1. The critical micelle concentration (CMC) of sodium dodecyl sulfate was determined using a conductivity method. The CMC was found to be approximately 0.003M based on a break point in the graph of conductivity versus concentration. 2. Various methods can be used to determine CMC including surface tension, dye, turbidity, light scattering, and changes in other solution properties at the CMC. Each method exploits changes that occur when micelle formation begins. 3. The conductivity method works because monomers are better charge carriers than micelles, so the rate of increase in conductivity decreases above the CMC where micelles begin to form.

2. Determination of
Criticle micells
concentration

3. Critical Micelle Concentration
The lowest concentration at which micelles first
appear is called the critical concentration for
micelle formation

4. A listing of some 70 methods of determining
CMC's encountered in the literature up to1966
has appeared recently

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

6. Some Commonly Used Methods

7. 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.

8. Graph
log 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.

9. 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.

10. 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.

11. 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.

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

13. 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).

14. 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.

15. 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 9.17
0.5 0.0007 27.2
1.0 0.0015 56.1
1.5 0.0023 67.3
2.0 0.0030 102.8
2.5 0.0036 108.4
3.0 0.0043 96.5
3.5 0.0049 137.8
4.0 0.0055 99.6
4.5 0.0061 160.4
5.0 0.0067 207.0
5.5 0.0072 204.0
6.0 0.0077 238.0
6.5 0.0083 247.0
7.0 0.0088 257.0
7.5 0.0092 245.0
8.0 0.0097 253.0
8.5 0.0101 272.0
9.0 0.0105 286.0
9.5 0.0011 301.0
10.0 0.0114 307.0
10.5 0.0118 313.0
11.0 0.0122 322.0
11.5 0.0126 331.0
12.0 0.0130 337.0

16. Volume of Stock Solution of SDS
added, V1 (ml)
Concentration of SDS in solution, M2 (M) Conductivity, (mS)
12.5 0.0133 344.0
13.0 0.0137 348.0
13.5 0.0140 354.0
14.0 0.0144 357.0
14.5 0.0147 365.0
15.0 0.0150 371.0
15.5 0.0153 375.0
16.0 0.0156 380.0
16.5 0.0159 384.0
17.0 0.0162 389.0
17.5 0.0165 393.0
18.5 0.0170 408.0
19.5 0.0175 418.0
20.5 0.0180 422.0
21.5 0.0185 425.0
22.5 0.0190 434.0
23.5 0.0194 440.0
24.5 0.0198 445.0
26 0.0204 454.0
27 0.0208 459.0
28 0.0211 464.0
29 0.0215 473.0
30 0.0218 475.0
31 0.0221 480.0
32 0.0225 483.0

17. Volume of Stock Solution of SDS
added, V1 (ml)
Concentration of SDS in solution, M2 (M) Conductivity, (mS)
33 0.0228 493.0
34 0.0231 714.0
35 0.0233 879.0
36 0.0236 907.0
37 0.0239 946.0
38 0.0241 964.0
39 0.0244 975.0
40 0.0246 988.0
41 0.0248 996.0
42 0.0251 1004.0
43 0.0253 1014.0
44 0.0255 1018.0
45 0.0257 1021.0
46 0.0259 1025.0
48 0.0263 1035.0
49 0.0265 1041.0
50 0.0267 1055.0

19. 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

20. 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), NaOSO­3C12H25 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.

21. 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.

22. • 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.

23. • 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.

24. Dye Method
There 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

25. 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

26. Light Scattering Method
o Micelle as dozens or more surfactant molecules or ions
complex, its size to wavelength range, and has a strong light
scattering
o 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.

27. 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

28. 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