Amphiphiles: Classification, Applications and Self Assemblies

Amphiphile a chemical compound possessing both hydrophilic & lipophilic (fat-loving) properties. 

Hi Friends, in this article we will learn about amphiphiles or surfactants classification, applications and self assemblies. 

Introduction

Amphiphile (from the Greek αμφις, amphis: both and φιλíα, philia: love, friendship) is a term describing a chemical compound possessing both hydrophilic (water-loving, polar) and lipophilic (fat-loving) properties. Such a compound is called amphiphilic or amphipathic. A typical amphiphilic molecule contains one hydrophilic group, usually referred as head, attached to hydrocarbon chain called tail. 

Figure 1: Typical structure of Amphiphile

The lipophilic group is typically a large hydrocarbon moiety, saturated or unsaturated, linear or branched. The hydrophilic group can be charged (cationic or anionic) or neutral. Surfactants are an example group of amphiphilic compounds. Their polar region can be either ionic, or non-ionic. Some typical members of this group are: sodium dodecyl sulfate (anionic), Benzalkonium chloride (cationic), Cocamidopropyl betaine (zwitterionic) and 1-octanol (long chain alcohol, non-ionic). Due to special property to accumulate, amphiphiles are widely used as a detergent, soaps, emulsifiers, wetting agents, foaming agents etc.

Figure 2 : Examples of Amphiphiles

Classification Of Amphiphiles

Based on number of polar head groups and nonpolar tails amphiphiles are categorized as follows.

• Conventional amphiphiles: Contains single polar head and single nonpolar tail

• Bola amphiphiles: Consists of two polar head groups connected with hydrophobic chain.

• Gemini amphiphiles: Consist of two polar heads and two nonpolar tails connected with linker group.

• Double or triple chain amphiphiles: Contains one polar head group and two or three lipophilic tails.

• Catanionic amphiphiles: This kind of amphiphiles forms by mixing 1:1 ratio of two appositely charged surfactants.

• Facial amphiphiles: Beyond the conventional amphiphiles there are other kind of amphiphiles such as peptide and proteins, amphiphilic polymers in which hydrophilic and hydrophobic groups are located on apposite faces of the molecule.

Figure 3 : Types of Amphiphiles

Self-aggregation of Amphiphiles

Amphiphiles have special property to self-organize to form poly-molecular assemblies in aqueous solutions due to their double affinity. The amphiphiles assemble in such way that the hydrophobic part remains inside of the aggregate whereas hydrophilic part remains at the surface. It is studied that in an amphiphile two opposing forces are responsible for the formation of such aggregates. The hydrophobic effect separates the hydrocarbon chain from water which helps in self organization of the molecules; whereas hydrophilic part due repulsive forces and steric effect avoids the formation of large three-dimensional structures hence create phase separation.

The self-aggregation of amphiphiles forms variety of molecular assemblies depending upon the structure of amphiphiles. The formation of aggregates also depends on the physical properties such as concentration, temperature, pH and ionic strength. Micelles and vesicles are known to be most common aggregation pattern shown by amphiphilic molecules.

Figure 4: Self-aggregation of Amphiphiles

Lets see each of the aggregation pattern in detail;

Micelles

Micelles are the spherical shaped assemblies typically formed by amphiphiles bearing single head and tail. The micelle consists of polar surface area made up of systematic arrangement of hydrophilic heads. The lipophilic tails arrange themselves in such a way that they separate from water and forms core of the micelle. Generally, a micelle comprises 50-100 monomers depending on the structure of the amphiphile. Micelle formation occurs above certain concentration which is called as critical micelle concentration (CMC). The CMC is important characteristic of a surfactant. The value of the CMC depends on temperature and pressure. Depending upon the structure of the amphiphiles shape of micelle may vary from spherical to ellipsoidal or sometimes long cylinders.

Langmuir monolayers

Amphiphiles forms Langmuir monolayers at interface of water-air, provided that they are not soluble or sparingly soluble in water. Typically, Langmuir monolayers are obtained by spreading dilute solution of amphiphile in volatile, water immiscible solvent on to the water surface. After evaporating solvent, polar heads of the amphiphiles move towards water surface and lipophilic tails arrange themselves in possible orientations above water surface.

Bilayers

Phospholipids form bilayers in biological systems often known as lipid bilayers. They are the main components of biological membranes. The amphiphilic nature of these molecules defines the way in which they form membranes. They arrange themselves into bilayers, by positioning their polar groups towards the surrounding aqueous medium, and their lipophilic chains towards inside of the bilayer.

Vesicles

Vesicles are the spherical or ellipsoidal structures formed by bilayer assemblies of amphiphiles. Vesicles consist of internal cavity filled with solvent in which they are dispersed. Many synthetic amphiphiles and surfactants which are able to form bilayers can form vesicles. Usually, vesicles are stable in aqueous solution. Energy is required to dissolve amphiphile in water in the form of stirring or sonication and consequently, amphiphiles arrange themselves in to bilayer which results in formation of vesicles. The formation of vesicles from bilayer occurs only at low concentration, at higher concentration bilayers form lamellar phase. Vesicles with one bilayer or lamella are known as unilamellar vesicles (ULV’s), whereas vesicles bearing several concentric bilayers are called as mutilamellar vesicles (MLV’s).

Applications of Amphiphiles in Supramolecular, Biological and Medicinal Chemistry

In addition to simple micelles and vesicles, amphiphiles have tendency to self-assemble in more organize in nanostructure such as fibers, ribbons, tubes, helices etc. Morphology of nanostructures is dependent on structure of amphiphiles and experimental conditions. Thus, amphiphiles can be used to control morphology of aggregate by changing structure of amphiphile and experimental parameters. The morphology of nanostructures has great effect on their function in biological systems and this is growing area of research in chemistry for the applications in functional materials, nano-chemistry and technology.

Amphiphiles originated from peptides or carbohydrates shows novel applications in biological and medicinal chemistry. Peptide based amphiphiles tends to form helical structures which can mimic bio molecules. Also, peptide based amphiphiles can be used as a potential candidate for drug design. Many new drugs candidates are bearing less solubility in aqueous medium. Hence there is requirement of drug delivery system which can solubilize drug molecule and carries it to the expected target without altering properties of drug molecule. Many of the soft delivery systems formed by surfactants are dependent on temperature, pH, concentration etc. Aggregates formed by synthetic surfactants are used as carriers for drugs and nanostructured lipids. The aggregates solubilize drug molecule due to water content and protect from enzymes which can degrade the drug molecule. 

Liposomal drug delivery systems became more popular in recent years because of their advantages over other drug delivery systems. The drug molecules can either be encapsulated in aqueous space or embedded into lipophilic hydrocarbon chain. Liposomes are biodegradable, low toxic and target specific. Additionally, liposomes carry both lipophilic and hydrophilic drugs without altering their properties. Liposomes are widely used in pharmaceutical and cosmetic industries as a carrier for various molecules. Besides these advantages, liposomes have few disadvantages as well, such as their production cost is high. Sometimes phospholipids suffer with oxidation or hydrolysis type reaction. Sometimes leakage or fusion of encapsulated drugs can happen. To overcome these difficulties researchers are getting attracted towards vesicles. Furthermore, availability of wide range of synthetic surfactant molecules can be used for making vesicles depending upon the application. 

Summary

Lastly, to summarize this part; we have discussed definition, classification and various applications of amphiphiles / surfactant molecules. 

Amphiphiles are the compounds which has components that are soluble in both water and lipid or oil. The amphiphiles are classified as ionic and non-ionic. Also there is another classification system that defines; Conventional amphiphiles, Bola amphiphiles, Gemini amphiphiles, Double or triple chain amphiphiles, Catanionic amphiphiles, Facial amphiphiles.

The amphiphiles has self aggregation properties which forms various three dimensional structures such as micelles, vesicles, bilayers and monolayers. 

Manmade amphiphiles has great application in biological, medicinal and materials chemistry.  

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