Determination of the critical micelle concentration (CMC) of an amphiphile by conductivity method

Title: Determination of the critical micelle concentration (CMC) of an amphiphile by conductivity method

Objective:

1. To learn the process involved in CMC determination of a surfactant

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

Introduction:

Surfactants are amphiphilic molecules that possess both hydrophobic and hydrophilic properties. A typical surfactant molecule consists of a long hydrocarbon ‘tail’ that dissolves in hydrocarbon and other non-polar solvents, and a hydrophilic ‘headgroup’ that dissolves in polar solvents (typically water). One example of a dualcharacter molecule having a head-group and a non-polar tail is sodium dodecyl sulphate (SDS), Na^{+ -}OSO₃Cl₂H₂₅. When a sufficient amount of SDS is dissolved in water, several bulk solution properties are significantly changed, particularly the surface tension (which decreases) and the ability of the solution to solubilise hydrocarbons, (which increases). These changes do not occur until a minimum bulk SDS concentration is reached. This concentration is called the critical micelle concentration (CMC). Several experiments, including light scattering and NMR, show that below the CMC, the surfactant exists mainly as solvated monomeric species, whereas above the CMC these monomers undergo self-assembly to form roughly spherical structures (having an overall diameter of ~5 nm) known as micelles (see Fig 1). Micelles are the simplest of all self-assembly structures.

Technically, a micellar solution is a colloidal dispersion of organised surfactant molecules. Non-ionic surfactant molecules can cluster together in micelles of 1000 molecules or more, but ionic species tend to form micelles of between 10 and about 100 molecules because of electrostatic repulsions between head-groups. One of the key aspects of micelle structure is that the interior of the micelle consists of an associated arrangement of hydrocarbon chains (an ‘oil droplet’). The exterior coat is constructed of the polar, ionic moieties (the OSO₃^- groups in the case of SDS). This ionic surface (which also contains associated water of hydration) is called the Stern layer. Surrounding this ionic mantle is a region that contains both counterions and oriented water molecules – the Gouy-Chapman layer. Together the Stern and Gouy-Chapman layers are known as the electrical double layer. But it is the oil-like interior of the micelle that gives it its many diverse and interesting properties. The hydrocarbon core (~3 nm in diameter) has the capacity to accommodate guest molecules. The most common application of micelles is as detergents but they can also act as micro-reaction vessels for organic syntheses and drug delivery agents

In this experiment you will determine some fundamental properties of the SDS micelle: the CMC and the free energy, enthalpy and entropy of micellisation. You will measure the CMC by measuring the conductivity of the system as a function of SDS concentration. The thermodynamic properties are obtained by determining the CMC at a variety of temperatures. You will need to pool your data – each member of the team will determine the CMC at a different temperature.

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). 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:

Table 1 Total volume of SDS added, concentration of SDS in solution and conductivity of solution

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
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
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

Graph 1 Conductivity against SDS concentration in the solution

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

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 ^-OSO­₃C₁₂H₂₅ 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 ^-OSO­₃C₁₂H₂₅ 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 ^-OSO­₃C₁₂H₂₅ 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 1 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.

Precaution Steps

1. The stirring is controlled not to be too fast during the experiment to avoid the formation of bubbles as bubbles can affect the conductivity.

2. SDS solution is added slowly to the water to prevent the formation of bubbles.

Recrystallization

Objectives

1. To separate benzoic acid from impurities by recrystallization.

2. To learn the technique of recrystallization.

3. To determine the percent recovery of benzoic acid from recrystallization.

Introduction

         A pure compound is a homogeneous sample that consisting only of molecules having the same structure. However, each substance believed to be pure may actually contain small amounts of contaminants. This includes the formation of side products during reaction, unreacted starting materials, inorganic materials, and solvents. However, recrystallization technique can be used to purify a solid and remove the impurities.

        Recrystallization is a method of purifying a solid which takes the advantage of differences in the solubility of the desired products and impurities to obtain the pure desired products. Almost all solute are more soluble in hot solvent than in a cold solvent. Thus, if a solid is dissolved in a hot solvent but is insufficient to dissolve it in the cold solvent, the crystals should form when the hot solution is allowed to cool. In the simplest case all of the impurities present in a solid sample will be so much more insoluble in the chosen solvent that all that remains in solution is the pure dissolved product (the solute).

Step 1: Choosing the solvent

An essential characteristic of a successful solvent is that the compound be soluble in the hot solvent but insoluble when the solvent is cold. Tests can be performed with small amounts of material in test tubes: a few drops of a solvent are added and if the material proves insoluble then the tube is heated to see if the material will dissolve at a higher temperature-if so, then a good solvent for recrystallization of that material may have been identified. A solvent should be rejected if the material appears readily soluble in cold solvent, is not soluble to any appreciable extent in the hot solvent even when the volume of solvent is increased, or requires an impractically large volume in order to fully dissolve the crystals.

Step 2: Dissolving the sample

An Erlenmeyer flask should be used of such a size that it will only be filled to around half-way when all the solvent has been added. The solid sample is introduced together with around 75% of the amount of solvent thought to be required. It is always advisable to use less solvent at this stage. The flask is heated on a hotplate until dissolution of the solute is complete, additional solvent can be added to the hot solution as necessary to ensure complete dissolution. A boiling wooden stick should be added to provide a nucleation side for bubbles to form and facilitate an even boiling process. A process of gradual addition of solvent to the flask will ensure that the sample has dissolved to form saturea and will deposit crystalline material once it is cooled. Using excessive amount of solvents will only decrease the percent recovery of the products.

Step 3: Hot filtration

Once the solute is fully dissolved, the remaining impurities can be removed by filtering the hot solution through a filter paper folded into a cone and placed inside a glass filter. A problem here is that the solution will cool rapidly as soon as the Erlenmeyer flask is removed from the hotplate. In most cases, this problem can be minimized or avoided entirely by using a stemless funnel placed on the top of beaker containing a few millimeters of the recrystallization solvent. The beaker is placed on the hotplate and the boiling solvent serves to heat the funnel and prevent the solute from crystalling during the filtration process.

Step 4: Cooling

Cooling the filtered solution will allow crystals to form and rate of cooling can determine the size of the crystals formed. Fast cooling generally produced more crystals of relatively small dimensions, but slow cooling might allow larger crystals to form. The solution usually is left to cool to room temperature before cooled in the ice-bath to ensure maximum recovery.

Step 5: Cool filtration

When the crystallization process is judged to be completed the crystals need to be collected by suction filtration. Both the funnel and suction flask should be chosen so that neither will become more than half full during the filtration process. It is preferable that all of the crystalline material is being transferred to the funnel as a suspension in the crystallization solvent, however it is sometimes hard to get all of the crystals moving freely by swirling the flask and occasionally it will be necessary to add more ice-cold solvent in order to transfer the last of the crystalline material. It may also be necessary to dislodge crystalline material from the sides of the flask with a spatula prior to filtration.

Step 6: Washing the crystals

Once the suction filtration process is completed, the collected crystals should be washed with a little more ice-cold solvent to remove final soluble impurities which would otherwise be left on the surface of the crystals. The solvent used for this final washing should be as cold as possible to minimize losses from the crystals re-dissolving.

Step 7: Drying the crystals

Once the crystals have been collected on the suction funnel they can usually be satisfactorily dried by continuing to draw air over them for a few minutes. The almost dry crystals should then be spread on a filter paper to allow the last traces of volatiles solvent to evaporate.

Apparatus and Materials

Erlenmeyer flask (125 mL), short-stemmed funnel, hot plate, boiling chips, benzoic acid and charcoal.

Procedures

1. 2.0 g of crude benzoic acid were weighed into a 125-mL Erlenmeyer flask.

2. 200 mL of water was heated to boiling in a beaker on a hot plate with boiling chips.

3. An Erlenmeyer flask with a little water in it with boiling chips also was heated on a hot plate, with a short-stemmed funnel resting in its neck.

4. A filter paper was fluted to fit the funnel.

5. A few boiling chips was added to the benzoic acid and the adding of hot water to the benzoic acid was started until the benzoic acid has dissolved.

6. About 0.2 g of decolorizing charcoal was added.

7. The hot solution was filtered through the fluted filter paper into the heated flask.

8. The original flask and the filter paper were rinsed with a little hot water.

9. The solution of benzoic acid was removed from the hot plate and allowed to cool to the room temperature.

10. The solution is then cooled in an ice bath after 15 minutes for 10 minutes.

11. The crystals were collected by suction filtration using Buchner funnel.

12. Vacuum is continued to pull on the funnel for 5 minutes.

13. The filter paper with crystals was transferred onto a fresh piece of filter paper, and the crystals are allowed to air-dry.

14. The percent recovery and the melting point of benzoic acid were determined.

Results & Calculation

Weight of crude benzoic acid = 2.0007 g

Weight of filter paper = 0.8459 g

Weight of benzoic acid crystals + filter paper = 1.8776 g

Weight of benzoic acid crystals = 1.0317 g

Melting point of benzoic acid crystals = 120 °C

Percent Recovery

= weight of compound recovered / weight of compound started with x100%

= (1.0317 g / 2.0007 g) × 100%

= 51.57%

Relative Accuracy of Melting Point

= melting point of benzoic acid/ melting point of recovered benzoic acid × 100%

= (120 °C / 122 °C) × 100%

= 98.36%

Discussion

The percentage recovery of benzoic acid is only 51.57% may be due to several factors that caused the loss of products. One of the factors is that the volume of water added to the solution is too much which making the solution not saturated enough to produce maximum yield of benzoic acid after cooling. Normally, larger volume of water used will tend to the products to dissolve more easily. The benzoic acid crystallized on the filter paper during the hot filtration. The additional hot water need to be added to dissolve the benzoic acid crystals on the filter paper which causes the solution to be more dilute. So that, the products is lost in the solution.

Besides, too much decolorizing charcoal is added to the solution is considered as one of the factors. Decolorizing charcoal functions to provide vacant sites to the organic compounds to accommodate to it in which it removes the unwanted colored impurities. However, this also caused the loss of products in this process because some of the benzoic acid also will be adsorbed onto the surface of charcoal. Generally, the charcoal added should be only about 1-5% of the weight of the sample being recrystallized. A little amount of charcoal is sufficiently to remove the colored impurities. Otherwise, excessive use of charcoal will only caused the products to be removed together with the colored impurities.

In this experiment, the benzoic acid is dissolved in hot water while only a little amount of benzoic acid are able to dissolve in cold water. The benzoic acid cannot dissolve well in cold solution because of its hydrophobic benzene ring. However, the carboxyl group, -COOH that attached to the benzene ring allows some of the benzoic acid solubilise in water. In hot solution, the increase in temperature causes the water molecules has more kinetic energy and move faster. As a result, it allows the water molecules to penetrate through the benzoic acid solid and hence solubilization of benzoic acid occurs. In addition, the charcoal and other impurities present in mixture cannot dissolve in water. So, water is a good solvent to be chosen in the experiment.

The boiling chips were added in the experiment. Boiling chips are small, insoluble, and porous stones made of calcium carbonate or silicon carbide. There is a lot of pores inside the boiling chips in which it provides nucleation site to trap air and creates space to allow the bubble of solvent to form. When the boiling chips are heated, it will release tiny bubbles which can prevent bumping and boiling over of the mixture so that the loss of solution can be avoided even it is boiled. The adding of boiling chips must be added before boiling of solution instead of after boiling. This is because adding boiling chips to a solution near its boiling point will induce flash boiling as well. The boiling chips are not soluble in the solvent and hence they can be filtered out by using filter paper, but they are not reusable.

During the hot filtration, most of the charcoal powders were removed and stay on the filter paper while the benzoic acid solution pass through the filter paper and goes into the conical flask. However, some of the charcoal powder was noticed in the conical flask as well, although in a small amount. This might be due to the size of charcoal powder is too small that can pass through the pores of filter paper. During the cooling process, the hot solution was allowed to cool slowly to the room temperature, and then only immersed in an ice-bath. The solution should be protected from contaminants by covering with a piece of filter paper. Fast cooling always produces relatively small crystals because the particles do not have sufficient time to arrange themselves in proper conformation, so it is not advisable to cool down the hot solution immediately in ice-bath. The small size of crystal form may trap impurities easily. Oppositely, slow cooling allowed the molecules to interact and arrange themselves properly and hence they form larger size of crystals. But, large particles may causes some solvent being trapped inside the crystals.

During the cold filtration, the water soluble impurities that might dissolve in water which was filtered out through the suction filtration. However, some of the impurities might be trapped on the surface of the benzoic acid crystals, so a small volume of ice-cold water should be used to wash the benzoic acid crystals to dissolve the particular impurities. The crystal was dried in the oven at 100 °C. A fresh piece of filter paper can be used to place under the filter paper with benzoic acid crystals.

The purity of a crystal can be determined by its melting point. A narrow range of melting point indicates high purity of the sample, otherwise broad range of melting point indicates the presence of impurities in the crystal. The melting point of the recovered benzoic acid obtained experimentally is 120 °C. Compared to the pure benzoic acid with 122°C of melting point, the purity of the recovered benzoic acid is very high which is98.36%. Although the accuracy is high enough, but it also means that the compound is slightly contaminated with impurities which included the charcoal powder or the water molecules that trapped inside the benzoic acid crystals. The melting point of recovered benzoic acid is lower because the crystals cannot arrange properly due to impurities.

Structure of benzoic acid

Suction filtration