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Deep Eutectic Solvents (DESs) and Their Applications Emma L. Smith,*,+,?DESs have also been proposed as environmentally benign alternatives for synthesis.DESs are systems formed from a eutectic mixture of Lewis or Bronsted acids and bases which can contain a variety of anionic and/or cationic species; in contrast, ILs are formed from systems composed primarily of one type of discrete anion and cation.INTRODUCTION Deep eutectic solvents (DESs) are now widely acknowledged as a new class of ionic liquid (IL) analogues because they share many characteristics and properties with ILs.Compared to classical ILs, the research into DESs is comparatively in its infancy, with the first paper on the subject only published in 2001.1 However, with publications from the past decade now approaching 200 an increasing amount of research effort is focusing on this emerging field.Transformations of Unprotected Sugars, Cellulose, and Starch 11077 4.5.Immersion and Electroless Coatings 11072 3.2.10.1.1.

Deep Eutectic Solvents (DESs) and Their Applications
Emma L. Smith,*,†,‡ Andrew P. Abbott,‡ and Karl S. Ryder‡
†
Department of Chemistry, School of Science and Technology, Nottingham Trent University, Nottingham NG11 8NS, United
Kingdom
‡
Department of Chemistry, University of Leicester, Leicester LE1 7RH, United Kingdom
CONTENTS

1. Introduction 11060
   1.1. Deep Eutectic Solvents 11060
   1.2. Brief Description of Ionic Liquids 11061


2. Properties of DESs 11062
   2.1. Phase Behavior 11062
   2.2. Density, Viscosity, Conductivity, and Surface
   Tension 11063
   2.3. Hole Theory and Ionicity 11064
   2.4. Electrochemical Behavior 11065
   2.5. Speciation 11066
   2.6. Green Credentials 11067


3. Metal Processing Applications 11067
   3.1. Metal Electrodeposition 11068
   3.1.1. Chrome Plating 11068
   3.1.2. Aluminum Plating 11068
   3.1.3. Copper Plating 11069
   3.1.4. Nickel Plating 11069
   3.1.5. Zinc Plating 11069
   3.1.6. Other Metal Coatings 11071
   3.1.7. Alloy Coatings 11071
   3.1.8. Composite Coatings 11072
   3.1.9. Immersion and Electroless Coatings 11072
   3.2.10. Synthesis of Metal Nanoparticles 11072
   3.2. Metal Electropolishing 11073
   3.3. Metal Extraction and the Processing of Metal
   Oxides 11073


4. Synthesis Applications 11074
   4.1. Ionothermal Synthesis 11076
   4.2. Gas Adsorption 11076
   4.3. Biotransformations 11077
   4.4. Transformations of Unprotected Sugars,
   Cellulose, and Starch 11077
   4.5. Purifying and Manufacturing Biodiesel 11077


5. Conclusions 11078
   Author Information 11078
   Corresponding Author 11078
   Notes 11078
   Biographies 11078
   Acknowledgments 11079
   References 11079


6. INTRODUCTION
   Deep eutectic solvents (DESs) are now widely acknowledged as
   a new class of ionic liquid (IL) analogues because they share
   many characteristics and properties with ILs. The terms DES and
   IL have been used interchangeably in the literature though it is
   necessary to point out that these are actually two different types
   of solvent. DESs are systems formed from a eutectic mixture of
   Lewis or Brønsted acids and bases which can contain a variety of
   anionic and/or cationic species; in contrast, ILs are formed from
   systems composed primarily of one type of discrete anion and
   cation. It is illustrated here that although the physical properties
   of DESs are similar to other ILs, their chemical properties suggest
   application areas which are significantly different.
   The research into ionic liquids (ILs) has escalated in the last
   two decades, ever since the potential for new chemical
   technologies was realized. In the period covered by this review,
   approximately 6000 journal articles have been published on the
   topic. Compared to classical ILs, the research into DESs is
   comparatively in its infancy, with the first paper on the subject
   only published in 2001.1 However, with publications from the
   past decade now approaching 200 an increasing amount of
   research effort is focusing on this emerging field. This is a
   comprehensive review encompassing all research areas and
   applications relating to DES publications between 2001 and the
   beginning of 2012. The chief focuses are the two major
   application areas of deep eutectic solvents (DESs): metal
   processing and synthesis media. A large proportion of research
   effort has been focused on the use of DESs as alternative media
   for metals that are traditionally difficult to plate or process, or
   involve environmentally hazardous processes. DESs have also
   been proposed as environmentally benign alternatives for
   synthesis.
   1.1. Deep Eutectic Solvents
   DESs contain large, nonsymmetric ions that have low lattice
   energy and hence low melting points. They are usually obtained
   by the complexation of a quaternary ammonium salt with a metal
   salt or hydrogen bond donor (HBD). The charge delocalization
   occurring through hydrogen bonding between for example a
   Received: April 18, 2012
   Published: October 10, 2014