Micelles uw1b

Applications of surfactants: cleaning/detergents (40%); textiles; cosmetics; pharmacy; paint; food; etc.

Surfactant micelles surfactant = surface active agent

30-100 molecules d-3-5 nm (association)

orientation → energy minimum Hardy-Harkins principle

HOW ARE SOAPS ARE PREPARED? Soaps are produced from complex esters:

animal fat The saponification of a triglyceride

Introduction to Surfactants

Presentation outline

• Basic Surfactant Chemistry • Surfactant Properties • Surfactant Classification • The Applications

Basic Surfactant Chemistry

Oil & water don’t mix !

Surfactant • Surfactants are compounds that stabilize mixtures of oil and water by reducing the surface tension at the interface between the oil and water molecules. • Surfactants are amphiphilic in nature i.e.; they contain 2 distinct structural units:

• Tail or hydrophobic group which has little affinity for water – this group is usually hydrocarbon (alkyl) chain • Head or hydrophilic group which has strong affinity for water & can be neutral or charged.

Surfactants at the air-water Interface

Surfactants - behavior

Surfactants reduce the surface tension of water by adsorbing at the liquid-gas interface. They also reduce the interfacial tension between oil and water by adsorbing at the liquid-liquid interface.  At higher concentrations surfactants have the tendency to assemble into aggregates. Examples of such aggregates are vesicles and micelles.  The concentration at which surfactants begin to form micelles is known as the critical micelle concentration or CMC.

Surfactant- micelle

In a micelle : The lipophilic tails of the surfactant molecules remain on the inside of the micelle  The polar heads of the micelle form a hydrophilic outer layer that in effect protects the hydrophobic core of the micelle.

Classification of Surfactants

Anionic Surfactant

The most commonly used anionic surfactants are alkyl sulphates, alkyl ethoxylate sulphates and soaps.

•

Cationic surfactants In solution, the head is positively charged. There are 3 different categories of cationic each with their specific application: •In household and bathroom cleaners, cationic surfactants contribute to the disinfecting/saniti zing properties.

Non-ionic surfactants These surfactants do not have an electrical charge, which makes them resistant to water hardness deactivation. They are excellent grease removers that are used in laundry products, household cleaners and hand dishwashing liquids. Most laundry detergents contain both non-ionic and anionic surfactants as they complement each other's cleaning action. Nonionic surfactants contribute to making the surfactant system less hardness sensitive. The most commonly used non-ionic surfactants are ethers of fatty alcohols

Amphoteric/zwitterionic surfactants These surfactants are very mild, making them particularly suited for use in personal care and household cleaning products. They can be anionic (negatively charged), cationic (positively charged) or non-ionic (no charge) in solution, depending on the acidity or pH of the water. How do surfactants work in detail?

Surfactants can work in three different ways: roll-up, emulsification, and solubilization. Roll-up mechanism The surfactant lowers the oil/solution and fabric/solution interfacial tensions and in this way lifts the stain of the fabric. Emulsification The surfactant lowers the oil-solution interfacial tension and makes easy emulsification of the oily soils possible. Solubilization Through interaction with the micelles of a surfactant in a solvent (water), a substance spontaneously dissolves to form a stable and clear solution.

CRITICAL MICELLE CONCENTRATION Certain molecules may be said to contain two distinct components, differing in their affinity for solutes. The part of the molecule which has an affinity for polar solutes, such as water, is said to be hydrophilic. The part of the molecule which has an affinity for non-polar solutes, such as hydrocarbons, is said to be hydrophobic. Molecules containing both types of components are said to be amphiphilic (illustration A).

Such molecules display distinct behavior when interacting with water. The polar part of the molecule seeks to interact with water while the non-polar part shuns interaction with water. There are two ways in which such a molecule achieve both these states. An amphiphilic molecule can arrange itself at the surface of the water such that the polar part interacts with the water and the non-polar part is held above the surface (either in the air or in a non-polar liquid) as shown in Figure B above. The presence of these molecules on the surface disrupts the cohesive energy at the surface and thus lowers the surface tension. Such molecules are called ‘surface active’ molecules or surfactants.

 Another arrangement of these molecules can allow each component to interact with its favored environment.

 Molecules can form aggregates in which the hydrophobic portions are oriented within the cluster and the hydrophilic portions are exposed to the solvent. Such aggregates are called micelles.  An example of a spherical micelle is diagrammed above (illustration C).

 The proportion of molecules present at the surface or as micelles in the bulk of the liquid depends on the concentration of the amphiphile.  At low concentrations surfactants will favor arrangement on the surface. As the surface becomes crowded with surfactant more molecules will arrange into micelles.  At some concentration the surface becomes completely loaded with surfactant and any further additions must arrange as micelles. This concentration is called the Critical Micelle Concentration (CMC).  It follows that measurement of surface tension may be used to find CMC. A graph of surface tension versus log of concentration of surfactant added will appear as follows:

In this graph you can see three phases: 1. At very low concentrations of surfactant only slight change in surface tension is detected. 2. Additional surfactant decreases surface tension 3. Surface becomes fully loaded, no further change in surface tension.

Micelles and the critical micelle concentration (CMC) • At low concentrations of soap, surfactant molecules exist in solution as monomers (individual molecules of surfactant). When a hydrophobic dye is added to this solution, the dye settles to the bottom and has little solubility • At higher concentrations of soap, the surfactant molecules saturate the surface and start forming aggregates in solution called micelles (micelles are formed once the soap/surfactant exceeds a given concentration called the critical micelle concentration, CMC). These micelles have a hydrophobic (oil-loving) core which can solubilize a hydrophobic dye. Thus when the dye is added to this solution, it is solubilized and results in a vivid change in the color of solution.

• This is the same type of mechanism which causes detergents and soaps to remove oil and grease stains from your dishes or clothes.

CMC Explanation Micelle

• The core of a micelle is hydrophobic and can solubilize a hydrophobic dye

Hydrophobic Dye

Micelle Solubilizing Hydrophobic Dye

Factors affecting critical micelle concentration • 1- In case of polar solvents(e.g water) • A)increasing

the length of hydrocarbon chain (minor interaction with water i.e facilitate the transfer from an aqueous phase to micelles) produces a decrease in the CMC • B)increased polarity of the polar portion (greater interaction with water i.e retard the transfer from an aqueous phase to micelles) produces an increase in CMC

2-In case of non-polar solvents(e.g chloroform) The increased polarity of the hydrophilic part will lower CMC whereas, the longer the hydrocarbon chain , the higher the CMC Effect of hydrophobic group. -if hydrophobic group is aromatic, micelle does not formed. -length of hydrocarbon chain is directly proportional to micelle size & inversely proportional to CMC. We express this in mathematical term, log [CMC] = A – Bm where, A & B are homologous series constant. m is the no. of carbon atom in chain.

Additives -With ionic micelles, electrolyte addition

reduces the repulsion between charged groups at the surface of the micelle CMC decreases as electrolyte increases.  Addition of electrolyte decreases the CMC &increases the micelles size.  Following are the effect of NaCl addition on micelle size of Dodesyl trimethyl ammonium bromide.

Conc. Of NaCl mole/dm3

CMC in mole/dm3

Aggregaton number

0

0.146

61

0.1

0.00428

74

0.502

0.00171

90

-Addition of organic molecules affect CMC. Influence water structure. Sugars structure makers lower CMC; urea and formamide structure breakers – increase CMC.

3-Nature of hydrocarbon chain The branched chain ring systems and double bonds tend to raise CMC. 4-Temperature the increased temperature increases CMC.  What is cloud point ?  At temperature up to the cloud point an increase in micelle size &a corresponding decrease in CMC is noted for many non ionic surfactant.

5) EFFECT OF THE COUNTRION 

In case of cationic surfactant as the counter ion is charged in series , Cl-, Br-, I- , micelle size is an increase in order for Cl-< Br-< I- .



In case of anionic surfactant as the counter ion is charged in series ,Na+, K+, Cs+, micelle size is an increase in order for Na+< K+


More weakly hydrated a counter ion larger the micelle formed

THERMODYNAMICS OF MICELLIZATION • Phase Separation Model Simplest approach treats micelles as a single phase. • Mass Action Model Association dissociation equilibrium between free surfactant & micelle phase.

PHASE SEPARATION MODEL • For non ionic surfactant • ΔG0M = RT ln [Xcmc]

• ΔG0M = 2.303 RT [log[CMC] - logW] where, ΔG0M -: standard free energy change W -: mole/dm3 of water

PHASE SEPARATION MODEL •

For ionic surfactant in case of ionic surfactant (1-α)

1.

ΔG0M = (2- α) RT ln [Xcmc]

2.

ΔG0M = (2- α)2.303RT [logcmc - logW]

MASS ACTION MODEL • For ionic micelle the ionic micelle, M++ is consider to be formed by the association of n surfactant ions, D+ & (n-p) counter ion X-. nD+ + (n-p)X- -> MP+ then std. free energy of micellization per mole of monomeric surfactant is ΔG0M = (2-p/n) 2.303 RT logXcmc

ENTHALPY AND ENTROPY OF MICELLIZATION • The enthalpy of micellization can be calculated from the variation of c.m.c. with temperature.

- ΔH0 = RT2 (dln [Xcmc]/dT) • The entropy of micellization can be calculated from the relationship between ΔG0M and ΔH0 i.e., ΔG0M = ΔH0 – TΔS0

Microemulsions  These are clear, thermodynamically stable, isotropic liquid mixtures of oil, water and surfactant, frequently in combination with a cosurfactant.  The aqueous phase may contain salt(s) and/or other ingredients, and the "oil" may actually be a complex mixture of different hydrocarbons and olefins.  In contrast to ordinary emulsions, microemulsions form upon simple mixing of the components and do not require the high shear conditions generally used in the formation of ordinary emulsions.  The three basic types of microemulsions are direct (oil dispersed in water, o/w), reversed (water dispersed in oil, w/o) and bicontinuous.

Microemulsions have many commercially important uses •Water-in-oil microemulsions for some dry cleaning processes •Floor polishers and cleaners •Personal care products •Pesticide formulations •Cutting oils.

Inverted micelle/ Reverse micelle The reversible formation of association colloids from surfactants in non-polar solvents leads to aggregates termed inverted (or inverse, reverse or reversed)micelles. Such association is often of the type:

The phenomenon of critical micelle concentration (or an analogous effect) is consequently not observed.

 In an inverted micelle the polar groups of the surfactants are concentrated in the interior and the lipophilic groups extend towards and into the non-polar solvent.

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