Interfacial Phenomena PDF

Interfacial phenomena 1  Interface: is the boundary between phases. Properties of molecules forming the interface are different than those in the bulk. Types of interfaces depend on the state of the two adjacent phases. The term surface is used for gas-liquid or...

Interfacial phenomena 1  Interface: is the boundary between phases. Properties of molecules forming the interface are different than those in the bulk. Types of interfaces depend on the state of the two adjacent phases. The term surface is used for gas-liquid or gas- solid interfaces. 2 3  Importance Adsorption and absorption of drugs. Penetration of drugs through membranes. Emulsion formation and stability. Suspension (despertion) formation and stability. Functions of surface active agents in the body (lungs). 4  Liquid interfaces Liquid- gas Liquid- liquid Liquid- solid 5  Surface and interfacial tension The forces (cohesive or adhesive) between molecules in the bulk are different than those at the interface. Cohesive forces: between similar molecules. Adhesive forces: between non-similar molecules 6  The net effect of unsimilarity of the forces between the bulk and the interface leads to formation of an inward force (pull) towards the bulk. This pull towards the bulk contracts the surface and results in surface (interfacial) tension. Liquid droplets assume a spherical shape due to this pull since a sphere has the lowest surface area per unit volume. 7 8  Surface and interfacial tensions Surface tension: is the force per unit length that must be applied parallel to the surface in order to counterbalance the net inward pull. Interfacial tension: Force per unit length existing at the interface between two immiscible liquids. 9  Units of surface and interfacial tension: Force(F)/length(l) – dyne/ cm – Dyne=1 g·cm/s² 1 Newton = 100000 dyne 1 dyne = 0.00001 Newton 10  Interfacial tensions are usually less than surface tension because of the adhesive forces at the interface reduce the pull towards the bulk. Two miscible liquid has no interfacial tension. 11 12  Surface tension as a force per unit area can be illustrated using Wire frame apparatus. – Three side wire frame with a movable bar act against surface tension. – F increase, surface tension increase. – Increase length, surface tension increase. 13  γ = Fb/2L Where, Fb is the force required to break the film. L is the length of the movable bar Note that there are two film in contact with the wire one on top and one on bottom. That is why the forces is divided by 2 in the equation. dW=F*ds= γ *2L*ds dW= γdA W= γΔA Where, dw is the work done to increase the surface area (A). Fig. 16-3. wire frame apparatus used to demonstrate the principle of surface tension. 14  Surface tension and bubbles (pressure difference against curved interface) Soap bubble Young- Laplace equation ΔP = 2γ/r Where, ΔP (P in- P out) is the pressure difference across the wall. r is the radius of the bubble at equilibrium. Increase r, decrease Δ P P=F/A As r decrease , P inside increase relative to out side. Fig. 16.4. schematic representation of the pressure difference across the 15 curved surface of a soap bubble.  Effect of temperature Surface tension of most liquids decrease with increase in temperature. – for example water has a surface tension of 75.6 dyne/cm at 0˚C, 72.8 dyne/cm at 20˚C, and 63.5 dyne/cm at 75˚C. 16 Measurement of surface and interfacial tension 17  Measurement of surface and interfacial tension Capillary rise method. – For surface tension determination only. – Not used for interfacial tension determination. DuNouy ring method. Others, drop weight and bubble pressure. – All experiments should be performed at constant predetermined temperature. 18 Capillary rise method When a capillary is immersed in a liquid that wets its surface, Then the liquid rises in the capillary to a certain distance that depends on Surface tension and density of the liquid, and radius of the capillary. The liquid rises due to adhesive forces (wetting) between the liquid and surface of the capillary. The liquid continues to rise in the capillary until the upward force equals the gravity (downward) force. 19 γ = (1/2) r h ρ g Where, r is the radius of the capillary. h is the liquid height in the capillary. ρ is the liquid density. g is the acceleration of gravity (981 cm/ sec2). 20 21  DuNouy ring method Using DuNouy tensiometer we determine the force necessary to detach a platinum-iridium ring immersed at the surface or the interface. 22 γ = (force reading/ 2 * ring circumference) * correction factor 23  Spreading If adhesion forces between molecules of a liquid and water are greater than the cohesive forces between the molecules of the liquid, – then it will spread on the surface of water (e.g. oleic acid). Spreading coefficient, S – S = γL1-(γL2+ γL12) γL1 is the surface tension of the sublayer liquid. γL2 is the surface tension of the spreading liquid. γL12 is the interfacial tension between the two liquids. 24  Spreading occurs if S is positive. If S is negative then spreading does not occur and globule or floating lens is formed on the surface. (mineral oil and water) 25 Interfacial tension affecting spreading 26  Adsorption at liquid interfaces Adsorption occurring at the liquid surface or interface due to molecules having preference to be on the surface or at the interface. Those molecules having such properties are called surface active agents or surfactants. 27  Adsorption at liquid interfaces A surfactant molecules (amphiphile) have certain parts are polar (hydrophilic – water loving) and other parts that are considered nonpolar (lipophillic-oil loving). 28 29  Surfactants Examples: – Ethanol: 2 carbons water soluble, Completely miscible no effect at the interface. – Amyl alcohol: 5 carbons reduced water solubility. It has surface activity and works as a surfactant. – Cetyl alcohol: 16 carbons strongly lipophylic. No surface activity. In order to have surface activity a balance between hydrophilic and hydrophobic parts is required. 30  Hydrophilic- Lipophilic Balance (HLB) A number system used to classify surfactant molecules according to their optimum use. Higher HLB value means that the surfactant is more hydrophilic. Lower HLB value means that the surfactant is more lipophilic. 31 th (16-18) (13-16) (3-8) 32 33  Required HLB (RHLB) is the HLB value for the surfactant used to obtain an o/w emulsion or w/o emulsion. Different oily materials have different required HLB values. If several oily materials are included in the formula then RHLB value for the oils in the formula is calculated according to their individual RHLB and proportion in the formula. Mixture of high and low HLB surfactants is used with amounts of each calculated to achieve RHLB level. 34 35  Use mixture tween 80 (HLB=15.0) and span 80 (HLB=4.3) So, 59% of the amount of surfactant (2.0 g) in the formula is tween 80 and 41% is span 80 36  Types of monolayer at liquid surfaces Soluble monolayers (e.g. amylalcohol in water) Insoluble films (e.g. cetyl alcohol in water) – Surface excess or surface concentration,( Γ) : is the amount of surfactant per unit area of surface in excess of that in the bulk. 37  Soluble monolayers c d =− RT dc Where, Γ is Surface excess or surface concentration (mole/cm2), R is gas constant (83143000 erg/(deg mole) , T is absolute temperature in Kelvin, dγ/ dc is the change of surface tension with change of bulk concentration, c) 38  1 d B point at which complete B-C linear region =− and surface concentration is constant RT d ln c 39  Insoluble film Amphiphilic molecules are water insoluble or slightly soluble. When such oils are added to water surface they spread to form a monolayer film. Film thickness is equal to the length of the molecules standing in a vertical position when the molecules are packed in closest arrangement Surface or film pressure, π, is the difference between of the pure substrate,γo , and that with a film spread on it, γ 40 41  Adsorption at solid interface Adsorption of gases on solids, used in: – Surface area calculation of solids. – Adsorption of bad odors from rooms. – Gas masks Adsorption from liquids on surfaces of insoluble solids, used in: – Adsorption chromatography. – Decolorizing solutions. – Detergency. – Wetting. 42  Adsorption of gases on solids Degree of gas adsorption on the surface of solid depends on: – Chemical nature of adsorbent (solid) and the adsorbate (gas). – Surface area of adsorbent. – Partial pressure of adsorbed gas. – Temperature. 43  Adsorption of gases on solids Desorption is removal of adsorbate from the surface of adsorbent probably by elevating temperature and/or reducing pressure. Physical sorption: physical bonding between adsorbate and adsorbent at the surface (reversible). Chemi-sorption: adsorbate molecules chemically bond to the adsorbent surface (irreversible). 44  Adsorption of gases on solids Isotherm: is a plot showing relationship between the amount of adsorbate adsorbed and the partial pressure of the gas at constant temperature and pressure (STP standard temperature and pressure). 45 Types of adsorption isotherms 46  Solid-liquid interface Adsorption of solute from solution onto the surface of a solid adsorbent (charcoal). Isotherms similar to solid –gas isotherms can be drawn. – Langmuir equation can be used: c 1 c = + y bym ym – Where, c is solute concentration in milligrams per 100 ml, – amount of solute adsorbed in milligrams per gram of adsorbent (ƴ = x/m). – ƴm is the milligrams of solute per gram of adsorbent that form monomolecular layer. – b is aconstant. 47 Smaller slope better adsorption 48  Activated charcoal Used as antidote for poisoning. It has large surface area per unit gram (e.g. 3000 m2/g). It has several functional groups on the surface that facilitate bonding with adsrobate molecules. 49  Wetting Contact angle: the angle between the wetting liquid and the solid surface. Wetting agent is a surfactant that lowers contact angle. 50  Electric properties of interfaces Particles dispersed in liquid medium may obtain charge due to: – Selective adsorption of particular ion on the surface (from an ion dissolved in water or it could be hydroxyl or hydronium ions. – Ionization of group at the surface of the particle (COOH) – Difference in dielectric constant between the particle and the medium. 51  Electric double layer 1.Tightly bound layer 2.Diffuse layer 52  Electric double layer Tightly bound layer contains less counter ion charge than that of the adsorbed layer. – Diffuse layer contains more negative charge (see figure) Tightly bound layer contains equal counter ion charge as that of the adsorbed layer. – Diffuse layer contains equal negative and positive charges. Tightly bound layer contains more counter ion charge than that of the adsorbed layer. – Diffuse layer contains more positive charge. – Zeta potential : is the charge that develops at the interface between a solid surface and its liquid meium. 53 1 2 3 54