Presentation on theme: "Dispersed Systems FDSC400 2004 Version. Goals Scales and Types of Structure in Food Surface Tension Curved Surfaces Surface Active Materials Charged Surfaces."— Presentation transcript:
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 cm 3 and a surface area 0.125 cm 2. As we need about 5000 droplets we would have a total area of 625 cm 2 We have 20 cm 3 of oil in 1 cm radius droplets. Each has a volume of (4/3. .r 3 ) 5.5 cm 3 and a surface area of (4. .r 2 ) 12.5 cm 2. As we need about 3.6 droplets we would have a total area of 45.5 cm 2
For a Fixed COMPOSITION Decrease size, increase number of particles Increase AREA of interfacial contact decrease area
Tendency to break 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
Surface Binding Isotherm ln Bulk concentration Surface concentration /mg m -2 Surface saturation No binding below a certain concentration
Surface Tension Lowering Bare surface (tension 0 ) Interface partly “hidden” (tension ) Surface pressure – the ability of a surfactant to lower surface tension
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.
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 Oil-in-water emulsion Water-in-oil emulsion Water Oil mm
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
Emulsion Size < 0.5 m 0.5-1.5 m 1.5-3 m >3 m
Number Distributions < 0.5 m 0.5-1.5 m 1.5-3 m >3 m Number Very few large droplets contain most of the oil
Median Polydispersity Large droplets often contribute most to instability (Volume in class Total volume measured) Note log scale
Volume Fraction =Total volume of the dispersed phase Total volume of the system Close packing, max Monodisperse Ideal ~0.69 Random ~0.5 Polydisperse Much greater
Emulsion Viscosity Emulsion droplets disrupt streamlines and require more effort to get the same flow rate Viscosity of emulsion Continuous phase viscosity Dispersed phase volume fraction
Creaming Buoyancy (Archimedes) Friction (Stokes-Einstein) Continuous phase viscosity density difference g Acceleration due to gravity d droplet diameter v droplet terminal velocity v s Stokes velocity
Flocculation and Coalescence Film rupture Rehomogenization Collision and sticking (reaction) Stir or change chemical conditions FLOCCULATION COALESCENCE
Aggregation Kinetics Droplets diffuse around and will collide often In fact only a tiny proportion of collisions are reactive 2P P2P2 G GG k slow =k fast /W 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?
Rheology of Flocculated Emulsions Flocculation leads to an increase in viscosity Water is trapped within the floc and must flow with the floc Effective volume fraction increased rgrg