6. 4. Properties of surface chemistry. Unit 6: Physical chemistry of spectroscopy, surfaces and chemical and phase equilibria

Transcription

1 6. 4 Properties of surface chemistry Catalysis is of critical importance to modern chemistry. For many large-scale industrial reactions, such as the aber or Contact process, the catalyst and the reacting substances are in different states and hence the reaction occurs at the surface of the catalyst. In this topic guide you will look at the nature and properties of such surfaces and the way in which the reacting gas molecules interact with these surfaces. You will also become familiar with two other key interfaces that between the components of colloidal systems and that between solids and liquids. These interfaces are important in the food, cosmetic and oil industries. On successful completion of this topic you will: understand the properties of surface chemistry (LO4). To achieve a Pass in this unit you need to show that you can: explain the solid-gas interface (4.1) explain the nature and properties of surface active agents (4.2) discuss features of the solid-liquid interface (4.3) discuss features of colloidal systems (4.4). 1

2 1 The solid-gas interface Many important reactions in industry involve the reaction of two gaseous molecules. A very simple example would be the hydrogenation of ethene: 6.4.1: The industrial hydrogenation of ethene. This occurs in the presence of a catalyst such as palladium or platinum. C C C C When hydrogenation of unsaturated hydrocarbon chains is carried out in industry, similar catalysts are used. The key to this reaction, as with other examples of heterogeneous catalysis, is the way in which the molecules interact with the surface. Case study: ydrogenation of unsaturated oils Margarine is a food product widely used as a cheaper and (allegedly) healthier alternative to butter. It is made by hydrogenating vegetable oils, which are triesters of glycerol, and unsaturated fatty acids, such as oleic acid or linoleic acid. The presence of these unsaturated fatty acids lowers the melting point of these oils, which explains why they are liquids at room temperature. By hydrogenating some of the double bonds in the fatty acid chains, the melting point of the oil is raised sufficiently to make them solid (like butter), and this solid product is known as margarine. Over two million tonnes of margarine are produced annually, using a process in which vegetable oils are passed over a finely divided nickel catalyst at 430 K. owever, this hydrogenation process converts any remaining C=C bonds into the trans-geometric isomer, and the resulting trans fats have been linked with raised cholesterol levels. Key terms eterogeneous catalysis: A reaction in which a catalyst is used and in which the catalyst is in a different state (or phase) from the reactants. Isotherm: An equation or graph showing the relationship between two variables at a constant temperature for example, surface coverage at different pressures or concentrations. Adsorption A solid surface exposed to a gas is being continually bombarded with gas molecules. These gas molecules will tend to become attached to the surface and form a layer covering the surface, a process described as adsorption. If the attachment is a result of the formation of weak forces, such as Van der Waals forces, this is described as physical adsorption. If chemical bonds (usually covalent) are formed between the gas molecule and the surface, then this is described as chemisorption. Isotherms The extent of the coverage of the surface depends on the pressure of the free gas above it. Generally, the higher the pressure, the greater the extent of the coverage. To maximise the rate of catalysis, the extent of coverage of the surface by reacting gas molecules must be maximised. To this end, chemical engineers use equations such as the Langmuir or BET isotherm to calculate percentage coverage at different conditions of pressure. Langmuir isotherm This is the simplest isotherm. It is derived by considering the adsorption process as a dynamic equilibrium involving gas molecules, A, and a solid surface, M, which consists of a fixed number of adsorption sites: 2

3 Activity For an adsorption process with K = 3, calculate a value for Ɵ for the following partial pressures of the gas being adsorbed: (a) 0.2 bar (b) 0.5 bar (c) 1 bar (d) 2 bar (e) 3 bar. Use the results of these calculations to plot a graph of Ɵ against partial pressure, p. A(g) M(s) AM(s) The rate constant for the forward reaction (adsorption) is k a, and for the backward reaction (desorption) is k d. The equilibrium constant, K, for this system is therefore given by: (1) K = k a k d If various assumptions are made, this leads to the Langmuir isotherm, (2), for the fractional coverage, Ɵ, of a gas at a partial pressure, p: K. p (2) Ɵ = 1 K. p Derivation and assumptions The Langmuir isotherm makes three assumptions: 1 adsorbed molecules form a monolayer (a layer of molecules on the surface only one molecule thick) 2 all adsorption sites are identical 3 there is no interaction between adsorbed molecules. Consider a surface with a total of N adsorption sites. The rate of adsorption, r a, will then be proportional to the partial pressure of the gas and the number of sites not already occupied: r a = k a N(1 Ɵ)p The rate of desorption is proportional to the number of gas molecules already adsorbed: r d = k d NƟ At equilibrium: k a N(1 Ɵ)p = k d NƟ So, cancelling N and rearranging we obtain: k a k d Replacing k a k d (1 Ɵ)p = Ɵ by K and rearranging we obtain Ɵ = K. p 1 K. p. When Ɵ is plotted against pressure for various values of K, graphs such as the ones in Figure are obtained. Two different curves are plotted for two very different values of K. Figure 6.4.2: The Langmuir isotherm predicts how the coverage of a surface depends on the partial pressure of the gas being adsorbed. 1.0 K = 10 K = Partial pressure, p, of adsorbed gas 3

4 Activity Toxic and unpleasant gases such as ammonia can be adsorbed by using activated charcoal. The charcoal may not be effective at high ammonia concentrations because too much of the surface will be covered by adsorbed ammonia molecules. N 3 (g) charcoal(s) charcoal N 3 (s) The equilibrium constant for this process at 0 C is 7.5 bar 1. Calculate the partial pressure of ammonia at which the charcoal surface will be half covered by ammonia molecules (Ɵ = 0.5). Take it further Elements of Physical Chemistry (Atkins and de Paula, 2009) has derivations of the equations referred to in the Case study and provides some worked examples to explain how they are used. Case study: Calculations from the Langmuir isotherm For a monolayer surface, the volume of gas adsorbed will be proportional to the fractional coverage. ence a plot of volume of gas adsorbed against pressure of gas will produce a curve with the same shape as in Figure The monolayer adsorption capacity of solids, V mon (the maximum volume that can be adsorbed by the solid) can therefore be estimated from experimental data of the volume of gas, V, adsorbed at different pressures. Plotting p V against p gives a straight line with slope 1. V mon Another useful way in which the Langmuir isotherm can be used is to relate the equilibrium constant K to the enthalpy of adsorption, Δ adsɵ. Plotting ln p against 1 T produces a straight line with slope Δ Ɵ ads. R Limitations of the Langmuir isotherm For many gases the Langmuir isotherm describes the adsorption of a gas reliably. It can be adapted to take account of dissociation of gas molecules on the surface or for the situation where two gases are present and competing for the same adsorption sites. It is most likely to remain accurate even at high pressures where chemisorption occurs between the surface at the gas molecules. owever, particularly when physical adsorption is involved, further molecules of gas can bind to the molecules already adsorbed and hence Ɵ, the fractional coverage, can take a value of >1 at high pressures. This also means that the volume of gas adsorbed can be greater than the monolayer adsorption capacity, V mon. Figure shows how this situation is described by an alternative isotherm, the BET isotherm. 4

5 Figure 6.4.3: At high values of partial pressure, the Langmuir isotherm may no longer be valid and the BET isotherm is a more accurate description of adsorption. Volume of gas adsorbed, V BET V mon Langmuir p* Partial pressure, p The BET isotherm The main assumptions used to derive the BET isotherm concern the enthalpy change when gas molecules bind to the molecules already bound to the surface: the enthalpy of adsorption is due to Van der Waals forces there is no interaction between molecules in the plane of the surface the enthalpy of adsorption of molecules onto the monolayer is equal to the enthalpy change of condensation of the gas. Take it further: The equation for the BET isotherm Using the assumptions above, an equation can be derived for the ratio of the adsorbed volume of gas, V, to the monolayer adsorption capacity, V mon V V mon = cx (1 x)[1 (1 c)x] Where: c is the ratio of the equilibrium constant for adsorption to the surface of the solid (K 0 ) compared to the equilibrium constant for adsorption to the monolayer (K 1 ); c = K 0 K 1 x is the ratio of the pressure of the gas, p, compared to the vapour pressure of the gas when sufficient has adsorbed that it can be considered to act as the surface of a liquid (p*); x = p p* Mechanisms of heterogeneous catalysis Knowledge of the way in which surfaces interact with molecules enables chemists to understand the mechanism of processes involving heterogeneous catalysts. ydrogenation of alkenes The example of the hydrogenation of alkenes mentioned at the start of this section provides a good example. Figure illustrates the mechanism of this reaction: 5

6 1 both the alkene and the hydrogen molecule chemisorb to the surface 2 the chemisorption of hydrogen results in dissociation of the hydrogen molecule 3 the chemisorption of the alkene results in the breaking of the π bond between the carbon atoms; new C bonds form, creating an alkane molecule 4 the alkane desorbs from the catalyst surface. Figure 6.4.4: The mechanism of the hydrogenation reaction of alkenes C C C C C C C C Although the hydrogenation of ethene is not industrially important, the hydrogenation of other molecules containing alkene groups is used in the production of hydrogenated fats (margarine) (see the Case study on page 2). Portfolio activity (4.1) Research an industrially important gas-phase reaction that uses heterogeneous catalysis. Research the conditions used and mechanism of the reaction and write a report about the reaction using ideas about the solid-gas interface. Suitable examples could include the aber process or the reaction between sulfur dioxide and oxygen (the Contact process). In your description you should: identify whether the adsorption involved is chemisorption or physical adsorption, distinguishing between these processes use the Langmuir and BET isotherms to describe how the volume of the gases adsorbed onto the surface of the catalyst would change with pressure, discussing the assumptions made in each case and considering which isotherm is likely to be more appropriate at the conditions used sketch out a suggested mechanism for the process. 2 Surface activity of liquids A liquid can form surface interfaces not only with solids and gases, but also with another liquid with which it is immiscible. Surface tension The effects of surface tension will be familiar to anyone who has looked closely at a droplet of water on a waxed surface (see Figure 6.4.5) or at the meniscus that forms when water is contained in a capillary tube. 6

7 Figure 6.4.5: Surface tension causes this phenomenon to occur in water and other liquids. Surface tension at the surface of a drop of water is a result of the imbalance of intermolecular forces on molecules at the surface. These have a net pull inwards, away from the surface, as shown in Figure Figure 6.4.6: The net inward forces on molecules at the surface of a liquid are responsible for the formation of spherical droplets. As a result, molecules at the surface have a higher potential energy than molecules in the bulk of the liquid. To minimise these forces, the liquid tends to take up a spherical shape because this minimises the surface area of the droplet. Molecules that have much weaker intermolecular forces do not behave in the same way a drop of petrol placed on a surface is more likely to spread out to form a thin film than to form spherical droplets. Interfacial tension Interfacial tension is a special case of surface tension and it occurs at the interface of two immiscible liquids. As in the example of the water droplet, to reduce the interfacial energy to a minimum, the surface area of the interface will be as low as possible, which, in this case, means that the two liquids will form separate layers, as shown in Figure Figure 6.4.7: Two immiscible liquids form separate layers in order to reduce the surface area at the interface. minimum surface area of interface If the liquids are shaken then, although initially one of the layers will break up into spherical droplets, the two layer structure will soon reform. Key terms ydrophobic: Substances that do not form forces of attraction to water molecules (and are therefore repelled by water molecules). ydrophilic: Substances that can form forces of attraction to water molecules. Surfactants Surfactants (a contraction of surface active agents ) are substances that reduce surface tension. Most of the applications of surfactants occur in aqueous systems, which will be discussed below. Structure of surfactants This property is associated with a specific type of molecular structure in which the molecule has two distinct ends, one consisting of a hydrophobic group (usually a long hydrocarbon chain) and the other a hydrophilic group: 7

8 Figure 6.4.8: The hydrophilic and hydrophobic groups are responsible for the surfactant properties of a molecule. hydrophobic hydrophilic Such surfactants can be of three types, depending on the type of group present at the hydrophilic end (see Table 6.4.1). Table 6.4.1: The three different types of surfactant. Type Name Structure anionic (negatively-charged group at hydrophilic end) alkyl benzene sulfonate O SO 3 cationic (positively-charged group at hydrophilic end) quaternary ammonium salt C 3 N C 3 C 3 non-ionic (polar group at hydrophilic end) alcohol ethoxylate O O C C Key terms Counterion: The ion that accompanies a charged species in order to achieve electrical neutrality. Micelle: Structures of submicroscopic size formed when surfactants cluster together in a liquid medium, usually water. The exterior surface of the micelle is formed from the hydrophilic groups of the surfactant molecule. Figure 6.4.9: The formation of micelles in a polar liquid system. Anionic and cationic surfactants are added to water in the form of neutral salts, thus counterions will also be present (usually Na in the case of an anionic surfactant and Cl or SO 4 2 in the case of a cationic surfactant). It has been shown that using different counterions can alter the overall surfactant properties. Mechanism of surfactant action Micelle formation When surfactants are present in liquids in sufficiently high concentration (called the critical micelle concentration, CMC), they tend to cluster together in small structures called micelles, as shown in Figure At low concentrations, surfactant molecules tend to occupy sites at the air-water interface, oriented in such a way that the hydrophobic groups of the surfactant molecule now form the actual interface. There is now very little attraction between the groups at the actual interface so surface tension is reduced. At higher concentrations of surfactant, spherical micelles form in the body of the liquid. The exterior surface of the micelle consists of the hydrophilic groups of the surfactant molecules while the hydrophobic groups align themselves in the interior of the micelle. 8

9 Once micelles are present in a system, they can help to solubilise non-polar substances, such as fats and oils. The non-polar substance becomes incorporated into the centre of the micelle (which is, of course, non-polar itself). This creates a stable emulsion. Key terms Solubilisation: The process by which two immiscible liquids form an emulsion by the action of a surfactant. Emulsion: A type of colloid in which one liquid is dispersed in another. Reverse micelles: Micelles formed when surfactants are present in a non-polar medium. The exterior surface of a reverse micelle is formed from the hydrophobic groups of the surfactant molecule. Detergent: A substance that has the ability to lift soil (dirt and grease) from a surface. Of course, if the liquid system was a non-polar solvent rather than water, the alignment of the surfactant molecules would be reversed, with the hydrophobic groups forming the micelle surface. Such micelles are known as reverse micelles however, these can only be formed if the surfactant is non-ionic as the repulsions between ionic groups in the centre of the micelle would create instability. Activity Sketch out a diagram, similar to the one in Figure 6.4.9, to show the formation of reverse micelles in a non-polar liquid system. Suggest the structure of a surfactant suitable for forming these reverse micelles. Wetting of surfaces As noted above, the high surface tension of water means that when water is placed on a non-polar solid surface it will tend to form a drop that is almost spherical in shape. The presence of surfactants, by reducing surface tension, causes the drop to spread out and wet the surface. The contact angle between the solid surface and the air-water interface is decreased (see Figure ). Figure : The wetting of a surface depends on the contact angle, α. A contact angle of 180 would mean that no wetting of the surface would occur. good wetting ( < 90 ) partial wetting ( > 90 ) water soil (oil or grease) fabric Figure : The roll-up mechanism. Applications of surfactants Detergency The most significant application of surfactants follows from their ability to act as detergents to possess the property of detergency. There are two main ways in which detergents operate, the roll-up mechanism and the solubilisation of the soil. Most detergents operate by a combination of the two mechanisms. Figure summarises the roll-up mechanism. By reducing the interactions between the soil surface and the water, the contact angle is increased and the soil rolls up into a spherical micelle that is released. The surfactant reduces the surface tension at the interface with the water and with the fabric, causing the contact angle to decrease and the soil droplet to roll-up into a sphere. This eventually detaches itself from the fabric surface. The process is also enhanced by the wetting effect of the surfactant, allowing the cleaning liquid to spread over a greater surface area of the surface. 9

10 Figure : Solubilisation if the surface of the micelle is charged, it can interact with polar solvent molecules, which will result in the dissolving of the micelles. Figure summarises the mechanism of solubilisation. The interaction between the charged surface of the micelle and polar water molecules causes the soil micelles to be soluble. Formulation chemist (detergents) I liaise with clients to produce a design brief for specialised detergent products. Each product is designed for the specific cleaning needs of the client removing ingrained dirt from concrete, destaining metal or cleaning out fuel oil tanks. Each product will contain suitable surfactants that lift off the soil, grease or oil as micelles, as well as alkalis or buffers to regulate the p, and building agents such as silicates which enhance the performance of the surfactants. Different environments will require different combinations of these basic materials for effective cleaning and to minimise the possibility of damage to the surface. I will discuss the type of surface to be cleaned, and the conditions under which cleaning will happen (for example, the temperature of the cleaning solution). This information gets passed back to the team who will develop the product for the client. Case study: Micelles in the cosmetic industry Surfactants are the key ingredient in many cosmetics. They are used in products that act as, for example, cleansers and conditioners. They are also essential for the formulation of many other products that are used as emulsions or solutions. Cleansers work by using surfactants to remove oil or grease from skin and hair by forming micelles which can then be washed clear of the skin using water. Conditioners are deposited on the surface of hair and skin and remain there, enhancing the feel and texture of the hair. air conditioners can specifically attach themselves to damaged hair because it frequently carries a positive charge. Suggest what type of surfactant molecule could be used as a hair conditioner. Take it further More information about the use of surfactants in the cosmetic industry can be found at: Case study: Micelles in the oil industry Crude oil (petroleum) will often adhere to surfaces and the removal of crude oil using surfactants is therefore important in cleaning up after accidental release of crude oil into the environment. It also helps in the enhanced recovery of crude oil from the porous rocks in which it is found. Ionic surfactants are often used as they will adhere well to the charged surfaces of many minerals. Suggest what type of surfactant molecule could be used in petroleum recovery from porous rocks with a positively-charged surface. 10

11 Portfolio activity (4.2) Explain the behaviour and uses of surfactants. In your answer you should: give the structure of a substance used as a surfactant, for example, an alkyl benzene sulfonate detergent molecule describe the features of surfactant behaviour explain how the structure enables it to act as a surfactant describe one way in which the molecule you have chosen can be used. Charged interfaces Micelles formed from anionic or cationic surfactants are arranged so that the charged groups are at the surface of the micelle. owever, the micelle is electrically neutral, so this charged surface will attract oppositely-charged ions usually the counterions of the surfactant compound, creating an electrical double layer. Electrical double layer The electrical double layer model is used to describe a range of situations in which a charged surface is in contact with an ionic environment: micelles formed from ionic surfactants metal electrodes (in contact with electrolytes) insulators with charged groups at the surface in contact with ionic solutions. Figure : In the elmholtz model, the electrical potential, φ, falls linearly as the distance away from the surface increases. elmholtz model The simplest model of the electrical double layer is the elmholtz model. In this model, the charged surface attracts a single layer of ions with the opposite charge to that of the surface (these will be the counterions from the surfactant in the case of micelles). This model is shown in Figure φ surface surface solution φ solution The electrical potential falls linearly to that of the bulk solution across this layer. Gouy-Chapman model A more sophisticated model, the Gouy-Chapman model, imagines that the oppositely-charged ions are not rigidly held on the surface but form a diffuse cloud of charge, with the concentration of these ions (and the electrical potential) decreasing exponentially away from the surface, as shown in Figure

12 Figure : In the Gouy- Chapman model, the electrical potential falls exponentially with distance from the surface. φ surface surface solution φ solution Stern model This model most accurately reflects the reality of the electric double layer, especially for highly charged surfaces. It suggests that the layer of ions at the surface resembles the layer of the elmholtz model but that further away from the surface there is a diffuse charge cloud, as in the Gouy-Chapman model. The inner, rigid layer of ions is known as the Stern layer, the outer diffuse layer as the Gouy-Chapman layer. The electrical potential falls linearly in the Stern layer and exponentially in the Gouy-Chapman layer, as shown in Figure Figure : The Stern model combines features of the elmholtz and Gouy-Chapman models. φ surface surface solution φ solution Portfolio activity (4.3) Describe the models that exist to describe the surface liquid interfaces. For each model you should detail: the main features in the structure of the model the observable characteristics of the model. Take it further Stern layer Gouy-Chapman layer Surfactants and the micelles formed by surfactants have significant roles in the solubilisation of drugs. A sample chapter from a textbook on the physicochemical principles of pharmacy is available at: and gives, among other detailed discussion of surface activity, an excellent description of how electric double layer models are applied to micelles. 3 Colloidal systems You saw an example of a colloidal system in Section 2 of this topic guide when the emulsion formed by micelles in water was discussed. In this section you will learn more about different types of such colloidal systems and their properties and applications. 12

13 Activity Other terms used to describe specific types of colloidal dispersion include: sol, gel and aerosol. Carry out research to identify the dispersion phase and dispersion medium in each of these cases, and give specific examples of each type of colloid. Key terms Association colloids: Systems in which the dispersed phase consists of clusters of molecules that have lyophobic and lyophilic parts. Lyophobic: A colloid in which there is little attraction between the dispersed phase and the dispersion medium. Lyophilic: A colloid in which the dispersed phase is attracted to the dispersion medium. Colloidal dispersions In a colloidal system (or colloid), fine particles (between 1 and 1000 nm in diameter) are dispersed evenly through another substance. The dispersed particles are known as the dispersed phase and the substance through which they are spread is known as the dispersion medium. In an emulsion, for example, the dispersed phase and dispersion medium are both liquids; in a foam the dispersed phase is a gas and the dispersion medium is a liquid. Special cases of colloidal systems Association colloid The emulsion formed by micelles has already been mentioned as an example of a colloid. Micelles are examples of association colloids, because the surfactants that form the micelles contain hydrophobic and hydrophilic groups. The hydrophobic group can also be described as lyophobic and the hydrophilic group as lyophilic so the micelles fit the definition of association colloids. Properties of colloidal systems Regardless of the type of colloidal system, several characteristic properties of colloidal systems can be readily observed. Two of the most significant and easily observed are optical effects: colloidal systems display turbidity; in other words, the system appears cloudy or hazy due to the scattering of light by the dispersed particles a related property is the Tyndall effect; if a beam of light is shone through a colloid it becomes visible laser displays at concerts make use of this effect when the laser beam becomes visible in the stage smoke. Factors affecting the properties of colloidal systems The scattering of light is affected by the particle size of the dispersed phase; most efficient scattering occurs when the particle size is approximately the same as the wavelength of light being scattered. Portfolio activity (4.4) Choose two examples of colloidal systems, including an association colloid and a colloidal dispersion. Suitable examples could be the micelles formed by surfactants in cosmetics or an aerosol such as smoke from a fire. Describe some of the observable properties of these systems and explain the factors that affect these properties. 13

14 Checklist At the end of this topic guide you should be familiar with the following ideas: the interactions of gas molecules with a solid surface are important in catalysis and may involve strong covalent bonds or weaker Van der Waals interactions the Langmuir and BET isotherms describe how the volume of gas molecules adsorbed by a surface depends on the pressure of the gas intermolecular forces in a liquid create surface tension and play an important part in determining the nature of liquid-surface and liquid-liquid interfaces the presence of surfactants causes the formation of micelles within a liquid-liquid system; they have important applications as detergents and emulsifiers if charged particles are in contact with a surface, a range of models exist to show the way in which the charged particles will arrange themselves close to the surface colloids are substances in which small particles are dispersed through a dispersion medium; a range of colloids exist depending on the nature of the dispersed phase and the dispersion medium colloids may differ in their stability but share characteristic properties such as turbidity. Further reading Elements of Physical Chemistry (Atkins and de Paula, 2009) covers solid surfaces in Chapter 18, including derivations of the Langmuir and BET isotherms and their application. Books published in the field of surface and colloid science are often written at a very high level and may not be appropriate as further reading for this course. owever, An Introduction to Interfaces and Colloids: The Bridge to Nanoscience (Berg, 2009) is more accessible than some, and includes sections on surfactants, micelles and colloid properties. Acknowledgements The publisher would like to thank the following for their kind permission to reproduce their photographs: Corbis: David Sutherland; Fotolia.com: ANK 7 All other images Pearson Education Every effort has been made to trace the copyright holders and we apologise in advance for any unintentional omissions. We would be pleased to insert the appropriate acknowledgement in any subsequent edition of this publication. About the author David Goodfellow studied Natural Sciences at Cambridge and spent 20 years teaching A-level Chemistry in a sixth-form college. e was lead developer for the OCR AS Science in 2008 and for several years was chief examiner for the course. e now works as a freelance writer and examiner alongside part-time work as a teacher. Publications include a textbook for the AS Science course, teaching materials to accompany Chemistry GCSE courses and contributions to textbooks for BTEC First Applied Science. 14