Toxicity and environmental aspects of surfactants

3.1 From branched alkylbenzene sulfonates to linear alkylbenzene sulfonates

The branched alkylbenzene sulfonates have a branched alkyl chain attached at the benzene ring through a Friedel-Crafts alkylation reaction. The branched alkyl moiety is usually a propylene oligomer and propylene tetramer soon became the alkyl chain of choice. The subsequent sulfonation results in the SO₃ group being almost exclusively in para position to the alkyl chain. The commercial product is a mixture of isomers, and a representative structure is shown in Figure 1.

Figure 1:

Branched (left) and linear (right) alkylbenzene sulfonate.

The surfactant usually comes with sodium as counterion but the calcium salt, as well as salts of quaternary ammonium ions, are also commercially available. The structures shown in the figure are just examples of the many isomers that the commercial surfactants are composed of.

The sodium salt of the branched alkylbenzene sulfonate was introduced on the market in the beginning of the 1930s and it soon became the most important synthetic surfactant in both household cleaning and industrial cleaning formulations. Its popularity was partly due to a relatively low price and partly to its excellent performance. The raw materials, propylene tetramer, benzene, and the sulfonating reagent, which was initially oleum, were readily available at the time and both the alkylation and the sulfonation reactions could be performed in high yields. The surfactant was a good wetting and foaming agent and was effective in solubilizing oily soil. It was insensitive to pH and relatively resistant to hard water.

The branched alkylbenzene sulfonate (BAS) was the work-horse surfactant for the detergent industry until the 1960s when problems related to poor biodegradation started to become obvious. Persistent foams that were attributed to BAS were found in rivers and lakes, as well as in sewage plants, Biodegradation tests, which were introduced at that time, showed that BAS was very resistant to degradation by microorganisms. Figure 2 shows an example of an unwanted foam. The high stability of the surfactant, which had been regarded as an asset when it was developed and introduced on the market, was now seen as a severe problem.

Figure 2:

Extensive foaming at the wrong place. Picture from Fremont, California taken from Wikipedia.

The industry’s response to the biodegradation problem with BAS was to replace it with its linear counterpart, linear alkylbenzene sulfonate (LAS), which is also shown in Figure 1. BAS is still used in certain industrial applications where biodegradability is not important, but the volumes are small.

The linear alkylbenzene sulfonates were originally made by alkylating benzene with an alkyl chloride using AlCl₃ as catalyst followed by sulfonation. In the process used today an olefin is used instead of the alkyl chloride and the catalyst is usually HF. The product obtained, the LAS, has a straight alkyl chain attached to the benzene ring by any position on the chain except the two terminal ones. Thus, LAS, like BAS, is a mixture of isomers. The number of carbon atoms in the alkyl chain can be varied but 12 is the most common. (In fact, since the alkyl chain is a distillation fraction, the number of carbon atoms is not an integer; the value is 11.7–11.8 for most commercial products today). This product, often called dodecylbenzene sulfonate, is now the largest synthetic surfactant in terms of volume, with an annual sale of 4 million tons. It is often the main constituent of household detergents and many other cleaning products, where it is usually combined with a nonionic surfactant. It is readily biodegradable. ^{4 , 5}

Thus, environmental concerns triggered a switch from one alkylbenzene sulfonate to another, both produced world-wide in very large scale. The transition could have been dramatic, but it was smooth. The technology to produce LAS was similar to that used since decades to produce BAS and the same surfactant companies could produce the new surfactant in the same plant as the old one. For some applications the highly branched structure of BAS, with as many as six methyl groups, gave a character to the surfactant that was different from what LAS could offer. Methyl groups provide hydrophobicity to the surfactant tail and the more such groups, the more hydrophobic is the tail. However, for most applications the difference in performance between BAS and LAS was small. In hard water LAS even proved to be superior to BAS in detergency. ⁶

There are many reports about the toxicity of BAS to bacteria and protozoa in sewage treatment processes. ^{7 , 8} Acute toxicity of BAS to various freshwater fish has also been documented. ^{9 , 10}

The biodegradation of alkylbenzene sulfonates is now believed to be well understood. The degradation starts with a breakdown of the alkyl chain followed by elimination of the sulfonate group and finally degradation of the benzene ring. ^{11 , 12} For a linear alkyl chain biodegradation is initiated by ω-oxidation of a terminal methyl group generating first a terminal alcohol, then an aldehyde and ultimately an acid. The chain is subsequently degraded by β-oxidation. For a branched alkyl chain, this mechanism does not apply; instead, an α- and β-oxidation mechanism has been demonstrated. ¹³ Combined α- and β-oxidation is rare in microorganisms, which explains why the branched alkylbenzene sulfonates degrade so slowly in the environment. However, with prolonged acclimatization, they will ultimately be degraded. ¹²

3.2 From alkylphenol ethoxylates to fatty alcohol ethoxylates

Nonyl- and octylphenol ethoxylates were introduced on the market at about the same time as the branched alkylbenzene sulfonates, i.e. during the 1930s. The alkylphenols were made by Friedel-Crafts alkylation of phenol with branched olefins using an acid catalyst and they were subsequently ethoxylated. Ethylene oxide had been known since 1859, when it was synthesized by treating 2-chloroethanol with potassium hydroxide. However, it took until the 1930s for industrial production to start based on the development of the direct oxidation of ethylene with oxygen or air over a silver catalyst. The ethoxylation is normally initiated with potassium hydroxide which transforms the alkylphenol to alkylphenolate. The phenolate anion attacks the oxirane ring, initiating the polymerization. The length of the oxyethylene chain will simply depend on the ratio of ethylene oxide to alkylphenol. The product is always a broad mixture of homologues, and the homologue distribution can to some extent be varied by the choice of catalyst.

Figure 3 shows the structure of nonylphenol ethoxylate. Since the nonyl group is a mixture of isomers, the resulting surfactant will be a mixture of species with different structures in the alkyl chain (besides the broad distribution of homologues in the polyoxyethylene chain). Figure 3 also shows a typical homologue distribution of an alkylphenol ethoxylate.

Figure 3:

Nonylphenol (left) and the homologue distribution, which is a Poisson distribution, for nonylphenol with 10 mol of ethylene oxide added (deca(ethylene glycol)monononylphenyl ether) (right). The structure shown in the left figure is just one example of the many isomers that the commercial nonylphenol is composed of.

There are many similarities between alkylphenol ethoxylates and the branched alkylbenzene sulfonates although the ethoxylates are nonionic and the sulfonates are anionic surfactants. Both became popular rapidly due to a combination of low price and good performance and both shared the undesirable property of being harmful to the environment.

During the 1970s and 80s it became increasingly clear that alkylphenol ethoxylates not only exhibited slow biodegradation, but they were also toxic to marine organisms. Thus, the surfactant failed in OECD’s two main criteria related to classification of surfactants: (1) rate of biodegradation, which states that >60 % of the surfactant should have degraded into CO₂, water and minerals by biological processes within 28 days, and (2) aquatic toxicity, where the LD₅₀ value for fish should be above 1 mg/L. Since the alkylphenol ethoxylates did not meet these criteria, they were gradually phased out of markets, starting with consumer products such as laundry detergents and continuing with formulations for industrial use. In addition to relatively slow biodegradability and high aquatic toxicity, alkylphenol ethoxylates were found to exhibit hormone-like effects. In mammals and in fishes they can cause endocrine disruptive effects as they appear to feminize juvenile males by acting like oestrogens. The endocrine effect is today regarded as the most serious and was the main driving force behind the ban on this type of products in Europe in 2001. ¹⁴

The alkylphenol ethoxylates have largely been replaced by fatty alcohol ethoxylates with similar hydrophilic-lipophilic balance (HLB). However, this transition was not as smooth as the transition from branched to linear alkylbenzene sulfonates, discussed above. The alkylphenol moiety contains many methyl groups, as can be seen in Figure 3, while a natural fatty alcohol has only one methyl. Synthetic alcohols may be branched but they rarely contain more than two methyl groups. As discussed in the previous section, the more methyl groups in a surfactant tail, the more hydrophobic is the tail. Another difference between alkylphenol ethoxylates and fatty alcohol ethoxylates is the geometry of the surfactant. The alkylphenol moiety is voluminous compared to a fatty alcohol chain, particularly if the chain is linear. Surfactants with voluminous tails have higher values of the critical packing parameter (CPP) than surfactants with a long, straight chain, which, in turn, make them align better at surfaces and interfaces. ¹⁵

A third difference between alkylphenol ethoxylates and fatty alcohol ethoxylates is that the former, but not the latter, contains π-electrons. Surfactants containing π-electrons may form charge-transfer complexes with surfaces that contain electron-deficient olefinic bonds. This type of attractive interaction may be of importance when a surfactant acts as a dispersing agent for organic particles, e.g. certain organic pigments. Figure 4 illustrates formation of a charge transfer complex between an alkylphenol ethoxylate and a surface containing an olefinic bond.

Figure 4:

Nonylphenol ethoxylate can interact with a double bond in a surface by formation of a charge-transfer complex.

A fourth and important difference between an alkylphenol ethoxylate and a fatty alcohol ethoxylate is that the homologue distribution obtained from the ethoxylation process is different. The phenolic hydroxyl group is more acidic than an alcoholic hydroxyl group. This means that the phenolic hydroxyl group will preferentially be deprotonated, which, in turn, means that there will be no, or little, unreacted alkylphenol left after the ethoxylation, even for surfactants with only few moles of ethylene oxide added. ¹⁶ While a typical fatty alcohol with four mol of ethylene oxide will contain (15–20) % unreacted fatty alcohol, nonylphenol with four mol of ethylene oxide contains less than 1 % unreacted nonylphenol. Unreacted fatty alcohol can constitute an odor problem.

From the above it is obvious that replacing alkylphenol ethoxylates by fatty alcohol ethoxylates has not been a straight-forward process. They are still used in applications such as emulsion polymerization, pigment dispersion, and textile processing; however, the quantities are small compared to what they used to be. As a result, the concentrations of alkylphenol ethoxylates and its degradation products found in rivers and estuaries are nowadays at least one order of magnitude lower than what they were in the 1980s.

In sewage plants and in the environment alkylphenol ethoxylates break down from the hydroxyl end giving metabolites with progressively shorter oxyethylene chains. The terminal hydroxyl groups of the homologues may oxidize to carboxyl groups. The rate of degradation diminishes as the oxyethylene chain becomes shorter because of increasing hydrophobicity of the molecule. Nonylphenol with one or two oxyethylene units are typical long-lived metabolites. ^{14 , 17} It is these degradation products that are responsible for the endocrine effects discussed above.

3.3 From dialkyl quats to diester quats

Quaternary ammonium compounds with two short and two long alkyl substituents, often called ‘dialkyl quats’, were introduced in the 1960s as the active ingredient in textile softeners, also named conditioners. The long alkyl chains are typically straight-chain hexadecyl or octadecyl (often named ‘hydrogenated tallow’) and the short substituents may be methyl, ethyl, or hydroxyethyl. The surfactants come with a counterion, which may be chloride, acetate, or methyl sulfate. These amphiphiles are very hydrophobic. In water they form gels and liquid crystals instead of micellar solutions. ²

Very hydrophobic surfactants often exhibit slow biodegradation and the diquats are no exception. Regardless of the choice of counterion the products did not meet OECD’s criterion for ‘readily biodegradable’. The values for aquatic toxicity for the dialkyl quats were not a problem but that is not enough. Without approved values for both biodegradation and aquatic toxicity a product will not be approved within the OECD countries. Consequently, the industry was forced to look for alternative structures with similar properties. This search resulted in amphiphiles with similar structure but with ester bonds connecting the long alkyl chains and the central quaternary ammonium group, so-called ‘diester quats’. The structures of a dialkyl quat and a diester quat are shown in Figure 5. The transition from the dialkyl quats to the ester quats happened during a period of around 10 years, from 1985 to 1995. This was much faster that the transition from BAS to LAS and from alkylphenol ethoxylates to alcohol ethoxylates and the reason for the quick move was a strong market pull. The market for these surfactants is almost entirely the personal care segment and environmental concern is a much stronger driving force for consumer products than for industrial products. The main customers for these surfactants were the ‘big soapers”, i.e. Procter & Gamble, Unilever, Henkel, etc., and a ‘green’ profile is of high priority for these companies.

Figure 5:

A dialkyl quat (left) and a diester quat (right).

As can be seen from the figure, the dialkyl quat and the diester quat have very similar structures. Their physical chemical behavior is also similar, and their softening performance is about the same. ¹⁸ However, the routes by which they are prepared differ widely. The dialkyl quats are made by reacting a long-chain nitrile with a long-chain amine to give a secondary amine, which is then methylated to a tertiary amine and subsequently quaternized with methyl chloride or some other alkylating agent. The diester quats, on the other hand, are produced by reacting a fatty acid (or a reactive derivative of the acid) with a diol containing a quaternary ammonium group.

Different equipments are used for the two syntheses, which means that from an industrial point of view the transition from dialkyl quats to diester quats was not as straight-forward as the change from branched to linear alkylbenzene sulfonates. However, as mentioned above, the strong demands from the market around 1990 made it a fast transition.

The diester quats break down readily in the environment. Lipases are efficient catalyst for the ester bonds and the generated fatty acids biodegrade quickly. ¹⁹

3.4 Toxicity problems with traces of ethylene oxide and 1,4-dioxane in ethoxylates

Ethoxylated surfactants are made by reacting a long-chain alcohol, acid, amine, etc., with ethylene oxide. As mentioned above, ethylene oxide has been used as an industrial chemical since the 1930s and ethoxylation is one of the major chemical processes today. Ethylene oxide, a three-membered ring, is highly reactive and it was soon realized that it is a chemical that should be treated with caution. One indication of its biological activity is that one of its early uses was as a sterilant in the healthcare industry. Today, we know that ethylene oxide can cause damage to the brain and nervous system. It is classified as both carcinogenic and mutagenic. ^{20 , 21}

The surfactant industry has a long tradition of handling ethylene oxide in a safe manner, so the problem is not there. The problem lies in the minute residues of unreacted ethylene oxide in the surfactants. Ethylene oxide is a volatile compound with a boiling point of 11 °C; thus, it is easily removed by evaporation. The surfactant industry takes advantage of the high volatility and has developed methods to reduce the level of unreacted ethylene oxide to extremely low values.

Low values of residual ethylene oxide are particularly important for ethoxylated surfactants intended for the food, feed, pharma and personal care sectors. The threshold value of ethylene oxide in such products is very low and has been reduced as the analytical methods to quantify ethylene oxide has become more sophisticated. Advanced GC-MS methods today typically report detection limits for ethylene oxide in surfactants in the range of (0.01–1) ppm, depending on the type of surfactant and the preparation techniques.

The detection limit for ethylene oxide in cosmetic products is also very low, often around 1 ppm. In the EU the limit today is as low as the detection limit. The maximum allowed level of ethylene oxide in surfactants intended to come into contact with food is 0.1 mg/kg (0.1 ppm). These levels are indeed low, and, in addition, ethylene oxide is produced in vivo when we metabolize ethylene, which is present naturally in our bodies. We inhale ethylene from various sources, such as burning of biomass, incomplete combustion of fossil fuels, forest fires, exhaust from vehicles, etc. ²² Both ethylene oxide and ethylene are also components of tobacco smoke, and smokers and nonsmokers therefore have different exposure profiles to ethylene oxide. The concentration of ethylene oxide in different organs have been determined in animal trials. ²³

Against this background one would, from a scientific perspective, regard the minute amounts of ethylene oxide from surfactants as a negligible problem; however, that is not the case. The consumers today are very observant of what is in the products they buy. The mere fact that there can be traces of a carcinogenic and mutagenic compound in a surfactant that goes into cosmetic products or into food is today a problem for ethoxylated products and opens the door for alternative surfactants, not based on ethoxylation.

Another toxicity issue for ethoxylated surfactants is that of 1,4-dioxane, a substance that is classified as potentially carcinogenic. 1,4-Dioxane (often referred to as just ‘dioxane’ since the 1,2- and 1,3-isomers are not important) has a long tradition as a solvent for textiles, paper, flame retardants, and adhesives. Its acute toxicity is low with an LD₅₀ of 5170 mg/kg in rats but there are indications that it may be a carcinogen. It was listed as “reasonably anticipated to be a human carcinogen” by the U.S. Department of Health and Human Services in 2016. ^{24 , 25} Consequently, its use as a solvent has dramatically decreased during the last decades.

Dioxane is produced commercially by acid-catalyzed dehydration of diethylene glycol. However, it is also generated as a biproduct in ethoxylated products. Dioxane is formed in the normal process of making fatty alcohol ethoxylates by treating a fatty alcohol with ethylene oxide but the amount of dioxane formed with the traditional alkaline catalyst is very low. ^{26 , 27 , 28} Ethoxylation of a fatty alcohol can also be made with a Lewis acid catalyst such as SnCl₄ or BF₃ and the level of dioxane formed is then much higher. ¹⁶ Partly for that reason the acid route is not much used today.

The big concern with 1,4-dioxane in surfactants relates to sulfated fatty alcohol ethoxylates, the so-called alcohol ether sulfates. The typical alcohol in these products is dodecanol (lauryl alcohol) and the number of oxyethylene units is only two or three. Laureth sulfate is a commonly used name for these products and its formula is given below, see Figure 6. Laureth sulfate is frequently used in personal care products, hand dishwashing liquids, and many other consumer products.

Figure 6:

Laureth sulfate, an important surfactant in many consumer products.

Laureth sulfate is made by adding two or three mol of ethylene oxide to dodecanol followed by sulfation. Under the strongly acidic sulfation step, dioxane will be generated. A fraction of the laureth sulfate will decompose into 1,4-dioxane, SO₃ and an alcohol ether sulfate that has lost two oxyethylene units. This means that if the intended laureth sulfate contains three oxyethylene units there will be a small fraction of dodecanol with only one oxyethylene unit and if the intended laureth sulfate contains two oxyethylene units the biproduct will be dodecanol. The reaction scheme is shown in Figure 7.

Figure 7:

Formation of 1,4-dioxane during the synthesis of laureth sulfate. (Redrawn from ref. 27).

1,4-Dioxane is a liquid with a boiling point of 101 °C. This means that evaporation under vacuum is an obvious way to reduce the content in the surfactant. However, such a treatment does not eliminate all dioxane and other methods, such as adsorption on specific high-surface materials, are used in order to reduce the levels further. However, in the end there will always be traces of dioxane left in the surfactant, which means that the formulated product will not be completely free from the potentially carcinogenic biproduct. A recent report stated that in the US 53 % of laundry detergents, 59 % of shampoos, 62 % of body cleansers, and 69 % of dish soaps contained 1,4-dioxane levels above 1 ppm. ²⁷

Measuring very low levels of 1,4-dioxane in water is nowadays relatively straight-forward using advanced gas chromatography techniques. Measuring the dioxane content in consumer products is much more complex, however. Consumer products are complex formulations containing a diversity of ingredient classes, including surfactants, solvents, dyes, pigment, perfumes, polymers, preservatives, and so forth. It is therefore a challenge to quantify the dioxane content in these products down to ppm levels. The best approach appears to be capillary GC with MS or possibly MS-MS detection. ²⁹

Thus, ethoxylated surfactants, and in particular fatty alcohol ether sulfates, suffer from a problem with residual traces of dioxane in the products. The levels are very low, probably so low that the hazard can be neglected on scientific grounds. However, the consumer does not always follow what science tells. Like with remaining traces of ethylene oxide in ethoxylated surfactants, the mere risk of having 1,4-dioxane, a potentially carcinogenic substance, in a surfactant for personal care products is something that a growing number of consumers will not accept. It is, therefore, a major concern for the surfactant industry today.

Taken together, the small amounts of unreacted ethylene oxide and generated 1,4-dioxane in surfactants made by ethoxylation are regarded as a problem and have become one of the main driving forces behind the ambition to replace ethoxylated surfactants with something else. The alternative can be synthetic amphiphiles with a polyhydroxyl moiety instead of a polyoxyethylene chain as polar headgroup. The polyol may be either sugar or polyglycerol. There is currently a rapidly growing activity both in academia and in the surfactants industry to develop nonionic surfactants with glycerol instead of ethylene oxide as building block for the polar headgroup.

Another alternative is to switch to biosurfactants, which may be produced by macroorganisms or microorganisms. Examples of the first category are saponins and lecithin. Saponins are obtained from a variety of plants, the most well-known being the soap bark tree, Quillaja saponaria. Lecithin can be of either plant or animal origin, examples being soybeen lecithin and egg lecithin, respectively. Lecithin has been a commercial surfactant since long back and has an established position on the market while saponins can be regarded as emerging amphiphiles with limited commercial use today.

Natural surfactants produced by microorganisms are also growing in importance, the most prominent types being sophorolipids and rhamnolipids. These biosurfactants are produced by fermentation of either yeast or bacteria. Sophorolipids are produced by many non-pathogenic yeasts, principally from Starmerella genus, and rhamnolipids are produced by the pathogenic bacterium Pseudomonas aeruginosa. These natural surfactants are commercial and have found use in a wide variety of areas, the personal care sector probably being the most important today. The volumes are still small compared to the synthetic surfactants but are steadily growing. The fact that they are completely free of ethylene oxide and dioxane is used for the promotion. The high production cost is the main obstacle for a more rapid expansion. Biotechnological production of surfactants is still expensive compared to chemical synthesis even when cheap carbon sources, such as sugar and vegetable oil, are used. The reaction time is long, the yield is relatively low and the work-up is cumbersome. It is likely that the price difference between biotechnological and chemical production will decrease in the future.

3.5 Use of specific zwitterionic surfactants to achieve formulations with low skin toxicity

All surfactants show some kind of biological activity. They are designed to adsorb at surfaces, and they are usually able to penetrate porous material. Skin is a porous material, and it is then not surprising that many surfactants, when applied to skin, elicit irritant reactions. Surfactants are capable of interacting both with proteins and lipids in the stratum corneum. By penetrating through this layer, surfactants are also able to affect living cells in deeper regions of the skin. ³⁰

It has been known for a long time that some classes of surfactants are more skin irritating than others. Anionic and cationic surfactants are generally seen as more problematic than nonionic and zwitterionic surfactants. Cationics, in particular, have a reputation of causing skin and eye irritation, which is not surprising since this surfactant class has a long history as bactericides. Hence, they are more biologically active than the other surfactant classes.

One of the reasons why nonionic and zwitterionic surfactants are less irritating than ionic surfactants is believed to be that they have much lower values of the critical micelle concentration (CMC). For the same length of the hydrophobic tail the nonionic and zwitterionic surfactants have two orders of magnitude lower CMC than the ionic surfactants. The reason for this is that they do not carry any counterions. There is a considerable entropy penalty involved in the micellization of ionic surfactants because it leads to an accumulation of counterions around the micelle. ¹⁵

When formulating a personal care product, e.g. a shampoo, one cannot only go for mildness. Performance, such as cleaning and degreasing for a shampoo, is essential so a compromise may have to be made between good performance and mildness to skin and eye.

The first shampoos on the market were relatively eye-irritating but this changed with the discovery that when a fatty alcohol ether sulphate, a so-called laureth sulfate, see Section 3.4, was combined with certain zwitterionic surfactants a very mild formulation was obtained, which also exhibited good technical performance. The choice of zwitterionic surfactant was critical. Only those that could switch between a zwitterionic and a cationic state would work. Betaine surfactants and amine oxide surfactants are the zwitterionic surfactant types most commonly used for the purpose.

The pronounced mildness of such formulations can be explained as follows (and illustrated in Figure 8 for a betaine surfactant): The betaine surfactant is zwitterionic at neutral pH but if the carboxylate group becomes protonated it will turn into a cationic surfactant. This happens if the pH is reduced but a shampoo or a cleanser should have a pH of 5–6, which is the normal pH of the hair and scalp. Deviations from this pH can be harmful, particularly for persons with dry skin. ³¹ A pH of 5–6 is above the pK _a of the carboxyl group, so one would then expect the betaine surfactant to stay zwitterionic. However, when put into an aqueous solution together with the laureth sulfate the possibility appears to form mixed micelles involving the anionic surfactant and the betaine surfactant in protonated form, i.e. in the form of a cationic surfactant. Formation of mixed micelles of one anionic and one cationic surfactant is thermodynamically favored for the same reason as why micelles of nonionic surfactants are favored: there are no counterions around the micelle and, hence, no entropy penalty related to counterions. Formation of such a mixed micelle is so favorable that it triggers protonation of the carboxylate group.

Figure 8:

A betaine surfactant may take up a proton and become a cationic surfactant when put into an aqueous solution together with an anionic surfactant, such as laureth sulfate, even at a pH higher than the pK _a of the carboxyl group. The driving force is the possibility to form mixed micelles of anionic and cationic surfactant.

This means that the micelles are composed of one anionic and one cationic surfactant, the laureth sulfate and the protonated betaine surfactant, respectively. Such mixed micelles have very low CMC values, which means that the concentration of the unimers is very low. The unimers are an anionic surfactant and a zwitterionic surfactant, which is a good cleaning and degreasing combination. The low unimer concentration is advantageous from a dermatological standpoint because it is the unimers, not the micelles, that are skin irritating. Micelles have no surface activity and will not interact with the skin surface. This is a formulation that possesses surfactant synergy, meaning that the combination is better than the individual constituents. Such a combination is milder to the skin and eye than either of the two surfactants alone. Anionic surfactants alone can be relatively skin irritating. In this type of formulation, the zwitterionic surfactant is said to impart mildness to the laureth sulfate.

3.6 Surfactant-based CO₂ foam flooding for both enhanced oil recovery and carbon storage

Among the many versatile applications of surfactants, such as detergents, emulsifiers, wetting agents, etc., foam generation ability is one of the most important and has been exploited extensively. It was well understood that when a liquid (usually water) is agitated with gas, foam will be generated by the formation of gas cells whose walls consist of thin liquid films called lamellae. If surfactants are present in the aqueous phase, the surfactant molecules tend to adsorb onto the gas-liquid interface with their hydrophobic chain oriented toward the gas and the hydrophilic group toward the liquid, resulting in a decrease of the gas-liquid interfacial tension and an increase of the film elasticity (Figure 9). This helps to prevent lamella rupture and bubble coalescence, which contributes significantly to stabilization of the foam. ³²

Figure 9:

Arrangement of surfactant molecules at the foam lamella. ³³

In the petroleum industry, gas injection and water alternate gas (WAG) injection are two stimulation techniques used by operators to increase oil recovery. ³⁴ However, due to the low gas density and the low viscosity and high mobility of the injected fluids, severe fingering and channeling are encountered during gas or WAG implementation. This leads to quick gas breakthrough and poor sweep efficiency. To address these issues, foam flooding, which holds much higher viscosity compared to gas and WAG, was developed. Laboratory and field tests have demonstrated that foam flooding is able to improve both areal and vertical sweep. ^{33 , 35} As oil displacement is strongly dependent upon sweep efficiency, the use of a viscous foam is a way to achieve efficient oil recovery. Among the gases used to generate foams, CO₂ is advantageous because of its relatively high solubility in oil, which gives reduction of the oil viscosity, facilitates reaching a miscible state (low minimum miscibility pressure), and decreases the interfacial tension. ^{36 , 37} In addition, the carbon capture and storage (CCS) strategy to handle the ever-increasing global temperature is gaining considerable attention across the world. CO₂ Enhanced oil recovery (EOR), as a CCS technique, has been considered a realistic way to reduce the amount of greenhouse gases in the atmosphere from an economical point of view. ^{37 , 38} In this process, CO₂ is used as a valuable resource to boost oil production, while at the same time storing large amounts of anthropogenic CO₂ permanently in the geological subsurface formations. Therefore, foam flooding with CO₂ is believed to be a viable way to address the growing climate change concerns.

As mentioned previously, surfactants are commonly used to generate foams. Depending on the target reservoir properties such as temperature, formation brine salinity, and formation minerology, different kinds of surfactants have been found to be optimal to generate sufficiently stable CO₂ foams for EOR purposes. For instance, Qichao et al. reported the use of sodium lauryl polyoxyethylene ether sulfate, an important anionic surfactant, to generate CO₂ foam for carbonate reservoirs. ³⁹ Roozbahani et al. tested another important anionic surfactant, sodium dodecylbenzene sulphonate, for CO₂ foam flooding for both EOR and CCS purposes. ³⁷ Sodium dodecylbenzene sulphonate is wildly used in EOR because of its excellent foaming ability, low tendency for adsorption on the reservoir rock, high detergency, and cost-effectiveness. Gauteplass et al. used an alpha olefin sulfonate, an anionic surfactant with high foaming ability, to generate viscous foams to maintain favorable mobility control in fractured reservoirs. ⁴⁰ Besides anionic surfactants, both zwitterionic and nonionic surfactants have been explored for the creation of CO₂ foams for EOR. Ming et al. studied a zwitterionic surfactant, the pentasodium salt of N,N′,N″-dodecyl diethylene triamine pentaacetic acid, to generate stable foams in a saline environment for post-polymer flooding EOR. The oligomeric surfactant possessed superior foaming performance compared to the corresponding monomeric surfactant. A pilot test was conducted based on the laboratory results and promising results were obtained. ⁴¹ Nonionic surfactants, which possess high tolerance to salinity, were also investigated by various researchers in the field of CO₂ foam EOR. Pandey et al. used Triton X-100 as a foaming agent to investigate the rheology, stability, and viscoelasticity of CO₂ foams, as well as the influence of different electrolytes on foam properties. ⁴²

During the past two decades, with the rapid development of nanotechnology and the deepened insight into foam formation mechanisms, use of nanoparticles has attracted considerable interest as a way to create more stable and robust CO₂ foams. Numerous studies have verified that the incorporation of specifically modified nanoparticles into conventional surfactant formulations can significantly improve foam stability under harsh reservoir conditions. The nanoparticles can irreversibly adsorb at the gas–liquid interface, thus retarding foam coalescence, disproportionation, and film drainage. ^{38 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50} It is anticipated that the synergism between surfactants and nanoparticles will push CO₂ foam flooding to a new stage, which beneficially affects the environment by sequestering large amounts of carbon in geological formations.