COMPETITIVE ADSORPTION AT THE GOLD-OILINTERFACE
3833
A Radiotracer Study of Competitive Adsorption at the Gold-Oil Interface’
by Morris L. Smith, Benjamin E. Gordon, and Richard C. Nelson Shell Development Company, Emeryville, California
(Received M a y 21, 1965)
The adsorption characteristics of an oleophilic detergent, calcium dinonylnaphthalenesulfonate tagged with radioactive sulfur-35, have been determined a t the gold-white oil interface at room temperature. The adsorption increases with time, requiring 150 min. to reach a maximum value. The adsorption is completely reversible, and exchange with untagged sulfonate is observed. The force-area curve of the sulfonate a t the air-water interface yields 125 A.2/molecule for a tightly packed film. Displacement of the adsorbed calcium dinonylnaphthalenesulfonate from the gold was achieved by using two nonionic polymeric detergents: (1) a polydisperse polyalcohol prepared by hydrolysis of a polyacetate made by copolymerizing vinyl acetate and Cs-Cle &-olefins; and (2) a monodisperse polyester, poly(dodecy1 methacrylate). The sulfonate-polymer competition is analyzed in terms of Langmuirian adsorption resulting in a close fit of the experimental data. A linear relation is obtained for a plot of the ratio of the surface coverage for each component vs. the ratio of their solution concentrations. The slopes of the lines are a measure of the relative “surface activity” of the two components. A comparison by weight shows that surface activity increases in the order: polyester < sulfonates < polyalcohol. However, the relative surface activity of the different polar groups (or adsorption sites for the polymers) increases in the order: ester < secondary alcohol < sulfonate.
A. Introduction Among the modern methods available for investigating interactions at surfaces, the use of radioactive tracer techniques has enjoyed considerable expansion in the last decade.2 Since the adsorption of molecules on a metal surface is a subject of both practical and fundamental interest, studies of long-chain organic polar compounds tagged with C1*or H3 have received much attention. 3-7 The current interest in detergency in the petroleum industry, particularly with respect to high molecular weight nonionic detergents, has led to several investigations of macromolecule adsorption at the solidliquid i n t e r f a ~ e . ~ - lIlost ~ of these works have dealt with single-solute systems in which the adsorbed film consisted of surfactant and solvent. However, of critical interest is the behavior of mixed films a t the solid-liquid interface. Although the literature abounds with studies of mixed films a t liquid-air and liquidliquid interfaces, the data available for solid-liquid interfaces are meager. In any study of mixed films, the competition for a limited surface is of importance,
for by a detailed analysis the relative “st,rength of adsorption” of the components may be assessed. The present study was initiated with this in mind and since the use of radiotracers is useful to this type of in(1) Presented in part at the 140th National Meeting of the American Chemical Society, Division of Colloid and Surface Chemistry, Chicago, Ill., Sept. 1961. (2) J. T. Davies and E. K. Rideal, “Interfacial Phenomena,” Academic Press Inc., New York, N. I-,, 1963. (3) (a) H. A. Smith and K. A. Allen, J . Phys. Chem., 58, 499 (1954); H. A. Smith and R. M. McGill, ibid., 61, 1025 (1957); H.A. Smith and T. Fort, Jr., ibid., 62, 519 (1958); (b) E. Rideal and J. Tadayon, Proc. Roy. SOC.(London), A225, 346 (1954). (4) J. W. Shepard and J. P. Ryan, J . Phys. Chem., 60, 127 (1956); 63, 1729 (1959). (5) H. E. Ries, Jr., and H. D. Cook, J . Colloid Sci., 9, 535 (1954); J . Phys. Chem., 63, 226 (1959). (6) D. C. Walker and H. E. Ries, Jr., J . Colloid Sci., 17, 789 (1962). (7) H. Schonhorn, ibid., 18, 445 (1963). (8) J. Koral, R. Ullman, and F. R. Eirich, J . Phys. Chem., 62, 251 (1958). (9) J. 5. Binford, Jr., and A. M. Gessler, ibid., 63, 1376 (1959). (10) H. L. Frisch, M. Y. Hellman, and J. L. Lundberg, J. Polymer Sci., 38, 441 (1959). (11) F. M. Fowkes, M. J. Schick, and A. Bondi, J . Colloid Sci., 15, 531 (1960).
Volume 69. Number 11 November 1965
3834
vestigat ion, this sensitive, unambiguous technique was employed. Calcium dinonylnaphthalenesulfonate tagged with sulfur-35 mas chosen as a reference since this compound approximates the present-day detergent petroleum sulfonate^.^'-'^ Two nonionic surfaceactive polymers were chosen for comparison: (1) a polydisperse polyalcohol prepared by hydrolysis of the product obtained from the copolymerization of vinyl acetate and Cs-C18 a-olefins; and (2) a monodisperse polyester, poly(dodecy1 methacrylate), Gold was chosen as the metallic substrate because of its low reactivity; i.e., it uniquely does not chemisorb oxygenlg although physical adsorption must certainly occur.
B. Experimental Section 1. Materials. a. Solvent. The oil used as solvent was a highly refined naphthenic white oil which was freshly chromatographed through dry silica gel immediately prior to use. b. Calcium DinonylnuphthalenesuIfonate-S35.Dinonylnaphthalene (b.p. 210-215 at 0.1 mm.) mas obtained from a distillation of crude nonylated naphthalene (from R. T. Vanderbilt Co., Inc.) through a 900 X 14 mm. Vartin spinning-band column. Ultraviolet analysis showed the product to be greater than 9574 disubstituted, with impurities consisting mainly of rnonoand trinonylated naphthalene. Although the structure of the dinonylnaphthalene is not precisely known, the two nonyl groups (from 1-a-nonenes derived from propylene trimerization) have been shown by infrared spectra to be substituted into the same ring, which is different from that containing the sulfonate group. The redistilled dinonylnaphthalene (0.1 mole) was then sulfonated with 0.065 mole of 101% sulfuric acid containing 110 mc. of S35as sulfuric acid. After neutralization of the reaction mixture with sodium hydroxide, the wdium sulfonates were separated from the unreacted starting material on a Florisil chromatographic column. The eluent for the sulfonate was a 50:50 mixture of dry benzene and methanol. The sodium sulfonates were converted to the calcium salt by treating a hot solution of 10% aqueous isopropyl alcohol with excess calcium chloride and washing the precipitate with water until the washings gave no evidence of reaction with silver nitrate. The product was dried in a vacuum oven at 110” for 48 hr. and was a dry, light tan powder with specific activity of 3.69 mc./g. (determined by liquid scintillation counting techniques using standards). The infrared spectrum was identical with that of known calcium dinonylnaphthalenesulfonate whose preparation and characterization have been describedaZ0 The
The Journal of Physical Chemistry
M. L. SMITH, B. E. GORDON, AND R. C. NELSON
yield based on sulfuric acid was 36.2% of the theoretica amount. Anal. Calcd. for C5sHs6CaSzOs: C, 70.05; H, 8.99; Ca, 4.18; S, 6.69. Found: C, 69.69; H, 8.80; Ca, 4.10; S, 6.42. c. Stearic acid-l-C’* was purchased from Tracerlab, Inc., and was used without further purification (m.p. 69.5). The specific activity was found to be 4.42 mc./g. d. The polyalcohol used in these studies was characterized and discussed in detail in a previous paperall This polymer is the unfractionated product from the hydrolysis of the copolymerization of a Cs-C18 aolefin and vinyl acetate. The average molecular weight in chloroform is 20,000 as determined by the ultracentrifuge. The average distribution of monomers was determined as four hydroxyls pel a-olefin. The “equivalent weight” of the polymer is then assigned as 424, the equivalents per mole as 47, and the number of hydroxyls per molecule as 188. e. The poly(dodecy2 methacrylate) was prepared by radical polymerization of redistilled dodecyl methacrylate in benzene initiated by a,&’-azodiisobutylnitrile. The unreacted monomer was removed by dissolution of the crude product followed by methanolinduced precipitation of the polymer. This was repeated several times followed by drying the finished polymer in a vacuum oven at 60’ for several hours. The weight-average molecular weight in chloroform is 50,000 as determined by the ultracentrifuge. The “equivalent weight” of the polymer was 254 (mole weight of dodecyl methacrylate) , and each polymer has ca. 196 ester groups per molecule. 2. Apparatus. The detector for measuring the adsorption of the tagged sulfonate was a modification of that originally suggested by Cook.21 It consisted of a conventional Tracerlab TGC-2 Geiger counter in which the mica window had been replaced by a thin (12) B. J. Fontana and J. R. Thomas, J. Phys. Chem., 65, 480 (1961); B. J. Fontana, ibid., 67, 2360 (1963). (13)C.Peterson and T . K. Kwei, ibid., 65, 1330 (1961). (14) A. Silberberg, ibid., 66, 1872, 1884 (1962). (15) R. R. Stromberg, E. Passaglia, and D. J. Tutas, J . Res. il’atl. Bur. Std., A67, 431 (1963); R. R. Stromberg and W. H. Grant, ibid., A67, 601 (1963); R. R. Stromberg, W. H. Grant, and E. Passaglia, A68, 391 (1964). (16)K. Shinoda and K. Kinoshita, J. CoEEoid Sci., 18, 174 (1963); K. Shinoda and J. Nakanishi, J. Phys. Chem., 67,2547 (1963). (17) F. M. Fowkes, ibid., 67, 1094 (1963). (18)S. Kaufman and C. R. Singleterry, J . CoEZoid Sci., 12, 465 (1957). (19) B. M . W. Trapnell, Proe. Roy. Soc. (London), A218, 566 (1953). (20) F. M. Fowkes, J . Phys. Chem., 66, 1843 (1962). (21) H.D.Cook, Rev. Sci. Inslr., 27, 1081 (1956).
COMPETITIVE ADSORPTION AT
THE
GOLD-OILINTERFACE
(0.8 mg./cm.Z) Mylar polyester film secured with EPON@resin glue. The gold was vacuum evaporated onto the Mylar and stored in a vacuum desiccator until use. While protected from gross contaminants in this way, the gold film presumably is covered with a physically adsorbed mixed layer of water and atmospheric gases. Nevertheless, the reproducibility of the isotherms was excellent a t f2’30, The entire tube was flushed with counting gas through two appropriately situated side arms. A constant rate of flow of gas was maintained through the tube a t a pressure of 25 mm. of water above atmospheric. The geometric area of the gold film was 8 cm.2, and each surface wab used only once. The signal from the counter was measured by a Tracerlab SC-18A Superscaler driving a Tracerlab SC-5F Tracergraph printing interval timer. With these instruments, the time required to reach a preset number of counts was printed on a tape followed by automatic resetting to zero between each interval. This assembly was much more precise than a rate meter and allowed data to be collected automatically a t short intervals over extended perjods. 3. Procedures. a. Adsorption studies were carried out by inserting the counter into the solution of the radioactive sulfonate contained in a Teflon beaker. Concentrations from 0.001 to 0.300% weight were studied, and all gave the same saturation value. As the radioactive surfactant was deposited on the gold, the increase in counting rate with time was followed until the rate became constant. This increase could be observed because of the very short range of the Sa5 p particles, i.e., about 0.1 mm. in oil; the bulk of the activity in the solution did not contribute to the solution background. A solution concentration of 0.03070 weight gave the most convenient counting rate and was used for most experiments. All experiments were at room temperature (22 f 1”). b. T h e thickness of the gold Jilm was measured by (1) directly weighing the amount deposited on a known area and (2) by @-ray adsorption of a standard C14 source. Both of these methods gave gold-film thicknesses of 500-1000 A. An electron micrograph showed the average size of the gold particles to be only 20-40 A. in diameter, making it highly probable that the film was continuous and none of the Mylar backing was exposed. From these considerations, it was valid to assume that the gold film was sufficiently thick to serve as an infinitely thick layer of metal. c. Determination of the roughness factor was achieved on the by depositing a monolayer of stearic a~id-1-C’~ gold film from its melt a t the melting point of the acid by the method of Zisman.22 The method of the
3835
molten drop ensured as nearly as possible the deposition of a condensed monolayer without concomitant problems arising from solvation of the carboxyl group or the “carry-out” difficulties noted by others.FI4,6 A direct count yielded the amount of acid absorbed. As a check on the mechanical technique, the acid layer was then desorbed in 200 ml. of reagent toluene, and the total acid initially adsorbed in the monolayer was found by extrapolation of the desorption curve to zero time. These two numbers agreed to within 5%, but better precision was obtained with the extrapolated value for the monolayer. Consequently, the desorption curve value was used in the following roughness factor calculation. From the chosen value of 24 A.z for the cross-sectional area of the carboxyl group, the true area of the gold film was calculated to be 9.9 X 10l6 A.2 compared to the geometric area of 8 X 10l6 Thus, the roughness factor was 1.2 based on the stearic acid. This factor was used in the calculations on calcium dinonylnaphthalenesulfonate. d. T h e force-area curve for calcium dinonylnaphthalenesulfonate spread from benzene solution on a 1 M aqueous solution of calcium chloride was carried out on a recording surface balanceF3thermostated a t 25O. No significant hysteresis was noted with this sulfonate. The reproducibility of the curve in Figure 1 is about 90% at film pressures less than 10 dynes/cm. but rises to 97% at film pressures greater than 30 dynes/ cm. e. T h e speciJic activity of the sulfonates in terms of Geiger counter efficiency was determined by spreading 25 to 100 pg. in benzene onto the slightly depressed gold-film tube followed by subsequent evaporation of the benzene. The specific activity was 972 c.p.m./ pg. It was found that self-adsorption was not serious until a total deposit of a t least 1000 pg. was present. f. Competition Experiments. The radioactive sulfonate was adsorbed on the gold film from white oil, and the counting rate was followed until equilibrium was reached. Known amounts of the inactive polymer were added from a concentrated stock solution in white oil, and the resulting decrease, if any, in counting rate attendant upon each increment of added polymer was recorded. There was a several thousandfold excess of both materials present a t all concentration ratios above that required to cover the gold surface completely. The volume of the added solution was so small (the total of all additions in any one run was less than 1.5 ml.) that the concentration of the original (22) R. L. Cottington, E. G. Shafrin, and W. A. Zisman, J. Phys. Chem., 62,513 (1958). (23) M.J. Schick, J. PoEymer Sci., 25, 465 (1957).
Volume 60, Number 11 November 1966
M. L. SMITH, B. E. GORDON, AND R. c. NELSON
3836
3000
2000
d
b V 1000
0
100
200
400
300
500
Time, min.
Figure 2. Adsorption of calcium dinonylnaphthalenesulfonate at gold-white oil interface: stirring in oil, at room temperature.
0
I
120
I
I
I
140
I
1 eo
1
180
200
A. ¶/molecule.
Figure 1. Force-area curve of calcium dinonylnaphthalenwulfonate a t air-water interface (1 M CaC12).
test solution remained sensibly constant. Sufficient time elapsed (25-48 hr.) between each addition to ensure that all data represented the equilibrium state.
Results and Discussions 1. Force-Area Curve. The force-area curve of calcium dinonylnaphthalenesulfonate at the waterair interface is shown in Figure 1. Sufficient similarity in the structure of adsorbed films is generally found between different interfaces to justify some extrapolation from one to the other. The shape of the curve indicates that the film is of the liquid-expanded type showing a maximum area per molecule of 200 A.z. The area per molecule when the film is tightly packed is about 125 &2at40 dynes/cm. 2. Adsorption Curves. A typical adsorption curve obtained with the calcium dinonylnaphthalenesulfonate-Sa5 in white oil is shown in Figure 2. The time required to reach apparent maximum adsorption vaned from ca. 4500 min. with no stirring to ca. 100 min. with rapid stirring. The reasons for these extremely slow adsorption times (compared to diffusion times) are not really understood, but have often been observed in solid-liquid solutions. Fowkes has noted this same phenomenon in rates determined by orientation potentials at the metal-ion interfa~e.~‘ The adsorption was shown to be reversible by replacing the active solution with a new solution containing nonradioactive calcium dinonylnaphthaleneThe Journal of Phyeical Chemhtry
sulfonate and observing the drop in counting rate to near background. Upon exchange of the solution again, the counting rate increased once more to the value illustrated by the plateau in Figure 2. 3. Monolayer Formation. From the net increase in counting rate illustrated in Figure 2 (1440 c.p.m.), the specific activity (975 c.p.m./pg.), the area per and the roughness factor (1.2), molecule (125 the number of monolayers is 1.25. Values ranging from 1.20 to 1.35 were obtained many times on different tubes. In the monolayer calculation, there is some uncertainty in the choice of 125 8 . 2 as the size of the molecule since the area is quite dependent on the film pressure. No information on film pressures at the gold-oil interface is available, so the point of film collapse was chosen for the calculation. In light of these uncertainties and those inherent in the roughness factor evaluation, these values establish that the film is essentially a monolayer. 4. Competition Studies. Figure 3 illustrates the displacement of the preadsorbed sulfonates from gold by the polyalcohol and the polyester. Since only the sulfonates are radioactive, the activity observed at any time should be proportional to the area occupied by this compound; the amount covered by the polymer is determined by difference. The concentrations studied are listed in Table I. If the Langmuir adsorption theory is applied to the “idealized” system represented by a gold surface, then for the ith component in a mixture of n components, all of which are simultaneously adsorbing, the rate of adsorption is adsorption rate = k,C,(l
- el + e2 +
...
and for desorption, the rate is merely (24) F. M . Fowkes,J . Phys. Chem., 64, 726 (1960).
+ 6,)
(1)
COMPETITIVE ADSORPTION AT THE GOLD-OIL INTERFACE
3837
desorption rate = k-8, (2) Analogously after Langmuir, at equilibrium we have
k,c,(i - el
+ e2 + . . . + e,)
=
(3) Now let K , = k,/k-, be a measure of the “surface activity” of the ith component, and the general expression for the isotherm becomes
K,c,
=
e,/(i - ze,)
(4)
which reduces simply to the classical Langmuir expression when n = 1. For two competing species, the relationship may be expressed as
of_ --- K,C,
(5)
K,C, with the slope of the line, K,/K,, being equal to the “relative surface activity” of i to j . If Ai is the radioactivity when only component 1 is adsorbed and A 61
0
- K2C2
2.0 C,/Cl.
5.0
2.5
i
(6)
KiCi
4.0
3.0
Figure 4. Langmuir plot of polyalcohol (2) us. calcium dinonylnaphthalenesulfonate: 0, C2/C1 by weight; 0, C2/Cl by moles, moles of hydroxyl per moles of sulfonates.
the observed activity when both 1 and 2 are adsorbed and only 1 is radioactive, then eq. 5 is expressed in experimental terms as
Ai - ---A A
1.0
2.0
1.5 G -.
1.0
70polymer. 0.5
Figure 3. Gold-oil interface, a t 21°, the effect of increasing polymer concentration to displace calcium dinonylnaphthalenesulfonates: 0, polyalcohol; 0, polyester.
Table I : Competitive Concentrations of Polymers” Polyalcohol, lo-* in 100 ml.
g. X
3.1 6.2 30.0 37.5 52.5
Polyester, g. X 10-8 in 100 ml.
7.5
0
10
20 CdCi.
30
40
Figure 5. Langmuir plot of poly(dodecy1 methacrylate) (2) calcium dinonylnaphthalenesulfonate: 0, C2/C1 by weight; 0, C&21 by moles, moles of ester per moles of sulfonates.
u8.
33.0 45.0 93.0
156.0 282.0
a The concentration of calcium dinonylnaphthalenesulfonate waa always 30.0 X lo-* g./100 ml.
The data for the polyalcohol us. calcium dinonylnaphthalenesulfonate applied through eq. 6 are illustrated in Figure 4. These yield straight lines and exhibit no deviation from linearity even out to 75% displacement of the sulfonate. Volume 69,Number 11 November 1966
M. L.SMITH, B. E. GORDON, AND R. C.NELSON
3838
When calcium dinonylnaphthalenesulfonate is displaced by poly(dodecy1 methacrylate), the Langmuir plot results in Figure 5. Again, the curves are linear up to 70y0 displacement. The classical interpretation of adsorbed monolayers seems to explain this idealized system adequately even though the competing species are themselves complex. 5. Surface Activity. The slopes of the lines of Figures 4 and 5 are measures of the relative “surface activity” of the competing species. Once the values of K 2 / K 1 were determined from the experiments, another ratio, i e . , polyester vs. polyalcohol, can be obtained indirectly. All of the values of K2/K1 are listed in Table 11. These values can be expressed in several ways depending on the concentration units chosen. The comparisons have been made by weight and by Table 11: Relative Strengths of Adsorption, KdKi (Goldail Interface, 21’) Comparison
Weight
Polar group
Polyalcohol us. Ca sulfonates Polyester us. Ca sulfonates Polyalcohol us. polyester
1.98 0.36
0.21
5.50
0.06 3.50
polar group. The former comparisons are of interest from an economic viewpoint while the latter possess
The J o u d of Phvsdcal Chemistry
theoretical value. The comparison of polar groups is significant, since a frequently cited mechanism of adsorption proposes that these are the sites of adsorption. Studies on the polyalcohol have shown” the polymer is uncoiled with most of the hydroxyls present at the interface. From the comparison given here, the relative strengths of adsorption are sulfonates > alcohol > polyester. It would indeed be helpful to be able to relate this order of surface activity to some other independent properties of the surfactants. However, such attempts in the past have resulted in little SUCC~SS.~~ It~ ~is* apparent that numerous variables need be assessed before elucidation of the nature of solid-solution interfacial phenomena is forthcoming. However, the results presented here are gratifying in that relative surface activities have been quantitatively assigned to surfactants in a complex, heterogeneous system.
Acknowledgments. The authors thank F. W. Anderson for preparing the gold-filmed Geiger counters and examining this surface with the electron microscope. They also much appreciate the aid received from Dr. F. M. Fowkes during numerous discussions. (25) See J. Arnold, J. Am. Chem. Soc., 61, 1911 (1939),for other leading references. (26) B. Eric, E. V. Goode, and D. A. Ibbitson, J. Chem. Soc., 55
(1960).
