Interfacial Phenomenon SB

Interfacial Phenomenon SB

• 1.
  Surface Tensions & Measurement of SurfaceTensions
• 2.
  Liquid Interfaces 2 • Interfaceis the boundary between two phases. • Surface is a term used to describe either a gas-solid or a gas- liquid interface. • Interfacial phase is a term used to describe molecules forming the interface between two phases which have different properties from molecules in the bulk of each phase.
• 3.
  Surface Tension • Moleculesin the bulk liquid are surrounded in all directions by other molecules for which they have an equal attraction (only cohesive forces). • Molecules at the surface can only develop cohesive forces with other molecules that are below and adjacent to them; and can develop adhesive forces with molecules of the other phase. 3 Inwardpull
• 4.
  Surface Tension This imbalancein the molecular attraction will lead to an inward force toward the bulk that pulls the molecules of the interface together and contracts the surface, resulting in a surface tension. 4
• 5.
  Definition • The surfacetension (γ) is the force per unit length acting along the surface of a liquid at right angles to counterbalance the net inward pull • A unit of surface tension in CGS system is dynes per centimetre (dyne cm–1).
• 6.
  Interfacial Tension • Interfacialtension is the force per unit length existing at the interface between two immiscible phases (units are dynes/cm or N/m). • The term interfacial tension is used for the force between: 6 Two liquids = γLL Solid liquids = γSL
• 7.
  Interfacial Tension • Theterm surface tension is reserved for the tensions: Liquid-vapor = γ Solid-vapor = γ • Interfacial tensions are weaker than surface tensions because the adhesive forces between two liquid phases forming an interface are greater than that between liquid and gas phase 7
• 8.
  Surface Free Energy Eachmolecule of the liquid has a tendency to move inside the liquid from the surface; therefore, when the surface is increased, the liquid takes the form with minimal surface and as a result, minimal surface energy: 8 Sphere shape
• 9.
  Surface Free Energy Thesurface layer of a liquid possesses additional energy as compared to the bulk liquid. If the surface of the liquid increases (e.g. when water is broken into a fine spray), the energy of the liquid also increases. Because this energy is proportional to the size of the free surface, it is called a surface free energy: 𝑾= 𝜸 ∆ 𝑨 𝑾 Surfacefree energy (ergs) 𝜸 surface tension (dynes/cm) ∆ 𝑨 increase in area (cm2). Therefore, surface tension can also be defined as the surface free energy per unit area of liquid surface.
• 10.
  Measurement of SurfaceTensions • Drop Count method • Drop weight method
• 11.
  Measurement of SurfaceTensions 𝜸 surface tension 𝒓 radius of capillary 𝒉 height 𝒑 density of the liquid 𝒈 acceleration of gravity This method cannot be used to obtain interfacial tensions. Capillary Rise Method When a capillarytube is placed in a liquidcontained in a beaker, the liquid rises up in the tube to a certain distance. By measuring this rise in the capillary, it is possible to using thedetermine the surface tension of the liquid formula: 𝜸 = ½ 𝒓 𝒉 𝒑 𝒈 11
• 12.
  Measurement of SurfaceTensions The DuNoüy Ring Method The force necessary to detach a platinum–iridium ring immersed at the surface or interface is proportional to the surface or interfacial tension. The surface tension is given by the formula: The DuNoüy tensiometer is widely used for measuring surface and interfacial tensions.
• 13.
  Spreading of Liquid & Spreading Coefficient
• 14.
  Spreading of liquid •When Oleic acid dropped on water, it immediately spreads on the surface of water • Oleic Acid – Spreading Liquid (L) • Water – Sub-layer Liquid (S) • Generally spreading occurs when adhesive force is more than cohesive force
• 15.
  Spreading Coefficient •Work of Cohesion (Wc) may be defined as the surface free energy increased by separating a column of pure liquid into two halves • Surface free energy change = γ dA • Wc = γL (dA+dA) = 2 γLdA • Here the column is of cross sectional area is 1cm2 (dA= 1cm2) • Wc = 2 γL
• 16.
  • Work ofAdhesion (Wa) may be defined as the surface free energy increased by separating a column of two immiscible liquids at its boundary into two sections • As two sections of immiscible liquids are already separated by a boundary, the energy requirement will be less by an amount γLS dA • Wa = γLdA + γS dA - γLS dA • Here the columns are of cross sectional area 1cm2 • Wa = γL + γS - γLS Spreading Coefficient
• 17.
  Spreading coefficient • Spreadingcoefficient (S) is the difference between work of adhesion and work of cohesion S = Wa –Wc = (γL + γS – γLS) - 2γL = γS – γL – γLS • S = γS – (γL + γLS) • γL - Surface tension of spreading liquid • γS - Surface tension of sublayer liquid • γLS - Interfacial tension
• 18.
  Spreading coefficient • Spreadingoccurs when spreading coefficient S is positive i.e., γS > (γL+ γLS). When free energy of the spreading liquid and the interfacial tension with the sub layer is less than that of sublayer the spreading becomes spontaneous to reduce free energy of sublayer. • If spreading coefficient S is negative ie, (γL+ γLS) > γS Spreading liquid forms globules or floating lens means spreading will not take place
• 19.
  Spreading • There maybe saturation of the liquid with the other and there may be change in the surface tension of the sublayer liquid • In that case the spreading coefficient may become negative after saturation, the spreading liquid coalesces and form a lens on the surface of the sublayer • In the case of a DUPLEX FILM if S become negative after saturation, it forms a monolayer and excess liquid remains as lens on the surface
• 20.
  Spreading • Fatty acidsand alcohols have high spreading coefficient • As non-polar chain length increases in an acid or alcohol spreading coefficient decreases • Propionic acid and ethyl alcohol having high spreading coefficient
• 21.
  Applications of spreadingcoefficients • Spreading of medicament from creams, lotions, etc. on skin • Stabilization of emulsions
• 22.
  Surface Active Agents
• 23.
  Surface Active Agents •Molecules and ions that are adsorbed at interfaces are termed surface-active agents or surfactants. • Surfactants have two distinct functional groups in their chemical structure, one of which is water-liking (hydrophilic) and the other of which is lipid-liking (lipophilic). • These molecules are referred to as amphiphile. 23
• 24.
  Surface Active Agents Whensuch molecule is placed in an air-water or oil-water system, the polar groups are oriented toward the water, and the nonpolar groups are oriented toward the air or oil.
• 25.
  Reduction of SurfaceTension Principle: When surfactants are dissolved in water they can reduce surface tension by replacing some of the water molecules in the surface so that the forces of attraction between surfactant and water molecules are less than those between water molecules themselves, hence the contraction force is reduced. 25
• 26.
  Classification of surfaceactive agents • Non-ionic surfactants Have low toxicity and high stability and compatibility, e.g. Sorbitan esters (spans) and Polysorbates (tweens). • Anionic surfactants Have bacteriostatic action e.g. Sodium Lauryl Sulphate • Cationic surfactants Have bactericidal activity e.g. benzalkonium chloride • Ampholytic Surfactants • Phospholipids 26
• 27.
  Reduction of SurfaceTension Effect of Structure on Surface Activity • The surface activity (surface tension reduction) of a particular surfactant depends on the balance between its hydrophilic and hydrophobic properties. • An increase in the length of the hydrocarbon chain (hydrophobic) of a surfactant increases the surface activity. • An increase in the length of the ethylene oxide chain (hydrophilic) of a non-ionic surfactant results in a decrease of surface activity. 27
• 28.
  HLB Hydrophilic-Lipophilic balance
• 29.
  HLB System • Definition:The hydrophile-lipophile balance (HLB) system is an arbitrary scale for expressing the hydrophilic and lipophilic characteristics of an emulsifying agent. • Agents with HLB value of 1-8 are lipophilic and suitable for preparation of w/o emulsion, • Those with HLB value of 8-18 are hydrophilic and good for o/w emulsion.
• 30.
  HLB System The oilphase of an o/w emulsion requires a specific HLB, called the required hydrophile– lipophile balance (RHLB). % emulsifier with high HLB = RHLB- HLB Low HLB High - RHLB
• 31.
  Micellization Solubilization/Detergency • Micelles areformed when the concentration of a surfactant reaches a given concentration called critical micelle concentration (CMC) in which the surface is saturated with surfactant molecules. • The main reason for micelle formation is to obtain a minimum free energy state. • In a micelle, polar or ionic heads form an outer shell in contact with water, while non polar tails are sequestered in the interior to avoid water. Concentration SurfaceTension
• 32.
  Micellisation Solubilization • Solubilization isthe process where water-insoluble substances are brought into solution by incorporation into micelle. • Solubilization does not occur until the milcells are formed (i.e. above CMC) • The amount of substance solubilized increases as the number of micelles increases.
• 33.
  Detergency • Detergency isa complex process involving the removal of foreign matter from surfaces. • Surfactants are used for the removal of dirt through the detergency effect. • Initial wetting of the dirt and of the surface to be cleaned is carried out by deflocculation and suspension or emulsification or solubilization of the dirt particles. It also involves foaming of the agent for entertainment and washing away of the particles. • When a wetting agent dissolved in water, lowers the advancing contact angle, helps in displacing an air pockets at the surface and replacing it with a liquid phase.
• 34.
  Applications of Detergency 1.Displacement of air from sulfur, charcoal and other powders for dispersing these drugs in liquid vehicles 2. Displacement of air from the matrix of cotton pads and bandages so that medicinal solutions may be absorbed for application to various body areas. 3. Displacement of dirt and debris using detergents in the washing of wounds. 4. The application of medicinal lotions and sprays to the surface of the skin and mucous membrane.
• 35.
  Adsorption on solid surface
• 36.
  Adsorption of solutefrom liquid to solid surface Types of Adsorption Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a interface. There are two general types of adsorption: 1. Physical adsorption, in which the adsorbate is bound to the surface through the weak van der Waals forces. 2. Chemical adsorption or chemisorption, which involves the stronger valence forces.
• 37.
  Factors Affecting Adsorption •Surface area of adsorbent: • Being surface phenomena extent of adsorption depends on available surface area of adsorbent. • Finely divided materials since has large surface area, more adsorption is observed on their surfaces. • Nature of adsorbate: • The amount of adsorbate adsorbed on solids depends on its nature; easily liquefiable gases adsorbed to greater extent. • Temperature: • As seen under the characteristics of physical adsorption, it decreases with increase in • temperature, while chemical adsorption increases with increase in temperature. • Pressure: • It was observed that increase in pressure increases adsorption.
• 38.
  Adsorption of Gas •Adsorption & Desorption (evaporation) • Physical adsorption, in which the adsorbate is bound to the surface through the weak van der Waals forces. • Chemical adsorption or chemisorption, which involves the stronger valence forces. • Adsorbate: material which get adsorb (Gas/solute) (x moles) • Adsorbent: material on which adsorption takes place (m grams) m x
• 39.
  Freundlich adsorption isotherm y= x m = kp1/n Log x m = Log k + 1 n Log p y = x m = kp1/n
• 40.
  Langmuir adsorption isotherm •Molecules of gas adsorb on active sites on adsorbent • Fraction of active sites occupied (at pressure p) are ϴ • Fraction of un-occupied sites are (1- ϴ) • Rate of adsorption is directly proportional to pressure and no. of un occupied sites • r1=k1(1-ϴ)p • Rate of desorption (evaporation) directly is proportional to no. of occupied sites • r2=k2ϴ • At equilibrium r2=r1
• 41.
  Langmuir adsorption isotherm•k2ϴ = k1(1-ϴ)p • ϴ = k1p k2 +k1 p • ϴ = (k1/k2)p 1+( k1 k2 )p • y = ymbp 1+bp • p y = 1 bym + p ym If we replace • (k1/k2) with b • ϴ with y/ym Where, • y= mass of gas adsorb per gram of adsorbent at pressure p • ym= mass of gas adsorb on adsorbent on 1 gram of adsorbent when monolayer is formed The plot of p/y against p gives straight line and ym and b can be obtained by slop and intercept
• 42.
  Langmuir adsorption isotherm
• 43.
  Other types ofadsorption isotherm Type I : It represents behavior of nitrogen or oxygen on charcoal. Type II : Sigmoidal when gas undergo physical adsorption on non-porous solid to form monolayer. Described by BET equation. Multilayer formation and condensation within pores. Type IV : Adsorption on porous solid. Capillary condensation Type III & Type V : Heat of adsorption of gas in first layer is less than latent heat of condensation in successive layers. Capillary condensation occur in type V Surface area can be determined where monolayer formation is detected … Type I, II & IV
• 44.
  Application of adsorption •Gas masks: Poisonous gases get adsorbed to at the surface of the mask and prevent its encounter when used by coal miners. • Production of vacuum: Traces of air are adsorbed on charcoal and removed from devices undergoing the process of evacuation. • Removal of moisture: Silica gel pellets are used for the adsorption of moisture in medicines and new plastic bottles in order to control humidity. • Removal of colour: The juice extracted from cane is treated with animal charcoal for the removal of the colouring agent in order to get a clear liquid solution. • As Catalysts: Suitable materials are used as a catalyst such that reactants get adhered to its surface, thus enabling the reaction to proceed at a faster rate and increasing the rate of reaction
• 45.
  Wetting
• 46.
  Wetting • Force ofattraction between solid and liquid play important role • Angle of contact range (0° to 180 °)
• 47.
  Wetting phenomenon • Contactangle is the angle between liquid droplet and surface over which it spreads. • Important action of wetting is to reduce angle of contact • Draves test for to test wetting property of wetting agent… time required to sink weighted cotton yarn of standard solution.
• 48.
  Young’s equation • γs=γSL - γL cos ϴ • Where ϴ= angle of contact • We know that spreading coefficient ‘S’ • S= γS – γL – γLS • Combining both these equations, substituting value of γs in second equation • S = γL (cos ϴ -1) • Surface tension when, cos ϴ =1 is known as critical surface tension
• 49.
  Steps of wetting •Adhesional Wetting • Immersional • Spreading
• 50.
  Thank you