SURFACE VISCOSITY OF LOKG-CHAIN ALCOHOL MONOLAYERS* LYMAN FOURT
AND
W. D. HARKINS
George Herbert Jones Chemistry Laboratory, University of Chicago, Chzcago, Illinois Received J u l y 1 , 1058 INTRODUCTION
This work was undertaken with the purpose of providing a set of quantitative standards of surface viscosity, to clear the m7ay for the more confident use of surface viscosity measurements in studies of molecular association and adsorption. Multivalent ions have very great effects on the viscosity and rigidity of fatty acid monolayers, and the interpretation that this is brought about by ionic bridges seems well supported. Films of macromolecules may show either great or very little shear resistance, and the adsorption of molecules beneath a film can sometimes be inferred from viscosity increases, as in the work of Langmuir, Schaefer, and Sobotka (4) with cholesterol and digitonin. The great desirability of more detailed knowledge of the relation of molecular structure to chemical reactivity and the special good fortune that the biological catalysts, the enzymes, are macromolecules makes it evident that a reliable method of interpreting viscosity in monolayers would be an important tool in the kit of the biophysicist. The underlying significance of surface viscosity may be seen from the fact that it is a measurable function related to the molecular binding energy within the plane of a monolayer, between molecules of specified orientation. In order to simplify the problem as far as possible me have attempted to reduce the variables to two,-molecular length and orientation. .As representatives of the two extreme types of molecular orientation, perpendicular and parallel to the surface, we hare used the normal alcohols and the linear polymers of w-hydroxydecanoic acid. The alcohols gave results of value for our purpose, but no viscosities large enough t o measure could be obtained for the w-hydroxydecanoic acid polymers. The alcohols have two special advantages as standards: ( I ) their force-area curves are simple, with two approximately straight lines, which indicates relative non-complexity of film structure; and ( 2 ) the effect of ions, etc., in the Presented at the Fifteenth Colloid Symposium, held a t Cambridge, Massachusetts, June 9-11, 1938. 897
898
LYMAN FOURT AND W. D. HARKINS
subphase should be less than on films with more reactive types of polar groups. METHOD AND THEORY
The basic method is that used in the long series of studies of superficial viscosity dating back to Plateau (8) and in particular the work of Stables and Wilson in 1883 (lo), Schutt in 1903 (9), Wilson and Ries in 1923 (13), Langmuir and Schaefer (3) in 1936 and 1937, and Myers and Harkins (6) in 1937. A circularly symmetrical body passes through the surface, and oscillates as a torsion pendulum about the vertical axis. The logarithmic decrement of the oscillations is proportional to the viscosity. Letting XIO stand for the common logarithm of the ratio of successive amplitudes, and u for the surface viscosity in C.G.S. units, we have
where I is the moment of inertia, P is the period, a is the radius of the oscillating body, and b the radius of the bounding vessel. The value of Ahlo gives the change of decrement between the clean surface and that covered by the film, that is, applies a correction for the water resistance on the assumption that all of the change in resistance is caused by the film. This may include in the viscosity ascribed to the film an effect of accompanying water, but this effect, if any, would be a monotonic function of the true film viscosity. The period remains practically constant throughout the compression of any alcohol thus far encountered. The slight increase found can be attributed in part to the damping itself, as well as to increase in the rotational inertia because of accompanying water. Rigid films, however, contribute to the elasticity of the oscillating system and thus shorten P. The outer radius, b, was always large with respect to the inner radius, and was taken as 12.5 cm., the distance to the three nearest points on the bounding rectangle. A film balance of the horizontal thrust type (Langmuir-Adam) was used for simultaneous forcearea control. The trough was 1.9 cm. deep. Myers and Harkins (6) employed a sharp edged ring to reduce the water resistance, and a fairly large diameter, 10 cm., to increase the film effect. The ring was placed so that its sharp edge just touched the surface. These experiments have been continued, using rings of 3-in. and 1-in. diameter. The comparative data on different substances we obtained by using a disc 1 in. in diameter, the same diameter as that used by Langmuir and Schaefer (3). This disc, the end of a brass cylinder, was lowered until it just touched the surface. Phosphor bronze wires served for suspensions. The whole system was enclosed in a box to eliminate drafts. TWOair jets were arranged to start or brake osciliation in either direction. The
SURFACE VISCOSITY OF MONOLAYERS
899
parts in contact with the surface were always dipped in molter, paraffin and allowed to drain freely while cooling, before each experiment. Oberbeck in 1880 (7) and Fourt in 1937 (2) used an oscillating vane, instead of a circularly symmetrical system. Such a system has some distinct advantages in rigid films where slippage is a factor. From the damping of an oscillating vane of length 1 a surface viscosity may be computed 4 X 2.31
u = Xro ___
PP
on the assumption that the resistance arises from flow past the ends of the vane. Since this assumption is open to question, experimental comparisons with the ring and disc devices were made. The vane consisted of a microscope cover slip clamped between the halves of a slitted cylinder. It was cleaned with hot chromic acid and washed with water before each experiment, then put in place and lowered so that it just touched the surface. The viscosity can also be computed from the torque required to maintain a uniform rotation. T o do this we used the shaft of an electric clock motor for the upper suspension. The twist between the two ends of the torsion wire is measured by arranging two optical levers to throw spots of light from the moving suspension and from the rotating device onto the same scale. With the torsion constant, r (dynes per radian), which is determined from oscillation trials, the torque can be obtained by timing the interval, t , between the passage of the two spots of light past a fixed mark. For true (Newtonian) viscosity, independent of the rate of shear, we have an equation in which the rate of rotation does not appear, although it must be constant. Here At is the change of interval caused by the film.
-&- );
u = Atr
4n
1
(3)
Anomalous viscosity or plasticity would cause a dependence on the rate of shear. Although the experimental work with constant rotation is only preliminary, we have examined the theory because the interpretation of results with plastic films should be simpler for a method in which the rate of shear is constant, than for the oscillation methods in which the velocity is always changing. Moreover, of all the methods described here it seems the best adapted to the determination of the unique viscosity of the film without the effect of accompanying water. Hydrodynamic considerations indicate that absolute measurements of film viscosity could be obtained by using a device such as a hemisphere, to secure the condition that the
900
LYMAN FOURT AND W. D. HARKINS
vertical velocity gradient in the underlying fluid should be zero in the plane of the surface. This condition could be approached, but the increase in non-film resistance requires greater sensitivity in measurement, and meniscus and centrifugal effects and the superposition of oscillation upon rotation may be expected to reduce the degree of realization of the theoretical conditions. However, the idea is adaptable, at least in theory, to interfacial measurements, by aligning the interface with the equatorial plane of the sphere. The choice of the parameter for molecular orientation n7as dictated by experimental circumstances. With small damping as many as ten oscillations may be required for a single determination of the decrement, and with the high degree of damping encountered in the plastic films, a series of decrements was needed for the comparison a t constant rate of shear. Hence an interval of several minutes was usually required for a single measurement. For the results to have significance with respect to molecular properties, it is desirable that the film structure remain the same throughout the measuring interval. The lower alcohols, however, showed a distinct tendency to disappear from the film, by collapse or, more probably, solution. That is, the force-area curve is dependent on the rate of solution, the rate of compression, and the elapsed time. A plot obtained by the rapid compression of a film of tetradecyl alcohol is paralleled by the curve obtained after an interval during which viscosities were measured. The area per molecule, computed on the basis that all of the alcohol spread remains on the surfare, is less; the type of force-area relations is the same, however, and, what is most important, the intersections of the two straight limbs of the force-area curves are a t the same pressure. For this reason the surface pressure is taken as the parameter to indicate the molecular state and resultant film structure. The area per molecule for the molecules actually present in the monolayer is presumably the same for these substances, a t a given pressure, in spite of the apparent change with time In the expprirnents the pressure was held constant by manual adjustment of the area. The impcrfection of this type of control is a major source of random error. However, of the substances used, tetraderyl alcohol is the only one rapidly soluble or expanded at the temperatures used in these experiments, which ranged from 21' t o 26°C. Iri the preliminary nork for the comparison of different devices, cetyl alcohol purchased from the Eastman Kodalr Co. was used. For the compari5on of the different alcohols we were privileged to use the preparations of L)r Jane D. Xeyer, which \\ere kindly furnished us by Ur. E. E. Reid. Reitistilled ligroin was used as a solvent in spieading these films. The a-liydroxydecanoic acid polymers TI ere kindly furriished by Drs. IT.H. Carothers, E 0. Kraemer, and F. J. \-an Atta of the du Pont Experimental Station The polymers were dissolved in redistilled benzene. All of the
SURFACE VISCOSITY OF MONOLAYERS
901
alcohol films were spread on 0.01 N hydrochloric acid solution, made up in redistilled water. RESULTS
All the alcohols from tetradecyl to heptadecyl showed viscosity relations of the same general type. The ltinlr point of the force-area curve is likewise a singular point in viscosity. At all lower pressures the films behave as Newtonian fluids. A t all higher pressures the films exhibit anomalous viscosity, that is, the apparent viscosity increases with decreasing rate of shear. Harkins and Myers (6) referred to this as plasticity, as did Wilson and Ries (13) for their adsorbrd films. However, we use the term 1.75
-\
w v) 0 a
w
1.0 0
V LL 4
a
3 Lo
0.25 0.0
0.1
0.2 0.3 RADIANS FIG.1. Representative family of curves to show the dependence of viscosity on the rate of shear above the kink point pressure. Ordinate, apparent surface viscosity; abscissa, mean amplitude. The open circle curves are all a t successively higher pressures and viscosities up to the viscosity maximum. The solid circles lie on curves for still higher pressures but decreasing viscosities.
“plastic” merely as a convenient expression for anomalous viscosity, because our methods do not give us any measure of a yield point. Moreover, sensitive tests for rigidity, by nieans of the oscillating vane, observation of talc mobilities, and the behavior of floating strips of paper (Langmuir and Schaefer (3)) fail to give any evidence of rigidity. The variation of apparent viscosity with rate of shear or amplitude, which is encountered above the kink point, has been treated by an empirical method designed to make the successive values in a given experiment comparable. To do this, the logarithmic decrement or the apparent viscosity was plotted against the sum of the amplitudes used in its computation. An example of the resulting families of curves is shown in figure 1. These
902
LYMAN FOURT AND W. D. HARKINS
curves do not cross each other, except for occasional irregularities, which shows that “greater than or less than” comparisons are justified. The values for the plastic films are those taken at a mean amplitude of 0.1 radian, but the greatest viscosities recorded in the plastic region were several-fold larger; the smallest was about half as large. The transition a t the kink point is very nearly free from hysteresis. Points taken by expanding a compressed film fall on ‘the same curve, to the accuracy of this work, as those obtained by successive compressions. This type of experiment was tried with tetra-, hexa-, and hepta-decyl alcohols. It is not impossible that a more nearly instantaneous method of measurement might reveal a hysteresis or sluggishness obscured by the time required for a series of oscillations. In the steep part of the curve a second series of oscillations at a given pressure usually gives a higher apparent viscosity. With two films of tetradecyl alcohol, low values of the viscosity were obtained above the kink point. Although this may TABLE 1 Dimensions of oscillation devices TYPE
Ring. . . . . . . . . . . . . . . . . . . . . . . .
Disc,. . . . . . . . . . . . . . . . . . . . . . . . Vane. . . . . . . . . . . . . . . . . . . . . . . .
RADIUS OR LENQTE
MOMENT OF INERTIA
PERIOD IN CLEAN SURFACE
em.
9. cm.’
nee.
5.00
2520
3.91 1.26 1.26 3.00 6.00
695 141 127 202 177
73.40 10.88 33.68 17.69 16.88 17.80 17.91
\LO I N CLEAB
SURFACE
0.042 0.0082 0.034 0.0051 0.0075 0.0030 0.051
LPPARATUS CONSTANT
i P
0.426 2.87 0.476 1.88 1.70 11.6 2.52
correspond to a metastable state, like a condition of undercooling or supersaturation, a comparatively large number of repeated experiments with this substance have not enabled us to specify the conditions under which this behavior may be observed. Except for octadecyl alcohol, the viscosity is Newtonian until well into the steep part of the transition curve. It is not a matter of the magnitude of the observed value of Xl0, since experiments with devices in which X ~ O is large and the factor small agree with those used in the comparative work. Octade-yl alcohol differed in showing a slight plasticity below the kink point. This effect is small compared with the difference between check experiments on different films, but could be seen clearly in an individual experiment. Octadecyl alcohol has the further peculiarity that the curves which show the viscosity as a function of the amplitude, although they rise with decreasing amplitude, are nearly straight, in contrast to the considerable curvature found with the other alcohols.
903
SURFACE VISCOSITY O F MONOLAYERS
The hexadecyl alcohol samples from the two sources (Eastman, and Meyer and Reid) gave nearly the same numerical values below the kink point, but above the kink point the Eastman sample showed values about half as large, though with a curve of similar shape. For a comparison of the different oscillation methods we may confine our attention to the values below the kink point. Table 1 shows the dimensions of the different systems used, the approximate value of Xl0 for the clean surface, and the factor “apparatus constant + P” by which the change in Xl0 is multiplied to obtain u.
0.3
I
> b
+ x
I x
0 0
I
I
0 0
4 L b
*
0:
3 v)
0. I
,
2 1 I
nn
I.”
0
2
4 6 8 DYNES PER CM.
1
0
I?
0
10
20 DYNES PER CM.
30
FIQ.2 FIQ.3 FIG.2. Surface viscosity of hexadecyl alcohol below the kink point, as measured by different devices. Ordinate, surface viscosity; abscissa, surface pressure. 0 , 10-cm. ring; A, 3-in. ring; 0 , I-in. ring; a, I-in. disc; f, 6-cm. vane; X , 3-om. vane. FIG.3. Surface viscosity of hexadecyl alcohol from constant rotation measurements. Ordinate, logarithm of viscosity; abscissa, surface pressure.
Figure 2 presents the results obtained with the different methods. Check experiments with each device show a range of values comparable to the difference between different devices; there is just a suggestion of a trend in that the largest values are obtained with the rings of largest diameter, and the smallest with those of smallest diameter. This may involve the approximation introduced by taking the half-width of the rectangular tray as an outer circular boundary. Most of the points with the IO-cm. ring were computed from the experiments of Myers (6). The values obtained with vanes 3 and 6 cm. long fall among the others. Above
904
LYMAX FOURT AND W. D. HARKINS
the kink point all of the devices give values similar in magnitude and in variation with prebsure, the agreement being of the same order as that exhibited below the kink. Figure 3 shon s some preliminary results obtained for tiexadecyl alcohol from the rotation oi the 10-mi. and 3-in rings, soine vvith stopnatch tiinillg and home by an automatic timing system. This apparatus could be adapted to automatic recording. Below the kink point these values are of thc same order as those obtained by the oscillation methods, but are tenfold smaller abo\-e The comparatively large angular velocity, 0.387 riditlri per second, compared with 0 0237, the average speed in oscillation at 0 1 radian, with P = 16.88 sec is in the direction to account for this differwce The points nere taken too far apart and the timing interval ala11 ,Lie was too coarse to do more than indicate the potentialities of the me I t I if(1 T! e choice of apparatus for the work with different substances \\as y The lower liniit of viscosity measurable by any of tion devices is about the same, asmay be seen from the constancy ‘apparatus constant + P” times unity in the place of figure of the clean surface Ala (given in table 1) For this icacon the work has been confined to a straightfornard application of methods, such as would he desirable for exploratory experiments and routine teiting. By an increased expenditure of time and a statistical analysis of the results any of these devices could be used to nieasure vicocities 3 8 Ion. as 0.0002 surface poise. The vi: cosities of the normal alcohols from Cld to (‘(18 inclusive are s1~oir.n in figure 4 The portions above and below the kink point are plotted on different arithmetic kcales of \ iscosity, and the very high pressures are s h o m c;n a contracted scale K i t h increasing chain length, the kink points lie a t increasingly higher pressures. The T iscosities a t low pressures are larger, the longer the chain. Belon the kink point the viscosity of each alcohol increases with increasing pressure, and the more rapidly as this singular point is approached The transition is more abrupt for the shorter chain Above the kink point the apparent viscosity of each alcohol tends to increase for a range of pressures, and finally, as collapse is approached, to fall off again. The scattering of values between different experiments on the same substance is more pronounced above than below the kink. Nevertheless, the trend of viscosity mith chain length is clear: the longer the chain, the less the apparent viscosity. With respect to what we may call the degree of plasticity, the rate of change of apparent viscosity .vrith amplitude, the trend is also reversed, octadeeyl alcohol being the only one plastic below the kink point, and the least so above No great degree of difference could be seen in the plasticity of the other alcohols, however.
SURFACE VISCOSITY OF MONOLAYERS
905
The whole range of the viscosities of all of the alcohols can be shown on a single figure by plotting the viscosities on a logarithmic scale (figure 5). The range of values-nearly 10,000-fold-and the magnitude of the transition a t the kink point is emphasized. Below the kink point the logarithm of the viscosity increases somewhat more rapidly than linearly with pressure. The insert in figure 5 presents the mean values taken from the logarithmic plot, plotted against pressure expressed as fraction of the kink point
a
0
4
8
1
DYNES
2
6 IO 14 PER CM.
18
22 25 40
FIG. 4. Surface viscosities of the normal alcohols from C I to ~ Cis. Left-hand part, below the kink point; right-hand part, above the kink point, showing apparent viscosities a t 0.1 radian mean amplitude. Ordinates, surface viscosity; abscissae, surface pressure. Pressures above 25 dynes per centimeter are shown on a reduced scale. On the low pressure side, the bottom of the plot represents -0.002 surface poise, to show that “negative viscosities” are not significant.
pressure. This corresponding states graph facilitates comparison by omitting the crossing over with the reversal of order a t the kink point. From this figure it is seen that below the kink point the viscosity increases with increasing chain length. Above the kink point pentadecyl alcohol is shown out of order; the fields of points represented by these median lines are so overlapping, though, for tetra-, penta-, and hexa-decyl alcohols, that only the general trend toward lower apparent viscosity with longer chain is significant.
906
LYMAN FOURT AXD W. D. HARKINS
0
IO
20
IO
30
0
-I 18
0
IO
m
W v) -
0
10
a. -2
I
IO
W
0
a LL
05
=
v,
-3
IO
-4
IO
0
5
IO
SURFACE
150
I 2 PRESSURE
3
FIG. 5. Surface viscosity of the normal alcohols from C l r to C I S . Ordinate, logarithm of surface viscosity; abscissa, surface pressure. I n the insert the surface pressure scale is in multiples of the mean kink point pressure.
907
SURFACE VISCOSITY OF MONOLAYERS
Neither with respect to kink point pressures nor with respect to viscosities is there any alternation evident between odd and even numbers of ombon atoms in $is series. This parallels the absence of alternation of melting point found by Meyer and Reid (5). The mean values of the kink points for the different alcohols are given in table 2, which also shows the values reported by Adam and Dyer (1). The mean values given are the midpoints of the transition curves of viscosity shown in figure 5 and are accurate within 0.5 dyne. Within this limit they agree with the kink points determined from force-area measurements on the same samples in this laboratory. The force-area curves of tetradecyl alcohol show a curved portion below 3 dynes pressure. This corresponds to the expanded type of b,which has too low a viscosity to measure, as may be seen in figure 5. All the other alcohols had measurable viscosities even at 0.1 dyne pressure, and TABLE 2 Transition pressures of the normal alcohols ALCOHOL
Cl,. .............................
............................ CIS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cl,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . CIS. ............................ c20. ............................ CIS.,
I
PREBBDREB, I N DYNE8 PER CENTIMETER ~
-
This work
Adam and Dyer
6.2 8.5 10.0 11.6
7.2
13.4
9.2
11 .o 13.5
these viscosities lie fairly well on the same smooth curve as those a t higher pressures. At quite large areas, though with the film at practically zero pressure, one frequently observes a very small “negative viscosity,” that is, a damping less than that of the original clean surface. This is, however, never much larger than the random fluctuation of the damping caused by the clean surface, and thus is hardly significant, as the arithmetic plot of figure 4 shows. This anomaly is less marked with the disc than with any of the rings. The Clz alcohol is quite soluble and has an entirely expanded force-area curve at room temperature. It showed no measurable viscosity, Likewise, we were unable to measure any viscosity for w-hydroxydecanoic acid polymers of molecular weights 780 and 16,900, spread on 0.01 N hydrochloric acid or sodium hydroxide, or on sodium hydroxide to which a calcium salt had been added. Only on collapse and compression to a visible scum could a change in damping be obtained. Collapse occurs at comparatively low pressures, however (ca. 3 dynes per centimeter).
90s
LYMAN FOURT AND W. D. HARKINS DISCUSSION
The relation of viscosity and of compressibility to the kink point is significantly different. To a close approximation, the two limbs of the force-area curve of the alcohols are straight lines, intersecting sharply, implying a discontinuity in compressibility. By contrast the change in viscosity, although large in magnitude and extreme in quality (reversal of relationship to molecular length) , is continuous and spreads out on either side of the main transition. Recent detailed studies of the region of the kink point for these substances, made in this laboratory by Dr. G. C. Nutting, show a previously unnoticed phenomenon; a slight increase in compressibility and an increased tendency of the pressure to fall off with time become evident just below the kink. This effect is found in a pressure range of about 0.5 dyne above and below the "kink" determined by the extrapolated curves. TABLE 3 Computed bulk viscosities of the noma1 alcohols
i
FILM THICKNESS AT
1
":r,{
ALCOEOL
A
surface poases
poasen
Cl'
I
CIS Ci6
GI7 CIS
_______-__
15 102 5 0 7 X lo1 5 1 8 X 104 I 3 3 2 X 10' 2 6 6 X IO4 0
I
I
1 2 X 3 X 3 X 6 X
106
lo6 106 IO6 IO6
Adam's distinction betu een close-packed heads and close-packed chains was suggested by the force-area relations. Without making such a detailed picture as is implied by those phrases, we may speak of low and high pressure condensed films, and of a transition range The state of aggregation of the molecules in the structure predominating above the kink point must be left unsettled. Plasticity is a more convenient expression than anomalous viscosity, but it must be emphasized that a solid, in addition to phenomena of non-Newtonian deformation or flow, should shorn a yield point and especially the property of shear elasticity. There is no necessity for an explanation of the decline of the apparent viscosity a t the higher pressures in terms of film structure For what they interpreted as a maximum strength phenomenon in monolayers, Talmud, Suclion olsltaja, and Lubman (11) have postulated lessened intert\\ining of the more perfectly oriented chains and consequent more ready
SURFACE VISCOSITY O F MONOLAYERS
909
slippage under shearing stress. The viscosity maximum observed in the present experiments could also arise from an instability of the film with the approach of collapse. Under shearing stress molecules could yield by escaping from the plane of the film, the more readily the higher the pressure. These experiments offer no basis for choice. A formal calculation of bulk viscosity may be made by dividing tke surface viscosity by the vertical thickness of the films. If we take 21.6 A2. as the area per molecule at 0.1 dyne pressure for all the compounds, and 19 i 2 . as the area a t the pressure of maximum viscosity, and use 0.82 as a rcund number for the density, the mean thickness of the monolayer can be computed for each alcohol. The corresponding ordinary viscosities are given in table 3. When we consider that the viscosity of water at room temperature is poises we see that the smallest values measured are from lo4 1 X to 106 times that, and that the maximum values are some 10*-fold larger than for water. These might be regarded as excessivelyhigh values because of the participation of accompanying water, not taken into account in computing the film thickness. However, there is no reason apparent why more water should accompany one alcohol than another. This supports the idea that the differences found are really in the plane of the monolayer. Trouton and Andrews (12) found the viscosity of shoemaker’s wax t o be 5 X lo6, of pitch around 1010, and of soda glass 1013 poises. This would tend to show that there is nothing intrinsically unreasonable in the magnitudes of the maximum values encountered in this work SUMMARY
Methods based on the damping of oscillating rings, discs, and vanes by a surface film give values of the surface viscosity which agree well enough to indicate the usefulness of such methods. A continuous rotation device, although not yet refined to the same degree of usefulness, gives similar values. Theory indicates that such a device would possess advantages for use with plastic films, and that it could be adapted to give absolute values of surface and interfacia1 film viscosities, free from subphase effects. The viscosity of expanded films of the normal alcohols is too low to measure by these means. Measurements were obtained on condensed films of the five alcohols from C14 to C18 inclusive. The low pressure linear part of the compression curve corresponds to low values of the surface viscosity, increasing more and more rapidly with increasing pressure. For this portion the longer the chain, the higher the viscosity. The high pressure linear part of the compression curve corresponds to high apparent viscosities, which are greater for lower rates of shear. These values, compared at a single average rate of shear, rise to a maximum as the pressure increases, then fall off with the approach of film collapse. Of the alcohols
910
LYbiAN FOURT AND W. D. HARKINB
examined] only octadecyl alcohol shows non-Newtonian viscosity below the kink point. The singular point or, as recent findings show, region of the force-area curves corresponds to the transition in viscosity. REFERENCES (1) ADAM,N.K.,AND DYER,J. W. W.: Proc. Roy. SOC. (London) A106, 694 (1924). (2) FOURT, L.:Am. J. Physiol. 119, 310 (1937). I.: Science 34, 379 (1936). (3) LANGMUIR, LANGMUIR, I., AND SCHAEFER, V. J.: J. Am. Chem. SOC.69, 2400 (1937). (4) LANGMIJIR,I., SCHAEFER,V. J., AND SOBOTKA, H.: J. Am. Chem. SOC.69, 1751 (1937). ( 5 ) MEYER,J. D., AND REID, E. E . : J. Am. Chem. SOC.66, 1574 (1933). W. D.: J. Chem. Phys. 6,601 (1937). (6) MYERS,R. J., AND HARKINS, A.: Wied. Ann. 11, 634 (1880). (7) OBERBECK, (8) PLATEAU, J.: Phil. Mag. [4]38, 445 (1869). (9) SCHUTT,K.:Ann. Physik (41 13, 712 (1904). (10) STABLES,W. H.,AND WILSON, A. E.: Phil. Mag. 16, 406 (1883). (11) TALMUD, D. L., SUCHOWOL~KAJA, S., AND LUBMAN, N.: z. physik. Chem. A161, 401 (1930). (12) TROUTON, F., AND ANDREWS,E.: Phil. Mag. 161 7, 347 (1904). (13) WILSON,R. E.,AND RIES, E. D.: Colloid Symposium Monograph 1, 145 (1923).
