CH-222

FOURTH SEMESTER CHEMISTRY PROJECT

Determination of Critical Micellar Concentration(CMC) of Sodium Dodecyl Sulphate in Hexane using Dyanamic Light Scattering (DLS) Method.

Done By:

Hemanta Sarmah (09MS001)Soumalya Sinha (09MS006)Ankan Bag (09MS030)Debtanu Chakraborty (09MS031)

INTRODUCTION:

Dynamic light scattering (also known as photon correlation spectroscopy or quasi-elastic light scattering) is a technique in physics, which can be used to determine the size distribution profile of small particles in suspension or polymers in solution. It can also be used to probe the behavior of complex fluids such as concentrated polymer solutions.

When light hits small particles the light scatters in all directions (Rayleigh scattering) so long as the particles are small compared to the wavelength (below 250 nm). If the light source is a laser, and thus is monochromatic and coherent, then one observes a time-dependent fluctuation in the scattering intensity. These fluctuations are due to the fact that the small molecules in solutions are undergoing Brownian motion and so the distance between the scatterers in the solution is constantly changing with time. This scattered light then undergoes either constructive or destructive interference by the surrounding particles and within this intensity fluctuation, information is contained about the time scale of movement of the scatterers.

The sample that we are going to analyze is Sodium Dodecyl Sulphate (C12H25SO4Na). Our solvent is n- Hexane (99.9 % pure) . Now since the solvent is non-polar so, there will be a reverse micelle formed, with the polar head groups pointing inwards and the non-polar tail protruding outwards. In such a system we cannot apply the normal conductometric method of determining the critical micelle concentration . So we resort to DLS method, that has already been described above.

OBJECTIVES:

1) To become familiar with the technique of Dynamic Light Scattering(DLS).2) To compare the closeness of CMC values of SDS in reverse micelle condition to

that in micelle condition.

MATERIALS AND EQUIPMENT:

• n-Hexane • 10 test-tubes • tes-tube stand• SDS (Molecular Weight = 288.5 g/mol)• DLS machine• quartz cubette• dipso-van syringe• Nylon filter

REAGENTS REQURED: (per 100 ml of solution)

1) Sodium Dodecyl Sulphate (SDS) (0.02885 x 36) gms = 1.0368 gms2) n- Hexane 10 ml

Procedure:

1) We calculate the molar mass of SDS and find it to be 288.5 g/mol.2) We measure 0.0288 gm of SDS using an electronic balance and then transfer it to

an empty test-tube. Add hexane in it to make the final volume to be = 10 ml.3) In the next test-tube we add ( 2x0.0288 )

g i.e 0.0576 g of SDS in 10 ml hexane.4) In the remaining 6 test-tubes, we now

keep adding 0.0288 g more than what we added in the previous one.

5) We then seal the test-tubes. It is necessary because hexane is volatile in nature and can easily escape in room temperature.

6) We then shake the test-tubes , such that the entire amount of SDS gets dissolved.

7) The DLS machine is switched on and, left for 20 minutes and then the calibration is done of n-hexane i.e our solvent.

8) Then we filter the individual solutions using a 45 μm nylon filter. For filtering we place the filter over the cubette and then with the help of a dispo-van push the solvent through the filter.

9) Parameters for DLS are set, viz: Room temp. = 250C, Equilibrating time= 60 secs.10) Data (Diameter of the particles vs frequency) is taken from the graphs that

emerge on the computer screen.

Result Analysis :

Table 1: Concentration vs. Size of particle

Sl. No Concentration of solution (N)

Size of the particle (I) (nm)

Size of the particle (I) (nm)

Average Size of I and II(nm)

1 0.02885 0.3921 0.3764 0.38425 2 0.0577 0.399 0.387 0.393 3 0.0865 0.3597 0.3106 0.33515 4 0.1154 0.4825 0.3597 0.4211 5 0.1442 0.3597 0.4841 0.4219 6 0.1731 6.772 5.05 5.911 7 0.2019 5.765 5.007 5.386 8 0.2308 6.4825 5.3597 5.9211

From the graph the CMC value can be calculated to be 0.16 mol/lit i.e = 0.16 M.

SOURCES OF ERROR:

1) SDS is sparingly soluble in n-hexane. Even at low concentrations it is very difficult to dissolve completely in solution, and as a result there might arise bulky particles. If these were to be scanned over a large number of particles, then the error will be very high.

2) No external dust should be falling after the filtration has been done3) Once a nylon filter is used, it should be discarded, before re-use. 4) Solvent being volatile, will always try to escape away. So seal the test-tubes

before shaking.

CONCLUSION: The critical micelle concentration was found out to be = 0.16 M

ACKNOWLEDGEMENTS:

We are very grateful for Prof. Parna Gupta Bhattacharya and Prof. Priyadarshi De and Prof. Mousumi Das for allowing us to do project titled “Determination of Critical Micellar Concentration(CMC) of Sodium Dodecyl Sulphate in Hexane using Dyanamic Light Scattering (DLS) Method.”. Their guidance came in a lot handy for doing the project successfully.

We are also thankful to Dr. Srikanth and Mr. Saroj (Lab Assistant) who provided us with chemicals and reagents.

Last but not the least we would like to thank Prof . Raja Shunmugam along with his Phd student Mr. Santu Sarkar, Department of Chemical Sciences , IISER Kolkata for having allowed us the space and equipment for working .

REFERENCES:

1. Physical Chemistry by Atkins.2. www.wikipedia.org 3. http://pubs.acs.org/doi/abs/10.1021/la051447u 4. Dynamic Light scattering : with applications to chemistry,biology and physics.