Surfaces and Interfaces PDF

Surfaces and Interfaces Dr. Lara Matia-Merino With many thanks to Prof. E. Dickinson School of Food and Natural Science Massey University © Massey University Recommended Reading P. Walstra (2003), “Physical Chemistry of Foods...

Surfaces and Interfaces Dr. Lara Matia-Merino With many thanks to Prof. E. Dickinson School of Food and Natural Science Massey University © Massey University Recommended Reading P. Walstra (2003), “Physical Chemistry of Foods”. Ed. Marcel Dekker, Inc. New York. E.Dickinson (1992), “An Introduction to Food Colloids”. Ed. Oxford University Press, Oxford. D. Myers ( 1999), “Surfaces, Interfaces and Colloids”. VCH Publishers, Germany.  Overview Surface / Interfacial Tension and Surface Free Energy Adsorption and Surface activity Surface Active Agents: surfactants Monolayers: Surface Pressure and Surface Rheology Wetting and Spreading: the contact angle Common Interfaces For a liquid-gas system, the interfacial region will be smooth with a narrow transition region and smooth concentration profile Definition of Surface Tension Molecular explanation Thermodynamic explanation Mechanical explanation Surface & Interfacial Tension The surface tension at the air-water interface, important factor in foam formation The interfacial tension at the oil-water interface, important factor in emulsion formation The smallest interfacial area for a given volume is given by a sphere; thus bubbles and droplets tend to be spherical to minimize the surface area. Molecular level In molecular terms, the surface (interfacial) tension is a manifestation of the imbalance in intermolecular forces at the boundary between two bulk phases of different density (composition). Molecular level The overall result of this asymmetric force on surface molecules is that: The surface of the liquid will rearrange until the least number of molecules are present on the surface, in other words the surface area will be minimized (a sphere has the smallest surface area to volume ratio). The surface molecules will pack somewhat closer together than the rest of the molecules in the liquid, thus, the surface will seem to have a "skin". The "inward" molecular attraction forces, which must be overcome to increase the surface area, are termed the "surface tension" Typically, the surface (interfacial) thickness is of the order of no more than a few solvent molecule diameters  In mechanical terms.. The surface (interfacial) tension is the force acting on an individual unit at the surface and directed inward. The resulting force on all surface units gives rise to the apparent existence of a tangential tension. Analogy→ child pulls down on a table cloth, perpendicular to the table surface, with a resultant horizontal movement of the items on the surface In thermodynamic terms.. The surface (interfacial) tension is the work required to increase the area by unit amount. This work against the molecular forces is called surface free energy (Joules)  G   Is the surface tension in J.m-2 or Nm-1  =   A V ,T ,n ' G Is the Gibbs free energy in J Some Values of Surface and Interfacial Tension--Liquids Mercury Marangoni effect This effect is due to the presence of surface tension gradients in a liquid surface. Example (b): tears of wine phenomenon due to the mixture of water + ethanol with very different surface tension: Marangoni effect: the mass transfer  Water = 72.8 mN.m-1 along an interface due to surface tension alcohol = 22 mN.m-1 gradient. Formation of wine tears https://www.youtube.com/watch?v=tgrTbvSnE50 General principles Interfacial tension are generally lower than surface tensions Interfacial & surface tension normally decrease with increasing temperature The more chemically dissimilar are the molecules making up the two liquid phases, the larger is the interfacial tension Methods to measure surface tension Capillary rise (only for surface tension of pure liquids) Gas is merged through Bubble pressure method the tube thus forming bubbles into the liquid. The pressure needed to form a bubble is Pendant drop measured. The surface tension is calculated from Laplace pressure. The liquid is injected from a needle so that it forms a drop on the tip of the needle. The drop is then optically observed and the surface tension is calculated from the shape of the drop. Wilhelmy plate method A thin plate (perimeter about 40 mm) is lowered to the surface of a liquid and the downward force directed to the plate is measured. Surface tension is directly the force divided by the perimeter of the plate. Adsorption Molecules with strong tendency to go to the surface are SURFACE ACTIVE. They reduce the surface tension when they accumulate at a surface in greater concentrations than in the rest of the system. Reversible adsorption = small molecules surfactants (equations of equilibrium thermodynamics). Gibbs adsorption applies below CMC Irreversible adsorption = macromolecules or particles. Adsorption cannot be described by Gibbs adsorption equation. Most common materials at food interfaces Water Air/oil Proteins and other Small molecule emulsifiers Surface active from fatty acids biopolymers Particles (few food grade examples) Surface active material (i) Small-molecule surfactants (“emulsifiers”) – Saponins fall within this category (ii) Macromolecules (proteins) (iii) Finely divide solids (protein or fat particles: casein micelles, fat crystals) 0.5–1 nm SURFACTANTS thin layers 1–5 nm PROTEINS 5–10 nm HYDROCOLLOIDS thick layers 10 nm to COLLOIDAL several m PARTICLES  Food emulsions are typically prepared with molecular emulsifying agents: proteins, surfactants, and some hydrocolloids These adsorb at oil–water interfaces ⎯ reducing the interfacial tension and facilitating droplet disruption When a mixture of these species is exposed to an interface, the different components compete to dominate that interface Hydrophobic tail Air or oil Aqueous Hydrophilic solution head Surfactant Protein  Small Molecule Surfactants: Emulsifiers Material that adsorb strongly at the interface, lowering the  at low concentrations Amphiphilic molecules with: Hydrophilic (polar or ionic) head group Hydrophobic (non-polar) hydrocarbon tail Packing efficiency depends on MOLECULAR STRUCTURE: – Size of cross-sectional area of hydrophobic chain or head groups – Straight/bulky chains – Orientation of surfactant (a second polar group, ether, ester, amides, hydroxyls… ) The Gibbs adsorption equation Hypothetical plane “Gibbs dividing surface” in the notional surface → s a – Surface excess concentration s nis i = g/m2 A b Is the amount of any component i in the surface phase in excess of that which would have been present had the bulk phases extended to the Gibbs surface with unchanging composition Gibbs adsorption isotherm: Links the change in surface tension to the concentration of adsorbing species at the interface. For non-ideal solution: 1 d  Slope gives 2 2 = − RT d ln x2 f 2 Log x2 The main use of the GIBBS adsorption equation is to calculate the amount adsorbed from measurements of the variation of surface/interfacial tension with concentration Critical Micelle Concentration Physical changes with surfactant concentration Above Critical Micelle Concentration (CMC) surfactants form: Micelles Bilayers Vesicles Micelles Reverse micelles Monolayers: Surface Pressure Insoluble monolayer: monomolecular film of adsorbed molecules where essentially all the surfactant is isolated at the interface due to its low solubility in the liquid phase  = 0 − Decrease in surface tension due to the monolayer Surface pressure: The surface pressure of a monolayer film, is defined as the difference between the surface tension of the pure supporting liquid,  0 and that of the liquid with an adsorbed film. Langmuir-Adam surface balance A known amount of material is added to a solution and the pressure is increased by slowly decreasing the available area Surface pressure measured in terms of the horizontal force exerted on the float and the area of the film is varied by the movable barrier Determination of π – A curve. Information about: Physical nature of the film Molecular characteristics of the adsorbed material Langmuir Trough 2:50 min  Saponins lipid‐soluble aglycone and water‐soluble sugar chain WHERE: Chilean soap bark tree (Quillaja Saponaria Molina) Saponins are a class of substances with a Garlic rigid skeleton of at least four hydrocarbon Oats rings to which sugars in groups of one or Ginseng two are attached (usually not more than 10 Legumes (beans and peas) units). Traditionally, they are subdivided into Leaves sugar beet triterpenoid and steroid glycosides Adsorption of macromolecules Interfacial Rheology Shear Rheology Dilatational Rheology Surface Rheology Surface viscosity is the change in the viscosity of the surface layer brought about by the monomolecular film. This leads to the production of persistent foams, stable emulsions, etc… Protein film surface rheology depends on: – Protein structure (globular protein more viscoelastic than caseins and k > a s1 >b) – Solvent conditions: pH, ionic strength…. (optimum intra/intermolecular cohesion within the film at the protein pI ) Complexation of protein and surfactants and subsequent displacement Adsorption of particles (Pickering stabilization) The putative role of dispersed particles in stabilizing food emulsions has long been part of the technical literature HOMOGENIZED MILK MAYONNAISE (3.5 vol% milk fat) (80 vol% vegetable oil) FAT ADSORBED OIL CASEIN MICELLES EGG YOLK OIL LIPOPROTEIN FAT PARTICLES Heertje & Pâques, 1995 Holcomb et al., 1990 Rapid growth of interest in Number of papers particle stabilization of published → emulsions (and foams) 0 20 40 60 80 100 2000 Binks and coworkers Web of Science search 2001 (University of Hull) for “Pickering 2002 stabilization/stability” 2003 2004 2005 2006 droplet 2007 or bubble 2008 2009 water 2010 or oil 2011 particle  Examples of particle-stabilized oil-in-water emulsions NANOPARTICLES Oil droplets stabilized by chitin nanocrystals (240 nm  20 nm) at pH = 3 (Tzoumaki et al., 2011) aggregates MICROPARTICLES of starch granules Oil droplets stabilized by hydrophobically modified d32  25 m starch particles ( 10 m) (Yusoff & Murray (2011) At boundary of colloidal and molecular length scales lie small nanoparticles (and also protein molecules) Viscoelastic protein monolayer cannot stabilize against bubble air growth or shrinkage by disproportionation But rigid layer of solid particles air can provide stability against disproportionation and coalescence behaves like rigid shell An unusual protein behaves rather like a surface-active nanoparticle... hydrophobin At boundary of colloidal and molecular length scales lie small nanoparticles (and also protein molecules) Relative Bubble sizes for bubble UNSTABLE size aqueous foams prepared with various surface- active agents Hydrophobin foam stable for several months Cox et al., 2009 Time (days)  PROTEINS SMALL SURFACTANS  SOLID PARTICLES HYDROCOLLOIDS Surfactants (fast adsorbing) — equilibrium (static) tension Hydrocolloids/proteins (slow adsorbing) — dynamic tension Particle emulsifiers — tension at bare oil–water interface To effectively stabilize an emulsion, the size of the stabilizing particle should be at lest an order of magnitude smaller than the emulsion droplet (Gould et al. 2013). For example to stabilize emulsion droplets of 0.5-10 µm, the particles must be in the sub- micron/nanometre size range (Dickinson, 2012)  Wetting and Spreading Wetting of a surface by a liquid and the spreading of that liquid….. Wettability of the surface by the liquid or the wetting ability of the liquid on the surface… The Contact Angle – Primary characteristic of any immiscible, 2 or 3 condensed phases system, at least one of which is a liquid. – Practical importance: contact angle of a liquid directly on a solid. – The quantitative measure of the wetting process is the contact angle θ Contact angle (a) The solid is completely wetted by the liquid (θ= 0º) liquid forms a uniform film (b) θ > 0º (30-89º) ‘partially wetting’ (c) θ > 90º system is nonwetting Young’s equation At equilibrium there must be a balance of surface forces at any point on the contact line (where the three phases meet). Considering the forces in the horizontal direction, the equilibrium condition implies that the interfacial cos  = ( SV −  SL ) /  LV tension γAS equals the sum of γ LS and the projection of γ AL on the solid surface. When liquid spreads completely over the solid surface, θ =0 →  LV =  SV −  SL  Macroscopic description — Location of predominantly hydrophilic spherical particle at oil–water interface WATER pw Young’s equation: cos  = (po – pw) / ow PARTICLE  ow OIL OIL po Interfacial tensions: po particle–oil Contact angle   90 favours pw particle–water stabilization of oil-in-water emulsion ow oil–water FINKLE’S RULE Contact angle  Work of cohesion: Work of adhesion  The Spreading coefficient (S)= Wa-Wc – Term that indicates from a thermodynamic point of view whether a given liquid—solid system will be wetting (θ = 0º) or nonwetting (θ >0º) Work of cohesion: S SLG =  SG −  SL −  LG – the reversible work required to separate two surfaces of unit area of a material with surface tension LG Wc = 2 LG Work of adhesion – the work required to separate unit area of interface between two different materials Wa =  SG +  LG −  SL  Wetting of milk powders The thermodynamics of wetting is determined by the value of the contact angle θ between the solid milk particle and the water. – θ = 20º skim milk powder – θ = 50º whole milk powder SMP: wet adequately in both warm and cold water WMP: Improved wetting in warm water (> 40ºC) – Wetting problem arises from disruption of protective layers around fat globules during spray-drying. The released fat forms a water-repellent layer around the dried solid particles. Sponge-like conglomerates (2-4 mm) fast wetting compared to big compact milk particles Interdisciplinary activity DIGESTION FOOD biology STRUCTURE ? medicine HEALTH physics chemistry engineering