Extended surfactants are molecules including an intramolecular extension that allow attaining high performance without the need for cosurfactant or linker alcohol. The polypropylene oxide chain intramolecular extension generates a polarity transition inside the molecule that produces more interactions with the oil and aqueous phases. The idea was developed in the 1990s, basically to fasten together the rather hydrophilic surfactant and the lipophilic linker, producing the same effect as the mixture without losing a part of the lipophilic linker going away from the interface. Since the lipophilic linker was an amphiphile with a small hydrophilic part located close to the interface, the single structure was developed to imitate the mixture situation. It contains a polar head located in water, then an intermediate slightly polar zone in the oil phase close to the interface, and finally, the surfactant classical hydrocarbon tail.
The synthesis of surfactants for the solubilization of complex oils, including crude oils and polar oils, has been widely researched since the late 1970s . After petroleum sulfonates were introduced , it was necessary to develop high-performance surfactants with reasonable production costs to advance surfactant flooding processes. Henceforth, ethoxylated oleyl sulfonates (EOS) were introduced  as the first surfactants that presented an extension from the usual alkyl chain—sulfate head surfactants. The first EOS surfactants had 2-3 ethylene oxide groups in their structure, which allowed a somewhat higher performance, although still with the need of cosurfactants to achieve high solubilizations and low IFT . They were tried with long-chain hydrocarbons as the oil phase, e.g., hexadecane and paraffin oils . Then, an additional intramolecular extension with four polypropylene oxide (PO) groups was added to sulfate surfactants , which allowed attaining high performance without the need for cosurfactant alcohol. 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 (SCPN)  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 .
Table 1. Molecular structure and classification of sulfate head extended surfactants according to its normalized characteristic parameter (SCPN) 
1Nomenclature: A: Alfoterra, S: Seppic, Chen and He are first authors of the papers where these surfactants were synthetized. A/10/18/2/S stands for Alfoterra/C10/PO18/O2/SO4. This is the same nomenclature as .
*SCPN is the surfactant classification parameter. Higher SCPN indicates a more important lipophilic part of the molecule (hydrocarbon tail and PO extension), a lower SCPN (more negative) indicates a more important hydrophilic head contribution.
The need for the solubilization of polar oils (triglycerides, oleic acids), which present very low solubilization with conventional surfactants, led to trying these new molecules. The polar oils tested included perchloroethylene oil  and ethyl oleate  with a (surprising at the time) very high performance. Miñana et al.  also tried the solubilization of triglycerides (which at the time was practically zero with conventional surfactants, even with cosurfactant alcohols) with C12POnEO2SO4 (n = 6, 10 or 14), showing for the first time high solubilization of these bulky polar oils. In the same year, Aoudia et al.  achieved high performance with crude oils and a very long PO = 15 extension in a C14PO15SO4 surfactant. These first state-of-the-art extended surfactants synthetized and proved for high solubilization of polar oils allowed a progressive advance of research . Nevertheless, it was not until the early 2000s that new molecules were developed at FIRP Lab. , including surfactants with a carboxylate head and others derived from triglycerides and natural sugars . These first trends concerning new extended surfactants for the high solubilization of polar oils allowed the progressive advance of research .
Several other surfactants that were tried since 1995 were used in different applications, including EOR and the solubilization of polar oils , but also other not-so-conventional applications, such as drilling fluids  and crude oil dewatering .
2. Historical Introduction on Formulation Concepts
It can be said that a century ago, the so-called Bancroft’s rule and its related research and development discussions  were the first attempt to attain a generalized practical approach for surfactant–oil–water (SOW) systems. However, it was only in the late 1950s that two researchers from industry tried to improve the practical aspects related to SOW systems.
Griffin  introduced the so-called hydrophilic-lipophilic balance parameter, called HLB. Sometime later , he proposed several numerical expressions to estimate the HLB number as a function of the chemical structure of nonionic surfactants, e.g., 20% of the polyoxyethylene weight for an ethoxylated alcohol. HLB was related to the surfactant effect and was thus the first numerical scale that could help compare cases and averaging effects. Even though it did not take into account the effect of other variables, it was the unique numerical criterion for 25 years because it was an extremely simple concept. Thus, it is still currently used as approximate information for people in the industry who do not require high accuracy in formulation work .
At the same time, but in a completely different research area, Winsor  proposed a complex model based on the ratio R of interactions between the surfactant adsorbed at the interface and the neighboring oil and water molecules on both sides of it, indicated explicitly as ACO and ACW in Figure 1.
Figure 1. Interaction of surfactant, oil and water molecules close to interface according to Winsor’s scheme.
The original model R = ACO/ACW was made more realistic by introducing the self-interactions between the surfactant, oil and water molecule as references, each in separated terms .
Thus, this more precise approach resulted in a new definition of the interaction ratio as R = (ACO − AOO − ALL)/(ACW − AWW − AHH), which is not discussed here because it is out of the scope of this review. Nevertheless, and as has been explained elsewhere , 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 AOO (between two n-alkane molecules) increases, in general, faster than ACO (between the surfactant tail and an oil molecule), while the other terms are unaltered. In what follows, R is taken as (ACO − AOO)/ACW for simplicity.
The Unidimensional Scan of a Formulation Variable
In his research in the late 1940s, Winsor proposed a basic method to study a surfactant–oil–water (SOW) system by determining its phase behavior versus a continuous variation (called a scan) of a variable susceptible to alter at least one of the interactions . In simple systems, these variables typically were the oil nature (e.g., the n-alkane carbon number ACN), the water NaCl salinity (S), the surfactant and co-surfactant type (head or tail nature), the temperature, and eventually the pressure , which is an essential variable in a petroleum reservoir.
Winsor reported that a three-phase behavior (central diagram in Figure 2 left part) was occurring exactly at R = 1 and was associated with a low interfacial tension, indicated as γ* in Figure 2, right part.
Figure 2. Variation of the interfacial tension (right) and of the phase behavior in a ternary diagram (left) along a formulation unidimensional scan (here, the salinity of the aqueous phase S). S* and γ* are the salinity and the interfacial tension at optimum formulation, respectively.
3. Multivariable Scans and Generalized HLD Expression for Optimum Formulation
Figure 3 displays the phase behavior transitions in the two-dimensional S-ACN space when all other variables are constant. In the left plot, numerical values are indicated in the ordinate for salinity and in abscissa for ACN. The gray area indicates the three-phase zone (WIII). A vertical variation of salinity at the ACN constant corresponds to Figure 2 salinity scan, while a horizontal variation of ACN at constant salinity is an ACN scan. The line at the center of the three-phase zone is the optimum formulation line in a bidimensional scan; it corresponds to the optimum ACN* at constant salinity and to the optimum S* at constant ACN. Hence, an increase in salinity results in the transition WI > WIII > WII, while an increase in ACN does the opposite. Figure 3 corresponds to a typical system for EOR, containing 1 wt% of a commercial alkylbenzene sulfonate with 3 vol% of 2-butanol co-surfactant at a fixed temperature and pressure. It can be seen in Figure 3 (right plot) that the optimum formulation line at the center of the three-phase zone becomes exactly a straight line if the LnS scale is used in the ordinate.
Figure 3. Bidimensional (water salinity and oil ACN) formulation scan . S* is the salinity at optimum ormulation.
Consequently, the equation of the optimum formulation line in this LnS-ACN space may be written as LnS = KA ACN + CST, where the constant term (CST) depends on the other variables that do not change, i.e., the surfactant and cosurfactant, the temperature and the pressure. Such a bidimensional variation of both oil ACN and water salinity (for instance, between the two square dots located on the optimum line) may be expressed as ΔHLD = ΔLnS − KA ΔACN = 0 at all other constant variables.
Numerous studies from various laboratories in the past 40 years  indicated that this linear relationship is essentially valid (at least over some range of ACN of about 10 units for all ionic surfactants). However, the ACN coefficient KA value was found to depend on the surfactant, particularly its head group. It is 0.16 for alkylbenzene sulfonate, 0.10 for n-alkyl sulfate or carboxylate, 0.17 for the dihexylsulfosuccinate, and 0.20 for alkyl trimethyl ammonium salts. For extended surfactants with several blocks, such as the alkyl polypropoxy ethoxy sulfates or carboxylates, this KA coefficient is particularly low, i.e., 0.05 to 0.10, and depends on the exact structure—in particular, the alkoxylated central block size .
A slight departure from linearity with LnS has been found for complex head groups, mainly when there is an internal or external mixture of ionic and nonionic parts . A linear relation between S (instead of LnS) and ACN generally better matches ethoxylated nonionic surfactant . However, the effect is much weaker than for ionic species and a partitioning inconvenience occurs with commercial surfactants, affecting the numerical data accuracy.
This double scan technique with an exact compensation of two opposite effects was introduced by the University of Texas group in 1977 and was later applied to many different pairs of variables susceptible to altering the interactions of the surfactant with both oil and water. The equations summarized in Table 2 indicate numerical results from a double scan change involving two variables, i.e., ΔHLD = 0, from our data and other publications cited in the two previous paragraphs.
Table 2. Different cases of equations indicating the changes of two formulation variables to deviate from optimum formulation and to return to it (ΔHLD = 0) with coefficients found in experimental data.
HLD Equation–Surfactant Type
ΔHLD1 = ΔLnS − 0.16 ΔACN = 0 for alkylbenzene sulfonates ΔHLD2 = ΔLnS − 0.19 ΔACN = 0 for alkyltrimethyl ammonium chlorides ΔHLD3 = ΔLnS − 0.07 ΔACN = 0 for alkyl hexapropyleneoxide diethylenoxide sulfates ΔHLD4 = 0.33 ΔSAT − ΔEON = 0 for ethoxylated n-alcohol with EON~5 and T~25 °C ΔHLD5 = 0.13 ΔS − ΔEON = 0 for ethoxylated n-alcohol with EON~5 and T~25 °C ΔHLD6 = 2.25 ΔSAT − ΔACN = 0 for ethoxylated n-alcohol with EON~5 ΔHLD7 = − 0.24 ΔACN − ΔEON = 0 for ethoxylated n-alcohol with EON~5 ΔHLD8 = − ΔT − 20 ΔACN = 0 for n-alkyl sulfates ΔHLD9 = − ΔT − 14.3 ΔACN = 0 for alkylbenzene sulfonates ΔHLD10 = ΔT − 4 ΔACN = 0 for ethoxylated nonionic (EON~5–6 & T~20–30 °C) ΔHLD11 = ΔT − 1.4 ΔACN = 0 for ethoxylated nonionic (EON~8–9 & T~70 °C) ΔHLD12 = ΔT − 0.90 ΔACN = 0 for ethoxylated nonionic (EON~10–11 & T~80–90 °C) ΔHLD13 = − ΔGN − 0.12 ΔACN = 0 for polyglyceryl monolaurate (GN~5–6) ΔHLD14 = ΔLnS − 0.14 ΔPON = 0 for alkyl polypropyleneoxide diethylenoxide sulfates
where S is the salinity in wt% NaCl, EON is the exact or average number of ethylene oxide groups, SAT is the surfactant n-alkyl tail length in carbon atom number, T is the temperature in °C, and GN is the number of glyceryl group in polyglyceryl monolaurate oligomers.
3.1. The Normalized Hydrophilic Lipophilic Deviation (HLDN) Equation
The previous data comes from many different experiments. It should be noted that the numerical values of the equivalence of the change between two variables are arbitrarily written as K1 ΔV1 − K2 ΔV2 = 0, which could be systematically written as ΔV1 = K21 ΔV2 with the coefficient K21 = K2/K1, as the linear equivalence in the ΔHLD double change.
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 . 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 ΔHLDNi Equations (1)–(3). The minus sign is explained by the fact that an increase in ACN tends to decrease the numerator of R, thus reducing the HLD.
Table 3. Various cases of equations indicating the changes of two formulation variables to deviate from optimum formulation and return to it (ΔHLD = 0) with the same −1 coefficient in front of the ΔACN so that all the equations have the same scale.
Thus, all the previous equations will be written as follows with the properly evaluated or calculated Ki coefficient before any Vi variable. The normalized equation implies ACN and at least one Vi variable to produce the double scan compensation but can include 3, 4, 5, etc.
ΔHLDN = 1 − 1 = 0 = − ΔACN + ∑ ± Ki ΔVi with i = 2, 3, 4 etc.
If the double change does not include a unit variation of ΔACN, it should include variations of (at least) two other variables that are equivalent to ΔACN = ±1, as in the ΔHLDNi equations numbers 4, 5, in the previous list, which indicate the proper coefficient Ki before all formulation variables.
The data displayed in Table 3 deserve some specific comments. When a − ΔACN term appears in one of the equations, a positive effect of another term indicates that the corresponding variable change increases the HLD. It means that it is increasing the interaction of the surfactant with oil or diminishing its interaction with water by an amount equivalent to a negative unit of ACN. Therefore, from the previous Table 3 data, diminishing the ACN by one unit has the same effect as adding 0.44 carbon atoms in the surfactant n-alkyl tail (SAT) or adding 0.83 propylene oxide group (PON) in the intermediate part of an alkyl polypropoxy extended surfactant, or removing 0.24 ethylene oxide group from the head (EON) of an ethoxylated alcohol.
Transforming expression (2) to a differential equation level and taking into account that the first partial derivatives are constant (over some range) 
dHLDN = 0 = −dACN + ∑ ± Ki ΔdVi with i = 2, 3, 4 etc.
and integrating it
HLDN = 0 = −(ACN−ACNref) + ∑ ± Ki (Vi − Vi ref) with i = 2, 3, 4 etc.
Including an integration constant which is zero when all the variables are at a reference in which there is an optimum formulation.
Using only four variables to describe a simple system, i.e., surfactant, oil, water and temperature, the equation HLDN = 0 at optimum formulation can be written as follows for ionic and nonionic surfactants :
In the past few years and with or without clear explanations, the references have been taken as follows. Sref = 1, because ionic surfactants are salts that thus provide a minimum salinity or equivalent salinity in wt% NaCl, which is in general much smaller than 1 wt% . What is favorable is that, consequently, LnSref becomes zero in the ionic surfactant equation. A Sref = 0 often provides the reference for nonionic surfactants, but it is unfortunately different from the ionic case. Thus, it might be more logical to use Sref = 1 in all the cases, the term KS Sref being almost negligible in most cases. ACNref = 0 also simplifies the equations, even if it is not a very logical choice, since it corresponds to no carbon atom in the n-alkane, i.e., something which does not exist. However, an equivalent EACN = 0 was found to be some approximation for benzene . The temperature reference is often taken as 25 °C, but also at T = 0 °C, which simplifies the equation. This occurs even though it often corresponds to a solid-state of oil. In this simplified equation, there is no particular term for the pressure effect nor for a co-surfactant, initially called f(A) ; however, if this is so, the references would be the atmospheric pressure and the absence of co-surfactant.
3.2. The Normalized Surfactant Characteristic Parameter (SCPN)
The SCPN abbreviation means the normalized surfactant contribution term , also called σ/KA for ionic and β/KA for nonionics in the literature , including a very comprehensive recent review . To avoid confusion, the SCPref term is always taken as zero. However, it could be a misleading value when it is related to a characteristic surfactant curvature since it depends on many aspects other than the surfactant . The term SCPN changes with the surfactant, i.e., it increases by 2.26 when a carbon atom is added to its n-alkyl tail, and it decreases by 6.67 when an EON group is added to its polyethoxylated head). Then, its value depends on the references for all the variables, which is the reason why it is not a surfactant characteristic.
HLDN = 0 equation makes it possible to numerically express the conditions for an optimum formulation in the same scale (ΔACN unit change). It thus allows to make comparisons between the corresponding SCPN values for various surfactants, and it also makes it possible to calculate the values of mixtures according to
HLDNmix = ∑Xi HLDNi or SCPNmix = ∑Xi SCPNi
With X indicating the molar fraction at the interface, which is often approximated as the weight fraction in the system, although such linearity could be erroneous in the presence of partitioning of the different species, as in many commercial surfactants. .
4. Lipophilic and Hydrophilic Linkers, and Extended Surfactants
4.1. The Lipophilic Linker
Since the main difficulty was to increase the interaction on the oil side, a special C1 highly lipophilic amphiphile was used to replace the C1 surfactant, as indicated in Figure 4b, while the C2 surfactant was only slightly hydrophilic, and thus was likely to mainly go to the interface. This C1 species was typically a n-decanol or dodecanol that was not really a surfactant adsorbed at interface but rather a polar oil segregated in the oil phase close to the interface because of the affinity of the OH end group with water .
This kind of substance was called a lipophilic linker  because it was not a co-surfactant (since it was not adsorbed at the interface) but was a species of the oil phase staying close to the interface, as shown in Figure 4b. As a consequence of its location, the lipophilic linker was more or less sticking to the surfactant tail and somehow extending it further in the oil phase. Therefore, it was increasing the interaction on the oil side, and thus improving the performance.
Figure 4. Interfacial arrangements (a) with two ordinary surfactants C1 and C1, (b) with a surfactant and a lipophilic linker, (c) with an ordinary surfactant and a polar oil, and (d) with an extended surfactant and a polar oil.
4.2. The Hydrophilic Linker
It was suggested that a similar intermediate be placed on the other side of the interface between the surfactant head and the water and was called a hydrophilic linker . Its benefit was, however, much less significant, mainly because it was not producing a much higher penetration of the surfactant head group into water.
Moreover, it was known that the solubilization of a polar oil with an ordinary surfactant having a head group and an alkyl tail, as indicated in Figure 4c, was not very good because of a lack of perfect matching of the hydrocarbon tail with the polar oil. This is why adding a lipophilic linker in the oil phase close to the interface produced an improvement of the interaction with polar oil.
Nevertheless, the lipophilic linker was actually an oil phase component; a significant amount of it was lost in the oil bulk far from the interface, and thus partitioning was even worse with a polar oil. It was thus necessary to find a means to cumulate the favorable effects and avoid the unwanted ones.
4.3. The Extended Surfactant with an Intramolecular PO Extension
The idea developed in the 1990s  was to fasten together the rather hydrophilic surfactant and the lipophilic linker, producing the same effect as the mixture without losing a part of the lipophilic linker deviating from the interface. Since the lipophilic linker was an amphiphile with a small hydrophilic part located close to the interface, the single structure shown in Figure 4d to imitate the mixture situation is a so-called “extended” surfactant. It contains a polar head located in water, then an intermediate slightly polar zone in the oil phase close to the interface, and finally, the surfactant classical hydrocarbon tail. Figure 4d shows that the slightly polar intermediate extension is bent and interacts with the water molecules, producing more ACW interaction.
The central extension was selected to be a polypropylene oxide (PO) because it was neatly lipophilic and was thus located on the oil side of the interface with good interaction with polar oil molecules. It also perfectly plays its main role of displacing the alkyl part of the tail further away from the interface, as shown in Figure 4d, without the usual precipitation problem at increasing the tail size. It is worth noting that our original extended surfactants, e.g., C12PONEO2SO4-Na+, had a much longer intermediate (N up 14) than the alkoxylated surfactants available at this time, which had very few units only. It should be noted that a 10-unit PO chain is about three times longer than a C12 n-alkyl group, so that the actual tail extension is considerable, even if it is not completely perpendicular to the interface .
The PO chain is in the zone where the polar oil molecules are segregated, while the alkyl hydrocarbon part of the tail could be far away from the interface where most of the oil could be the non-polar one. This was an extra matching situation between the surfactant tail and the oil phase containing polar molecules. Sometimes, a very short ethoxylation (2 groups) is placed at the end of the PO chain, not really as a hydrophilic linker part, but rather because it makes an easier addition of the sulfate or other hydrophilic group .
The improving performance with polar oil was reported first by Miñana , particularly in mixtures of extended surfactants with ordinary species that were not solubilizing at all like triglyceride vegetable oils. Most research carried out in the following years used different anionic and nonionic heads sometimes connected by two EO groups to the propoxylated extension .
4.4. The Increased Performance of Extended Surfactant Systems with Polar Oils and Crude Oils
The general results obtained in the last 20 years have been extensively reviewed very recently  and should be examined in detail because of the large amount of available data. The main aspects can be summarized as follows. First of all, the PO chain is significantly lipophilic, and only sightly hydrophilic, i.e., much less than a polyethylene oxide chain. The PO unit is three times longer than a (CH2) methylene group in a surfactant typical n-alkyl tail; it thus considerably extends the surfactant penetration in oil and increases the interaction, particularly with polar oil segregated close to the interface. The first 2–4 PO groups stay close to the water phase, so the molecule is twisted in this zone, and is not really straight and perpendicular to the interface, as can be seen in Figure 4d . In some cases, the accumulation of the PO groups outside the water can be considerable, in particular at the air surface where they are said to form rugby balls . This arbitrary gathering of the first PO group in a short and twisted hydrated chain close to the water phase produces some disorder that helps avoiding the surfactant’s rigidity and precipitation. A branched structure, such as the Guerbet double tail, was found to improve the extended surfactant interaction with oil phases, in particular with di/triglycerides . An increasing number of PO groups from five to 15 definitively increases the performance, which practically stays constant at higher PON. A PON term may be introduced in the HLD correlation but not as a linear term since it is also dealing with the surfactant hydrophobe part length (SAT) . A more continuous change from hydrophilic to lipophilic parts, including an intermediate with an additional butylene oxide block between the alkyl tail and the propylene oxide block, improves, even more, the performance . This is probably because it results in a very wide zone with a smooth variation of hydrophilicity, which is particularly appropriate to interact with crude oils containing many polar species containing functional groups bearing nitrogen, sulfur and oxygen atoms, such as asphaltenes, naphthenic acids, etc. .
The last benefit found in an extended surfactant is that being an intramolecular mixture between ionic and nonionic parts, a proper adjustment of the different blocks could present new opportunities such as insensitivity to temperature  or insensitivity to surfactant concentration  and robustness to electrolyte concentration and mixture variations, including new applications with complex biobased polar oils .
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