Simple, Reversible Emulsion System Switched by pH on the Basis of Chitosan without Any Hydrophobic Modification

Abstract

Chitosan without hydrophobic modification is not a good emulsifier itself. However, it has a pH-tunable sol-gel transition due to free amino groups along its backbone. In the present work, a simple reversible Pickering emulsion system based on the pH-tunable sol-gel transition of chitosan was developed. At pH > 6.0, as adjusted by NaOH, chitosan was insoluble in water. Chitosan nanoparticles or micrometer-sized floccular precipitates were formed in situ. These chitosan aggregates could adsorb at the interface of oil and water to stabilize the o/w emulsions, so-called Pickering emulsions. At pH < 6.0, as adjusted by HCl, chitosan was soluble in water. Demulsification happened. Four organic solvents (liquid paraffin, n-hexane, toluene, and dichloromethane) were chosen as the oil phase. Reversible emulsions were formed for all four oils. Chitosan-based Pickering emulsions could undergo five cycles of emulsification-demulsification with only a slight increase in the emulsion droplet size. They also had good long-term stability for more than 2 months. Herein, we give an example of chitosan without any hydrophobic modification to act as an effective emulsifier for various oil-water systems. From the results, we have determined that natural polymers with a stimulus-responsive sol-gel transition should be a good particulate emulsifier. The method for in situ formation of pH-responsive Pickering emulsions based on chitosan will open up a new route to the preparation of a wide range of reversible emulsions.

Chitosan: Gels and Interfacial Properties

“…Oil was added to the chitosan solutions at pH 4 and followed by a pH adjustment to pH > 6 and upon subsequent homogenization O/W Pickering emulsions were formed. One of the most interesting findings in this work is the reversibility of this emulsion system; reversion was repeated more than five times by the alternate addition of HCl and NaOH [59].…”

“…Chitosan is usually not mentioned in the group of polysaccharides with good emulsification properties, but several studies have shown that chitosan can in fact act as the sole emulsifier and produce emulsions of acceptable stability [55][56][57][58][59][60].…”

A pH responsive Pickering emulsion stabilized by fibrous palygorskite particles

“…Recently, the synthesis and application of novel stimuli-responsive micro-particles as an emulsifier for the controlled formation of Pickering emulsions have attracted increasing attention (Liu et al, 2012). Among available stimuli, pH is a convenient method to manipulate emulsions.…”

“…Among available stimuli, pH is a convenient method to manipulate emulsions. However, most of these pH-responsive emulsifiers usually require tedious synthesis labor (Liu et al, 2012). In the present work, the pH sensitive behavior of Pickering emulsions stabilized by palygorskite was investigated during the in-situ preparation of toluene/water emulsions by using water with different pH values.…”

Self-assembly and emulsification of dopamine-modified hyaluronan

“…Although scientists have tried using naturally occurring materials, such as viruses (Russell et al, 2005) and protein particles (Fujii et al, 2009), to stabilize Pickering emulsions, the large-scale application of these materials will be limited by their high cost and complicated synthesis process. Therefore, much interest has been shown for investigation of particulate emulsifiers using natural macromolecular polysaccharides such as chitosan, cellulose, and starch (Kalashnikova, Bizot, Cathala, & Capron, 2011;Liu, Wang, Zou, Wei, & Tong, 2012;Tan et al, 2012).…”