Introduction to Dispersed Systems FDSC400 09/28/2001

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

SolidLiquidGas SolidSome glasses SolSmoke LiquidEmulsionAerosol GasSolid foam Foam Dispersed phase Continuous 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 cm 3 and a surface area 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 Tension -molecular scale-

Surface Tension -bulk scale- Area, A Force,  Interfacial energy Interfacial area Slope 

Curved Surface Highly curved surface Slightly curved surface

Curved Surfaces Molecules at highly deformed surfaces are less well anchored into their phase

Laplace Pressure Surface pressure pulls inwards increasing pressure on dispersed phase pressure Increased pressure Surface tension radius

Curved Surfaces -Consequences- Dispersed phase structures tend to be round Small fluid droplets behave as hard spheres Solubility increases with pressure so… Large droplets may grow at the expense of small (Ostwald ripening) –Depends on the solubility of the dispersed phase in the continuous

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 COSTENTROPY COST

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