Dispersed Systems FDSC400 2004 Version
Goals • Scales and Types of Structure in Food • Surface Tension • Curved Surfaces • Surface Active Materials • Charged Surfaces
COLLOIDAL SCALE
Dispersed Systems A kinetically stable mixture of one phase in another largely immiscible phase. Usually at least one length scale is in the colloidal range.
Dispersed Systems Dispersed phase Continuous phase Interface
Continuous phase Dispersed phase
Properties of Dispersed Systems • Too small to see • Affected by both gravitational forces and thermal diffusion • Large interfacial area • SURFACE EFFECTS ARE IMPORTANT
Increased Surface Area The same oil is split into 0.1 cm radius droplets, each has a volume of 0.004 cm3 and a surface area 0.125 cm2. As we need about 5000 droplets we would have a total area of 625 cm2 We have 20 cm3 of oil in 1 cm radius droplets. Each has a volume of (4/3.p.r3) 5.5 cm3 and a surface area of (4.p.r2) 12.5 cm2. As we need about 3.6 droplets we would have a total area of 45.5 cm2
For a Fixed COMPOSITION • Decrease size, increase number of particles • Increase AREA of interfacial contact decrease area
LYOPHOBIC Weak interfacial tension Little to be gained by breaking e.g., gums LYOPHILIC Strong interfacial tension Strong energetic pressure to reduce area e.g., emulsions Tendency to break
Surface Tension-molecular scale-
Surface Tension-bulk scale- Force, g Slope g Interfacial energy Area, A Interfacial area
Surface Active Material • Types of surfactant • Surface accumulation • Surface tension lowering
Types of Surfactant-small molecule- Hydrophilic head group (charged or polar) Hydrophobic tail (non-polar)
Types of Surfactant-polymeric- Polymer backbone Sequence of more water soluble subunits Sequence of less water soluble subunits
Surface Binding Equilibrium ENTHALPY COST ENTROPY COST
Surface Binding Isotherm Surface saturation Surface concentration /mg m-2 No binding below a certain concentration ln Bulk concentration
Surface Tension Lowering Bare surface (tension g0) Interface partly “hidden” (tension g) Surface pressure – the ability of a surfactant to lower surface tension p = g-g0
Summary • Small particles have a large surface area • Surfaces have energy associated with them (i.e., they are unstable) because of their interfacial tension • Dispersions will tend to aggregate to reduce the interfacial area • Proteins and small molecule surfactants will adsorb to the surface to reduce surface tension and increase stability.
Example Dispersion: Emulsions
Emulsion A fine dispersion of one liquid in a second, largely immiscible liquid. In foods the liquids are inevitably oil and an aqueous solution.
Types of Emulsion mm Water Oil Oil-in-water emulsion Water-in-oil emulsion
Chemical Composition Interfacial layer. Essential to stabilizing the emulsion Oil Phase. Limited effects on the properties of the emulsion Aqueous Phase. Aqueous chemical reactions affect the interface and hence emulsion stability
< 0.5 mm 0.5-1.5 mm 1.5-3 mm >3 mm Emulsion Size
Very few large droplets contain most of the oil Number Distributions Number • < 0.5 mm • 0.5-1.5 mm • 1.5-3 mm • >3 mm
Large droplets often contribute most to instability Median (Volume in class Total volume measured) Polydispersity Note log scale
Volume Fraction f=Total volume of the dispersed phase Total volume of the system Close packing, fmax Monodisperse Ideal ~0.69 Random ~0.5 Polydisperse Much greater
Emulsion droplets disrupt streamlines and require more effort to get the same flow rate Emulsion Viscosity Dispersed phase volume fraction Viscosity of emulsion Continuous phase viscosity
Emulsion Destabilization • Creaming • Flocculation • Coalescence • Combined methods
Creaming Buoyancy (Archimedes) h Continuous phase viscosity Dr density difference g Acceleration due to gravity ddroplet diameter v droplet terminal velocity vs Stokes velocity Friction (Stokes-Einstein)
Flocculation and Coalescence Collision and sticking (reaction) Stir or change chemical conditions FLOCCULATION Rehomogenization Film rupture COALESCENCE
Aggregation Kinetics • Droplets diffuse around and will collide often • In fact only a tiny proportion of collisions are reactive DG 2P G kslow=kfast/W P2 Function of energy barrier
Interaction Potential • Non-covalent attractive and repulsive forces will act to pull droplets together (increase flocculation rate) or push them apart (decrease flocculation rate)
Van der Waals Attraction • Always attractive • Very short range
Electrostatic Repulsion • Repulsive or attractive depending on sign of charges • Magnitude depends on magnitude of the charge • Gets weaker with distance but reasonably long range
Steric Repulsion Droplets approach each other Protein layers overlap Proteins repel each other mechanically & by osmotic dehydration What happens when protein molecules on different droplets are reactive?
Flocculation leads to an increase in viscosity Water is trapped within the floc and must flow with the floc Effective volume fraction increased Rheology of Flocculated Emulsions rg
Gelled Emulsions Thin liquid Viscous liquid Gelled solid
