Langmuir 1986,2,625-630 Neumann equation of state.17 The application of eq 2, on the other hand, is known to result in false predictions of particle behavior in a number of ~ y s t e m s . ~ ~ ' ~ The findings reported here give rise to some regrets. The idea of subdividingsurface tensions into components which arise from the action of the individual intermolecular forces is very appealing. A completely successful theory of this nature would be an immensely valuable link between macroscopic and microscopic thinking. It should be understood that the experimental results presented here do not preclude the existence of such a theory. While it has become apparent that present day values are not dispersion components of surface tension, such components might well exist and contribute to the overall surface tension, although probably not in an additive fashion. We see the Fowkes approach as a decisive first step in the development of this area of surface science. (19) Smith, R. P. Ph.D. Thesis, University of Toronto, 1984.
625
Conclusions The contact angles of pairs of liquids on a single solid substrate were found to be equal when the surface tensions of the liquids were the same, regardless of the relative magnitudes of the dispersion forces comprising each of the two liquids. This observation is in direct conflict with the theory of surface tension components. It is, however, consistent with the expectations of the equation of state approach, in which the solid-liquid interfacial tension is only a function of the total solid and liquid surface tensions. Acknowledgment. Supported in part by the Natural Science and Engineering Research Council of Canada (A8278), the Medical Research Council (MT 5462, M T 8024, MA 9144), and the Ontario Heart and Stroke Foundation (4-12, AN-402). D.R.A. acknowledges gratefully support of the Ontario Heart Foundation through the receipt of a Senior Fellowship.
Interactions in Concentrated Nonaqueous Polymer Latices Ivana Markovie, R. H. Ottewill,* and Sylvia M. Underwood+ School of Chemistry, University of Bristol, Bristol B S 8 1 TS, U.K.
Th. F. Tadros ICI Plant Protection Division, Jealott's Hill Research Station, Bracknell, Berkshire, U.K. Received February 25,1986. I n Final Form: J u n e 25, 1986 Particles of poly(methy1methacrylate) stabilized by poly(l2-hydroxystearic acid) can be used to form dispersions of high volume fraction in dodecane. In the present work volume fractions between 0.23 and 0.42 were examined by small-angleneutron scattering. From these measurements both the structure factor, S(Q),and the pair correlation function, g(r), were derived. By use of a model interaction potential function, based on the mean spherical approximation, it was found that the particle-particle interaction was soft at low volume fractions and became harder at high volume fractions, indicating that in the latter condition some compression of the stabilizing layer occurred.
Introduction Following the introduction of the concept of dispersion polymerization in nonaqueous media1g2methods have been developed for the preparation of stable monodisperse polymer latices in hydrocarbon^.^-' These have proved useful model colloidal dispersions for fundamental studies in media of low dielectric constant. A system of this type, which we have used in previous s t ~ d i e s ~is, ~composed ,~*~ of a core particle of poly(methy1 methacrylate) with an outer layer of a "comb" polymer, poly( 12-hydroxystearic acid), covalently attached to the core particle so that the oleophilic "teeth" project into the dispersion medium. In hydrocarbon media such as dodecane it seems reasonable to conclude that electrostatic effects are absent and hence the particles are sterically stabilized. Consequently, the range of interaction forces between these particles would not be expected to be significant beyond a range of about twice the length of the stabilizer chains,1° in the present case the "teeth" of the comb polymer. Although a significant number of theories have been proposed to account for the phenomena of steric stabili-
* To whom correspondence should be addressed. t Present
address: Dulux Australia Limited, Clayton, Victoria,
Australia.
0743-7463/86/2402-0625$01.50/0
zation, and these have been well surveyed by Napper," there is still a sparsity of direct experimental information on the nature of the interaction potentials between sterically stabilized particles. However, our previous preliminary study on this type of system using small-angle neutron scattering4 and subsequent theoretical developm e n t ~ ' ~has - ~indicated ~ that this technique provides a (I) Osmond, D. W. J.; Walbridge, D. J. J. Polym. Sci. 1970, (230,381.
(2) Barrett, K. E. J. Dispersion Polymerization I n Organic Media; Wiley: London, 1975. (3) Cairns, R. J. R.; Ottewill, R. H.; Osmond, D. W. J.; Wagstaff, I. J. Colloid Znterface Sci. 1976,54, 45. (4) Cebula, D. J.; Goodwin, J. W.; Ottewill, R. H.; Jenkin, G.; Tabony, J. Colloid Polym. Sci. 1983,261,555. (5) Antl, L.; Goodwin, J. W.; Hill, R. D.; Ottewill, R. H.; Owens, S. M.; Papworth, S.;Waters, J. A. Colloids Surf. 1986, 17,67. (6) Dawkins, J. V.; Taylor, G. Colloid Polym. Sci. 1980,258, 79. (7) Everett, D. H.; Stageman, J. F. Faraday Discuss. Chem. SOC.1978,
--.(8) Cairns, R. J. R.; van Megen, W.; Ottewill,R. H. J.Colloid Interface
65. 2.70.
Sci. 1981, 79, 511.
(9) van Megen, W.; Ottewill, R. H.; Owens, S. M.; Pusey, P. N. J. Chem. Phys. 1985,82, 508. (10)Koelmans, H.: Overbeek, J. Th. G. Discuss. Faraday SOC.1954. 18,52. (11) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions;
Academic Press: London, 1983. (12) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. 0 1986 American Chemical Society
626 Langmuir, Vol. 2, No. 5, 1986 Table I. Preparation of PMMA Latices material amount added n-hexane 200 cm3 134 cm3 n-dodecane methacrylic acid 4.8 g methyl methacrylate 50.8 cm3 1.76 cm3 octyl mercaptan ADIB 1.64 g
useful means of obtaining fundamental information on particle-particle interactions. An advantage of the method is that the interaction of neutrons with matter occurs via the nuclei of the atoms. Hence there is very little attenuation of the radiation and multiple scattering effects can be minimized. This means that concentrated colloidal dispersions can conveniently be examined by neutrons, particularly, as the wavelength of the beam is convenient for the spatial correlations in dispersions of small colloidal particles. The present work constitutes an examination of concentrated poly(methy1 methacrylate) latices of small particle size (diameter ca. 400 A) in dodecane in order to examine the form of the structure factor, S(Q)and the pair correlation function, g(r),as a function of volume fraction. The results are then compared with theoretical calculations using an assumed pair potential. The results indicate that the particles behave as "soft" spheres at low volume fractions and "hard" spheres at high volume fractions. Experimental Section Materials. Hexane (aromatic free), dodecane, and azobis(isobutyronitrile) (ADIB) were BDH materials. Octyl mercaptan (Honeywill and Stain) and 2-(dimethy1amino)ethanol were technical grade materials which were obtained via IC1 PLC. Methyl methacrylate and methacrylic acid were also obtained from IC1 PLC. Before use each monomer was distilled under nitrogen at low pressure at 40 "C and condensed in a container surrounded by solid carbon dioxide and ethanol. The particle stabilizer was a comb copolymer of poly( 12-hydroxystearic acid) "teeth" with a backbone composed of a block copolymer of glycidyl methacrylate and poly(methy1 methacrylate). It was supplied by IC1 PLC, Paints Division, as a 40% solution of the stabilizer in a low-boiling aliphatic hydrocarbon-ester mixture. Preparation of the Polymer Latices. The latex was prepared by a method similar to those previously de~cribed."~ The reactants as listed in Table I were measured into a glass flask fitted with a reflux condenser and a n anchor-type stirrer; the latter was rotated during the preparation a t 150 rpm. After heating to 75 "C the ADIB was washed in and the polymerization allowed to proceed for 2.5 h. At this stage the stabilizer was chemically bonded to the surface by the following method. 2-(Dimethylamino)ethanol, 1.0 g, was washed into the reaction vessel with 25 cm3 of filtered dodecane and the condenser changed from the reflux to the distillation position. The mixture was then heated on a n oil bath at 130 "C for 2.25 h. As the hexane was distilled off, dodecane was gradually added to replace it; a total of 120 cm3 of dodecane was added during this process. After cooling, the excess unbound stabilizer was removed by successive sedimentations of the particles in heptane and decantations of the supernatants. The "dry" weight fraction of the latex was determined by drying a weighed sample of the dispersion in a vacuum oven to obtain the weight of latex particles and by difference the weight of dodecane present. The approximate volume fraction of the latex was calculated by assuming the average density of the particle (13) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982,46, 651. (14)Cebula, D.J.; Goodwin, J. W.; Jeffrey, G. C.; Ottewill, R. H.; Parentich, A,; Richardson, R. A. Faraday Discuss. Chem. SOC.1983,76, 37. (15) Ottewill,R. H. Ber. Bumenges. Phys. Chem. 1985,89, 517. (16)Neutron Beam Facilities at the High Flux Reactor, Institut Laue-Langevin, Grenoble, 1983.
Markovic' et al. Table 11. Samples Used for Neutron Scattering Experiments designation volume fraction dilute 0.023 PMMA/23 PMMA/28 PMMA/33 PMMA/36 PMMA/42
0.23 0.28 0.33 0.36 0.42
to be 1.09 g ~ m - A ~ .confirmation of the effective volume fraction was obtained from fits to the experimental small angle neutron scattering results on the concentrated dispersions (see later). The samples and the corresponding volume fractions are listed in Table 11. The size of the poly(methy1 methacrylate-poly( 12-hydroxystearic acid) particles was determined by electron microscopy from carbon replicas of the particles. The microscope was calibrated by using a carbon replica of a diffraction grating. The number-average diameter of the particles by this procedure was found to be 370 8, with a coefficient of variation on the mean of 8%. S m a l l Angle N e u t r o n Scattering. The neutron scattering experiments were all carried out a t the Institut Laue-Langevin (ILL), Grenoble, France, using the neutron diffractometer D17.17 For these experiments the dispersions were contained in optical-quality quartz cells having a path length of 1 mm. The intensity of scattering was measured over a range of scattering vectors, Q, which for elastic scattering can be defined bY Q = (4r/X) sin (0/2) with 9 = the scattering angle and X = the wavelength of the incident beam. Measurements were made using a sampledetector distance of 2.84 m with X = 18.0 A and 6X/X = 5%. This gave an experimentally accessible Q range of ca. 0.006 to 0.06 A-1. For the dilute samples the standard ILL computer programs were used to process the basic data to give the intensity of scattering a t a particular Q, namely, I(Q), as a function of Q relative to the scattering from water. The appropriate background was subtracted from the scattering patterns of each of the samples and corrections were also made for the attenuation of the beam due to absorption.16 For the concentrated dispersions an extension of these procedures, which allowed for the presence of a high concentration of the dispersed phase, was used.18
Theory and Results Latex Particles i n a Dilute Dispersion. The latex particles were composed of a poly(methy1 methacrylate) (PMMA) spherical core and stabilized by an outer shell of covalently bound poly(l2-hydroxystearic acid) (PHS). Hence, for the purpose of analyzing the small-angle neutron scattering results we have assumed that the particles have a concentric sphere geometry with a core particle of radius R1 and an overall particle radius of Rz giving the thickness of the PHS layer as Rz - R1. As shown previously4J9the intensity of scattering at a scattering vector Q, for a dispersion of noninteracting particles, can be written in the form,
(P, -
(1)
where A. = an instrumental constant and Np= the number of particles per unit volume. P A = the coherent neutron scattering length density of the outer layer, p p = the co(17) Ghosh, R. E. A Computing Guide for Small-Angle Scattering Experiments; Institut Laue-Langeuin: Grenoble, 1978; 78GH 247T. (la) MarkoviE, I.; Ottewill, R. H. Colloid Polym. Sci. 1986,264, 65. (19) MarkoviE, I.; Ottewill, R. H.; Cebula, D. J.; Field, I.; Marsh, J. F. Colloid Polym. Sci. 1984,262, 648.
Langmuir, Vol. 2, No. 5, 1986 627
Interactions in Nonaqueous Polymer Latices loo
Table 111. Coherent Neutron Scattering Length Densities 0
50t 9
material
designation
p/10lo cm-2
PMMA PHS (1)" PHS (2)B dodecane-hz6
PP PA PA Pm
1.07 -0.06 0.075 -0.43
Based on the value of stearic acid by using as the density 0.847 g cm-3.21*Calculated by using the monomer unit of PHS and the density of stearic acid.
I
nnnn..
0.02
0.01
0.03
Q /a-1
Figure 1. 1(Q) against Q for a dilute poly(methy1methacrylate) experimental points latex, volume fraction 0.023, in dodecane: (0) (-) fitted curve. herent neutron scattering length of the particle, and pm = the coherent neutron length of the medium. The volume fraction of the particles in the dispersion can be written as
4 = N,4rRz3/3
(2)
For a spherical particle we can write the general expression f(QR) = 3
(
sin QR - QR cos QR Q3R3
Equation 1assumes that the particles are monodisperse. In general, this is not so and an allowance for the distribution of particle sizes is required. In the present work a log-normal probability distribution was used in the form
to allow for the polydispersity of the core particles. R, = the modal mean radius and uo = a parameter describing the width and skewness of the distribution.20 The mean value of R1 taken as R , is given by In R = In R,
+ 1.5~~'
(5)
and the standard deviation, u, by u = R,[exp(4u02)
- exp(3~,~)]'/~
(6)
For computational purposes the value of R1 was taken as R1 = R f na/2 (7) where n was taken from 0 to 6 in order to obtain a 13-step histogram. The variation in wavelength 6X/h and the Q resolution were not taken into account. The small angle neutron scattering results obtained on the PMMA-PHS latex in dodecane-hZ6at a volume fraction of 0.023 are shown in Figure 1. The line was calculated by using the above procedures and the values of PA, p,, and pm listed in Table 111, namely, pp = 1.07 X 1O'O cm+ ,' pA = -0.06 X 1O'O cm-2, and pm = -0.43 X 1O'O cm-2 with R1 .= 150 A and R2 = 250 A and with a coefficient of variation of 5%. The fit did not appear to be particularly sensitive to values of polydispersity between 5% and 10% nor to the choice of either of the values listed in Table I11 for PHS. The value of R1 obtained from the fit should be reliable because of the magnitude of (p, - pm); however, (20) MarkoviE, I.; Ottewill, R. H. Colloid Polym. Sci. 1986,264, 454. (21) Handbook of Chemistry and Physics, 55th ed.;Chemical Rubber
Publishing
Co.: Boca Raton, FL, 1974.
0.01
0.02
0.03
0.04
Q/V Figure 2. I ( Q )against Q for concentrated dispersions of poly(methyl methacrylate) in dodecane: (A) $I = 0.23, (O), $I = 0.36; ( 0 )4 = 0.42.
in view of the closeness of P A and p,, the value of Rz may be subject to some error. Latex Particles in a Concentrated Dispersion. In the previous section it was assumed that particle-particle interactions could be neglected in the case of dilute dispersions and that the scattering was simply that of a lognormal distribution of particles of total number concentration N,. In concentrated dispersions, however, the particle positions become correlated and interparticle scattering effects become important in addition to intraparticle scattering. S(Q) is the interparticle term and is denoted as the structure factor. It is given by 4rNP S(Q) = 1 + g J m r [ g ( r ) - 11 sin Qr dr (8) where r = the distance between the particle centers and g(r) is the pair correlation function. If I(Q)dd is determined at a dilute volume fraction 4dil, a t which the interaction effects can be neglected, and I(Q)conis determined for a concentrated dispersion of volume fraction, in which interaction effects are present, we can write
It is assumed in the above equation that the intraparticle scattering is essentially the same in both the dilute and concentrated cases. It is also assumed that multiple scattering effects are negligible and that the data in both dilute and concentrated dispersions have been corrected for background scattering of the dispersion medium and cell as well as attenuation of the primary beam.16 Hence S(Q), obtained by this procedure, is an experimental quantity which can then be compared with theoretical predictions. In the present work the scattering from concentrated dispersions in the volume fraction range 0.23-0,42 has been determined. The scattering curves obtained are shown in Figures 2 and 3 in the form of I(Q)conagainst Q. As can be seen, unlike the dilute dispersion illustrated in Figure 1, well-defined peaks were observed.
628 Langmuir, Vol. 2, No. 5, 1986
300
I
-
Markovic‘ et al.
-
-
0 03
0.02
0.01
7
0.04
QiP’
Figure 3. Z(Q) against Q for concentrated dispersions of poly(methyl methacrylate) in dodecane:).( @ = 0.28, (0) @ = 0.33.
:.I :I1
I
I
I
400
800
1200
I 1 1600
r/B,
Figure 5. g(r) against r for dispersions of varying volume fraction: (-) PMMA 23; ( - - - ) PMMA 28; (-*-) PMMA 36; PMMA 42. (**e)
1
SI01 2.0
t 1 0.010
1
J
0.020
all-’
Figure 4. S(Q) against Q curves for dispersion. Experimental results: (0)PMMA 23; ( 0 ) PMMA 28; (0)PMMA 36; (A) PMMA 42; (-) represents theoretical computations.
From the procedure suggested by eq 9 and with the Cp = 0.023 dispersion to provide the form of Z(Q)dethe curves of S(Q) against Q were determined. These are shown in Figure 4. Comparison of Experimental S ( Q ) Results with Theory. In a previous experiment4 using a PHS-PMMA latex and a volume fraction of 0.35 it was found that modeling the interactions, by assuming them to be of the hard-sphere type and using the approach of Ashcroft and Lekner,22’23 reasonably good agreement was obtained between the experimental results and those predicted theoretically. In the present work, which covers a much wider range of volume fractions, it was found that although good fits could be obtained using the hard-sphere model at the high-volume fractions, the fit was not as good at the lower volume fractions. In order to approach a fit it was found necessary to change both the core radius slightly and to vary the hard-sphere radius of the particle. This suggested that the hard-sphere model did not adequately describe the results. In order to fit the experimental details, a softer potential appeared to be required and the potential chosen was of the form V(r) = A exp(-Br)/r (10) This was a useful potential in that it allowed the strength of the interaction to be determined by A and the softness to be regulated by B. Moreover, the electrostatic analogue had been used in this form in the mean spherical approximation by Hayter and Penfold,12 Hansen and Hayter,13 and our~elves.’~ For the nonaqueous dispersions the constants A and B are regarded as curve-fitting constants. (22) Perms, J. K.;Yevick, G.J. Phys. Reo. 1958, 110, 1. (23) Ashcroft, N. W.; Lekner, J. Phys. Reo. 1966, 45. 33.
At the moment they have not been directly related to either solvent or particle parameters. The continuous lines shown in Figure 4 were obtained by using the mean spherical approximation to calculate S(Q), and, as can be seen, reasonable agreement was obtained with the experimental results. Moreover, this agreement extended over all the volume fractions examined. In the calculations a value of 150 A was taken for the core particle radius. It should be noted that B is the analogue of the DebyeHuckel parameter in the electrostatic case and moderates the fall off of the potential with distance and consequently controls the effective distance of interaction. The Pair Correlation Function g ( r ) . The pair correlation g ( r ) is directly related to S(Q) by Fourier transformation of eq 8 giving 1 g(r) = 1 + -L-[S(Q) 2a2rNp - 1]Q sin Qr dQ (11) Once the curves of S(Q) against Q had been obtained the corresponding curves of g(r) against r were generated by using a fast Fourier routine.12 The results are shown in Figure 5 for four volume fractions of the latex. It becomes clear from these curves that as the volume fraction of the latex increases so the slope of the initial region of the curve becomes steeper. This suggests that the potential becomes “harder” with increase in the volume fraction of the latex. It is also clear that the particles are able to approach more closely at the higher volume fractions. Moreover, the increase in magnitude of the peaks and the troughs of g(r) indicates an increase in structure in the system as stronger interactions ensue.
Discussion The current experimental results obtained on the dilute sample of latex, although not specifically optimized to obtain the layer thickness, indicate the thickness of the PHS layer to be ca. 100 A. This is consistent with the fact that the PHS used had an average of five ester linkages per chain, with a small spread of chain lengths, giving a stretched length of about 100 A. Hence the model of the particle previously suggested4 of a core particle of poly(methyl methacrylate) with the PHS chains anchored to the core at one end thus leaving the branched chains projecting into the dispersion medium appears to be reasonable. The results obtained on the more concentrated PMMA-PHS latices over the volume fraction range 0.234.42 show by the form of the curves of S ( Q )against Q (Figure 4)that strong repulsive interactions occur between the particles. For example, a well-defined primary peak, S(QImaX, is observed which increases in magnitude
Langmuir, Vol. 2, No. 5, 1986 629
Interactions in Nonaqueous Polymer Latices 4
a A/106
S‘Q’,,,
//’ 1
__--I
I
I
I
I
I
b 5
-
B / ~ o - ~
4
-
3
-
2
-
1
-
O0.01 ’ O L
0.1
0.2
0.4
0.3
0.5
0.6
$
Figure 6. (a) S(Q), volume fraction 4.
against volume fraction, 4. (b) Q , against
and moves to a higher Q, Q ,, with increasing volume fraction. These effects are illustrated in Figure 6. An extrapolation of the curve of S(Q),, against 4 suggests that S(Q), would reach a value of 2.4 at a volume fraction of ca. 0.55. The latter value is often considered to be that at which a system of hard spheres would show a liquidsolid transition, i.e., freeze. Thus it would be of considerable interest to extend the measurements of S(Q)against Q to higher volume fractions in order to examine this phase transition in a nonaqueous dispersion and to provide confirmation of the hard-sphere-like nature of the particle-particle interactions at very high volume fractions. Attempts to use a hard-sphere model following the procedure of Ashcroft and LekneP to compute theoretical curves of S(Q) against Q showed that at the lower volume fractions the experimentally determined primary peak was much broader than that calculated by the model. However, a t the higher volume fractions a very much better fit was obtained. In view of the poor fit a t the lower volume fractions we have made a preliminary investigation of a mean field approach to interpret the results obtained on nonaqueous systems. The pair potential used is given in eq 10 and was based, in principle, on the model used for electrostatic interactions between spherical colloidal particle^.^^*^* For the present, the preexponential term A and the exponential decay factor B have not been associated with the parameters often used to describe steric stabilization, e.g., as used by Cairns et al.8 Their use has been confined to that of fitting parameters. Despite this limitation direct evidence for the form of the interaction can be obtained from the values of A and B. These are plotted in Figure 7 as a function of the volume fraction of the latex. The preexponential constant increases very steeply over the volume fraction range examined, and this combined with the increase in the exponential decay coefficient B leads to a much harder potential at the higher volume fractions. This is illustrated in Figure 8 which shows the potential energy curves, evaluated from eq 10, (24) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948.
I
I
0.1
0.2
*
0.3
0.4
I
I
0.5
0.6
Figure 7. Constants A and B from eq 10 used to fit the experimental data, plotted as a function of volume fraction.
Ir-----\
-.*..
I ....-._ .-
.. ...-.__
-- - - -
1
LO
40
80
60
100
~
-- - _ _ 120
0
h/i
Figure 8. V(r)/kT against interparticle core surface separation distance, h: PMMA 23; ( - - - ) PMMA 33; (-) PMMA 42. (-a)
for three different volume fractions. In the most dilute case, 4 = 0.23, the change in potential energy of interaction with distance is very gradual, and taking V(r)/ltTi= 1.0 as being about the energy of translation of a colloidal particle, we find this corresponds to an h value, where h is the surface to surface separation distance of the core particles, of ca. 106 A. However, at 4 = 0.33 this is reduced to 63 A and at 4 = 0.42 to 24 A. It can be seen that at 4 = 0.42 the form of the potential energy curve is approaching that of a hard-sphere-type interaction; this is consistent with the suggestion, expressed earlier, that a hard-sphere liquid-solid transition could be anticipated to occur at about 4 = 0.55. Overall the evidence obtained in the present work from both the S(Q) against Q and g(r) against r curves suggests that at low volume fractions, when there is a large spacing between the particle, the PHS chains are fully extended and interaction only occurs between the most extended
630
Langmuir 1986,2, 630-638
parts of the chains. With increase in volume fraction and consequentlyless free space in the system it appears either that the PHS chains compact as a consequence of a change in solvency of the PHS layer or that the greater frequency of collisions between the particles causes the solvent to be squeezed out from between the chains so that the surface layer becomes much more compact. This suggestion of changing interaction with volume fraction could not be explained with the system used either by desorption of the stabilizing molecules or by extensive lateral mobility since
the PHS chains were chemically bonded to the surface of the core PMMA particle.
Acknowledgment. We express our thanks to SERC and IC1 PLC for support of this work. We also gratefully acknowledge the use of neutron beam facilities at the Institut Laue-Langevin, Grenoble. Registry No. PMMA, 9011-14-7;12-hydroxystearicacid, copolymer with glycidyl methacrylate and methyl methacrylate, 103817-71-6.
Quantitative Investigations of Adsorption of tert -Amyl Alcohol at the Gold(110)-Aqueous Solution Interface Jocelyn Richer, Lorne Stolberg, and Jacek Lipkowski* Guelph- Waterloo Centre for Graduate Work i n Chemistry, Guelph Campus, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario N l G 2W1, Canada Received March 10, 1986. I n Final Form: July 7, 1986 Chronocoulometry was used in the quantitative investigation of the physical adsorption of tert-amyl alcohol on the (110) face of a gold single crystal. The amount of surfactant adsorbed was determined from the charge density at the electrode surface. A computerized system was used to obtain the charge density and perform the data treatment. The film pressure, Gibbs surface excess, and free energy of adsorption were determined as functions of the electrode potential, the charge density, and the potential drop across the inner layer. The congruency of the adsorption data with respect to charge and potential is discussed.
Introduction Modern spectroscopic techniques such as surface-enhanced Raman scattering (SERS), electromodulated infrared spectroscopy (ELMIRS), and subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) offer new possibilities to probe molecular properties of organic surfactants adsorbed at the solidsolution interface.' In principle, information about the orientation of the adsorbed molecules, absence or presence of a partial charge transfer, interactions between adsorbates, solvent, and surface, etc. could be obtained by these techniques. However, for the proper interpretation of the spectroscopic experiments as well as for the better understanding of electrocatalytic processes, macroscopic data such as the surface concentration, free energy of adsorption, and electrosorption valency are needed. Unfortunately, in contrast with the fast development of the microscopic techniques, little progress has been made in the acquisition of macroscopic data on surfactant adsorption on solid electrodes. However, quantitative information about the adsorption of organic molecules has already been obtained on platinum by Hubbard et al.2-5 Recently, we demonstrated that adsorption of organic surfactants at the metal-solution interface can be determined quantitatively from the charge density at the electrode surface and that the charge density corresponding to the adsorption equilibrium can be determined by chronocoulometry.6--8 (1)Pons, S.J.Electrounul. Chem. 1983,150, 495. (2)Soriaga, M.P.; Hubbard, A. T. J. Am. Chem. Soc. 1982,104,2735. (3)Soriaga, M.P.; Stickney, J. L.; Hubbard, A. T. J.Mol. Cutal. 1983, 21, 211. (4)Soriaga, M.P.;White, J. H.; Hubbard, A. T. J . Phys. Chem. 1983, 87,3048. (5)Soriaga, M.P.; Hubbard, A. T. J.Phys. Chem. 1984,88,1089,1758.
In the present paper, chronocoulometry is used to investigate the adsorption of tert-amyl alcohol (t-AA)on the gold(ll0) single-crystal plane. The objective of this work is to show how the Gibbs surface excess, the free energy of adsorption, and the potential drop across the inner layer can be obtained for adsorption of an organic surfactant on a solid electrode. The surfactant chosen was t - A A because (1)it does not oxidize on the gold electrode and (2) its adsorption on mercury is well described, allowing for comparison with the data obtained on gold. The Au(ll0) surface is energetically homogeneous, it is ideally polarizable over a large potential range, and has known electrocatalytic p r o p e r t i e ~ . ~ J ~ The technique of data acquisition and analysis developed on this model system could be used later to obtain results needed for the interpretation of the spectroscopic experiments or to explain the mechanisms of electrocatalyzed reactions.
Experimental Section (i) Solutions. Solutions were prepared with Milli-Q water (Waters) with a resistivity higher than 16 MQ cm. The KCIOl (ACS Certified from Fisher) was calcinated at 300 "C, twice recrystallized, and dried before use. tert-Amyl alcohol (BDH Chemicals Ltd. Gold label 99+%)was used without further purification. The supporting electrolyte consisted of a 0.05 M KCIOl solution. The t-AA solutions were prepared by spiking a known volume of the supporting electrolyte solution with a small amount of a concentrated solution of alcohol. The experiments were (6) Richer, J.; Lipkowski, J. J. Electrochem. SOC.1986,133,121. (7)Stolberg, L.; Richer, J.; Lipkowski, J.; Irish, D. E. J.Electrounul. Chem. 1986,207,213. (8) Lipkowski, J.; Van Huong, C. N.; Hinnen, C.; Dalbera, J. P.; Parsons, R. J . Electround. Chem. 1983,143,375. (9)Schwank, J. Gold Bull. 1983,16, 103. (10) Wachs, I. E. Gold Bull. 1983,16,98.
0743-7463/86/2402-0630$01.50/0 0 1986 American Chemical Society
