Lecture 1 Interfacial Phenomenon 1 PDF

Interfacial Phenomena I  Phases Gas Liquid Solid Interfaces and Surfaces Boundary between binary system Interface Examples of Interfaces Gas/gas No interface Gas/liquid Liquid surface, body of water exposed to atmosphe...

Interfacial Phenomena I  Phases Gas Liquid Solid Interfaces and Surfaces Boundary between binary system Interface Examples of Interfaces Gas/gas No interface Gas/liquid Liquid surface, body of water exposed to atmosphere Gas/solid Solid surface, table top in contact with atmosphere Liquid/liquid Interface, emulsion (oil/water) Liquid/solid Interface, suspension (solid suspended in a liquid) Solid/solid Mixture of powder particles in contact. I. The liquid /gas interface Molecular theory of surface tension Liquid Interfaces On a molecular level Bulk molecules Surface mobile molecules Symmetric forces: Cohesive Cohesion or cohesive attraction or cohesive Inward pull of the force in chemistry is the intermolecular surface molecules attraction between like-molecules. to the bulk Asymmetric forces: Adhesive (imbalance) Adhesive force is the force of attraction between molecules of different substances. The surface of a liquid is on a condition of tension: Due to unbalanced forces on the surface and the inward pull Least number of molecules on the surface Tendency to contract in area the sphere has the least surface for a given volume liquid drops are spherical to minimize its surface area  Air Liquid drop Surface Smallest surface Inward pull contraction area/unit volume (surface tension) (drop)  energy Contracted surface Enlargement Why energy is required? Molecules from the bulk To the surface Surface free energy Surface free energy “”: The work “w”required to increase the surface area of a liquid by 1 cm2, its units is erg/cm2 Total work is “W” in ergs WαA W =   A = erg/cm2 x cm2 = erg  = W/  A = erg/cm2 Surface tension “”: The force acting at right angles to any line of 1 cm length in the liquid surface, and its units are dyne/cm. The surface free energy and surface tension are numerically equal and dimensionally equivalent. W erg dyne x cm dyne = = = = A cm2 cm2 cm Therefore, water has a surface tension of 72.8 dyne/cm and a surface free energy of 72.8 erg/cm2 Interfacial tensions The force per unit length existing at the interface between two immiscible liquid phases (A & B) and has units of dyne/cm. Substance Surface tension Interfacial tension Water 72.8 --- Benzene 28.9 35.5 Chloroform 27.1 32.4 Ethanol 22 --- Spreading Coefficient “S” Oil Water Droplets Film Spreading Adhesion > Cohesion  1 cm2 L L L Work of adhesion LS (Wa) S S S LS prevents miscibility between the two liquids (Wa) is equivalent to the extent of attraction between the two liquids LS is destroyed after separation and L and S are created In general, W=A For 1 cm2 W= Therefore, the work required for separation Wa: Wa = S + L – LS  1 cm2 L L L Work of cohesion LL = 0 (Wc) L L L LL = 0, because no interface exists between like molecules The work required for separation Wc: Wc = 2 L The spreading coefficient “S” = Wa –Wc L = S.T of spreading liquid S = S + L – LS – 2 L S= S.T of sublayer liquid = S – (LS + L) If “ S ” is positive spreading E.g.: Oleic acid If “ S ” is negative no spreading E.g.: Mineral oil Spreading 1. Alternatively we can describe the likelihood of spreading in terms of the work of cohesion and adhesion. 2. Balance between: adhesional and cohesional forces Wadhesion Wcohesion Which of these two forces has the greatest influence?  Spreading 1. If Wadhesion > Wcohesion for B Spontaneous spreading of B over surface A 2. If Wadhesion < Wcohesion No spreading 3. Spreading coefficient S S = Wadhesion-Wcohesion Spreading will occur if S ≥ 0  Spreading: two liquids hexane: 18.0 dyne/cm γ water/hexane: 50.8 dyne/cm decane: 23.9 dyne/cm γ water/decane: 52.3 dyne/cm oleic acid: 32.5 dyne/cm γ water/oleic acid: 15.6 dyne/cm benzene: 28.9 dyne/cm γ water/benzene: 35.0 dyne/cm water: 72.8 dyne/cm Will hexane; decane, benzene, and oleic acid spread on water? Answers: Measurement of surface and interfacial tension Several methods are available Capillary rise Drop weight/volume du Nuoy tensiometer  Measurement of Surface Tension 1) Capillary rise method: Based on : Measuring the height to which a liquid rises in a capillary tube. Exp: If we put a glass capillary in a beaker containing a liquid w` wets the glass It spreads on the inside of the tube By wetting: Adhesive F between water and glass cause it to rise to certain distance.  The liquid rises in the capillary tube and rises till 2 opposing forces oppose each other. Where γ = S.T R=inner radius of capillary tu D=density of liquid G=acceleration due to gravity A)Upward force V= volume of liquid (SURFACE TENSION) The force of surface S.T w` B) Down ward force cause the liquid to rise Gravitational F w` pull γ = F/L The liquid downward F = γ.L F = m.g = v.d.g =γ.2П r = Пr2.h.d.g when the liquid stops Upward force= downward force γ.2П r = Пr2.h.d.g γ. = 1\2 r.h.d.g dyne\cm For surface tension only 2- Drop weight or drop volume method The drop falls when 2 forces are balanced a)S.T(upwards) (F=γ.2Пr) b)Dp wt (Acts downwards) (F=m.g) γ.2Пr=m g 1=2  We repeat the experiment for 2 liquids one of them is water which act as reference. 2П r γ1=m 1 g 2П r γ2=m 2 g γ1/ γ2= m 1 /m 2 (m= v.d) v= volume of the drop= V /n V (Total volume of the liquid) n ( no of the drops) (V/n1)*d1 1/2 = (V/n2)*d2 = n2d1/n1d2 ! If Density Not Given, For simplicity d1=d2 For surface tension and interfacial tension 3- Ring method (The DuNouy tensiometer): For surface tension and interfacial tension 3- Ring method (The DuNouy tensiometer):  Dial reading (dyne)  = x CF 2 * ring circumference CF depends on: - Radius of the ring - Radius of the wire of the ring - Volume of liquid raised out of the surface Surface Active Agents (Surfactants) Structure of SAA molecule Polar head Nonpolar tail  Why an amphiphile is adsorbed at the interface? CH3 - Amyl alcohol CH-CH2-CH2 -OH. CH3 Nonpolar group Polar group  Non polar group Weak adhesion force with water Weak solubility Rejected from water Interference with the cohesive force between water molecules  Polar group is directed to the bulk  The interface satisfies both tendencies Orientation of surfactant molecules At low SAA Conc. At high SAA Conc. Air or oil Air or oil Water Water Examples: - Ethanol CH3-CH2 -OH Miscible with water in all proportions CH3 Reduced water - Amyl alcohol CH-CH2-CH2-OH solubility and surface active (interface) CH3 - Cetyl alcohol Cl6H33 - OH Insoluble in water A relatively large hydrophobic portion is required for significant surface activity. Traubs rule: “In dilute aqueous solutions of compounds belonging to anyone homologous series, the molar concentration required to produce equal lowering of the surface tension of water decreases threefold for each additional CH2 group in the hydrocarbon chain of the solute. Example: To reduce  water from 72.8 dyne/cm to 50 dyne/cm we need either: 2.03 moles ethanol 2C or 0.6 moles propanol 3C or 0.18 moles butanol 4C or 0.06 moles pentanol 5C or 0.02 moles hexanol 6C Understanding surfactants Traube’s rule The concentration required for an equal lowering on surface tension for a homologous series of surfactants decreases by a factor ~ 3 for each additional CH2 group Factors Affecting surface tension 1- Effect of temperature on surface tension: Increase in temperature Decrease in surface tension Reduced intermolecular forces Structure loss 2- Surface active agents Increase adhesive force Decrease surface tension 3- Electrolytes Increase Inward pull Increase surface tension Thank You