Hemin J Majeed MSc. Pharmaceutical sciences Physical Pharmacy Lab 4- Surface Active Agents Hemin J Majeed MSc. Pharmaceutical sciences

Surface Active Agents: Solute molecules or ions that are adsorbed at surface/ interface and reduce the surface or interfacial tension are called surfactants ( surface active agents). Surfactants molecules are termed amphiphiles because contain a polar head and non polar tail and as a result they have certain affinity for both polar and non polar solvents. Depending on the number and nature of the polar and non polar groups present, the amphiphile may be predominantly hydrophilic (water-loving), lipophilic (oil-loving), or be reasonably well-balanced between these two extremes.

When a surfactant is added to water Hydrophilic head will be oriented towards water Lipophilic end will be oriented away from water When the same solute is added to an immiscible system Hydrophilic head will be oriented towards aqueous phase (away from oil) Lipophilic end will be oriented towards oil phase (away from water)

At lower concentration of surfactant molecules will be distributed at the surface As concentration increases the surface is filled up and saturated with surfactant with marked decrease in the surface tension After surface saturation, the surfactant molecules will move into bulk of the liquid forming small aggregation called micelles.

Critical Micellar concentration (CMC) CMC is the minimum concentration of surfactant in bulk of liquid at which the formation of micelles appear.

The process of micelles formation is called micellization typical micelle is spherical in structure which contain 50-100 monomers Number of monomers that formed micelle is termed aggregation number Micellization technique is used as drug delivery carrier system It should be sufficiently stable in blood circulation. Formation of micelles in bulk liquid lead to change the physical properties of system like conductivity, density and viscosity.

Micellar solubilization Micelles can increase the solubility of materials that are, normally insoluble, or only slightly soluble, in the dispersing medium used. This phenomenon is known as Micellar Solubilization.

Experimental Work: To see the effect of increasing the concentration of S.A.A (Tween 80) on the solubility of slightly soluble substance (salicylic acid). Procedure: 1. Prepare 50 mls of different concentrations of Tween 80 solutions: 0%, 1%, 2%, 3%, and 4%. 2. Place 25 ml of each concentration in a conical flask of (50 ml) then add 0.25 g salicylic acid to each flask.

3. Shake the flasks for 10 minutes then do a filtration using a funnel and filter paper. 4. Withdraw 10 ml of filtrate, and titrate with standardized sodium hydroxide solution (0.05 N) using phenothaline as indicator. 5.The end point is a point when the color changes from yellow to pink. Measure the end points. 6. Plot the total solubility (g/10m1) of salicylic acid VS conc. of Tween 80.

Calculation: 1 molecule of SA + 1 molecule of NaOH give the product 1 M.wt of SA = 1 M.wt of NaOH 1 eq. wt SA = 1 eq. wt NaOH 138.1 g = 1 L of 1 N NaOH 138.1 g = 1000 ml of 1 N NaOH 138.1/1000 x 0.05 = 1 ml of 0.05 N NaOH Each 1 ml of 0.05 N NaOH is equivalent to 0.0069 g SA. So End point x 0.0069= g SA /10 ml