Surface Tension and Surfactants 2024 (PDF)

MPharm Programme Surface tension & surfactants Dr Paul Carter Slide 1 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Interface Definition: An interface is the transition region wher...

MPharm Programme Surface tension & surfactants Dr Paul Carter Slide 1 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Interface Definition: An interface is the transition region where two immiscible phases contact each other i.e. the contact area between two phases e.g. Liquid-vapour (‘gas’) interface (e.g. liquid surface in air) liquid-liquid (e.g. emulsion) solid-liquid (e.g. suspension) solid-vapour (e.g. solid surface in air) solid-solid (e.g. solid mix) Where one phase is liquid and the other gaseous = ‘surface tension’ Where both phases liquids = ‘interfacial tension’ Between solids, physical phenomena such as adhesion and friction. Slide 2 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Interface Surface Tension Interfacial Tension Liquid – liquid interface Gas – liquid interface Slide 3 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Surface free energy Excess energy the surface has compared to the bulk Surface molecules have higher energies and reactivities to the same molecules in the bulk Why? Think about attractive forces on molecules at surface and in bulk – different. Nature acts to attain a state of minimum total free energy (more stable). Since the surface has higher energy than the bulk, reducing surface area an efficient way of reducing energy Slide 4 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Surface free energy Water droplets are spherical (smallest surface area for given volume). Occurs spontaneously to reduce energy. Solids can’t deform though! Higher surface area = more reactive. Higher energy. For solids, reducing particle size (milling) creates a increased surface area and therefore more surface free energy. Fine particles are very difficult to handle. To increase surface area of a solid/liquid, energy must be supplied. Example – emulsion formation. Slide 5 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Surface tension Surface of water demonstrates a ‘skin’ that resists puncture Inward force experienced by surface molecules creates a tendency for surface to contract. Slide 6 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Exaggerated diagram - Cohesive forces between molecules in bulk are shared with all neighbouring molecules. On surface, no molecules above – net pull downwards. Surface molecules exhibit stronger attractive forces between them on the surface Slide 7 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Surface tension Surface molecules have higher energy and experience an inward pull. Leave surface and enter bulk – dynamic equilibrium. Typical lifetime of a molecule at the surface is 1 microsecond (1 x 10-6 s) Definition: surface tension is the force acting parallel to the surface and at right angles to a 1m line drawn anywhere in the surface (N m-1) If area of surface/interface is to be increased, energy must be supplied Definition: surface free energy is the work required to increase the surface area isothermally and reversibly by 1 m2 (J m-2) Surface tension and surface free energy are numerically equivalent Slide 8 of 24 Surfactants and Their Use in Pharmaceutical Preparations  Surface tension Surface free energy and surface tension have symbol γ with subscript referring to system e.g. γw/a So, liquid/vapour surface tension = γl/v interfacial free energy of an o/w system = γo/w The surface tension of water is 72 mN m-1 at 250C. The surface tension of water decreases significantly with increase in temperature Slide 9 of 24 Surfactants and Their Use in Pharmaceutical Preparations  Surface tension versus temperature Approximately linear. Hot water better cleaning agent than cold – a lower surface tension makes hot water a better ‘wetting agent’ to get into pores/fissures rather than bridging them (with surface tension effect). Soaps/detergents further lower the surface tension 70 Surface tension (mN m-1) 60 (59 mN m-1) 50 20 40 60 80 100 Temperature 0C Slide 10 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Surface tension values γl/v (mN m-1 at 250C) water 72 glycerol 63 ethanol 22 octanol 27 liquid paraffin 35 mercury 480 Note: presence of impurities reduces surface tension Slide 11 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Wetting phenomena – solid liquid interface hydrophobic hydrophilic Float on surface SINK !! Slide 12 of24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Contact angle No wetting Absolute wetting θ = 0o θ =180o θ θ θ θ < 90o θ = 90o θ > 90o Slide 13 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Measurement of surface tension Wilhelmy Plate Thin, rectangular plate (glass, mica) attached to a torsion balance, dips into liquid under investigation F vapour θ = 00 liquid Slide 14 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Wilhelmy plate Force, F, to detach plate is calculated. The force of surface tension (γl/v) acts around the perimeter, P. Must be completely wetted i.e. zero contact angle to ensure surface tension act vertically. γl/v = F/P Absolute method i.e. no correction factor May use in static mode to measure change in force to keep plate at constant depth – useful when measuring change in surface tension with time or other variable. Slide 15 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Calculation example The force needed to detach a Wilhelmy Plate from liquid A at 200C was 3 mN. The rectangular plate was 2.2 cm length and 2.5 mm thickness. Calculate the surface tension of A at 200C in mN m-1. Give answer to 2 d.p. γl/v = F/P F = 3 mN, convert dimensions to metres: Length = 0.022 m, thickness = 0.0025 m. Perimeter = 0.049 m Therefore, 3/0.049 = 61.22 mN m-1. Slide 16 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  du Nouy tensiometer (ring method) measures force to detach a platinum ring from surface/interface Slide 17of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  du Nouy tensiometer Detachment force = surface tension x perimeter F = 2π (R1 + R2) γ R1 and R2 are the inner and outer radii for the ring Zero contact angle needed. Cleaning and flaming. During detachment, all forces do not act vertically and therefore need correction factor for accurate determinations (in lab – use apparent surface tension) Slide 18 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  What are surfactants? SURFace ACTive AgenNT A substance which has both water loving and oil loving structural components in the same molecule Slide 19 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Surfactants Amphipathic Hydrophobic group (usually a carbon chain) – no affinity for aqueous solvents Hydrophilic group – affinity for water Because surfactants have two regions, once in solution they will orientate at surface/interface with hydrocarbon group away from aqueous phase Slide 20 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Surfactant structure Hydrophobic ( lyophilic, Hydrophilic ( lyophobic, water-fearing ) tail water-loving) head containing a containing a charged hydrocarbon chain functional group Slide 21 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Surfactant action The longer the chain, the more energetically favourable to adsorb at surface/interface. The longer the chain, the greater the tendency to escape the aqueous environment. Traube’s rule = for dilute solutions of a homologous series of the aliphatic alcohols, e.g. CnH2n+1OH, the ratio of the concentration at the surface layer to that in the bulk increases by a factor of approximately 3 for each additional –CH2 group This adsorption lowers surface tension – surface active molecules replace water molecules - disruption of the water-water bonding. Reduces contractile nature and therefore reduces surface tension. Dynamic equilibrium Water-water attractive forces > water-hydrocarbon attractive forces > hydrocarbon-hydrocarbon attractive forces cause Slide 22 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Liquid/Liquid interface A colloidal system comprised of two or more immiscible liquids is called an Emulsion. In cosmetics, as one phase is usually water, they are called water-in-oil or oil-in-water emulsions. Slide 24 of 24 Surfactants and Their Use in Pharmaceutical Preparations  Surfactant action Surfactants used as emulsifying agents, detergents, solubilizing agents, wetting agents, foaming & antifoaming agents, flocculating agents Example – liquid paraffin & water emulsion Initially, spontaneous separation of oil & water – reduce interfacial free energy (area of interface) – oil floats since less dense o o w w Shaking gives temporary emulsion. Oil droplets dispersed in water – large increase in interfacial area. High energy situation = unstable. When stop shaking, system reverts spontaneously back to give lowest interfacial area. So how can we form a stable o/w emulsion? Slide 23 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Example – liquid paraffin & water emulsion Add surfactants to form stable system Water soluble surfactant e.g sodium dodecyl sulphate Oil soluble surfactant e.g. cetostearyl alcohol Form mixed monolayer – droplets very small (due to reduction in surface tension). Tendency to coalesce is reduced since complex stable film, that is charged (- ve). Slide 24 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations  Example – liquid paraffin & water emulsion The overall effect is that the interfacial tension between the phases is reduced. Which means that with ɣAB now lower, the work needed to create a surface is less. With a surfactant on the surface of the internal particles, they can act as a barrier to coalescence, either by steric hindrance or electrostatic repulsion Slide 24 of 24 MPharm Surfactants and Their Use in Pharmaceutical Preparations