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International audience; Hydrophilicity is a key property of surfactants, but it is difficult to quantify in absolute terms since it depends both on their molecular structure and on external parameters, especially salinity and temperature. When it comes to complex mixtures of technical grade surfactants, as the ones used in industrial applications, it is even more complicated. The PIT-slope and the SPI-slope are surfactants characterization methods based on the perturbation of a well-defined reference system by the surfactant under study. They allow quantifying the hydrophilic/lipophilic ratio and the sensitivity of surfactants as regards to temperature (PIT-slope) and salinity (SPI-slope). The study herein applies these methods to characterize technical grade surfactants used in chemical Enhanced Oil Recovery (EOR), which belong to the classes of polyethoxylated alcohols, alkyl benzene sulfonates and alkoxylated sulfates and carboxylates. The surfactants are positioned in a SPI-slope/PIT-slope map and we hypothesize that two surfactants, alone or in mixture, with the same position in this map show analogous interfacial behaviors. We illustrate how to use this tool to identify several surfactant mixtures providing the “optimal formulation” at the same salinity, for a given crude oil at the reservoir temperature. From a known binary surfactant mixture that forms a three-phase system (Winsor III) with a given crude oil, two other systems with the same position in the SPI-slope/PIT-slope map are formulated and evaluated.

International audience; A novel method is described to extract crude oil surface active species. It is based on the formation of balanced three-phase microemulsions (optimum Winsor III systems) with a well-defined temperature-sensitive nonionic surfactant. The crude oil surface-active species are concentrated in the middle-phase microemulsion, which is separated. The interfacial material is then recovered in the surfactant-rich aqueous phase after cooling. This method was applied to five crude oils and the interfacial materials were then analyzed by Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS). This novel extraction method compares well with the established “wet-silica” method, as evidenced by the analysis in the negative electrospray ESI(−) mode of the interfacial materials of the same crude extracted by the two methods. The analytical data have been related to the interfacial activity of the crude oils, namely, their tendency to form stable emulsions with water or viscous phases in surfactant/oil/water systems at equilibrium, and to their equivalent alkane carbon numbers (EACN). The interfacial materials are confirmed to be responsible for the increased stability of water-in-oil emulsions.

The transitional phase inversion of BrijL4/isopropyl myristate/water emulsions is detected by monitoring the visible light backscattering (LBS) using an optical fiber. For comparison, electrical co...

Hypothesis The salinity at which the dynamic phase inversion of the reference system C10E4/n-Octane/Water occurs in the presence of increasing amounts of a test surfactant S2 provides quantitative information on the hydrophilic/lipophilic ratio and on the sensitivity to NaClaq of S2. Experiences The Salinities causing the Phase Inversion (SPI) of the reference system mixed with 12 ionic and 10 nonionic well-defined surfactants are determined in order to quantify the contributions of the nature of the polar head and of the alkyl chain length. Findings The SPI varies linearly upon the addition of S2. The slope of the straight variation with the molar fraction of S2 is called the “SPI-slope”. It quantifies the hydrophilic/lipophilic ratio of S2 in saline environment and its salt-sensitivity with respect to the reference surfactant C10E4. The SPI-slopes of C12 surfactants bearing different polar heads are found to decrease in the following order: C12NMe3Br > C12E8 > C12E7 ≥C12SO3Na ≈ C12COONa ≥ C12SO4Na > C12E6 > C12E5 > C12E3. This classification is different from that obtained when the phase inversion is caused by a change in temperature (PIT-slope method) because the addition of NaCl in significant amounts (3 to 10 wt%) partially screens the ionic heads and diminishes their apparent hydrophilicities. A simple model, valid for all types of nonionic surfactants, is developed on the basis of the HLDN equation (Normalized Hydrophilic-Lipophilic Deviation) to express the SPI-slope as a function of the hydrophilic/lipophilic ratio (PACN2) and the salinity coefficient (δ2) of S2. All studied surfactants are positioned on a 2D map according to the values of their SPI-slope and their PIT-slope to graphically highlight their hydrophilic/lipophilic ratio and their salt-sensitivity. Finally, a linear model connecting the PIT-slope and the SPI-slope is derived for nonionics, emphasizing that the thermal partitioning of C10E4 towards n-octane is much greater in the PIT-slope than in the SPI-slope experiments.

Hypothesis The insertion of polyether spacers between the anionic head and the alkyl chain of ionic surfactants significantly improves their salt-tolerance. The aim of this work is to study whether the petro-based polyethoxy spacer can be replaced by a glyceryl ether group for high salinity applications. Experiments A series of amphiphilic sodium salts of alkyl glyceryl ether carboxylates are synthesized with different alkyl chain lengths from 4 to 12 and various spacers between the glyceryl and the carboxylate groups. Their aggregation behavior is studied by tensiometry and their amphiphilicities are assessed by the PIT-slope method. The dramatic effect of the methylation of the glyceryl spacer on the salt-tolerance is highlighted, and rationalized by DFT calculations and molecular dynamics. Findings In contrast to the corresponding sodium soap, n-C6H13-CO2Na, and to the non-methylated counterpart, the sodium salt of 1-pentyl-3-methyl glyceryl ether methylene carboxylate ([5.0.1]-CH2CO2Na) exhibits an excellent salt-tolerance since it remains water-soluble with NaCl or CaCl2 concentrations greater than 20 wt% at 25 °C. Amphiphiles with short alkyl chains (

The aqueous solubility of anionic surfactants is generally limited by their Krafft point. However, the insertion of non-ionic spacers such as ethoxy or propoxy groups between the alkyl chain and the anionic head is known to improve the water solubility of these surfactants in pure water or in the presence of electrolytes. In this work, the amphiphilicity of a series of ethoxylated or propoxylated sulfate surfactants is assessed by the PIT-slope method. Then, the phase behavior of these surfactants in concentrated aqueous solutions of NaCl or CaCl2 is investigated and compared with that of the typical nonionic (C12EO7) and ionic (SDS) surfactants. Surprisingly, sodium dodecyl sulfate presents a cloud point at high salinities and temperatures. Adding ethoxy or propoxy groups greatly enhances the resistance to electrolytes due to the “disordered” structure of non-ionic spacers that hinders crystallization, leading to Krafft point under 0 °C. Increasing the number of hydrophilic ethoxy groups increases the salt tolerance in contrast with the decrease observed with the slightly hydrophobic propoxy groups. Finally, the salt effect of CaCl2 and Ca(NO3)2. are compared pointing out the salting-out effect of Cl− and showing a weak salting-in effect of Ca2+ on nonionics and its strong interaction with sulfate anionic function.

Five new bio-based surfactants have been synthetized by coupling hexahydrofarnesol with mono and di-saccharides. Hexahydrofarnesol (3,7,11-trimethyl-dodecan-1-ol) is a by-product of the industrial production of farnesane, a sustainable aviation fuel obtained by a fermentation process from sugar feedstocks. Using hexahydrofarnesol as the lipophilic starting material allows obtaining 100% bio-based surfactants while valorizing an industrial by-product. Moreover, the C15-branched alkyl chain brings unique properties to the surfactants. This paper presents a physicochemical characterization of these new surfactants including their behaviors in water (water solubility, critical micellar concentration and surface tension) and in oil/water systems (interfacial tension against model oil and ternary phase behavior). Their hydrophilicities have been determined thanks to the PIT-slope method and compared to the ones of standard surfactants with linear alkyl chains, in order to distinguish the contributions of the sugar polar heads and of the branched hexahydrofarnesyl lipophilic chain. This novel class of surfactants combines the properties of sugar-based surfactants (low sensitivity to temperature and salinity, ability to form Winsor III microemulsion systems over a wide range of salinity), along with specificities linked to the branched alkyl chain (lower Krafft temperature, low surface tension).

The addition of water to hydrophobic alkyd resin containing a hydrophilic nonionic surfactant (polyethoxylated-20-oleyl alcohol) was used to produce the emulsion morphology change from W/O to O/W in a small thermostated reactor (50 mL). The so-called catastrophic phase inversion of the water/alkyd resin system was detected by monitoring both, the viscosity by torque measurement and the electrical conductivity, until an equal weight fraction of water and resin is attained (fw = 0.5). The two methods provide similar values of the phase inversion point (PIP). Although the determination of the PIP exhibits a small inaccuracy, it is clear that different torque profiles and PIP are observed depending on the resin neutralization level, the surfactant concentration and the water addition rate. For the non-neutralized resin, both, PIP and emulsion drop size, decrease with surfactant concentration increase. The completely neutralized resin could be emulsified using a minimum of 2 wt% of surfactant in the final emulsion. In such a case the water fraction at which inversion occurs (fw,Inv = 0.36) is independent of the surfactant concentration and a small resin drop diameter (

Highly viscous hydrophobic isosorbide biobased polyester O/W emulsions are prepared through catastrophic phase inversion. The process is followed in situ with two different methods: torque and light backscattering (LBS). Considering high viscosity of the system, only discontinuous conductivity monitoring is performed for comparison. Torque and LBS allow to highlight the emulsion inversion point (EIP) with relatively close water weight fraction values (fw≈0.20). The torque and LBS signals are rather noisy before inversion (evolution of different structures) and more smooth after phase inversion (continuous aqueous phase). Torque gives a more macroscopic information, representative of the global state of the dispersion. Consistent conductivity and torque measurements suggest indeed an inversion pathway through multiple o/W/O emulsions leading to multiple complex structures before getting continuous aqueous final emulsion. This hypothesis is confirmed with continuous LBS monitoring and microscopic observations. LBS signal seems more complete because it combines the information of conductivity and torque and allows to clearly follow in situ the inversion from the beginning to the end of the process.

In this paper the Phase Inversion Temperature (PIT) of the C10E4/n-Octane/0.01 M NaCl(aq) emulsion was used as a reference to determine the changes introduced by adding small amounts of a second surfactant S2. The so-called PIT-slope method allowed the calculation of the dPIT/dx2 value, i.e. the linear variation of PIT with the molar fraction x2 of second surfactant S2. For nonionics polyethoxylated surfactants CiEj, dPIT/dx2 showed a linear dependency respect to the Preferred Alkane Carbon Number (PACN), one of the parameters of the HLD equation based on phase behavior. We calculated here the PACN from PIT and x2 data for CiEj and other nonionic surfactants derived from isosorbide or glycerol. When possible, the experimental PACN is determined from n-alkane scans at 25 °C, and used to validate the proposed estimation method. The effects of changing the oil nature and water salinity on dPIT/dx2 are discussed in terms of the HLD equation and validates the hypothesis that allows calculate the PACN.

The standard HLD (Hydrophilic-Lipophilic-Deviation) equation expressing quantitatively the deviation from the “optimum formulation” of Surfactant/Oil/Water systems is normalized and simplified into a relation including only the three more meaningful formulation variables, namely (i) the “Preferred Alkane Carbon Number” PACN which expresses the amphiphilicity of the surfactant, (ii) the “Equivalent Alkane Carbon Number” EACN which accurately reflects the hydrophobicity of the oil and (iii) the temperature which has a strong influence on ethoxylated surfactants and is thus selected as an effective, continuous and reversible scanning variable. The PACN and EACN values, as well as the “temperature-sensitivity-coefficient”τ of surfactants are determined by reviewing available data in the literature for 17 nonionic n-alkyl polyglycol ether (CiEj) surfactants and 125 well-defined oils. The key information used is the so-called “fish-tail-temperature” T* which is a unique data point in true ternary CiEj/Oil/Water fish diagrams. The PACNs of CiEj surfactants are compared with other descriptors of their amphiphilicity, namely, the cloud point, the HLB number and the PIT-slope value. The EACNs of oils are rationalized by the Effective-Packing-Parameter concept and modelled thanks to the COSMO-RS theory.

Hypothesis The attainment of ultralow interfacial tensions between crude oil and injected aqueous surfactant mixtures is a prerequisite for an effective chemical Enhanced Oil Recovery (EOR). The dynamic Phase Inversion Temperature (PIT) of SOW emulsified systems is very close to the “optimum temperature” currently identified with equilibrated SOW systems. Therefore, the PIT could be a tool to track the “optimum formulation” and determine EACN of crude oils. Additionally, the PIT-slope method could be used to characterize EOR surfactants. Experiments The PIT of 3% C10E4/crude oils/water emulsions are compared to the PIT for n-alkanes in order to estimate crude oils EACN. The “PIT-slope” method is applied to different non-ionic and ionic extended EOR surfactants to assess their amphiphilicity. The conductivity profiles of different EOR surfactants/crude oil/NaCl(aq) emulsions at fw = 0.5 are determined at different salinities. Findings Considering the PIT shifts and shapes, it is possible to infer relevant information on the crude oil such as precise EACN and relationships between optimum salinity and temperature. The “PIT-Slope method” allows ranking EOR surfactants according to their amphiphilicity. Mixing both results allows a faster determination of key parameters used in EOR compared to studies with equilibrated system.

The standard HLD (Hydrophilic-Lipophilic-Deviation) equation expressing quantitatively the deviation from the "optimum formulation" of Surfactant/Oil/Water systems is normalized and simplified into a relation including only the three more meaningful formulation variables, namely (i) the "Preferred Alkane Carbon Number" PACN which expresses the amphiphilicity of the surfactant, (ii) the "Equivalent Alkane Carbon Number" EACN which accurately reflects the hydrophobicity of the oil and (iii) the temperature which has a strong influence on ethoxylated surfactants and is thus selected as an effective, continuous and reversible scanning variable. The PACN and EACN values, as well as the "temperature-sensitivity-coefficient"τ of surfactants are determined by reviewing available data in the literature for 17 nonionic n-alkyl polyglycol ether (C

Apparue en 1990, la technologie microfluidique a ouvert des voies de comprehension de differents phenomenes dans des disciplines variees. Elle a en meme temps permis de developper des nouveaux systemes complexes pour de nouvelles applications. Cet article etablit l’etat de l’art de la microfluidique dans le domaine des emulsions (simples, multiples, de Pickering) et des systemes colloidaux complexes (microcapsules, microgels, vesicules), en partant d’une description des generalites dans chaque domaine jusqu’a detailler certaines applications.

In conventional chemical Enhanced Oil Recovery (EOR), surfactant mixtures must be optimized to obtain the so-called “optimum formulation” characterized by a three-phase behaviour (WIII) and an ultra-low interfacial tension between crude oil and injected water. To attain this condition, discontinuous salinity scans of Surfactant-Oil-Water (SOW) systems are usually performed in a series of sealed pipettes maintained at the reservoir temperature until complete phase separation. Furthermore, some SOW mixtures do not give a WIII, but instead they form viscous phases which are very detrimental for EOR applications. The study herein describes a novel method called dynamic Salinity-Phase-Inversion (SPI) to quickly and accurately determine the optimum formulation and to anticipate whether the surfactant mixture will provide, by the conventional static method, a WIII or a viscous phase. This method consists in continuously modifying the salinity of a stirred SOW system at WOR in the vicinity of 1, in order to detect the phase inversion of the emulsion by a sudden drop in conductivity. The effectiveness of the method is illustrated by seeking the optimum formulation for two crude oils in the presence of a mixture of an ionic EOR surfactant (sulfated propoxylated n-alcohol) and a nonionic co-surfactant (ethoxylated i-alcohol), with or without alkali. Continuous salinity scan of the stirred SOW system maintained at constant temperature allows the identification of the optimal salinity within 30 min. This method therefore significantly accelerates the optimization of surfactant blends suitable for a given oil reservoir.

Formulation and process variables in surfactant/oil/water (SOW) systems can strongly modify the water/oil interfacial tension leading to different behaviors. Pre-equilibration time of the SDS/toluene/1-butanol/NaCl(aq) system was studied as a process variable and the flow regimes were described at different continuous and dispersed flow rates through a flow focusing geometry. Salinity and oil nature for the SDS/n-alkane/alcohol/NaCl(aq) equilibrated system were studied as formulation variables at fixed dispersed flow rates, changing the continuous flow rate and the formulation variable in order to get O/W or W/O emulsions in hydrophilic and hydrophobic chips, respectively. Pre-equilibration time modifies the flow regimes depending on the differences observed in the interfacial tension at t = 0 and t = 24 h of equilibration. Under very hydrophilic conditions, when γt=24 > γt=0, the entire droplet area is slightly higher for equilibrated systems. At WIII condition, when γt=0 >> γt=24, droplets are formed only in jetting regime for not equilibrated systems. For salinity scan, monodispersed oil droplets were obtained at low salinities, S = 0−0.25 % and in the oil scan, water droplets were obtained when using cyclohexane and cyclohexane-hexane mixtures. In both cases, flow regimes are reported into the formulation maps, using the Hydrophilic Lipophilic Deviation as formulation variable (HLD vs flow rate ratio). Droplet size was fitted into the same equation ( d i ≈ α C a c - 0.264 ), and it is noteworthy that the interfacial tension in CaC is lower than in most of reported works using flow-focusing geometries and both emulsions (W/O and O/W) were obtained with the same surfactant.

Highly viscous hydrophobic isosorbide biobased polyester O/W emulsions are prepared through catastrophic phase inversion. The process is followed in situ with two different methods: torque and light backscattering (LBS). Considering high viscosity of the system, only discontinuous conductivity monitoring is performed for comparison. Torque and LBS allow to highlight the emulsion inversion point (EIP) with relatively close water weight fraction values (f

The influence of 13 linear alcohols from methanol to hexadecanol and 20 fragrance molecules on the phase inversion temperature (PIT) of the Brij30/ n -octane/water emulsion system was investigated and sorted out using the hydrophilic lipophilic deviation (HLD) concept. Effect of the hydrophobic chain structure and nature of polar function is discussed. Among the various fragrance molecules, alcohols are very efficient to reduce the PIT of emulsion whereas acetals, esters and ketones have no effect and even sometimes, some of them increase it a little. Phenols exhibit an even more dramatic effect since only 0.4% decrease the PIT of 19 °C. Experimental results are rationalized in terms of structural parameters of fragrance molecule.

Hypothesis In contrast to anionic and nonionic amphiphilic substances, bio-based cationic ones are very rare. Cationic amphiphiles are mostly based on quaternary ammonium, pyridinium or imidazolium groups that are either badly biodegradable or have toxic residues even after degradation. In the search for green alternatives to cationic hydrotropes and amphiphiles, natural l -carnitine could be a promising candidate for a cationic headgroup. Experiments By esterification of carnitine in one step and with low cost, cationic molecules with alkyl chain length of n = 2–14 could be obtained. Their thermal properties, aggregation behaviour and cytotoxicity were determined. Hydrophobic compounds were solubilized in their aqueous solutions and the PIT-slope method was applied to determine a relative hydrophilicity. Findings It was found that some pure carnitine ester bromides were liquid at room temperature and thus can be classified as ionic liquids. They are highly water-soluble, and in aqueous solutions, they showed hydrotrope or surfactant behaviour depending on their alkyl chain length. Their high hydrotropic efficiency was demonstrated by solubilizing Disperse Red 13, while also biomolecules, like vanillin, could be dissolved in reasonable amounts. In all tests, they performed at least as good as the tested reference substances, while showing similar cytotoxicity towards human skin keratinocytes, thus demonstrating their potential as green functional amphiphilic molecules of positive charge.

International audience; The influence of various carboxylic acids (from C2 to C18) one equilibrated and emulsified surfactant/oil/water (SOW) systems is investigated. The PIT-slope method, based on the influence of an additive on the PIT of the C10E4/n-octane/10−2 M NaCl(aq) reference emulsion, shows that from butanoic to hexadecanoic acids, the PIT of the system is decreased. A breakdown on dPIT/dx (considering the molar fraction x) or a minimum on dPIT/dC (taking into account the weight percentage C) is obtained with octanoic acid. The phase behavior of the C10E4/carboxylic acid/n-octane/0.01 M NaCl(aq) system maintaining a molar fraction of carboxylic acid xCA = 0.2 shows that from octanoic acid, the solubility of the system is clearly enhanced explaining the breakdown obtained in the PIT-slope. The Span 80/carboxylic acid/oil/10−2 M NaCl(aq) system at a water-to-oil ratio equal to 1 is able to form O/W emulsions in the presence of some carboxylic acids, of which the alkyl chain modifies the partition of the different oligomers of Span 80. Finally, the PIT-slope method is applied to mixtures of lauric acid/sodium laurate providing a linear behavior of the PIT-slope dPIT/dx as a function of the acid/salt ratio.

The equilibrium phase behaviour of the tetraethyleneglycol decyl ether (C10E4)/n-octane/1% peptide hydrolysates of bovine haemoglobin systems at fw = 0.5 has been studied. Fish diagrams have been completely built for two hydrolysates with 3 and 15% degree of haemoglobin hydrolysis and they do not differ much from those having NaCl 0.01 M as aqueous phase. From the knowledge of these diagrams a new method is proposed to separate selectively peptide hydrolysates from haem. Most important parameters in extraction, range of temperatures and surfactant concentrations have been established. Five extractions using 5 wt.% C10E4/n-octane/1% hydrolysates (degree of haemoglobin hydrolysis DH = 15%) at 27 °C allowed to eliminate 98% of starting haem, which is concentrated in the middle microemulsion phase, while maintaining 94% of the starting peptide population. In the same conditions, the hydrolysate DH = 3% presents 99.6% of haem elimination and maintains 85.6% peptides in aqueous phase. Well defined feasibility of the selective extractions and process conditions are established to provide depleted haem aqueous phase.

The relationship between the phase behavior and the type of emulsion formed under stirring has been studied for a well-defined surfactant/oil/water (SOW) system (ultrapure C10E4/n-octane/water) at three surfactant concentrations (1%, 3%, and 7%). The phase behavior was determined from systems equilibrated at constant temperature. The type of emulsions formed when these pre-equilibrated systems were stirred was established from conductivity measurements, and the so-called “standard inversion frontier” between the two emulsion morphologies (O/W and W/O) was plotted on temperature–water/oil proportion and temperature–surfactant concentration maps (χ and γ cuts, respectively, of the SOW–T prism). Dynamic phase inversions, produced by imposing a temperature variation under continuous stirring, were also observed. In the χ and γ maps, an exact correspondence between phase behavior and emulsion type was not observed, under either standard or dynamic conditions, and some regions of the formulation–composition map...

Brij30/isopropyl myristate/water model emulsions were used to study the inversion morphology change from O/W to W/O. The transitional phase inversion was detected by monitoring the electrical conductivity during a heating–cooling cycle while the viscosity of the emulsion was followed under constant shear rate with a cone–plate rheometer equipment. The two methods provide similar values of phase inversion temperature. However, different rheological profiles are observed depending on the surfactant concentration and water fraction. Viscosity maps of formulation-composition maps (specifically temperature–surfactant concentration and temperature–water fraction) pointed to the occurrence of both transitional and catastrophic inversion processes. A complete formulation–composition map with all different emulsion morphologies and isoviscosities contours for the 9% Brij30/IPM/water is presented. The use of a commercial cone–plate rheometer exhibits several advantages over the classical conductivity measurement; i...

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.

The phase behavior of well-defined C 10 E 4 /ester oil/water systems versus temperature was investigated. Fifteen ester oils were studied and their Equivalent Alkane Carbon Numbers (EACNs) were determined from the so-called fish-tail temperature T * of the fish diagrams obtained with an equal weight amount of oil and water ( f w = 0.5). The influence of the chemical structure of linear monoester on EACN was quantitatively rationalized in terms of ester bonds position and total carbon number, and explained by the influence of these polar oils on the “effective” packing parameter of the interfacial surfactant, which takes into account its entire physicochemical environment. In order to compare the behaviors of typical mono-, di-, and triester oils, three fish diagrams were entirely plotted with isopropyl myristate, bis (2-ethylhexyl) adipate, and glycerol trioctanoate. When the number of ester bonds increases, a more pronounced asymmetry of the three-phase body of the fish diagram with respect to T * is observed. In this case, T * is much closer to the upper limit temperature T u than to the lower limit temperature T l of the three-phase zone. This asymmetry is suggested to be linked to an increased solubility of the surfactant in the oil phase, which decreases the surfactant availability for the interfacial pseudo-phase. As a consequence, the asymmetry depends on the water–oil ratio, and a method is proposed to determine the f w value at which T * is located at the mean value of T u and T l .