Gel - Mixed gels and filled gels Flashcards
Type of mixed gels (two polymers A and B): Type I, explain
Type I: only one of the component forms gel. Polymer A forms a space spanning network, and polymer B no. This is due to the fact that the concentration of polymer B is below the gelation concentration of polymer B. Not enough B chain present to form a network. There are no attractive interactions present between A and B. The properties of the final gel is going to be determined only by the network of polymer A.
Type of mixed gels (two polymers A and B): Type II, explain
Type II: both components form a gel. There are three different types of networks:
IIA) Coupled network: both polymers contribute to one network. A-A junction, B-B junction, A-B junctions. Properties of the network depends on the properties of both polymers. Can be electrostatic attraction or salt bridges. E.g.: gelatin + alginate, gelatin +pectin
IIB) Interpenetrating network: two independent network are present, Only A-A junctions and B-B junctions.
IIC) Phase separated network: B form gel in pores of gel A. Only A-A junctions and B-B junctions. Example: gelatin + agar. Deplition interaction.
FILLED GELS, what are they ?
Filled gels are gels in which particles are incorporated in poly network. Examples: starch gels (amylose gel with granular particles ), polysaccharide gels incorporating protein micelles or aggregates.
Filled gel, rheological properties on the network?
Effect of particles on the strength of the network depends on:
• Size of the particles vs network pore size
• Volume fraction of particles
• Interaction between particles and network polymers (depletion, H-bonding, hydrophobic interactions, …..)
YOGURT: filled gel. EXAMPLE
*Particles: fat droplets or fat globules.
*Network: aggregation of large casein micelles.
PREPARATION: lactic acid, decreases the pH of the system which reduces the electrostatic repulsion between micelles. Micelles starts to aggregate, leads to formation of weak gel.
Effect of homogenization? Example: yogurt
*When the sample is homogenized, small droplets: incorporated in
network gel becomes stronger.
*When the milk is not homogenized, large droplets: not
incorporated, disrupts network, gel becomes weaker.
How do we measure the strength of the gel? Example: yogurt
*Change in pH? 1) addition of culture (circles) 2) Addition of GDL (glucono delta-lactone–>ability to decrease the pH), (squares).
GDL fast drop, and the culture slow drop in pH. Due to this, the casein micelles aggregates and a network is formed.
*Measure increase G’(storage modulus): start of the aggregation
Start of aggregation is determined by pH.
Starter culture slower drop in pH, so slower aggregation, network is formed later.
*Tan delta: development of the network over time =G´´(viscous liquid)/G´(elastic solid). Decrease in tan delta, indicates that the network becomes stronger over time and becomes more solid behaviour.
When the strain increases, the storage modulus decreases. This is an effect of the breakdown of the network structure, after which the yogurt starts to flow.
