ROBERTR. STROMBERG AND LESLIEE. SMITH
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Conformation of Polystryrene Adsorbed on Liquid Mercury’
by Robert R. Stromberg and Leslie E. Smith Inetitute for Materials Research, National Bureau of Standards, Washington, D . C. 90934 (Received November 7, 1966)
Measurement of the extension of polystyrene adsorbed on liquid mercury from cyclohexane near the B temperature indicates that the molecule is attached a t a relatively large number of sites and that the conformation remains constant during most of the adsorption period. In addition, the extension is approximately independent of molecular weight for the range studied (537,000-3,300,000). These results are in contrast to previous measurements on polycrystalline metallic surfaces. The behavior may be attributed in part to the large value of the contribution of the London dispersion forces to the surface free energy of mercury as compared to other metals. Other possibilities are also discussed.
Introduction In a previous communication2 from this laboratory the results of a study of the conformation of polystyrene molecules adsorbed on several metal surfaces a t the Flory 0 temperature were reported. The conformation was investigated by ellipsometric measurements of the extension of the adsorbed molecule normal to the surface. This technique permitted measurement of the adsorbed layer while immersed in the polymer solution to prevent any change in conformation resulting from drying. The extension was found to be dependent on surface population, increasing with adsorption time, indicating an initial flat conformation for the early arrivals which then changed to a more extended form as additional molecules adsorbed. The equilibrium extension increased with increasing solution concentration until a plateau was reached, again indicating a conformation dependence on the number of molecules competing for adsorption sites. For the molecular weight range 76,000-3,3OO,OOO, the extension at these plateau regions was approximately proportional to the square root of the molecular weight except for the highest molecular weight sample studied. This relationship was interpreted to indicate that the polymer was adsorbed under these conditions as a random coil. The conformation of the adsorbed molecule on the several metallic surfaces studied, which included gold, chromium, silver, and steel, was independent of the specific metal. Although the surfaces were “clean” and reproducible, most probably an oxide rather than The Journal of Physical Chemistry
the metal itself comprised the surface for some of these metals. In addition, an adsorbed gas film was initially present on these surfaces prior to immersion in the polymer solvent. In the present paper we will describe the results of a study by means of ellipsometry of the conformation of polystyrene molecules adsorbed on a well-characterized pure metal surface. For this purpose we selected mercury, a metal that is relatively easy to purify, store, and transfer in a pure form. In addition, it provides a flat, nonporous adsorbing surface.
Experimental Procedure The mercury used for the adsorbing surface was purified by repeated passage through dilute nitric acid, then through distilled water, followed by vacuum distillation into ampoules equipped with break seals. An open tube was joined to the break-seal arm and filled with cyclohexane. Dissolved gases were removed from the cyclohexane by boiling in the tube immediately prior to use. With this technique, the mercury in the ampoule was covered with the immersion liquid and transferred into the optical cell without exposure to the atmosphere. Spectrograde cyclohexane was freshly distilled and passed through a column of silica gel and molecular sieve immediately before use. Three samples of (1) Supported in part with funds provided by the Army Research Office (Durham). (2) R. R. Stromberg, D. J. Titus, and E. Passaglia, J . Phys. Chem., 69, 3955 (1965).
CONFORMATION OF POLYSTYRENE ADSORBED ON LIQUIDMERCURY
polystyrene of molecular weight 537,000, 1,300,000, and 3,300,000, were used. They were obtained from Dr. H. W. RlcCormick of the Dow Chemical Co. and had been prepared by an anionic polymerization technique. All of the original samples had relatively narrow molecular weight distributions and were further fractionated by us. The ratio of Mw/M, for the fractionated material was probably lower than 1.OB. The concentration ranges of the polymer solutions studied were: mol w t 537,000, 0.10-2.44 mg/ml; mol wt 1.3 X lo6,0.45-2.26 mg/ml; and mol wt 3.3 X 1 0 6 , l . O l mg/ml. The measurement technique, ellipsometry, has been previously described3v4as has been its application to polymer adsorption ~ t u d i e s . ~The , ~ method is based on the measurement of changes in the state of polarization of light upon reflection from a surface. From the changes measured the optical constants of a surface can be calculated directly. Additional changes in the properties of the light caused by a thin film overlaying that surface allow calculation of the thickness and refractive index of the film. The optical constants of the mercury were determined under cyclohexane. The stability of a mercury surface in this environment was checked for periods of time as long as 100 hr with no change occurring in the ellipsometer readings during this period. The optical constants for the surfaces used for the adsorption studies are reported elsewhere.6 After obtaining the optical constants the cyclohexane was withdrawn from the cell with a hypodermic syringe to a level closely above the mercury and replaced by polymer solution without exposing the mercury to air. Measurements were made during the adsorption process. All adsorption measurements were made under polymer solution. Only negligible changes in solution concentration occurred as a result of adsorption. The measurements were carried out a t 35", which is approximately the 0 temperature for polystyrene in this solvent.
247 1
thickness value calculated by the Drude equations to a root-mean-square average thickness, t,,,, for an exponential distribution. These values for the extension of the adsorbed polymer molecule normal to the surface, were obtained2 by dividing the reported here as t,, value obtained using the Drude equation by 1.5, A typical result for the root-mean-square extension of the adsorbing film on the mercury surface as a function of time is shown in Figure 1. There was no measurable change in extension with time; the first measurement was taken about 30 min after the solvent was replaced by polymer solution. Measurements at shorter times were imprecise apparently owing to low concentration of polymer in the adsorbed film. The vertical lines for each point represent the uncertainty for the individual measurement. Similar results were obtained for all polymer solution concentrations and for the other molecular weight samples studied. Although the extension remained constant, the refractive index of the film increased monotonically showing that adsorption was not completed within the first 30 min, requiring approximately the same time periods to achieve maximum values as had been observed for other metal surfaces.2 This is shown in Figure 2; the increase in adsorbance with time, with
0
I 50
I 100
I
I
150 200 TIWF,rnin
I 250
3W
Figure 1. Root-mean-square extension of adsorbed polystyrene molecule (mol wt 1,29.5,000)normal to mercury surface during the adsorption period. The solution concentration was 2.26 mg/ml.
Results and Discussion
(3) See F. L. McCrackin, E. Passaglia, R. R. Stromberg, and H. L. Steinberg, J. Res. Nail. Bur. Std., A67, 363 (1963),and references
The Drude' equations, which are used for the calculation of the thickness and refractive index of a film, are for a homogeneous film with discrete boundaries. The polymer concentration in an adsorbed polymer film, however, would be expected to decrease with dis-
given.
(4) E. Passaglia, R. R. Stromberg, and J. Kruger, Ed., "Ellipsometry in the hleasurement of Surfaces and Thin Films,, (Symposium Proceedings), National Bureau of Standards Publication 256. U. S. Government Printing Office, Washington, D. C., 1964. (5) R. R. Stromberg, E. Passaglia, and D. J. Tutas, J. Res. Natl' B ~Std.. ~ A67.431 . (1963).
Volume 7'1, Number 8 July 1967
ROBERTR. STRONBERG AND LESLIEE. SMITH
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I
Ol
0
I
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I
I
1
1 (23hnl
I
I
I
I
I
50
100
150
200
250
TIME, rnin
b
Figlire 2. Adsorbance of polystyrene (mol wt 537,000)on mercury surface during the adsorption period. The solution concentration was 2.44 mg/ml.
maximum adsorbaricc occurring within 24 hr, is typical for all the molecular wcight samples and solutions studicd here. The final concentration of polymer in thc adsorbcd film, calculated5 from the measured value of n and the refractive index increment, dnldc, was approximately 9%, very close t o that measured on the other metals. At all solution concentrations studicd for a given molecular weight sample, the values of the cxtciision remained constant with time, as shown in l‘igurc 1. Morcover, the equilibrium value of t he cxtctision was independent of concentration for each molccular weight samplc within the concentration range studied, any small differences being random arid cxperimental. The equilibrium values of the adsorbaiice (or rcfmctive index) were also independent of concentration for thcse concentration ranges. Possibly the most significant result of this study is the finding that the exterision of the adsorbed polymer molcculc on the mercury surface is independent of thc mo1ecul:w weight for the molecular weight range studicd here. These results are shown in Figure 3, along with thc curvc previously reported2 for other surfaces. 111 both cases the polymer, solvent, and tempcr:Lturc wcre the same. The approximately cotist:Lnt v:ilue of 310 A for thc extension on the mercury surface for the rnolccular weight range of 537,0003,300,000 is compared to a range of exterisions of 420S30 A on the other metallic surfaces for the same molecular weight range. It would appear, then, that the conformation of the adsorbed polystyrciie molecule on the mercury surface is considerably different from that previously reported for :L number of metallic surfaces. In the case of the mercury surface, it appears that the polymer niolcculc attains its final conformation relatively early in the adsorption period aiid remains in this conformation :is additional adsorption occurs. This conformation is rclativcly ‘(flat” and thc early arrivals do not
Figure 3. Root-mean-square extension of adsorbed polystyrene as a function of the square root of the molecular weight: -, results on polycrystalline metal surfaces;’ -0-, results on liquid mercury surface.
change into more extended conformations as the surface population increases. Rather, as shown by changes in the refractive index of the adsorbed film, ‘Lholes’’in the adsorbed layer are filled with later arrivals, the concentration of this adsorbed layer increasing with time, until an equilibrium value is attained. At equilibrium the refractive index of the adsorbed film on mercury and, therefore, its polymer concentration are also independent of molecular weight. In addition, the interaction between the mercury surface and the polystryene molecule is such that the value of p , the fraction of segments attached, probably remains approximately constant and independent of molecular weight as well as surface population. In order for the extension to be proportional to the square root of the molecular weight, as was observed for the other metallic surfaces, the value of p must decrease with increasing molecular weight. Thcrc are several possible reasons for the dissimilarities between the adsorption of polystyrene on the liquid mercury surface and the adsorption on the other metallic surfaces. As mentioned earlier, the preparation and cleaning techniques employed for the gold, chromium, silver, and steel surfaces were such that gas molecules, either from the air or from a flame, were probably adsorbed on the surface prior to immersion in the cyclohexane. If such an adsorbed layer were not replaced by cyclohexane or polymer, it is possible that adsorption of the polymer occurred in part on such a layer rather than entirely to the metallic surface itself. This could account for the similar extension and adsorbance values observed on the several surfaces.
CONFORMATION OF POLYSTYRENE ADSORBED ON LIQUIDMERCURY
Fowkes1l,l2has calculated the value of the contribution of the London dispersion forces, yd, to the surface free energy for liquid mecury. He calculated the value of ydHs from the interfacial tensions with respect to a number of hydrocarbons, including benzene arid npropylbenzene, and found the value, 200 dynes/cm at 20°,to he relatively independent of the hydrocarbon used. As there were no differences in y d ~ gvalues obtained with saturated hydrocarbons and with aromatics, he concluded that the interfacial forces between aromatic hydrocarbons and mercury were entirely dispersion forces. calculation^^^^^^ have also been made for the dispersion force component of the surface energy for a number of solids. These are given in Table I for several surfaces, together with the extension of the adsorbed polystyrene molecule as measured in this laboratory. Table I : Comparison of Values of yd and Extension of Adsorbed Polystyrene (Mol W t 537,000) for Various Metallic Surfaces yd,a
Rootmeansquare thickness,
surface
dynes/cm
A
Copper Copper oxide Silver Silver oxide Iron Ferric oxide Gold Mercury
60 67 74, 85 76 108 110 122 200
460 510 440 (steel) 410 300
a The values of yd were obtained from ref 11, 12, and 13; the roobmeari-square thickness values (except for mercury) were from ref 2.
Except for copper, which exhibited other abnormal behavior,* there is a decrease in extension normal to the surface with an increase in the value yd. Although the small difference in the measured extension for gold and steel is certainly not significant, the larger differences of approximately 100 A between silver and steel or gold and 200 A between silver and mercury are significant. It is probable that the adsorbent surfaces of chrome, silver, and steel are oxides. However, as shown in Table I, the values of yd for these oxides are very close to those of the metals themselves. Since, as discussed by Fowkes, hydrocarbons, including aromatics, have only dispersion force interactions with mercury, it is reasonable to use the values of yd of the metal as a relative measure of the interaction energy between polystyrene and the various metallic surfaces if the induced dipole interactions with the other metals
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are small compared with y“. It is possiblc arid consisterit with the results of scvcral thcorctical s t ~ d i e s g ~that ~ ~ thc ~ ~ 5higher interaction cricrgy of thc mercury surface results in a highcr valuc of p arid smaller loops than for the other mctal surfaccs studied. However, this explariatiori docs not accouri t for thc lack of dependence of extcnsion 011 molecular wcight for the polymer on thc mercury surface. In addition to thc previously reported cxpcrimcrital rcsults shown in Figure 3, a theoretical study by Hoevel6 has shown a linear relation betwccri the root-mcari-squurc distance of segments from thc jntcrface arid the squarc root of the molccular wcight for adsorption from a e solvent. All thc metal surfaces previously studied were polycrystalline and hetcrogcncous with rcspcct to the energy of interaction at pfJSSihle adsorption sites on a crystal face as compared to grairi boundaries, dislocations, and geomctric irregularjtics. Although tho interaction energy at such boundaries arid dislocations would be relatively high, thc availability of such sites is low compared to the entirc surface area arid tho value of p for an adsorbing molcculc would be low, accounting for the largc loops. Attachmerit to a crystal face itself may also bc possible for some surfaces; however, although such sites arc riumer~usarid closc, the interaction cricrgies may be sufficicntly low that for such attachments thc value of p is also low. If p is small the loop sizc will be very scrrsitivc to small changes in p . Therefore, for a dccrcasc in p with increasing molecular weight, the size of thc loops may very well be a dircct function of thc coil dimerisions in the solution and directly related to thc squarc root of the molecular weight. In the case of adsorption on liquid mercury, a Smooth homogeneous surface is presented to the polymcr, as well as a higher interaction energy. It is reasonablc, thcn, that thc valuc of p is increased arid thc lOOp size decreased. In this casc thc value of p would bc approximatcly independent of molecular weight arid the size of the loops unaffected by the coil dimcrisions in the solution. As a result of this highcr intcraction energy and the correspondingly higher equili brium value of p , any changes in the exterisiorr normal to (11) F. M. Fowken, J. Z’hye. Chem., 67,2538 (1963). (12) F. M. Fowkes, I n d . Eng. Chem., 56, 40 (1964). (13) E. Thelen, Office of Saline Water liewearch and Development, Report No. 184, U. 6 . Government Printing Ofice, Washington, D. C. 20402, April 1966, p 61. (14) C. A. J. Hoeve, E. A. DiMarzio, and P. I’eyccer, J . Chem. I’hya., 42,2558 (1965). (15) A. Silherherg,
J. Phys. Chem., 66, 1872 (1962). (16) C. A . J. Hoeve, J. Chem. Phye., 44, 1505 (1966). Volume 71, Number 8 July 1007
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F. ACCASCINA, F. P. CAVASINO, AND S. D’ALESSANDRO
the mercury surface during the adsorption period would be expected to be small and perhaps beyond experimental detection in this system. The independence of extension with molecular weight is not necessarily in conflict with Hoeve’s findings.’e The example that he discussed corresponds to a limiting case of weak interactions and large loop sizes, whereas in the case of mercury the interaction is probably strong.
We are proceeding with further studies on “pure” homogeneous surfaces to determine whether the difference in behavior between the two studies discussed above results from differences in interaction energies between surfaces or within a surface or whether the mercury surface, perhaps because of its liquid nature, provides other factors which have not been considered here.
Mechanism of Formation of FeN,2+and of Other Monocomplexes of Iron(II1)
by F. Accascina, F. P. Cavasino, and S. D’Alessandro Inatitule of Physica2 Chemistry, Unisersily of Palernto, Palernto, Italy Accepted and Tranrnitted by The Faraday Society
(June 7, 1966)
The kinetics of the formation of the monoazide complex of iron(II1) in aqueous solution has been studied by the temperature-jump method at various temperatures and a t ionic strength 0.1 M . A general mechanism is proposed which accounts for the available kinetic data on the formation of iron(II1) monocomplexes. The proposed mechanism and the activation parameters confirm the previous suggestion that the rate-determining step of iron(II1) complex formation is the release of a water molecule from the inner coordination sphere of the metal ion.
Introduction Kinetic studies on the formation of complexes of bivalent transition and alkaline earth metals in aqueous solution have shown that the rates of complex formation “e essentially independent of t~~~entering ligand and that the rate-determining step of these reactions is the release of a water molecule from the inner coordination sphere of the metal A different behavior seems to be shown by the iron(111) ; in fact the rate constants for the reactions of Fea+ with various anions (Cl-, Br-, CNS-, S04’-, F-, Na-) appear to be dependent On the nature Of the ligand, increasing with the basicity of the anion. In order to account for this dependence, two different The Journal of Physical Chemiet.ry
interpretations have been formulated. According to the first interpretation,‘-’ the ion pair formed in the first step of the reaction is supposed to undergo an (1) M. Eigen and L. De Maeyer in “Technique of Organic Chemic try,” Vol. VIII, 2nd ed, S. L. Friess, E. S. Lewis, and A. Weissberger, Ed., Interscience Publishers, Inc., New York, N . Y., 1963, Part 2, 895, (2) F. P. Cavasino, Rk. Sei. Rend., AB, 1120 (1966). (3) F. P. Cavasino, J . Phye. c h . ,69, 4380 (1965). For other references see this pawr. (4) M. Eigen in “Advances in the Chemistry of the Coordination Compounds,” The nlacduan co.,New York, N. y., 1961, p 371. (5) M. Eigen, Pure Appl. C h m . , 6 , 97 (1963). (6) H.Wendt and H. Strehlow, 2.Eleklrochem., 66, 228 (1962). (7) F. P.Cavasino and M. Eigen, ~ ksci. . Rend., A4, 509 (1964).
