Emulsifying ability and cross-linking of silk fibroin microcapsules containing phase change materials
Introduction
Phase change materials (PCMs) have been well known for their high latent heat storage [1], [2], [3]. They are capable of absorbing or releasing great amount of energy in a form of latent heat during phase transitions between solid–solid or solid–liquid phases over a narrow temperature range. Among the various phase change materials of interest, the use of paraffin waxes is particularly attractive due to non-corrosive and chemically stable merits, little sub-cooling, high latent heat per unit weight and low vapor pressure [1], [4], [5]. However, paraffin waxes also have disadvantages in low thermal conductivities, flammability and high changes in volume during phase change. Thus, for overcoming the defects and promoting the ease of handling, research groups have developed many encapsulating and storage methods, such as impregnating PCMs into various foams, shape stabilizing by embedding PCMs into a matrix and microencapsulating PCMs with organic/inorganic shells [5], [6], [7]. Microencapsulation of PCMs has been shown as effective engulfing method by increasing heat transfer areas and preventing PCMs leakage and the interaction between PCMs with ambient environment [8].
Up to now, there have been many methods for various wall materials to encapsulate PCMs, such as in situ polymerization for melamine–formaldehyde [9], [10], interfacial polymerization for urea–formaldehyde [11], suspension-like polymerization for methyl methacrylate-based polymer [2], [12], [13], sol–gel solution for SiO2 [14], [15] and TiO2 [16]. Complex coacervation has also received considerable attention in recent years for the microencapsulation of PCMs with natural and biodegradable polymers as shells [8], [17], [18], [19], [20]. Polysaccharides and proteins are mostly used in literature [20], [21], such as gum acacia, hydrophobically modified starch, alginate, carboxymethylcellulose, whey proteins, soy proteins and sodium caseinate. Hawlader et al. [19] prepared paraffin wax/gelatin-acacia microcapsules by spray-drying and complex coacervation. The encapsulated paraffin wax with a thermal energy storage/release capacity of about 145–240 J/g showed a good potential as a solar-energy storage material.
Onder et al. [18] also explored the influence of process parameters on the microencapsulation of paraffin waxes with gum arabic–gelatin mixture as the shell material using complex coacervation method. Regenerated silk fibroin (SF) was firstly used for the microencapsulation of paraffin wax by Basal et al. via complex coacervation method [22]. Span-20, an nonionic surfactant, was introduced to be the emulsifier for the formation of paraffin waxes/SF emulsion in their report. SF is an ionic surfactant, not only composing of hydrophobic and hydrophilic segments, but also possessing of negative and positive charges. With the suitable surfactants, the properties of paraffin waxes /SF microcapsules, such as the surface morphology and energy storage density, may be improved. In this manuscript, n-octadecane, one type of paraffin waxes, was used as PCMs. Nonionic, ionic and mixed surfactants were respectively applied to investigate the emulsion stability and SF microencapsulating capacity of n-octadecane. Effects of different types of surfactants on diverse properties of PCM microcapsules including morphology, energy storage density, mechanical strength and thermal stability were discussed.
SF microspheres and microcapsules have been explored in pharmaceutical and medical technology due to their unique combination of self-assembly, mechanical stability, controllable structure and morphology [23], [24], [25], [26]. Among various preparation methods, the assembly process of SF particles is relatively simple and avoids the additions required in templating approaches [23]. Some inorganic salts, such as potassium phosphate and sodium chloride, pH variation and organic solvents, such as methanol and ethanol, can induce the conformation transition of SF from Silk I to Silk II, with the result of phase separation [24], [27]. In this report, ethanol was applied to induce the assembling of SF walls after the formation of oil-in-water emulsion.
Section snippets
Preparation of SF aqueous solution
Cocoons of Bombyx mori (Zhejiang province, China) were used to prepare SF aqueous solution as previously described [28]. Briefly, cocoons were boiled twice for 30 min in an aqueous solution of 0.5 wt% Na2CO3 solution at 100 °C, and washed with deionized water to remove sericin. After air drying, the degummed fibers were dissolved in a 9.0 M LiBr aqueous solution at 40 °C for 2 h yielding a 10% (w/v) solution. After being centrifuged and filtrated, the solution was dialyzed against deionized water
Results an discussion
n-octadecane/SF MicroPCMs were successfully prepared by SF self-assembly. It was feasible to form microcapsules starting from n-octadecane/SF (aqueous solution) emulsions in the presence of emulsifiers. Ethanol was used to induce the conformation transition of silk fibroins from α-helix/random coil to β-sheets, with the result of phase separation of silk fibroins.
The prerequisite for manufacturing MicroPCMs with good characteristics is the ability to produce stable droplets with a uniform
Conclusion
SF MicroPCMs were successfully prepared by means of SF self-assembling method. Nonionic, ionic and mixed surfactants were applied to increase the emulsion stability and the encapsulating capacity of SF microcapsules. Comparing with nonionic or ionic surfactants, mixed surfactants promoted significantly the formation and stability of n-octadecane/SF emulsion. With the effects of co-emulsifiers, mixed surfactants acted simultaneously as excellent emulsifiers and cross-linkers in SF
Acknowledgments
We would like to gratefully acknowledge the financial supports from Shenzhen government project JCYJ20140417115840245 and ZDSY20120619140933512.
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The two authors contributed equally to the work.