CO2-switchable Materials

Solvents, Surfactants, Solutes and Solids

Philip G Jessop, Michael F Cunningham

• Publisher: Royal Society of Chemistry
• Year: 2020
• Print ISBN: 9781782628767
• eBook ISBN: 9781839160356
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

CO2-responsive materials are a relatively recent innovation. In general, stimuli-responsive materials exhibit reversible changes in their physical or chemical properties in response to external triggers such as temperature, pH, light, or voltage. However, there are often limitations in applying these triggers including economic and environmental costs, and product contamination. The realization that CO2 can be used as an effective trigger for stimulating changes in material properties has prompted a surge in interest in this area within the past few years, with numerous new studies currently underway in several countries. CO2 is an ideal trigger for switchable or stimuli-responsive materials because it is benign, inexpensive, green, abundant, and does not accumulate in the system. Many different CO2-responsive materials including polymers, latexes, solvents, solutes, gels, surfactants, and catalysts have been prepared.

Summarizing recent progress in the preparation, self-assembly, and functional applications of CO2-responsive materials, this book explores the physical chemistry of CO2-switching, including constraints on structural design and process conditions, together with applications. With emphasis on the environmental, health, and safety advantages and disadvantages compared to conventional materials, it is ideal for researchers and industrialists working in green chemistry, chemical engineering, and polymer chemistry.

CHAPTER 1

Introduction to CO₂-switchable Materials

Switchable materials are so common in our everyday lives that we rarely appreciate how green they are. We switch on the lights when we enter a room because we need the illumination, but we switch off the lights when we exit in order to save energy. The switchability of lights makes them greener and less energy-consuming than nonswitchable lights. Cars, ovens, computers, cellphones, doors – we are surrounded by switchable items that save energy and reduce environmental impact by their switchability. But should we not demand the same from our solvents, surfactants, drying agents, and coatings? A switchable solvent, one that dissolves a solute when needed and later releases the product when dissolution is no longer wanted, could make processes more efficient and less environmentally harmful. Switching the solvent “off” would precipitate the product without the solvent needing to be removed by energy-intensive distillation. Perhaps such a solvent could also be more easily recycled? Similar arguments can be made for the expectation that many switchable materials would, other factors being equal, be greener than their nonswitchable predecessors.

Unfortunately, other factors are rarely equal. If a switchable material is far more complex to make, more toxic, more depleting, or in any other way significantly more harmful than the material it could replace, then the potential for environmental benefit is reduced. Harm reduction can best be achieved by adding the extra functionality of switching with little, if any, increase in the environmental impact of the material itself. Switchable materials belong in the field of green chemistry because of their great potential for reductions in energy and materials usage, but like any chemicals they have the potential for environmental harm if they are not designed and used wisely. Starting with the choice of the trigger, and continuing with the design of the switchable molecules and the processes and products in which they will perform, the principles of green chemistry must govern their design. The metrics of green chemistry, such as life cycle assessment, must also be used to evaluate whether the switchable technology reduces environmental harm.

What triggers can be used for stimuli-responsive or “switchable” materials? Until recently, CO₂ has rarely been considered a possibility and is generally ignored in books and reviews on the topic. Instead, the most frequently studied triggers have been light, voltage, oxidants/reductants, and acids/bases. Heat, too, can be a trigger but it differs from the others in generally causing physical rather than chemical changes, such as raising or lowering the temperature of a mixture through a phase transition, although exceptions include heat-induced chemical changes like isomerizations or changes in spin state.¹ Less commonly used triggers include applied magnetic fields and ultrasound.^2–4 All triggers have advantages and disadvantages (Table 1.1), and each is likely to be appropriate in some applications and entirely inappropriate in others. Reviews and books about stimuli-responsive^5–7 and smart materials^8–10 are available.

Table 1.1 Advantages and disadvantages of the various triggers or stimuli that can be used to reversibly switch materials.

Trigger	Advantages	Disadvantages
Light	Nontoxic trigger Trigger will not accumulate in the sample	Requires the material to be transparent or translucent Requires thin material due to absorption of light Light-responsive groups are typically expensive
Voltage	Nontoxic trigger Trigger will not accumulate in the system	Requires system to be conductive Some redox-active groups are expensive, toxic, and ecotoxic
Oxidants/reductants	Does not require a transparent or conductive material	Potentially toxic trigger Trigger will accumulate in the system Some redox-active groups are expensive, toxic, and ecotoxic
Temperature	Does not require a transparent or conductive material Innocuous trigger Relatively innocuous trigger-responsive functional groups Trigger will not accumulate in the system	Typically induces a phase change rather than a chemical change Separations triggered by temperature change may be poor unless a polymer is used Nonbiodegradable polymer often used May not be practical for large volume applications
Acids/bases	Does not require a transparent or conductive system Inexpensive and relatively innocuous trigger-responsive functional groups	Trigger will accumulate in the system Standard acids/bases are caustic or corrosive. Requires acidic/basic functional groups, which may cause health or environmental impacts
CO2	Does not require a transparent or conductive system Trigger will not accumulate in the system Innocuous trigger Inexpensive trigger-responsive functional groups	Mass transfer of CO2 into and out of the system may be slow for large volumes or viscous media Acidic/basic functional groups may cause harm

Why, then, should we use CO₂ as a trigger? As summarized in Table 1.1, the key advantages are:

• it does not require a transparent system, as light-responsive materials require,
• it does not require a conductive system, as voltage-responsive materials require,
• CO₂ addition and removal do not cause the accumulation of materials in the system, as occurs when acid/base or redox-responsive materials are switched,
• the functional groups that respond to CO₂ as a trigger (amines and carboxylate anions) are typically inexpensive compared to light- or voltage-responsive functional groups, and less toxic than most voltage-responsive functional groups, although amines and carboxylic acids can be harmful to skin and eyes, and
• CO₂ is far more benign to health, environment, and equipment than most of the acids, bases, oxidants, and reductants used in acid/base or redox-responsive materials.

A comment on the environmental impact of using CO₂ is warranted. While it is the molecule most responsible for the biggest environmental threat of our time, global warming, the use of CO₂ as a trigger, even if the CO₂ is released afterwards, can be beneficial for the environment compared to the technologies it could replace. First, the CO₂ is recycled waste material; in almost all of the technologies described in this volume, the CO₂ is obtained from an external source, mostly as a byproduct of ammonia production but sometimes also from power production or fermentation processes. Therefore, use of CO₂ as a trigger does not generate CO₂. Second, commercialized CO₂-switchable materials will almost always be replacements for incumbent technologies that lack the energy- and materials-saving advantages possessed by switchable materials. Those energy and materials savings will inherently reduce environmental impact, including CO₂ emissions. This is one of the ironies of CO₂ utilization – the environmental benefit is not directly from the use of CO₂ itself but rather from the avoided harm associated with energy and materials that are no longer needed. For that reason, permanent chemical conversion of CO₂ into a product is not necessary for CO₂ utilization to be beneficial for the environment.

CO₂-switchable materials were first invented by nature, not humans. The opening and closing of stomata, the mouth-shaped pores on plant leaves that allow gases and moisture to enter and exit the leaf (Figure 1.1), are controlled by CO₂-switchable guard cells. The stomatal pore opens when CO₂ concentration is low and closes when CO₂ concentration is high. This makes it possible for the plants to exchange gases with the surrounding atmosphere without losing too much moisture. While the system is elegant, the biochemistry is complex.¹¹ Plants developed this advanced technology by the Silurian era, over 400 million years ago.^1...

Table of contents

• Cover
• Title
• Copyright
• Preface
• Acknowledgements
• Contents
• Chapter 1 Introduction to CO2-switchable Materials
• Chapter 2 The Chemistry of CO2-triggered Switchable Materials
• Chapter 3 CO2-switchable Aqueous Solutions
• Chapter 4 CO2-switchable Solutes
• Chapter 5 Switchable-polarity Solvents
• Chapter 6 CO2-switchable Hydrophilicity Solvents
• Chapter 7 CO2-switchable Surfactants
• Chapter 8 CO2-switchable Particles
• Chapter 9 CO2-switchable Surfaces and Coatings
• Chapter 10 CO2-switchable Gels, Microgels, and Adhesives
• Chapter 11 Analysis and Future Outlook
• Subject Index