PH1002 Experiment 4 Determination of the CMC of Sodium Lauryl Sulphate copy PDF

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PH1002: Physical Pharmacy I 4. Determination of the Critical Micelle Concentration of Sodium Lauryl Sulphate

Name: Joseph O’Shea Student Number: 14311653 Date: 16/12/2014

Introduction: The aim of this experiment is to determine the critical micelle concentration of Sodium Lauryl Sulphate.

This substance is a surfactant. Surfactants are compounds which lower the surface tension between two liquids (or between a liquid and a solid) and promote wetting. These drugs have a hydrophobic tail group and a hydrophilic head group, i.e. the head group is polar and the tail will not be polar. When inserted into the water, the hydrophilic head group will stay in the water whereas the hydrophobic tail will stick out of the surface of the water. As the surfactant lines up against the surface of the water, the bonds between water molecules grow weaker and therefore reduces the surface tension. Surface tension is measured using a tensiometer. There is a point in the experiment when the surface tension will not lower beyond a certain value. This concentration is known as the critical micelle concentration (C.M.C.) and is a measure of surfactant efficiency (a lower C.M.C indicates less surfactant is needed to saturate interfaces and form micelles). At this stage, there is an excess of surfactant in the water. There is not sufficient space for any more surfactant molecules on the surface of the water. To compensate for this, the surfactant forms a structure called a micelle to ensure that the hydrophobic tail is not in contact with the water. Micelles are an aggregate of molecules/ions in a colloidal solution. A colloid is a substance which has insoluble, microscopic particles dispersed in it and are suspended throughout another substance. Unlike a solution, whose solute and solvent make up only one phase, a colloid has a dispersed phase (the suspended particles) and a continuous phase (the medium of suspension). The dispersed-phase particles have a diameter of between approximately 1 and 1000 nm. A colloidal solution is one in which the

material is evenly suspended in a liquid. The formation of colloidal-sized micelles explain the physico-chemical properties of solutions of certain substances. It is characteristic of the substances that produce micelles to have a sharp change in the properties of their solutions at the concentration at which micelles are formed. (Below this concentration, the substance in solution behaves like a true solution- with various properties being related directly to the concentration of the substance. Above this concentration, properties are not simply related to concentration of the substance). Therefore, a graph of surface tension against concentration of such substances will show a change of slope at the C.M.C. The C.M.C can be estimated by observing other changes, such as the difference of colour of a dye dissolved in solutions of such substances above and below the concentration of micelle formation. The cationic dye, Pinacyanol is added to solutions of deionised water and sodium lauryl sulphate of varying concentrations. By observing the colour that the dye takes, the critical micelle concentration can be determined. The addition of electrolytes will lower the concentration of surfactant required to reach the critical micelle concentration. In this experiment, the C.M.C of sodium lauryl sulphate was determined by the surface tension and dye colour change methods. The results obtained from the two experiments and the quality of the two methods were compared.

Method:

1. Surface Tension Method: Solutions containing 100 mL distilled water and 10 mg, 25 mg, 50 mg, 100 mg, 150 mg, 200 mg, 250 mg, 300 mg and 400 mg of sodium lauryl were prepared. The surface tension of each solution of sodium lauryl was determined and the mean of three replicate readings for each surface tension were calculated and recorded. The surface tension was measured using two different methods. The first was the Wilhelmy Plate Method, and the second was the Du Nouy Ring Method. A) The Wilhelmy Plate MethodA digital tensiometer was used to measure the surface tension of the liquid samples. The tensiometer was first calibrated. (How?). The Wilhelmy Plate was then cleaned and placed in the support and suspended. The container of liquid of a certain concentration was then placed on the lab jack and lifted up until the surface was close to the lower side of the plate. As the container was raised, a thermometer was held in place inside to liquid to ensure temperature was kept constant throughout. The temperature is kept constant because the surface tension decreases with increasing temperature- surface tension and temperature are inversely proportional. The tensiometer is only ready to execute the measure once the zero key has been pressed. The container was then lifted very slowly and carefully until the surface touches the lower side of the plate. The highest value shown on the display was then recorded as the surface tension of the sample. This method was repeated for all of the samples until a complete set of results was obtained. At the end of each test, the container was lowered and replaced with another one of different concentration, and the plate removed and cleaned. B) Du Nouy Ring MethodThe surface tension at each concentration was measured with the Lauda TD 1M tensiometer. The ring was carefully cleaned and suspended by the hooked support. The sample (at 20 BC) was then placed on the manual jack and raised until the surface of the sample was near to the lower side of the ring. The mode key was then pressed four times followed by the enter key and finally the zero key. The tensiometer was then ready for use. The sample was raised until the ring was plunged into the sample, and then lowered slowly and carefully until the maximum value was observed on the display just before the ring breaks away from the liquid. The corrected surface tension (mN/m) was calculated. The liquid density was then calculated selecting the Un2 function. The cylinder was filled with the sample until the level was reached and then both the floater and the thermoprobe were introduced in the liquid. The liquid was stirred to homogenise the temperature. Care was taken to ensure that no air bubbles were present on the floater. The floater was then hooked, ensuring that it did not touch the internal walls of the cylinder or the thermoprobe. The density of the liquid then appeared on the display and was recorded. The values obtained were then used to obtain a value for the surface tension using this ring method- using the surface tension calculator excel file (the PC calculates directly the liquid surface tension value at 20 BC in mN/m). The correction value is given by: (2)F = 0.8759 + [(0.0009188 x STunc) / D]

(Where: F = correction value, STunc = Uncorrected surface tension and D = density of the test liquid in g/cm3) The STunc is specific to the ring on the apparatus. Therefore, the following formula must be used: STunc = Fmax / (2 x π x d) (Where: Fmax = average force as displayed on the measuring system and d= diameter of the ring in cm. Also note that the value for STunc is given in nM/m) STabs = STunc x F A graph of mean surface tension versus concentration of sodium lauryl sulphate is plotted. (Note: The order in which calculations should be done is shown in brackets).

2. Dye Colour Method: Solutions containing 100 mL distilled water and 10 mg, 25 mg, 50 mg, 100 mg, 150 mg, 200 mg, 250 mg, 300 mg and 400 mg of sodium lauryl were prepared using 10-4 molar solution of Pinacyanol in distilled water as a solvent. The change in colour was observed and noted to determine the CMC of sodium lauryl sulphate. The colour changes from red to blue at the C.M.C. (The C.M.C is when the red disappears, as the dye is red in water containing sodium lauryl sulphate in concentrations below the C.M.C and blue in concentrations above the C.M.C).

Results and Conclusions:

Surface Tension Method: 1. Wilhelmy Plate MethodConcentration (% w/v) 0.01 0.025 0.05 0.1 0.15 0.20 0.25 0.30 0.40

Surface Tension (mN) 48 20.3 36.9 33.3 34.9 34.6 37.3 22.9 24.1

Conclusion:

The graph obtained from these results was in no way similar to the expected graph for this experiment. The expected graph of surface tension against concentration should show a steady decline in surface tension (decreasing slope) with increasing concentration, and then a sudden change of slope (graph plateaus), indicating the critical micelle concentration at the point where this change occurs. However, the values obtained for the surface tension at the different concentrations increased and decreased, resulting in a completely different graph as to what was to be expected, and therefore the CMC could not be determined from the graph. 2. Du Nouy Ring Method-

Lauda TD 1M Readings:

Concentration(%w/ v) Measurement 1

0 53.5

Measurement 2

54.0

Measurement 3

51.9

Average

53.1 3

0.0 1 39. 0 39. 2 39. 4 39. 2

0.02 5 35.0

0.05

0.1

0.15

0.2

0.25

0.3

0.4

33.4

31.0

32.3

32.7

31.7

35.5

33.6

30.3

32.2

32.5

31.9

35.9

33.7

30.1

32.2

33.0

32.5

35.4 6

33.5 7

29. 5 29. 8 30. 1 29. 8

30.4 6

32.2 3

32.7 3

32.0 3

28. 9 31. 1 31. 0 30. 3

STunc = Fmax / (2 x π x d)

Calculations for 0 %w/v and all calculations were done in the same manner: STunc = the average reading from the tensiometer as the Lauda apparatus factors the formula in already

Concentration(%w/v) STunc (mN/m)

0

0.0 0.025 0.05 0.1 0.15 0.2 0.25 0.3 1 53.13 39.2 35.46 33.57 29.8 30.46 32.23 32.73 32.03

0.4 30. 3

F = 0.8759 + [(0.0009188 x STunc) / D] F = 0.8759 + [(0.0009188 x 53.13) /1000000] D = 1g/ cubic centimetre as the test liquid is water. 1 gram/cubic centimetre = 1000000 gram/cubic metre. The figure is converted as force is in metres. F = 0.8759 + [(0.0009188 x 53.13) /1000000] = 0.875900048815844

The Correction Value for each concentration was calculated but is not stated due to space issues: Stabs = STunc x F Concentration(% w/v) STabs (mN/m)

0

0.01

46.5 4

34.3 4

0.02 5 31.0 6

0.05

0.1

0.15

0.2

0.25

0.3

0.4

29.4 0

26.1 0

26.6 8

28.2 3

28.6 7

28.0 6

26.5 4

Conclusion: The ring method proved to be more accurate than the plate method, as the values obtained for the surface tension, apart from 3 of the values, decreased with increasing concentration, which resulted in a graph which was similar to what was the be expected- allowing determination of the CMC of sodium lauryl sulphate.

Colour Dye MethodThe Pinacyanol is violet in water, red in concentrations below the CMC and blue in concentrations above the CMC. The critical micelle concentration is reached when the colour in the flasks change from red to blue along the line. Flask 5 (0.1 % w/v) was red while flask 6 (0.15 % w/v) was blue. The CMC lies between these two concentrations.

Relationship Between Surface Tension and Concentration of Sodium Lauryl Sulphate Surface Tension (Absolute)[mN/m]

50 45 40 35 30 25

STabs (mN/m)

20 15 10 5 0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Concentration of Sodium Lauryl Sulphate (%w/v)

Suitability and Advantages of Both Methods: The colour dye method is less time consuming than the use of the tensiometer. However, the colour dye is inaccurate and the critical micelle concentration can lie within a large range. For example, in the investigation carried out the CMC is within 0.1 and 0.15 %w/v. The tensiometer is more accurate as we see the surface tension so can investigate more thoroughly. Also, the colour dye method can be wasteful....