Advances in the use of microgels as emulsion stabilisers and as a strategy for cellulose functionalisation
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REVIEW PAPER
Advances in the use of microgels as emulsion stabilisers and as a strategy for cellulose functionalisation Katherine S. Lefroy . Brent S. Murray
. Michael E. Ries
Received: 19 July 2020 / Accepted: 17 November 2020 Ó The Author(s) 2020
Abstract Microgel particles have recently emerged as an alternative route to emulsion stabilisation. Classed as soft colloidal particles, their ability to swell to differing degrees in certain solvents and to rearrange once attached to an interface makes them highly suitable for systems requiring long-term stabilization, such as formulations in the food, agricultural, cosmetic and pharmaceutical industries. Microgels made with biocompatible polymers such as proteins and polysaccharides in particular offer an environmental advantage and currently form a very active area of research. Cellulose, being a natural, biodegradable polymer, is an attractive ingredient for gels and microgels. However, its use as a functional material is often somewhat hindered by its insolubility in water
and most other organic solvents. Furthermore, the surface activity of cellulose has proven difficult to harness and therefore its ability to act as an emulsion stabiliser has been almost exclusively applied to oilin-water (O/W) emulsions, with very few reports on its water in oil (W/O) activity. This review aims to summarise some of the recent progress made in the microgel field including their ability to act as emulsion stabilisers, with a focus on cellulose microgels (CMGs). A brief overview of cellulose processing is also given, describing the dissolution and reprecipitation routes used to functionalise cellulose without covalent modification and the potential for cellulose particles and CMGs to act as O/W and W/O emulsion stabilisers.
K. S. Lefroy B. S. Murray (&) Food Colloids & Bioprocessing Group, School of Food Science & Nutrition, University of Leeds, Leeds LS2 9JT, UK e-mail: [email protected] M. E. Ries Soft Matter Physics Group, School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
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Cellulose
Graphic abstract
Keywords Cellulose Microgel Stabiliser Emulsion Ionic liquid
Microgel particles Microgel particles (also referred to as ‘‘microgels’’) are a class of soft colloidal species which are receiving increasing interest in the area of emulsion stabilisation (Dickinson 2015). Originally, the term microgel was introduced in 1949, with the word ‘‘micro’’ referring to the gel particle size and ‘‘gel’’ meaning the ability of the particles to swell in organic solvents (Baker 1949; Pelton and Hoare 2011). They are often now referred to as ‘‘colloidal dispersions of gel-like particles’’, in which each individual microgel exists as a long-lived, kinetically stable particle (Dickinson 2015). Microgels are made up of a covalently, or strong physically cross-linked polymer network that must be dispersed in and swollen by the solvent. Their swelling ‘state’ is to an extent governed by the cross-link density,
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