Microcapsules coacervate formation

In this chapter, novel method for microencapsulation by coacervation is presented. The method employs polymer-polymer incompatibility taking place in a ternary system composed of sodium carboxymethyl cellulose (NaCMC), hydroxypropylmethyl cellulose (HPMC), and sodium dodecylsulfate (SDS). In the ternary system, various interactions between HPMC-NaCMC, HPMC-SDS and NaCMC-(HPMC-SDS) take place. The interactions were investigated by carrying out detailed conductometric, tensiometric, turbidimetric, viscosimetric, and rheological study. The interactions may result in coacervate formation as a result of incompatibility between NaCMC molecules and HPMC/SDS complex, where the ternary system phase separates in HPMC/SDS complex rich coacervate and NaCMC rich equilibrium solution. By tuning the interactions in the ternary system coacervate of controlled rheological properties was obtained. Thus obtained coacervate was deposited at the surface of dispersed oil droplets in emulsion, and oil-content microcapsules with a coacervate shell of different properties were obtained. Formation mechanism and stability of the coacervate shell, as well as stability of emulsions depend on HPMC-NaCMC-SDS interaction. Emulsions stabilized with coacervate of different properties were spray dried and powder of microcapsules was obtained. Dispersion properties of microcapsules, and microencapsulation efficiency were investigated and found to depend on both properties of deposited coacervate and the encapsulated oil type. 

Emulsions of different oils in ternary HPMC/NaCMC/SDS mixture having 0.7% HPMC, 0.3% NaCMC and three characteristic SDS concentrations i.e. 0.00% (no HPMC-SDS complex formation), 0.35% (maximum of HPMC/SDS interaction), 1.00% (end of HPMC-SDS interaction) were prepared and spray dried. Dispersion properties (mean diameter and standard deviation) of the emulsions and suspensions of microcapsules in water are shown in Table 1. As it can be seen in Table 1 oil type influences dispersion properties of emulsions, and thus suspensions. Largest droplet diameter in all emulsions is obtained at 0.00% SDS, i.e. where no HPMC/SDS complex and coacervate formation takes place. On the other hand, largest diameter of suspended microcapsules occurs at 1.00% SDS. 

Several other investigators have reported microencapsulation methods based upon polyelectrolyte complexes [289, 343]. For example, oppositely-charged polyelectrolytes (Amberlite IR120-P (cationic) and Amberlite IR-400 (anionic)) were recently used along with acacia and albumin to form complex coacervates for controlled release microcapsule formations [343]. Tsai and Levy [344,345] produced submicron microcapsules by interfacial crosslinking of aqueous polyethylene imine) and an organic solution of poly(2,6 dimethyl... 

Dobetti and Pantaleo (38) investigated the influence of hydrodynamic parameters per se on the efficiency of a coacervation process for microcapsule formation. They based their work on that of Armenante and Kirwan (39) who described the size of the smallest eddies or vortices generated in a turbulent regime on a microscopic scale in the vicinity of the agitation source, i.e., microeddies, as... 

Burgess, D.J. (1994). Complex coacervation microcapsule formation. In Dubin, P., Bock,... 

The polymer employed to prepare microspheres must be characterized in terms of molecular weight and purity,however this topic is beyond the scope of this article. Characterization of the materials may have implications for the formation of the microspheres. The viscosity and film-forming properties of the polymers used should be known. Viscosity can affect the tendency to form microspheres, their size, and even their shape. Burgess and coworkers have shown that albumin-acacia coacervates do not form microcapsules under certain conditions of pH and ionic strength, if the viscosity of the coacervate phase is too high. Burgess and Carless developed a method to predict the optimum conditions for complex coacer-vation based on the charge carried by the two polymers involved. 

Coacervation or coprecipitation of host and guest and suspension of the guest molecule in polysaccharide gels, followed by drying, is another common procedure. Microcapsules can be made on the formation of polysaccharide-protein complexes in the presence of a potential host. Preswelled granular starches are potential natural microcapsules (Lii et al., 2001b). 

Leclercq S., Harlander K.R., and Reineccius G., 2009. Formation and characterization of microcapsules by complex coacervation with liquid or solid aroma cores. Flavour Fragrance Journal, 24, 17-24. 

Burgess, D.J., Dubin, PL., Bock, J., Davis. Schulz. Complex Coacervation Microcapsule Formation in Macromolecular Complexes in Chemistry and Biology, Springer-Verlag Berlin, Germany, 1994, pp. 285-300. 

Other encapsulations utilize more or less similar methods for the formation of the capsule wall. Complex coacervation utilizes the reaction of an anionic water-soluble polymer with a cationic material to form the shell wall that separates from the solution. As the coacervate separates from the solution, it will tend to coat suspended particles with a protective shell. The shell wall is then hardened with a cross-linking agent. In situ polymerization is used to form urea formaldehyde or melamine formaldehyde shells by using heat to cross-link the monomers forming the shell waU. Interfacial polymerization with isocyanates via hydrolysis is another method to form a shell wall at an organic-water interface. In this case, water acts to hydrolyze some of the polyisocyanate to an amine, which cross-links to form the polyurea microcapsule waU. 

Currently, two methods for coacervation are available, namely simple and complex processes. The mechanism of microcapsule formation for both processes is identical, except for the way in which the phase separation is carried out. In simple coacervation a desolvation agent is added for phase separation, whereas complex coacervation involves complexation between two oppositely charged polymers. 

Many pairs of oppositely charged Polyelectrolytes (qv) are able to form a liquid complex coacervate suitable for microcapsule formation. Normally, gelatin is the positively charged polyion, because it is readily available and forms suitable complex coacervates with a wide range of polyanions. Polyanions typically used include gum arabic, polyphosphate, poly(acrylic acid), and alginate. 

Microcapsules are present in a number of personal care and cosmetic products (80). For example, deodorants may contain capsules that release their core contents due to moisture developed because of sweating. Microcapsules are incorporated in various cosmetic creams, powders, and personal cleansing products. Kiyama (81) summarizes the preparation and use of poy(vinyl alcohol) microcapsules in cosmetic products. Miyazawa and co-workers (82) describe the formation of agar capsules designed for cosmetic use. They note that residual glutaraldehyde in capsules with gelatin shells formed by complex coacervation may be an issue for microcapsules intended for cosmetic applications. 

Microencapsulation by coacervation is a common method for microcapsules production. It can be achieved by employing different methods, where the most common one is formation of an insoluble complex of two oppositely charged polymers and its subsequent deposition at surface of dispersed particles (e.g. emulsified oil droplets). In this way, microcapsules with coacervate shell are formed. Composition and microstructure of the coacervate shell are key to determine properties and application of microcapsules. 

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