Surface Chemistry of Surfactants and Polymers

Bengt Kronberg, Krister Holmberg, Bjorn Lindman

• Publisher: Wiley
• Year: 2014
• Print ISBN:
  9781119961246
• eBook ISBN:
  9781118695982
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

This book gives the reader an introduction to the field of surfactants in solution as well as polymers in solution. Starting with an introduction to surfactants the book then discusses their environmental and health aspects. Chapter 3 looks at fundamental forces in surface and colloid chemistry. Chapter 4 covers self-assembly and 5 phase diagrams. Chapter 6 reviews advanced self-assembly while chapter 7 looks at complex behaviour. Chapters 8 to 10 cover polymer adsorption at solid surfaces, polymers in solution and surface active polymers, respectively. Chapters 11 and 12 discuss adsorption and surface and interfacial tension, while Chapters 13- 16 deal with mixed surfactant systems. Chapter 17, 18 and 19 address microemulsions, colloidal stability and the rheology of polymer and surfactant solutions. Wetting and wetting agents, hydrophobization and hydrophobizing agents, solid dispersions, surfactant assemblies, foaming, emulsions and emulsifiers and microemulsions for soil and oil removal complete the coverage in chapters 20-25.

1
Types of Surfactants, their Synthesis, and Applications

Definition of a Surfactant

Surfactant is a widely used contraction for surface active agent, which literally means active at a surface. The term surface active means that the surfactant reduces the free energy of surfaces and interfaces. Expressed differently, they reduce the surface and the interfacial tensions. This is not an unique quality, however. Most water-soluble organic compounds give a reduction of the surface and interfacial tensions when added to an aqueous solution but the effect is normally much less pronounced than for surfactants. The unique behavior of a surfactant is that it self-assembles at interfaces and forms tightly packed structures: monolayers at the air–water and the oil–water interface, and monolayers and aggregates at the solid–water interface. Such self-assembled layers drastically change the character of the interface. Surfactant self-assembly at the air–water interface, commonly referred to as the “surface,” is dealt with in Chapter 12; the assembling at the oil–water interface, which is key to formation of emulsions, is treated in Chapter 24; and assembly at the solid–water interface, adsorption, is described in depth in Chapter 8. Surfactants also self-assemble in water, usually forming micelles at very low concentration and other aggregates, called surfactant liquid crystals, at higher concentration. These are treated in Chapters 4 and 6, respectively.

The term surfactant is usually associated with relatively low molecular weight substances. The molecular weight is typically below 500 Da but may be larger for nonionic surfactants with long polyoxyethylene tails. There also exist polymeric surface active agents and these may be called polymeric surfactants. However, more often they are referred to as surface active polymers and that terminology is used in this book. Several chapters deal with surface active polymers.

Surfactants are amphiphilic molecules. The word has a Greek origin with amphi meaning “both” and phil meaning “like”; that is, surfactants are molecules that like both a polar and a nonpolar environment. This is due to their structure. All surfactants have at least one polar headgroup that wants to be in water and at least one hydrophobic tail that prefers to be in an apolar environment; hence, the tendency to go to interfaces. Figure 1.1 shows the structure of a surfactant with one polar head group and one hydrophobic tail.

Figure 1.1 Schematic illustration of a surfactant

A surfactant may be viewed as a molecule consisting of a lyophilic and a lyophobic part. The lyophilic moiety is soluble in a specific fluid whereas the lyophobic moiety in insoluble in this fluid. When the fluid is water, which is usually the case, the terms hydrophilic and hydrophobic parts are normally used.

The term amphiphile or amphiphilic compound is sometimes used in the same sense as the word surfactant. Amphiphilic compounds are also very common in nature. All biological systems contain surface active substances. However, these molecules are usually called polar lipids rather than surfactants. Thus, implicit in the name is that a surfactant is a man-made compound in some sense, although the molecule may have a natural origin, as is discussed later in this chapter.

The hydrophobic tail is almost always based on one or more carbon chains and the chains may be linear, branched, or cyclic. The only exception to carbon chains as hydrophobic backbone is siloxane chains. There exist both high molecular weight and low molecular weight amphiphilic compounds based on the –Si–O– unit; these are discussed in Chapters 10 and 20, respectively.

Surfactants Adsorb at Interfaces

The term interface denotes a boundary between any two immiscible phases; the term surface indicates that one of the phases is a gas, usually air. Altogether five different interfaces exist:

1. Solid–vapor surface
2. Solid–liquid
3. Solid–solid
4. Liquid–vapor surface
5. Liquid–liquid

The driving force for a surfactant to adsorb at an interface is to lower the free energy of that phase boundary. The interfacial free energy per unit area represents the amount of work required to expand the interface. The term interfacial tension is often used instead of interfacial free energy per unit area. Thus, the surface tension of water is equivalent to the interfacial free energy per unit area of the boundary between water and the air above it. When that boundary is covered by surfactant molecules, the surface tension (or the amount of work required to expand the interface) is reduced. The denser the surfactant packing at the interface, the larger is the reduction in surface tension.

This book is concerned with events at interfaces that involve at least one liquid phase, which means three out of the five interfaces listed above. The liquid is usually, but not always, water. Phenomena occurring at these three interfaces, as well as in the bulk liquid, may be referred to as wet surface chemistry. Dry surface chemistry is also very important—heterogeneous catalysis is a prime example—but that science is not discussed here. Examples of the different “wet” interfaces and of products in which these interfaces are important are given in Table 1.1.

Table 1.1 Examples of interfaces involving at least one liquid phase

Interface	Type of system	Product
Solid–liquid	Suspension, sol, slurry	Solvent-borne paints, colloidal silica, cement slurries
Liquid–liquid	Emulsion	Bitumen emulsions, milk, cream
Liquid–vapor	Foam, aerosol	Shaving cream, fire fighting foams, clouds

The systems indicated in Table 1.1 are all examples of dispersions, that is, systems with one phase, called the dispersed phase, finely distributed in another phase, the continuous phase. In many formulated products several types of systems are present at the same time. Water-based paints and paper coating formulations are examples of familiar but, from a colloidal point of view, very complicated systems, containing both solid–liquid (dispersed pigment particles) and liquid–liquid (latex or other binder droplets) interfaces. In addition, foam formation is a common (but unwanted) phenomenon at the application stage. All the interfaces are stabilized by surfactants.

As mentioned above, the tendency to accumulate at interfaces is a fundamental property of a surfactant. In principle, the stronger the tendency, the better is the surfactant. The tende...

Table of contents

• Cover
• Title page
• Copyright page
• Preface
• Acronyms
• 1 Types of Surfactants, their Synthesis, and Applications
• 2 Environmental and Health Aspects of Surfactants
• 3 Two Fundamental Forces in Surface and Colloid Chemistry
• 4 Surfactant Self-Assembly
• 5 Introduction to Phase Diagrams
• 6 Surfactant Self-Assembly
• 7 Surfactants and Polymers Containing Oxyethylene Groups Show a Complex Behavior
• 8 Surfactant Adsorption at Solid Surfaces
• 9 Polymers in Solution
• 10 Surface Active Polymers
• 11 Adsorption of Polymers at Solid Surfaces
• 12 Surface and Interfacial Tension
• 13 Mixed Surfactant Systems
• 14 Surfactant–Polymer Systems
• 15 Surfactant–Protein Mixtures
• 16 Surfactant–Polymer Mixtures at Interfaces
• 17 Microemulsions
• 18 Colloidal Stability
• 19 An Introduction to the Rheology of Polymer and Surfactant Solutions
• 20 Wetting and Wetting Agents, Hydrophobization and Hydrophobizing Agents
• 21 Solid Dispersions
• 22 Surfactant Assemblies as Templates
• 23 Foaming of Surfactant Solutions
• 24 Emulsions and Emulsifiers
• 25 Microemulsions for Soil and Oil Removal
• Index
• End User License Agreement