Influence of process parameters on microcapsule formation from chitosan—Type B gelatin complex coacervates

Highlights

• Bio-based PCM microcapsules were produced by coavervation of CH:GB complex.

• The narrowest size distribution and greatest coavervation yield were achieved at 1:5 CH:GB ratio and 30 min homogenization.

• Thicker and more smooth shells were observed by increasing GB fraction in complex.

• No interaction between paraffin core and CH:GB shell was observed.

• The CH:GB-1:5 ratio exhibited the highest core content in the experimental condition.

Abstract

A series of chitosan/gelatin based microcapsules containing n-hexadecane was synthesized through complex phase coacervation from chitosan (CH) and type-B gelatin (GB), and crosslinked by glutaraldehyde (GTA). This research was conducted to clarify the influence of different parameters on the encapsulation process, i.e., the emulsion formation and the shell formation, using zeta potential and surface tension measurements, attenuated total reflectance (ATR), and thermal analysis such as differential scanning calorimetry (DSC). The optimal values of biopolymer ratios (TBP), crosslinker amount, emulsion time and feeding weight ratio of core/shell polymer (RCS) were identified. The stability of the emulsion was depended on the surface activity and TBP ratio, which also affected the droplet size distribution and the thickness of the shell. Furthermore, core content, encapsulation efficiency and thermal properties of the microcapsules were related to TBP and RCS; with the lowest RCS giving the best microcapsules features.

Introduction

Microencapsulation is a technique by which a polymeric shell can be formed around a liquid droplet or core material acting as reservoir, with the main aim of protecting the core material from the surrounding environment. The formation of microcapsules can be carried out by various processes such as interfacial polycondensation (Pan et al., 2012), coacervation of biopolymers (Bayés-García et al., 2010), spray drying (Kalušević et al., 2017; Tao, Zhang, Hu, Wan, & Su, 2013), sol-gel (Ciriminna, Sciortino, Alonzo, Schrijver, & Pagliaro, 2011), in situ polymerization (Salaün, Devaux, Bourbigot, & Rumeau, 2009), etc. Coacervation is one of the simplest techniques, based on phase separation from an oil-in-water emulsion without using any organic solvents. Under specific temperature and pH of the medium, the process leads to a polymer rich phase and polymer poor phase. The simple coacervation process involves only one type of polymer, whose dissolution is controlled by maintaining the proper pH, temperature or by adding salts (Chatterjee et al., 2014). The complex coacervation process involves two or more polymers dissolved in the aqueous solution and phase separation occurs by electrostatic interactions, hydrogen bonds and hydrophobic interactions among the polymer hydrocolloids (Roy, Salaün, Giraud, Ferri, & Guan, 2017b). To achieve microencapsulation, emulsification and coacervation are followed by deposition of the coacervates on the emulsified droplets in the continuous phase. The deposited coacervates form a thin layer around the active core material, and the thermo-mechanical properties of the polymer shell can be improved by adding a suitable crosslinker.

The selection of shell materials is one of the important steps in microencapsulation, which also depends on the final application. For textile functionalization, the thermo-mechanical properties of the microcapsule shell should be sufficiently high to resist conventional finishing processes, i.e. padding, grafting, coating, etc. The material used to form the shell can be selected from synthetic or natural polymers. Nevertheless, biopolymers such as proteins and polysaccharides are particularly interesting due to their diversified functionality and biocompatibility. Moreover, biopolymer mixtures have attracted much attention due to their improved mechanical strength, reduced tendency of water dissolution, increased porosity of the membrane compared to individual biopolymers (Rahman, Pervez, Nesa, & Khan, 2013). Therefore, many polymer composite systems such as chitosan and fucoidan (Pinheiro et al., 2015) Gelatin/Arabic Gum, Agar-Agar/Arabic Gum (Bayés-García et al., 2010), Chitosan/Arabic Gum (Butstraen & Salaün, 2014), sepiolit/chitosan (Konuklu & Ersoy, 2016) have demonstrated improved properties of the polymer shell compared to single polymer shells. For the growing concerns regarding health and environmental issues, environmental friendly processes and non-toxic product development are highly demanding for consumers and companies. Therefore, chitosan (CH), a polysaccharide and type B gelatin (GB), a protein, can be selected as shell forming materials in encapsulation of liposoluble liquid core due to their availability, low cost, biodegradability, non-toxicity and various functionality (Rawat & Bohidar, 2014; Roy, Salaün, Giraud, Ferri, & Guan, 2017a). Both CH and GB are produced from the waste of the fishing and meat industry respectively. Hence, these biopolymers are very cheap and available for industrial applications such as food, cosmetics, textile, pharmaceutical and agriculture. CH is the only positively charged polysaccharide below pH 6.0 while GB carries negative charges above the isoelectric point (IEP) at pH 4.8 (Basu, Kavitha, & Rupeshkumar, 2011; Wang & Heuzey, 2016). Chitosan shows antibacterial and excellent film forming properties; CH film is permeable to specific gases such as O₂ and CO₂ but it collapses in the swollen state very quickly (Cui, Beach, & Anastas, 2011; Ferreira et al., 2016; Rahman et al., 2013). Moreover, CH has the ability to act as emulsifier to stabilize the oil/water system and to coat colloidal system for oral, nasal and intravenous administration since exhibiting no toxicity (Schulz, Rodríguez, Del Blanco, Pistonesi, & Agulló, 1998). Gelatin shows high surface activity which allows to emulsify oil with good viscoelastic properties but it exhibits very high dissolution in aqueous medium (Liu, Wang, Barrow, & Adhikari, 2014). It was observed that the elongation at break of a chitosan-gelatin composite film increased when three times more gelatin was used in the blend of chitosan and gelatin. In addition, the glass transition temperature of chitosan-gelatin film raised to higher temperature compared to pure gelatin film (Rahman et al., 2013). In emulsification, chitosan-gelatin complex formed an extensively stable pickering emulsion by forming a stronger barrier around the oil droplets compared to the individual chitosan or gelatin (Wang & Heuzey, 2016). Water solubility of the composite film was tuned by adjusting the amount of pure bovine gelatin and chitosan, obtaining moderate swelling upon mixing of both bioplymers (Gómez-Estaca, Gómez-Guillén, Fernández-Martín, & Montero, 2011; Yin, Yao, Cheng, & Ma, 1999). Concerning microencapsulation ability, it was observed that chitosan/gelatin shell material allows to entrap 50% (maximum) of core substance with 66% encapsulation efficiency when using 1:1 biopolymer ratio (Hussain & Maji, 2008). In addition, crosslinking of the chitosan-gelatin composite film by glutaraldehyde, which involves Schiff base and acetylation reactions, was found to increase the film mechanical properties, (Qian, Zhang, Chen, Ke, & Mo, 2011).

Furthermore, since the two last decades microencapsulation for textile applications has gained huge interest (Salaün, 2016), especially for cosmeto-textile (Chatterjee, Salaün, Campagne, Vaupre, & Beirão, 2012), flame retardant textiles (Butstraen, Salaün, & Devaux, 2015) and thermal comfort of apparel (Salaün, Devaux, Bourbigot, & Rumeau, 2010). Therefore, the novelty of the work was to develop a surfactant free emulsion system, which turned into microcapsules with hard biopolymer shell and compatible for the finishing process on textile. A two-step microencapsulation process has been developed. The first step is the liquid–liquid dispersion of n-hexadecane-water and the second step is the microencapsulation of n-hexadecane by complex coacervation of CH-GB biopolymers. The first step, namely the dispersion of n-alkane in the continuous phase, is the determining step in defining the size distribution of the final microcapsule formulation. However, to control the morphology and to prepare microcapsules with desired physical properties, it is necessary to know the membrane or shell forming mechanism. The dispersion of the organic phase in the continuous aqueous phase is affected to a great extent by the physico-chemical properties of the two immiscible phases as well as by the characteristics of the mixing system. Variations in these physical and design parameters influence the size distribution of the dispersed organic phase. Moreover, it is essential that the dispersed liquid droplets are stable during the time before the formation of the polymer shell. Therefore, a well-controlled microencapsulation process should typically lead to the production of microcapsules having a narrow capsule size distribution. On the one hand, experiments were conducted to characterize the influence of the parameters governing the emulsion step. On the other hand, the effect of CH-GB mixing ratio on the membrane formation mechanism was investigated. The formation of the coacervates allows to stabilize the oil/water interface through a Pickering particle formation mechanism, which depends on the amount of total biopolymer or CH-GB mixing ratio. The formation of the shell was completed by adding glutaraldehyde to crosslink the CH-GB complex, obtaining thermally stable microcapsules in the aqueous medium. The microcapsules were then dried and analysed in terms of surface morphology and thermal stability, using Fourier transform infrared spectroscopy in the attenuated total reflectance mode (ATR-FTIR), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC).