Multiphase systems, preservative

Composite materials are multiphase systems consisting of two or more components that preserve their structure and properties within the composite formulation. 

Surface active compounds are often effective to potentiate the activity of the microbiddes. It is observed that the lower the content of volatile organic carbon in a paint the more important is the formulation of a preservative to transport the active ingredient to the location within the multiphase system where it is needed. Frequently, an optimum balance of membrane-activity and reactivity of the second active ingredient against microorganisms in complex matrices has to be found. It can be demonstrated that surface active compounds [II, 18.] like e.g. quaternary ammonium salts, fatty amine derivatives, ethoxylated alcohols or dodecylbenzene sulfonic add potentiates the activity of bactericides like BIT, MIT. 

Due to the nonmiscibility between blocks of different chemical nature (see Section 4.4), block copolymers are generally segregated in multiphase systems in which each phase preserves its properties. In that respect, they are definitely different from statistical copolymers having the same overall composition. 

In a multiphase formulation, such as an oil-in-water emulsion, preservative molecules will distribute themselves in an unstable equilibrium between the bulk aqueous phase and (i) the oil phase by partition, (ii) the surfactant micelles by solubilization, (iii) polymeric suspending agents and other solutes by competitive displacement of water of solvation, (iv) particulate and container surfaces by adsorption and, (v) any microorganisms present. Generally, the overall preservative efficiency can be related to the small proportion of preservative molecules remaining unbound in the bulk aqueous phase, although as this becomes depleted some slow re-equilibration between the components can be anticipated. The loss of neutral molecules into oil and micellar phases may be favoured over ionized species, although considerable variation in distribution is found between different systems. 

Both the chemical reactions and the phase separation proceed under nonequilibrium conditions simply because they proceed simultaneously. After some degree of chemical conversion and cross-linking is reached, microphase separation is impeded and the system freezes in a nonequilibrium structure characterized by incomplete phase separation. Thus, by the completion of IPN formation, reactions proceed in two evolved phases. The real structure of an IPN is a multiphase one, which is determined by the coexistence of at least three phases (not in a true thermodynamic sense). Two phases are formed by networks due to phase separation. Each phase may be considered as an independent IPN in which phase separation did not take place (the state of forced compatibility), and in which mixing on the molecular level is preserved. The composition of these two phases is determined by the reaction rate and the temperature. Each phase has an average composition that does not correspond to the network ratio in the entire IPN. The third phase is the nonequilibrium transition zone from one phase to another its size depends on the conditions of phase separation. This zone may be called mesophase and may be considered as a nonequilibrium IPN of some transition composition, since the molecular level of mixing should also be preserved. For spinodal decomposition there is no sharp border between coexisting phases. The transition zone may be arbitrarily chosen in such a way that its composition corresponds to the average composition of the IPN. 

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