Measurement of the Critical Micelle Concentration of a Surfactant

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EXP 2.13: MEASUREMENT OF THE CRITICAL MICELLE CONCENTRATIONOF A SURFACTANT ABSTRACT The aim of this experiment is to measure the critical micelle concentration of sodium dodecysulphate. This will be done by using the conductivity method which will involve measuring the conductivity of a solution during a titration. The second part of the experiment will involve using spectrophotometric method to determine the maximum wavelengths at different concentrations. INTRODUCTION in colloidal and surface chemistry the critical micelle concentration (CMC) of a surfactant is the concentration where micelles begin to form [1]. Surfactants are compounds that contain hydrophobic and hydrophilic groups. A surfactant therefore has both water soluble and water insoluble properties. These substances diffiuse into water in such a way that the water soluble head remain in the water phase of the aqueous liquid while the water insoluble hydrophobic group extends out of the water. Micelles are an accumulation of surfactant molecules dispersed in a liquid colloid. Typical micelle in solution usually combine with the hydrophilic head region in conctact with surrounding solvent, which leaves the hydrophobic tail regions in the micelle centre. An example of a surfactant is dodecyl sulphate (SDS) which has a structure: H3C-CH2-CH2-CH2 –CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-O-S(=O)2-O-Na+ [1] This soap molecule has an end which is highly polar (hydrophilic) and the other end is non- polar (hydrophobic). When forming micelles the water soluble head remains on the outside while the tail is enclosed inside forming a sphere like structure. This is what gives soap its properties. When enough amount of SDS is dissolved in water, some properties of the bulk solution are significantly changed. For example, the surface tension decreases and the ability of the solution to dissolve hydrocarbons

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