Determination of Criticle micells concentration

Critical Micelle Concentration

The lowest concentration at which micelles first appear is

called the critical concentration for micelle formation

A listing of some 70 methods of determining CMC's encountered in the literature up to1966

has appeared recently

The surfactant CMC determination methods were collected from ISI data based from 1962-2010 and were classified as:

Some Commonly Used Methods

Surface Tension Method Determine surface tension of solution towards several different

concentrations, and then draw a graph of surface tension against log C the logarithm of concentration, and the turning

points is the CMC.

Technique

Various technique available one of which is Will Helmy side method

Principle

Force is required to detach a thin microscopic slide of known thickness and width from the surface of liquid is measured.

Graphlog of Concentration Vs Surface tension

For assaying CMC by measurement of surface tension is simple and straightforward. A graph of surface tension versus log

concentration is produced. The CMC is found as the point at which two lines intersect; the baseline of minimal surface tension and the

slope where surface tension shows linear decline.

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.

Advantages:

1. simple and convenient

2. Applicable to all kinds of surface active agent. i.e.; it can be use for ionic and non-ionic surfactant.

3. Can be use for very low concentration.

4. The sensitivity is not affected by the surfactant type concentration, activity level whether the effect of inorganic salts and other factors.

5. Is generally believed that the surface tension method is the standard method for the determination of surfactant CMC.

Conductivity method

By the relationship between conductivity of ionic surfactant solution and concentration, draw curves of conductivity versus concentration or molar

conductivity on which turning points is the CMC.

Graph

Molar Conductivity vs. concentration

Precautions

Clean electrode after each determination through ethanol and rise rapidly with demonized water and dry

Advantages

Simple

Limitation

Can not used for non- ionic surfactant.

EXAMPLE

To determine the critical micelle concentration of the amphiphile sodium dodecyl sulphate (SDS)

Conductometric Determination of the CMC

Below the CMC, the addition of surfactant to an aqueous solution causes an increase in the number of charge carriers ( (aq) Na+ and (aq) -OSO3Cl2H25 ) and consequently, an increase in the conductivity. Above the CMC, further addition of surfactant increases the micelle concentration while the monomer concentration remains approximately constant (at the CMC level). Since a micelle is much larger than a SDS monomer it diffuses more slowly through solution and so is a less efficient charge carrier. A plot of conductivity against surfactant concentration is, thus expected to show a break at the CMC (Figure 1).

Apparatus: beaker, pipette, conductivity meter, glass rod

Materials: SDS, deionised water

Procedure:

1. 50ml of an approximately 0.04M aqueous stock solution SDS was prepared.

2. 25ml of deionised water was pipetted into a 200ml beaker.

3. 0.5ml of SDS stock solution was pipetted into water and stir.

4. The conductivity was recorded.

5. Repeat steps 3 and 4 until all the SDS have been added into the beaker.

6. A conductivity as a function of the SDS concentration was plotted and CMC was estimated.

7. The standard change in Gibbs free energy was calculated.

Results and calculation: Volume of Stock Solution of

SDS added, V1 (ml)Concentration of SDS in solution, M2

(M)Conductivity, (mS)

0.0 0.0000 9.170.5 0.0007 27.21.0 0.0015 56.11.5 0.0023 67.32.0 0.0030 102.82.5 0.0036 108.43.0 0.0043 96.53.5 0.0049 137.84.0 0.0055 99.64.5 0.0061 160.45.0 0.0067 207.05.5 0.0072 204.06.0 0.0077 238.06.5 0.0083 247.07.0 0.0088 257.07.5 0.0092 245.08.0 0.0097 253.08.5 0.0101 272.09.0 0.0105 286.09.5 0.0011 301.0

10.0 0.0114 307.010.5 0.0118 313.011.0 0.0122 322.011.5 0.0126 331.012.0 0.0130 337.0

Volume of Stock Solution of SDS added, V1 (ml)

Concentration of SDS in solution, M2 (M) Conductivity, (mS)

12.5 0.0133 344.013.0 0.0137 348.013.5 0.0140 354.014.0 0.0144 357.014.5 0.0147 365.015.0 0.0150 371.015.5 0.0153 375.016.0 0.0156 380.016.5 0.0159 384.017.0 0.0162 389.017.5 0.0165 393.018.5 0.0170 408.019.5 0.0175 418.020.5 0.0180 422.021.5 0.0185 425.022.5 0.0190 434.023.5 0.0194 440.024.5 0.0198 445.026 0.0204 454.027 0.0208 459.028 0.0211 464.029 0.0215 473.030 0.0218 475.031 0.0221 480.032 0.0225 483.0

Volume of Stock Solution of SDS added, V1 (ml)

Concentration of SDS in solution, M2 (M) Conductivity, (mS)

33 0.0228 493.034 0.0231 714.035 0.0233 879.036 0.0236 907.037 0.0239 946.038 0.0241 964.039 0.0244 975.040 0.0246 988.041 0.0248 996.042 0.0251 1004.043 0.0253 1014.044 0.0255 1018.045 0.0257 1021.046 0.0259 1025.048 0.0263 1035.049 0.0265 1041.050 0.0267 1055.0

At the critical micelle concentration (CMC), the conductivity of the solution is approximately 100, hence the concentration of the SDS solution is approximately 0.003M. and provided the value of p/n = 0.3.

ΔG M, m° ≈ RT (2 – p/n) ln [CMC]

= 8.3145 J mol-1 K-1 x (25 + 273) K x (2 – 0.3) ln 0.092

= - 10.05 kJ mol-1 ΔG M, m = molar Gibbs energy of micellization

R= universal gas constant= 8.3145 J/mol·K

T=is the absolute temperature=25°C=25+273=298

CMC critical micelle concentration=0.092

Discussion:

In this experiment, the critical micelle concentration (CMC) of sodium dodecyl sulfate was determined by using the method of conductivity. Sodium dodecyl sulfate (SDS), NaOSO 3C12H25 is known as amphiphilic surfactant which possesses both hydrophobic and hydrophilic properties. SDS was ionized in the aqueous solution to form Na + and -OSO 3C12H25 ions in the solution. Self-dissociation of SDS into micelle is strongly cooperative and occurs at the defined concentration called critical micelle concentration. Below CMC, the amphiphile dissolves as monomers. Once the concentration beyonds CMC, the monomers concentration remains unchanged while the micelle concentration increases. The CMC can be determined by the conductivity method of the SDS solution. Na + and -OSO 3C12H25 ions are known as charge carriers which will increase the conductivity of the solution when ionization takes place.

At the beginning of the experiment, a small amount of SDS is added into the distilled water. In a SDS dilute solution, the concentration of SDS is below its CMC, hence it behaves as normal electrolyte and ionizes to give out Na + which soluble in the aqueous phase while -

OSO 3C12H25 ions solubilize its hydrophilic head in the water and hydrophobic tail extent out the water surface. The ions exist as solvated monomer instead of micelle due to low SDS concentration. The number of monomers was increased as the amount of the SDS solution was added into the solution. At the same time, the increase of conductivity that had been detected due to the increase of SDS ions carried more charges within the solution. Once the amount of SDS solution added into the aqueous solution is equals to the CMC, the first micelle start to form spontaneously in the solution.

• The micelle formation occurs at the above of CMC which the monomers undergo self-assembly to form aggregate in the solution. This caused the solution converted from true solution to become a colloidal system. The micellar solution is known as a colloidal dispersion (association colloid) of organized surfactant molecules. The micelle formed in the solution is a spherical structure which the hydrophilic head groups were exposed to the solution while the hydrophobic tails were faced toward the interior of the micelle structure.

• The exterior of the micelle is built up from the ionic –OSO3 groups which form the Stern layer which associated by water molecules. The further layer that surrounding the Stern layer is composed of the positive counter ions and oriented water molecule called Gouy-Chapman layer. Both Stern layer and Gouy-Chapman layer are known as electric double layer. This double layer will maintain the stability of the colloidal system.

• The higher concentration of SDS caused nucleation for the micelle to form increased and hence more micelle was formed in the solution. Above the CMC, the concentration of micelle definitely increases. However, the concentration of monomers almost remained unchanged in the solution. Monomers tend to form the micelle at the same time the added SDS solution ionized in the solution to replace the monomers that used to build micelle. But, the charge carriers could be increased slowly because the rate of micellisation is slower than the rate of monomers were used in the building of micelle and hence the conductivity of the solution increased at a slower rate in an ideal condition. This can be noticed in the graph which shows the increasing rate of conductivity had became slower obviously. This is because the formation of micelle required the ionic monomers and some of the ions had been attracted towards the micelle surrounding to form the electric double layer. As a result, some monomers are no longer free in the solution but for those ions are not strongly attracted still can carry charge in the solution. Hence, the conductivity of the solution increased slower. However, at the final part in graph shows a sudden increase in the conductivity of the may be due to the formation of bubbles inside the solution. Above the CMC, when bubbles start forming, micelles will be broken down to form monomers to expand the bubbles. As more SDS monomers being formed back, the conductivity shoot up because SDS monomers is a more effective charge carrier than micelles.

Dye MethodThere are obvious differences between the color properties of some dyes in water and

micelles in, determined by CMC titration method.Methods

first in the high concentration (>CMC) by adding a small amount of dye surfactant solution, the dye solubilization in micelles, presents a kind of color. Method and

titration, the solution is diluted with water, until the color change, when the solution concentration is CMC.

Graph

Absorbance VS Molecular concentration Advantages

As long as to find a suitable dyes, this method is very simple.But sometimes the color change is not obvious, the CMC is not easy to determine, at

this time can be used to improve the accuracy of spectrometer instead of visual observation.

Application

Used for various ionic and non-ionic surfactant

Turbidity Method

Non polar organic compounds such as hydrocarbons

in surfactant solution Turbidity surface activity observed by adding proper amount of hydrocarbon solution with surfactant concentration, the concentration of turbidity point mutation is

the surface active agent CMC.The experiment can use visual or turbidimetric endpoints. This means that there are solubilizate effect of surfactant CMC, the

general is to enable CMC to reduce, reduce the degree of different for different types of hydrocarbons. If using benzene

as solubilizate, sometimes CMC can be reduced by 30

Light Scattering Methodo Micelle as dozens or more surfactant molecules or ions complex,

its size to wavelength range, and has a strong light scatteringo The mutation point scattering light intensity and concentration

curves can be measured CMC.o The method in determination of CMC, aggregates, micelles can

also determine the micelle shape and size;o The method in determination of CMC, the aggregation number,

micelle can also determine the micelle shape and size;o For solution is very clean, any dust particle has a significant

impact on the determination of.

Other Properties Of Solution

In principle as long as the solution properties of micelles in solution with changed, the turning point in a concentration curve, so as to obtain the critical micelle concentration by drawing

References:

1. Mukerjee, P., and K. J. Mysels, Critical Micelle Concentrations of Aqueous Surfactant Systems, NSRDS—NBS-36; Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. USA, 1971. Now available from National Technical Information Center, 5285 Port Royal Rd., Springfield, VA 22161,

2. http://151.fosu.edu.cn/hxsy/wulihuaxueshiyan/my%20web/biaomianhuoxingji%20y.htm

3. http://www.86wiki.com/view/65969.htm

4. http://1chemistry.blogspot.com/2011/08/determination-of-critical-micelle.html

5. AULTON 3rd EDITION