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PAPER No. 10: Physical Chemistry –III (Classical Thermodynamics, Non-Equilibrium Thermodynamics, Surface Chemistry, Fast Kinetics) MODULE 26 : Surfactants

Subject Chemistry

Paper No and Title 10; Physical Chemistry –III (Classical Thermodynamics, Non-Equilibrium Thermodynamics, Surface Chemistry, Fast Kinetics)

Module No and Title 26, Surfactants

Module Tag CHE_P10_M26

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TABLE OF CONTENTS

1. Learning Outcomes

2. Surfactants: Surface Active Agents

3. Composition and Structure: Surfactants are Amphiphilic

4. Dynamics of surfactants in solution

5. Classification of Surfactants

5.1 Anionic surfactants

5.2 Cationic surfactants

5.3 Nonionic surfactants

5.4 Zwitterionic surfactants

6. Applications of Surfactants

7. Summary

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1. Learning Outcomes

After studying this module, you shall be able to:

• Know about surfactants and their behavior in the solution. • Learn about formation of surfactants and their solubility behaviour in different phases. • Classify the surfactants by their process of formation • Know about the various applications of surfactant in different fields.

2. Introduction : Surfactant (Surface Active Agent)

Surfactants are surface active agents, which literally means active at surface. Surfactants are those compounds which lower the surface tension (or interfacial tension) by their tendency to get adsorbed at surface and interfaces. Interface is a link of any two immiscible phases and the term surface is used when any one phase is a gas, i.e., air. Some examples of surface and interface are as follows: Solid – Vapour (surface) Solid – Liquid (interface) Solid – solid (interface) Liquid – Vapour (surface) Liquid – liquid (interface) The extent of work required to expand the interface is termed as interfacial free energy per unit area and is usually termed as interfacial tension. The surface tension of water is equal to the interfacial free energy per unit area of the boundary between water and the air above it. When that boundary is covered by surfactant molecules, surface tension (or the amount of work required to expand the interface) is reduced Fig. 2.1. Denser the surfactant part at the interface, greater will be the reduction in surface tension.

Figure 2.1 Adsorption of surfactant at the surface of water

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Surfactant may absorb at all surfaces and interfaces given above. Generally, we have interfaces which have water as the liquid phase. Variouse examples for the different interfaces and products are given in Table 2.1. Table 2.1 Examples of interface involving a liquid phase Interface Type of system Product Solid – liquid Suspension sand in water Liquid – liquid Emulsion Milk, Cream Liquid – Vapour Foam Shaving cream As we have seen, the fundamental property of the surfactants is their tendency to accumulate at interface. The concentration of surfactant at a boundary usually depends on the surfactant structure and on the nature of the two phases which are in contact at the interface. Therefore, no universally good surfactant is potent for all uses. The choice depends on the application. A good surfactant should have weak solubility to the bulk phases. Some surfactants (and several surface active macromolecules) are only soluble at the oil-water interface. Such compounds are hard to carry but are very efficient in reducing the interfacial tension.

3. Composition and Structure: Surfactants are Amphiphilic

The word amphiphile first introduced by G.S. Hartley, is taken from the Greek word amphi, which means ‘both’. Mainly these amphiphilic organic compounds consist of contain 2 parts, one part i.e., the lyophilic part which is soluble in a specific fluid while the other part which is insoluble is the lyophobic part. The hydrophilic part is denoted as the head group and the hydrophobic part is referred as the tail (see figure 3.1). In case of water as a fluid, the water soluble part is called hydrophilic and water insoluble part is called hydrophobic.

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Figure 3.1 Diagrammatic illustration of a surfactant

The hydrophobic part is usually linear or a branched chain of 8 to 18 carbons, which can be aliphatic, aromatic, or a mixture of both the structures. The sources of hydrophobic part are generally fats, oils, petroleum fractions, short synthetic polymers, or relatively high molecular weight synthetic alcohols. The chain branching degree, the polar group position and the chain length are parameters of importance for the surfactant physiochemical properties. The surfactant polar part may be ionic or non-ionic, which give the primary classification to the surfactants which we will discuss in Section 5. The choice of polar group and comparative size of the hydrophobic and polar groups are important factors in determining the properties and surfactant physiochemical behavior in water. A surfactant usually contains one polar group. In recent times, there have been considerable research interest in certain dimeric surfactants, which contain two hydrophobic tails and two head groups linked together with a short spacer. These species are termed as Gemini surfactants or doubled chain surfactant. They show several interesting physicochemical properties, such as high efficiency in lowering surface tension and very low CMC. A typical Gemini surfactant is shown in Figure 3.2.

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Figure 3.2 (a) Gemini surfactant (b) Dimer of sodiumdodecylsulfate Natural Surfactants:

Nature’s own surfactants are termed as polar lipids. These are abundant in all living organisms. In the biological system, the surface active agents play the same role as surfactants in technical systems: to overcome solubility problems, as emulsifiers, as dispersants, to modify surfaces, etc. Some examples of this in biological system are: bile salts are extremely efficient solubilizers of hydrophobic components in the blood, while mixtures of phospholipids pack in ordered bilayers liquid crystal of surfactants and such structures constitute cell membranes,. The important example of a phospholipid is lecithin: phosphatidylcholine.,illustrated in fig. 3.3. Lecithin is extracted from phospholipid-rich sources such as soybean and egg.

Figure 3.3 lecithin: phosphatidylcholine

(a) (b)

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One characteristic feature of surfactant is their tendency to dissolve in water and adsorb at interface of air and water or at the interface of oil & H2O (a case where H2O is mixed with oil). The hydrophobic group which is water insoluble may extend out from the water phase which is in bulk with the air or with the oil phase. Whereas water soluble head group is linked with the water phase. The alignment at the surface of surfactant improves the water properties –air or water-oil interface. In addition to this, surfactants have a special property that of having a monomer in solution which tends to form aggregates known as micelles. The Micelle formation or the process micellization is an alternative mechanism of adsorption where the hydrophobic group is removed from the water at the interface, thereby reducing the system’s free energy. It is an important process as surfactant molecules when present in micelles form behave differently in comparison to free monomers in solution. Only surfactant monomers take part on the surface, whereas the lowered interfacial tension and the dynamic phenomena i.e., wetting and foaming is checked by concentration of free monomers in the solution. The micelles formation is considered as a reservoir for the monomers surfactant. The exchange rate of a surfactant molecule between micelle and bulk solution may vary by many orders of magnitude depending on the size and structure of the surfactant. The dynamics of surfactant in solution is shown in figure 4.1.

The middle figure corresponds to reverse micelle? In micelle the surfactant hydrophobic group is linked with the interior of the cluster and the polar head group is linked with the solvent. The micelle, therefore, is a polar aggregate of high water solubility and without much surface activity. When a surfactant adsorbs from aqueous solution at a hydrophobic surface, it normally orients its hydrophobic group towards the surface and exposes its polar group to the water. The surface becomes hydrophilic and, as a result, the interfacial tension of the surface and water is reduced. Adsorption at hydrophilic surface often result in more complex surfactant assemblies.

4. Dynamics of surfactants in solution

Figure 4.1. Dynamics of surfactant in solution

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5. Classification of Surfactants

The classification of primary surfactants is made on the basis of the charge on the polar head group. If the charge is negative, the surfactant is termed as anionic; if the charge is positive, it is termed as cationic. If a surfactant contains a head with two oppositely charged groups, it is known as zwitterionic. We can categorize surfactants into the classes - anionic, cationic, nonionic and zwitterionic.

Figure 5.1 Surfactants classification according the composition of charges on them.

5.1 Anionic surfactants Anionic surfactants consist of anionic functional groups at their head, i.e., sulfate, sulfonate, phosphate, and carboxylate. We use anionic surfactants in greater volume than other surfactant class, and they are used in most detergent formulations and the best detergency is obtained by alkyl and alkyl-aryl chains in the C12-C18 range. Soaps which are the largest single type of surfactants are anionic surfactants obtained by saponification of natural oils and fats. Soap is a gene name indicating the metal salt of an alkali carboxylic acid originated from animal fats or vegetable oils. Soap bars are usually based on fatty acids mixtures produced from tallow, coconut

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and palm oil. Over the last 50 years, soaps have been replaced with more efficient substances like alkylsulphates, alkyl sulphonates and alkyl benzene sulphonates. Anionic surfactants are very sensitive to hardness of water .The counter ions most commonly used are sodium, potassium, ammonium, calcium and various protonated alkyl amines. Sodium and potassium impart solubility in water, whereas calcium and magnesium give solubility in oil. Amine / Alkanol amine salts give products which are both oil and water soluble.

Table 5.1. Few examples of Anionic surfactants Classifications Examples Alkyl sulfates • Ammonium lauryl sulfate

• Sodium lauryl sulfate (SDS, sodium dodecyl sulfate)

Alkyl-ether sulfates • Sodium laureth sulfate or sodium lauryl ether sulfate (SLES) • Sodium myreth sulfate.

Sulfonate • Dioctyl sodium sulfosuccinate, perfluorooctanesulfonate (PFOS) • Perfluorobutanesulfonate • Linear alkylbenzenesulfonates (LABs).

Alkyl carboxylates • Sodium stearate • Sodium lauroylsarcosinate

Carboxylate-based fluorosurfactants

• Perfluorononanoate • Perfluorooctanoate (PFOA or PFO).

5.2 Cationic surfactants In this category the hydrophilic part is positively charged. This group contains no wash activity effect, but it fastens to the surfaces where they may give softening, antistatic, soil repellent, and antibacterial or corrosion inhibitory effects. The various typical applications are their use as softeners (fabric softeners) and antistatic. The counter ion of cationic surfactants is generally a halide or methyl sulfate. The primary, secondary, or tertiary amines are pH dependent: (Primary and secondary amines are positively charged with pH < 10).

Table 5.2 Some common examples of cationic surfactants

Permanently Charged quaternary ammonium cation

• Hexadecyltrimethyl ammonium bromide • Cetrimonium bromide • Dioctadecyldimethylammonium bromide

(DODAB)

• Cetylpyridinium chloride (CPC) • Cetyltrimethylammonium bromide (CTAB)

• Benzethonium chloride (BZT) • Cetyltrimethylammonium chloride (CTAC)

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• 5-Bromo-5-nitro-1,3-dioxane Dimethyldioctadecylammonium chloride

• Benzalkonium chloride (BAC)

5.3 Nonionic surfactants

A surfactant of uncharged hydrophilic part is non-ionic. These substances are suitable for cleaning purposes and are insensitive to water hardness. They have a wide application among cleaning detergents and they include groups like fatty alcohol polyglycosides, alcohol ethoxylates etc. The long chain alcohols show some surfactant properties. Most prominent among these are fatty alcohols, cetyl alcohol, stearyl alcohol, and cetostearyl alcohol (consisting predominantly of cetyl and stearyl alcohols), and oleyl alcohol.

Table5.3 Some common examples of nonionic surfactants Polyoxyethylene glycol alkyl ethers (Brij): CH3–(CH2)10–16–(O-C2H4)1–25–OH:

• Octaethylene glycol monododecyl ether

• Pentaethylene glycol monododecyl ether

Polyoxypropylene glycol alkyl ethers: CH3–(CH2)10-16–(O-C3H6)1–25–OH

Glucoside alkyl ethers: CH3–(CH2)10–16–(O-Glucoside)1–3–OH

• Decylglucoside, • Lauryl glucoside • Octylglucoside

Polyoxyethylene glycol octylphenol ethers C8H17–(C6H4)–(O-C2H4)1–

25–OH: • Triton X-100

Polyoxyethylene-glycol alkylphenol ethers: C9H19–(C6H4)–(O-C2H4)1–25–OH:

• Nonoxynol-9

Glycerol alkyl esters: • Glyceryllaurate

Polyoxyethylene-glycol sorbitan alkyl esters:

• Polysorbate

Sorbitan alkyl esters: • Spans • Cocamide MEA, • cocamide DEA • Dodecyldimethylamine

oxide

Block copolymers of polyethylene glycol and polypropylene glycol:

• PoloxamersPolyethoxylated tallow amine (POEA)

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5.4 Zwitterionic surfactants Zwitterionic surfactant consists of two oppositely charged groups. Zwitterions are often known as ‘amphoterics’ but these terms are not same. An amphoteric surfactant is one that converts to net cation via Zwitterion to net anion by going from low to high pH. Neither the acid nor the base site is permantly charged, i.e. the compound is only Zwitterionic over a limit pH range. At the iso-electric point, the physicochemical behavior often resembles that of non-ionic surfactants. Below and above the iso-electric point there is gradual shift towards the cation and anion character, respectively. Zwitterion is a group that is followed by their excellent dermatological properties. They cause low eye and skin irritation. Therefore, these are suitable for use in shampoo and various personal care (cosmetic) products. Zwitterionic (amphoteric) surfactants consists of both cation and anion centers attached with the same molecule. The cation part depends on primary, secondary, or tertiary amines or quaternary ammonium cations. The anion part can vary and it include sulfonates, i.e. CHAPS (3-[(3 Cholamidopropyl)dimethylammonio]-1-propanesulfonate). Other anionic groups are sultaines. Sultaines are the inner sulfonic acid salt of a strong inorganic acid and are commonly termed as‘sulfobetaines.’ They are similar to betaines, which are inner carboxylic acid salt of a weak organic acids. Both molecules are considered zwitter-ionic at pH7 where the nitrogen at the hydrophobic tail is quaternary (cationic); the polar head groups are anionic and add to the hydrophilic properties of the molecule. The nitrogen which is quaternary always positive, these molecules do not get anionic nature at any pH and they are not truly amphoteric, although they are commonly referred to some common types of Zwitterionic surfactants that are N-alkyl derivatives of simple amino acids, such as glycine (NH2CH2COOH), betain (CH3)2NCH2COOH) and amino propionic acid (NH2CH2CH2COOH). Betaines, e.g. cocamidopropylbetaine. Phosphates: lecithin.

6.Applications of Surfactants

The surfactant plays an important role in cleaning, wetting, dispersing, emulsifying, foaming and anti-foaming agents with many practical applications and products, including a wide variety of consumer products which are given below:

� Shampoos � Dish detergents � Laundry detergents � Conditioners � Fabric softeners � Diapers � Contact lens cleaners � Cosmetics � Toothpastes � Biocides (sanitizers) � Fabric softeners

Surfactants are also widely used in the following industries:

� Emulsions

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� Paints � Adhesives � Recycled papers deinking, in flotation, washing and enzymatic processes � Heavy and tertiary oil recovery � Ore flotation � Dry cleaning � Pesticide and herbicide applications � Water repellency

Others:

� Ski waxes, snowboard wax � Quantum dot coatings � Pipelines, liquid drag reducing agent � Laxatives � Inks

8. Summary

• Surfactants are Surface Active Agents. • They may be acting as detergent, wetting agents, emulsifiers, foaming agents and

dispersants, due to their tendency to adsorb on the surface and on the interfaces. • They are composed of two groups hydrophilic (water loving) and hydrophobic (water

hating). • These two groups imparts some dissolving properties in polar as well as non polar

environment. • Along with the adsorption in the interface/ liquid surface, they also form aggregate in the

wider phase called ‘micelle’, at higher concentration. • Primary classification of surfactants depends on the charges present on the polar head

groups such as anionic, cationic, nonionic and zwitterionic. • Surfactants have many applications in personal household and commercial field, which is

increasing day by day.

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