Chapter 15. Interfacial Phenomena

Contents Liquid Interface Adsorption at Liquid Interfaces Adsorption at Solid Interfaces Applications of Surface Active Agents Electric Properties of Interfaces

Interface & Surface Interface Surface The boundary between two phases (faces) Surface Ether a gas-solid or a gas-liquid interface

LIQUID INTERFACES

Liquid Interfaces Fig. 15-1

Surface & Interfacial Tension Surface tension A force pulls the molecules of the interface together Unit : dyne/cm Interfacial tension The force per unit length existing at the interface between two immiscible liquid phases

Surface Tension () Fig. 15-3  = f / 2L

Surface Free Energy W =  A  : surface tension (dyne/cm) W : The work done or surface free energy (ergs)  : surface tension (dyne/cm) A : The increase in area (cm)

Measurement of Surface and Interfacial Tension Capillary rise method DuNoüy ring method Drop weight ….  Temperature   Surface tension   Critical Temperature  Surface Tension = 0

Capillary Rise Method 2r = r2hg  = 1/2 rhg Fig. 15-5

DuNoüy Ring Method Fig. 15-6

Work of Adhesion Fig. 15-7 Wa = L + S - LS

Work of Cohesion Fig. 15-8 Wc = 2L

Spreading Coefficient S = Wa - Wc = (L + S - LS) - 2L S = S - (L + LS) S > 0 : spreading S < 0 : form globules or a floating lens

ADSORPTION AT LIQUID INTERFACES

Adsorption at Liquid Interfaces Surface free energy The work that must be done to increase the surface by unit area Adsorption A phenomenon, where the added molecules are partitioned in favor of the interface Absorption The liquid or gas being absorbed penetrates into the capillary spaces of the absorbing medium

Surface-Active Agent Surface-active agent (= surfactant, amphiphile) Molecule and ion that are adsorbed at interfaces The molecule or ion has a certain affinity for both polar and nonpolar solvent

Hydrophilic-Lipophilic Balance ( HLB ) Fig. 15-11 An arbitrary scale of values to serve as a measure of the hydrophilic-lipophilic balance of surface-active agents

HLB S : The saponification number of the ester Nonionic surfactant HLB = E/5 E : Percent by weight of nonionic surfactant Polyhydric alcohol fatty acid esters S : The saponification number of the ester A : The acid number of the fatty acid

Required Hydrophile-Lipophile Balance (RHLB) The specific HLB that the oil phase of an oil-in-water(O/W) emulsion required s : The density of the surfactant mixture  : The density of the dispersed phase Q : The percent of the dispersant of the emulsion

ADSORPTION AT SOLID INTERFACES

The Solid-Gas Interface Adsorbent : The material used to adsorb the gas Adsorbate : The substance being adsorbed

Adsorption Physical adsorption Chemisorption the adsorbent) Van der Waals forces Reversible Temperature, pressure  desorption (the removal of the adsorbate from the adsorbent) Chemisorption Chemical bond Irreversible

Adsorption Isotherm The relationship between the amount of gas physically adsorbed on a solid and the equilibrium pressure or concentration at constant temperature Fig. 15-18

Freundlich Isotherm y : the mass of gas x adsorbed per unit mass m of adsorbent k, n : constants

Langmuir Isotherm y : the mass of gas adsorbed per gram of adsorbent  y : the mass of gas adsorbed per gram of adsorbent at pressure p and at constant temperature ym : the mass of gas that 1 gram of the adsorbent can adsorb when the monolayer is complete

Adsorption Isotherm Fig. 15-20

BET Equation P : the pressure of the adsorbate in mmHg at which the mass y of vapor per gram of adsorbent is adsorbed p0 : the vapor pressure when the adsorbent is saturated with adsorbate vapor ym : the quantity of vapor adsorbed per unit mass of adsorbent when the surface is covered with a monomolecular layer

The Solid-Liquid Interface Langmuir equation c : the equilibrium conc. In milligrams of alkaloidal base per 100mL solution y : the amount of alkaloidal base x in milligrams adsorbed per gram m of clay

Wetting Wetting agent : A surfactant that, when dissolved in water, lowers the advancing contact angle Fig. 15-24

Young’s Equation s = SL + L cos  S = L ( cos  -1 ) Wa = WSL = L ( 1- cos  )

Critical Surface Tension Fig. 15-25 Surface tension at cos  = 1 (c)

APPLICATIONS OF SURFACE ACTIVE AGENTS

Applications of Surface Active Agents Emulsifying agents Detergents Wetting agents Solubilizing agents Antibacterial agents Protective agents Aids to absorption of drug

ELECTRIC PROPERTIES OF INTERFACES

The Electric Double Layer Fig. 15-28

Nernst and Zeta Potentials Fig. 15-29 Nernst potential (E) : aa’ Zeta potential () : bb’

Effect of Electrolytes Concentration of electrolyte   Screening effect of the counterion   Potential falls off more rapidly with distance