- Experiment 2 Critical Micelle Concentration Of Surfactant 1 Objective 1 1 To Determine Critical Micelle Concentration 1 (127.94 KiB) Viewed 72 times
- Experiment 2 Critical Micelle Concentration Of Surfactant 1 Objective 1 1 To Determine Critical Micelle Concentration 2 (43.03 KiB) Viewed 72 times
Experiment 2 Critical micelle concentration of surfactant 1. Objective 1.1. To determine critical micelle concentration (CMC) of an anionic surfactant, sodium dodecyl sulphate. 1.2. To learn the conductometric method for measuring CMC of ionic surfactants. 2. Introduction Surfactant is an amphiphilic substance that consists of both polar and non-polar parts. When a small amount of surfactant is dissolved in water, it mainly exists as monomers dispersed in water. Based on its structural properties, the surfactant will be adsorbed on the surface of the solution with a preferred orientation. As the concentration increases, it will aggregate and form micelles. To make itself stable in solution, the surfactant changes in two ways: one is to leave the hydrophilic groups in the water and to let the hydrophobic groups facing the oil phase or the air, another is to collect the hydrophobic groups of the surfactant in order to reduce the interfacial area with ater. The former reduces the surface tension and leads to the adsorption of the surfactant on the interface and the formation of an oriented mono-molecular membrane. The latter leads to the formation of micelle. Since the hydrophilic groups of the micelle constitute its exterior coat interacting with the water molecules, the surfactant is stable in solution. As the surfactant concentration further increases, the spherical micelle may change to clavicorn micelle, or even to lamellar micelle. The minimum concentration of surfactant at which micelle formation begins is referred to as the critical micelle concentration, abbreviated to CMC which is a very important property for surfactants. Above the CMC, the physical and chemical properties (such as surface tension, conductance, osmotic pressure, turbidity, optical property) change abruptly with concentration owing to the change in solution structure. This phenomenon is an important characteristic of surfactants, and also the base of CMC measurments. In this experiment, the conductivities (or molar conductivities) of sodium dodecyl sulphate of different concentrations are measured using a conductometer. The CMC is obtained from the discernible break in the relation between conductivity and concentration. 3. Apparatus and Reagent Conductometer; thermostatic water bath; volumetric flask; conical flask; sodium dodecyl sulphate. 4. Procedure 4.1. Prepare a series of sodium dodecyl sulphate solutions with different concentrations with pure water 4.2. Turn on the thermostatic water bath and adjust the temperature of the thermostat to 25 °C. 4.3. Measure the conductivities of the above solutions. Wash the electrode with distilled water. Rinse the conical flask with the test solutions over three times before each measurement. Let the flask and the solution warm for 10 min before measurement.
4.4. Tabulate the data of the conductivities of the solutions. 4.10. Wash the tubes and electrodes with distilled water after experiment. 5. Data Analysis 5.1. Plot the conductivities against concentrations, and find out the CMC from the turning point in the plot. 5.2. Reference data: at 25 °C, CMC of sodium dodecyl sulphate is about 8.0 x 10 mol/L. 6. Notes 6.1. Electrodes must be wiped to dry after washing to ensure the precision of the concentrations of solutions. 6.2. The conductometer must be adjusted before measurement. 7. Exercises 7.1. What are the factors that influence the measurement of CMC of ionic surfactant? 7.2. To identify whether the measured CMC is correct, what kind of experimental technique can be used?