Supplementary Information
Detailed derivation of the equation 3.
Equation (2) may be integrated to give
Eq. 2.a
wherel is the membrane thickness. The penetrant distribution between the penetrant and the polymer phase is described by the Nernst distribution law as,
c = KC Eq. 2.b
wherec is the absorbed concentration, C is the ambient permeate concentration in contact with the polymer surface and K is the partition coefficient of active between the core and the shell material (also called sorption coefficient) and depends on temperature and c.
Combining altogether one will obtain for liquids:
Eq. 2.c
In case of transport of gases and vapours, pressure p is used instead of surface concentration. According to Henry's law,
c = SpEq. 2.d
whereS is the solubility coefficient. The combination of gives the well-known permeation equation,
Eq. 2.e
wherep1and p2are the ambient pressures on two sides of a membrane of thickness l.
This equation is routinely applied for the characterization of gas barrier properties of packaging films. The product DS is called [1] the permeability coefficient P, so that
P = DS
Barrier films in food packaging: general considerations
The barrier properties of food packaging materials are perhaps studied to the most extent. Food package design requirements can be summarized as the following (please note that they are very similar to the requirements usually set for microcapsules):
-adequate mechanical stability,
-adequate barrier properties towards moisture, oxygen, light and malodours,
-adequate cost,
-environmental aspects,
-food safety.
In most cases, it is not possible to meet all the packaging design requirements using a single monolayer material (even on the macroscale). Nowadays, multilayered structures with up to 11 layers became a standard (fig. SI1). These single packaging structures combine the different properties of different base materials in order to meet these design requirements. Thus, most of the food packages combine paper (mechanical strength), polyethylene (protection from water) and aluminium foil (gas barrier) in order to achieve desired characteristics.
Figure SI1 A. The structure of a conventional high barrier food packaging (e.g. juice, milk). B. Possible structure of a microcapsule with advanced barrier properties
An adequate barrier to moisture and gases is prerequisite for many packaging materials. The different barriers provided by some commonly used base materials are illustrated in fig. SI2, which shows their permeabilities to water vapour and oxygen [2].
Figure SI2Permeabilities to water vapour and oxygen of some base materials used for packaging (100 µm thick films) [3].
It will readily be seen that moisture barrier and oxygen barrier do not go hand in hand. For example, LDPE has a good moisture barrier but only a moderate oxygen barrier. This is due to the fact that water vapor and oxygen have different solubility and diffusion coefficients in the LDPE. Very noticeable is position of metals and ceramics on the graph: they are very impermeable to both, water vapour and oxygen. Generally, glass, ceramics and metals are known for very good barrier properties for eventually all kind of molecules. This is extensively exploited: almost every multilayer package contains an aluminium foil layer. It is worth noting that the diffusion in these materialsmostly happens through defects (cracks, holes and pits); therefore, the integrity of the layer is one of the very important characteristics for this material class.
In principle, one could think that if the solution for diffusion prevention already exists, one can “simply” use the same solution for the production of microcapsules. The reason why it is not happening is that production methods for flat films (like packaging) are very different from the methods used for the production of microcapsules (self-assembly processes). They are so different that even the same materials cannot be used.
There are many ways of combining different materials to form packaging structures; the main two are co-extrusion and lamination. In both processes the preformed polymers are molten and then formed into a film. The film of non-plastic materials (e.g. aluminium foil) is also formed in advance. The multilayer package is obtained by “gluing” two or more flat films together (fig. SI3).
Figure SI3 Illustration of coextrusion and lamination processes
There is no need to say that the same approach cannot be used in the case of microcapsules, because they are not flat macroscopic films, but rather, fine objects that are complex in shape. Many materials used for packaging cannot be used for microencapsulation, since encapsulation requires the materials to be either dissolved or formed in situ. Most of the materials used for packaging are not soluble or swellable in organic solvents and cannot be synthesized in relevant for microencapsulation conditions and mediums.
In order to be able to produce microcapsules in large quantities, one still has to rely on the typical for the microencapsulation processes, which assume either dissolution of wall-forming materials or employing chemical reactions leading to the formation of wall forming materials in situ.
Nevertheless, two important things should be learned from packaging materials:
- The concept of multilayered structure, in which each layer has a particular function (mechanical stability, barrier function or adhesion between the two layers) as shown in fig. SI1.
- The classes of materials useful for prevention of diffusion: glass, ceramics and metals must be considered in the future.
1.Ahmad Fauzi Ismail, T.D.K., Azeman Mustafa, Understanding the Solution Diffusion Mechanism in Gas Separation Membrane for Engineering Students. Regional Conference on Engineering Education (RCEE 2005), Johor, Malaysia, 2005.
2.J.Dixon., Packaging Materials 9. Multilayer Packaging for Food and Beverages.ILSI Europe Report Series 2011: p. 1-43.
3.