPIT/dxCA (i.e., the PIT variation versus the molar fraction of the acid) has been investigated. The effect of the lauric acid/sodium laurate ratio on dPIT/dxCA has been examined using the same method. The phase behavior of C10E4/CA/n-octane/water systems at equilibrium has also been studied in order to evaluate the impact of carboxylic acids on the solubilizing capacity of microemulsion systems. Finally, evolution of the morphology of a Span 80/oil/water emulsion in the presence of the different CAs points out the complex behavior of such formulations, based on commercial oil (Creasil IH CG) and surfactant (Span 80) mixtures.
Section snippets
Chemicals
Pure tetraethyleneglycol monodecyl ether (C10E4) reference surfactant was synthesized according to a method described elsewhere [23], [24]. Its purity was assessed by NMR and GC analyses (>99%) and by comparing its cloud point temperature [25] (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 carboxylic acids: acetic acid C2(99%), propionic acid C3 (99.5%), hexanoic
Effect of carboxylic acids chain length on SOW systems
The influence of carboxylic acids (CAs) at the interface as a function of the CA chain length was determined using the PIT-slope method. Thus, the PIT of the 3 wt.% C10E4/n-octane/10−2 M NaCl(aq) reference system was measured in the presence of increasing amounts of 12 linear carboxylic acids(CAs) from acetic acid (C2) to stearic acid (C18) and one unsaturated compound, i.e. oleic acid. Fig. 1 describes the linear evolution of the PIT values as a function of the weight concentrations for some
Conclusion
The role of carboxylic acids in SOW systems is complex and it can be compared in some aspects to the alcohols effect. They are able to modify both the polarity of the oil phase and also the interfacial layer behavior. PIT-slope experiments allow to demonstrate that most of them (except acetic acid) enhances the lipophilicity of the C10E4/n-octane/water emulsion. The temperature scan for equilibrated systems at a fixed molar fraction of carboxylic acid of 0.2 confirms the PIT-slope results. Even
Acknowledgments
Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche, Région Nord Pas de Calais and FEDER are acknowledged for supporting and funding part of this work. The authors would like to thank Prof. J-M. Aubry who kindly provided helpful comments for the discussion.
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