Langmuir 1988,4, 140-144
140
.-0 0.6
-
0.4
-
c L
LL
-0 E,
8
i
0
A
0 0
0
:
Figure 12. Variation of micelle composition with monomer composition: ( 0 )LiFOS-LiTS, (A)LiFOS-LiDS, (W) LiFOSLiDeS. The l i e is computed from the group contribution method. evidence to resolve the issue of the coexistence of two kinds of mixed micelles or not. In Figures 9-12, the monomer concentration (see Figures 9-11) and micellar pseudo-phase composition (see Figure 12) are plotted versus the composition in the monomer phase for mixtures of LiFOS with LiTS, LiDS, and LiDeS,
respectively. The predicted curves from the group contribution method are shown for comparison. We have already proposed that the mixture cmc can be successfully given by the group contribution method.’ An important feature of the group method is to be able to describe the series of mixed systems containing the same functional groups by the use of the same interaction parameters. In contrast, the regular solution theory cannot make a prior prediction of the micelle composition even for the series of mixed systems containing the same functional groups. In conclusion, ultrafiltration is a useful method to indicate the variation of monomer concentrations and compositions beyond the mixture cmc. The micelle demixing region was increased by the increase of the alkyl chain length as expected. The fixed monomer composition, which corresponds to the azeotropic condition, was interpreted by the occurrence of micelle demixing.
Acknowledgment. We are grateful to Dainippon Ink Chemical Industry Co., Ltd., for providing the fluorocarbon surfactant. Registry No. LiFOS, 29457-72-5; SPFO, 335-95-5; SDS, 151-21-3;LiDeS, 2044-55-5; LiDS, 2044-56-6;LiTS, 52886-14-3.
Ellipsometric Observation of the Adsorption of Sodium Dodecyl Sulfate Gregory J. Besio,t Robert K. Prud’homme, and Jay B. Benziger* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 Received March 16, 1987. In Final Form: July 14, 1987 In this paper we report the observation of the formation of surface micelles, or hemimicelles, using ellipsometry. SDS is adsorbed onto a platinum electrode from solutions with concentrations below the critical micelle concentration (crnc). Formation of the surface micelles is initiated by applying a potential at the electrode, inducing local concentrations above the critical hemimicelle concentration (chmc) near the electrode surface. The thickness of the adsorbed layer, as measured by ellipsometry, based on a film refractive index of 1.44,at low potentials is 10-15 A, which corresponds to the extended dimension of the SDS molecule. At higher potentials, transitions to films 2-3 times the initial film thickness are observed.
Introduction In solution, surfactants spontaneously aggregate, as a result of their unique hydrophilic/ hydrophobic nature. The aggregation is a function of the solution concentration of the surfactant. At low concentrations, typically lesa than 1 mM, the surfactant molecules exist in solution as monomers and dimers. Above a certain transition concentration, known as the critical micelle concentration or cmc, the surfactant will form micelles. As the surfactant solution concentration increases further, the number of micelles in the solution increases until a second transition occurs in which the aggregation number of the micelles and the shape of the micelles change. Surfactant solutions in contact with solid surfaces exhibit similar transitions to their pure solution anal~gs.l-~ A solid surface in contact with a dilute surfactant solution will have individually adsorbed surfactant molecules. As the solution concentration is increased, surface aggregation +Currentaddress: General Electric CRD, PO Box 8, Building K-1, 4B33, Schenectady, New York 12301
will occur in parallel to the solution aggregation. At a concentration of approximately 0.1 cmc, surface aggregates termed “hemimicelles” will begin to form. Hemimicelles are the adsorbed equivalent of micelles. Above the critical hemimicelle concentration (chmc),5-7 the surface coverage rapidly increases until the entire surface is coated with a surfactant bilayer, a t which point the surface is saturated, and no further appreciable adsorption will occur. The (1) Schamehorn, J. F.;Schecter, R. S.;Wade, W. H. J. Colloid Interface Sci. 1982,85(2),463. (2) Schamehorn, J. F.; Schecter, R. S.;Wade, W. H. J. Colloid Interface Sci. 1982,85(2),479. (3) Nunn, C. C.;Schecter, R. S.;Wade, W. H. ACS Symposium on Chemistry and Enchanced Oil Recovery, Atlanta, GA, 1981. (4) Nunn, C. C.;Schecter, R. S.;Wade, W. H. J . Colloid Interface Sci. 1981, 80(2), 598. (5) Harwell, J. H.; Schecter, R. S.; Wade, W. H. AIChe. J. 1985,31(3), Helfferich, F. G.; Schectr, R. S. AIChe. J. 1982,28(3), 415. Harwell, J. H.; 448. .
(6) Lim, H. K.;Fernandez, M. E.; Schamehorn, J. F.; Nunn, C. C.; Wade, W. H.; Schecter, R. S.,to be published. (7) Harwell, J. H.; Hoskins, J. C.; Schecter, R. S.; Wade, W. H. Langmuir 1985, 1, 251.
0 1988 American Chemical Society 0~43-~463/8S/2404-0~40$01.50/0
Langmuir, Vol. 4, No. 1, 1988 141
Ellipsometric Observation of SDS Adsorption
IO0
7
/,yb’
w
0.1
CHMC CMC log Concentration
I . Henry’s Law Adsorption (C(CHMC) 1 1 . Rapid Adsorption (CHHCCMC)
Figure 1. Schematic and graphical illustration of the adsorption conformations and relative adsorption amounts for surfactants adsorbed on solid surfaces.
adsorption saturation plateau occurs when the solution concentration is a t or above the cmc. Because the adsorption of surfactant increases rapidly above the chmc, the forrmation of the surface bilayer is almost instantaneous. In a batch type experiment, in which an adsorbing solid is introduced into a surfactant solution, the adsorbed amount of surfactant will “jump” to the plateau level if the solution concentration is great enough. This behavior has been c o n f i i e d experimentally in a number of experimental systems.’-’ The adsorption transition behavior is illustrated schematically and pictorially in Figure 1. The purpose of the experiments described here is to investigate the applicability of ellipsometry to the study of the surface films formed on a surface in contact with a surfactant solution with particular emphasis on the identification of the formation of hemimicelles. Ellipsometry provides a direct measurement of the thickness of the film formed and an estimate of the refractive index of the film, which is related to surface concentration. The technique of ellipsometry has been applied to the study of adsorbed monolayers of gas, with an accuracy of fl A.8 Additionally, a number of applications of ellipsometry to electrochemistry have been reported in the l i t e r a t ~ r e , “ ~where the formation of adsorbed layers formed electrochemically under the influence of a surface potential were studied ellipsometrically. In addition to ellipsometry, we apply cyclic voltametry (CV) to monitor the adsorption of ions onto the platinum surface. CV is shown to provide a way to prepare reproducible, clean surfaces for the surfactant adsorption measurements. The surfactant chosen for the study was sodium dodecyl sulfate (SDS),because it has been studied extensively and is available in extremely pure form. Ad(8) Bootama, G. A,; Meyer, F. Surf. Sci. 1969, 14, 52. (9) Kruger, J. Aduances in Electrochemistry and Electrochemical Engineering; Muller, R. H., Ed.; Wiley: New York, 1973; Vol. 9. (IO) Chiu, Y. C.; Genshaw, M. A. J.Phys. Chem. 1969, 73, 3571. (11) Chiu, Y. C.; Genshaw, M. A. J.Phys. Chem. 1968, 72,4325. (12) Reddy, A. K. N.; Genshaw, M. A.; Bockris, J. OM. J.Chem. Phys. 1968, 48, 671. (13) Minc. S.; Misiura, A. Elektrokhimiya 1979, 15(1), 147. (14) Brodskii, A. M.; Kudryavtseva, Z. I.; Urbakh, M. I.; Zhuchkova, N. A.; Shumilova, A. Elektrokhimiya 1980,16(2), 208. (15) Gottefeld, S.; Babai, M.; Reichman, B. Surf. Sci. 1976,57, 251.
ditionally, the cmc of SDS is well defined,16-19thus simplifying experiment design.
Experimental Section Materials and Apparatus. The SDS used was obtained from J. T. Baker (electrophoresis grade, 99.99% pure, lot no. 132252). The sodium perchlorate used for the buffer solutions was obtained from Fisher Chemical (reagent grade). Both were used as received. Solutions were prepared from laboratory distilled water that had been filtered through 0.22-pm Millipore filters. The platinum electrode was prepared from polycrystalline platinum film, polished with successively finer alumina grits, ending with 0.05-pm particle size. After being polished, the foil was cleaned in a 50/50 mixture of sulfuric and nitric acid to remove any residual adsorbed organic material. After the acid bath, the foil was rinsed with copious amounts of distilled water and stored in a sodium perchlorate solution until use. The experiments were performed in an ellipsometry cell fabricated from Kel-F (poly(trifluoroethylene), 3M, Minneapolis, MN). A three-electrode setup was controlled with a Bioanalytical Systems CV-1B voltammetry unit. The reference electrode was a Ag/3 M AgCl electrode. The auxiliary electrode was a platinum wire. The working electrode was the previously described platinum foil, which also acted as the adsorbing substrate. The ellipsometer used in the experiments was built in our laboratory by using a combination of commercial and custom fabricated components. The device built was a rotating compensator ellipsometer (RCE);%the principles of operation of the RCE are described by Hauge?I The light intensity from the ellipsometer was digitized in synchronization with an optical encoder; 256 points per revolution of the compensator were obtained. Typically, data from 30 consecutive revolutions were averaged to improve signal to noise. The wave forms were obtained with an IBM pC and were Fourier transformed. The ellipsometric parameters, A and $, were determined from the second and fourth harmonic Fourier coefficients. From this system the ellipsometric parameters could be determined every 30 s to an accuracy of f0.03°. Experiment. A number of experiments examining the cyclic voltamagrams for the oxidation and reduction of HzOon platinum were run to establish a cleaning procedure for the Pt surface. The sweep limits were set to vary between the electrolysis of water to hydrogen or oxygen as indicated by a steep increase in the measured current. By this process, it was determined that in 0.1 M sodium perchlorate the onset of oxidation of the surface occurred at +1.0 V and the onset of reduction at -0.5 V. These values agreed reasonably with published values for acidic and basic environments at similar concentrations.12,22The cleaning procedure for the Pt surface was then established to be the following: the surface was brought to a potential of 1.2 V and held there for 2-3 min; the potential was then switched to -0.6V and again held for 2-3 min. The cycle was repeated until constant values for the measured ellipsometric parameters A and were reached at both extremes. This procedure would oxidize or reduce any adsorbed organic material to gaseous constituents, which would then diffuse away from the surface.22 The reproducibility of the optical constants measured after the electrolysis was taken to be an indication of a clean surface. Adsorption of SDS was carried out at three concentrations and five imposed potentials. The concentrations were chosen to be 1order of magnitude less than the chmc, on the order of the chmc,
+
~
~~
~~
(16) Vijayan, S.; Woods, D. R.; Vaya, H. Can. J. Chem. Eng. 1977,55, 718. (17) Vijayan, S.; W d , D. R.; Vaya, H. Can. J. Chem. Eng. 1978,56, 103. (18) Vijayan, S.; Woods, D. R.; Vaya, H. Can. J. Chem. Eng. 1979,57, 496. (19) Vijayan, S.; Woods, D. R.; Vaya, H. Can. J. Chem. Eng. 1980,58, 485. (20) Hauge, P. S.; Dill, F. H. Optics Commun. 1975, 14(4), 431. (21) Hauge, P. S. J. Opt. SOC.Am. 1978, 68(ll), 1519. (22) Benziger, J. B.; Pascal, F. A.; Bernasik, S. L.; Soriga, M. P.; Hubbard, A. T. J.Electroanal. Chem. 1986, 198, 65. (23) Besio, G. J. Ph.D. Thesis, Princeton University, Princeton, NJ, 1986.
142 Langmuir, Vol. 4, No. 1, 1988 105
A
[degrees]
Besio et al.
,
105
I
A [degrees]
104
103
-0
2
1
= 0.01 mM
104
I
0
C,
I
,
1
2
Time [hours]
Time [hours] 105
105
1-E
A [degrees]
A [degrees]
104
,--
,
I
0
1
104
-
-0.2v
-.
I
1
I
2
0
1
2
Time [hours] Time [hours] Figure 2. Measured ellipsometric parameter A as a function of time for applied potentials of 0,0.2,0.4,0.6,and 0.8V for buffered SDS solutions of concentration (a, top left) 0.0, (b, top right) 0.01,(c, bottom left) 0.1, and (d, bottom right) 1 mM. Table I. Eauilibrium A and dA Values for SDS Adsorotion ExDeriments applied electrode potential, V vsAg/AgCl -0.2 0 0.2 0.4 0.6 0.8
A, deg
104.40 104.16 104.02 104.03 103.52 103.51
cSDS = 0.01 mM dA, deg -0.24 -0.38 -0.37 -0.88 -0.89
cSDS
t, 8,
7 11 11
26 16
A, deg
105.20 104.84 104.65 104.76 104.28 104.06
and on the order of the cmc. The cmc of SDS is given in a number of citations and is 8 mM a t 25 OC.lG-lg Subsequently, the adsorption experiments were carried out a t 0.01,0.1, and 1 mM. Adsorption was carried out at applied potentials of 0.0-0.8 V in steps of 0.2 V. The protocol for the experiments was as follows. Fresh SDS solutions in 0.1 M perchlorate solution were introduced into the ellipsometer cell, and the platinum electrode was cleaned electrochemically. At the end of the f d reduction cycle, the potential was switched to the measuring potential and the ellipsometric measurement initiated. Again, it was assumed that the products of the oxidation/reduction cleaning cycles diffused away from the interface as dissolved gasses. The ellipsometer was programmed to average 30 revolutions per pass, yielding one measurement every 30 s. Adsorption was monitored for 2 h. Plots of the A values for the four concentrations as a function of time and potential are shown in parts a 4 of Figure 2;tabulated values of the steady-state (as measured after 2 h) A values are given in Table I. The values of A show a rapid drop followed by a gradual leveling off. After 2 h, the A values appeared to reach a near constant value, although a t the highest applied potentials A still seems to be decreasing. The values of $ show
= 0.1 mM dA, deg
-0.36 -0.55 -0.44 -0.92 -1.14
CQDS =
t,,8,
11
16 13 27 34
A, deg
104.73 104.27 104.20 104.29 103.17 103.17
1 mM dA, deg
t,, 8,
-0.46 -0.53 -0.44 -1.56 -1.56
14 16 13 46 46
the reverse trend of h; they show an initial increase followed by a leveling off. The changes in the t j values were much less than those observed for A. While A decreased by approximately lo, $ increased by only 0.2O. Previous researchers have measured adsorption equilibrium times ranging from 4 h4to 2 days.24 Allara used a variety of techniques and found that if the and Nuz20~~ solution concentration is high enough the bilayer forms within a matter of hours; however, defects still exist in the surface assembly, and the last 1520% of the bilayer forms more slowly. The data indicate the equilibrium values of A decrease with increasing applied potential.
Discussion In addition to the evidence collected by Scheder et aL-' regarding the formation of surface micelles and surface bilayers in surfactant systems, there is ample evidence in t h e literature suggesting the formation of multilayer adsorption films for similar low molecular weight molecules. (24) Allara, D.L.;Nuzzo, R. G. Langmuir 1985, 1, 45.
Ellipsometric Observation of SDS Adsorption
1
2
3 40
--
O . O l m Y SOS 0.1mM SOS 1mY
SOS
Adsorbed Layer . Thickness
[AI
20
Langmuir, Vol. 4, No. 1, 1988 143
- -1.0 c!4
-
[degrees]
0
1 0.0 0.0
0.2
0.4
0.6
0.8
Applied Potential [V vs. Ag/AgCI]
Figure 3. Adsorption of SDS as a function of applied potential. The scale on the right-hand axis shows the equilibrium dA value; the scale on the left-handaxis shows the corresponding thickness of an adsorbed layer of refractive index 1.44 calculated for the film.
The formation of multilayer films on platinum was first observed by Bockris and Swinkels26*26 using a radiotracer adsorption experiment. The adsorption of n-decylamine was found to exceed a surface coverage of 8 = 1 a t certain applied potentials. The authors postulated that a second layer adsorbed on the hydrophobic surface formed by the Fist adsorbed layer, with the hydrophobic surface formed by alkane chains from the first layer projecting perpendicular to the surface. Their evidence of multilayer adsorption was the calculated surface excess concentration above that required for a simple monolayer coverage. Recently, Allara and N u z ~ have o ~ ~examined the adsorption of n-alkanoic acids with carbon numbers ranging from 8 to 25 using nulling ellipsometry. They deduced that these acids formed close-packed monolayers when adsorbed from dilute solutions ( c 5 mM) onto oxidized aluminum surfaces. The thickness was found to correspond to the length of the carbon chain, and the surface coverage 8 was equal to 1. Multilayer adsorption was observed in the systems with carbon numbers less than 12, where the film thickness measurements exhibited large scatter. They repeated the experiments a t a lower concentration (c 0.5 mM) and found that the ultimate layer thicknesses were unchanged, though the adsorption kinetics were slowed. Again, the multilayer adsorption for chains with carbon numbers less than 12 was noted. The results of our experiments on SDS adsorption are summarized in Figure 3 and show dA as a function of applied potential for the three solution concentrations. The data show a plateau adsorption thickness up to an applied potential of 0.4 V vs Ag/AgCl followed by a jump in the adsorbed layer thickness to a t least double the low-potential plateau. The dA plot was constructed by subtracting the equilibrium A value from the steady-state value (measured after 2 h, as shown in Figure 2) a t each of the applied potentials. The solutions had been degassed, and since perchlorate has been shown to have negligible adsorption on Pt electrodes,'O the optical constants of the platinum substrate measured a t potential were used as a base line.
-
-
(25) Bockris, J. O'M.; Swinkels, D. A. J. J.Electrochem. SOC.1964, 11(6),736. (26) Bockris, J. O'M.; Swinkels, D. A. J. J.Electrochem. SOC.1964, 11(6), 743.
The refractive index of the adsorbed layer may be calculated by using the aggregation number and volume of an SDS micelle, as measured by Almgren and S ~ a r u p . ~ ' For SDS, the micelle aggregation number is 70 and the micellar radius is 20 A. From these values, the concentration of SDS in a micelle is calculated to be 1 g/cm3. The dn/dc value for SDS was measured in our laboratory and was found to be 0.110 cm3/g. Therefore, the refractive index of an SDS micelle would be approximately 1.44. This value of the refractive index may be used with the ellipsometric parameters to determine the thickness of the adsorbed layer.28 This calculation was done with a program developed a t NBS.29 The layer based on the measured A values is approximately 10-15 A for the potentials of 0.4 V or below, corresponding to the observations of who measured on adsorbed layer thickness of -16 A for an n-alkanoic acid with a carbon number of 12. The measured values for +, although not used for any further calculation, were also found to be of the correct sign and magnitude when compared to the calculated values. Although in theory one could use both # and A to determine both the film refractive index and the film thickness, we found the refractive index contrast between the adsorbed film and the solution was not sufficient to give both ellipsometric parameters with adequate precision for such a determination. As A is approximately linearly related to film thickness for a constant refractive index, and it showed a significant change, we chose to assume a refractive index and calculate the film thickness based on the measured values of A. A second complication in comparing the data to model calculations concerns the fact that a t potentials above 0.2 V a hydroxide layer is formed on the electrode. This can be modeled with a two-layer model in the NBS program. We could not provide all the necessary parameters for such a model. To circumvent this problem, we used the dA values and a one-layer model. For the applied potentials of 0.6 V or greater, the thickness of the adsorbed layer is doubled for the 0.01 and 0.1 mM solutions and tripled for the 1 mM solution. The thickness values indicate multilayer adsorption. Specifically, for the two lower concentrations, the formation of a surface bilayer or hemimicelle is strongly indicated. For the concentration which approaches the cmc, the formation of more complex surface entities is indicated. From Schechter e t al.,l the observed thickness changes are explained in terms of a surface excess vs solution concentration plot as presented in Figure 1. A t very low concentrations, 1 order of magnitude or more below the cmc, adsorption is very low and increases linearly with solution concentration. A transition occurs in adsorption near a concentration of 0.1 cmc, or the critical hemimicelle concentration, in which a rapid increase in surface concentration occurs. Finally, surface adsorption plateaus at the cmc. All of our experiments were carried out in solutions with concentrations below the cmc. What our experiments show is that the applied potential at the surface can be used to induce the transition to saturation surface coverage by inducing local concentrations near the electrode surface which are above the CHMC. The results show that surface potential is very important to adsorption, as a shift in surface potential can dramatically increase the amount of adsorption, especially in dilute solutions. The observation (27) Almgren, M.; Swarup, S. J. Colloid Interface Sci. 1983,92(1), 256. (28)Aspnes, D. E. In Optical Properties of Solids: New Developments; Seraphin, B. O., Ed.; North-Holland Amsterdam, 1976; p 776. (29) McCrackin, F. L. "A FORTRAN Program for the Analysis of Ellipsometry Data";NBS Technical Note No. 479, 1969.
144
Langmuir 1988,4, 144-147
of the adsorbed layer thickness supports the conclusions of Schechter et al.
Summary Ellipsometry was used to detect adsorption on a platinum electrode. Using electrochemistry we were able to reproducibly clean the electrode for use in optical experiments, in which the adsorption of SDS could be studied. The SDS experiments indicated the formation of multi-
layer films above certain applied surface potentials. The formation of multilayers is predicted by previous experiments in which the adsorption was measured indirectly.
Acknowledgment. We would like to acknowledge financial support from Du Pont for apparatus construction and from the National Science Foundation and American Cyanamid Co. for fellowship support for G. J. Besio. Registry No. SDS, 151-21-3;Pt, 7440-06-4.
Characterization of Gold and Silver Sols by Sedimentation Field-Flow Fractionation Larry E. Oppenheimer* and Gregory A. Smith Photographic Research Laboratories-Photographic Products Group, Eastman Kodak Company, Rochester, New York 14650 Received June 5,1987. In Final Form: August 14, 1987 Sedimentation field-flow fractionation (SFFF) has been used to investigate particle size distributions of metal sols containing particles with diameters extending well below 0.01 Fm. Phenomena such a~ particle growth and flocculation are readily observed by this method, which avoids the problems of ambiguous aggregation information frequently encountered with electron microscopy. Since SFFF separates partioles by mass, it is insensitive to both shape variations and the presence of adsorbed material on the particle surface. Substantially less than 1 h is needed to obtain a distribution by this method.
Introduction Colloidal metals, such as the silver sols described by Carey Lea,l are of interest in such diverse fields as photographic science, catalysis, and spectroscopy. Numerous papers have appeared recently describing work in which colloidal silver was used as the substrate for surface-enhanced Raman spectroscopy,which is described with other work on the optics of colloidal silver in ref 2. Photographic applications include use as optical filters and in antihalation coating^.^ The desired properties of these materials are usually a function of the particle size and shape4or the specific surface area of the dispersed phase. The particle size distributions of these materials are often broad, and in a recent paper Jolivet e t al.5 showed colloidal silver to be subject to aggregation and/or particle growth on aging in the absence of a protective colloid. Such aggregation strongly affects the optical properties of the sols. Understanding these phenomena therefore requires knowledge of the particle size distributions of the colloidal metal sols. Particle size distributions of these materials are typically obtained by electron microscopy. This technique is labor-intensive, with results that are difficult to interpret when the particles are nonspherical, and often provides ambiguous information about the state of aggregation of the sol. We have used sedimentation field-flow fraction(1) Carey Lea, M. Am. J. Sci. 1889, 37, 476. (2) Kerker, M. J. Colloid Interface Sci. 1985, 105, 297. (3) The Theory of the Photographic Process; James, T. H., Ed.; Macmillan: New York, 1977; Vol. 4. (4) Siiman, 0.;Bumm,L. A.; Callaghan, R.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983,87, 1014. (5) Jolivet, J. P.; Gzara, M.; Mazieres, J.; Lefebvre, J. J.Colloid Interface Sci. 1985, 107, 429.
ation (SFFF)to examine colloidal silver and gold dispersions and have found good agreement between mass distributions obtained by this method and those derived from electron micrographs. SFFF is useful for both spherical and nonspherical particles because the elution time generally depends only on the mass of the eluting particle, not its shape! This has the additional advantage that the presence of adsorbed materials, such as stabilizing colloids, on the particles has little effect on the SFFF results because they do little to change the mass of the particle. SFFF may therefore be used in situations where methods which depend on measurement of the diffusion coefficient or sedimentation rate will give misleading results. The unique capabilities of SFFF for the characterization of the size distributions of colloidal systems are demonstrated by the variety of materials which have been examined by this method. These include, for example, polymer latices, pigments, mineral samples, silica sols, liposomes, and biological materials.6-12
Experimental Section Materials. Silver sols (type I) made by reduction of AgN03 with FeS04in the presence of sodium citrate13 either were used (6)Kirkland, J. J.; Schallinger, L. E.; Yau, W. W. Anal. Chern. 1985, 57, 2271. (7) Kirkland, J. J.; Yau, W. W.; Doerner, W. A.; Grant, J. W. Anal. Chern. 1980,52, 1944. (8)Kirkland, J. J.; Yau, W. W.; Szoka, F. C. Science (Washington, D.c.)i9sa,215, 296. (9) Giddinas, J. C.; Karaiskakis, G.;Caldwell. K.D.; Mvers, M. N. J. Colloid Interface Sci. 1983, 92, 66. (10) Yang, F. S.; Caldwell, K. D.; Giddinga, J. C. J . Colloid Interface Sci. 1983, 92, 81. (11) Janca, J.; Kleparnik, K.; Jahnova, V.; Chmelik, J. J.Liq. Chromatogr. 1984, 7(S-1), 1. (12) Oppenheimer, L. E.; Mourey, T. H. J.Chromatogr. 1984,298,217.
0743-7463/88/2404-0144$01.50/00 1988 American Chemical Society
