Review on Development and Application of Eco-Benign Surfactants

Abstract

The development of eco-benign surfactants is the foundation of ingredients utilized in the pharmaceutical, laundry, household, and personal-care cleaning products. The types of surfactants, such as anionic, cationic, nonionic, zwitterionic, properties, toxicity, and thermodynamic feasibilities, are reviewed. The properties, such as critical micelle concentration (CMC), Kraft temperature, critical packing parameter (CPP), and foaming form the basis of solubility of these surfactants in reaction mixtures. The critical micelle concentration (CMC) is the fundamental concentration at which micelle formation is observed or noticed. It helps in the thermodynamic evaluation of surfactant activities in reaction mixtures. However, the Kraft temperature, which is also referred to as the melting point of micelles, is the foundation of the solubility of surfactants. The Kraft temperature is the point of intersection between the critical micelle concentration and solubility curves. The application of eco-benign surfactants is a developing trend in pharmaceutical, laundry, households, personal care, and remediation processes.

1. Introduction

Surfactant is a special chemical substance employed in the development and production of classified products utilized in the pharmaceutical, laundry, personal-care handling, metallurgy, oil and gas, and phenomena, etc. Surfactants are described as a special class of chemical substances that lower surface tension between two liquids, gas and liquid, oil and water, and water and oil [1].

Based on their chemical composition, it may act as detergents, wetting agents, emulsifiers, demulsifiers, foaming agents, or dispersants. Typically, it is composed of organic compounds having a hydrophilic head and a hydrophobic tail (see Figure 1). According to Obi & Idowu [2], surfactants are described as multipurpose organic compounds with tremendous applications.

Figure 1.

Diagrammatic representation of surfactant molecule.

The interaction of surfactants with aqueous medium constitutes the micellar environment. This aqueous orientation generates the different forms of micelles (Figure 2). The hydrophilic region (head), which is polar in nature, may be classified as anionic, cationic, neutral, or zwitterionic. However, the hydrophobic region (tail), which is nonpolar, is usually compound of hydrocarbons of one or more chains with varying lengths.

Figure 2.

Micelle formation [3].

In aqueous phase, these surfactants dissolve completely or with the aid of other chemical species at very low concentrations but above a certain limit called the critical micelle concentration (CMC) to form micelles (see Figure 2). The mechanism allows for detergency, which helps in cleaning processes [4]. The micelles formed vary in size and shape depending on the number of aggregation (N). The value of CMC can be monitored by varying the physicochemical properties of the surfactant solutions as the concentration of the surfactant molecules increases. This has been the foundation in drug formulations and management [5].

The utilization of surfactant is dated back to 1929 when the Swiss physiologist Kurt Neergaard in one of his experiments achieved a breakthrough in neonatology. The surfactant replacement therapy was actually a milestone in neonatology.

The application of surfactants is broad and varied. In the dyeing industries, that is. textiles, it helps in the penetration of dyes on the fabric [6]. Surfactants play significant role in preparation of different drug delivery systems. For drugs that are partially soluble in aqueous phase, pharmaceutically matchable surfactants (biosurfactants) or cosolvents are utilized to enhance solubility [7]. The presence of surfactants in drugs also improves their dissolution profile, permeation, and stabilization. Some examples of pharmaceutical surfactants include sodium lauryl sulfate (SLS), cetyl pyridinium chloride (CPC), sulfobetaine, and polyoxyethylene sorbitan fatty acid esters (polysorbate and tween), etc.

Surfactants are widely used as soaps and detergents in industrial, domestic, and personal-care handling activities [8]. Some of the common surfactants used in soap making include sodium stearate (SS), 4-(5-dodecyl) benzene sulfonate (DBS), dioctyl sodium sulfosuccinate (DSSA), alkyl ether phosphates (AEB), and benzalkonium chloride (BAC), etc.

Surfactants also find application in the cosmetics industries (cosmetology). Cosmetic formulations are composed of mixtures of surfactants with emulsifying, solubilizing, wetting, foaming, and dispersion properties [9]. Typical examples of surfactant used in this regard include polyethylene glycol ethers (PGE), sophorolipids, andrhamnolipids (biosurfactant), etc.

Surfactants are also referred to as oil field chemicals. They are used in oil recovery and enhancement processes and in the inhibition of corrosion during the transportation of crude oil [10]. The groups of surfactants in this category are known as emulsifiers and demulsifiers [11].

Emulsifiers and demulsifiers are amphiphilic substances. Its operations tend to promote dispersion of the phase in which they do not dissolve very well. Emulsifiers that are more soluble in water generally form oil-in-water emulsions, while those that are more soluble in oil form water-in-oil emulsions.

However, demulsifiers participate in the breaking of crude oil into oil and water phases. The operation of demulsifier is a criterion for the desirability of crude oil and the specification of pipelines. Some examples of emulsifiers include polyethylene glycol, carboxymethyl cellulose, and acacia, etc., while demulsifiers include ethoxylated phenols, ethoxylated alcohols, ethoxylated amines, and sulphonic acid salts, etc.

The application of surfactants (frothers) in mineral processing is based on contact angle. A contact angle is the angle at which hydrophobic materials form at the interphase between the liquid and the solid. The binding capacity of the mineral particles depends on wettability.

The flotation of hydrophilic or weak hydrophobic mineral particles is enhanced by the adsorption of suitable surfactants (chemical reagents) in order to lower the Gibb’s free energy. Some of the examples of surfactants used as frothers (reagents) include sodium ethyl xanthate (ionizing collectors), potassium isopropyl xanthate (nonionizing collectors), and inherently hydrophobic surfactants such as kerosene, creosote, and grease, etc. However, collectors are grouped into anionic, cationic, mixed, amphoteric, bio-collectors, and ionic, etc., [12].

Recently, surfactants have been applied in the stabilization of hydrophobic nano materials in water. The successes in its application in nanotechnology have led to the development of colorimetric sensors, which have varied and massive impact on numerous interdisciplinary applications.

Surfactants are widely used in biological genetics and engineering in formulating new and in modifying existing natural surfactants, in the production of proprietary medicines, and for chemical flooding in sandstone oil reservoirs. All this is understandable since the properties of surfactants are to greater extent responsible for the existence and the performance of cellular membranes, emulsification, solubilization, and of transport of compounds that are otherwise insoluble in living tissues.

The demand for eco-benign surfactant popularly referred to as biosurfactant has generated global concerns. These groups of new generations of eco-benign surfactant molecules often directly or indirectly obtained from renewable building precursors can be broadly termed as sustainable surfactants, which are increasingly becoming popular in many application activities such as microbial enhanced oil recovery (MEOR), reduction of carbon dioxide (CO₂) emission, metals remediation, laundry industry, cosmetic industry, and food processing industry, etc.

2. Types of surfactants

There are basically two major types of surfactants based on the charge on their hydrophilic end namely the nonionic surfactants, which have neither charge nor ion attached to the hydrophilic end, and the ionic surfactants with different types of charges or ions, either positive or negative or both attached to the hydrophilic end. Ionic surfactants are further grouped into three types based on the particular type of ion attached to the hydrophilic end: anionics, cationic and amphoteric surfactants.

2.1 Nonionic surfactants

These are surfactants without any charge or ion attached to their hydrophilic end and as a result do not ionize in aqueous solutions. They are usually neutral and compatible with other types of surfactants. They are polar and their polarity is derived from the rich oxygen molecule of the hydrophilic end and the large organic molecule end of the hydrophobic end. They also possess properties such as wetting, spreading, emulsifying, solubilizing, detergency, and foaming actions. Some nonionic surfactants are non-foaming which makes them effective materials for low-foaming detergents. Some nonionic surfactants possess high ratio of hydrophilic and hydrophobic moieties and as a result, have no cloud point, while most others possess cloud point. They are used in shampoos, conditioners, and shower gel. Examples of nonionic surfactants include ethylene oxide, propylene oxide, and alcohol ethoxylates (Figure 3). Some natural nonionic surfactants have been synthesized from sugar-based molecules to provide safer, milder, better, and nontoxic effects, for examples include saponins such as soapwort, soap nut, and alkyl poly glucosides such as coco glucosides, decyl glucosides, and lauryl glucoside (see Figure 4).

Figure 3.

Lauryl glucoside (pubchem.ncbi.nlm.nih.gov., [13]).

Figure 4.

Examples of cationic surfactants [14].

Other anionic surfactants include all cleansing or foaming products such as shampoo, shower gel, foam for bathing, micellar water, and shaving cream, etc.

2.2 Cationic surfactants

Cationic surfactants are surfactants containing positively charged ions on their hydrophilic end. They are usually used where foaming is not required as they cannot produce foam on their own. They are incompatible with anionic surfactants because the opposing charges do not combine in effect, they form insoluble and ineffective compounds, they are rather compatible with the amphoteric and the nonionic surfactants. Cationic surfactants are not used as hard surface cleaners as the positive charges have the tendency to be attracted to hard surfaces.

They are used as disinfectants because of their ability to serve as antibacterial and antifungal agents, their positive charge enables them to be used as antistatic products such as fabric softeners. They are used in conditioning hair as cleansing conditioner, conditioning balm, antifreeze, and detanglers, etc. Some common examples include cetyl pyridinium chloride (CPC), cetyl trimethyl ammonium bromide (CTAB), and dimethyl dido decyl ammonium bromide (DDAB) (see Figure 5), etc.

Figure 5.

Typical examples of amphoteric surfactants [15].

2.3 Amphoteric surfactants

Amphoteric surfactants also referred as zwitterionic surfactants contain both positive and negative charges on their hydrophilic end which cancel each other by creating a zero net charge. The reaction of amphoteric surfactants depends on the pH of a given solution, if the pH of the solution is acidic, the amphoteric surfactants become positively charged and have similar behavior as cationic surfactants with low foaming, but if alkaline, they develop negative charge and behave as anionic surfactants by providing high foaming and cleansing capabilities, at neutral pH, they have similar behavior as nonionic surfactants. They are compatible with all other surfactants, the anionic surfactants which they also stabilize their foam. They are widely used with nourishing and conditioning properties. Amphoteric surfactants when used alone give a gentle and milder cleansing. They are used in all cleaning, foaming, conditioning, solubilization, and antifreeze products such as cosmetics and shampoos. Common examples include amino oxides and natural ones include betaines such as cocoidopropyl betaine and amphoacetates such as sodium cocoamphoacetate and sodium lauroamphoacetate (see Figure 5).

3. Development of surfactants

Surfactants and synthetic detergents are the major foundation of industries in the 1930s. These two special compounds have begun to gain weight since the mid of the eighteenth century, that is. from mutton oil to green oil (petroleum sulfonic acid). Experts have predicted that the doubling of population from 2000-–2060 will bring about a corresponding increment in the development and consumption of detergent from 50 million to 120 million (a net increase of 1.4 cultures [16].

Surfactant design requires a thorough consideration of the selection of the hydrophile and hydrophobe pair such that they can be readily synthesized with minimal purification and posses optimum properties for a given application [17].

There have been massive works by researchers in changing the pathway of surfactant synthesis from chemical to greener and eco-benign route. In addition, there is a paradigm shift from the convectional pattern to a more sustainable production using renewable natural products.

The new era of eco-benign surfactants synthesis considers the stability of the ecosystem by choosing renewable natural raw materials.

The properties keenly looked at include renewability, cheapness and availability, less toxicity, and commercial feasibility with respect to kinetics and thermodynamics, etc.

There is a lot of synthesis of eco-benign surfactants in the literature such as preparation of alkyl polyglucoside (APG) using D-glucose and octanol [5] and consequent modification of natural surfactants from plants such as the modification of cashew nutshell liquid (CNSL) surfactant with triethanolamine used for chemical flooding in sandstone oil reservoirs [18].

Research team led by Professor Julian Eastoe has discovered a new magnet soap for oil spill remediation. The researchers broke this record by dissolving iron in various inert surfactant materials, composed of chloride and bromide ions similar to those found in products such as mouthwash and fabric conditioner. This ultimately resulted in a soap structure each with metallic center. This product was tested in the laboratory by covering test tube with a less dense layer of an organic solution. When a magnet was introduced from above, the soap reacted by breaking the surface tension between it and the organic solution, rising through that solution to reach the magnet.

4. Properties of convectional surfactants versus biosurfactants

The properties of surfactants to a large extent depend on the charge on the hydrophilic head (which could be positive, negative, neutral, or zwitterionic) and the ratios of the hydrophilic to lipophilic balance (HLB) [19]. The following properties are discussed in this chapter.

4.1 Critical micelle concentration (CMC) and micellar growth

This is a fundamental characteristic of surfactant development. It is the minimum concentration at which micelles are observed [20]. It is also the concentration in water at which individual molecules of surfactant begin to aggregate [21].

The aggregation formed occur within a narrow concentration range thereby causing some physicochemical properties such as surface tension, osmotic pressure, solubility, conductivity, absorbance, etc., of the surfactant solution change abruptly (see Figure 6).

Figure 6.

Variation of absorbance with surfactant concentration [5].

The aggregation of surfactant molecules by way of propagation initiates micellar growth. The higher the CMC value, the larger the growth of the micelles. Importantly, a high CMC value signifies that the surfactant is hydrophilic and vice versa. At concentrations above CMC, surfactants solubilize more hydrophobic organic compounds. CMC is also a determinant factor in defining surfactants’ antibacterial properties.

According to Perinelli et al. [22], surfactants with lower CMC demonstrate higher germicidal activity. Evidently noticed at concentrations above CMC, the freely dissociated surfactant molecules in the water bulk phase begin to form closed micellar structures that can be spherical, oblate, prolate, tablet, and rod-like (cylindrical) in shapes. The actual radii of these micellar structures formed are determined by surfactant concentration and geometry.

However, researches have shown that biosurfactants have low CMC and are generally effective in the lowering of surface/interfacial tension (Aranda et al., 2007; [23, 24]). It was also reported that their efficiencies can occur in extreme conditions such as temperature, pH, and salinity, etc. A group of researchers evaluated the interfacial and micellization behavior by mixing surfactin biosurfactant and synthetic surfactant sodium dodecylbenzene sulfonate (SDOBS). The result revealed that the associated molecules of surfactin and SDOBS demonstrated reduced electrostatic repulsion and promoted hydrophobic interaction between the molecules. The mole fraction of the biosurfactant in these mixed micelles was found 86% greater than its mole fraction in the bulk solution, indicating higher interfacial and self-assembly characteristics of surfactin relative to SDOBS [25, 26, 27].

4.2 Solubility and Kraft temperature

These two properties are key to making sure that surfactant molecules dispersed homogeneously on the surface or substrate. It is a chief factor in the application of surfactants in pharmaceutical and emulsion industries. Owing to the nature of the surface, that is. hydrophobic or hydrophilic, surfactants have limited solubility, which needs to reach the level of the CMC of the surfactant to be able to allow maximum performance.

The point of intersection between the solubility curve and the CMC curve of surfactants is referred to as the Kraft temperature (also called the melting point) (see Figure 7) [28].

Figure 7.

Variation of CMC and solubility as a function of temperature [2].

This is a point that shows the ability of an insoluble surfactant to become soluble. The Kraft point increases irregularly with increased alkyl chain length. The Kraft point can also be enhanced by addition of salt or decreased by the presence of other co-solutes. The solubility is equally enhanced by the manipulation of HLB values. A low value of HLB indicates a hydrophobic surfactant, while a high HLB value indicates a hydrophilic surfactant.

Experiment has shown that the solubility of hydrophobic organic compounds in the presence of biosurfactant depends on their relative concentrations, as well as the pH and salinity of the aqueous solutions. For example, the solubility of styrene in water was enhanced 30 times (from 3.1 to 84.3 mg/dL) when the rhamnolipid concentration in solution exceeded the CMC (0.2 mg/dL) [29, 30]. Rhamnolipids, a biosurfactant, can enhance solubilization of hydrophobic compounds starting at the CMC, with solubilization increasing with increasing hydrophobicity of the micelles [31].

4.3 Foaming

This process involves the lowering of surface tension in the presence of a surfactant. The initial foam height produced by the dispersion of surfactant solution onto another identical surfactant solution per unit time is a measure of foamability. However, the decay of the foam per unit time is a measure of foam stability. The foamability and stability of foam are commonly measured using the methods of Ross-Miles [32] and Bikerman [33]. Both parameters are intrinsic in different applications.

Pradhan & Bhattacharyya [34] are proposing for eco-benign surfactants with efficient surface and foaming characteristics.

4.4 Emulsification and critical packing parameter (CPP)

Emulsification is a thermodynamic process, which involves the formation of a liquid–liquid interface. Based on the structure of surfactants, microemulsions contain both nonpolar and polar regions, allowing the numerous industrial applications. The HLB and phase inversion temperature (PIT) are the two basic parameters in determining the behavior of surfactant in emulsions. The HLB has been discussed above. However, PIT depends on the structure of the surfactant and the nature of oil. It is noted that above PIT, the emulsion is of water-in-oil type, and below PIT, the emulsion is of oil-in-water type.

Jonsson et al. [35] noted that formation of thermodynamically stable microemulsions requires that the surface tension is extremely low or else emulsion may inform instead. He further noted that water-in-oil microemulsions in equilibrium with excess oil is called a Winsor I system, an oil-in-water microemulsion in equilibrium with excess water is referred to as Winsor III system, and a microemulsion in equilibrium in both excess water and oil is called Winsor II system.

Israelachvili [36] observed that CPP is a geometrical concept that is defined as the ratio of the volume (V) of the hydrophobic tail of the surfactant to the product of the optimal areas (a) of the hydrophilic head and the length (L) of the hydrocarbon chain. It is expressed mathematically thus;

$$CPP=\frac{\text{V}}{\text{a}\times \text{L}}$$

The CPP greatly influences the phase behavior of the surfactant. He revealed the following:

1. A low CPP ($<\frac{1}{3})$ indicates that spherical micelles can be formed (i.e., favoring oil-in-water emulsions).

2. A high CPP ($>1)$ favors the formation of reversed micelles (i.e., favoring water-in-oil emulsions).

3. An intermediate CPP ($\frac{1}{3}-1)$ will favor the formation of hexagonal, lamellar, and cubic micelles.

It is reported that hydrophobic surfactants will, thus, have high CPP values and weak hydrophobic will have lower CPP values. Addition of smaller hydrophiles to the surfactant solution has the tendency of increasing the CPP.

The CPP and PIT are somehow connected since an increase in the CPP decreases the PIT.

Extremely unfavorable packing possibilities might result in an inability of some amphiphilic substances to form micelles; such compounds have no CMCs, and not surfactants in a practical sense, but might still function as hydrotropes.

However, pieces of evidence have shown that biosurfactants as anti-agglomerants for ice, and hydrates provide improved efficiency for oil production [37]. The implication is that anti-agglomerants yield water-in-oil emulsions that limit hydrate growth to water droplets dispersed in oil [37]. The low CMC values and high emulsifying activity (formation of biofilms) (E₂₄ > 70%) exhibited by biosurfactants may make them attractive for applications where low surfactant concentrations are desired [38, 39].

4.5 Detergency and dispersion stability

The ability of the surfactant molecules to solubilize with the solid (dirt) in the aqueous phase is referred to as detergency. It is a fundamental property of soaps and most detergents used in laundry, domestic, and personal-care handling processes. The spreading abilities of the aggregates of surfactant molecules are referred to as dispersion ability.

The dispersion ability of surfactant molecules is estimated in terms of amounts of particles/aggregates (N) per amount of surfactant (gram per gram) that can be dispersed.

The detergency is achieved through adsorption and desorption. Detergency occurs through the following mechanisms:

1. Formation of emulsions

2. Solubilization

3. Creation of repulsive forces between fabric and the oily dirt.

Emulsion is best performed at the PIT of the particular oil-in-water surfactant system.

However, research has also shown that biosurfactant such as lipopeptide (Iturin A) showed enhanced resistance to flocculation and creaming instabilities in emulsion compared with surfactin and fencygin [40].

4.6 Adsorption and wetting

The adsorption process encountered by surfactant molecules is easily explained by using regular solution theory and Langmuir equation. The thermodynamics and kinetics of micellization are clearly described by adsorption. The thermodynamics of surfactant systems is very important, both in theory and in practice. This is because surfactant systems represent a system that is intermediate between ordered and disordered states. The thermodynamics of micelle formation is based on the interactions arising between micellar chains within the micellar core, polar head constituents, and the surrounding medium. It is a temperature-dependent evaluation. The thermodynamic parameters associated with this type of interactions include Gibb’s function of micellization, enthalpy change of micellization, and entropy change of micellization.

Obi & Idowu [2] reported that increase in temperature invokes a reduction in the hydration of the hydrophilic oxyethylene group, which in turn favors micelle formation.

The wetting process is achieved by the adsorption phenomenon. A wetting process strictly depends on properties of a surface of three phases (i.e., air, liquid, and solid). It can be enhanced by the addition of surfactant. Wetting to a large degree also depends on the physical properties of wettable solid. The equilibrium is achieved much quicker on irregular flat surface than porous surface.

The surface is characterized by high wetting, and the degree of wetting is determined by kinetics and thermodynamics of the wetting properties phenomenon. It can occur in three ways namely:

1. Spreading

2. Immersion

3. Adhesion

In spreading, a liquid placed on a solid surface increases contact surface, displacing a gas phase from a solid surface.

In adhesion, the liquid–solid contact surface does not change. The contact angle created will increase with the addition of surfactant, down to an optimum minimum of 0°, which means complete droplet spreading.

It has been established that adsorption increases as the CPP increases, although that is not always the case.

Generally, an increase in the hydrophobicity of a surfactant (biosurfactant) will increase the wetting ability of the surfactant toward a hydrophobic surface. Hence, biosurfactants use wettability property by creating a suitable environment for the attachment of bacterial adhesion thereby forming biofilms. Biofilms are the microbial communities that produce extracellular matrix [41].

5. Mechanism of biosurfactant synthesis

Biosurfactant mechanisms refer to step-by-step processes of actions, for example. Synthesis, reactions, dispersion processes, and other activities. They can be produced by enzymes, microorganisms, plants, and animals on microbial cell surfaces. Biosurfactants contain both hydrophobic and hydrophilic tail ends and act by accumulating at the interface between two immiscible liquids or between a liquid and a solid and lower their surface and interfacial tensions thereby lowering the repulsive forces between the two different phases, allowing the different phases to mix and interact more readily with themselves.

Based on the structural makeup of biosurfactants, two synthetic pathways were proposed by Sourav et al. [42]. Firstly, one pathway leads to synthesis of hydrophiles (enzymatic synthesis). However, the second pathway leads to the synthesis of hydrophobes (microbial synthesis).

The hydrophobic components are synthesized by simple lipid mechanism. However, the hydrophile components display a greater degree of structural complexity. Therefore, wide varieties of synthetic routes are adopted.

According to Syldatk and Wagner [43], four principal sequences of biosynthesis of such biosurfactant molecules (amphiphiles) can be adopted:

1. Both the hydrophilic and hydrophobic components are synthesized de novo by two separate pathways.

2. The hydrophilic component is synthesized de novo, whereas the synthesis of the hydrophobic component is induced by substrate.

3. The hydrophobic component is synthesized de novo, whereas the hydrophilic component is made substrate dependent.

4. The synthesis of both hydrophobic and hydrophilic components depends on the type of substrate utilized.

Sourav et al. [42] noted that the recovery of the biosurfactants after synthesis is simple, cost-effective, and eco-benign (see Table 1).

S/N	Recovery technique	Biosurfactants recovered
1.	Solvent extraction	Liposan and trehalolopids, etc.
2.	Centrifugation	Trehalolipids
3.	Adsorption	Glycolipids and lipopeptides, etc.
4.	Crystallization	Glycolipids and cello biolipids
5.	Ultrafiltration	surfactin, glycolipids
6.	Foam separation and precipitation	surfactin

Table 1.

Recovery techniques and types of biosurfactants recovered [42].

Biosurfactants are basically synthesized following enzymatic and microbial pathways [41]. Enzymatic synthesis can be prepared via biocatalysis involving the use of enzymes as catalyst. They can be enhanced to possess the desired physicochemical properties with respect to the applications. Enzymes, such as lipases, glucosidases, and phospholipases, etc., are majorly adopted in the synthesis of surfactants [44]. The various examples of enzymatic synthesis that lead to production of biosurfactants include synthesis of monoglycerides [45], synthesis of sugar esters [46], synthesis of alkyl glycosides [47, 48], and synthesis of lysophospholipids [45], etc.

However, microbial synthesis involves the use of microorganisms to produce varieties of surfactants mostly on cell surfaces or excreted extracellularly. Microbial synthesis of biosurfactants generally follows separate pathways to form the hydrophilic and hydrophobic moieties, which are then subsequently combined [43]. These two moieties may be formed through de novo synthesis, or they may be derived from carbon substrates available to the cells from their environment. Hence, because of the simplicity, cost-effectiveness, eco-benignity, and availability of these microbes, we are going to dwell more on their synthetic pathway.

5.1 Production of biosurfactants by microorganism

Microorganisms naturally produce microbial-derived surfactants called biosurfactants. Microorganisms produce biosurfactants due to their ability to survive on some hydrophobic substances and also the desorption processes from the hydrophobic substances, which allow direct contact with the cell, which hitherto increases the bioavailability of the insoluble substances and changes the properties of the microorganisms’ cell surfaces. Microorganisms require some sources, such as carbon, nitrogen, minerals, vitamins, and growth factors, etc., to propagate and form products. These microbially derived surfactants have different chemical structures and hence diverse chemical properties. Ramkrishna et al. [44] noted that every biosurfactant has its natural role in the life cycle of the microorganism that produces it.

Biosurfactants-producing microorganisms can produce biosurfactants in aqueous medium using carbon sources such as glucose, fructose, glycerol, olive oil, agricultural waste such as sugarcane molasses and corn steep liquor [49, 50, 51], etc., and nitrogen sources such as urea or dried stalks as part of cultivation left over as substrates. In formulating the production medium, the first step required is the balanced equation based on the cell and product generation as:

Carbon and energy source + Nitrogen source + Heat + Nutrients → Carbon dioxide + Water + Biomass + Products.

Many types of biosurfactants are in existence, which includes glycolipids, phospholipids, lipopeptides, fatty acids/neutral lipids, and polymeric biosurfactants. Three examples of glycolipids include rhamnolipids, trehalolipids, and sophorolipids [52]. Rhamnolipids are produced by Pseudomonas aeruginosa and P. aeruginosa, Trehalolipids are produced by Mycobacterium tuberculosis, Rhodpococcus erythropolis, Arthrobacter spp., Nocardia spp. and Corynebacterium spp. and Sophorolipids are produced by Torulopsis bombicola, Torulopsis petrophilum, and Torulopsis apicola. Typical example of phospholipids includes phosphatidylethanolamine produced by Acinetobacter spp. and Rhodococcus erythropolis. Example of fatty acid biosurfactant is Spiculisporic acid produced by Penicillium spiculisporum. Examples of Lipopeptides biosurfactants include surfactin produced by Bacillus subtilis (Figure 8), and Lichenysin produced by Bacillus licheniformis. Examples of Polymeric biosurfactants include Emusan produced by Acinetobacter cacoaceticus RAG-1, Alasan produced by Acinetobacter radioresistens KA-53, Liposan produced by Candida lipolytica, etc. [54].

Figure 8.

Typical structures of biosurfactants produced by microbes [53].

Formulating the culture media is done by introducing pure compounds essential for the growth of microorganisms with properties such as high selectivity toward the target products over the unwanted products, ability to form a persistent product, and easily disinfected, etc. In developing a suitable medium for biosurfactants, two important considerations are made firstly, it should be noted that different microorganisms require different quantities of different nutrients and that each microorganism has a different metabolic pathway and also a different way of factoring the sugar and lipid moieties in its chemical structure, secondly, some microorganisms have high salt intolerance so the use of adequate quantity of chelating agents in the medium to facilitate the solubilization of the iron components to avoid the inhibition of their metabolic pathways.

To produce a hydrophilic moiety of biosurfactants for the growth of microorganisms, water-soluble substrates of carbohydrates are used, and the hydrophobic substrates, such as fats and oils, are used for building the hydrophobic moiety. Some of the pathways for the production of precursors of biosurfactants productions require carbon sources that are natural in the culture medium. The movement of carbon can be regulated via lipogenic pathway and glycolytic pathway restrained by microbial metabolism. A water-soluble substrate such as glucose is broken down to glucose-6- phosphate (G6P) by glycolytic pathway. In the formation of hydrophilic moieties, a variety of enzymes are used in catalyzing G6P, while in the formation of hydrophobic moiety, glucose is oxidized to pyruvate that can transform into CoA for the synthesis of malonyl-CoA react with oxaloacetate that can be converted into fatty acid for the production of lipid (Figure 9) [56].

Figure 9.

Metabolic pathways of biosurfactants using hydrocarbon substrate [55].

To complete the process of biosurfactant formation, some multienzyme complexes are needed after the production of lipid and sugar moieties. The two most common biosurfactants are rhamnolipids produced by P. aeruginosa and surfactin synthesized by B. subtilis. Rhamnolipids production is done using two glycosyl transfer reactions, with each catalyzed with a different rhamnosyltransferase. Surfactin production is catalyzed non-ribosomally by a large multienzyme peptide synthetase complex called surfactin synthetase [57].

5.2 Mechanisms of action for biosurfactants

The four mechanisms of action for biosurfactants include the following:

1. Membrane disruption

2. Interference with quorum sensing (QS)

3. Inhibition of swarming motility and biofilm formation

4. Inhibition of tissue adhesion

Membrane Disruption: Some compounds get bound to the cell membrane through electrostatic and hydrophobic interactions. Some peptides disrupt cells by forming pores in the cell membrane, which induces the thinning of the bilayer and bilayer disorder, which eventually disrupts the cell and leads to the death of cells. Example surfactin when bound to cell membrane becomes sensitive to membrane dipole potential, negatively charged lipids, and acyl chain length causing degradation of the polar region and disordering effects leading to solubilization and leakage and eventual death of cell; Iturin A when bound to the cell membrane is sensitive to Mg²⁺ and sterols, lipid phase state and acyl chain length, while Fencygin is sensitive to Ca^2+, pH and ionic strength, preferential electrostatic interaction with PE, lipid aggregation and intrinsic curvature all these cause dehydration of the polar region and disordering effects leading to solubilization and leakage and eventual death of the cell [58].

Interference with quorum sensing (QS): Quorum sensing is a process of cell–cell communication, which allows bacteria to share information about cell density and adjust gene expression [59]. It is also seen as a form of intercellular communication used to coordinate the physical processes and cooperative activities of bacteria at the population level [60]. It depends on the production, secretion, and detection of small diffusible autoinducers, such as acyl-homoserine lactones, auto-inducing oligo-peptides, and auto inducer 2, which increase in concentration as a function of cell density. Steps in quorum sensing range from production of small biochemical signal molecules to the passive or active release of the signal molecules to the recognition of the signal molecules by specific receptors and finally to changes in gene regulations. Using Pseudomonas aeruginosa the bacteria that produce rhamnolipids as example, P. aeruginosa virulence traits are controlled by 2-distinct lux-type quorum sensing systems termed las and rhi named after their influence on elastase and rhamnolipid production, respectively. The las system regulates the rhi system as virulence regulators. The hierarchical circuitry controls multiple virulence traits beyond elastase and rhamnolipid, including exoprotease and toxin production, motility, and biofilm formation [61]. The Las and rhi system has been seen to significantly reduce virulence animals’ infections.

Inhibition of swarming, motility, and biofilm formation: Bacterial swarming motility and exopolysaccharide contribute to biofilm formation. In P. aeruginosa, Psl (a repeating pentasaccharide) and Pel (a repeating pentasaccharide) are two major exopolysaccharides that serve as biofilm matrix components. Psl contains l-rhamnose, which is an essential matrix component used by P. aeruginosa to initiate biofilm formation and the maintenance of biofilm structure [52]. Loss of Psl production greatly attenuates P. aeruginosa biofilm formation [62, 63].

Inhibition of tissue adhesion: Adhesion mechanisms involve appendages capable of binding to some chemical species on certain surfaces [64].

According to Karlapudi et al. [41], microbes that produce biosurfactant undergo physiological metamorphosis as represented in Figure 10.

Figure 10.

Biosynthetic pathway of bacteria producing biosurfactant [13].

5.3 Emerging application of biosurfactant in oil spill remediation

Oil spillage is a natural or artificial phenomenon that pollutes the fragile ecosystem. In fact, it is a devastating effect to aquatic domain.

Surfactants, which are the key to the dispersant’s effectiveness and efficiency, are compounds comprising of two different kinds of chemical groups: one that is oil-compatible (lipophilic or hydrophobic) and one that is water-compatible (hydrophilic). The lipophilic end of the surfactant molecule has London dispersion forces similar to those of the oil, allowing this portion of the molecule to be soluble in the oil layer. On the other hand, the hydrophilic end has dipole–dipole forces and/or hydrogen bonding similar to that of water, allowing this portion of the molecule to be soluble in the water. This dual action helps in the lowering of the interfacial tension between the oil and the water, allowing the oil to break up into small droplets surrounded by surfactant molecules. The selection of surfactants with the appropriate hydrophilic–lipophilic balance (HLB) values is determinant in the proper remediation of spilled oil.

The use of chemical surfactants (dispersants) had been reported for their toxicity to aquatic domains, so were, referred to as unsuitable for oil spill remediation.

However, biosurfactants are currently used in enhanced oil recovery such as microbial enhanced oil recovery (MEOR) [41, 65, 66] and chemical flooding in sandstone oil reservoirs [18].

MEOR activates the stimulation of native reservoir microbes (hydrocarbonoclastic bacteria) or mechanically injecting specially selected natural bacteria into the oil reservoir to produce specific metabolic events promoting the synthesis of biosurfactants, which ultimately lead to improved oil recovery [67, 68, 69]. This is achieved through the reduction of oil viscosity and interfacial tension, which results from a partial breakdown of the large molecular structure of crude oil, making it more fluid-like. This process leads to the production of CO₂ gas as a byproduct of microbial metabolism both pressurize the reservoir and move upward, displacing oil in the well. Additionally, the production of biomass that accumulates between the oil and the rock surface of the well displaces the oil, making it easier to recover from the well [68].

It is also applied in the recovery of residual oil with low permeability or high viscosity [67].

According to Bach et al. [70], biosurfactant adsorbs the oil by altering the wettability capacity of the porous media (spilled site). They noted that emulsion produced by Acinetobacter venetianus (ATCC 31012) at 0.1 mg/mL removes 89% of crude oil, which had been reabsorbed to the samples of limestone, and 98% of removal was achieved at 0.5 mg/mL.

Yakimov et al. [71] also noted that many bacterial species that produce biosurfactants had been described for the microbially enhanced oil recovery in situ applications that belong to Bacillussps. because of their thermal and halotolerance ability. A typical Bacillus strain was grown and produced lichenysin by both anaerobic and aerobic processes at relatively high temperatures ranging from 40 to 60°C.

6. Challenges and way forward

The simultaneous manipulation of surfactant’s structure with respect to functionality, economics, and eco-benignity possess great difficulty in the replacement of convectional surfactants (i.e., petroleum source). Surfactants used as soap are obviously sustainable under appropriate condition but show difficulty at lower temperatures and insensitivity toward hard water. This weakness is a fundamental driving force in the development of synthetic and eco-benign surfactants. The solubility of surfactants in aqueous phase also poses great challenge in its applications. The toxicity of convectional surfactants has posed serious danger to their applications. The toxicity of these surfactants is based on their structural conformation [72]. The elevated concentrations of surfactants and their degraded products interfere with microbial dynamics, and their important biogeochemical phenomena hinder plant-surviving processes and their ecological habitant, and retard the human organic and systemic functionalities [73]. The following are few toxicities X-rayed in this discussion.

Skin infection toxicity: Research has shown that ionic surfactants are the most toxic if they are soluble in water [74]. They assert that crystalline ionic surfactants of low solubility show low toxicity. However, for nonionic surfactants, the nature of the chemical bonding linking the polar head region, and the alkyl chain hydrocarbon has great impact on skin toxicity.

Aquatic perturbation toxicity: Surfactants are new emerging contaminants (EC), about 15 million tons of surfactants produced globally are routed directly into wastewater streams [75]. Aquatic toxicity varies strongly among living organisms such as fish, daphnia, algae, or bacteria. Above listed, bacteria appear to be the least sensitive species in case of acute and chronic toxicity. Examples of such surfactants that affect aquatic habitant include branched alkylbenzene sulfonates (ABS), ditallow dimethyl ammonium chloride (DTDMAC), and nonionic nonylphenol ethoxylates (NPEO), etc.

Fire-control chemicals toxicity: These are chemicals used to protect forest resources from wildland fires. These chemicals contain several components including foam. The foam concentrates are typically composed of anionic surfactants, stabilizers, and solvents. These chemicals in large concentrations and applications affect both aquatic and human ecosystems [76]. Examples of the surfactant component of the foam include sodium dodecyl sulfonates (SDS) and linear alkyl benzene sulfonates (LABS), etc.

Cellular toxicity: This occurs as a result of surfactants used as herbicides or pesticides. The composition of the herbicide (i.e., surfactants, solvent, and anti-freeze) affect the membrane activity, metabolic activity, mitochondrial activity, and total protein synthesis rate in a cell culture [77]. Research has shown that sodium lauryl ether sulfate (SLES) and polyoxyethylene lauryl ether significantly destroyed the cellular activities.

However, literature has shown that biosurfactants are generally low in toxicity [41]. In addition, several studies have explored biosurfactant toxicity in plants, aquatic life, and human cell lines. For example, Santos et al. [78] reported that biosurfactants produced from C. lipolytica were found to have no impact on seed germination or root length of various plant species at concentrations ranging from 0.5 to 2 times the CMC. Santos et al. [78] also reported that brine shrimp also showed no signs of cytotoxicity when exposed to a glycolipid biosurfactant.

This study encourages researchers to rise up to the challenges by propounding seamless and greener route for the development/synthesis of biosurfactants through modification of sustainable hydrophilic head groups and hydrophobic tail groups and designing other natural moieties that will be compatible to extreme conditions such as temperature, pH, and salinity, etc. [34, 79, 80]. It is reported that the utilization of low-cost microbial substrates through genetic engineering combined with improvements in the downstream purification could significantly scale up the economics, eco-benignity, and the availability of biosurfactants for large scale applications [42].

7. Conclusion

This chapter has in a nutshell evaluated the synthesis of biosurfactants which is the way forward and exposed in general the properties, applications, toxicities, and challenges of the old and new trends. It is believed that the adoption of eco-benign route of synthesis will be a panacea in solving several toxicities experienced by both human and aquatic habitants.

Therefore, this study encourages researchers to develop toxicity testing procedures and cost-effective pathways of production and recovery that will not limit its use.

Declaration of interest

The authors state no conflicts of interest concerning the preparation of this manuscript.