CH-222 FOURTH SEMESTER CHEMISTRY PROJECTDetermination 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-elasticlight scattering) is a technique in physics, which can be used to determine the sizedistribution profile of small particles in suspension or polymers in solution. It can alsobe used to probe the behavior of complex fluids such as concentrated polymer solutions.When light hits small particles thelight scatters in all directions(Rayleigh scattering) so long as theparticles are small compared to thewavelength (below 250 nm). If thelight source is a laser, and thus ismonochromatic and coherent, thenone observes a time-dependentfluctuation in the scattering intensity.These fluctuations are due to the factthat the small molecules in solutionsare undergoing Brownian motion andso the distance between the scatterersin the solution is constantly changingwith time. This scattered light thenundergoes either constructive or destructive interference by the surrounding particlesand within this intensity fluctuation, information is contained about the time scale ofmovement 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, therewill be a reverse micelle formed, with the polar head groups pointing inwards and thenon-polar tail protruding outwards. In such a system we cannot apply the normalconductometric method of determining the critical micelle concentration . So we resortto 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 filterREAGENTS REQURED: (per 100 ml of solution) 1) Sodium Dodecyl Sulphate (SDS) (0.02885 x 36) gms = 1.0368 gms 2) n- Hexane 10 mlProcedure: 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 particleSl. No Concentration Size of the particle Size of the particle (I) Average Size of I and II of solution (N) (I) (nm) (nm) (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 done 3) 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 MACKNOWLEDGEMENTS:We are very grateful for Prof. Parna Gupta Bhattacharya and Prof. Priyadarshi De andProf. Mousumi Das for allowing us to do project titled “Determination of CriticalMicellar Concentration(CMC) of Sodium Dodecyl Sulphate in Hexane using DyanamicLight Scattering (DLS) Method.”. Their guidance came in a lot handy for doing theproject successfully.We are also thankful to Dr. Srikanth and Mr. Saroj (Lab Assistant) who provided uswith chemicals and reagents.Last but not the least we would like to thank Prof . Raja Shunmugam along with his Phdstudent Mr. Santu Sarkar, Department of Chemical Sciences , IISER Kolkata for havingallowed 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.