Download Surface Thermodynamics: Effect of Surfactants on Interfacial Tension and more Exercises Civil Engineering in PDF only on Docsity! Objectives_template file:///E|/courses/colloid_interface_science/lecture21/21_1.htm[6/16/2012 1:10:36 PM] Module 4: "Surface Thermodynamics" Lecture 21: "" The Lecture Contains: Effect of surfactant on interfacial tension Objectives_template file:///E|/courses/colloid_interface_science/lecture21/21_2.htm[6/16/2012 1:10:36 PM] Module 4: "Surface Thermodynamics" Lecture 21: "" Surface Thermodynamics: Roles of Surfactants and solutes Effect of surfactant on interfacial tension A surfactant or a surface active agent is an organic molecule which is amphiphilic in nature, that is, contains two distinct parts. 1) Hydrophilic part (the head group) 2) Hydrophobic part (the tail group) Fig 6.1: Schematic of a surfactant molecule The hydrophilic (water-loving) head is attracted to water molecules while the lipophilic (fat-loving) tail attaches itself to oil and grease, repelling water. An effective surfactant is usually considered to be one with a hydrophobic tail > 8 Carbon atoms. While surfactants may be further classified as nonionic (eg. Fatty alcohols), cationic (eg. Cetyl trimethyl ammonium bromide or CTAB), anionic (eg. Sodium dodecyl sulfate or SDS) or zwitterionic (Cocamidopropyl hydroxysultaine) based on the charge of the head group let us focus here on its effect on surface tension that renders it invaluable to us. Objectives_template file:///E|/courses/colloid_interface_science/lecture21/21_5.htm[6/16/2012 1:10:37 PM] Hence, (6.2) Module 4: "Surface Thermodynamics" Lecture 21: "" Now, we calculate the effect of surfactants on interfacial tension. Gibbs Duhem equation for bulk is Gibbs Duhem equation for interface is given by Assuming system is in thermal and chemical equilibrium at constant temperature . We get, where is concentration. Therefore Objectives_template file:///E|/courses/colloid_interface_science/lecture21/21_6.htm[6/16/2012 1:10:37 PM] (6.3) Module 4: "Surface Thermodynamics" Lecture 21: "" If we assume that the interface is a flat surface then from the Young Laplace equation Without loss in generality we can assume species ‘1’ as the solvent for the system assuming it is present in excess amount. where = surface excess concentration of species with respect to 1. Objectives_template file:///E|/courses/colloid_interface_science/lecture21/21_7.htm[6/16/2012 1:10:37 PM] (6.4) Module 4: "Surface Thermodynamics" Lecture 21: "" We choose the interface such that then . The idea is, change in surface tension should not depend on the solvent concentration (species 1) i.e should be independent of This is known as the Gibb’s convention. It can be proved that calculated above is independent of the position of the surface. we have Thus,
