chemistri ii

Experiment 3

Aim of the Experiment

To determine critical micelle concentration (CMC) of a given surfactant (Sodium Dodecylsulfate, SDS) using conductivity method.

Principle

The term colloid (which means “glue” in Greek) was first introduced in 1861 by Thomas Graham to describe the “pseudosolutions” in aqueous systems of silver chloride, sulfur, and Prussian blue. Amongst these systems, the lyophobic (“liquid – hating”) colloids, composed of insoluble or immiscible components; e.g. colloidal gold sols, which involve solid particles in water. Other examples of lyophobic colloids are milk (liquid fat dispersed as fine drops in an aqueous phase), smoke (solid particles dispersed in air), fog (small liquid droplets dispersed in air), paints (small solid particles dispersed in liquid), jelly (large protein molecules dispersed in water), and bone (small particles of calcium phosphate dispersed in a solid matrix of collagen). Other class of colloids is the lyophilic (“liquid – loving”) colloids, which are solutions that form spontaneously and are thermodynamically stable. These systems consist of solute molecules that are polymers (i.e., of much larger size than the solvent molecules), and as such form a large and distinct area of research (polymer science).

Another major group of colloidal systems is that of the association colloids. These are aggregates of amphiphilic (both “oil and water – loving”) molecules that associate in a dynamic and thermodynamically driven process. Such molecules are commonly termed “SurfActAnts”, a contraction of the term Surface Active Agents. Surfactants are a versatile class of chemicals and are important in various fields of interfacial science and continue to be critical in many applications such as agriculture, water treatment, oil recovery, fire fighting, paper and plastic manufacturing.

Surface-active agents are organic molecules that, when dissolved in a solvent at low concentration, have the ability to adsorb at interfaces, thereby altering significantly the physical properties of those interfaces. This adsorption behavior can be attributed to the solvent nature and to a chemical structure for surfactants that combine both a polar and a non – polar (amphiphilic) group into a single molecule. Owing to their dual nature, amphiphiles therefore adsorb at interfaces so that their lyophobic moiety stays away from strong solvent interactions while the lyophilic part remains in solution. Since water is the most common solvent, and is the liquid of most academic and industrial interest, amphiphiles are described with regard to their “hydrophilic” and “hydrophobic” moieties, or “head” and “tail” respectively.

In addition to forming oriented interfacial monolayers, surfactants can aggregate to form micelles, provided their concentration is sufficiently high. Micelles are typically clusters of between 50−200 surfactant molecules. Micelle formation occurs over a fairly sharply defined region called the critical micelle concentration (CMC). The mechanism of micelle formation is as illustrated below:

Above the cmc, additional surfactant forms the aggregates, whereas the concentration of the unassociated monomers remains almost constant. As a result at the cmc, there is an abrupt change in physical properties of the solution like molar conductivity, osmotic pressure, surface tension etc. For determination of cmc, the most common method is to measure molar conductivity or surface tension as a function of concentration of surfactant. A break in the conductance-concentration or surface tension-concentration plot corresponds to the cmc of the surfactant.

The critical micelle concentration is the simplest means of characterizing the colloid and surface behavior of a surfactant, which in turn determines its industrial usefulness and biological activity and also gives a measure of solute–solute interactions.

• Procedure

• Prepare 10 mL 0.1 M SDS solution (stock solution) in conductivity water. Pipette out exactly 25 mL of conductivity water in a beaker and dip the conductivity cell into it. Measure the conductance of conductivity water. Add 0.2 mL of 0.1M SDS solution using a micropipette in the beaker and note down the conductance. Repeat the same by addition of 0.2 mL SDS solution each time till the total volume in the beaker becomes 28 mL. Plot a graph of Specific conductance (κ) versus concentration of surfactant (C). A point representing the intersection of two straight lines denotes the CMC of SDS at given temperature.

• 
  

Observation:

Room Temperature :___________________ °C

Cell Constant :___________________

• Result

• The CMC of given surfactant at ________________ °C is _________________ mili Moles.

• (The literature value of CMC of SDS at 25 °C is 8.1 x 10^-3 moles)

Reference Material

1. Practical Physical Chemistry by B. Viswanathan and P. S. Raghavan

Questions

1. What is the difference between soaps and detergent?

2. What is CMC?