2210
The Journal of Physical Chemlstry, Vol. 82, No. 20, 1978 approximately 0.6in the 60% DO , solvent we used. (See P. M. Laughton and R. E. Robertson, “Solvent Isotope Effects in Equillbria and Reactions” in “Solute-Solvent Interactions”, J. E. Coetzee and C. D. Rltchie, Ed., Marcel Dekker, New York, N.Y., 1969,Chapter
7).
I n terms of the true equilibrium constant Kat and the activity coefficients, K , can be equated to Kalyaquo/yH+yh droxo, where the subscripts “aquo” and “hydroxo” refer to the tekaaquo and triaquo-hydroxo complexes, respectively. Assuming that the yaqua/ ymxo ratio remains relatively constant, one woukl expect a decrease in K, on increasing the ionic strength from 1.0to 5.0 M, since yH+ increases rapidly in this high ionic strength range. It has been noted (ref 26)that K, for the cis(py), complex is probably about lo-‘, while for Fac(NH a value of about lo4 is indicated (ref 318 31),with values of about 10and lo-‘ applicable to the cis(NH,),
Gunter Richard Jopplen and (NH& species, respectively (ref 27). (31) (a) J. N. Bronsted and K. Volqvartz, Z. Phys. Chem., 134,97 (1928); (b) G. Schwarzenbach,J. Boesch, and H. Egli, J. Inorg. Nucl. Chem.,
33, 2141 (1971). (32) J. March, “Advanced Organic Chemistry”, McGraw-Hill, New York, N.Y., 2nd ed, 1977,p 640. (33) D. D. Thusius and H. Taube, J . Phys. Chem., 71,3845 (1967);J. D. White and H. Taube, ibid., 74,4142 (1970). (34) R. D. Gillard, Inorg. Chem. Acta, 11, L21 (1974);Coord. Chem. Rev., 16,67 (1975). (35) M. L. Tobe, Acc. Chem. Res., 3,377 (1970). (36) D. J. Francis and R. B. Jordan, Inorg. Chem., 11, 461 (1972). (37) Elemental analyses in this work were performed by Instranal Laboratory, Inc., Rensselaer, N.Y. 12144.
Characterization of Adsorbed Polymers at the Charged Silica-Aqueous Electrolyte Interface Gunter Richard Joppien Institut fur Technische Chemie der Universitat Stuttgati, 0-7000Stuttgart 80, West Germany (Received April 24, 1978)
The mode of adsorption of poly(oxyethy1ene)(PEO), poly(acry1ic acid) (PAA), poly(4-vinylpyridine)(PVP), and isobutyric acid (IBA) at the charged Aerosil silica-aqueous electrolyte interface was studied by surface charge, adsorption density, and electrophoresis investigations. Due to the dissociation of surface silanol groups the silica surface becomes increasingly negatively charged when the suspension pH is raised above -3.5. Adsorption of PEO reduces the surface charge density considerably. While PEO is only partially desorbed from a silica surface with increasing surface charge, complete desorption of PAA and of IBA occurs along with their dissociation to form anions. Protonated PVP is readily adsorbed around the point of zero charge, the adsorption density diminishing with decreasing pH. While PEO adsorption drastically reduces the electrophoretic mobility of silica particles, adsorbed PAA contributes to a slight increase, but adsorbed protonated PVP reverses the direction of particle motion in an electric field.
Introduction Polymer molecules can undergo considerable conformational changes upon adsorption at solid-liquid interfaces. Such changes frequently affect the properties of polymer-modified systems and therefore have become a matter of growing interest. Particularly challenging are polymer adsorption studies at solid-aqueous interfaces where adsorbing polymer segments interfere with electrical double layers. The adsorption of electroneutral and charged polymers has been studied in detail at the mercury-aqueous electrolyte interface.l Insight into the conformational arrangements of polymer segments at this idealized model interface could be achieved. Of more practical importance are relevant studies in aqueous dispersions of lyophobic colloids, where polymer adsorption can highly affect colloid stability, as has been shown theoretically2-6 and experiment all^.^-^ A most suitable colloid system for model investigations of polymer-double layer interactions is aqueous silver iodide s 0 1 , ~ ~However, J~ its practical applicability is small. More important in a practical sense are aqueous dispersions of inorganic oxides, but little is known about the interactions of polymers with their charged ~urfaces.l~-’~ In the present work a survey on the mode of adsorption of electroneutral as well as electrically charged polymers at the charged silica-aqueous electrolyte interface has been undertaken with the goal of assessing the general trends of polymer-surface interactions involving like and opposite charges. In order to achieve as much information as possible, surface charge density (restricted to PEO adsorption), polymer adsorption 0022-3654/78/2082-2210$01 .OO/O
density, and electrophoretic mobility investigations were combined. H+ and OH- are the potential-determining ions for the aqueous silica system. Since they are no direct constituents of the silica surface phase and the silica is slightly water soluble, the Nernst equation may not be valid for this system.15 Hence it was not intended to overexert the data obtained in terms of doubtful double layer interpretation.
Experimental Section Materials. The silica was pyrogenic Aerosil 200 (Degussa A.G.), supplied as a finely divided powder of high purity (>99.8% SiOJ with an average particle size of 1 2 nm. Its nonporous surface had a specific nitrogen BET surface area of 189 m2 g-l. On samples pretreated under vacuum of mbar at 200 OC the concentration of surface hydroxyl groups was determined as 2.54 f 0.05 SiOH/nm2 by chemical reaction with CH3MgI and subsequent volumetric measurement of the evolved CH4. Prior to the preparation of aqueous silica suspensions the substrate was baked a t 200 “C in air to remove volatile organic impurities. Poly(oxyethy1ene)s of mol wt 400 and 20 000 (Merck), poly(acry1ic acid) of mol wt 750000 (Roehm GmbH), poly(4-vinylpyridine) of mol w t 550 000 (Polysciences Inc.), and isobutyric acid (Merck, Schuchardt) were used without further purification. The water was first refluxed with dissolved KMnO? in order to oxidize organic impurities, then deionized by triple distillation under nitrogen in a quartz vessel. Aqueous 0 1978 American Chemical Society
Characterization of Adsorbed Polymers
standard solutions were prepared from NaCl Suprapur and from HCl and NaOH Titrisol (Merck) by dilution. Determination of Charge vs. p H Curves. Suspensions of 5 g of SiOz in 200 mL of aqueous NaCl solutions of different ionic strengths were subjected to potentiometric acidlbase titrations in the presence or absence of dissolved polymers. An Ingold 405-90 glass electrode with integrated AglAgCl reference electrode and salt bridge served for pH determinations. The electrode was calibrated by use of several NBS standard buffer solutions. pH readings were taken with a Digi 510 digital pH meter (Wissenschaftlich Technische Werkstatten) with a precision of better than fO.01 pH units after the attainment of adsorption equilibria. Equilibration times of 36-60 min following the last addition of acid or base were normally sufficient. All titrations were carried out in a 400-mL double-wall Pyrex cell, maintained at 25.0 "C by externally thermostatted water circulation. A tight-fitting glass cover provided bore holes for a buret dispensing the titrant (0.1 M HC1 or NaOH), for the glass electrode, for a moist C02-free nitrogen flow, and for an auxiliary platinum electrode which connected the suspension to ground. All suspensions were vigorously stirred during the titrations. The difference between the actual pH change after addition of acid or base and the theoretical pH change which should be observable in the absence of the solid served as the measure to calculate the surface density (rH+ - rOH-) of potential-determining ions. These calculations were based on the equations given by BBrub6 and De BruynlGfor mass balance in titration system. They were performed by means of a Fortran computer program on the Cyber 174 computer a t the University of Stuttgart Rechenzentrum. Control experiments with titrated blank solutions showed only negligible deviations. For calculations of absolute adsorption densities a point of zero charge a t pH 3.5 was assumed for the silica. The surface charge of silica was derived from the adsorption density by multiplication with the Faraday constant. Determination of the Adsorbed Amounts of Polymer. The adsorbed amounts were determined indirectly from concentration changes in the supernatant solutions. Samples of the silica suspensions containing polymer were removed from the titration cell after 30-60-min equilibration time. They were centrifuged 1 h at 38000g to completely settle the solid. In a LI 3 laboratory interferometer (Jenoptik Jena GmbH) with glass double cell of matched 10-mm path lengths, thermostatted in a water bath a t 20 "C, the refractive indices of the supernatant solutions were compared with those of test solutions of known concentrations. The relevant concentrations were then evaluated from calibration curves which had correction terms for concentration changes of the salt solutions due to added amounts of titrant, for the pH changes during titrations, and for the states of dissociation of the polyelectrolytes which all affected the refractive index. While the refractive index determinations had an experimental error of only f 2 X lo4 refractive index units, the errors for the absolute polymer concentration determinations were f 2 % for PEO, and f5'% for PVP and PAA due to the small adsorbed amounts of the latter compounds. Longer equilibration times (1-72 h) of the suspensions only caused deviations within the stated error margins. In some experiments the concentration determinations were reproduced with the nephelometric method described by Attia and Rubio.17 PEO was precipitated by tannic acid and PAA by Hyamine 1622 respectively and the produced turbidities were compared with those of standard solutions
The Journal of Physical Chemistry, Vol. 82, No. 20, 1978 2211
-4
';"
E-3 2 3 oJ I s
=.
A
I +1
1
2
3
L
5
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1 0 PH
Figure 1. The adsorption density of potentialdeterminingions on Si02 (lefl ordinate) and the surface charge of SiO, (right ordinate) dispersed in aqueous NaCl solutions as a function of pH: 1 (0),0.1 M; 2 (O), 0.01 M; 3 (A), 0.001 M NaCI; SOp concentration 25 g dmF3.
in a nephelometer. However, this method gave less reliable concentration results compared to the interferometer technique. Determination of Electrophoretic Mobilities. Single particles and agglomerates of Aerosil silica after dispersion in an aqueous medium by ultrasonics and mechanical stirring (Janke and Kunkel Ultraturrax T 45 at 10000 rpm) are too small to be subjected to the microelectrophoretic technique. The electrophoretic mobilities of the turbidity boundaries of Si02dispersions in contact with aqueous solutions of approximately the same electrolyte and polymer concentrations and with carefully adjusted pH values were thus measured in a glass U-tube of 700-mm effective path length by moving boundary electrophoresis. An electric field of 90 V was applied across the tube via two platinum electrodes. Two auxiliary electrodes served to measure the applied potential without current in order to avoid errors due to possible electrode polarizations. At least six readings of the positions of the moving boundaries at different times were first taken in one direction and then on reversing the polarity of the applied field in the opposite direction.
Results and Discussion Adsorption of Potential-Determining Ions at the Silica-Aqueous Electrolyte Interface. The silica surface becomes increasingly negatively charged when the dispersion pH is gradually increased from 3.5 to 10 by the additions of NaOH solutions (Figure 1). This effect is more pronounced at higher electrolyte concentrations. However, there is only a small tendency of the silica surface to accept positive charges when the pH of the dispersion is lowered to 2 by addition of HC1 solutions. Some of the adsorption density or surface charge values must be considered with reservation as they could be affected by titration errors. This particularly applies to
2212
The Journal of Physical Chemistry, Vol. 82, No. 20, 1978
the values in the pH region 2-3 since they have been obtained as rather small differences of high H+ion activities. On the other hand, results obtained for H+ ion adsorption are consistent with some18yet not a W 2 1of the literature data. Other possibilities for titration errors are given at high pH values by the increasing dissolution of silica and by the sol concentration effect. Both effects are much more pronounced for silica dispersions having low NaCl concentrations and may have slightly affected the values of curve 3, Figure 1,in the pH region from 8 to 10. Particularly the sol concentration effect could not be eliminated completely a t this lowest electrolyte concentration. The results are best compared with previous literature investigations which intended to clarify the mode of adsorption of OH- ions at the silica-aqueous electrolyte interface and the maximum obtainable surface charge density in relation to the maximum density of surface silanol groups. Heston, Iler, and Sears,19 Bolt,20 and Abendroth21 observed relatively low negative suface charge densities on nonporous silica surfaces even at highest pH and highest salt concentration. The saturation values did not exceed possible silanol concentrations. Accordingly, the process of establishment of the negative surface charge could be fully explained by OH- adsorption to an existing SiOH group followed by dehydration, leaving the surface group SiO- with a negative charge. Tadros and Lyklema reported specific surface charge densities on a microporous silica sample which exceeded other literature data by a factor of 2-3.18 They postulated OH- penetration into the subsurface structure of the porous oxide, along with cation penetration. The highest surface charge density in Figure 1 is approximately 37 kC/cmz for a silica suspension in 0,l M NaCl solution slightly below pH 10. This corresponds to an adsorption density of 2.31 OH- ions/nm2 of the BET surface area and is still below the silanol surface density of 2.54 SiOH/nm2 as determined chemically. These results are thus only consistent with previous results obtained for nonporous silicas in support of the assumption that the Aerosil silica surface has no microporosity. Furthermore it can be assumed that formation of additional silanol groups on the surface of Aerosil by opening of siloxane bridges under the possible catalytic action of OH- ions must be negligible below pH 10 when moderate concentrations of NaCl are present. The adsorption of OH- ions onto existing silanol groups followed by dehydration apparently is the prevalent charging effect. Due to the screening action of Na+ ions closely located to the SiOsurface sites already formed the OH- adsorption density can increase with increasing Na+ ion activity. Poly(oxyethy1ene) Adsorption at the Charged SilicaAqueous Solution Interface and Its Effect on Surface Charge Density. Adsorption isotherms of poly(oxyethy1ene)s of mol wt 400 and 20 000 at the silica-aqueous 0.01 M NaCl solution interface at three different pH values are shown in Figure 2. While the isotherms of the oligomer PEO 400 gradually reach saturation plateaus, those of the polymer PEO 20 000 exhibit very sharp initial rises followed by slight maxima. The adsorbed amounts of polymer exceed those of the oligomer by almost a factor of 3. The general differences in shape of the two sets of isotherms are typical for polymer compared to oligomer adsorption and consistent with previous findingse2’ Polymers are more readily adsorbed from solutions of extremely low concentrations onto solid surfaces due to their capability of multiple contact-making. Low molecular
Gunter Richard Joppien
lzor
I
A
PEO 20000
I
.E
60
PEo
Y
pH3
equilibrium concentration in g.drn-’ Flgure 2. Adsorption isotherms of PEO 400 and PEO 20 000 on SiOp dlspersed in aqueous 0.01 M NaCi solutions of the polymers at three dlfferent pH values.
weight compounds and oligomers can only interact with one or a few surface sites, adsorption saturation being obtained at much higher solution concentrations. The sharp initial rises of the isotherms of PEO 20 000 indicate the formation of multiple adsorption contacts. According to infrared investigations of poly(oxyethy1ene) adsorption from solution in CCll onto silica the specific interaction proceeds via hydrogen bond formation from surface hydroxyl groups as donors to ether oxygen atoms as acceptors.23 Hydrogen bonds are likewise responsible for the specific adsorption of biopolymers from aqueous solutions onto silica as proved by IR in deuterated aqueous adsorption systemsz4 and can be regarded as the predominant specific interaction of PEO with the aqueous electrolyte-silica interface in this work. The dependence of the adsorption saturation plateaus on the molecular weight of PEO is strongly indicative of different conformational arrangements of the adsorbed samples. In accordance with observations in other systemsz5the extension of polymer segments from the surface increases with increasing molecular weight of the adsorbed samples. It should also be mentioned that conformational differences between oligomeric and polymeric ethylene oxides have been reported for the crystalline state,26 While the former have a zigzag chain conformation, the structure of the latter is a meandering helix. If such highly ordered structural domains might be formed in PEO 400 and PEO 20 000 adsorption layers an increase in maximum adsorption capacity by approximately a factor of 2 could well be accounted for. The most significant evidence to be drawn from the adsorption isotherms in Figure 2 is a dependence of the adsorption capacity on solution pH. At higher pH values the adsorption plateaus are considerably lower. This pH dependence is more clearly depicted in Figure 3, where the adsorption densities of PEO 20000 and of PEO 400, adsorbed from aqueous solutions of different initial concentrations onto silica, are plotted as a function of pH. Adsorption densities are expressed as the concentrations in monomoles per unit of surface area, a monomole being defined as the molecular weight of the repetitive unit
The Journahof Physical Chemisfry, Vol. 82, No. 20, 1978 2213
Characterization of Adsorbed Polymers
u2
3
4
5
6
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8
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PH
10
Figure 3. Adsorption densities of PEO 20 000 and PEO 400 on SO, dispersed in aqueous 0.01 M NaCl solutions of the polymers as a function of pH: 1 (A and A), 2.5 g dm-3 PEO; 2 (V and V),5 g dm-3; 3 (W and 0), 10 g dm-3; 4 (0 and 0), 15 g dm-3.
[CH2-CH2-0] in grams. All eight plots show steady decreases of the adsorption densities with increasing pH. PEO adsorption in a reverse way significantly reduces the surface charge density of silica. This can be seen in Figure 4 for PEO 20000 adsorption, curves 1-4 corresponding to the upper four adsorption density curves in Figure 3 and curve 5 being a magnified section of the surface charge density vs. pH plot 2 of Figure 1. Evidently the decrease of surface charge density is stronger for larger adsorbed amounts of polymer. This follows from a comparison of curves 2 and 4, Figure 3, with the corresponding curves in Figure 4. The general trend obtained from the data is consistent with polymer adsorption results a t charged mercury and silver iodide surfaces. Adsorption of poly(viny1 alcohol) a t the charged silver iodide-aqueous solution interface likewise reduces the surface charge density at high pAg values.lOJ1However, a reported shift of the point of zero charge to more positive values is not observed for PEO on silica, probably due to the almost horizontal form of the surface charge vs. pH curves at low pH values. A reduction of the surface charge density must be equivalent to a decrease of the differential capacity of the electrical double layer on silica. Such a reduction of the capacity has been directly observed a t the mercury-aqueous electrolyte interface by several a ~ t h o r s . ~ These ' ? ~ ~ investigations also indicate that PEO is desorbed from highly charged mercury surfaces. According to LyklemalO one would expect maximum adsorption of PEO at a charge uOm, characterized by a common intersection of surface charge plots. Unfortunately such an intersection cannot be located in the PEO + silica system. Hydrogen bond interactions are not involved in polymer adsorption interactions with mercury or silver iodide. Hence a comparison of the results with literature data on charged oxides would be desirable. There are only a few such data available. Kavanagh, Posner, and Quirk13 studied the adsorption of poly(viny1alcohol) at the charged
Flgure 4. Effect of PEO 20 000 on the adsorption density of potential-determining ions on SOp (left ordinate) and on the surface charge of SOp (right ordinate) dispersed in aqueous 0.01 M NaCl solutions of the olymer as a function of pH: 1 (0),2.5 g dm-3 PEO; 2 (A),5 g dm- , 3 (v),10 g dm-3; 4 (0),15 g dm-3; 5 (O), polymer-free solution.
5:
alumina-aqueous electrolyte interface. They found no capacity changes upon adsorption, and the polymer adsorption density was almost independent of the pH. On the other hand, Tadros12 found a decrease of poly(viny1 alcohol) adsorption on silica with increasing pH, and recently a confirmation of this pH dependence was also given for the PEO silica system by Rubio and Kitchener.14 The latter work had, however, not been extended to investigations of the effect of PEO adsorption on the surface charge density. The mode of desorption of PEO with increasing pH is an open question. Tadros12suggested that it might be due to the decrease in concentration of SiOH groups as the potential adsorption sites. More likely might be the hypothesis given in the paper of Rubio and Kitchener14 that hydrated counterions associated with ionized surface sites prevent PEO segments from approaching the surface. Adsorption of Poly(Cuiny1 Pyridine), Poly(acry1ic Acid), and Isobutyric Acid at the Silica-Aqueous Electrolyte Interface. The effect of polyelectrolyte adsorption on the surface charge density of the silica-aqueous electrolyte interface cannot be studied by the potentiometric technique. The following exploratory investigations are therefore restricted to observations of the dependence of polymer adsorption density on pH. As shown in Figure 5 , the adsorption densities (expressed in monomole concentration per unit of surface area) of the three substances on Si02 are lower compared to those of PEO 20 000 and PEO 400. PVP which is sufficiently water soluble only in the pH range below -4.2, becomes more protonated a t lower pH values. PAA and IBA (pK = 4.84) increasingly dissociate at higher pH values. As indicated in the graph, PAA and IBA are completely desorbed from the surface with increasing negative surface charge and increasing dissociation. It is not well understood why IBA desorption is complete even after the first addition of NaOH to the system. However, the general trend reflects the mutual electrostatic repulsion effect. Ionized PVP is not desorbed at very low pH values, where the silica surface could possibly carry positive charges. According to Kargin and Rabinovich the isoelectric point of extremely pure silica is at pH 7,29 and it could well be that some strongly acidic impurity ions on the surface of Aerosil are stable a t very
+
2214
The Journal of Physical Chemistry, Vol. 82,No. 20, 1978
"i
Gunter Richard Jopplen
7
P3
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PEO 400
" 4 c 0
82 1
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IO
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Figure 5. Adsorption densities of PEO 20 000, PEO 400, PVP 550 000, PAA 750000, and IBA on SiOl dispersed in aqueous 0.01 M NaCl solutions of the organic adsorptives as a function of pH; adsorptive concentrations 5 g d m 3 ; Si02 concentration 25 g dm-3.
low pH values, acting as adsorption sites for PVPH+ segments. Effect of Polymer Adsorption and p H on the Electrophoretic Mobility of Silica in Aqueous Electrolyte Solutions. The electrophoretic mobility of dispersed silica in aqueous NaCl solutions, but in the absence of polymers, has very small negative values below pH 3. An almost linear increase to larger negative values is observable in the higher pH region (Figure 6, curve 1). PEO 20000 adsorption reduces the mobility drastically (curve 2), while the presence of PAA contributes to a slight mobility increase at lower, to a slight decrease a t higher pH values (curve 4). PVP adsorption reverses the direction of the electrophoretic mobility of silica to relatively high positive values (curve 3). These findings can be reasonably well interpreted by referring to the adsorption density data of Figure 5. (The rheological changes induced by the polymers might slightly affect these interpretations.) The small negative mobilities for the silica below pH 3 in the absence of polymers do not explicitly support the questionable evidence for positive charging of the surface (Figure l),although electrophoretic mobility data may only partially reflect the ionic composition of the Stern layer on silica. Reversely these mobilities can provide an explanation for the incomplete desorption of PVPH' at very low pH values. When neutral PEO is adsorbed a t the silica surface, polymer segments attached as trains and even more the segments protruding into the solution must displace the electrophoretic shear plane away from the surface. This could then bring about a decrease in {potential and, hence, in electrophoretic mobility as it is observed. PAA adsorption can modify the ionic composition of the electrical double layer on silica. The adsorption density in the pH region 2-7 (Figure 5) is low, however, and the small contribution of adsorbed poly(acry1ic acid) anions to the surface charge and mobility can possibly overcompensate the expected mobility decrease, due to a displacement of the shear plane and a higher solution viscosity. The latter probably gives rise to the slight
-2
tL 1
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I
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Figure 6. The electrophoretic mobility of the turbidity boundary of S102 dispersions in aqueous 0.01 M NaCl solutions (1 (0)) and in the presence of PEO 20 000 (2 (A)), PVP 550 000 (3 (A)),and PAA 750 000 (4 (0)) as a function of pH; polymer concentrations 15 g dm-3; SiOp concentration 25 g ~ l m - ~ .
mobility decrease (confirmed by only one point) in the pH region above pH 7 , where PAA is completely desorbed. The sign of the { potential of silica with adsorbed PVP is positive, owing to the positively charged polymer. Most likely the smaller PVP surface concentration at lower pH (Figure 5) is outweighed by a higher protonation of the residual adsorbed molecules, thus leading to an increase in electrophoretic mobility.
Acknowledgment. The author gratefully acknowledges financial support of part of this work by the Deutsche Bundesministerium fur Wirtschaft, assistance in the experimental work by Miss K. Teufel and Miss A. Stumpp, and construction of the titration cell by Mr. H. Amende. The Aerosil silica was kindly provided by Degussa A.G., Hanau, and the poly(acry1ic acid) was a gift of Roehm GmbH, Darmstadt. References and Notes (1) I. R. Miller and D. Bach in "Surface and Colloid Science", E. Matijevie, Ed., Vol. 6, 1973, p 185. (2) B. Vincent, J . Colloid Interface Sci., 42, 270 (1973). (3) P. Bagchi, J . Colloid Interface Sci., 47, 86 (1974). (4) J. B. Smltham, R. Evans, and D. H. Napper, Discuss Faraday Soc., 59, 285 (1975). (5) A. Vrij, Pure Appl. Chem., 48, 471 (1976). (6) A. Sommerauer, D. Sussman, and W . Stumm. Kolloid-Z. Z . Polvm., 225, 147 (1968). (7) G. J. Fleer, L. K. Koopal, and J. Lyklema, Kolloid-Z. 2.Polym., 250, 689 (1972). (8) D. Dollimore and T. A. Horridge, J. Colloid Interface Sci., 42, 581 (1973). (9) G . J. Fleer and J. Lyklema, J. Colloid Interface Sci., 48, 1 (1974). (10) L. K. Koopal and J. Lyklema, Discuss. Faraday Soc., 59, 230 (1975). (11) L. K.Koopal, Meded. Landbouwhogesch. Wageningen, 78-12 (1978). (12) Th. F. Tadros, J . ColloldInterface Sci., 48, 528 (1974). (13) B. V. Kavanagh, A. M. Posner, and J. P. Quirk, Discuss. Faraday Soc., 59, 242 (1975). (14) J. Rubio and J. A. KRchener, J. CollokjInteffaceSci., 57, 132 (1976). (15) S. Levine and A. L. Smith, Discuss. Faraday Soc., 52, 290 (1971). (16) Y.G. BBrubB and P. L De Bruyn, J. ColloldInterface Sci., 27, 305 (1968).
Excess Electron Energy in Rare Gas-Propane Systems
(17) Y. A. Attia and J. Rubio, Br. Polym. J . , 7,135 (1975). (18) Th. F. Tadros and J. Lvklema. J. Nectroanai. Chem.. 17. 267 (19681. ilQj W.M. Heston: Jr.,-Rli
