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THE OTHER STATES OF MATTER
10 THE OTHER STATES OF MATTER The forces between atoms, molecules and ions have been shown to lead to certain types of structure in many solids—structures based on units which repeat indefinitely in three dimensions. The principles which have emerged are equally applicable to the other states of matter, even although the regularity of the crystalline state is then lacking.
T H E G A S E O U S STATE F r o m a structural point of view there is little to say about the gaseous state. Bonding and forces within isolated molecules have been fully discussed in previous chapters. The starting-point of the kinetic theory of gases is the perfect or ideal gas, with no forces between its molecules. Such a gas would never condense, so that for this reason alone there must be forces between the molecules of real gases. The nature of these forces has been dis cussed in connection with molecular crystals (p. 214). As far as the gaseous state itself is concerned, their importance is the way in which they determine the P - V - T relationships of real gases.
L I Q U I D S A N D GLASSES The Liquid
State
The liquid state has often been regarded as more akin to the gaseous than to the solid state. Liquids and gases, for instance, are collectively referred to as "fluids", for both have the property of
254
The Other States of Matter
255
flow, taking up the shape of the containing vessel. At first sight, solids seem clearly marked off from liquids and gases through their definite crystalline form; from some physico-chemical points of view also, there is a clearer dividing line between solids and liquids than between liquids and gases. Thus, all distinction between liquid and vapour disappears above the critical point, but efforts to establish the existence of a critical point for solids and liquids have so far failed. On the other hand, liquids are undeniably closer to solids than to gases in some physical properties. Density is an obvious example; the volume increase on fusion seldom exceeds about 10 per cent, showing that the molecules of a liquid are not much further apart than those of the solid. F o r a number of substances the volume actually decreases on fusion, showing that the atoms or molecules must then be more closely-packed in the liquid than in the solid. The compressibility of a liquid is much closer to that of a solid than to the very large compressibility of a gas, and the specific heat of a liquid just above the melting point, is usually not much different from that of the solid just below the melting point. The results of X-ray diffraction studies on liquids, although difficult to interpret, have shown that in many cases a liquid has a definite structure. When the molecules are approximately spherical this structure is only rudimentary, but in other cases it may be more pronounced. Whatever the degree of structure thus revealed, it is only an average one. The molecules of a liquid have complete translational freedom, so that the overall pattern is continually changing. In each instantaneous arrangement, how ever, the average environment of a particular molecule is probably not very different from what it is in the corresponding crystal. When, for instance, a close-packed metal or rare gas melts, each " s n a p s h o t " picture of the liquid would reveal a situation in which each molecule has perhaps ten, eleven or twelve nearest neigh bours. But a snapshot taken the next instant, would reveal a completely different general picture, though the same types of short-distance groupings would be seen (see Fig. 76).
256
Chemical Binding and Structure
Of course, with a small volume change on melting, the packing of the atoms or molecules in the liquid cannot differ much from that in the solid, so perhaps the above result is only to be expected. Sometimes, though, X-ray diffraction has shown the existence of a very definite degree of structure in the liquid. The best example is water. Ice has the oxygen atoms of the water molecules held by hydrogen bonds in a tetrahedral arrangement, rather like diamond. O
n
° ° 0
°°°0°
o ° n o o o o ° ° no 0 °°o°o o° FIG. 7 6 . Short-range order in liquids.
On fusion, appreciable regions of this structure persist, so much so that water just above the melting point is denser than ice just below. Many of the properties of liquid water are due to this high degree of structure, but the pattern constantly changes, as with all liquids. Between the extremes of a liquid metal and water comes a whole range of intermediate cases. Whatever the details, it remains true that while in a liquid there may be a considerable degree of shortrange order, there is no long-range order. That is, there is n o definite relationship between the individual small ordered regions which are, of course, many orders of magnitude smaller than the crystallites of a poly-crystalline solid. If it is permissible to regard a solid crystal, however small, as an extended three-dimensional " w a l l p a p e r " , then a liquid must be considered rather as a threedimensional mosaic. But it is a mosaic in which the separate tiles are being constantly rearranged, and, indeed, constantly broken up and re-made. F r o m the structural point of view, then, a liquid is certainly better regarded as a disordered solid than as a condensed gas.
The Other States of Matter
Liquid
257
Crystals
Certain organic compounds with long molecules, melt to a turbid liquid which retains the property of double refraction, characteristic of many crystals (see p . 361). At a definite higher temperature the double refraction disappears, and the liquid becomes clear. For instance, p-azoxyanisole " m e l t s " to a liquid crystal at 116°C, and this becomes clear and loses its double refraction, at 135°C. F o r /?-methoxycinnamic acid, the corre sponding temperatures are 170°C and 186°C: >CH 3 CH 3 0<
C H : CH.COOH
/7-methoxycinnamic acid
Molecules of such compounds are parallel to one another in the crystalline state, and remain parallel on melting, although the regular disposition within rows or layers is then lost (Fig. 77).
Liquid
FIG. 7 7 . Differences between crystals, liquid crystals and liquids.
The molecules of a liquid crystal have translational freedom, so that the parallel arrangement is retained only in regions of limited extent, and the population of each such region is not stationary. The optical properties are due to the effect on light of these small
Chemical Binding and Structure
258
oriented regions. Liquid crystals (also known as mesomorphic phases) are in a sense intermediate between crystals and liquids; part of the regularity of the crystal has been lost, and part retained.
The Vitreous
State
Most liquids can be cooled to at least five or ten degrees below the freezing-point before crystals form. The temperature then rises at once to the freezing-point, since this is the only tempera ture at which solid and liquid can co-exist. (The latent heat evolved on solidification brings about the temperature rise.) On the other hand, no solid can ever be superheated above its melting point. This reflects the fact that the change from ordered solid to disordered liquid is easy—like shuffling a pack of cards— whereas the reverse process is more difficult. Crystallisation, whether from solution or from a melt, involves two distinct stages (a) the formation of nuclei; (b) their subsequent growth. In a perfectly clean liquid, nucleation is spontaneous, and more likely the lower the temperature. Otherwise, particles of dust or foreign crystals may provide the necessary element of pattern, and thus induce crystallisation. If, for some reason, nucleation is particu larly difficult, it may be possible to supercool the liquid very greatly. As the temperature is lowered in such a case, translational motions become weaker and weaker until, within a small tempera ture range, the viscosity rises very rapidly indeed and all atomic and molecular translational motion is lost. The liquid has now solidified to a glass, but it has not crystallised. In effect, one of the ever-changing configurations of the liquid state has become frozen into place. At this and lower temperatures it is now very difficult for crystallisation to occur, since there is little thermal energy available for the necessary atomic and molecular movements. Most of the characteristic properties of crystals are absent in
The Other States of Matter
259
glasses. Like a single crystal and like a liquid (but unlike a polycrystalline material where multiple refractions occur) a glass is transparent, because it forms a continuum. It is more correct to regard a glass as a supercooled liquid which has lost its fluidity, than as a true solid.
Glass
FIG. 7 8 . Crystalline and vitreous forms of boron trioxide.
A glass has no sharp melting point; instead, there is a gradual softening over a range of temperature, unaccompanied by any latent heat, or sharp changes of density and specific heat, as are found for the fusion of a crystal. In fact, a glass is always a metastable system; it is unstable with respect to the corresponding crystal, and exists only because of the extreme difficulty of the change to this stable phase. Crystallisation does occasionally happen (old glass will sometimes "devitrify" on standing, or o n heating to a temperature below the softening range) but this is exceptional behaviour. The types of compound which form glasses are those in which the immediate environment of each a t o m is more significant than the overall pattern in the crystal. F o r instance, silica readily forms a glass, and ordinary glass is a mixture of silicates with the same general arrangement of silicon and oxygen atoms as silica itself.
260
Chemical Binding and Structure
As long as each silicon atom has four oxygens around it, the precise disposition of the S i 0 4 tetrahedra with respect to each other is comparatively unimportant. Similarly, boron trioxide and borates can be obtained as glasses, based on a triangular B 0 3 unit (Fig. 78); so also can beryllium fluoride, which consists of B e F 4 tetrahedra.
HIGH POLYMERS Glasses are examples of amorphous solids, not because they are without external crystalline form (many solids with ordered internal structures do not appear crystalline) but because there is n o long-range regularity in the arrangement of their atoms and molecules. Other substances falling into this category, though for not quite the same reasons, are compounds of very high molecular weight, both natural and synthetic. Only the synthetic variety will be considered here, and an attempt will be made to show how the characteristic properties of thermo-plastic and thermo-setting materials, fibres and rubbers, are related to the chemical nature of the very long molecules of which all these types of material are composed. In order to understand the behaviour of very long molecules, that of somewhat shorter ones will be first examined. Solid paraffin hydrocarbons have a regular crystalline structure, with the molecules more or less parallel to one another, and sharp melting points. For the lower paraffins there is the normal increase of melting point with molecular weight—the larger the molecule, the larger are the intermolecular forces. As the molecules become very long, however, the melting point approaches a limiting value (Table 20). Beyond a certain length, one end of a long-chain molecule evidently has little to do with the other end, and such molecules essentially melt in segments. This is even more true for very long molecules like polythene, with hundreds of carbon atoms in each chain. Here, it is impossible
The Other States of Matter
261
TABLE 20 M E L T I N G P O I N T S O F N O R M A L P A R A F F I N S , C « H 2n + 2
η 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
m.p. (°C) -182-6 -1720 -187-1 -1350 -129-7 940 90-5 56-8 53-7 29-7 25-6 9-6 60 5-5 100 181
η
m.p. (°C
17 18 19 20 21 22 23 24 25 30 35 40 50 60 64 70
220 280 320 36-4 40-4 44-4 47-4 511 53-3 660 74-6 810 920 990 1020 1050
for one end of a chain to have any influence on the other, and it is possible, and natural, for there to be regions of regular arrange ment in the crystal, joined u p by amorphous regions in which there is no such regularity. A very long molecule may easily pass through several such regions (Fig. 79). On heating, the different regions will " m e l t " with differing ease, so that there is a gradual
A Crystalline region
B Amorphous region
FIG. 7 9 . Crystalline and amorphous regions in a solid high polymer.
262
Chemical Binding and Structure
ν ν ν \ / \ κ\ / \ / \ / \ Cl
'
Η
Η
CH 2
Cl
Η
Cl Cl χ
CH 2
CH
/
Η
Η
/ Cl
CH 2
CH 2
Ι
α
I
CH 2
/ V
\/ V
CH 2
.CH
^
CH 2
ν/ V \ χ
;C^ X
η
α
H
C
softening, and no sharp melting point. It c his { virtually impossible for a very long molecule to/ be / \ \ ordered / \ regularly in relation to its rl Cl Cl Η neighbours, alongClits Ηwhole length, and such solid systems are FIG. 8 0 . Polyvinylchloride. essentially amorphous. A further complication is that the polymerisation techniques mostly used, seldom give molecules all of the same kind. In the polymerisation of a compound like vinyl chloride, for instance, the molecules are normally all arranged in the same direction in the chain—a head-to-tail linkage. But chain branching frequently occurs, two growing chains may combine head-to-head, and the chlorine atoms are arranged at random on either side of the axis of the chain (Fig. 80). Polymer molecules, quite apart from the continuous distribution of chain length, therefore have many different shapes; accordingly, a polymer system cannot really be said to consist of one pure compound. The absence of a regular arrangement and of a sharp melting point, is therefore only to be expected. With the catalysts introduced by Ziegler and Natta during the last decade a much more regular polymer chain is obtained, pack ing in the solid is correspondingly closer, and the product is more crystalline, with a higher softening-point. For instance, poly ethylene made with the new catalysts has both a higher density and a higher softening-point than the polyethylene of the conventional high pressure process. Moreover, substituent groups are usually oriented regularly along the chain, which is thereby able to adopt a spiral configuration, leading to an even more compact packing
263
The Other States of Matter CH3
Η
CH3 Η
Η
CH3 CH3 Η
Random or atactic
Η
CH3 Η
CH3 Η
structure
ν ν ν ν ν ν \ H
ch
2
x
2
x
ch2
ch
Regular or
CH3
xh
2
Isotactic
ch2
2
V xh
n 2
structure
FIG. 8 1 . Regular and random forms of polypropylene.
and a denser, stronger structure. Fig. 81 shows the difference between the random and regular structures, in the case of poly propylene. Thermosetting
and Thermoplastic
Materials
All these systems consist basically of very long molecules; their particular properties depend on the nature of the polymer chains, and the extent of cross-linking, i.e. of bonds between the chains. Thermo-setting plastics such as bakelite (Fig. 82) have such a
OH
HO
OH
O M
OH
OH
O M
FIG. 8 2 .
OH
OH
Bakelite.
OH
OH
OH
OH
264
Chemical Binding and Structure
high degree of cross-linking that there is no possibility of the chains separating at elevated temperatures. They are joined by bonds of the same type as those within the chains, and it is just as easy to break the chains as to separate them. Softening does not occur on heating, and continued rise of temperature leads only to decomposition. With such materials the polymerisation must not be allowed to proceed too far in the initial stages of manufacture, and the "moulding p o w d e r " supplied to the fabricators does still soften on heating. The final cross-linking is then carried out during the actual moulding of the finished product. The vulcanisation of rubber is another example; the rubber, in its mould, is heated with sulphur, atoms of which form the links between the long rubber molecules. When there is no great degree of cross-linking, the material is "thermoplastic". It is supplied to the fabricator in its final chemical form, and either melted or softened sufficiently by heat to take u p the shape of moulds, to be drawn into rod or tube, or to be formed into sheet. Perspex (poly-methyl methacrylate), ch3
coocH3
polyvinyl chloride, and polystyrene — f C H 2 — C H C g H ^ - , are ex amples of thermoplastic polymers. Compounds such as dioctyl phthalate are often added as "plasticisers". The large, inert mole cules of the plasticiser penetrate between the polymer chains and help these to slide over one another, thus making the material more flexible. Fibres Most thermoplastic materials form weak fibres, in which the long polymer chains have been caused to lie parallel or nearly parallel to one another. F o r a fibre to be strong enough to be
265
The Other States of Matter
useful, there must be both a high degree of orientation of the chains, and large forces between them. Nylon and terylene molecules (Fig. 83), both condensation polymers, cannot be branched or cross-linked, so that they may easily be made parallel. Among vinyl polymers, those with highly polar side-groups are most likely to give strong fibres, e.g. polyvinyl alcohol —4-ch¿—CHOH-f^-, -HN-lCH^^NHOCÍCH^CO-HNÍCHp^HOCÍCH^CO-HN-ÍCH^NHNylon
-O-OC^
^)CO-OCH 2CH ¿0 0C<^
^CO-0-CH 2-CH¿0-OC^
^C0-0-
Te r y Ie η e FIG. 8 3 . Nylon and terylene. (N.B.—the actual shapes of these chains are not shown)
and polyacrylonitrile (Acrilari) —f c h ¿ — c h c n - ^ - , b u t n o t perspex or polystyrene. Polypropylene made by the Ziegler process forms a very strong fibre, because the regular arrangement of the methyl groups along the chains facilitates the formation of compact spirals, as explained already. In the manufacture of synthetic fibres, the long filaments are usually obtained by extruding the molten polymer through fine holes in a spinneret. T h e filaments are then subjected t o tension, in order to increase the degree of orientation of the molecules. Mono-filament fibres are made by twisting a number of the primary filaments together, whereas staple fibre is obtained by cutting the primary filaments into short lengths. The staple fibre can then be processed on ordinary textile machinery, and gives a softer fabric.
266
Chemical Binding and Structure
Rubbers All molecules continually execute internal vibrations, and either torsional oscillations or free rotations about single bonds. Because of the free rotation, long-chain molecules can exist in a great many different configurations, some of which are shown diagrammatically in Fig. 84. Any one molecule passes through many such configurations in the course of time, but for most of the
ΛΛΛ/WWWWV
FIG. 8 4 . Possible configurations of a polymer chain.
time it will have a form closely similar to the equilibrium con figuration, whose nature depends on both intermolecular and intramolecular forces. If the latter are large, a coiled-up form may be the most stable one, whereas large intermolecular forces favour a more extended shape for the molecule. At a low enough temperature the extended form is likely to be the stable one for any molecule, since intramolecular motions are then small. Appli cation of mechanical force to a specimen of a compound at a temperature at which the equilibrium state is the coiled-up one tends to pull the molecules into the extended form; they then lie parallel, or nearly parallel, to the direction of the force, and the length of the specimen increases. If the extension is not too great, the molecules return to the coiled-up equilibrium form, when the force is removed. X-ray diffraction measurements in fact show a
The Other States of Matter
267
much higher degree of crystallinity in stretched than in unstretched rubber; this would be expected from the orientation of molecules by stretching. Rubber-like properties are therefore most likely to be found in polymers without highly polar groupings, as these would favour the extended form. Natural rubber and many artificial rubbers are based on a purely hydrocarbon chain. (Natural rubber has isoprene, 2-methyl butadiene, as the unit, while butadiene itself is the basis of some artificial rubbers.) As for the effect of temperature, rubber is well known to lose its elasticity and become brittle if cooled to liquid oxygen temperature, whereas perspex and other thermoplastic materials become rubbery on heating. For a full account of these topics, see another volume in this series: Introduction to Polymer Chemistry by G. C. East and D . Margerison.
SOLUTIONS O F NON-ELECTROLYTES Forces between covalent molecules (p. 214) are usually non specific, and are rather weak unless the molecules are highly polar. When foreign molecules A are introduced into a covalent liquid B, the forces between A and Β do not differ much from those between A and A or Β and B. Consequently, covalent liquids usually mix completely with one another, and molecular crystals usually dissolve quite freely in covalent liquids. Hydroxylic compounds are associated in the liquid state through hydrogen bonding, giving definite groups of molecules, even though these have only a transient existence. The forces between two hydroxylic molecules are therefore much greater than those between a hydroxylic and a non-hydroxylic molecule. F o r this reason, water and the lower alcohols are completely miscible, but often form only partially miscible systems with other covalent liquids. When a high polymer is added to a low molecular weight liquid the solvent molecules penetrate between the polymer chains, and the polymer swells. The swelling is greater, the sn^Uer is the
268
Chemical Binding and Structure
degree of cross-linking between the chains, and sometimes a great deal of solvent may be taken u p without the polymer altogether losing its solid form. With a high degree of cross-linking, the polymer never goes into solution. In other cases, the configuration of the dissolved polymer molecules depends on the solvent-polymer forces. In a " g o o d " solvent these forces are at least as large as the forces between different segments of the polymer chain, and the fully-extended form is favoured. This produces, among other effects, a high viscosity for the solution. With a " p o o r " solvent, on the other hand, solvent-polymer interactions are weak, and different parts of the polymer chain tend instead to attract each other. Consequently, a coiled-up configuration is favoured, and the viscosity of the solution is less different from that of the solvent.
AQUEOUS SOLUTIONS O F ELECTROLYTES One way of regarding ionic solutions is to take the hydrogenbonded structure of liquid water as the starting-point. This is to some extent perturbed when ions are introduced, for in the neighbourhood of the ions, the water molecules are oriented in a special manner. Hydroxyl and hydroxonium ions are in a unique position, since they can to all intents and purposes join on to the + already existing structure. The abnormally high mobility of H 3 0 and O H " in an electric field is due to shifts of charge in such a hydrogen-bonded aggregate, as shown in Fig. 85. Alternatively, attention may be focused on each ion, which is surrounded by a more or less permanent shell of water molecules —cations being more heavily hydrated than anions. F o r the highly electropositive metals, the attraction is largely electrostatic, and the membership of each hydration shell changes rather rapidly. F o r those transition metals which form strong complexes, there are probably definite six-co-ordinated groups of water molecules around each cation. Strictly speaking, then, the properties of a
269
The Other States of Matter
I
Η Η
Η
1}
„ν ν i ft Λ
Η Η
X
ft ft
Η
Λ
-
e t c
X
r H^H
H''°\
ft ft etc. FIG. 8 5 . Mechanism of movement of hydroxy I and hydroxonium ions in water.
solution of (say) chromic chloride, are due not to the presence of 3+ + Cr but of C r ( H 2 0 ) o . F o r instance, it is incorrect to say that chromic ions are violet, and that colour changes occur on complex formation. A more accurate statement is that hydrated chromic ions are violet, and that there are accompanying colour changes when water molecules are displaced by chlorine atoms, ammonia mole + cules, etc., to give such ions as C r ( H 2 0 ) 4 C l 2 , C r ( H 2 0 ) 3 ( N H 3 ) 3 ,
Chemical Binding and Structure
270
etc. (cf. p . 146). Again, the acid reaction of a solution of chromic chloride is now considered t o be d u e t o progressive dissociation of the aquo-cation: +
Cr(H20)| ^Cr(H20)5OH
2+
+
+
+ Η ^ α ( Η 2 0 ) 4 ( Ο Η ) + + 2 H etc. +
Although the hydrogen ion is written H in the above equilibria it is always hydrated in aqueous solution (cf. p . 188). Indeed, there + is reason to believe that the hydroxonium ions H 3 0 so formed are usually joined by hydrogen bonds t o three more water molecules + to give an eifective unit H 3 0 ( H 2 0 ) 3 , in all but the most con centrated solutions.
The Solubility of Salts in Water In spite of well known rules about the solubility of salts (all nitrates and sodium salts are soluble, nearly all carbonates and phosphates are insoluble, etc.) it is impossible t o give a simple discussion of the factors which determine solubility. The solubility of a salt, given in some concentration unit, expresses the position of the equilibrium between solid salt and solution. Like all equilibrium constants, its magnitude is governed by two distinct factors, the energy and the entropy factors, as explained in Chapter 7. Even a small change in either of these quantities, on going from one salt t o another, may cause a large change in the position of the solubility equilibrium. Further, each factor is composite, since the dissolution of a salt in water may be resolved into the following stages: AB(s)
+
A ( g ) + B"(g)
+
A ( a q ) + B-(aq)
Thus, the lattice energy AHL of a compound of the charge type of 2 + 2 M g 0 " , is considerably larger than that of a compound of the + charge type of N a C l ~ , and this makes for a smaller heat of solution. On the other hand, the larger hydration energies ΔΗΗ of d i v a l e n t than o f m o n o v a l e n t i o n s work in the opposite direction.
The Other States of Matter
271
In any case, as already pointed out, the heat of solution is not the only factor, for many highly soluble salts dissolve with absorption of heat.
T H E A D S O R B E D STATE Adsorption at Interfaces Accurate measurements of pressure and volume show that when a gas is admitted to a vessel containing a clean metal surface, molecules of the gas are in some way held on the surface of the metal. (The phenomenon is by no means restricted to metals.) It is called " a d s o r p t i o n " , as distinct from " a b s o r p t i o n " , which occurs when there is an actual penetration into the interior of the absorbing material. The greater the pressure of the gas the more is adsorbed, u p to a limit beyond which further increase of pressure has very little effect. Calculations show that the state of saturation corresponds to a complete monomolecular layer of gas molecules on the surface of the metal. The energy of adsorption is often considerable, and the adsorbed layer difficult to remove; in such cases the adsorbed molecules are probably joined to the atoms of the surface layer by valency-type forces. Oxygen and hydrogen adsorbed on metals, for instance, may in effect form surface oxides and hydrides. In other cases the energy of adsorp tion is much less, as it is also for adsorption beyond the limit of a monomolecular layer. Van der Waals' forces probably then operate. Adsorbed molecules have a surprisingly high degree of surface mobility, so that in some respects the adsorbed state resembles a two-dimensional liquid or gas. The catalysis of gas reactions brought about by solid surfaces is well known to involve adsorption of the reactant molecules on the surface, as an essential preliminary to reaction. Adsorption also occurs at liquid surfaces. F o r instance, the variation of the surface tension of solutions with concentration is due to the varying differences in concentration between the bulk of
212
Chemical Binding and Structure /Torsion
wire
FIG. 86. Surface balance for investigating monolayers on liquids.
FIG. 8 7 . Orientation of fatty acids and their salts at interfaces.
The Other States of Matter
273
the solution and the surface layer. The existence of a surface layer is shown even more clearly by experiments with films of insoluble compounds such as long-chain fatty acids. These films can be confined by barriers, and their surface pressure measured by a torsion arrangement (Fig. 86). In this way force-area curves have been determined, analogous to pressure-volume curves for gases. The area per molecule can be deduced, and a monomolecular layer shown to be formed. The behaviour is just that expected for twodimensional solids, liquids and gases. The carboxyl groups are attracted into the water, while the hydrocarbon chains project vertically from the surface (Fig. 87). Similar surface effects explain the action of soaps and detergents. These compounds also have a polar end to a long-chain hydrocarbon molecule, and form a completely oriented layer between water and oil globules, thus dispersing the latter in the former (Fig. 87).
The Colloidal
State
This may be defined as the state of matter in which surface properties are of overriding importance. F o r particles in a solution to be colloidal, the surface area/volume ratio must have a minimum value. Characteristic colloidal properties are found for 5 7 particles in the size range 1 0 " to 1 0 " c m ; for still smaller diameters the dispersion is a molecular solution. In colloidal solutions, thermal agitation would normally lead to an aggrega tion or coagulation of the particles to a coarse sediment, if it were not for electrical charges on the particles, due either to inherent features of molecular structure, or to adsorption of ions from solution. At one time colloids were distinguished sharply from crystalloids, but this antithesis is quite false for solid-in-liquid dispersions. Broadly speaking, colloidal particles are either single very large molecules (e.g. proteins or polystyrene) or very small portions of a crystal lattice (e.g. ferric hydroxide or arsenious sulphide sol).
274
Chemical Binding and Structure
Solid high polymers and other macromolecular compounds do, however, have many colloidal properties, and since these are certainly not crystalline in the ordinary sense (p. 262), the distinction is perhaps then valid. The colloidal state is considered in detail, in another volume of this series: Chemical Kinetics and Surface and Colloid Chemistry by A. F . Trotman-Dickenson and G. D . Parfitt.
T H E G A S E O U S STATE F r o m a structural point of view there is little to say about the gaseous state. Bonding and forces within isolated molecules have been fully discussed in previous chapters. The starting-point of the kinetic theory of gases is the perfect or ideal gas, with no forces between its molecules. Such a gas would never condense, so that for this reason alone there must be forces between the molecules of real gases. The nature of these forces has been dis cussed in connection with molecular crystals (p. 214). As far as the gaseous state itself is concerned, their importance is the way in which they determine the P - V - T relationships of real gases.
L I Q U I D S A N D GLASSES The Liquid
State
The liquid state has often been regarded as more akin to the gaseous than to the solid state. Liquids and gases, for instance, are collectively referred to as "fluids", for both have the property of
254
The Other States of Matter
255
flow, taking up the shape of the containing vessel. At first sight, solids seem clearly marked off from liquids and gases through their definite crystalline form; from some physico-chemical points of view also, there is a clearer dividing line between solids and liquids than between liquids and gases. Thus, all distinction between liquid and vapour disappears above the critical point, but efforts to establish the existence of a critical point for solids and liquids have so far failed. On the other hand, liquids are undeniably closer to solids than to gases in some physical properties. Density is an obvious example; the volume increase on fusion seldom exceeds about 10 per cent, showing that the molecules of a liquid are not much further apart than those of the solid. F o r a number of substances the volume actually decreases on fusion, showing that the atoms or molecules must then be more closely-packed in the liquid than in the solid. The compressibility of a liquid is much closer to that of a solid than to the very large compressibility of a gas, and the specific heat of a liquid just above the melting point, is usually not much different from that of the solid just below the melting point. The results of X-ray diffraction studies on liquids, although difficult to interpret, have shown that in many cases a liquid has a definite structure. When the molecules are approximately spherical this structure is only rudimentary, but in other cases it may be more pronounced. Whatever the degree of structure thus revealed, it is only an average one. The molecules of a liquid have complete translational freedom, so that the overall pattern is continually changing. In each instantaneous arrangement, how ever, the average environment of a particular molecule is probably not very different from what it is in the corresponding crystal. When, for instance, a close-packed metal or rare gas melts, each " s n a p s h o t " picture of the liquid would reveal a situation in which each molecule has perhaps ten, eleven or twelve nearest neigh bours. But a snapshot taken the next instant, would reveal a completely different general picture, though the same types of short-distance groupings would be seen (see Fig. 76).
256
Chemical Binding and Structure
Of course, with a small volume change on melting, the packing of the atoms or molecules in the liquid cannot differ much from that in the solid, so perhaps the above result is only to be expected. Sometimes, though, X-ray diffraction has shown the existence of a very definite degree of structure in the liquid. The best example is water. Ice has the oxygen atoms of the water molecules held by hydrogen bonds in a tetrahedral arrangement, rather like diamond. O
n
° ° 0
°°°0°
o ° n o o o o ° ° no 0 °°o°o o° FIG. 7 6 . Short-range order in liquids.
On fusion, appreciable regions of this structure persist, so much so that water just above the melting point is denser than ice just below. Many of the properties of liquid water are due to this high degree of structure, but the pattern constantly changes, as with all liquids. Between the extremes of a liquid metal and water comes a whole range of intermediate cases. Whatever the details, it remains true that while in a liquid there may be a considerable degree of shortrange order, there is no long-range order. That is, there is n o definite relationship between the individual small ordered regions which are, of course, many orders of magnitude smaller than the crystallites of a poly-crystalline solid. If it is permissible to regard a solid crystal, however small, as an extended three-dimensional " w a l l p a p e r " , then a liquid must be considered rather as a threedimensional mosaic. But it is a mosaic in which the separate tiles are being constantly rearranged, and, indeed, constantly broken up and re-made. F r o m the structural point of view, then, a liquid is certainly better regarded as a disordered solid than as a condensed gas.
The Other States of Matter
Liquid
257
Crystals
Certain organic compounds with long molecules, melt to a turbid liquid which retains the property of double refraction, characteristic of many crystals (see p . 361). At a definite higher temperature the double refraction disappears, and the liquid becomes clear. For instance, p-azoxyanisole " m e l t s " to a liquid crystal at 116°C, and this becomes clear and loses its double refraction, at 135°C. F o r /?-methoxycinnamic acid, the corre sponding temperatures are 170°C and 186°C: >CH 3 CH 3 0<
C H : CH.COOH
/7-methoxycinnamic acid
Molecules of such compounds are parallel to one another in the crystalline state, and remain parallel on melting, although the regular disposition within rows or layers is then lost (Fig. 77).
Liquid
FIG. 7 7 . Differences between crystals, liquid crystals and liquids.
The molecules of a liquid crystal have translational freedom, so that the parallel arrangement is retained only in regions of limited extent, and the population of each such region is not stationary. The optical properties are due to the effect on light of these small
Chemical Binding and Structure
258
oriented regions. Liquid crystals (also known as mesomorphic phases) are in a sense intermediate between crystals and liquids; part of the regularity of the crystal has been lost, and part retained.
The Vitreous
State
Most liquids can be cooled to at least five or ten degrees below the freezing-point before crystals form. The temperature then rises at once to the freezing-point, since this is the only tempera ture at which solid and liquid can co-exist. (The latent heat evolved on solidification brings about the temperature rise.) On the other hand, no solid can ever be superheated above its melting point. This reflects the fact that the change from ordered solid to disordered liquid is easy—like shuffling a pack of cards— whereas the reverse process is more difficult. Crystallisation, whether from solution or from a melt, involves two distinct stages (a) the formation of nuclei; (b) their subsequent growth. In a perfectly clean liquid, nucleation is spontaneous, and more likely the lower the temperature. Otherwise, particles of dust or foreign crystals may provide the necessary element of pattern, and thus induce crystallisation. If, for some reason, nucleation is particu larly difficult, it may be possible to supercool the liquid very greatly. As the temperature is lowered in such a case, translational motions become weaker and weaker until, within a small tempera ture range, the viscosity rises very rapidly indeed and all atomic and molecular translational motion is lost. The liquid has now solidified to a glass, but it has not crystallised. In effect, one of the ever-changing configurations of the liquid state has become frozen into place. At this and lower temperatures it is now very difficult for crystallisation to occur, since there is little thermal energy available for the necessary atomic and molecular movements. Most of the characteristic properties of crystals are absent in
The Other States of Matter
259
glasses. Like a single crystal and like a liquid (but unlike a polycrystalline material where multiple refractions occur) a glass is transparent, because it forms a continuum. It is more correct to regard a glass as a supercooled liquid which has lost its fluidity, than as a true solid.
Glass
FIG. 7 8 . Crystalline and vitreous forms of boron trioxide.
A glass has no sharp melting point; instead, there is a gradual softening over a range of temperature, unaccompanied by any latent heat, or sharp changes of density and specific heat, as are found for the fusion of a crystal. In fact, a glass is always a metastable system; it is unstable with respect to the corresponding crystal, and exists only because of the extreme difficulty of the change to this stable phase. Crystallisation does occasionally happen (old glass will sometimes "devitrify" on standing, or o n heating to a temperature below the softening range) but this is exceptional behaviour. The types of compound which form glasses are those in which the immediate environment of each a t o m is more significant than the overall pattern in the crystal. F o r instance, silica readily forms a glass, and ordinary glass is a mixture of silicates with the same general arrangement of silicon and oxygen atoms as silica itself.
260
Chemical Binding and Structure
As long as each silicon atom has four oxygens around it, the precise disposition of the S i 0 4 tetrahedra with respect to each other is comparatively unimportant. Similarly, boron trioxide and borates can be obtained as glasses, based on a triangular B 0 3 unit (Fig. 78); so also can beryllium fluoride, which consists of B e F 4 tetrahedra.
HIGH POLYMERS Glasses are examples of amorphous solids, not because they are without external crystalline form (many solids with ordered internal structures do not appear crystalline) but because there is n o long-range regularity in the arrangement of their atoms and molecules. Other substances falling into this category, though for not quite the same reasons, are compounds of very high molecular weight, both natural and synthetic. Only the synthetic variety will be considered here, and an attempt will be made to show how the characteristic properties of thermo-plastic and thermo-setting materials, fibres and rubbers, are related to the chemical nature of the very long molecules of which all these types of material are composed. In order to understand the behaviour of very long molecules, that of somewhat shorter ones will be first examined. Solid paraffin hydrocarbons have a regular crystalline structure, with the molecules more or less parallel to one another, and sharp melting points. For the lower paraffins there is the normal increase of melting point with molecular weight—the larger the molecule, the larger are the intermolecular forces. As the molecules become very long, however, the melting point approaches a limiting value (Table 20). Beyond a certain length, one end of a long-chain molecule evidently has little to do with the other end, and such molecules essentially melt in segments. This is even more true for very long molecules like polythene, with hundreds of carbon atoms in each chain. Here, it is impossible
The Other States of Matter
261
TABLE 20 M E L T I N G P O I N T S O F N O R M A L P A R A F F I N S , C « H 2n + 2
η 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
m.p. (°C) -182-6 -1720 -187-1 -1350 -129-7 940 90-5 56-8 53-7 29-7 25-6 9-6 60 5-5 100 181
η
m.p. (°C
17 18 19 20 21 22 23 24 25 30 35 40 50 60 64 70
220 280 320 36-4 40-4 44-4 47-4 511 53-3 660 74-6 810 920 990 1020 1050
for one end of a chain to have any influence on the other, and it is possible, and natural, for there to be regions of regular arrange ment in the crystal, joined u p by amorphous regions in which there is no such regularity. A very long molecule may easily pass through several such regions (Fig. 79). On heating, the different regions will " m e l t " with differing ease, so that there is a gradual
A Crystalline region
B Amorphous region
FIG. 7 9 . Crystalline and amorphous regions in a solid high polymer.
262
Chemical Binding and Structure
ν ν ν \ / \ κ\ / \ / \ / \ Cl
'
Η
Η
CH 2
Cl
Η
Cl Cl χ
CH 2
CH
/
Η
Η
/ Cl
CH 2
CH 2
Ι
α
I
CH 2
/ V
\/ V
CH 2
.CH
^
CH 2
ν/ V \ χ
;C^ X
η
α
H
C
softening, and no sharp melting point. It c his { virtually impossible for a very long molecule to/ be / \ \ ordered / \ regularly in relation to its rl Cl Cl Η neighbours, alongClits Ηwhole length, and such solid systems are FIG. 8 0 . Polyvinylchloride. essentially amorphous. A further complication is that the polymerisation techniques mostly used, seldom give molecules all of the same kind. In the polymerisation of a compound like vinyl chloride, for instance, the molecules are normally all arranged in the same direction in the chain—a head-to-tail linkage. But chain branching frequently occurs, two growing chains may combine head-to-head, and the chlorine atoms are arranged at random on either side of the axis of the chain (Fig. 80). Polymer molecules, quite apart from the continuous distribution of chain length, therefore have many different shapes; accordingly, a polymer system cannot really be said to consist of one pure compound. The absence of a regular arrangement and of a sharp melting point, is therefore only to be expected. With the catalysts introduced by Ziegler and Natta during the last decade a much more regular polymer chain is obtained, pack ing in the solid is correspondingly closer, and the product is more crystalline, with a higher softening-point. For instance, poly ethylene made with the new catalysts has both a higher density and a higher softening-point than the polyethylene of the conventional high pressure process. Moreover, substituent groups are usually oriented regularly along the chain, which is thereby able to adopt a spiral configuration, leading to an even more compact packing
263
The Other States of Matter CH3
Η
CH3 Η
Η
CH3 CH3 Η
Random or atactic
Η
CH3 Η
CH3 Η
structure
ν ν ν ν ν ν \ H
ch
2
x
2
x
ch2
ch
Regular or
CH3
xh
2
Isotactic
ch2
2
V xh
n 2
structure
FIG. 8 1 . Regular and random forms of polypropylene.
and a denser, stronger structure. Fig. 81 shows the difference between the random and regular structures, in the case of poly propylene. Thermosetting
and Thermoplastic
Materials
All these systems consist basically of very long molecules; their particular properties depend on the nature of the polymer chains, and the extent of cross-linking, i.e. of bonds between the chains. Thermo-setting plastics such as bakelite (Fig. 82) have such a
OH
HO
OH
O M
OH
OH
O M
FIG. 8 2 .
OH
OH
Bakelite.
OH
OH
OH
OH
264
Chemical Binding and Structure
high degree of cross-linking that there is no possibility of the chains separating at elevated temperatures. They are joined by bonds of the same type as those within the chains, and it is just as easy to break the chains as to separate them. Softening does not occur on heating, and continued rise of temperature leads only to decomposition. With such materials the polymerisation must not be allowed to proceed too far in the initial stages of manufacture, and the "moulding p o w d e r " supplied to the fabricators does still soften on heating. The final cross-linking is then carried out during the actual moulding of the finished product. The vulcanisation of rubber is another example; the rubber, in its mould, is heated with sulphur, atoms of which form the links between the long rubber molecules. When there is no great degree of cross-linking, the material is "thermoplastic". It is supplied to the fabricator in its final chemical form, and either melted or softened sufficiently by heat to take u p the shape of moulds, to be drawn into rod or tube, or to be formed into sheet. Perspex (poly-methyl methacrylate), ch3
coocH3
polyvinyl chloride, and polystyrene — f C H 2 — C H C g H ^ - , are ex amples of thermoplastic polymers. Compounds such as dioctyl phthalate are often added as "plasticisers". The large, inert mole cules of the plasticiser penetrate between the polymer chains and help these to slide over one another, thus making the material more flexible. Fibres Most thermoplastic materials form weak fibres, in which the long polymer chains have been caused to lie parallel or nearly parallel to one another. F o r a fibre to be strong enough to be
265
The Other States of Matter
useful, there must be both a high degree of orientation of the chains, and large forces between them. Nylon and terylene molecules (Fig. 83), both condensation polymers, cannot be branched or cross-linked, so that they may easily be made parallel. Among vinyl polymers, those with highly polar side-groups are most likely to give strong fibres, e.g. polyvinyl alcohol —4-ch¿—CHOH-f^-, -HN-lCH^^NHOCÍCH^CO-HNÍCHp^HOCÍCH^CO-HN-ÍCH^NHNylon
-O-OC^
^)CO-OCH 2CH ¿0 0C<^
^CO-0-CH 2-CH¿0-OC^
^C0-0-
Te r y Ie η e FIG. 8 3 . Nylon and terylene. (N.B.—the actual shapes of these chains are not shown)
and polyacrylonitrile (Acrilari) —f c h ¿ — c h c n - ^ - , b u t n o t perspex or polystyrene. Polypropylene made by the Ziegler process forms a very strong fibre, because the regular arrangement of the methyl groups along the chains facilitates the formation of compact spirals, as explained already. In the manufacture of synthetic fibres, the long filaments are usually obtained by extruding the molten polymer through fine holes in a spinneret. T h e filaments are then subjected t o tension, in order to increase the degree of orientation of the molecules. Mono-filament fibres are made by twisting a number of the primary filaments together, whereas staple fibre is obtained by cutting the primary filaments into short lengths. The staple fibre can then be processed on ordinary textile machinery, and gives a softer fabric.
266
Chemical Binding and Structure
Rubbers All molecules continually execute internal vibrations, and either torsional oscillations or free rotations about single bonds. Because of the free rotation, long-chain molecules can exist in a great many different configurations, some of which are shown diagrammatically in Fig. 84. Any one molecule passes through many such configurations in the course of time, but for most of the
ΛΛΛ/WWWWV
FIG. 8 4 . Possible configurations of a polymer chain.
time it will have a form closely similar to the equilibrium con figuration, whose nature depends on both intermolecular and intramolecular forces. If the latter are large, a coiled-up form may be the most stable one, whereas large intermolecular forces favour a more extended shape for the molecule. At a low enough temperature the extended form is likely to be the stable one for any molecule, since intramolecular motions are then small. Appli cation of mechanical force to a specimen of a compound at a temperature at which the equilibrium state is the coiled-up one tends to pull the molecules into the extended form; they then lie parallel, or nearly parallel, to the direction of the force, and the length of the specimen increases. If the extension is not too great, the molecules return to the coiled-up equilibrium form, when the force is removed. X-ray diffraction measurements in fact show a
The Other States of Matter
267
much higher degree of crystallinity in stretched than in unstretched rubber; this would be expected from the orientation of molecules by stretching. Rubber-like properties are therefore most likely to be found in polymers without highly polar groupings, as these would favour the extended form. Natural rubber and many artificial rubbers are based on a purely hydrocarbon chain. (Natural rubber has isoprene, 2-methyl butadiene, as the unit, while butadiene itself is the basis of some artificial rubbers.) As for the effect of temperature, rubber is well known to lose its elasticity and become brittle if cooled to liquid oxygen temperature, whereas perspex and other thermoplastic materials become rubbery on heating. For a full account of these topics, see another volume in this series: Introduction to Polymer Chemistry by G. C. East and D . Margerison.
SOLUTIONS O F NON-ELECTROLYTES Forces between covalent molecules (p. 214) are usually non specific, and are rather weak unless the molecules are highly polar. When foreign molecules A are introduced into a covalent liquid B, the forces between A and Β do not differ much from those between A and A or Β and B. Consequently, covalent liquids usually mix completely with one another, and molecular crystals usually dissolve quite freely in covalent liquids. Hydroxylic compounds are associated in the liquid state through hydrogen bonding, giving definite groups of molecules, even though these have only a transient existence. The forces between two hydroxylic molecules are therefore much greater than those between a hydroxylic and a non-hydroxylic molecule. F o r this reason, water and the lower alcohols are completely miscible, but often form only partially miscible systems with other covalent liquids. When a high polymer is added to a low molecular weight liquid the solvent molecules penetrate between the polymer chains, and the polymer swells. The swelling is greater, the sn^Uer is the
268
Chemical Binding and Structure
degree of cross-linking between the chains, and sometimes a great deal of solvent may be taken u p without the polymer altogether losing its solid form. With a high degree of cross-linking, the polymer never goes into solution. In other cases, the configuration of the dissolved polymer molecules depends on the solvent-polymer forces. In a " g o o d " solvent these forces are at least as large as the forces between different segments of the polymer chain, and the fully-extended form is favoured. This produces, among other effects, a high viscosity for the solution. With a " p o o r " solvent, on the other hand, solvent-polymer interactions are weak, and different parts of the polymer chain tend instead to attract each other. Consequently, a coiled-up configuration is favoured, and the viscosity of the solution is less different from that of the solvent.
AQUEOUS SOLUTIONS O F ELECTROLYTES One way of regarding ionic solutions is to take the hydrogenbonded structure of liquid water as the starting-point. This is to some extent perturbed when ions are introduced, for in the neighbourhood of the ions, the water molecules are oriented in a special manner. Hydroxyl and hydroxonium ions are in a unique position, since they can to all intents and purposes join on to the + already existing structure. The abnormally high mobility of H 3 0 and O H " in an electric field is due to shifts of charge in such a hydrogen-bonded aggregate, as shown in Fig. 85. Alternatively, attention may be focused on each ion, which is surrounded by a more or less permanent shell of water molecules —cations being more heavily hydrated than anions. F o r the highly electropositive metals, the attraction is largely electrostatic, and the membership of each hydration shell changes rather rapidly. F o r those transition metals which form strong complexes, there are probably definite six-co-ordinated groups of water molecules around each cation. Strictly speaking, then, the properties of a
269
The Other States of Matter
I
Η Η
Η
1}
„ν ν i ft Λ
Η Η
X
ft ft
Η
Λ
-
e t c
X
r H^H
H''°\
ft ft etc. FIG. 8 5 . Mechanism of movement of hydroxy I and hydroxonium ions in water.
solution of (say) chromic chloride, are due not to the presence of 3+ + Cr but of C r ( H 2 0 ) o . F o r instance, it is incorrect to say that chromic ions are violet, and that colour changes occur on complex formation. A more accurate statement is that hydrated chromic ions are violet, and that there are accompanying colour changes when water molecules are displaced by chlorine atoms, ammonia mole + cules, etc., to give such ions as C r ( H 2 0 ) 4 C l 2 , C r ( H 2 0 ) 3 ( N H 3 ) 3 ,
Chemical Binding and Structure
270
etc. (cf. p . 146). Again, the acid reaction of a solution of chromic chloride is now considered t o be d u e t o progressive dissociation of the aquo-cation: +
Cr(H20)| ^Cr(H20)5OH
2+
+
+
+ Η ^ α ( Η 2 0 ) 4 ( Ο Η ) + + 2 H etc. +
Although the hydrogen ion is written H in the above equilibria it is always hydrated in aqueous solution (cf. p . 188). Indeed, there + is reason to believe that the hydroxonium ions H 3 0 so formed are usually joined by hydrogen bonds t o three more water molecules + to give an eifective unit H 3 0 ( H 2 0 ) 3 , in all but the most con centrated solutions.
The Solubility of Salts in Water In spite of well known rules about the solubility of salts (all nitrates and sodium salts are soluble, nearly all carbonates and phosphates are insoluble, etc.) it is impossible t o give a simple discussion of the factors which determine solubility. The solubility of a salt, given in some concentration unit, expresses the position of the equilibrium between solid salt and solution. Like all equilibrium constants, its magnitude is governed by two distinct factors, the energy and the entropy factors, as explained in Chapter 7. Even a small change in either of these quantities, on going from one salt t o another, may cause a large change in the position of the solubility equilibrium. Further, each factor is composite, since the dissolution of a salt in water may be resolved into the following stages: AB(s)
+
A ( g ) + B"(g)
+
A ( a q ) + B-(aq)
Thus, the lattice energy AHL of a compound of the charge type of 2 + 2 M g 0 " , is considerably larger than that of a compound of the + charge type of N a C l ~ , and this makes for a smaller heat of solution. On the other hand, the larger hydration energies ΔΗΗ of d i v a l e n t than o f m o n o v a l e n t i o n s work in the opposite direction.
The Other States of Matter
271
In any case, as already pointed out, the heat of solution is not the only factor, for many highly soluble salts dissolve with absorption of heat.
T H E A D S O R B E D STATE Adsorption at Interfaces Accurate measurements of pressure and volume show that when a gas is admitted to a vessel containing a clean metal surface, molecules of the gas are in some way held on the surface of the metal. (The phenomenon is by no means restricted to metals.) It is called " a d s o r p t i o n " , as distinct from " a b s o r p t i o n " , which occurs when there is an actual penetration into the interior of the absorbing material. The greater the pressure of the gas the more is adsorbed, u p to a limit beyond which further increase of pressure has very little effect. Calculations show that the state of saturation corresponds to a complete monomolecular layer of gas molecules on the surface of the metal. The energy of adsorption is often considerable, and the adsorbed layer difficult to remove; in such cases the adsorbed molecules are probably joined to the atoms of the surface layer by valency-type forces. Oxygen and hydrogen adsorbed on metals, for instance, may in effect form surface oxides and hydrides. In other cases the energy of adsorp tion is much less, as it is also for adsorption beyond the limit of a monomolecular layer. Van der Waals' forces probably then operate. Adsorbed molecules have a surprisingly high degree of surface mobility, so that in some respects the adsorbed state resembles a two-dimensional liquid or gas. The catalysis of gas reactions brought about by solid surfaces is well known to involve adsorption of the reactant molecules on the surface, as an essential preliminary to reaction. Adsorption also occurs at liquid surfaces. F o r instance, the variation of the surface tension of solutions with concentration is due to the varying differences in concentration between the bulk of
212
Chemical Binding and Structure /Torsion
wire
FIG. 86. Surface balance for investigating monolayers on liquids.
FIG. 8 7 . Orientation of fatty acids and their salts at interfaces.
The Other States of Matter
273
the solution and the surface layer. The existence of a surface layer is shown even more clearly by experiments with films of insoluble compounds such as long-chain fatty acids. These films can be confined by barriers, and their surface pressure measured by a torsion arrangement (Fig. 86). In this way force-area curves have been determined, analogous to pressure-volume curves for gases. The area per molecule can be deduced, and a monomolecular layer shown to be formed. The behaviour is just that expected for twodimensional solids, liquids and gases. The carboxyl groups are attracted into the water, while the hydrocarbon chains project vertically from the surface (Fig. 87). Similar surface effects explain the action of soaps and detergents. These compounds also have a polar end to a long-chain hydrocarbon molecule, and form a completely oriented layer between water and oil globules, thus dispersing the latter in the former (Fig. 87).
The Colloidal
State
This may be defined as the state of matter in which surface properties are of overriding importance. F o r particles in a solution to be colloidal, the surface area/volume ratio must have a minimum value. Characteristic colloidal properties are found for 5 7 particles in the size range 1 0 " to 1 0 " c m ; for still smaller diameters the dispersion is a molecular solution. In colloidal solutions, thermal agitation would normally lead to an aggrega tion or coagulation of the particles to a coarse sediment, if it were not for electrical charges on the particles, due either to inherent features of molecular structure, or to adsorption of ions from solution. At one time colloids were distinguished sharply from crystalloids, but this antithesis is quite false for solid-in-liquid dispersions. Broadly speaking, colloidal particles are either single very large molecules (e.g. proteins or polystyrene) or very small portions of a crystal lattice (e.g. ferric hydroxide or arsenious sulphide sol).
274
Chemical Binding and Structure
Solid high polymers and other macromolecular compounds do, however, have many colloidal properties, and since these are certainly not crystalline in the ordinary sense (p. 262), the distinction is perhaps then valid. The colloidal state is considered in detail, in another volume of this series: Chemical Kinetics and Surface and Colloid Chemistry by A. F . Trotman-Dickenson and G. D . Parfitt.