The Structure of Surface Films on Water

molecules in the film possessed approximately the shape indicated by their organic structural formulae,and also that they were oriented perpendicular...
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T H E STRUCTURE OF SURFACE FILMS ON WATER BY N. K. ADAM

In 1899 Rayleigh (4)came to the conclusion that the films formed by the spreading of fatty substances on water are one moleeule thick. Later workers accepted this conclusion, (though in some cases with reserve) ; but prior to 1917 no serious discussion of the more detailed structure of the films was published. In 1917, Langmuir (12) accepted that the film was one molecule thick, and, expressing his results in terms of the average area in the surface occupied by each molecule of the film, was led to the conclusion that the molecules in the film possessed approximately the shape indicated by their organic structural formulae, and also that they were oriented perpendicular to the surface, in much the same manner as that which Hardy (Proc. Roy. SOC.88 A, 303 (1913)) had previously discovered from the study of the properties of interfaces between two liquids. I n this way a most important and direct connection was established between organic chemistry and capillarity, and the relation between chemical constitution and capillary properties has from the adoption of a new point of view, become most satisfactorily clear. The development of experimental methods for the study of these surface films began with Miss Pockels’ observation (I) that the surface of water can be cleaned from contamination, even of molecular dimensions, by sweeping with a solid barrier. Such barriers may be used to confine a film within a known area of a surface, and have been employed by Rayleigh and Hardy, and by Devaux, Marcelin, and other workers in France. Barriers of the type employed by earlier workers are not, however, efficient means of keeping a film confined within a given area of surface, but allow it to leak slowly. Mainly for this reason it has been necessary to modify the apparatus, in order to obtain accurate results. Langmuir was the first to measure directly the force which acts on the film, in the plane of the surface, although Rayleigh and others studied the relation between free surface energy and surface concentration. By a modification of Langmuir’s apparatus, I have succeeded in eliminating leaks past the barriers practically completely, and in conducting experiments of fairly long duration. In this paper the present method of experiment will be described, and an attempt will be made to arrange the results so that the argument leading to conclusions regarding the molecular force-fields is developed more consistently than has been possible in the original papers. The experiments cover some ground already investigated by Langmuir, but are probably more accurate. The portions of the theory for which I am indebted to others are

88

N. K . ADAM

generally indicated in the preceding remarks. The theory, it will be noticed, is based on Langrnuir’s, but is, in some respects, carried further.l The apparatus2 (Fig. I) consists of a rectangular trough filled to the brim with water and levelled. Glass strips, CD, are used as barriers. The sides of the trough are at least 3/8-inch thick and are flat on the top. A brass trough built up of rectangular section brass on a plate has been used for most of the experiments.

1FIG.I

The trough and barriers must be clean so as to avoid contamination of the water, but since water spreads over clean surfaces, a perfectly clean trough is in a state in which the barriers cannot confine the film, since there is a pool of water round the point where the barriers touch the sides of the trough, and the film can move freely along the surface of these pools from one side of the barriers to the other. This difficulty is overcome by coating the barriers and the sides of the trough with good paraffin wax, which prevents spreading, but does not contaminate the water surfaces. The surface is first cleaned by sweeping up all contamination to the left hand end of the trough by the barriers. The film is then deposited on the cleaned surface, by dropping a known number of drops of a solution of the desired substance in clean benzene or toluene, from a calibrated pipette. Using a fine pipette the error can be reduced to about 2%. The solvent must be purified till it leaves practically no trace on the surface after evaporation of a few drops. One end of the film is bounded by the barrier CD; a t the other there j s a strip AB (usually of thin copper, paraffined), floating on the surface. The horizontal force on this strip is measured by the balance, the vertical arms of which pass loosely through holes in the floating strip. The strip is about I mm. shorter than the width of the trough, to allow for free movement, and Part I, 1 References to my own papers mill be made simply to the number of the part. Proc. Roy. SOC.,99A, 356 (1921);Parts I1 and III,lOlA, 452, 516; Parts IV and V, 103A, 676, 687. Several figures are reproduced from these papers, by kind permission of the Council. * For full details see Part I, 337, Part 11, 469.

‘

STRCCTURE O F SURFACE FILbIS Oh- WATER

89

the escape of the film through the narrow channels a t its ends is prevented by a current of air from the jets shewn in the figure. The weight of the substance put on the surface being known, and its molecular weight, the number of molecules put on is deduced, assuming the number of molecules in a gramme molecule to be 6.06 X 1 0 ~ 3 . The principal results have been obtained by considering the curves relating the force exerted by the float AB on the film, to the area per molecule. Weights in the balance pan obviously cause the float AB to exert a compressive force on any continuous film of floating objects between AB and CD. The films for this work are necessarily of substances so insoluble in water that they do not leave the surface appreciably during an experiment. Being

FIG. 2 Curve I Fatty acids on distilled mater (final curve); amides; triglycerides; ureas above the transition temperature. I1 Alcohols on water, acid, and alkaline solutions. I11 Ethyl, methyl, and allyl esters of saturated acids, on water and acid solutions. IV Ethyl iso-oleate. V Fatty acids on dilute HCl. VI1 Iso-oleic acid on dilute HCl. VI1 CISaldoxime. VI11 CIS and C I aldoximes. ~ I X Hexadecyl phenol and allied substances. X Ureas above the transition temperature. X I Nitriles. XI1 Final curves of the longer chain a bromo-acids, on dilute HC1. XI11 Bromo-palmitic and bromo-margaric acids, on dilute HC1.

confined to the surface, the molecules composing the film may correctly be considered as floating objects; hence the float AB exerts a compressive force on the film ABCD. Before putting on the film, the barrier CD is moved so as to allow more room than is found by experience to be necessary to accommodate the film.

90

S . K. ADAM

Next, the barrier is moved up until the float begins to move away to the right, indicating a compressive force on the film. At this point the film must be complete, and since it is the point a t which the floating molecules first transmit a thrust, a t this point the film is one molecule thick. By the study of the variation in compression as the area is still further reduced, it has been found that the films always remain one molecule thick, until they finally buckle up altogether. Not one case has been found, in thousands of experiments, in which a film could be considered as two or more molecules thick. The Measurements of Molecules The curves of Fig. 2 shew the relation between the area per molecule, plotted horizontally, and the compressive force on the film, plotted vertically, for a number of different substances. In the first six of these curves, which have been obtained on a fairly large range of substances having nothing in common except the long hydrocarbon chain, in their constitution, the same line GH is found. It is exceedingly steep, cuts the abscissa at about 2 0 . 7 A.U.,I and is found in the most accurate experiments to be slightly steeper in the upper part than in the lower. If the density of the film be assumed to be not far removed from that of the substance in bulk, the thickness of a film of stearic acid can be calculated from the area and is about 26 A.C. This thickness is of the same order of magnitude as, though somewhat greater than, that found for the length of this molecule in the crystal, by Muller (J. Chem. Soc. 123,2043 (1923)). Using this value as the thickness of the film, the slope of the line GH indicates that the compressibility of the surface film is about the same as that of a higher liquid paraffin in bulk. (Part 11, p. 458.) Now when a liquid is compressed in bulk, the molecules cannot rearrange themselves under compression so as to occupy less space. Therefore the low compressibility in GH shews that, in this condition of the film, lateral compression does not alter the packing arrangement of the molecules. The line GH occurs with many different long chain substances, and areas smaller than GH have never been found with stable films. Clearly long chain compounds cannot pack together closer than the area required by the chain will allow; so that the state of packing where the line GH is found is probably that in which the hydrocarbon chains are closely packed. Thus the cross section of a hydrocarbon chain as packed in the films is 2 0 . 7 A.C., within a few tenths of a unit. The orientation of the molecules follows from the fact that in almost all these curves the shape of the curve is entirely unaffected by the length of the hydrocarbon chain, provided this is greater than a certain minimum. One curve is characteristic of the acids, others of the alcohols, the nitriles, esters, and so on. This constancy of the area per molecule, for molecules of different lengths, shews that the molecules must be oriented in the films, at a definite and constant angle to the vertical. A force-field, however, which would 1

The unit of length used is

I

A X . = I G - ~ cm., and of area

I

A.U.

= 1 ~ i - lsq. ~

cm.

STRUCTURE O F SURFACE FILMS ON WATER

91

orient a molecule of one length of chain, say of the series of alcohols, at a certain angle to the vertical, would be most unlikely to orient longer and shorter members of the same series also a t the same angle, as well as molecules of other series of compounds. The evidence seems therefore conclusive that the molecules are oriented perpendicular to the surface. The polar end of the molecule must moreover be directed towards the water; it is found that those groups which confer solubility on aliphatic substances of low molecular weight also confer the property of forming stable films on long chain compounds (Langmujr, 1 2 , p. 1863). Also it will be seen later that a stable film may be rendered unstable by methylating, or otherwise modifying, the end group, so as to diminish its attraction for the water. All the curves in Fig. 2 shew, at the lowest compressions, a part which tends t o become horizontal and t o depart from the general direction of the

FIG.3

FIG.4

curve (these portions have been omitted, for clearness, from curves VI1 to XIII). The meaning of these lower portions is not a t present clear. But apart from these, in curves I1 and VI, there is a definite lower portion, below GH, which in the case of each group of compounds cuts the abscissa a t a definite area. This is 2 2 A.U. for the ethyl, methyl, and allyl esters; 2 1 . 7 for the alcohols, 27.5 for the nitriles, 2 5 . 1 for the acids on dilute HCI. These areas are characteristic of definite end groups in the molecule, and must be due to a different packing in the films from that of close packed chains. These areas must be the cross sections of the end groups as packed in the films. The structure of the films in this state may be represented diagrammatically as in Fig. 3, the chains being not quite in contact. If the heads of the molecules are not too bulky, they may be expected to be capable, under constraint, of fitting into recesses in the chains of nejghbouring molecules, and from the curves I1 to VI it is seen that often simple lateral compression causes the films to change their structure from the state of close packed heads into close packed chains. If the heads, as seems perhaps most probable, do not change their proper cross sectional area much on compression, then there must be a gradual change from the packing of Fig. 3 t o that of Fig. 4, as the compression is increased from that a t the lowest portion of the curves I1 to VI, to the point a t which these curves join the line GH. This change is found to be reversible, for on removing the compression the area increases along the original curve of compression.

N. K . ADBM

92

Katurally, if the head of the molecule is too large, or fits accurately into the heads of the neighbouring molecules, simple compression will not alter the structure to that of close packed chains. Curves X I , XII, and XIII, obtained on the nitriles and LY bromo-acids, illustrate this point. The bulky bromine atom not only increases the size of the head of the molecule, but, possibly because jt renders the head very unsymmetrical, makes quite a number of different packings possible. Actually it was found on a series of bromo-acids that the heads packed into areas from 26 to 32 A.U., according to circumstances (Part V, p. 6 8 7 ) . I n other cases, as with benzene derivatives of the type C16H33 /-‘OH,

\-/

the head has not a very large cross section, but it appears to have too much length to fit into recesses in the chains. Here the arrangement of close packed heads shewn in Fig. 3, appears to be stable against compression. The observed curve (IX) shews a compressjbility which is actually of the same order of magnitude as that of benzene in bulk, which agreeE with the supposition that the benzene rings are taking the whole of the strain of the compression, without rearrangement. In these films the molecules appear to be packed with the hydroxyl group to the water, the benzene rings closely packed in one plane, and the long chains vertically above the rings. The structure of the layer formed by the benzene rings jn this film appears to be identical with that of the monomolecular layer which would result from a series of cleavage: of the crystal of an aromatic hydrocarbon. The area, 23.8 A.U., is in very good agreement with 23.3 A.U., the best value that can be deduced a t present for the corresponding cross section of the benzene ring, from the X-ray measurements (Part IV, p. 6 7 7 ) . An intereeting effect on the cross section of the head is produced by in. troducing an ethylenic linkage in the 01 position to the carboxyl group Curves IV and VI shew the cross section of the heads of ieo-oleic acid and iti ester to be 28.7 A.U., and these are considerably greater than the cross section: of the heads of the saturated acid (25.1) and the saturated esters ( 2 2 ) . I1 seems probable that the chain of the molecule is bent into an elbow a t thf region of the double bond, and this makes packing more difficult than wit1 the saturated compounds. Where the double linkage is in the middle of tht chain, as with elaidic, erucic, and brassidic acids, there is no such enlargemeni of the head of the molecule in the films. In the middle of the chain, a doublt bond does not seem to alter the area of packing of the molecule (Part 11, p 457).

Table I gives the principal measurements of the cross section of groupr which have been made. These are of course the cross sections to which tht groups pack in the films, and are not necessarily the cross sections in fref space, of a single molecule.

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STRUCTURE O F SURFACE FILMS ON WATER

TABLE I Cross section. A.L--.

Group

Hydrocarbon chain -CH2 CHZ COOH - C H = C H COOH -CH2 CH2 COO C2Hb1 - C H = C H COO CzCHs - CHzOH -CONH2

20.7 2j.I 28.7 22 28.7 21.7

less than

-CN

21 27.5

-CHz NH’CO NH, -C6H4 OH -C6H4 K H CO CH, Triglycerides Glycol dipalmitate Cholesterol Hydrolecithin

25.5 23.8 28: 2

or

25.P

63 42

39 53

Two instances have been found of a kind of two-dimensional allotropy in the films, in which the packings change at a definite transition temperature which is altered by compression. The substituted ureas, RKHCONH2, give the arrangement of Fig. 3 below about 30’ (for octadecyl urea), and that of Fig. 4 above this temperature (Part 11, p. 464). Hexadecyl acetanilide gives two packings like Fig. 3, but of different closeness, one stable above z g O , the other below 24’ (Part IV, p. 681). Such phenomena are natural when the heads of the molecules reach a pufficient degree of complexity, and seem to be precisely analogous to the changes in crystalline structure of many solids at a definite temperature, usually called allotropy. The thickness of the films is frequently given, biit it must be remembered that what is always measured is the area of the film, and the thickness can only be calculated from this by an assumption as to the density. This can as a rule be considered correct only as to order of magnitude.

The Forces on Individual Molecules The two-dimensional and oriented state of matter in these films not only allows of the calculation of the size and shape of molecules, that is of the contour of their repulsive force fields, but it is one of the most favourable cases which can be found for the study of the attractive fields of force round the molecules. The problem of reducing observed forces to the actual forces on the individual molecules is enormously simplified when, as in the films, there is no need to allow for more than one orientation of the molecules or more than one position in a vertical direction. Up to the present, however, only qualitative information has been obtained. 1

Ethyl, methyl, and allyl esters pack into the Same area.

* According t o temperature.

94

K. Ihesurface. Kumerous other instances of films being unstable, where the polar group at the end of the molecule is partly blocked, have been found (Part IV, p. 682). If the compression is increased sufficiently, eventually some of the molecules must be forced out even from a stable film. It mould be expected from the fact that _hydrocarbons possess cohesion, that the chains packed parallel to each other in the films should have a mutual OH OCH lateral attraction. To expel a single molecule from the 3 film would mean doing work against these lateral attracFIG.5 tions as well as against the normal attraction of the end group for the water. But if an area containing a considerable number of molecules is buckled a t one time, it becomes only necessary to overcome the attraction of the end groups on the water. Indeed in practice probably the work required to buckle a film is even smaller than this, for the film of Fig. 3 can fold up into one or more double layers as in Fig. 6, work being gained by the approach of the polar end groups to one another. Double layers of this nature probably form the fundamental laminae out of which are built up the thin flakes in which most of these long chain compounds crystallise. Raising the temperature has also given information as to the attractive forces. The heads of the molecules are in a sense dissolved in the water, and are in intimate contact with the water molecules. They therefore partake of the thermal movement. Actually it is found that increasing the temperature may affect the film in one of two ways. The less common way is for the vertical component of the thermal movement to help the molecules to leave the surface film, and collapse as in Fig. 6. This effect of the molecular motion only predominates when the film is, a t any temperature, rather unstable, that is, when the attraction of the end group for the water is only just sufficient to hold the molecules there temporarily. I n such cases, usually a rise of temperature increases the rate a t which the films collapse. Much more frequently, the effect of the horizontal coniponents of the motion breaks up the lateral cohesion of the film, long before the vertical agitation assists the molecules to leave the film altogether. The films a t a certain temperature increase greatly in area, a phenomenon first observed by Labrouste; the explanation of its meaning was given almost simultaneously by Marcelin ( I O ) and myself (Part 111). Fig. 7 shews the general course of

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95

the change in area, under a constant small compression. The changes in the curves relating compression with area, as the temperature rises, are shewn in Fig. 8. As the temperature rises, with myristic acid on HC1, from 3.5" to 32.s0, the compression curves change from the general compression curve of Fig. 2 for the condensed films of fatty acids on HCI, to one ehewing much more compressibility, which repembles the isothermal of a gas. Marcelin ( I I ) claims t o have detected a compression on a film of oleic acid at an area which he does not state in definite measure, but which inust have been of the order of magnitude of 2 0 0 A.U., using a very delicate means of detecting compressive forces. The curves intermediate between 3 .so and 3 2 . 5 " bear some resemblance to the isothermals of a vapour near the critical point. The main difference is that the area increase in the expanded films is not so great as the volume in-

FIG.6 crease when a liquid vaporises. Above the temperature at which the inflexions first disappear from the curves, further rise of temperature only has the effect of moving the curves slightly to the right and upwards; at constant compression there is a gradual further increase in area as the temperature rises above that at which the main expansion is complete (Parts 111, p. 519, and Y, p. 689). This expansion of the films a t a definite temperature is a general phenomenon; it has been observed in every case, except when collapse of the film set in before expansion, or when it was experimentally impossible t o reach the temperature of expansion on account of its being too high or too low. It takes a slightly different form with some substances; for instance the corresponding curves t o Figs. 7 and 8 for the esters of fattyacids are given in Fig. 9, and the area of an expanded film of a fatty acid under given compression depends on the solution on which it is examined (Part 111, p. 521). The temperature of expansion however rises regularly with the length of the chain, in each homologous series. On ten different homologous series, the fatty acids, alcohols and their acetates amides, nitriles, ureas, methyl and ethyl esters, aldoximes, and bromo-acids, the same law has been found for the variation of the temperature of expansion as a given number of CH2

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N. K. ADAM

groups is added to the chain; the addition of one CH2raises the temperature of expansion by about ten degrees near oo and about seven degrees near 6o0C (Part 111, Tables I and 11, and Part V.). As, on the foregoing theory, the expansion is an overcoming of the lateral attractions between the chains, it is to be expected that lengthening the chains will increase the temperature requisite for expansion; and also that where the disruption of the film is into similar units (single chains) the influence on the temperature of expansion caused by an equal lengthening of the chains should be the independent of the nature of the end group. The only exception yet found to the rule just given for the influence of the addition of one CH2 to the chain on the expansion temperature, is with the triglycerides, where the expansion temperature

AREA5 PER MOLECULC

FIG.7

only increased about half the usual amount on passing from tripalmitin to tristearin (Part I11 p. 523). This is to be expected from the fact of these substances having the chains linked permanently together in groups of three. The expanded films are two-dimensional gases. I n the condensed films an impulse is transmitted from one end to the other by actual contact between the molecules. In the expanded films there appears to be considerable space vacant in the surface; the molecules are in constant motion along the surface, and exert pressure on the barriers by reason of their momentum. An impulse is transmitted in such a film by the transfer of momentum from one molecule to another in collisions. The film is not however to be treated as an isolated two-dimensional gaseous system, for the film molecules are in constant contact with the close packed water molecules, and are influenced by their motions. Hence the problem of working out the kinetic theory of these twodimensional gases is not easy. It is a problem which is related closely to the theory of the interactions of solvent and solute molecules, for the ends of the film molecules are in much the same relation to the water as are dissolved

STRUCTCRE OF SURFACE FILMS OK WATER

97

molecules to a solvent. It is already clear that there are differences in the motions of the film molecules according to the nature of the end groups of the film molecules and the substance dissolved in the water (Parts 111 and V). It is probable that in the expanded films, as in the condensed, the molecules are oriented perpendicular to the water surface. The area of the expanded films does not increase with lengthening chains, as it would do if the molecules were not vertical; on the contrary, it is often found that the area of an expanded film is actually less (at the same temperature) for a longer chain compound than for a shorter one of the same series (Part 111, p. 5 2 5 ) . This effect seems t o be due t o the greater attraction of the long chains in the expanded films causing a correction to the “simple gas laws” which is greater with longer than with shorter chains.

FIG.8

An ethylenic linkage in the molecule in the middle of the chain is found t o diminish the lateral attraction between the chains, and the attraction is considerably less if the stereochemical configuration of the double bond if as in oleic and erucic acids, than if it is as in elaidic and brassidic. The double bond of the oleic type lowers the expansion temperature by nearly 7ooC,that of the elaidic type only by about 40°, comparison being made with the saturated acid. I n Langmuir’s paper, the suggestion was made that because oleic acid occupied a greater area on the surface than the saturated acids he examined, jt was therefore doubled up with the double linkage in the water; unfortunately this most interesting suggestion is found to be incorrect now that i t is known that both saturated and unsaturated acids can exist in the condensed and expanded states, at suitable temperatures, and that there is very little difference, except in the temperature of expansion, between films of saturated and unsaturated acids.1 Wote added, November, 1924. Dr. Langmuir has pointed out to me that the expanded films consist of molecules oriented vertically and projecting from the surface, these molecules wi!l have a high potential energy, and may tend to lie down flat on the water surface. If this is so, they must overlap, as there is not room for the whole area of all the molecules lylng flat. There is the more probable alternative, however, that as soon as the molecules have separated, through expansion of the films, they sink in among the water molecules so as to satisfy the attractive forces of the chains. If this is eo, they will probably remain vertical. Nevertheless it must be remembered that the pressure in two dimensions in the surface is only about one sixth of that expected from the gas laws, and no satisfactory explanation of this fact is a t present available

98

’

N. K . ADAM

Applications of the Foregoing Theory A few examples of the way in which the ideas gained as to the force-fields about long molecules are useful in explaining different phenomena may be of some interest. A strong tendency of the long chains to pack closely together side by side has been revealed; this lateral adhesion being overcome when a suitable temperature is reached, which is higher, the longer the chains. There is also a strong attraction between such groups as OH, COOH, COOCH, CN, etc., and water. These properties, considered together with the melting points of the crystals, sufficed to predict some of the principal points of the structure of crystals of the fatty acids and their esters, which have since been shewn, by the X-ray investigations of Muller and Shearer, to exist in these crystals (Part 10.

ETHYL PALMITATE

IS

-

20

-

‘5

-

..

L

*‘

E ’

= 2 5 -

IO

I

I

20

10

40

50

60

70

BOZU

Areas per molecule

FIG.9

111, p. 5 2 8 ) . I n the acids, the melting points rise with increasing length of the chains, shewing probably a lateral close packing of the chains. But the esterification of the COOH group immediately lowers the melting point, shewing that in the esters the COOH groups of adjacent molecules attract each other and help to stabilize the crystal. Hence the crystal is probably composed of laminae made up of pairs of monomolecular films like those which form on the surface of water, the COOH groups being placed together in the centre of such a lamina. The esters containing long chains in both acidic and alcoholic groups rise in melting point as the alcoholic chains are lengthened, so that in these probably both sets of chains lie side by side, and therefore there is probably not any juxtaposition of the oxygen-containing groups of several molecules in the centre of the laminae, as there is with the acids. In the unit cell, the acids have two molecules end to end: the higher esters only one molecule. It becomes easy to see why the salts of the higher fatty acids have a tendency to form colloidal ions of high molecular weight and mobility, and to suggest a possible structure for these colloidal ions or “Micelles.” A single anion of a soap of a monovalent metal may exist temporarily in the free state in a liquid, but if it comes into contact with another will naturally pack close

STRUCTURE OF SURFACE FILMS ON WATER

99

to it, the chains side by side; and the final result of the close packing of soap anions in this manner will be an aggregate having the chains packed together in the centre, and t,he carboxyl groups all presented outwards to the water, Fig.10. These carboxyl groups, being free to dissociate, will confer 5t considerable charge on the colloidal particle, and thus the soap molecules naturally build themfelves up into aggregates possessing the known properties of the colloidal ions of soap solutions. That the fatty acids with chains shorter than a certain minimum do not form these aggregated ions is probably due to the thermal agitation of the water molecules disrupting the aggregates, as it does the condensed films, when the lateral attraction between the chains is not sufficient. Harkins (J. Am. Chem. SOC.39, 592 (1917))~ has suggested that in the formation of the layer of soap a t an interface between aqueous and hydrocarbon phases, a layer which is most important in determining the stability of emulsions, the “orientation and the form of the molecules together with adsorbed ions” may be of importance in determining the curvature of the interface. With the reservation that, since there is not yet any actual knowledge of the shapes of the soap molecules nor of their mode of packing, the suggestion must be revised when such knowledge is obtained, it can now be suggested that a calcium soap, or soap of a divalent metal, may have the polar FIG.I O end of less cross section than the two chains, while the monovalent soaps may have their polar ends larger than the chains, since with them there is one metallic atom to each chain instead of one to two chains. Molecules larger a t their polar ends will naturally pack into a curved film having the hydrocarbon side concave and the water attracting side convex, and such a film will fit the surface of an emulsion of oil dispersed in water. On the other hand molecules such as soaps of the divalent metals will be tapered in the opposite sense and so will tend to fit the curvature of, and to stablize, emulsions of water in oil. These are well known to be the effects of mono- and divalent s0aps.l Moreover the molecules which are narrower a t their polar ends than a t their hydrocarbon ends ill not probably form such stable aggregates of the type of Fig. IO, as the monovalent soaps which do form these colloidal ions. 1 These remarks were written before reading the interesting paper of Finkle, Draper, and Hildebrand (J. Am. Chem. Soc. 45, 2780, ( 1 9 2 3 ) ) ~which presents almost identical ideas. It is perhaps however worth allowing my presentation t o stand, as it brings out more clearly the relation with the information deduced from the study of mono-molecular films, with the explanation of phase reversal given by Rancroft, and with the solubility of the different classes of soaps.

IO0

N. X. ADAM

Taking inlo consideration only the ease of packing of molecules of the shape we are now assuming the divalent soaps to be, it is probable that these molecules will pack with their metallic ends in the centre and hydrocarbon chains outwards; and under certain circumstances of precipitation the insolubility of the group formed by the divalent metal and the COO groups may assist this packing of the polar groups within. Such an aggregate as this will present nothing but hydrocarbon groups outwards to the water, and therefore will probably be soluble in hydrocarbon solvents, not in water. Actually it is sometimes found that the soaps of the divalent metals form colloidal solutions in benzene and oil. It would seem therefore to be possible that the shapes of molecules may, under favourable circumstances, play a deciding part in determining the solubility of substances in different solvents. In this instance the shapes cawe packing into aggregates, in the one case packed so as to prePent a water attracting face outwards; in the other, the packing produces a surface which attracts hydrocarbons. In the explanation of the forces on the floating copper strip, a t the beginning of this paper, it has been shewn that the simplest way of regarding the force is as a thrust from the molecules of the surface film. But these molecules reduce the free surface energy, or “surface tension,” of the surface; so that what is commonly called “diminution of surface tension” appears, in a complicated surface like this, to be really an effect of the crowding of the molecules together laterally in the film, with compression. In the case of the tapered molecules, the crowding may be considered as greatest a t the wider end of the molecules, so that we may regard the curvature of the interface as being taken up in order to counteract the effect of the greater crowding at one end of the molecules. Bancroft (Applied Colloid Chemistry, p. 261 (1921), has suggested that the reason for the curvature is that the surface tends to become convex towards that phase in which the surface tension is most diminished by the adsorption of the soap. If we read, as we may, “greatest crowding of the film molecules,” for “greatest diminution of surface tension,’’ we see that Bancroft’s explanation and mine are identical, although a t first sight they might have appeared utterly different. The detergent action of soap also receives a simple explanation on this theory. Nearly all foreign matter which is insoluble in water, and therefore requires soap to remove it is of an oily nature, or at least has a less attraction for water than for hydrocarbons. Therefore, in a solution of soap, the soap molecules will be adsorbed on the surface with their hydrocarbon ends to the dirt, and their water attracting ends to the water. Hence the surface of the particle is rendered strongly water attracting, and, if the particle is small enough, it will go into colloidal solution. Pickering (J. Chem. Soc. 111, 86,

STRUCTVRE O F SURFACE FILMS ON WATER

IO1

(1917)) has shewn that small amounts of paraffin can be incorporated with soap and caused t o dissolve in water.l Sorby Research Laboratory University of iChe$ldtl.

Principal Papers on Monomolecular Films on Aqueous Solutions (I) Pockels: Nature, 43, 437 (1891). ( 2 ) Rayleigh: Proc. Roy. SOC.47, 364 (1890). (3) Rayleigh: Proc. Roy. Soc. 48, 127 (1890). (4) Rayleigh: Phil. Mag. 48, 331 (1899). (5) Devaux: “Ann. Rep. Smithsonian Institution,” 261 (1913). (6) Hardy: Proc. Roy. SOC.86A, 610 (1912). (7) Hardy: Proc. Roy. Soc. 88A, 313 (1913). (8) Marcelin: Ann. Phys. 1, 19 (1914). (9) Marcelin: Compt. rend. 173, 38, 79 (1921). ( I O ) Marcelin: Compt. rend. 175, 346 (1922). (11) Marcelin: Compt. rend. 176, 502 (1923). (12) Langmuir: J. Am. Chem. Soc. 39, 1848 (1917). (13) Langmuir: Met. Chem. Eng. 15, 468 (1016). (14) Labrouste: Ann. Phps. 14, 164 (1920). (IS) Adam: Proc. Roy. SOC.QQA,336 (1921). (16) Adam: Proc. Roy. Soc. 101A, 452] 516 (1922). (17) Adam: Proc. Roy. Soc. 103A, 676, 687 (1923). (18) Ehrenfest: Rec. Trav. chim. 42, 784 (1923). (19) Leathes: Proc. Phpsiol. Soc. Sov. (1923). 1 This explanation of detergent action has been suggested by Zsigmondy (Kolloidchemie, 314 (1920)), though apparently without the author recognizing the evidence, from other sources, for the orientation of the soap molecules at the surface.