Emulsifying ability and cross-linking of silk fibroin microcapsules containing phase change materials

https://doi.org/10.1016/j.solmat.2015.12.012Get rights and content

Highlights

  • Silk fibroins were used to microencapsulate PCMs using a facile method.

  • Mechanism of influence of surfactants on formation of MicroPCMs was indicated.

  • Mixed surfactants acting as emulsifiers improved thermal performances of MicoPCMs.

  • Mixed surfactants acting as crosslinkers enhanced mechanical strength of MicroPCMs.

Abstract

The microencapsulation of phase change materials (PCMs) with regenerated silk fibroin (SF) as a shell by means of SF self-assembling was studied. Nonionic, ionic and mixed surfactants were applied to increase the emulsion stability and enhance encapsulating capacity of SF microcapsules. Effects of different types of surfactants on diverse properties of PCM microcapsules including morphology, energy storage density, mechanical strength and thermal stability have been investigated. It was observed that 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 microencapsulation processing. Adding mixed surfactants to n-octadecane/SF system improved the surface morphology and energy storage density, along with the mechanical strength.

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.

References (29)

Cited by (25)

  • Microencapsulation of phase change materials for thermal energy storage systems

    2020, Advances in Thermal Energy Storage Systems: Methods and Applications
  • Fully bio-originated latent heat storing calcium alginate microcapsules with high coconut oil loading

    2018, Solar Energy
    Citation Excerpt :

    In the other approach, the carrier material is of biological origin, for example chitosan, a low-price and eco-friendly polymer, was used to synthesize graphene-based carbon aerogel that was filled with 1-hexadecanol in order to prepare a form-stable PCM for thermal energy storage (Fang et al., 2017). Silk fibroin was utilized for embedment of paraffin PCM (Luo et al., 2016). n-octadecane microcapsules with gelatin-gum arabic shell were prepared by complex coacervation and crosslinking with glutaraldehyde (Li et al., 2012).

  • Novel metal coated nanoencapsulated phase change materials with high thermal conductivity for thermal energy storage

    2018, Solar Energy Materials and Solar Cells
    Citation Excerpt :

    In order to improve the heat transfer properties of encapsulated PCMs, adoption of highly thermal conductive shell materials as well as decrease of capsule sizes are two key factors. Traditionally, various organic polymers were utilized to encapsulate PCMs, such as melamine-formaldehyde (M-F) resin [10,11], polystyrene (PS) [12], polymethylmethacrylate (PMMA) [13,14], and so on [15,16]. These shell materials possess desirable sealing characteristics and good chemical and thermal stability, but the thermal conductivity is fairly low.

  • The role of vegetable oil in water based phase change materials for medium temperature refrigeration

    2018, Journal of Energy Storage
    Citation Excerpt :

    Paraffin is also chemically stable during phase change process [28]. However, paraffin waxes have disadvantages due to their low thermal conductivities, low latent heat, flammability and high change in volume [29]. Fatty acid (CH3 (CH2)2nCOOH) has advantages compared to paraffin.

  • Review of current state of research on energy storage, toxicity, health hazards and commercialization of phase changing materials

    2017, Renewable and Sustainable Energy Reviews
    Citation Excerpt :

    The techniques, materials used for PCM encapsulation and the properties of the resulting material are summarized in Table 8. Luo et al. [111] analyzed different surfactant types for their suitability as emulsifying agents for Silk fibrions (SF)/n-octadecane microcapsules. Non-ionic compounds couldn’t hold the oil-water emulsion in the SF while ionic reacts too hard with SF leading to cross linking.

View all citing articles on Scopus
1

The two authors contributed equally to the work.

View full text