2360
B. J. FONTANA
Conclusions From the agreement between the conventional diffusion coefficients and those estimated by the evaporation procedure follows that the solute buildup a t the surface of evaporating solutions is diffusion controlled. This holds equally for electrolytes and surface active compounds. When the radioactive source is a weak p
Vol. 67
emitter, the surface activity increase approximates the surface concentration increase. After a short initial transition period, both increase continuously and virtually linearly with the time of evaporation. Away from the surface the concentration falls off approximately exponentially from its highest value a t the surface to the bulk concentration.
THE CONFIGURATIOX OF AX ADSORBED POLYMERIC DISPERSANT BY INFRARED STL‘DIES BY B. J. FONTAXA California Research Corporation, Richmond, California Received May 6, 1963 The fraction of attached ester segments of an alkyl methacrylate-polyglycol methacrylate copolymer molecule adsorbed onto silica was determined directly by infrared spectrometry. The exclusion of ester segments from attachment to the surface because of preferential adsorption of the polyglycol ether segments is demonstrated. It is suggested that the resultant polymer configuration is thus more extended away from the surface and that this accounts for the enhanced colloid stabilization properties reported for such polar-substituted polymers.
Introduction The steric mechanism of stabilization of colloids requires the adsorption on the particles of a surfactant film of a thickness sufficient to increase the distance of closest approach and thus decrease the van der Waals energy of attraction to below the thermal energy. Such an effect has been demonstrated experimentally by van der Waarden’ and others and treated theoretically by Mackor aiid van der Waals and by T‘oldS2 Adsorbed macromolecules would be expected to possibly function well in this respect. Heller and Pugh3 have postulated such a mechanism for aqueous gold sols stabilized by high molecular weight polyethylene glycols. With regard to hydrocarbon media, it has been found that appropriate modification of alkyl methacrylates can result in outstanding dispersants for use in automotive lubricating oils. The introduction of a relatively small fraction of polar substituents into the polymer molecule results in a great enhancement of the dispersance. In a previous study5 it has been shown that polylauryl methacrylate, PLMA, adsorbed on silica, is attached to the surface through about 40% of the segments. In t’his case then, a relatively flat adsorbed film results, estimated to be -30 A.thick. Incorporating about 17 mole % N-vinyl-2-pyrrolidoiie into PLMA as a copolymer, PAM-VP, increased the adsorption a t t’otalsurface coverage and gave a thicker film (-200 A. as determined from sedimentation measurements) .6 The PAM-VP was shown to adsorb uia both t’he ester and pyrrolidone carbonyl groups. The enhanced dispersant property of the PARII-VP4 then is apparently due to the thicker adsorbed film, aiid it appears reasonable to assume that the thicker film results from the preferential adsorption of the smaller number of more strongly polar pyrrolidone groups. The latter behavior is in accord with the predictions of the recent’ statistica,l (1) ill. van der Waarden, J. Colloid Sci., 6 , 317 (1950); 6, 443 (1951). (2) E. L. Maokor, ibid., 6 , 492 (1951); E. L. RIackor and J. H . van der Waals, ibid., 7, 536 (1952): hl. J. Vold, ihid., 16, 1 (1961). (3) W. Heller and T. L. Pugh, J. Polymer Sci., 47, 203 (1960). (4) A. L. Lyman and F. W. Kavanaph, Proc. A m . Petrol. Inst., Sect. III, 39, 296 (1959). (6) B. J. Fontana and J. R. Thomas, J . Phys. Ch6m., 6 6 , 480 (1981).
analysis of polymer adsorptioii by Silberberg.6 It was not possible to demonstrate this effect in the previous studies because of the overlapping of the pertinent infrared bands. In the present study the effect has been quantitatively determined for an alkyl methacrylatepolyglycol methacrylate copolymer, PA3I-PG. The latter belongs to a class of outstanding dispersants used as ashless detergents in engine oils and specifically designed to be efficient hydrogen bonding detergents.’ Experimental P-4RI-PG was prepared by direct copolymerization of the alkyl methacrylate and polyethylene glycol tridecyl ether methacrylate with 2,2’-azobis(2-methylpropionitrile) initiator in refluxing benzene. The alkyl groups were of mixed length averaging 14 carbon atoms. The polyethylene glycol residue [CH&H20] was 1600 average molecular weight and was capped Kith a 13carbon alkyl. The present sample was a narrow fraction obtained by solvent precipitation. The mole ratio of alkyl methacrylate to polyethylene glycol in this fraction was 20:l as determined by both infrared and elemental analysis. The copolymer molecular weight Yas 4 1 0 , 0 0 0 as determined from intrinsic viscosity (29 in isooctane, 85 in benzene at 2 5 ’ ) . The measurement of the adsorption isotherms and of the infrared spectra of the adsorbed species on silica is described in a previous publication.6 The details of the determination of the pertinent extinction coefficients and calculations of the fraction of adsorbed ester segments, p, are also given therein. The carbonyl extinction coefficient, e, for free polymer PAM-PG in n-dodecane solution was 1120 as compared to 1465 for PLMA (and 1180 and 1510, respectively, in decalin). This ratio (0.773 0.009) is accounted for by the difference in equivalent weights. Hence, the e,% value used for adsorbed carbonyl was corrected accordingly ew
(PARI-PG)
=
1650 X 0.773
2
1275
The infrared frequencies for the free and adsorbed carbonyl had exactly the same values as previously observed for PLlIA.
Results and Discussion In comparing the behavior of the polyglycol substituted polymer with the simple polyalkyl methacrylate, it should be noted that both have identical methacrylate “backbones.” The basic structural formula of PARI-PG is given on the following page. (6) A. Silberbarg, zbzd., 66, 1872 (1982). (7) W. T. Stewart, F. 4 Stuart, and J. A . Miller, Division of Petroleum Chemistry. American Chemical Society, Preprints, Vol 7, No. 4, 1962, P.
B-19.
Sov., 1963
CONFIGURATION OF ADSORBED POLYMERIC DISPERSAKT BY INFRARED STUDIES
2361
TABLE I DETERMINATION OF THE FRACTION OF ADSORBED ESTERSEGMENTB Adsorbed polymer mg./g. silica, u
Fraction of surface covered, 8
PAM-PG (n-dodecane)
586 283
0.69 .33
0.040 ,026
PAM-PG (decalin)
652 500 301 178
.95 .73 .44 .26
.017 ,029 ,024 .019
Polymer (solvent)
PLMA (both) ... 0.16-0.96 Weight per unit area in the spectrometer beam, see ref. 5. on OH data, see text. PLMA data are from ref. 5. a
--ddsorbed From CO data
-Mg.polymer/cm.lasegmentsFrom OH data
Total adsorbed polymer
Fraction of adsorbed oarbonyl groups,b p
0.151 .098
0.486 ,276
0.08 (0.31) ,094 ( .35)
.156 ,150 . 110 ,084
.508 ,517 ,293 .180
,033 ( .31) .06 ( .29) .082 ( .38) ,106 ( .47) ... .40 ( .46) ... ... Based on CO data; values in parentheses are the “apparent p” based
TABLE I1 IKTRINSIC VISCOSITY AND ADSORPTION Polymer
PAM-PG PLMA
where R = C14Hz9,atiid X = [CHZCH~O]~&H.X. The results obtained for PAM-PG are given in Table I. Comparison of these data should be made with those of ref. 5 . The pertinent values in the latter have been averaged aiid summarized in the last line of Table I. Substitution of the long polyglycol chains into the alkyl methacrylate polymer results in a 2.4- to 2.6-fold increase in the amount of polymer adsorbed a t total coverage, uo (Table 11). Accompanying this effect is a fourfold or greater decrease in p , the fraction of the total number of ester carbonyl groups in an adsorbed polymer molecule which are directly attached to the silica surface hydroxyls via hydrogen bonds. The increase in uo and decrease in p must be interdependent if the effects are interpreted as being due to preferential adsorption of the ]polyethylene oxide groups to the exclusion of the ester carbonyl groups. Correlatively, the change iia uo should be smaller than that in p , as found, if the polyethylene oxide groups are also adsorbed a t the interface. The large increase in uobrought about by introduction of a small fraction of more polar segments has been demonstrated elsewhere with partially hydrolyzed polymebhyl methacrylate8 and polyvinyl acetate.g The fraction 0 of the surface covered is here the ratio of the amount of adsorbed polymer u to the amount adsorbed uo at the plateau of the adsorption isotherm a t high polymey concentrations. The results in Table I shorn a trend in the values of p decreasing with increasing corerage 0. No such effect was apparent with PLMA.5 This is not unreasonable in view of the apparent configurations of the adsorbed polymers. The PLMA is relatively flat and extended parallel to the surface with 40% of the ester segments attached, while the PAlI-PG is relatively extended away from the surfacewith only 10% oir less of the ester segments attached. In the latter case, of course, some of the preferentially ( 8 ) S. Ellersteint and R. Ullman, J . Polymrr Sei., 66, 123 (1961). (9) J. Koral, R. Ullman, ctnd F. R. Eisich, J . Phye. C h e m , 62, 461 (1968).
--[VI a t 25ODodec- Deoa- Inverse ane lin ratio 24 74
56 95
2.3 1.3
--u0
Dodeoane
855 329
a t 25O--Decalin Ratio
685 282
1.3 1.2
adsorbed but less populous polyglycol segments are also attached to the surface. The data also show (Table 11) more definitively than previously for PLMA6 that the state of the polymer in the solvent phase (voluminosity as indicated by the intrinsic viscosity) has only a weak, if any, effect on the configuration a t the surface. The decrease in uo in going from dodecane to decalin as solvent is about the same for PLMA and PAM-PG. This suggests that competition with the solvent for surface sites is mainly involved. The spectral data for bound hydroxyl a t the surface cannot be used as before5 to calculate a supplementary value for the fraction of attached ester segments if, as suggested, the polyglyco’lsegments are also involved in the attachment to the fiilica surface hydroxyls. That the latter is, indeed, the case can be shown by calculating the apparent adsorbed carbonyl from the bound hydroxyl peak intensity enhancement on t’he same basis as previously (that is, assuming for the purpose of this calculation, that H-boding by carbonyl oxygen only is contributing to the observed effect on the OH peak). These values (in parentheses in Table I) are seen to be about fourfold great’er than the “true” value obtained from the carbonyl data. This can only mean that a notable fraction of the polyethylene oxide groups are attached to the surface via H-bonding of the ether oxygen to surface hydroxyl. KOadditional significance should be attached to tlhe apparent p values since the quantitative effect of ether oxygen on the hydroxyl extinction is not known. The preferential adsorption observed here of the polyethylene oxide groups requires, of course, that ether oxygen form a stronger H-bond than ester carbonyl oxygen. The available data in homogeneous systems’o,11 support this view. Conflicting enthalpy data have apparently been resolved in recent work by Joesten and Drago.’* The OH stretching frequency shift (decrease) (10) G. C. Pimentel and A . L. McClellan, “The Hydrogen Bond,” Freeman and Company, San Francisco and London, 1960, PF. 86, 91, and 362. (11) R . E’. Blanks and J . 31. Pravmitz, J . Chem. Phys., 88, 1500 (1963): this recent study of H-bonding with propanol of polybutyl methacrylate and polypropylene oxide is especlially pertinent, (12) M. D. Joesten and R. S. Drago. J . Am. Chsm. Soc., 84, 3817 (1962).
AND Af. T. ROGERS R. SUMMITT, J. J. EISCH,J. J. TRAINOR,
2362
is invariably greater for H-bonding with ethers (for example, by over 100 cm.-l with phenol as the Hdonor). The latter is taken t o mean that ethers are more powerful electron donors than esters with respect to H-bond formation.13 The present results appear to concur with this view. The broad bound OH peak observed a t about 3420 cm.-l with PLMA shifts only about 17 cm.-l lower with PAM-PG; however, the peak broadens considerably (80 to 105 cm.-l a t the halfintensity width) on the low frequency side. (13) W.Gordy, J. Chem. Phys., 7, 93 (1939); 9, 215 (1941).
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The data presented here, support the view that increased film thickness occurs with polymeric dispersants because of the preferential adsorption of the more polar segments. This results in a configuration more extended away from the surface than with a monophyletic polymer. This is in accord with the consequent enhancement of the effectiveness as colloidal dispersants. Acknowledgment.-The author is obligated to Dr. J. R. Thomas for past contributions in our Laboratory to the study of colloid stabilization.
PROTON MAGNETIC RESONANCE SPECTRA OF VINYLSILANES BY
ROBERTSUYMITT,~ JOHN J. EISCH,~ JAMES T. TRAINOR,3
AND &[AX
T. R O G E R S 4
Corniny Glass Works, Corning, New York, Kedzie Chemical Laboratory, Michigan State University, East Lansing, Michigan, and the Department of Chemistry, University of Michigan, Ann Arbor, Michigan Received M a y 11, 1963 The proton magnetic resonance spectra of a series of vinylsilanes, RaSiCH=CH2 (where R = chloro, alkyl, or a substituted phenyl group), have been analyzed to obtain the spin coupling constants and chemical shift parameters for the vinylic protons. The small variation in the coupling constant sum, ZJi,, supports the view that the silicon atom effectively shields the vinyl group from inductive electronic effects exerted by the substituent R. On the other hand, the approximate correlation noted between the electronic nature of R and the trans proton chemical shift is consistent with a variable d,-p, resonance effect between the silicon atom and the vinyl group. Possible reasons for the failure of 6-cis and a-protons to show a similar trend are discussed.
Introduction Considerable attention has focused on the question of bond overlap between the n-cloud of vinyl and propargyl groups and available d-orbitals of elements from the second or higher rows of the periodic table (d,-p, ~ v e r l a p ) . ~ -Studies ~ of acid and base strengthsa and the addition of hydrogen halides to vinylsilanes9 have been interpreted in terms of such d,-p, bonding, and measurements of dipole moments10 and infrared1’-1s and proton magnetic resonance (p.m.r.)14s‘5 spectra have been used to investigate this problem. P.m.r. spectra of a few vinylsilanes, R,Si(CH= C H Z ) ~where - ~ R = CH3 or C&, have been analyzed and the results discussed with reference to possible d,-p, bonding.l4~l6We have synthesized a series of vinylsilanes, R3SiCH=CH2, in which the group R can be varied to achieve differences in the electronic environment at the silicon atom.13 Examination of the infrared spectra of these compounds has revealed a (1) Research and Development Division, Corning Glass Works, Corning, N. Y. (2) Department of Chemistry, Catholic University of America, Washington, D. C. (3) Research Laboratories, Raybestos Division, Raybestos-Manhattan, Inc., Stratford, Conn. (4) Kedaie Chemical Laboratory, Michigan State University, East Lansing, Mich. ( 5 ) P. D. George, M. Prober, and 3.R. Elliot, Chem. Rev., 66,1065 (1856). (6) C. Eaborn, “Organosilicon Compounds,” Academic Press, Inc., New York. N. Y.,1960,pp. 91-103. (7) D. Seyferth,”Progress in Inorganlo Chemistry,” Vol. 3, F. A. Cotton, Ed., Interscience Publishers, New York, N. Y.,1962,p. 129 ff. (8) R. A. Benkeserand H. R. Krysiak, J . A m . Chem. Soc., 75,2421 (1953). (9) L. H. Sommer, D. L. Bailey, G. M. Goldberg, C. E. Buck, T. S. Bye, J. F. Evans, and F. G. Whltmore, zbad., 78, 1613 (1954). (IO) H. Soffer with T. DeVnes, %bid.,78,5817 (1951). (11) W. J. Potts and R. A. Nyquist, Spectrochzm. Acta, 15,679 (1959). (12) H. Buchert and W. Zeil, ebid., 18, 1043 (1862). (13) J. J. Eisch and J. T. Trainor, J. Orp. Chem., 28,487 (1963). (14) R. T. Hobgood, Jr., J. H. Goldstein, and G. S. Reddy, J. Chem. Phys., 85,2038 (1961). (15) R. T.Hobgood, Jr., and J. H. Goldstein, Spectrochim. Acta, 19, 321 (1963).
correlation between the CH2 “wag” frequency of the vinyl group and the electron-withdrawing power of the substituent R.13 The p.m.r. spectra of these vinylsilanes have now been obtained and analyzed to learn whether they reflect a similar sensitivity to the electronic nature of R. This paper presents the results and discusses the vinylic spin coupling constants and chemical shift parameters in relation to possible substituent effects. Experimental The preparation and purification of the vinylsilanes empIoyed in this study (Table I ) have been given e1se~here.l~ P.m.r. spectra of the vinylsilanes listed in Table I were recorded on a Varian A-60 analytical spectrometer a t 25”. Chemical shifts of spectral lines were measured relative to tetramethylsilane (TMS) as an internal standard (ca. 5% concentration) using a 500-C.P.S. sweep width. These measurements and the chemical shifts relative to TMS computed from them are accurate to fl C.P.S. Higher resolution spectra of the vinylic region were recorded using a 100-c.p.s. sweep width. Reproducibility in the measurement of relative chemical shifts of these lines was f0.05 c.P.s., and relative chemical shifts and spin coupling constants appear to be accurate to about f0.3 c.p.s. All samples were 15% concentration by weight in carbon tetrachloride, because small concentration shifts were noticed.lB
Results With one exception, spectra in the vinylic region were of the ABC type.17 The spectra of (C6Hs)aSiCH=CH2, (CBH6)2C1SiCN=CH2,and (CsH6CHz)aSiCH=CH2 are reproduced in Fig. 1. These examples serve to illustrate the’forms of the spectra and the assignment used in computation. The spectra of (CaH6)a(16) Strictly speaking, one would prefer data from spectra at several concentrations which could be extrapolated to obtain the chemical shifts a t infinite dilution. However, the observed solvent effects were small (for affected lines, differences of less than 0.5 0.p.s. between a dilute sample, w., Ei%, and pure liquid (CHs)rSiCH=CH*), and since these effects are not understood well, no effort was made t o obtain these data. (17) H. J. Bernstein, J. A. Pople, and W. G. Schneider, Can. J . Chsm., 85, 65 (1957).
