Determinación de la vida útil de los filetes de trucha recubiertos con la película biodegradable y la adición de aceite esencial de muña

An understanding of the flocculation mechanism of bio-colloidal material with polymers is essential for the selective aggregation of unwanted material from bacterial homogenates. Bio-colloidal material may be hydrophilic or hydrophobic in nature; whilst water soluble polymers can be either charged or uncharged, and thus classified as polyelectrolytes or non-ionic polymers respectively; PEI has been classified as a weak cationic polyelectrolyte (Horn, 1980). The stability or otherwise of colloidal particles can be described in terms of a balance between electrical repulsion and van der Waals attraction, this is usually referred to as DLVO theory (Gregory, 1978b). The electrical repulsion between charged particles in dilute electrolyte solutions can be sufficient to prevent a close approach of the particles. By increasing the charge on the particles, or by increasing the ionic strength of the solution, the electrical repulsion can be reduced enough to allow the particles to approach close enough for van der Waals forces to operate, and allow agglomeration

to occur. Flocculation of particles may be the result of bridging between particles by the adsorbed polymer chains; or if the particles and polymer have opposite electrical charge, then charge neutralisation may be responsible.

Effective flocculants are usually homopolymers with a linear or branched structure; they may be non-ionic but more commonly have ionisable groups. The important characteristics of polymeric flocculants are the molecular mass (degree of polymerisation), relative charge density, sign of charge and an extended configuration when dissolved. From the wide range of flocculants that are available (Kitchener, 1972), it is obvious that no specific structure is required other than the properties stated above; selectivity can be controlled by a selection of those properties and conditions. For the mechanism of flocculation to be understood the properties of the flocculant along with a detailed consideration of the surface properties of the material to be coagulated are required. It has been noted (Gregoiy, 1969) that certain polyelectrolytes with a high charge density and low molecular weight have good de-fiocculant (dispersant) properties. The size and shape of polymer molecules in solution is of considerable interest from the standpoint of flocculation. These depend upon the ionic strength of the solution and the degree of ionisation. Repulsion between the segments causes the chains to expand, but this effect is reduced as the ionic strength of the solution is increased, because the charges are screened by counter ions. Unfortunately, most reported work (Gregory, 1978b) on polymeric flocculation has been undertaken with poorly characterised materials usually of commercial origin. This is especially with regard to quoted molecular masses which are invariably average values (derived from viscosity data), and represent broad distributions of molecular masses.

The configuration and behaviour of macromolecular polyelectrolytes at the solid- liquid interface will depend upon a number of factors, especially the interaction energy between polymer segments and solvent molecules and the number of surface sites and their distribution on the particle’s surface. The attachment of a long chain polymer to a surface involves the individual adsorption of polymer segments (trains) onto surface site’s; whilst other segments extend into solution (loops and tails). The size and number of loops and tails will depend on the interaction of the surface and polymer as well as its flexibility. Since the system is dynamic, small polymers may be

displaced by larger ones. Short chain polymers tend to adsorb in a flat configuration, whereas high molecular weight linear polymers adsorb by segments on the surface and loops extending into solution. The two main parameters which characterise the polyelectrolytes behaviour are molecular weight (chain length) and charge density. The charge density or fraction of monomer units that carry charge will determine the configuration of the adsorbed polymer chain. When the strength of interaction between the polymer molecule and the surface is weak (few cationic charges), the adsorbed polymer will consist of numerous long loops and tails. Only a few segments will be adsorbed onto the surface, and the extension of loops and tails will increase with molecular weight. In the case of strong interaction, loops and tail sizes are small, and the majority of the segments are either attached or very near to the surface, in a flat configuration. Hence the actual flocculation mechanism will be determined by the charge density on the polymer. The mechanisms by which flocculation can occur can be divided into two main groups; charge neutralisation and bridging flocculation.

1.2.2 CH A RG E NEUTRALISATION.

In many applications, it has been found that the most effective flocculants are those of opposite charge to the particles. The strong interaction of the polyelectrolyte with the colloid possibly allows it to adsorb in a flat configuration, thus reducing the possibility of polymer bridging. The neutralisation of the particle charge by adsorption may alone be responsible for the destabilisation due to the compression o f the electrical double layers which permit the van der Waals forces to draw the particles together. If charge neutralisation alone were the flocculation mechanism, then high molecular weight polymers would not be required since polyelectrolytes with a high charge density would be more effective.

The most direct evidence that charge neutralisation governs some flocculation processes, comes from measurements of electrophoretic mobility (Dixon et a l, 1967; Bentham, 199o; Gregory, 1978b). It has been noted that the optimum dosage of some cationic polyelectrolytes corresponds with the quantity required to give near zero electrophoretic mobility {ie\- to neutralise the particle’s charge). From DLVO theory, the charge neutralisation mechanism should only occur when the zeta potential is low

enough to eliminate repulsion between particles; flocculation at higher zeta potentials might indicate bridging through a repulsive barrier. Gregory (1969) has shown that for the flocculation of anionic latex with two cationic polymers, the critical flocculation concentration (CFG) of the lower molecular weight polymer occurred at a low zeta potential, hence charge neutralisation was thought to take place; also the polyelectrolyte was quantitatively adsorbed up to the point of charge neutralisation. The electrophoretic mobility could be reduced by lowering the pH; this resulted in less flocculant being required. For higher molecular weight polyelectrolytes, the CFG occurred at a considerably more negative zeta potential, indicating a bridging mechanism. At low surface coverages, positively charged segments would extend for a considerable distance allowing bridging despite considerable electrical repulsion; low surface coverage would also increase the chances of attachment of other particles. For both molecular weights, the amount required to give zero mobility was directly related to the charge densities on the polymers. The low molecular weight polymers appeared to be completely adsorbed up to the point of charge neutralisation. Dixon et al, (1967) showed that the electrophoretic mobility of silica is sensitive to the presence of low concentrations of cationic polymer over a wide range of molecular weights. Also the charge on the silica changed from negative to positive in the polymer concentration range, which coincided with the range where the best flocculation occurred. Additional polymer caused the mobility to increase rapidly, but had little effect on the flocculation; at higher concentrations redispersion of the silica occurred.

An extension of the charge neutralisation theory has been proposed (Gregory, 1973) to account for some effects of molecular weight and ionic strength. The 'charge- patch* model recognises that many cationic polymers have a high charge density, whilst the particles’ surfaces may have a low negative charge density. Hence even when the particle has an overall neutral charge, there may still be areas of negative charge on the particles’ surfaces, along with localised patches of adsorbed polymer carrying excess positive charge. Particles with this ’mosaic’ distribution of charges which come into contact, are likely to coagulate through electrostatic attachment. This uneven charge distribution would lead to extra attractive contribution to the interaction energy. Aggregate strength is likely to depend on the charge difference between the patches on the particles. It has been noted (Gregory, 1973) that cationic

CATIONIC