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The sizes of molecules adsorbed at the benzene-water interface
JOURNAL OF coLLoID SCIENCE 11, 51--59
(1956)
THE SIZES OF MOLECULES ADSORBED AT THE BENZENE-WATER INTERFACE N. Pilpel Research Organisation B. I. Callender's Cables Ltd., 38Wood Lane, London W. 12, England Received September 22, 1955 J~.BSTRACT T h e force area characteristics of adsorbed films of some ketones and alcohols at the benzene-water interface have been investigated. It has been found possible to relate the film characteristics to the sizes of the molecules concerned. T h e technique m a y thus be of assistance in identifying other surface-active compounds.
INTRODUCTION The structures of surface-active compounds can often be determined by studying the characteristics of their monolayer. The structures of acids, esters, sterols, hormones, and other complex organic molecules have all been investigated by this technique. In the past, most of the work has been carried out on monolayers at the air-water interface. Relatively little has been done at the oil-water interface. The reason for this is the experimental difficulty involved. There are few compounds which are insoluble in both oil and water and whose monolayers at this interface can therefore be studied directly with a Langmuir Trough. Again, the operation of this instrument at the oil-water interface is a matter of considerable difficulty (I). In the case of surface-active compounds which are soluble in one or other of the two phases it is necessary to calculate forcearea characteristics of the adsorbed film. This involves employment of the Gibbs' adsorption equation and may, in certain cases, result in considerable errors. The oil-water interface, however, is of considerable interest in both biological and nonbiological systems. It frequently happens that it is the more convenient one to use. Thus in trying to categorize the hydrophilic compounds which occur in mineral oils, it is convenient to float the oil on water and study the film which is adsorbed at the interface between the two liquids. Very'little work of this nature has so far been published. The aim of the present work has been to extend the experimental data, and to see whether a convenient relationship between monolayer characteristics and molecular structure can be discovered. Such a relationship at the 51
52
N. P I L P E L
oil-water interface might assist considerably in identifying the hydrophilie compounds which develop in mineral oils. EXPERIMENTAL
The experiments now reported were carried out at the benzene-water interface. Ketones and alcohols were used to produce the adsorbed films, and the characteristics of these determined by application of the Gibbs' equations. Preparation of Materials. High-grade commerical materials were purified by standard methods. B.D.H. "molecular weight" benzene was used without further purification. The water for interracial tension measurements was distilled twice through all-glass apparatus. Melting points, boiling points, and refractive indices of the materials finally employed agreed well with reliable literature values. Measurements. Solutions of known concentration were prepared in benzene. Interracial tensions were measured at 20°C. with the Du Noiiy tensiometer using the platinum ring. The scale reading at rupture of the interfacial film was converted into interfacial tension by substituting into the following equation (2): = P{0.725 + (0.0145 P/C2(D -- d))½}.
[1]
Here 7 = interfacial tension in dynes/cm. P = scale reading at rupture in dynes/cm. C = circumference of platinum ring (here 4 cm.) D = density of water at 20°C. d = density of sample at 20°C. Freezing points of the solutions were measured in the standard form of Beckman apparatus. RESULTS
The experimental results are set out in Table I. In these tables Ns is the molar concentration of the solute; "F the interfacial tension; and 0 the freezing point depression. I' denotes the surface excess of the adsorbed molecules, the superscripts 1, N, and U referring to the three conventions used for defining the position of the oil-water interface. These conventions have been fully explained elsewhere (3, 4). The relevant equations now used for calculating the surface excesses are
r a) _
N~ To~ a~. No L T O0 ' r~(m = F~(~)/(1 + N~/No);
[2]
v~(U) = (Vs(~)Ao-{- Ns)/(NoAo + NsAs).
[4]
[3]
53
MOLECULES ADSORBED AT THE BENZENE-WATER INTERFACE TABLE I
I n t e ~ a c i a l T e n s i o n s a n d F r e e z i n g P o i n t D e p r e s s i o n s ~ S o l u t i o n s i n Benzene N•
7,
0,
~
dynes/cm.
°C.
--0"0
34.2 30.8 29.5 28.0 26.2 24.3 23.4 22.5
0 0.781 1.418 2.210 3.723 6.196 7.496 9.250
-2.85 1.93 1.55 1.05 0.612 0.491 0,394
l'S (1)
F8 (N)
r,.q(U)
gram moles~era3X 101°
F~
A,A~
dynes/cm.
316 250 200 156 140 132 121
3.4 4.7 6.2 8.0 9.9 10.8 11.7
324 131 72.1 76.0 74.6 56.9
1.1 3.2 7.1 8.8 10.4 12.1
A cetophenone 0.000 0.0106 0.0191 0:0288 0.0514 0.0855 0.1048 0.1311
-0.812 0.995 1.21 1.52 1.52 1.53 1.58
-0.805 0.975 1.18 1.43 1.39 1.37 1.37
0,529 0. 663 0.828 1.07 1.19 1.26 1.38
-0.485 1.19 2.12 1.94 1.86 2.31
0.513 1.27 2.31 2.20 2.24 2.94
Benzil 0.0000 0.0036 0.0106 0.0258 0.0360 0.0530 0.0855
34.0 32.9 30.8 26.9 25.2 23.6 21.9
0 0.238 0.686 1.702 2.375 3.470 4.713
-5.09 4.25 3.12 2.04 1.32 1.02
-0.485 1.20 2.19 2.01 1.96 2.53
Benzophenone 0.0000 0.0040 0.0085 0.0211 0.0427 0.0655 0.0810 0.1109
33.8 32.6 31.9 30.4 28.6 27.4 26.9 26.1
0 0.273 0.580 1.442 2.934 4.473 5.441 7.336
-2.76 2.28 1.38 1.02 0.661 0.475 0.345
0.0000 0.0018 0.0039 0.0086 0.0196 0.0471 0.0630 0.0800
34.0 33.8 33.6 32.9 31.6 29.1 27.8 26.9
0 0.114 0.246 0.544 1.278 2.859 3.621 4.225
-2.19 2.08 1.87 1.74 1.69 1.45 1.21
0.0000 0.0050 0.0181 0.0333 0.0602 0.0798
33.9 31.9 28.9 26.6 23.6 21.9
0 0.365 1.287 2.312 4.118 5.310
-4.28 2.66 1.99 1.53 1.28
-0.293 0.518 0.790 1.21 1.23 1.11 1.14
-0.291 0.514 0.776 1.16 1.14 1.00 0.916
-
-
0,319 0.572 0.925
1.45 1.60 1.56 1.69
w
_
_
521 290 180 115 104 107 98.6
1.2 1.9 3.4 5.2 6.4 6.9 7.7
0.2 0.4 1.1
2.4
2.0 5.0 7.3 10,3 12.0
Benzoyl acetone -0.105 0.217 0.431 0.925 2.22 2.59 2.78
-0.105 0.215 0.427 0.905 2.12 2.43 2.50
2.45 2.86 3.05
1410 682 341 160 67.7 58.1 54.5
-0.571 1.29 1.75 2.42 2.71
0.609 1.42 1,97 2.85 3.28
274 118 84.4 58.5 50.8
0.!18 0,244 0.489
1:04
4.9 6.2 7.1
Cyclohexanone -0.571 1.31 1.82 2.58 2.95
54
,' ~
:~
:' "
N.
P1LPEL
TABLE NE
7,
dynes~era.
"
0,.
,,H,
0.0000 0.0046 0.0110 0.0351 0.0664 0.0910 0.1100
34.2 30.5 27.0 24.0 20.6 18.8 16.3
I~8(1)
o%9
., ,
34.0 0 33.0 0.294 32.2 0,696 30.0 :.:2¢t80 27.8 : :4.t25 26.5 :,:5,847 25.7 ~7.t66 ~.
0.0000 0,0050 0.0098 0,0194 0.0380 0.0655 0.1079
OT
°C.
I~Continued
FB(N)
I~S( U )
A,A~
. 0.283 0.520 1.24 1.70 1.52 1.45
0. 314 0.596 1.48 2.16 2.18 2.23
541 279 113 76.9 76.3 74.6
1.0 1.8 4.0 6.2 7.5 8.3
-1.99 3.50 3.84 4.81 6.18 6.38
-2.02 3.58 4.07 5.30 6.65 7.21
82.5 46.7 40.9 31.5 25.1 23.1
3.7 7.2 10.2 13.6 15.4 17.9
-2.00 2.81 3.31 3.65 5.07 5.88
-2.03 2.89 3.46 3.90 5.51 6.59
82.4 :57.7 48.2 42.8 30.2 25.4
4.0 9.5 12.6 14.3 15:9 18,0
gram. moles~era.2 X 101°
F, dynes~era
Phorone
. 2.32 1.77 1.34 0.965 0.652 0.548
:
.
. 0.283 0,522 1,28 1,82 1.73 1,79
Decanol
:0 " -0.339 15.0 0.592 13.5 .0.961 7.71 ~:~1.400 5100 i i : 978 3.54 z2.636 2.20
-2.00 3.54 3.94 5.21 6.60 7.08 Oclanol
O.0000 0.0040 0.0106 0.0212 0.0355 0.0592 0.0991
34.0 0 30.0 0.278 24.5 0.638 21.4 !: t.021 19.7 :~'1.401 18.1 .'1.925 16.0 •2,745
-18.8 10.0 5.85 3.90 3.16 2.21
-2,00 2.85 3.36 3.78 5.28 6.45
To L T As
is the freezing p o i n t of the s o l v e n t benzene. its l a t e n t h e a t of fusion. the absolute temperature. is the area per molecule of t h e polar solute which m a y b e t a k e n as 22 A. ~ w i t h o u t significant error. A0 t h a t of the b e n z e n e molecule which is t a k e n as 24 A. 2. No is t h e mole f r a c t i o n of the solvent. T h e area, A, per polar molecule i n t h e adsorbed film is given b y : 1 A = F(~),
[5]
t h e force, ~, i n the surface b e i n g o b t a i n e d from :
~
=
70
-
~,
[6]
where ~o is t h e interfacial t e n s i o n of t h e p u r e solvent. As the force ~r is. increased t h e area per polar molecule decreases. T h e
MOLECULES
ADSORBED
AT
THE
BENZENE-WATER
INTERFACE
55
15
10
'~
\~.
5
0
I
I
100
200
I
300 Area/moleculein A 2
I
I
400
500
FIG. 1. Force-area curves for ketches adsorbed at the benzene-water interface. /~ Acetophenone; [] Benzil; X Benzophenone; O Benzoyl acetone; * Cyclohexanone; ~7 Phorone. 20
15
-o i 0 o% 5 ' :' i?
0
2'0
4'o
6'o
8'o
100
Area/molecule in A 2
FIG. 2. Force-area curves for alcohols adsorbed at the benzene-water interface. ® Octanol; X Decano]. resulting ~ A curves have been plotted in Figs. 1 and 2 for the compounds now studied. T h e curves are subject to two main sources of error. F i r s t l y there are the errors of measurement. Freezing points are proba b l y correct to within ±0.005°C., interfacial tensions to within ± 0 . 2 d y n e s / c m . B u t the error involved in deriving a~//00 f r o m the slope of the versus 0 curve is p r o b a b l y of the order of 1 5 % and m a y be greater when the slope is steep.
56
!
.
:
:
N.
PILPEL
,
:.
Secondly the equations used for calculating the surface excesses are strictly valid only under ideal conditions. These are not realized in practice. One important condition is that only very dilute solutions ( <0.01 M) are used. At concentrations >0.05 M the assumption that the solute molecules are unassociated is no longer true. This is shown by departure from linearity of the plots of freezing point against concentration, particularly for the alcohols. Some allowance however, is made for this by the 0 term in Eq. [2]. The fact, however, that the ~rA curves for the simple straight-chain alcohols become asymptotic at about 21 A. 2 supports the validity of the treatment, since direct measurement with the Langmuir Trough at the air-water interface has shown this to be the area at close packing of these molecules (5). DISCUSSION
The ~rA curves ob~/ined by the present method are incomplete. For the ketones the greatest f bree t h a t can be applied to the adsorbed film is about 12 dynes/cm., while for the alcohols it is about 18 dynes/cm. Nevertheless some useful deductions can be made. The ketones give smooth ~rA curves and there is no indication of a change of state in the adsorbed film. The alcohols, however, exhibit points of inflexion between 12 and 14 dynes/cm, and this is ascribed to a change of state from liquid expanded to liquid condensed. Langmuir has shown (6) that expanded fihns at the air-water interface obey an equation of the type (r
-
r0)(A
-
A0)
=
C.
[7]
In this 7r0 is a measure of the van der Waals' forces between nonpolar groups, while A0 is the co-area of the polar heads. This equation has been shown to apply also to insoluble films of proteins at the oil-water interface (7). TABLE II Cross-Sectional Areas of Molecules 7r0,
Substance
dynes/cm.
.:
Acetophenone
Benzil Benzophenone Benzoyl acetone Cyclohexanone Phorone Decanol Octanol
-0.8 +0.5 +0.5 +0.7
' : ' ~.
+0.4
:
~
C
Ao, A 2
Calculated area, As
1188 194 350 94 421
30.3 47.1 47.3 42.1 18.2
35.4 56.2 47.8 49,0 26.5
+0.5
245
42.2
44.2
+1.9 --2.3
92 405
29.7 21.5
---
MOLECULES ADSORBED
AT THE
BENZENE-WATER
INTERFACE
57
I t is now found that over the liquid expanded region the present 7rA curves obey the Langmuir equation quite satisfactorily, Numerical values of ~r and A taken from the curves have been substituted into Eq. [7]. The resulting values of 7r0, A0, and C are set out in Table II. It is seen that in all eases, 7r0 numerically is small. This is to be expected since the hydrophobic tails of the adsorbed molecules are dissolved in the oil phase. The negative value of 7r0 in the case of acetophenone must be attributed to a combination of experimental error, weakness in the method of calculation, and departure from the ideal behavior predicted b y the Langmuir equation. If it is assumed that condensation of the films takes place when the spheres of influence of the molecules begin to overlap, it follows that A0 should be a measure of the area occupied b y each molecule when these begin to "touch." The ketones now used have relatively rigid molecules and for them "touching" is more or less synonymous with close packing. This explains why their curves exhibit no condensed region. In the case of the alcohols however, the molecules have a long chain of C and H atoms, which can be tilted from the vertical at the benzene-water interface. Interaction between the molecules can commence before close packing and this accounts for the region in which the adsorbed film is condensed. It is now possible to see whether the values of A0 obtained from the Langmuir equation do in fact agree with the cross-sectional areas of the molecules concerned. For illustration the case of acetophenone is considered. At the benzene-water interface the adsorbed molecules will be orientated as shown in Fig. 3. The molecules are planar. If one assumes an angle of 110 ° between the benzene ring and the methyl group, and uses published values of bond lengths (8, 9), it is found that the distance x x in Fig. 3 is X
X
~ :
~
o
'
~
H~c/H
\o/\.. ~
~
FI6. 3. Aeetophenone molecule at the benzene-w~ter interface.
58
N. PILPEL
about 6.6 A. Multiplying this by 5.4 A. (the assumed thickness of the benzene ring 1) gives a cross-sectional area of 35.6 A. 2 This compares with an A0 value of 30.3 A. ~. The molecules of the remaining ketones have been assumed to have the same orientation at the interface as acetophenone. The thickness of the phorone molecule: has been taken as 4.6 A. (the distance between the centers of neighboring - - C H 2 - - chains), while the C~-----C--CH3 angle is assumed to be 124° . The cross-sectional areas thereby calculated are compared in Table I I with A0 values derived from the vA curves. It is seen that for the six ketones the calculated areas differ from the values of A0 by less than 10 A. 2. This agreement is considered very reasonable in view of the several assumptions that have been made in deriving rA curves, in calculating values of A 0, and in calculating the crosssectional areas of the molecules concerned. (For example, the C - - C - - C angle in cyclohexanone has been taken as 109 ° (9). At the interface it is likely to be greater than this as a result of deformation of the molecules (10). This would account for the low value of A0 obtained). We now consider the ~A curves of the two long-chain alcohols. Both exhibit regions in which the adsorbed film is presumably condensed (11). The condensed portions of the curves tend to a minimum area of about 20 A. 2, which is near to the figure of 21 A. ~ for close-packed straight-chain alcohols. The expanded regions of the curves obey the Langmuir equation fairly well, though not as well as the ketones. For oetanol A0 is 21.5, while for decanol it is 29.7. These admittedly rather approximate values are both greater than 21 A. 2, which indicate that at the interface the molecules are tilted from the vertical, so that interaction between the nonpolar tails occurs before close:packing of the OH heads. On this assumption one would expect the values of As to coincide with the points of inflexion in the ~A curves. That they do not must be attributed to departure from ideal behavior of the expanded films. It appears then, for the compounds now studied, that A0 gives some indication of the area occupied by each molecule when interaction between them commences. In the case of the rigid ketones interaction coincides with "touching" and Ao can be taken as the cross-sectional areas of the molecules. In the case of long-chain alcohols tilting occurs and As is thus greater than the cross-sectional area. For such compounds the required area is given by the value of A when the rA curve becomes asymptotic. This correlation between ~A characteristics and the sizes of the molecules may be of assistance in identifying unknown compounds adsorbed at the oil-water interface. 1This assumption leads to vMues of the cross-sectional areas of benzene, eholestanol, etc., in reasonable agreement with the accepted values.
MOLECULES ADSORBED AT THE BENZENE-WATER INTERFACE
59
~kCKNOWLEDGMENTS
The author wishes to thank the directors of British Insulated Callender's Cables for permission to publish this work. He is indebted to Mr. T. Platts Mills for carrying out the measurements. SUMMARY
:
i. ~. Z.:!~':
The force-area characteristics of adsorbed films of some ke't~)nes and alcohols at the benzene-water interface have been investigated on the assumption that the adsorbed film is monomolecular and that Gibbs' equations hold. For small, relatively rigid molecules it has been possible to relate the film characteristics to the sizes of the molecules concerned, and the method may therefore be of assistance in identifying other surface-active molecules adsorbed at the interface. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
ASKEW, F. A., AND I)ANIELLI, J. F., Proc. Roy. Soc (London) A155, 695 (1936). A.S.T.M. Standards, p. 1116 (1949). G~GOENHEIM, E. A., AND ADA~I, N. K., Proc. Roy. Soc. (London) A139, 218 (1933). HUTCmNSON, E., J. Colloid Sci. 3, 219 (1948). ADAM, N. K., "Physics and Chemistry of Surfaces." Oxford University Press, Oxford, 1952. LANGMTJIR, I., J. Chem. Phys. 1, 756 (1933). ALEXANDER, A. E., AND TEORELL, T., Trans. Faraday Soc. 35,727 (1939). SYRKIN, Y. I~., AND DYATKINA, M. E., "Structure of Molecules," Ch. 9. Butterworths, London, 1950. PINSKER, Z. G., " E l e c t r o n Diffraction," Ch. 11. Butterworths, London, 1953. ADAM, N. K., "Physics and Chemistry of Surfaces," p. 78. Oxford University Press, Oxford, 1952. HUTCHINSON, E., AND RANDALL, D., J. Colloid Sei. 7, 151 (1952).
(1956)
THE SIZES OF MOLECULES ADSORBED AT THE BENZENE-WATER INTERFACE N. Pilpel Research Organisation B. I. Callender's Cables Ltd., 38Wood Lane, London W. 12, England Received September 22, 1955 J~.BSTRACT T h e force area characteristics of adsorbed films of some ketones and alcohols at the benzene-water interface have been investigated. It has been found possible to relate the film characteristics to the sizes of the molecules concerned. T h e technique m a y thus be of assistance in identifying other surface-active compounds.
INTRODUCTION The structures of surface-active compounds can often be determined by studying the characteristics of their monolayer. The structures of acids, esters, sterols, hormones, and other complex organic molecules have all been investigated by this technique. In the past, most of the work has been carried out on monolayers at the air-water interface. Relatively little has been done at the oil-water interface. The reason for this is the experimental difficulty involved. There are few compounds which are insoluble in both oil and water and whose monolayers at this interface can therefore be studied directly with a Langmuir Trough. Again, the operation of this instrument at the oil-water interface is a matter of considerable difficulty (I). In the case of surface-active compounds which are soluble in one or other of the two phases it is necessary to calculate forcearea characteristics of the adsorbed film. This involves employment of the Gibbs' adsorption equation and may, in certain cases, result in considerable errors. The oil-water interface, however, is of considerable interest in both biological and nonbiological systems. It frequently happens that it is the more convenient one to use. Thus in trying to categorize the hydrophilic compounds which occur in mineral oils, it is convenient to float the oil on water and study the film which is adsorbed at the interface between the two liquids. Very'little work of this nature has so far been published. The aim of the present work has been to extend the experimental data, and to see whether a convenient relationship between monolayer characteristics and molecular structure can be discovered. Such a relationship at the 51
52
N. P I L P E L
oil-water interface might assist considerably in identifying the hydrophilie compounds which develop in mineral oils. EXPERIMENTAL
The experiments now reported were carried out at the benzene-water interface. Ketones and alcohols were used to produce the adsorbed films, and the characteristics of these determined by application of the Gibbs' equations. Preparation of Materials. High-grade commerical materials were purified by standard methods. B.D.H. "molecular weight" benzene was used without further purification. The water for interracial tension measurements was distilled twice through all-glass apparatus. Melting points, boiling points, and refractive indices of the materials finally employed agreed well with reliable literature values. Measurements. Solutions of known concentration were prepared in benzene. Interracial tensions were measured at 20°C. with the Du Noiiy tensiometer using the platinum ring. The scale reading at rupture of the interfacial film was converted into interfacial tension by substituting into the following equation (2): = P{0.725 + (0.0145 P/C2(D -- d))½}.
[1]
Here 7 = interfacial tension in dynes/cm. P = scale reading at rupture in dynes/cm. C = circumference of platinum ring (here 4 cm.) D = density of water at 20°C. d = density of sample at 20°C. Freezing points of the solutions were measured in the standard form of Beckman apparatus. RESULTS
The experimental results are set out in Table I. In these tables Ns is the molar concentration of the solute; "F the interfacial tension; and 0 the freezing point depression. I' denotes the surface excess of the adsorbed molecules, the superscripts 1, N, and U referring to the three conventions used for defining the position of the oil-water interface. These conventions have been fully explained elsewhere (3, 4). The relevant equations now used for calculating the surface excesses are
r a) _
N~ To~ a~. No L T O0 ' r~(m = F~(~)/(1 + N~/No);
[2]
v~(U) = (Vs(~)Ao-{- Ns)/(NoAo + NsAs).
[4]
[3]
53
MOLECULES ADSORBED AT THE BENZENE-WATER INTERFACE TABLE I
I n t e ~ a c i a l T e n s i o n s a n d F r e e z i n g P o i n t D e p r e s s i o n s ~ S o l u t i o n s i n Benzene N•
7,
0,
~
dynes/cm.
°C.
--0"0
34.2 30.8 29.5 28.0 26.2 24.3 23.4 22.5
0 0.781 1.418 2.210 3.723 6.196 7.496 9.250
-2.85 1.93 1.55 1.05 0.612 0.491 0,394
l'S (1)
F8 (N)
r,.q(U)
gram moles~era3X 101°
F~
A,A~
dynes/cm.
316 250 200 156 140 132 121
3.4 4.7 6.2 8.0 9.9 10.8 11.7
324 131 72.1 76.0 74.6 56.9
1.1 3.2 7.1 8.8 10.4 12.1
A cetophenone 0.000 0.0106 0.0191 0:0288 0.0514 0.0855 0.1048 0.1311
-0.812 0.995 1.21 1.52 1.52 1.53 1.58
-0.805 0.975 1.18 1.43 1.39 1.37 1.37
0,529 0. 663 0.828 1.07 1.19 1.26 1.38
-0.485 1.19 2.12 1.94 1.86 2.31
0.513 1.27 2.31 2.20 2.24 2.94
Benzil 0.0000 0.0036 0.0106 0.0258 0.0360 0.0530 0.0855
34.0 32.9 30.8 26.9 25.2 23.6 21.9
0 0.238 0.686 1.702 2.375 3.470 4.713
-5.09 4.25 3.12 2.04 1.32 1.02
-0.485 1.20 2.19 2.01 1.96 2.53
Benzophenone 0.0000 0.0040 0.0085 0.0211 0.0427 0.0655 0.0810 0.1109
33.8 32.6 31.9 30.4 28.6 27.4 26.9 26.1
0 0.273 0.580 1.442 2.934 4.473 5.441 7.336
-2.76 2.28 1.38 1.02 0.661 0.475 0.345
0.0000 0.0018 0.0039 0.0086 0.0196 0.0471 0.0630 0.0800
34.0 33.8 33.6 32.9 31.6 29.1 27.8 26.9
0 0.114 0.246 0.544 1.278 2.859 3.621 4.225
-2.19 2.08 1.87 1.74 1.69 1.45 1.21
0.0000 0.0050 0.0181 0.0333 0.0602 0.0798
33.9 31.9 28.9 26.6 23.6 21.9
0 0.365 1.287 2.312 4.118 5.310
-4.28 2.66 1.99 1.53 1.28
-0.293 0.518 0.790 1.21 1.23 1.11 1.14
-0.291 0.514 0.776 1.16 1.14 1.00 0.916
-
-
0,319 0.572 0.925
1.45 1.60 1.56 1.69
w
_
_
521 290 180 115 104 107 98.6
1.2 1.9 3.4 5.2 6.4 6.9 7.7
0.2 0.4 1.1
2.4
2.0 5.0 7.3 10,3 12.0
Benzoyl acetone -0.105 0.217 0.431 0.925 2.22 2.59 2.78
-0.105 0.215 0.427 0.905 2.12 2.43 2.50
2.45 2.86 3.05
1410 682 341 160 67.7 58.1 54.5
-0.571 1.29 1.75 2.42 2.71
0.609 1.42 1,97 2.85 3.28
274 118 84.4 58.5 50.8
0.!18 0,244 0.489
1:04
4.9 6.2 7.1
Cyclohexanone -0.571 1.31 1.82 2.58 2.95
54
,' ~
:~
:' "
N.
P1LPEL
TABLE NE
7,
dynes~era.
"
0,.
,,H,
0.0000 0.0046 0.0110 0.0351 0.0664 0.0910 0.1100
34.2 30.5 27.0 24.0 20.6 18.8 16.3
I~8(1)
o%9
., ,
34.0 0 33.0 0.294 32.2 0,696 30.0 :.:2¢t80 27.8 : :4.t25 26.5 :,:5,847 25.7 ~7.t66 ~.
0.0000 0,0050 0.0098 0,0194 0.0380 0.0655 0.1079
OT
°C.
I~Continued
FB(N)
I~S( U )
A,A~
. 0.283 0.520 1.24 1.70 1.52 1.45
0. 314 0.596 1.48 2.16 2.18 2.23
541 279 113 76.9 76.3 74.6
1.0 1.8 4.0 6.2 7.5 8.3
-1.99 3.50 3.84 4.81 6.18 6.38
-2.02 3.58 4.07 5.30 6.65 7.21
82.5 46.7 40.9 31.5 25.1 23.1
3.7 7.2 10.2 13.6 15.4 17.9
-2.00 2.81 3.31 3.65 5.07 5.88
-2.03 2.89 3.46 3.90 5.51 6.59
82.4 :57.7 48.2 42.8 30.2 25.4
4.0 9.5 12.6 14.3 15:9 18,0
gram. moles~era.2 X 101°
F, dynes~era
Phorone
. 2.32 1.77 1.34 0.965 0.652 0.548
:
.
. 0.283 0,522 1,28 1,82 1.73 1,79
Decanol
:0 " -0.339 15.0 0.592 13.5 .0.961 7.71 ~:~1.400 5100 i i : 978 3.54 z2.636 2.20
-2.00 3.54 3.94 5.21 6.60 7.08 Oclanol
O.0000 0.0040 0.0106 0.0212 0.0355 0.0592 0.0991
34.0 0 30.0 0.278 24.5 0.638 21.4 !: t.021 19.7 :~'1.401 18.1 .'1.925 16.0 •2,745
-18.8 10.0 5.85 3.90 3.16 2.21
-2,00 2.85 3.36 3.78 5.28 6.45
To L T As
is the freezing p o i n t of the s o l v e n t benzene. its l a t e n t h e a t of fusion. the absolute temperature. is the area per molecule of t h e polar solute which m a y b e t a k e n as 22 A. ~ w i t h o u t significant error. A0 t h a t of the b e n z e n e molecule which is t a k e n as 24 A. 2. No is t h e mole f r a c t i o n of the solvent. T h e area, A, per polar molecule i n t h e adsorbed film is given b y : 1 A = F(~),
[5]
t h e force, ~, i n the surface b e i n g o b t a i n e d from :
~
=
70
-
~,
[6]
where ~o is t h e interfacial t e n s i o n of t h e p u r e solvent. As the force ~r is. increased t h e area per polar molecule decreases. T h e
MOLECULES
ADSORBED
AT
THE
BENZENE-WATER
INTERFACE
55
15
10
'~
\~.
5
0
I
I
100
200
I
300 Area/moleculein A 2
I
I
400
500
FIG. 1. Force-area curves for ketches adsorbed at the benzene-water interface. /~ Acetophenone; [] Benzil; X Benzophenone; O Benzoyl acetone; * Cyclohexanone; ~7 Phorone. 20
15
-o i 0 o% 5 ' :' i?
0
2'0
4'o
6'o
8'o
100
Area/molecule in A 2
FIG. 2. Force-area curves for alcohols adsorbed at the benzene-water interface. ® Octanol; X Decano]. resulting ~ A curves have been plotted in Figs. 1 and 2 for the compounds now studied. T h e curves are subject to two main sources of error. F i r s t l y there are the errors of measurement. Freezing points are proba b l y correct to within ±0.005°C., interfacial tensions to within ± 0 . 2 d y n e s / c m . B u t the error involved in deriving a~//00 f r o m the slope of the versus 0 curve is p r o b a b l y of the order of 1 5 % and m a y be greater when the slope is steep.
56
!
.
:
:
N.
PILPEL
,
:.
Secondly the equations used for calculating the surface excesses are strictly valid only under ideal conditions. These are not realized in practice. One important condition is that only very dilute solutions ( <0.01 M) are used. At concentrations >0.05 M the assumption that the solute molecules are unassociated is no longer true. This is shown by departure from linearity of the plots of freezing point against concentration, particularly for the alcohols. Some allowance however, is made for this by the 0 term in Eq. [2]. The fact, however, that the ~rA curves for the simple straight-chain alcohols become asymptotic at about 21 A. 2 supports the validity of the treatment, since direct measurement with the Langmuir Trough at the air-water interface has shown this to be the area at close packing of these molecules (5). DISCUSSION
The ~rA curves ob~/ined by the present method are incomplete. For the ketones the greatest f bree t h a t can be applied to the adsorbed film is about 12 dynes/cm., while for the alcohols it is about 18 dynes/cm. Nevertheless some useful deductions can be made. The ketones give smooth ~rA curves and there is no indication of a change of state in the adsorbed film. The alcohols, however, exhibit points of inflexion between 12 and 14 dynes/cm, and this is ascribed to a change of state from liquid expanded to liquid condensed. Langmuir has shown (6) that expanded fihns at the air-water interface obey an equation of the type (r
-
r0)(A
-
A0)
=
C.
[7]
In this 7r0 is a measure of the van der Waals' forces between nonpolar groups, while A0 is the co-area of the polar heads. This equation has been shown to apply also to insoluble films of proteins at the oil-water interface (7). TABLE II Cross-Sectional Areas of Molecules 7r0,
Substance
dynes/cm.
.:
Acetophenone
Benzil Benzophenone Benzoyl acetone Cyclohexanone Phorone Decanol Octanol
-0.8 +0.5 +0.5 +0.7
' : ' ~.
+0.4
:
~
C
Ao, A 2
Calculated area, As
1188 194 350 94 421
30.3 47.1 47.3 42.1 18.2
35.4 56.2 47.8 49,0 26.5
+0.5
245
42.2
44.2
+1.9 --2.3
92 405
29.7 21.5
---
MOLECULES ADSORBED
AT THE
BENZENE-WATER
INTERFACE
57
I t is now found that over the liquid expanded region the present 7rA curves obey the Langmuir equation quite satisfactorily, Numerical values of ~r and A taken from the curves have been substituted into Eq. [7]. The resulting values of 7r0, A0, and C are set out in Table II. It is seen that in all eases, 7r0 numerically is small. This is to be expected since the hydrophobic tails of the adsorbed molecules are dissolved in the oil phase. The negative value of 7r0 in the case of acetophenone must be attributed to a combination of experimental error, weakness in the method of calculation, and departure from the ideal behavior predicted b y the Langmuir equation. If it is assumed that condensation of the films takes place when the spheres of influence of the molecules begin to overlap, it follows that A0 should be a measure of the area occupied b y each molecule when these begin to "touch." The ketones now used have relatively rigid molecules and for them "touching" is more or less synonymous with close packing. This explains why their curves exhibit no condensed region. In the case of the alcohols however, the molecules have a long chain of C and H atoms, which can be tilted from the vertical at the benzene-water interface. Interaction between the molecules can commence before close packing and this accounts for the region in which the adsorbed film is condensed. It is now possible to see whether the values of A0 obtained from the Langmuir equation do in fact agree with the cross-sectional areas of the molecules concerned. For illustration the case of acetophenone is considered. At the benzene-water interface the adsorbed molecules will be orientated as shown in Fig. 3. The molecules are planar. If one assumes an angle of 110 ° between the benzene ring and the methyl group, and uses published values of bond lengths (8, 9), it is found that the distance x x in Fig. 3 is X
X
~ :
~
o
'
~
H~c/H
\o/\.. ~
~
FI6. 3. Aeetophenone molecule at the benzene-w~ter interface.
58
N. PILPEL
about 6.6 A. Multiplying this by 5.4 A. (the assumed thickness of the benzene ring 1) gives a cross-sectional area of 35.6 A. 2 This compares with an A0 value of 30.3 A. ~. The molecules of the remaining ketones have been assumed to have the same orientation at the interface as acetophenone. The thickness of the phorone molecule: has been taken as 4.6 A. (the distance between the centers of neighboring - - C H 2 - - chains), while the C~-----C--CH3 angle is assumed to be 124° . The cross-sectional areas thereby calculated are compared in Table I I with A0 values derived from the vA curves. It is seen that for the six ketones the calculated areas differ from the values of A0 by less than 10 A. 2. This agreement is considered very reasonable in view of the several assumptions that have been made in deriving rA curves, in calculating values of A 0, and in calculating the crosssectional areas of the molecules concerned. (For example, the C - - C - - C angle in cyclohexanone has been taken as 109 ° (9). At the interface it is likely to be greater than this as a result of deformation of the molecules (10). This would account for the low value of A0 obtained). We now consider the ~A curves of the two long-chain alcohols. Both exhibit regions in which the adsorbed film is presumably condensed (11). The condensed portions of the curves tend to a minimum area of about 20 A. 2, which is near to the figure of 21 A. ~ for close-packed straight-chain alcohols. The expanded regions of the curves obey the Langmuir equation fairly well, though not as well as the ketones. For oetanol A0 is 21.5, while for decanol it is 29.7. These admittedly rather approximate values are both greater than 21 A. 2, which indicate that at the interface the molecules are tilted from the vertical, so that interaction between the nonpolar tails occurs before close:packing of the OH heads. On this assumption one would expect the values of As to coincide with the points of inflexion in the ~A curves. That they do not must be attributed to departure from ideal behavior of the expanded films. It appears then, for the compounds now studied, that A0 gives some indication of the area occupied by each molecule when interaction between them commences. In the case of the rigid ketones interaction coincides with "touching" and Ao can be taken as the cross-sectional areas of the molecules. In the case of long-chain alcohols tilting occurs and As is thus greater than the cross-sectional area. For such compounds the required area is given by the value of A when the rA curve becomes asymptotic. This correlation between ~A characteristics and the sizes of the molecules may be of assistance in identifying unknown compounds adsorbed at the oil-water interface. 1This assumption leads to vMues of the cross-sectional areas of benzene, eholestanol, etc., in reasonable agreement with the accepted values.
MOLECULES ADSORBED AT THE BENZENE-WATER INTERFACE
59
~kCKNOWLEDGMENTS
The author wishes to thank the directors of British Insulated Callender's Cables for permission to publish this work. He is indebted to Mr. T. Platts Mills for carrying out the measurements. SUMMARY
:
i. ~. Z.:!~':
The force-area characteristics of adsorbed films of some ke't~)nes and alcohols at the benzene-water interface have been investigated on the assumption that the adsorbed film is monomolecular and that Gibbs' equations hold. For small, relatively rigid molecules it has been possible to relate the film characteristics to the sizes of the molecules concerned, and the method may therefore be of assistance in identifying other surface-active molecules adsorbed at the interface. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
ASKEW, F. A., AND I)ANIELLI, J. F., Proc. Roy. Soc (London) A155, 695 (1936). A.S.T.M. Standards, p. 1116 (1949). G~GOENHEIM, E. A., AND ADA~I, N. K., Proc. Roy. Soc. (London) A139, 218 (1933). HUTCmNSON, E., J. Colloid Sci. 3, 219 (1948). ADAM, N. K., "Physics and Chemistry of Surfaces." Oxford University Press, Oxford, 1952. LANGMTJIR, I., J. Chem. Phys. 1, 756 (1933). ALEXANDER, A. E., AND TEORELL, T., Trans. Faraday Soc. 35,727 (1939). SYRKIN, Y. I~., AND DYATKINA, M. E., "Structure of Molecules," Ch. 9. Butterworths, London, 1950. PINSKER, Z. G., " E l e c t r o n Diffraction," Ch. 11. Butterworths, London, 1953. ADAM, N. K., "Physics and Chemistry of Surfaces," p. 78. Oxford University Press, Oxford, 1952. HUTCHINSON, E., AND RANDALL, D., J. Colloid Sei. 7, 151 (1952).