Surface Thermodynamics Lecture 22: Effect of Surfactants on Interfacial Tension, Exercises of Civil Engineering

Download Surface Thermodynamics Lecture 22: Effect of Surfactants on Interfacial Tension and more Exercises Civil Engineering in PDF only on Docsity! Objectives_template file:///E|/courses/colloid_interface_science/lecture22/22_1.htm[6/16/2012 1:10:07 PM] Module 4: "Surface Thermodynamics" Lecture 22: "" The Lecture Contains: Examples on Effect of surfactant on interfacial tension Objectives_template file:///E|/courses/colloid_interface_science/lecture22/22_2.htm[6/16/2012 1:10:08 PM] Module 4: "Surface Thermodynamics" Lecture 22: "" Example Consider liquid, its vapors and the solute as a two-phase two-component system. Let the properties of the vapor be represented by []' and the properties of the liquid phase by []". (1) represents solvent and (2) represents solute. If the solute is non-volatile (surfactants are usually non-volatile), then . Further, since concentration of solvent in vapor phase is very small compared to that in the bulk liquid phase, we have . From the above discussion we have If we consider the bulk solubility of the surfactant to be small then This is a limiting case implying that all the surfactant has gone to the surface. Objectives_template file:///E|/courses/colloid_interface_science/lecture22/22_5.htm[6/16/2012 1:10:08 PM] Module 4: "Surface Thermodynamics" Lecture 22: "" Any solute which is extremely hydrophilic (like common salt and sugar) would have a negative surface concentration implying it is present more in the bulk than at the surface. Fig. 6.5: Profile for a hydrophilic solute Fig. 6.6 : Interfacial tension versus solute concentration. Objectives_template file:///E|/courses/colloid_interface_science/lecture22/22_6.htm[6/16/2012 1:10:09 PM] Module 4: "Surface Thermodynamics" Lecture 22: "" Fig 6.6 shows the effect of adding different materials on the surface tension of water . On adding hydrophilic solutes that are present more in the bulk than the interface, the surface tension increases. While lyophobic solutes, that dislike the bulk aqueous phase, decrease the surface tension somewhat, huge reduction of surface tension takes place through surfactant addition.Cosurfactant addition further reduces the surface tension, while it is seen that alcohol, if added along with surfactant and cosurfactant can lead to ultra low surface tension. At high concentration of solute(surfactants), interfacial tension does not changemuch due to formation of micelles. Hence may not be a good representation and needs to be replaced with activity. Micelles form only after a particular concentration of solute (i.e surfactants) called critical micelles (delete) concentration (CMC). CMC is achieved because there is an energy penalty in micelles formation. Before reaching the CMC, thesurface tension changes strongly with the concentration of the surfactant. After reaching the CMC, the surface tension change becomes very small. Thus from a graph of concentration of surfactant versus the change in surface tension, the CMC for the system can be evaluated. [2] For , micelles form as a result of saturation and the activity of monomer surfactant remains constant. Objectives_template file:///E|/courses/colloid_interface_science/lecture22/22_7.htm[6/16/2012 1:10:09 PM] (6.6) Module 4: "Surface Thermodynamics" Lecture 22: "" Example Consider an electrolyte adsorbing at water air interface. It can be represented as chemical equation as where represents . Using Gibbs equation we can write Assuming surface is neutral i.e. equal number of anion and cation at the surface so is mean ionic activity which is practically more easy to measure then and . (6.7) where is the concentration(amount) of salt put in. Reduction in surface tension depends on how much molecules are adsorbed on the surface.