LAB 5: CRITICAL MICELLE CONCENTRATION (CMC)

OBJECTIVES

To determine the critical micelle concentration of surfactant by using spectrophotometry.

THEORY

Surfactant is a species that accumulates at the interface of two fluids (one of which may be air) and modifies the properties of the surface. A typical surfactant molecule consists of a long hydrocarbon tail that dissolves in hydrocarbon and other nonpolar materials, and a hydrophilic head group, such as a carboxylate group (COO), that dissolves in a polar solvent (typically water). The mode of action of a surfactant in a detergent, and of soap, is to dissolve in both the aqueous phase and the hydrocarbon phase where their surfaces are in contact, and hence to solubilize the hydrocarbon phase so that it can be washed away.

Surfactants can be mainly classified into 2 groups according to their charge:

1. Ionic surfactants

1.1 Anionic surfactants

Sodium dodecyl sulphate,

or sodium lauryl sulfate; C12H25SO4Na

Ammonium dodecyl sulfate, or ammonium lauryl sulfate; C12H29NO4S

Dioctyl sodium sulfosuccinate; C20H37NaO7S

Sodium stearate; C18H35NaO2

2

1.2 Cationic surfactants

Benzalkonium chloride

Benzethonium chloride; C27H42ClNO2

Bronidox; C4H6BrNO4

Dimethyldioctadecylammonium chloride; C38H80ClN

1.3 Zwitterionic or Amphoteric surfactants

3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS)

Cocamidopropyl betaine (CAPB)

2. Nonionic surfactants

1-hexadecanol; C16H34O

Dodecyldimethylamine oxide; C14H31NO

Octyl glucoside; C14H28O6

Cocamide monoethanolamine

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Only above a particular concentration known as critical micelle concentration (CMC) and temperature known as the Kraft temperature, surfactant molecules in solvent can spontaneously form colloid-sized clusters called micelles, in which the hydrophobic tails of the molecules tend to congregate while their hydrophilic heads provide protection (Fig. 1).

(a) (b)

Fig. 1 (a) The internal structure of a spherical micelle surrounded by water molecules. Black circles represent the ionic head groups, whereas the long tails depict the hydrophobic alkyl chains. SDS (C12) and cetyltrimethylammonium bromide (C16) are typical anionic and cationic micelle-forming surfactants, respectively. (b) The 3D-illustration of a spherical micelle.

The CMC can be determined by measuring physical properties of the surfactant solution as a function of surfactant concentration. At the CMC, a discontinuity in a given physical property of the surfactant solution is observed (see Fig 2). The physical properties of the solution commonly measured for the CMC determination are, for instance, molar conductivity, surface tension, absorbance, or osmotic pressure. The hydrocarbon interior of a micelle is like a droplet of oil. Nuclear magnetic resonance shows that the hydrocarbon tails are mobile, but slightly more restricted than in the bulk. Micelles are important in industry and biology on account of their solubilizing function: matter can be transported by water after it has been dissolved in their hydrocarbon interiors. For this reason, micellar systems are used as detergents, for organic synthesis, froth flotation, and petroleum recovery.

Fig. 2 Relationship between physical properties and concentration of surfactant.

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Ionic surfactants dissociate into surface-active molecules and counterions when they are dissolved in water. These counterions significantly influence the properties of surfactant solutions, e.g., CMC, micelle size, micellar catalytic activity etc. For aqueous systems, the increased binding of the counterion to surfactants causes a decrease in the CMC of the surfactant. The extent of binding of the counterion increases with an increase in the polarizability and valence of counterions and decreases with an increase in its hydrated radius. Thus, in aqueous solution, for the anionic dodecyl sulphate (DS), the CMC decreases in the order

. +++ >> Cs Na Li The enthalpy of micelle formation reflects the contributions of interactions between micelle chains within the micelles and between the polar head groups and the surrounding medium. Consequently, enthalpies of micelles formation display no readily discernible pattern and may be positive (endothermic) or negative (exothermic). Many nonionic micelles form endothermically, with H of the order of 10 kJ mol1 of surfactant. That such micelles do form above the CMC indicates that the entropy change accompanying their formation must then be positive, and measurements suggest a value of about +140 JK1 mol1 at room temperature. The fact that the entropy change is positive even though the molecules are clustering together shows that there must be a contribution to the entropy from the solvent and that its molecules must be more free to move once the solute molecules have herded into small clusters. This interpretation is plausible, because each individual solute molecule is held in an organized solvent cage, but once the micelle has formed the solvent molecules need form only a single (admittedly larger) cage. The increase in energy when hydrophobic groups cluster together and reduce their structural demands on the solvent is the origin of the hydrophobic interaction that tends to stabilize groupings of hydrophobic groups in biological macromolecules. The hydrophobic interaction is an example of an ordering process that is stabilized by a tendency toward greater disorder of the solvent.

INSTRUMENT AND EQUIPMENT

1. Spectrophotometer (Spectronic 20)

CHEMICALS

1. Surfactant: Sodium dodecyl sulphate (SDS), C12H25SO4Na (MW 288.38 g/mol)

2. Dye probe: Acridine orange (AO), C17H20N3+, which are obtained by dissolving acridine orange zinc chloride double salt, C17H20Cl3N3Zn (MW 438.11 g/mol) in water.

Fig. 3 Acridine orange zinc chloride double salt

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EXPERIMENTAL PROCEDURE

1. Preparation of two stock solutions:

1.1) Prepare 100 mL of M acridine orange stock solution, by dissolving 0.0026 g of acridine orange zinc chloride double salt in 100 mL of H

5105.9 2O using a volumetric flask.

1.2) Prepare 100 mL of 0.0205 M sodium dodecyl sulphate stock solution, by dissolving 0.5912 g of SDS in 100 mL of H2O using a volumetric flask.

2. Preparation of AO-SDS mixtures:

2.1) Pipet 5.00 mL of the stock AO solution into each of the 25.0 mL volumetric flasks.

2.2) Use a 5.00 mL bulb-pipette and make sure that each flask contains the same amount of AO.

2.3) Transfer the following volume of SDS stock solution into the volumetric flasks, using a graduated pipette:

Flask number 1 2 3 4 5 6 7 8 9

Amount of SDS (mL) 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 10.00

2.4) Make up each flask to the mark with distilled water.

2.5) Measure the temperature of each AO-SDS sample.

2.6) Fill the cuvette with each sample and measure the absorbance at 500 nm using the spectro-photometer (Spectronic 20). Also note the color variation in the sample series.

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TREATMENT OF DATA

1. Plot the absorbance against the final concentrations of the SDS solutions ( ). 2/V112 VCC =2. Determine the critical micelle concentration (CMC) using the graphical method (see Fig. 4).

Abs

orba

nce

(at 5

00 n

m)

Final SDS concentration [M]

CMC

Abs

orba

nce

(at 5

00 n

m)

Final SDS concentration [M]

CMC

Fig. 4 Graphical method used in the CMC determination. Two data regions are represented by two straight lines of different slopes. The CMC is the concentration at the intersection of these two straight lines.

3. Report the concentration of SDS where there was a change in the colour of the solution. Such the concentration should be near the CMC obtained from the graphical method. Compare the two values.

4. Compare your CMC with the value found in the literatures and evaluate the relative error (in percent). Discuss the causes of the error.

QUESTION

Explain why there is a change in the absorbance of the solution in the region of the CMC and why there is no further increase in the absorbance with increasing concentration of SDS beyond this region?

REFERENCES

1. C. Tanford, "The Hydrophobic Effect", J. Wiley, New York (1973).

2. E.H. Cordes (Ed.) "Reaction Kinetics in Micelle", Plenum Press, New York (1973).

3. D.J. Fender and J.H. Flendlar, Adv. Phys. Org. Chem., 8, 271 (1970).

4. M. Rujimethabhas and P. Wilairut, J. Chem. Educ., 55, 342 (1978).

5. http://www.merck-chemicals.com/thailand/acridine-orange-zinc-chloride-double-salt-c-i-46005/MDA_CHEM-115931/thai/p_vu.b.s1LZb8AAAEWZuEfVhTl

6. http://www.pnas.org/content/99/8/4805/F1.expansion.html

1. Ionic surfactants2. Nonionic surfactantsCHEMICALSEXPERIMENTAL PROCEDURE2. Preparation of AO-SDS mixtures:2.1) Pipet 5.00 mL of the stock AO solution into each of the 25.0 mL volumetric flasks. 2.2) Use a 5.00 mL bulb-pipette and make sure that each flask contains the same amount of AO. 2.3) Transfer the following volume of SDS stock solution into the volumetric flasks, using a graduated pipette: