Open access peer-reviewed chapter
Abstract
Amphiphiles form a large group of supramolecular structures can aggregate and be adsorbed spontaneously at the interface. Amphiphilicity is a feature of polar contrast between the groups that make up a molecule and their spatial separation. The most important classes of amphiphiles are surfactants, lipoproteins, and polymers that have hydrophilic and hydrophobic chemical moieties covalently bonded and spatially separated. Since surfactants are widely used in various industrial fields, we decide to focus on surfactants in addition to a brief review of the other amphiphiles. Surfactants are used in industrial applications and consumer products, from medical to cosmetics and food industry. Various industries require new surfactants from sustainable and renewable raw materials with improved performance, biocompatibility and minimal environmental impact. For example, liquid phase exfoliation and dispersion methods using surfactants in the solvent media have recently gained lots of attention because of their great potential for large-scale production. Notably, an ideal exfoliation for reaching desired graphene and CNTs may be achievable by molecular engineering of surfactants to improve the quality of molecular interactions. This chapter experimentally and theoretically highlighted physico-chemical characteristic parameters, and interactions of the components, which are essential to design and discover efficient exfoliation and dispersion systems.
Keywords
- amphiphilic
- conventional amphiphiles
- advanced amphiphiles
- hdyrophilicity
- hydrophobicity
- surfactants
- critical micelle concentration
1. Introduction
Since amphiphiles mainly form a large group of supramolecular structures, it is important to study their structure and properties. Amphiphilicity is one of the important structural features because two opposing parts (hydrophilic and hydrophobic) exist simultaneously in the structure of a molecule. This property causes such molecules to be able to spontaneously assemble at the interface of polar and non-polar two phases or to be forced to self-assemble to be in one phase. According to Figure 1, amphiphiles are molecules consisting of a hydrophilic “head” and a hydrophobic “tail”, which the two parts are linked on the basis of covalent bonds [1]. As can see in Figure 2, their hydrophilic heads can contain ionic, polar, non-ionic groups or form hydrogen bonds that lead to their dissolution in water. Their hydrophobic tail may also include saturated or unsaturated hydrocarbon chains or cyclic hydrocarbons. Their dual nature leads to spontaneous placement at the boundary between two phases, such as liquid–liquid, liquid–gas or solid–liquid, as a result reducing the surface tension and surface energy between the phases. Amphiphilicity is a scalable property that occurs in a molecular-level, nanoscale system and can probably be seen on a micro scale. The scalability of amphiphilic properties is a fundamentally new and hot topic and is considered in basic research [2, 3, 4].
Figure 1.
The structure of amphiphiles and pseudo-amphiphiles.
Figure 2.
Schematic structure of amphiphiles.
2. Brief historical account of amphiphiles
The word amphiphile comes from the Greek words amphis, meaning “both,” and philia, meaning “love.” Surfactants are found as amphiphiles in plants such as tomatoes, onions, mulberry leaves, ginseng, licorice, and all plants that contain glucosides. In the past, humans used mulberry leaves to clean their hands, which were stained with blackberry juice, without being aware of the dual nature of these substances and their washing.
Soap is another amphiphilic substance that has been made and used since ancient times in Mesopotamia [5]. Soap is a surfactant of fatty acids, and in the past it was made by natural synthetic methods from natural raw materials such as tallow, olive, argan or palm oil, and alkali-rich ash, the ash left over from wood burning. After the preparation of soap in Mesopotamia, its production has spread throughout continental Europe (Figure 3). In addition to helping to improve the quality of life, soap can eliminate or reduce many diseases in densely populated areas, and contribute to the development of urban life. Additionally, many Mediterranean regions, such as Provence in the south of France, or Castile in Spain, Florence in Italy, earn money by producing soap and cosmetics due to the rich resources of natural olive oil they have [6]. Olive oil is rich in palmitic, stearic and unsaturated oleic, linoleic, and unsaturated linolenic acids, which leads to the production of quality soaps. Sulfate oils were the first synthetic surfactants to be prepared after soap in 1834 by Runge via mixing olive oil and sulfuric acid. In 1875 sulfate castor oil, also known as Turkey red, was prepared as a dye additive. Additives were used in the textile industry [7]. Sulfate oil is not a pure surfactant, but a mixture of sulfate esters, water and fatty acid surfactants. In 1935, Colgate-Palmolive developed the first soap-free shampoo using mono surfactants and sulfated diglycerides [7]. Despite the long history of amphibians, these compounds are still of interest to scientists, who are looking to design new types of amphiphiles for use in a variety of fields.
Figure 3.
Time axis for the evolution of surfactants toward the present and future. Sources: Ash photo: (Pile of ashes after the fire went out. Ash texture. Burned out ashes grunge texture. Australia Shutterstock stock photo ID: 1611309511 (
3. Types of amphiphiles
As mentioned earlier, amphiphilic molecule is a general term for a compound that contains two parts with different covalent bonds and affinity for polar and non-polar solvents. The polar part has a high affinity for polar solvents (such as water) and the non-polar part has a high affinity for non-polar solvents such as hydrocarbons, ethers and esters. Surfactants, simple fatty acids, polymeric amphiphiles, and some lipid molecules, containing hydrophilic and hydrophobic components, are typical examples of amphiphilic molecules. Now a dayes, due to the use of amphiphilic compounds in various industrial and medical fields, scientists are looking to design and synthesize new types of these compounds. The hybrid of conventional building blocks with biomolecular and mineral components has led to new structures of these compounds with unique self-assembly properties. For example, the combination of an alkyl chain and a short peptide sequence results in the synthesis of amphiphile peptides that can aggregate spontaneously into various one-dimensional structures [8]. Other examples include giant amphiphiles from a single-protein hybrid [9]. Polygonal oligomeric silescoyoxanes [10] or polyoxometates [11], which spontaneously accumulate in new structures such as toroids and other complex architectures, show interesting features that are significantly different from ordinary amphiphiles. Interestingly, these new designs have led to the synthesis of giant amphiphiles (supraamphiles). Unlike conventional amphiphiles based on covalent bonds, new amphiphiles are designed based on non-covalent interactions or dynamic covalent bonds [12, 13, 14, 15]. In supraamphile, functional parts can be connected by non-covalent bonds. In this way, these compounds can be synthesized without the need for boring chemical syntheses. Building blocks for superaphiphiles can be small molecules or polymers. The development of supraamphiphiles not only enriches the common amphiphilic family, but also provides a new type of building block for complex self-assemblies, including hierarchical self-assemblies and functional nanostructures. Since surfactants are the most obvious and widely used group of amphiphiles, after a brief review of the types of amphiphiles, we will try to explore this group of amphiphiles in more detail. Amphiphilic molecules can be divided into two major categories based on the type of bond that connects all parts of the molecule as follows.
3.1 Conventional amphiphiles
In this type of molecule, all the parts are connected only by covalent bonds.
3.1.1 Bioamphiphiles
Many biological molecules, such as proteins, phospholipids, cholesterol, glycolipids, and bile acids, are also made up of two parts, hydrophilic and hydrophobic, and have a dual amphibian property called bioamphophiles.
3.1.1.1 Amphiphilic proteins
Amphiphilic proteins are composed of sequences of polar and non-polar parts of amino acids. For example, a protein may consist of hydrophilic moieties of polar amino acids (such as Asp-Ser, Tyr-Glu) and hydrophobic moieties of nonpolar amino acids (such as Gly-Pro, Ile-Pro-Met). This is the structure of membrane proteins in biological membranes. Their hydrophobic nature causes them to place themselves in the hydrophobic and non-polar regions of a biological membrane and expose their hydrophilic part to the polar environment. These hydrophilic parts of the protein lead to their interaction with polar molecules. Most of these amphiphilic proteins have these seemingly opposite interactions due to their amphiphilic helix structure. An amphiphilic helix is a protein helix that has two opposite sides. The face along the long axis of the spiral is hydrophilic, while the opposite face is hydrophobic. Thus, it can separate the hydrophobic and hydrophilic parts of a protein, resulting in a protein–protein interaction. Amphiphilic helices are a common structural feature in proteins. Ion channel membrane proteins, lung surfactant proteins, and apolipoproteins are examples of proteins with this structure [16].
3.1.1.2 Phospholipids
Phospholipids are also bio-amphiphilic molecules which are lipids with a glycerol group attached to two fatty acids and a phosphate group. Glycerol forms a phospholipid hydrophilic head by binding to a negatively charged phosphate group. The phosphate group may bind more to hydrogen, choline, serine, ethanolamine, or inositol, thus converting to phosphatidic acid, phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol phosphatidyl, respectively. Two long chains of fatty acid form the hydrophobic tail of phospholipids, which is lipophilic. The amphiphilic nature of phospholipids has made them an essential component of biological membranes. For example, the plasma membrane consists mainly of two phospholipid layers. Phospholipids can interact with different molecules depending on their polarity. Phospholipid heads readily interact with water and other polar molecules. Conversely, phospholipid tails avoid water and other polar interactions. Thus, phospholipids in water accumulate by directing their tails toward each other while exposing their heads to the aquatic environment. In fact, it is the amphiphilic nature of phospholipids that leads to the formation of the bilayer structure of the plasma membrane. Phospholipid tails are oriented in such a way that their tails are placed inside the plasma membrane while the head of phospholipids is placed outwards.
3.1.1.3 Cholesterol
Cholesterol, which is composed of a hydrophilic hydroxyl group (-OH) and a bulky hydrophobic steroid and hydrocarbon chain, is also a bioamphiphile. Cholesterol is found in the plasma membranes of animals. Its hydrophilic part interacts with the aqueous medium and with the polar heads of phospholipids, and its hydrophobic part, in turn, is located next to the hydrophobic tail of phospholipids and the chain of non-polar fatty acids of other lipids in the membrane.
3.1.1.4 Glycolipids
Glycolipids, which are composed of hydrophilic sugar groups attached by covalent bond to the hydrophobic lipid tail, are considered amphiphiles. They are also present in plasma membranes like cholesterol. Their carbohydrate portion extends to the outer surface of the cell while the lipid portion lies in the lipid bilayer. The residues of the sugar group, which are located on the outer surface of the cell, allow carbohydrate-carbohydrate interaction.
3.1.1.5 Bile acids
Bile acids are steroid-structured amphiphiles consisting of four rings and a side chain ending in carboxylic acid and hydroxyl groups. Bile acid salts, like surfactants, can form micelles by accumulating around lipid droplets. They can also emulsify lipids and prevent fat droplets from accumulating into larger fat particles.
3.1.1.6 Saponins
Saponins, which are abundant in plants, are made up amphiphilic compounds composed of a glycoside, which forms their hydrophilic moiety, and a triterpene, or steroid derivative, which is their hydrophobic moiety. These compounds are bitter and toxic and reduce the good taste of plants.
3.1.2 Polymer amphiphiles
Polymer amphiphiles are copolymers that consist of a block as a hydrophobic tail and a block as a hydrophilic head. While most copolymer amphiphiles are composed of two blocks, some may consist of three blocks and other geometric shapes and exhibit amphiphilic behavior. There are many polymer amphiphiles, for example, Pluronics, also known as poloxamer, is a synthetic block copolymer composed of hydrophilic poly (ethylene oxide) (PEO) and hydrophobic poly (propylene oxide) (PPO), which are located in a triple structure A-B-A, and form PEO-PPO-PEO.
3.1.3 Surfactants
According to Figure 4, surfactants are usually organic compounds that are amphiphilic. These compounds are synthetic or biological (biosurfactant) or may be extracted from natural materials such as some plants (glucosides or natural surfactants). Careful studies show that surfactants have unique applications in nanotechnology, such as their use in the synthesis, modification of properties, stabilization, self-aggregation, drug deliveries, and so on. Therefore, it can be said that they play a key role in empowering future science and technology. Surfactants are promising new areas of research in various industries of the future, as they can be used in the preparation of important materials that form the basis of various industries, such as carbon nanotubes or graphene. What makes surfactants attractive is that, unlike other molecules that remain there after being dissolved in water, they will always look for an adsorption interface after dissolution, whether solid–liquid, liquid–liquid, or liquid–gas. They can be said to be a kind of “smart” molecule. This behavior leads to their ability to assemble into extraordinary structures called self-aggregation. For this reason, in this chapter, among the different types of amphiphiles, surfactants will be explored in detail as part 3.
Figure 4.
The chemical structure of some of surfactants.
3.2 Advanced amphiphiles
These compounds are also called supermolecular amphiphiles (or superamphiphyls). They are a new bridge between colloidal science and supramolecular chemistry and provide a context in which we can make full use of our imagination. In these amphiphiles, all segments are connected by non-covalent such as electrostatic interaction, π-π stacking, charge-transfer interaction, hydrogen bonding, and host−guest interaction [14, 15] or dynamic covalent bonds such as imine and disulfide bonds, which are similar to noncovalent interactions under certain condition [17, 18]. In other words, these types of amphiphiles, unlike conventional amphiphiles, have hydrophilic and hydrophobic parts that are connected by physical bonding; consequently, these compounds and their aggregations are smart to external stimuli such as gas, light, temperature, etc.
3.2.1 Hybrid amphiphiles
Hybrid amphiphiles are compounds whose hydrophilic and hydrophobic moieties can be organic or inorganic compounds that are joined by non-covalent or dynamic covalent bonds. These compounds can exhibit additional functional properties such as special magnetic or catalytic properties or new properties that none of the components alone can have [19].
3.2.2 Janus amphiphiles
The word “Janus” in Greece is a word that is the opposite of itself. Thus, Janus amphiphiles, which are usually much larger than conventional surfactants, have both hydrophilic and hydrophobic components on the same (hard) particle in their structure. In recent years, Janus amphiphile particles have attracted a lot of attention due to their asymmetric and multifunctional structure [20, 21, 22, 23, 24]. In contrast to particles with uniform surface wettability, the surface of Janus amphiphile particles is divided into two parts with different wettability. Since Janus amphiphile particles have a unique structure, they exhibit special behaviors.
4. Principal types of surfactants
Surfactants are classified in different ways based on the chemical structure of the head group, the chemical structure of the tail group, the electrical charge of the head group, the type of application in industry, and the raw material from which they are derived.
On the one hand, the structure of the components of surfactants affects their application in various fields, thus its study is of particular importance. On the other hand, this book focuses on the self-assembled structures of materials and for the accumulation of surfactants, the chemical structure and charge of the head group and the chemical structure of the tail group are important, hence in this section we try to classify them from this perspective.
4.1 The electrical charge of the head groups
4.1.1 Anionic
Anionic surfactants are surfactants that have a negatively charged head group. In fact, any negatively charged organic compound with a positively charged ion (counter-ion) introduces an anionic surfactant.
4.1.2 Cationic
Cationic surfactants are surfactants whose head group has a positive charge. This charge may be permanent or only within a certain pH range. According to Figure 5, the negative charge of anionic surfactants is mostly on the oxygen atom and is available, while the positive charge of cationic surfactants is on the nitrogen group surrounded by alkyl groups. Consequently, the distances from the micellar core-water interface to the charges on anionic (SDS) and cationic (CTAB) surfactants, dch,SDS and dch,CTAB, respectively, need not be the same.
Figure 5.
The distances from the micellar core-water interface to the charges on anionic (SDS) and cationic (CTAB) surfactants, dch, SDS and dch, CTAB.
A new model has been proposed for estimating the surface potential and electrostatic free energy (gelec) using the capacitive model due to the difference in the location of the charge of anionic and cationic surfactants. Compared to the work of others, it is applicable to accurately study the changes in electrostatic energy and free transfer energy in transition of the accumulation of surfactants to each other [25].
4.1.3 Zwitterionic
Zwitterionic or amphoteric surfactants are surfactants whose functional group has both positive and negative charges. Like cationic surfactants, these charges can be permanent or pH dependent.
4.1.4 Nonionic
Nonionic surfactants are surfactants with polar head groups without electric charge. They usually have a functional group that can be slightly deprotonated. Therefore, despite its good solubility in polar solvents such as water, it does not work well as a Brønsted acid. In general, nonionic surfactants, unlike ionic surfactants, do not have good solubility and cannot change the pH of the solution.
4.2 The chemical structure of the head groups
4.2.1 Carboxylic acid salt, RCOO: M+
Carboxylates or carboxylic acid salts are surfactants in which the head group (hydrophilic group) is a carboxylate group, COO−. Figure 6 shows the structure of some of these surfactants.
Figure 6.
The structure of some of carboxylic acid salt surfactants.
4.2.2 Sulfonic acid salt, RC6H4SO3−M+
The second most important class of anionic surfactants are sulfonic acid-based surfactants. Figure 7 shows some of the most important compounds in this class. One of the most important surfactants in this category is sodium dodecyl sulfate, which is the sodium salt of dodecyl sulfonic acid. This surfactant has many industrial and medical applications. In addition to the surfactant made from aliphatic sulfonic acid, sodium dodecyl benzene sulfonate, one of the most common anionic surfactants, is made from aromatic sulfonic acid.
Figure 7.
The structure of some of sulfonic acid salt surfactants.
4.2.3 Sulfuric acid ester salt (sulfate salt), ROSO3−M+
Sulfate salts consisting of a predominantly linear alkyl chain having a terminal sulfate ester anion neutralized with an opposite ion. The most common structural feature of these surfactants is the presence of a predominantly linear aliphatic hydrocarbon chain with a neutral polar sulfate with an opposite ion (e.g. Na+, K+, NH4+, or an alkanolamine cation; Figure 8). The hydrophobic hydrocarbon chain (with lengths between C8 and C18) and the polar sulfate group provide surfactant properties in these compounds and allow the commercial use of these materials as anionic surfactants.
Figure 8.
The structure of sulfuric acid ester salt surfactants.
4.2.4 Phosphoric and polyphosphoric acid ester, R(OC2H4)xOP(O)(O−M+)2 and (R(OC2H4)xO)2P(O)O−M+
Anionic surfactant-phosphate ester-contains phosphate fraction as a hydrophilic head. Phosphate ester surfactants are produced by the reaction of alcohols with a phosphoric acid derivative. The property of phosphate ester depends in part on the structure of the primary alcohol used in its preparation. From the reaction of phosphoric acid with alcohol, polyphosphate ester surfactants can be prepared. Figure 9 shows the structure of anionic surfactants of phosphate ester and polyphosphate ester.
Figure 9.
The structure of phosphoric and polyphosphoric acid ester surfactants.
4.2.5 RNH3+Cl− (salt of a long-chain amine), RN (CH3)3+ Cl− (quaternary ammonium chloride)
Cationic surfactants have almost exclusively nitrogen-containing functional groups such as amines, quaternary ammonium, and almost imidazolium, and more recently pyridinium functional groups (Figure 10). Ammonium surfactants are active and at low pH values when the amine group is protonated, are cationic and hydrophilic. Thus, the amine group is a potential factor in surfactants for making pH-responsive amphiphiles.
Figure 10.
The structure of some of cationic surfactants.
4.3 The chemical structure of the tail groups
The chemical structure of the tail group of surfactants is divided into perfluoroalkyl, alkyl, alkyne, silicon, saturated, unsaturated, etc.
5. Surfactants self-assembly in solution
In the hydrophobic phenomenon, the hydrophobic tail of a surfactant molecule is not soluble in water due to the presence of hydrogen bonds between water molecules and the lack of strong bonds between surfactant molecules and water molecules. According to Figure 11, when the surfactant is placed at the air/water interface, its hydrophilic head group is in the water (liquid) phase, and its hydrophobic tail is in the air, which is a more suitable environment for it. Since it takes less energy to bring a surfactant molecule to the interface than a water molecule, adsorption is a spontaneous process. In other words, the surfactant molecule overcomes the hydrogen bond between the water molecules at the water surface by adsorption at the interface, thereby reducing surface tension. For each surfactant, depending on the structure and charge of the head group and the structure of its tail group and according to environmental conditions at a certain concentration after saturation of the interface, the tail of surfactant molecules assembles in a water bulk and forms aggregation. This specific concentration is called the critical micelle concentration (CMC). Since the sphere is the most stable geometric shape, the first micelle formed will be spherical. With the tail accumulation of surfactants together, the entropy of the system decreases thermodynamically. This accumulation, which occurs mainly due to the hydrophobicity of the tail group of surfactants, minimizes their undesirable contact with water molecules because it is more suitable for the tailings of surfactants to be placed together in bulk. As a result, the combination of these factors will lead to the stability of the system and the formation of aggregation. In other words, although the surfactant phase accumulates more regularly (lower entropy), water molecules that had an unfavorable orientation around the surfactant sequences are now released, leading to a net increase in the net entropy of the universe. Therefore, the process of micelle formation is a spontaneous process. At concentrations below CMC, the adsorption of the surfactant molecule at the interface between air and water depends on the surfactant concentration, and increasing the concentration leads to an increase in adsorption and thus a decrease in surface tension. However, after reaching the CMC, the additional surfactant molecules that are added to the bulk phase simply form the micelles, because this path is energy efficient and the surface tension of the liquid phase will remain constant after the CMC point and does not alter with the concentration changes.
Figure 11.
The schematic of (a) surfactant adsorption at the air-water interface and (b) surfactant aggregation in bulk solution.
5.1 Aggregation and packing parameter
As mentioned earlier, the CMC point is the first concentration at which it is thermodynamically preferred that surfactants accumulate in the form of spherical micelles. This is a phenomenon known as self-aggregation, and almost all amphiphiles spontaneously form regular structures that are thermodynamically stable. In these self-aggregating structures, the aggregation number is an important parameter. In fact, it shows the average number of surfactants in a micelle. On the one hand, experimental studies prove that the radius of the micelles cannot be longer than the length of the alkyl surfactant chain. On the other hand, these studies show that the diameter of the micelle is very close to the length of its alkyl chain, so the inside of the micelle is very compact and water or any other polar liquid cannot be placed in it.
The aggregation number can be obtained in two ways:
From the ratio of the volume of micelle to the volume of a surfactant molecule:
From the ratio of micelle area to the area of the surfactant:
The above two terms are equal thus we have:
This parameter, which is
This parameter indicates that, in the structure of a typical surfactant, the alkyl chain has the largest segment in the volume of the molecule and the dimensions of the micelle, and the hydrophilic head group exclusively contributes to the interface of the micelle and water. Therefore, the above equation can be interpreted as the volume of alkyl chains divided by the area of the hydrophilic group at the common level multiplied by the length of the alkyl chain.
According to Tanford equations [26], the hydrophobic chain volume of the micelle, v, and the critical chain length, lC, can be obtained from.
and
where nC is the number of carbon atoms in the hydrophobic chain of the surfactant. It has been proposed [27] that the surface area per headgroup, a0, is the most important factor controlling aggregate size. According to Figure 12, the packing parameter determines whether micelles are spherical (P < 1/3), nonspherical (1/3 < P < 1/2), vesicles or bilayers (1/2 < P < 1), or [28] “Inverted” structures (P > 1).
Figure 12.
Graphic representation of individual surfactant geometry and the corresponding aggregate structure [
In fact, the packing parameter is responsible for the curvature of the self-assembled structure based on the surfactant geometry. Studies show that for charged ionic groups, a0 is not the area occupied by the hydrophilic group, but the area of interaction of the two electrical layers determined by the length of Debye. The volume of the hydrophobic micelle core is mainly affected by the branching and length of the alkyl chain. The main weakness of the compaction parameter, which limits its practical use, is that first the type of self-assembly structures produced by a surfactant must be determined, then the compaction parameter must be calculated. For example, when this parameter is
Eventually, the two most important parameters for identifying a surfactant and preparing its solution are the Kraft point and the cloud point. The Kraft point is the temperature at which the value of CMC is the same as the solubility of the surfactant. The Kraft temperature for a given hydrophilic group increases with increasing carbon content in the hydrophobic group and decreases with branching. For a given hydrophobic group, the Kraft temperature decreases with increasing number of hydrophilic groups and for ionic surfactants decreases with increasing degree of counter-ion hydration [34]. To prepare a low temperature surfactant solution, it is critical to select surfactants with a low Kraft temperature. Cloud point is the temperature at which non-ionic surfactants in water are insoluble, resulting in the solution becoming a cloudy suspension that scatters light.
5.2 Inter-aggregation (long distance) and intra-aggregation (short distance) interactions
In this section, we will try to examine the interrelationships between the interactions that are responsible for the self-aggregation of surfactant molecules and amphiphiles and the interactions that occur between aggregations as they approach each other.
Essentially, there are four types of interparticle interactions: hard-sphere, soft (electrostatic), van der Waals, and steric. The hard-sphere interactions are repulsive and only significant when the particles are close to each other at distances of slightly less than twice the radius of the hard sphere [35].
5.2.1 DLVO theory: soft (electrostatic) and van der Waals interactions
The interactions between soft particles (electrostatic) and van der Waals (vdW) are considered in a theory independently proposed by Derjaguin and Landau [36] and Verwey and Overbeek [37], hence the so-called DLVO theory. According to this theory, in order for particles to disperse, a balance must be struck between attractive and repulsive energies. This theory states that the stability in the solution of surfactant (colloidal solution) is determined by the sum of the electrostatic repulsion forces associated with the double layer around the aggregates formed by these materials and the attraction and vdW forces between them. This theory suggests that a potential barrier to repulsive force prevents two particles from coming close to each other and adhering to each other. Therefore, if the repulsive particles are high enough, the dispersion will resist coagulation and the colloidal system will be stable.
Repulsive interactions are believed to be either due to double electrical layers with the same charge surrounding the particles or due to particle-solvent interaction. In other words, attractive interactions are mainly due to van der Waals forces between particles. For particle dispersion, repulsive interactions must be large enough to overcome attractive interactions. To accumulate particles, it must be reversed.
5.2.2 Steric interaction
One of the effective interactions in the formation of self-assembly structures is the interaction caused by steric hindrance [35], which is also known as non-DLVO interaction. This interaction creates two effects: (1) mixing effect and (2) entropic effect. The mixing effect is due to solvent-tail interactions and the high concentration of tails in the tail-solvent contact area. This effect becomes noticeable when the interaction of the tail of the surfactant with the solvent is stronger than the hydrophobic interaction between the tails, in this case the tendency of the particles to accumulate will increase and then the free energy of the system will decrease. In the other hand, if the tail-tail interaction is more than the solvent-tail interaction, then the hydrophobic interaction between the tails leads to the formation of aggregation. The entropic effect is due to the limited motion of tails that expand in the liquid phase when adjacent particles approach each other.
Both effects lead to an increase in the accumulation in the liquid phase by increasing the number of tails and increasing their length. However, the length of the tails should be optimized, because the possibility of aggregation increases with the increase in the length of the surfactant tail and the increase in the hydrophobicity phenomenon.
In cases where the liquid phase is a liquid other than water, the steric hindrance has the best effect on the aggregation when the head group has limited solubility in the liquid phase, as a result, despite the steric hindrance of the tails, monomers prefer to accumulate to reduce the free energy of the system. In general, it can be concluded that since the formation and deformation of self-assembled structures of surfactants largely depends on electrostatic, vdW, hydrophobic and steric interactions [38, 39] and the balance between them; consequently, structural parameters such as head group, chain length and counter ion on the surface charge density of micelles and the interaction between them and as a result have an effect on the size and shape of the aggregates. Also, environmental effects including ionic strength, temperature, electrolyte conditions such as salts and ionic liquids also have an effect on the interactions and as a result the size and shape of the aggregates.
Generally, it is possible to control the CMC parameter and the packing parameter by properly designing the head and tail structure of surfactants in such a way that in addition to reducing the CMC, different forms of aggregations can be produced based on their application. For example, atoms such as nitrogen or oxygen that can form intramolecular hydrogen bonds can be used in the surfactant head group structure. Investigations show that the formation of intramolecular hydrogen bonds leads to a decrease in CMC and an increase in aggregation number [40].
Since ions with the opposite charge are attached to the micelle surface, the nature of these ions affects the properties of micelles. For example, the size of micelles formed with a cationic or anionic surfactant increases with increasing counter ion size. Micelle size is not only a function of the size and electronegativity of the counter-ion, but also depends on the size of the hydration layer surrounding the counter-ion. So that weakly hydrated ions (smaller and highly electronegative) that are adsorbed closer to the micelle surface can neutralize the surfactant charge more effectively and lead to the formation of smaller micelles.
6. Complex self-assemblies beyond the conventional self-assemblies
The chirality of a compound, which is its lack of mirror symmetry, is a fundamental concept that affects the assembly, creation, and biological, chemical, mechanical, and optical properties of materials, and is intricately involved in the structure of life. Helical self-assembled structures with controllable twists have recently attracted considerable attention because they mimicks biological helices and can be used to design new chiral materials. However, coordination polymers (CPs) with helical morphology have rarely been discovered so far. In particular, chirality inversion via external actuation has not been achieved in helical CPs. Based on this, Zhang et al. have been able to synthesize coordination polymers (CPs), superhelices, which consist of metal ion nodes and organic linkers. In this study, they succeeded in providing a new way to fabricate helical CP nanostructures with desirable chirality, which have potential applications in chiral catalysis, chiral separation, and chiroptics [41]. In another work done by Diao et al., they were able to prepare hierarchical helical structures through a multi-step aggregation pathway [42]. Using experimental techniques, they were able to show that increasing the concentration of the solution leads to the production of multi-scale helical aggregation of twisted polymers. In other words, polymers progress from the nano scale to the micron scale. This study could pave the way for the field of chiral optoelectronics. Because the emergence of hierarchical chirality in non-chiral conjugated polymers can profoundly affect the way these polymers interact with light and the transmission of signals resulting from biomolecular interactions and cause properties that were not even imagined before. These new and advanced applications of complex self-assembly structures prompted us to further study these compounds.
In the previous sections, the phenomenon of self-assembly and the factors affecting it were explored in detail. This phenomenon forms the basis of many natural and biological processes such as protein folding and cell membrane formation. These processes are controlled by complex intermolecular and intramolecular interactions such as metal coordination, hydrophobic effect, hydrogen bonds, electrostatic interactions, π–π interactions and van der Waals forces. While the self-assembly of amphiphiles into conventional structures such as micelles or vesicles has attracted much attention for many decades due to their wide application in materials science, medicine, and gene delivery, with recent advances in nanoscience, the way to the emergence of supramolecular self-assembly structures has opened up the development of complex and hierarchical structures. In the other words, amphiphilic molecules, in addition to micelles and vesicles, according to Figure 13 can be hierarchically assembled in the form of more regular nanostructures such as fibers, helices, superhelices, ribbons and tubes, which are of interest to supramolecular chemists. In these new nanostructures, despite knowing the basic modes of supramolecular interaction, it is difficult to predict the shape of the self-assembly system. So far, studies have shown that some self-assembled structures are formed by creating a balance between hydrophilic and hydrophobic components, while recent studies have shown that chirality may play an important role in controlling the shape of self-assembled nanostructures. Hence, the phenomenon of chirality can play an important role in the complexity of these structures and functions at the molecular level. Also, these complexities have been observed in right-handed alpha-helical structures of proteins or double-helix DNA. Research has shown that these helical structures can be single, double, triple or quadruple strands and exist as linear or circular sets [45].
Figure 13.
The diversity of chiral nanostructures constructed through hierarchical self-assembly. Starting from the molecular level, various primary structures can be formed through plausible packing modes. Hierarchical self-assembly of higher order or complex structures can be attained by using these primary nanostructures as building blocks. In the clockwise direction, (c1) zero-dimensional nanospheres can form helical sphere chains and microspheres; (c2) one-dimensional nanofibers can further form single-helical fibers, double-helical fibers, helical bundles with two fibers and multiple fibers, and chiral spirals; (c3) one-dimensional nanorods can assemble into helical and twisted nanorods; (c4) two-dimensional nanobelts can form twisted ribbons, dendritic twists, helical ribbons, and chiral tubules; (c5) two-dimensional nanosheets can assemble into helically arranged petals and microspheres; (c6) three-dimensional nanotubes can form microtubule flowers and helical tubes [
Some research has shown that chirality can play an important role in controlling the shape of self-assemblies such as tubes and the morphology of helical ribbons. In order to design new supramolecular self-assembly structures for using in nanotechnology, in addition to understanding the form of chiral self-assembly, it is necessary to control the size and shape of the aggregates by adjusting the chemical composition, interactions, and conditions under which self-assembly occurs. In this regard, the important step is to know how the three parameters of the elastic force, chirality and the direction of membrane inclination control the structure and shape of the resulting aggregation. Previously, presented models show that aggregates of cylindrical tubes or spiral bands are formed due to chiral interactions, along with molecular tilt [46]. Since this model only predicts the formation of ideal structures, as a result, other models have been presented, including the theory that has been proposed for the effect of chirality on the shape of nanostructures resulting from complex supramolecular self-assembly [47, 48, 49]. According to these theories, the existence of some chiral restrictions in the systems leads to the non-parallel arrangement of macromolecules during the formation of self-aggregates. In other words, the macromolecules are forced to accumulate at a non-zero angle near to their closest neighbor, as a result, they will prefer a specific orientation in the double layer structure, which causes double layer deviation and as a result, the formation of a hollow cylinder will be followed by a fibrous morphology.
Nowadays, metamaterials with better mechanical, optical and even electromagnetic properties can be produced by manipulating the morphological chirality. Also, studies have shown that chirality has a significant effect on physiological properties and medicinal effects, so it can be said that chirality has been proposed as one of the most important research from a biological and chemical point of view. However, many features in nature, such as tunable and hierarchical chirality, which can provide electromagnetic control of light polarization and enhancement of mechanical properties in man-made structures, are still an important challenge. In this section, an attempt has been made to examine the concept of chirality and how it changes during the formation of aggregate structures and the parameters affecting it, so that the way to solve the existing challenges may be smoothed. Parameters such as the intensity of interaction between components in the aggregate structure, their electronic properties, steric effects, geometric preferences, stoichiometry, as well as external factors such as polarity, temperature, and pH play a central role in how chirality affects the shape of aggregates [50]. In 2020, Chen et al. have provided inspiration from origami techniques [51]. They have been able to propose egg-box-based chiral units for the construction of homogeneous and heterogeneous chiral structures and show a theoretical approach to adjust the chirality of these structures by adjusting their geometrical parameters and to achieve chirality change through the branching mechanism. Gradually, they placed a helical bond between the chiral units and were able to design hierarchical structures with chirality that is transferred from the structural elements to the morphological level. Their proposed method can lead to the development of artificial metamaterials with chiral properties, further these metamaterials may be used in engineering applications, including switchable electromagnetic metamaterials, morphing structures, and bionic robots. Put simply, chirality indicates the asymmetric property of an object that has at least one asymmetric center in its structure, in which case the object cannot conform to its mirror image. On the scale of macrochirality, it can usually be morphologically represented by a helix, Create a structure that exists in most biological and synthetic materials. The presence of chirality in materials leads to unusual mechanical, optical and electromagnetic properties and functions [52, 53, 54, 55], such as negative refraction, asymmetric electromagnetic waves transmission and superior elasticity.
Investigations show that chirality translation, in other words, the conversion of right-handed and left-handed chiralities into each other is an important phenomenon. On the one hand, the transformation of these configurations is important in biological [56, 57] and chemical processes [58, 59], and on the other hand, it is very important for engineering materials. Because this conversion can help to control the electromagnetic polarization of the light and also improve the mechanical properties. For example, materials with different handedness have different transmission effects on circularly polarized electromagnetic waves. It is extremely valuable to pay attention to this feature in the construction of light dividing devices and detectors. Various external stimuli can induce this transformation in natural or synthetic chiral materials. However, the chirality transfer coupled with the control of structural reconfiguration is a very difficult and complex task. For example, with a photoactive medium, chirality can be changed without any structural change in a chiral artificial metamolecule with only light [60]. Therefore, the chirality in multiple scales in a structure is called hierarchical chirality. This type of chirality is very useful for the synthesis and properties of materials, for example, to control symmetry in morphogenesis and increase mechanical properties [61, 62]. Eventually, both chirality transfer and hierarchical chirality provide effective ways to tune the mechanical, optical, or electromagnetic properties of a material.
7. Conclusion and future perspective
Amphiphiles, especially surfactants, due to their unique structure, have been able to make significant progress in industry, various fields of medicine and nanoscience. Since these materials can create aggregated structures in addition to reducing surface/interfacial tension by controlling concentration, structural design and controlling environmental conditions, they can be used as templates or drug carriers. Also, these compounds can be used for household cleaning and other industrial applications, and it is predicted that with the increase in the world population along with industrialization and the increase in living standards in developing countries, the demand for the production of these compounds will increase sharply. On the one hand, the widespread use of these compounds and on the other hand their chemical structure creates environmental concerns regarding their toxicity to mammals, aquatic animals and their accumulation in the soil. Therefore, considering that these materials should be used in some industries because of their unique structure, the design of functional surfactants with less risk for the environment is not far from expected.
The purpose of this chapter is to introduce the structure and assembly behavior of surfactants, trying to find a way so that if we cannot eliminate the use of surfactants in a particular industry due to more disadvantages than its benefits, at least we can reduce the amount of disadvantages by changing its structure. On the other hand, with detailed information about their structure and its effect on their properties, we can suggest the best surfactant with the highest efficiency and minimum environmental and economic harm for a specific application. For example, in 1970, the use of phosphate detergents was banned because phosphate is a nutrient for algae. The entry of the waste of these detergents into the water led to the excessive growth of algae and as a result, the reduction of oxygen in the water, followed by the death of aquatic animals and fish [63]. Since substitutes for these surfactants were found, especially in the current situation, using natural surfactants, so removing or limiting phosphate surfactants will not create a problem in the detergent industry or other industries. But another group of surfactants with chains that have fluorine atoms in their structure, such as perfluorooctane sulfonates (PFOS) and perfluorooctanoic acid (PFOA), are toxic to mammals and are suspected of causing cancer in these organisms [64]. The prohibition on the use of these types of surfactants also forced manufacturers to produce fluorinated surfactants with shorter linear chains. Later studies showed that these types of surfactants have the same performance or in some cases better than the previous surfactants, but their toxicity is less. In some industries, these fluorinated surfactants are not yet easily replaceable because there are no high-performance alternatives. Fluorinated surfactants usually have lower surface tension with excellent wetting ability [65]. Therefore, in order to reduce their toxicity, we should seek to change their structure.
In general, it can be said that with the advancement of computational methods along with complex experimental methods such as X-ray or neutron scattering, nuclear magnetic resonance and electron transfer scanning, the surfactant structure can be designed according to the type of application. In other words, considering that the chemical and thermodynamic laws governing the structures of aggregates formed by surfactants are well understood and documented and the related theories are well advanced, it is possible to use the results of experimental and computational studies and their simultaneous application to design different structures. Surfactants are used for their application in various fields such as nanomaterials, drug delivery, energy efficiency and environmental cleaning.
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