Cap 1 Barnes

1

1.1 The importance of interfaces

Interfaces are everywhere: in our bodies, in the food we eat and the drinks we drink, in plants, animals, fi sh, insects and microbes, in our cars, in the soil, in the atmosphere, in manufacturing and chemical factories. Occasionally, the presence of an interface does not appreciably alter the behaviour of a system, but in many cases it does have a signifi cant effect and, in some cases, it domi-nates the behaviour.

Think for instance of the catalytic converter, which has been an integral part of cars produced over the last decade. As the raw exhaust gases from the engine pass over the solid metal and metallic oxide catalyst surface, a large number of reactions take place, leading to more complete oxidation of the gases and, hence, to a cleaner environment. The gases spend only milliseconds in contact with the surface and yet this has a dramatic effect on the composi-tion of the fi nal exhaust mixture that is vented to the atmosphere. It is the surface of the catalyst that is crucial.

Another important example concerns the inner lining of the lung. A layer of fl uid lines the alveoli and, at the surface of this fl uid, in contact with the air, is a layer that is only one molecule thick, composed mainly of phospho-lipids. This mixture of phospholipids and some proteins is known as lung surfactant. Lung surfactant serves to lower the amount of work required for

Introduction

1.1 The importance of interfaces

1.2 Surfaces and interfaces

1.2.1 Introduction

1.2.2 Types of interface

1.2.3 Defi ning the interfacial region

1.3 Stable interfaces

1.4 Key concepts

1.4.1 Surface tension and surface pressure

1.4.2 Wetting

1.4.3 Adsorption

1.4.4 Emulsions

1.4.5 Colloids

1.4.6 Membranes

1.5 Organization of the book

OUP UNCORRECTED PROOF – REVISES, 11/26/10, NEWGEN

01_Insc_Chap01.indd 101_Insc_Chap01.indd 1 11/26/2010 7:59:32 PM11/26/2010 7:59:32 PM

2 | 1 INTRODUCTION

the action of breathing, a function that is so important that if the surfactant is not fully developed, unaided breathing is impossible. Because lung surfactant is only produced late in gestation, most infants who are born before 30 weeks’ gestation must be treated immediately after birth to ensure their survival. Again, it is the properties of the very outermost surface that determine the functioning of the entire system (see also Section 10.7).

In our day-to-day lives we are familiar with the behaviour of liquids, espe-cially the way they fl ow, being controlled largely by gravity. However, in space vehicles where the gravitational fi eld strength is very low, hydrostatic pressure is negligible, and liquids behave very differently—in fact, the forces arising from the interface between the liquid and the air dominate their fl ow.

Again and again we encounter similar examples of the importance of sur-faces. With the increasing emergence of new technologies relying on miniatur-ization, surface properties are growing in importance. The new and exploding fi eld of nanotechnology is an obvious case where the solid surfaces of, for example, nanoparticles and mesoporous materials, are the places where the processes of interest take place. An understanding of processes occurring at surfaces is therefore relevant to many new developments.

Even though surfaces are increasingly important to modern technology, they have, in fact, been studied for a very long time. Reports dating from Roman times describe the calming of water by spreading oil on the surface, but arguably the beginning of the fi eld as a scientifi c discipline dates from the experiments of Benjamin Franklin, reported to the Royal Society in 1774, where he describes the spreading of oil on a pond in Clapham Common, on the outskirts of London. He observed that placing as little as one teaspoon of oil on the surface calmed the ripples on a small area that quickly extended to about half an acre (2 × 103 m2).

Before we can describe the complex processes that occur at surfaces, it is important to describe exactly what we mean by this term.

1.2 Surfaces and interfaces

1.2.1 Introduction

Where two homogeneous bulk phases meet there is a region of fi nite thickness where the properties change, often markedly, as we move from one bulk phase to the other. Such regions are known as surfaces or interfaces (although the term interphase would be a better description). Although we may commonly think of a surface as being of negligible thickness, in fact, when we are discuss-ing phenomena at a molecular level the thickness of the interfacial region is signifi cant and defi nitely non-zero.

The properties of the interfacial region are particularly important when one of the phases is dispersed as many very small particles in the other phase, because of the dramatic increase in surface area. The two phases are usually referred to as the disperse (or dispersed) phase and the continuous phase. Examples include colloids, emulsions, aerosols, and some natural and syn-thetic polymers. Often the particles are below the resolution limit of the opti-cal microscope (<0.5 μm), but above the size range of all except the largest molecules. Table 1.1 illustrates how the surface area increases if we take a



1.2 SURFACES AND INTERFACES | 3

disperse phase of total volume 1 cm3 and subdivide it into smaller and smaller cubic particles.

Thus, for example, 1 cm3 of disperse phase divided into cubic particles with sides of 0.1 μm (within the size range of most colloids) has an interfacial area of 60 m2 and with sides of 10 nm the area is 600 m2.

1.2.2 Types of interface

Because there are three types of bulk phase, it is possible to classify interfaces based on the nature of the bulk phases that lie on either side of the interface. Thus, there are fi ve types of interface:

1 2

1 2

gas liquid G Lfluid interfaces

liquid1 liquid 2 L L

gas solid G S

liquid solid L S non-fluidorsolid interfaces

solid1 solid 2 S S

− − ⎫⎬− − ⎭

− − ⎫⎪− − ⎬⎪− − ⎭

Of course all gases mix with one another so there are no gas–gas interfaces. Usually with this notation the less dense phase will be shown fi rst: G–L rather than L–G, for example.

When three bulk phases meet in a line, this line is known as a triple

interface.

1.2.3 Defi ning the interfacial region

Taking an intensive property, such as density, and scanning its value from, say, a liquid phase through the interface to a gas phase would give a plot such as that in Figure 1.1.

In this case, the density shows a smooth transition from the high density of the liquid to the much lower density of the gas. The bulk phases can be separated from the interface by two surfaces parallel to one another, and positioned so that the bulk phases are homogeneous and uniform (uniform

Table 1.1 Effect of subdivision into cubic particles on the surface area of a disperse phase with a total volume of 1 cm3

Number of particles

Particle volume/m3 Length of cube edge/m

Total surface area/m2

1 10−6 10−2 0.0006

103 10−9 10−3 0.006

106 10−12 10−4 0.06

109 10−15 10−5 0.6

1012 10−18 10−6 6

1015 10−21 10−7 60

1018 10−24 10−8 600



4 | 1 INTRODUCTION

density in this case), while the inhomogeneity and non-uniformity are con-tained entirely within the interfacial region lying between the two surfaces. The dashed lines in Figure 1.1(c) illustrate this point.

We will see later that, for some properties, the transition from one bulk phase to another does not follow a smooth monotonic transition, such as that in Figure 1.1. For example, the concentrations of some solutes (particularly marked with the surfactants, see Section 4.6) at the gas–solution interface may reach values very much higher than those in either bulk phase and exhibit a profi le such as that in Figure 1.2.

The solute in Figure 1.2 is said to be adsorbed at the interface. This term will be defi ned and discussed in more detail in Chapter 3.

1.3 Stable interfaces

For any system at equilibrium, the free energy is at a minimum. If the system contains an interface, it is reasonable to expect that the interface would con-tribute to the free energy, and that this contribution would be a function of the area, A, of the interface. We might expect that this contribution would take the form:

G = γA + other terms. (1.1)

In this equation, the coeffi cient γ is known as the surface tension or interfa-

cial tension. If the system is stable it follows that γ must be positive, for if it were negative, an increase in area would lead to a lowering of the free energy and, therefore, the surface area would spontaneously expand. This would ultimately lead to the dissolving of one substance in the other. Figure 1.3 illustrates how this process might occur for two liquids. In fact, the opposite occurs for immiscible liquids, suggesting that for a stable interface γ must be positive and that at equilibrium the interfacial area will tend to a minimum in order to minimize the free energy. This process provides the driving force for many of the phenomena that will be discussed in following chapters.

Equation (1.1) also indicates that processes that lower the value of the interfacial tension would also be thermodynamically favoured. This is, then,

Fig. 1.1 Density profi le across the gas–liquid interface. The graph in (b) corresponds to the sample (a) and in (c) it has been rearranged into a more customary orientation. Dashed lines in (c) indicate the interfacial region.

Gas

Liquid

(a)

Density

Hei

ght

(b)

Solution Interface Gas

Height

Den

sity

(c)

Fig. 1.2 Concentration profi le for a solute at the gas–solution interface.

Sol

ute

conc

entr

atio

n

Solution Interface Gas

Height



1.4 KEY CONCEPTS | 5

a second force capable of driving interfacial processes. The phenomenon of adsorption is an important example.

1.4 Key concepts

Although the topic of interfacial science is vast, we have attempted to intro-duce the crucial areas in the subsequent chapters. In this section, we give a broad overview of the fi eld.

A dictionary of the terms used in interfacial science has been developed by Schramm (2001).

1.4.1 Surface tension and surface pressure

Probably, the most important concept of all, particularly when dealing with interfaces in which both bulk phases are fl uids, is that of surface tension. The existence of surface tension, and the effects that arise as a consequence, play a major role in the behaviour of systems containing interfaces. Because surface tension represents extra energy, which is proportional to the area, systems attempt to minimize their surface area, resulting in the familiar fact that drops of liquid in air and bubbles are spherical. Many effects are more subtle, how-ever, and are only evident if the interface is curved.

Surface pressure is frequently used in situations where the surface tension has been changed by events such as adsorption (see Section 1.4.3 and Chapter 3) or the addition of an insoluble monolayer to the surface (Chapter 5). It is defi ned as the reduction in surface tension arising from the event:

Π = γo + γf (1.2)

where γo is the original surface tension of the liquid, and γf is the surface ten-sion after the event. It follows that as the surface tension is normally reduced by adsorption, surface pressure typically increases from zero to a positive value.

1.4.2 Wetting

The shapes of liquid droplets and the wetting of solid surfaces are determined principally by the forces acting at the relevant interfaces. These forces deter-mine, for example, whether a liquid drop will spread over a solid surface or roll up into a ball, whether liquid will rise up the narrow gaps between the fi bres of a wick, whether water will penetrate through the weave of an umbrella cloth and contribute to the rise of sap in the stems of plants.

We are all familiar with the non-stick properties of Tefl onTM, which benefi ts from the tendency of liquids not to wet the surface, and fabrics designed to be non-wetting have the potential to be self-cleaning and never need washing!

1.4.3 Adsorption

A major consideration of interfaces is that the two bulk phases normally have quite different properties, and frequently materials that are soluble in one or

Fig. 1.3 The spontaneous increase in surface area that would occur between two liquids if the surface tension were negative. This does not occur for immiscible liquids!

Liquid A

Liquid B



6 | 1 INTRODUCTION

both phases will fi nd it energetically favourable to concentrate (or deplete) at the interface. The main example is the class of materials called surfactants, which are molecules that are designed to have a part that prefers one phase and another part that prefers the other phase. Such materials tend to concen-trate at the interface and are said to adsorb. This is crucial to a number of processes, such as detergency, in which oils can be dispersed in water when without detergent they are very insoluble. Such adsorbed layers are one mol-ecule thick giving a thickness of 1–3 nm. They could, therefore, be described as nanofi lms.

Adsorption is not restricted to fl uid interfaces: it occurs at all interface types. For example, gases adsorb on to solids, a fact which is exploited in the routine measurement of the surface areas of powders, and is an essential part in the catalysis of many gas phase reactions. For example, the reactions that occur in the catalytic converters of nearly all cars. Such catalysis is also essential in many aspects of the chemical industry.

Adsorption also leads to methods for the fabrication of thin fi lms, as thin as a single molecular layer. Methods exist which begin with a single layer at the air–water interface to build up, by the Langmuir–Blodgett technique, multilayer fi lms, one layer at a time (see Section 5.4). Other methods use the ability of molecules to adsorb and self-assemble at the interface between a solution and a solid surface, forming self-assembled monolayers and multi-layers (see Section 9.13). Monolayers on an air–water interface can also be used to retard the evaporation of water, which is a major concern with open water-storage dams in dry climates (see Section 5.9.1).

1.4.4 Emulsions

Emulsions, defi ned loosely as small droplets of one immiscible liquid in another, are very common, although frequently we might not recognize their presence. Common household products in which emulsions are frequently present are foods, paints, and cosmetics. To take paints as an example, these days most are water based. The water acts as a dispersion medium for an emulsion of polymer particles. After application, the water evaporates and the polymer particles coalesce to form a protective fi lm.

Because emulsions are inherently unstable, preferring to minimize the sur-face area of the interface by individual droplets merging together, there is an enormous amount of science in the stabilization of emulsions in order to improve the effectiveness and marketability of products. This fi eld is closely related to adsorption and surfactants, as the stabilization of emulsions relies on the use of surfactants in nearly all cases (see Chapter 6).

1.4.5 Colloids

Colloids are everywhere: in the ground on which we walk, in the air we breathe, in the food we eat, in the liquids we drink, and in many parts of our bodies. The term colloid refers to certain heterogeneous systems and the essen-tial aspect is size. The International Union of Pure and Applied Chemistry (IUPAC) (Everett, 1972) states that particles of the dispersed material must



1.5 ORGANIZATION OF THE BOOK | 7

have at least one dimension in the approximate range 1 nm to 1 μm or that there are discontinuities in the system in that range. Only one dimension need be in the range, so fi bres and thin fi lms may be classifi ed as colloidal. Also the units need not be discrete, so networks such as foams may be regarded as col-loidal provided that the foam lamellae fi t the size range.

A colloidal dispersion (often simply called a colloid) consists of discrete particles of colloidal size (the disperse or dispersed phase) dispersed in a con-tinuous phase. The particles may be solid, liquid, or gas, and must all have the same values of their intensive properties. The term, dispersed phase, implies that the material in the particles has the same properties as the bulk phase of the same composition. This is not always the case as the particle surface will sometimes cause major modifi cations. Many colloidal dispersions are disper-sions of nanoparticles.

Colloids are very common in industry, household products, and foods (Dickinson, 1972), and even particulates in air pollution represent a colloidal dispersion. The major issue with colloids is their tendency to aggregate and achieving stability of the dispersion is usually the goal in the formulation of products involving colloidal components. This requires a detailed under-standing of the charged nature of the interface, the effects of added salts, and other conditions.

1.4.6 Membranes

No biological organisms as we know them would exist without cell mem-branes that separate two liquid phases (usually aqueous) that would otherwise mix. The other function of membranes is to allow the controlled movement of molecules into and out of cells, selective permeability, which is a remarkably complex task achieved by equally remarkable materials. These are primarily molecules known as lipids and special proteins called membrane proteins.

Knowledge gained from the study of biological membranes is increasingly being used to design better drug delivery systems.

1.5 Organization of the book

The book opens with three general chapters in which the basic concepts of interfacial science are introduced and discussed, and some important equa-tions developed. We then go on to consider the various types of interface as shown above in Section 1.2.2. Fluid interfaces are free of the complications that come from the surfaces of solids so these are considered fi rst.

However, there are two notable and important omissions from the treat-ment. Interfacial science is heavily involved in electrochemistry, but the topic is mentioned only briefl y because there are numerous excellent texts on this subject. The solid–solid interface provides the foundation of modern electron-ics and we did not consider that it would be possible to treat adequately the immense amount of material now available on this topic in a short chapter.

Finally, in the last chapter we discuss some of the applications of interfa-cial science in biological systems. This is also a very large topic so we have



8 | 1 INTRODUCTION

concentrated on a few aspects where the relevance of the earlier chapters can be demonstrated.

The literature of interfacial science

There is an extensive literature dealing with surface chemistry and its various manifes-tations. This ranges from reports from Roman times of calming waves by the spreading of oil; through Benjamin Franklin’s experiments (1774) with monolayers on a pond in London; Faraday’s (1857) preparation and study of gold sols; the development of the fi rst fi lm balance by Agnes Pockels (1891) in the kitchen of her parents’ home; the development of a theory to describe the adsorption of gases on solids by Brunauer, Emmett, and Teller (1938); the formulation of a theory for the stability of lyophobic colloids by Deryaguin and Landau (1941) and Verwey and Overbeek (1948); cloud-seeding experiments by Langmuir and Schaefer (1946) and by Vonnegut (1946); and the experiments of Mansfi eld and Vines (1955–1962) on the retardation of water evaporation from large storages; to the many research and review papers currently being published in international journals of high repute.

A large number of books have been published on various aspects of interfacial sci-ence. Many of these will be found in the Further reading sections near the end of each chapter of this book. Also listed under this heading are relevant review articles, mostly from one of the review journals or series devoted to our topic. These lists are not exhaustive so it would always be worthwhile to browse nearby library shelves.

Original research is generally published in specialist journals, while there are also a small number of papers in more general journals. The titles of these journals can be seen in the references at the end of each chapter.

Chapter references

In each chapter the references are shown in the text by author and year of publication, for example, Peng et al. (2001). A book reference may also include a chapter or page number. At the end of the chapter the references to books and major review articles are listed alphabetically under ‘Further reading’ followed by other references under ‘References’.

No attempt has been made to provide an exhaustive list of references. Key refer-ences are given, and also references to further data or more detailed information, or to provide support for a possibly contentious argument.

Dickinson, E. (1992). An Introduction to Food Colloids. Oxford University Press, Oxford.

Everett, D.H. (1972). Defi nitions, Terminology and Symbols in Colloid and Surface Chemistry: Part I. Pure and Applied Chemistry, 31, 579–638.

Schramm. L.L. (2001). Dictionary of Colloid and Interface Science. Wiley-Interscience, New York.

FURTHER READING

REFERENCES



1.5 ORGANIZATION OF THE BOOK | 9

1.1 Find and describe other examples where an interface makes an important contribution to the behaviour of a system. [Could be a class discussion]

1.2 List the colloidal systems that can be found in and around the home and state why they can be considered as colloidal. [Could be a class discussion]

EXERCISES



Documents

Cap 1 Barnes