1900
Crescenzl
It should be noted that the method proposed in this work is much easier than the hydroperoxide method for the determination of the cross propagation rate constants. In the latter method, one needs separate experiments to determine the termination rate constant, 2ktBB9and the rate of initiation. Moreover, the observed rate of oxygen absorption must be corrected for oxygen absorbed and nitrogen evolved by the initiator, AIBN, and for oxygen evolved by the self-reactions of tert-butylperoxy radicals. As pointed out by Howard and Chenier,ll this correction is quite large at low substrate concentration, though low substrate concentration is essential to chain termination only by the self-reaction of tert-butylperoxy radicals. These complications are not involved in the method we described, which is based on eq 23. No knowledge of the rate constants such as kgtBBnor of the initiation rate, Ri, is necessary. All assumptions can be checked out by the linearity of the plot of (d[02]/dt)/[B02.]vs. [BO,.], as has been done in Figure 6. When the determination is based on eq 24, we need the value of kgBB,which can easily be determined by the method described above. In this case, only thing to do is to find the reaction conditions, in which neither generation nor absorption of oxygen occurs during the decomposition of the hydroperoxide in the presence of a hydrocarbon and to determine the radical concentration under that condition. A method for determination of the rate constants for chain transfer, ks, and cross termination seems to have been never established. For example, Howard et al.l used the trial and error method to determine the “best value” for the chain transfer constant in the co-oxidation of tetralin and cumene, and tedious calculations had to be
et al.
done to obtain the cross termination constant. Niki et ala3 made the assumption @ = ktm/ (ktrrkFB)l/z = 4, which has no theoretical basis, in order to estimate the value of kFB, the cross termination rate constant between the tertbutylperoxy radical and the tetralin peroxy radical (4.7 X 105 M-I s-1 at 333 K) and used this value for calculating the rate constant for chain transfer. Our method provides an explicit way to determine the rate constants of these elementary reactions, since all assumptions or experimental errors can be checked by the form of the linearity of the plot based on eq 32. Acknowledgment. We thank Professor T. Keii for helpful discussions. References and Notes (1) J. A. Howard, W. A. Schwalm, and K. U. Ingold, Adv. Chem. Ser., No. 75, 6 (1968). (2) (a) J. A. Howard, K. U. Ingold, and M. Symonds, Can. J . Chem., 48, 1017 (1968); (b) J. A. Howard and K. U. Ingold, ibM., 48, 2655, 2661 (1968); (c) ibid., 47, 3809 (1969); (d) E. Niki, Y, Kamiya, and N. Ohta, Bull. Chem. SOC.Jpn., 42, 2312 (1969); (e) J. A. Howard and K. U. Ingold, Can J . Chem., 48, 873 (1970); (f) J. A. Howard and S. Korcek, /bid., 48, 2165 (1970). (3) E. Niki, K. Okayasu, and Y. Kamiya, Int. J. Chem. Kinet., 8, 279 (1974). (4) J. A. Howard and J. H. B. Chenier, Int. J. Chem. Kinst.,8, 527 (1974). (5) S. Fukuzumi and Y. Ono, J. Chem. Soc., Perkin Trans. 2, 784 (1977). (6) S. Fukuzumi and Y. Ono, J. Phys. Chem., 80, 2973 (1976). (7) S. Fukuzumi and Y. Ono, J. Chem. Soc., ferkin Trans. 2,622 (1977). (8) S. Fukuzumi and Y. Ono, J. Chem. Soc., Perkin Trans. 2,625 (1977). (9) J. A. Howard and K. U. Ingold, Can. J . Chem., 45, 793 (1967). (10) J. A. Howard, K. Adamic, and K. U. IngoM, Can. J. Chem., 47, 3793 (1969). (1 1) S.Korcek, J. H. B. Chenier, J. A. Howard, and K. U. Ingold, Can. J . Chem., 50, 2285 (1972). (12) J. A. Howard and K. U. Ingoid, Can. J . Chem., 47, 3797 (1969). (13) J. A. Howard and K. U. Ingold, Can. J . Chem., 48, 2655 (1968).
Thermodynamics of Polycarboxylate Aqueous Solutions. 4. Special Features of Hydrophobic Maleic Acid-Olefin Copolymers J. C. Fenyo,’ F. Delben, S. Paoletti, and V. Crescenrl** Laboratorio di Chimica delle Macromolecole, Istituto di Chimica, Universlti di Trleste, 34 127 Trleste, Ita/y (Received December 14, 1976)
Dilatometric,potentiometric, and calorimetric data are reported on the ionization behavior of a few hydrolyzed copolymers of maleic anhydride (MA)and substituted a olefins in aqueous solution. In the case of MA-2methylpentene-1 and of MA-2,4,4-trirnethylpentene-l the whole set of data reveals a strikingly anomalous behavior reconcilable with a pH-induced passage of the chains from compact to expanded, coiled conformations similar to what was already found for other hydrophobic polycarboxylates.
Introduction In previous studies from this laboratory a thermodynamic characterization has been achieved for the ionization in aqueous solution of three different 1:l copolymers of maleic acid (MA) with the simple a olefins ethylene (MAE), propene (MAP), and isobutene (MAiB).3-6 We wish to report here dilatometric, potentiometric, and calorimetric data on the ionization in water at 25 OC of 1:l copolymers of MA with the more hydrophobic olefins, 2-methylpentene-1 (MAMPe) and 2,4,4-trimethylpentene-l (MA3MPe), respectively. The aim of our studies was to obtain a more complete description of the influence the increasing complexity of The Journal of Physical Chemistty, Vol. 8 1, No. 20, 1977
apolar chain substituents may have on the polyelectrolytic behavior of typical chainlike dicarboxylic acids in aqueous solution. Experimental Section Materials. MAE, MAP, MAiB, and MAMPe were Monsanto products. MA3Ml’e was a kind gift of Professor G. Barone of the University of Naples. The molecular weight, A&,, of the Monsanto copolymers was ca. 106;7that of MA3MPe should be lowera8 Stock solutions of the polyelectrolytes were prepared as previously describedS3:7MA3MPe was first hydrolyzed in NaOH solution under a stream of nitrogen with continuous
1901
Thermodynamics of Polycarboxylate Aqueous Solutions
stirring at 80-90 "C for 24 h, then purified by precipitation with ethanol from basic aqueous solution, redissolution in water, and reprecipitation with HC1. A slightly alkaline solution of the polymer was passed through anion and cation exchange columns. An aliquot was then titrated with standard NaOH solution and the remainder stored at 4 "C for all subsequent use. Stock MA3MPe solutions prepared in such a way (polymer concentration 2 X lo-' monomol/L) exhibited a marked tendency to foaming and a slight turbidity which disappeared, however, upon dilution or partial neutralization. Hydroxide solutions were certified 0.1 N Normex from C. Erba, Milan (NaOH), Normadose Prolabo (KOH), or Eastman Kodak solutions (tetra-n-butylammonium hydroxide), respectively. Tetra-n-butylammonium hydroxide (Bu4NOH) solutions were purged of carbon dioxide by passage through an anion exchange column in the basic form. HC1 solutions were prepared by dilution of certified 0.1 N Normex (C. Erba) solutions. NaCl was a C. Erba RP product. It was kept in an oven at 105 "C for 24 h and then stored under vacuum over 0
p4010.
n-Heptane used in the dilatometric measurements was a C. Erba RP product. It was purified as previously de~cribed.~ Methanol was a C. Erba RP product, and was used without further purification. All the solutions were prepared using freshly doubly distilled water with a specific conductivity of approximately ohm-l cm-l. Methods. Potentiometric measurements were performed using a Radiometer PHM52 digital pH meter equipped with a Radiometer combined electrode (Type GK2301C or GK2321C). Standardization was checked at pH 4.01 and 7.00 (Radiometer buffer solutions). Titrations of 5 x iO-3-iO-2 monomol/L (C,) polyelectrolyte solutions were carried out at 25 f 0.1 "C. Nz flux was not necessary since values obtained at alkaline pH conditions were not used in further calculations. From the degree of neutralization a, Le., the stoichiometric concentration of added base over the total (monomolar) polymer concentration (C,) and the pH value for each neutralization stage, the degree of dissociation, a 01 = C y [H+]/C,
+
and the negative logarithm of the apparent dissociation constant, pK,, of the polyelectrolyte
pK, = pH
+ log
">
-
were immediately calculated. In fact, no correction for the very weak second dissociation of the polyelectrolyte was necessary in the a range of our concern (0 6 a < The calorimetric experiments were carried out at 25 "C using a LKB Model 10700-1flow-type microcalorimeter. In a typical experiment, a polyelectrolyte solution (C, N at a given degree of neutralization (ai)was allowed to mix in the flow calorimeter first with water (to determine the heat of dilution of the polymer) and then with 5X N HC1. The flow rates, controlled volumetrically, were so adjusted to lead to a decrease of ai by approximately 0.06 units in each protonation stage. Treatment of the calorimetric data was the same as already described in detail e l s e ~ h e r e . ~ - ~ J @ ~ ~ The enthalpy of dissociation, (kcal/mol of H+), was calculated as the ratio between the observed heat exchange and the moles of H+effectively bound by the polymer, calculated with the aid of the potentiometric data. l).597,8
02
04
06
08
Lx
10
Flgure 1. Volume changes In milliliterslmonomol on proton addition to the Bu.,N+ (full symbols) and K+ (open symbols) salt form of MA3MPe (a, squares), MAMPe (b, circles), MAE (C, triangles), and MAlB (c. hexagons) in water at 25 "C. The final polymer concentration was 5X monomol/L in each case. The absclssae are to be read to the left.
The enthalpy of dilution, Al?&, (cal/monomol), has been corrected for the heat effects due to the change of degree of dissociation of the polyacids upon dilution (using the data reported in Figures 6 and 7). The dilatometric measurements were performed as previously d e s ~ r i b e d . ~ ? ~
Results and Discussion (a) Dilatometric Data. The experimental data on the volume changes upon protonation of MAE, MAiB, MAMPe, and MA3MPe in water at 25 "C are reported in Figure 1. Data of Figure 1 clearly show that: (1)for MAMPe and MA3MPe the AVdissagainst a plots in the range 0 C a! C 1 exhibit a break, as opposed to the case of MAE, MAiB, and of all other MA-a-olefin copolymers studied so far;5i6J3(2) the volume change on dissociation is seen to depend on the cation of the neutralizing base. The AVdissvalues result in fact systematically higher with Bu4N+ counterions. In addition, the anomalies in the trends of the AVdissagainst a plots for MAMPe and MA3MPe are influenced by the nature of the counterions (i.e., K+ or Bu4N+ions). Without too much demand on the accuracy of our dilatometric data (the maximum uncertainty in each AVdiss value is 0.2 mL/monomol) further interesting evidence may be drawn. The initial linear portions of all the plots of Figure 1 (small a values) would have in fact nearly the same slope. According -to our data the differential volume of dissociation, AVdiss,of the sparingly ionized samples thus results close to -9 mL/mol of H+, nearly independent of the nature of the counterions.14 For MAE and MAiB this AVdissvalue applies over the whole first dissociation range (0 6 a! < 1). On the contrary, in the case of MA3MPe it is evident that - a d i s s assumes rather abruptedly (and well before a = 1)larger values depending on the counterions, namely, 28 (MA3MPe-Bu4N+), 19 (MA3MPe-K+), 20 (MAMPe-Bu4N+), and 13 (MAMPe-K+). All values are in mL/mol of H+, with a maximum estimated uncertainty The Journal of Physical Chem/sfy, Vol. 81, No. 20, 1977
Crescenzi et al. __
60
PKa
5.5
a 50
45
't
1
5
7
pH
t
4
f+-
6 -1
8
1 C
5.0
6 pKa
4
4 5
3 0
0.2 0.4
0.6 08
10
1.2
- 1.4
a
Figure 2. Typical plots from the potentiometric titrations of MA3MPe (a), MAMPe (b), and MAiB (6) in water at 25 OC: titrating base (-) KOH, (--) Bu,NOH; polymer concentration 5 X monomol/b in each case. Titration curves with Me4NOHalways lie within those with KOH (not reported to avoid overcrowded plots).
of about f0.4 m%/mol of H+. In view of the very weak second dissociation of the copolymers, the increase in before = 1cannot be ascribed even partially to an increasing population of doubly ionized groups along their chains. Relying also on other evidence, some of which shall be discussed in the following sections, we conclude that the anomalies in the AVdi,, against a plots of Figure 1 for MA3MPe and W M P e chains (for the latter, at least upon neutralization with Bu4NOI-I) have to be attributed to pH-induced Conformational changes.15-" Taking into account the hydrophobic nature of the polyelectrolytes inclines us to believe that these changes should bring the chains from coiled, compact conformations to extended and more solvated 0nes.7~8JO-12318-20 The negative excess volume of dissociation resulting for MA3MPe (Bu4N+or K') and for MAMPe (Bu4N+)is well compatible with the hypothesis that such conformational changes cause the exposure to water of apolar groups, and the breaks in the dilatometric points would thus monitor their onset (at a E 0.5 and a C.L 0.4 for MA3MPe-Bu4N+ and for MASMPe-K+ respectively, and at a e 0.3 for MAMPe-Bu4N+). In the case of MAMPe partially neutralized with KOH the situation is somewhat different and it may be wondered whether a linear interpolation of the data points of Figure 1could be simply made, as for MAE and MAiB. However, were a single straight line assumed for the whole a range the probable error of the function would be significantly larger than that of the three linear sections drawn in Figure 1, which is very close to that found in all other cases. In the approximate a ranges 0-0.2, Q.2-0.65,and 0.66-1.8 the estimated values are 8, 13, and 9 (mL/mol of H+),respectively. One may therefore tentatively assume that the conformational change of MAMPe(K+)would be confined in the middle range, Le., within 0.2 < a 4 0.65, approximately, and be characterized by a volume change, AVC,l5of about -2 mL/monomol. For the other systems we similarly estimate that -AVc should be about 4 (MA3MPe-M+), 7 ( M ~ M ~ e - B u 4 ~and + ) ,larger than 9
-ndiss
The Journal of Physicel Chemistry, V d . 81, No. 20, 1977
40
35 0
02
04
0 6
0 8
10
OL
Figure 3. Apparent pK, dependence on a for MA3MPe (a), MAMPe (b), and MAiB (c) aqueous solutions titrated with KOH at 25 "e. The polymer concentration was 5 X
monornolll
In each case.
(MA3MPe-Bu4N+) mL/monomol. (b) Potentiometric Data. Typical titration curves for MAMPe and MA3MPe in salt free aqueous solution at 25 "C are reported in Figure 2. The pH inflection at = 1 is rather sharp in each case. The second stage of the neutralization cannot be detected by conventional titration because of the very small second ionization constants of MAMPe and of MA3MPe.7ssv21 Therefore there is practically no overlap in the titration of first neighboring carboxyl groups along the polymer chains which lends confidence in the plots of pK, against a,constructed as indicated in the experimental part and reported in Figures 3 and 4. In the same figures potentiometric data for are also given for comparison purposes. For each copolymer a number of potentiometric titrations have been carried out both with KOH and Bu4NOH. For the polyelectrolyte concentration chosen (5 >s monomol/L), careful and frequent control of the calibration of the pH meter with standard buffer solutions has yielded highly reproducible pH data for all a values using different electrodes. The pK, values given in Figwes 3 and 4 may thus be considered accurate to within f0.02 pK, units. Examination of the potentiometric plots clearly reveals that MAiB and MA3MPe represent two opposite, extreme cases. For MAiB the very small dependence of pK, on a is remarkable and suggests a small, gradual increase in electrostatic free energy upon neutralization only slightly influenced by the nature of the counterions (Lev,K' or Bu4N+ions; Figures 3 and 4). On the contrary in the case of MA3MPe the potentiometric data clearly suggest that a conformational transition takes place upon neutralization, in close analogy with what was already found on the basis of potentiometric evidence for other poly~ a r b o x y l a t e and s ~ in~ qualitative ~ ~ ~ ~ ~agreement ~ ~ ~ ~ with indications derived from our dilatometric data (Figure 1). The conformational transition of MA3MPe chains woul
1903
Thermodynamics of Polycarboxylate Aqueous Solutions I
'
I
'
I
'
I
"I
6.0
OI
PKa
55
-40
-80
5 0
55
4 5
PKa
-0
45
4.0
-120
E
0 C
0
50
0
E
\
-z-
-200
5
-400
v
- 600
50 PK,
45
-- 1'000 800
40
1
t 0
3 5 L '
I
0
02
'
I 04
'
I 06
'
I 0 8
'
I 10
I
,
I
A
Figure 4. Apparent pK, dependence on a for MA3MPe (a), MAMPe (b), and MAiB (c) aqueous solutions tirated with Bu4NOHat 25 OC. The monomol/L in each case. polymer concentration was 5 X
start around a N 0.3 when neutralized with KOH, and around a N 0.6 when neutralized with Bu4NOH. The Bu4N+counterions would thus stabilize the initial, more compact conformational state of the polymer, in agreement with potentiometric evidence concerning other hypercoiling polyelectrolyte^.^^^^^ Spotting even approximately the completion of the significant conformational change of MA3MPe is more difficult inasmuch as it should take place a t a values for which the pK, values begin to reflect the presence of small amounts of doubly ionized groups along the chains (see Figures 3 and 4). The potentiometric behavior of MAMPe is undoubtedly anomalous, at least when compared with the plain behavior of MAiB, but gives by itself no clear cut evidence of the occurrence of a conformational transition. However, recalling the dilatometric data (see preceding section and Figure 1) one may conclude that neutralization of MAMPe with Bu4NOH may indeed induce a conformational change from globular coils to expanded coils of the type invoked above for MA3MPe, and starting at around a N 0.3. Finally, it has to be pointed out that we are unable to calculate quantitatively AG,, the free energy change for conformational transition, even in the case of MA3MPe, for the following reasons. In our case computation of "reference" curves from a = 0 to the a values marking the end of the transition and hence evaluation of extrapolated pKo values (the intrinsic dissociation Constants) applying an equation such as the Henderson-Hasselbalch one to the potentiometric data beyond the conformational transition is made difficult by the scarcity of data in such ill-defined region. We have found moreover that the pKa-a plots for MA3MPe and MAMPe depend somewhat on polymer concentration, with a linear dependence of pH on log C,, similar to what was reported for other aqueous polyelectrolyte solutionszkz8(the shapes of the titration curves remain however essentially unchanged). In other words,
02
04
06
08
a
10
Flgure 5. Enthalpy changes on dilution (cal/monomol) of MA3MPe (squares), MAMPe (circles), MAiB (hexagons), and MAP (triangles) partially neutralized with KOH (open symbols) and Bu4NOH(full symbols) in water at 25 OC. Initial and final average polymer concentrations monomol/L, respectively (for each given a). were lo-* and 5 X
evaluation of limiting AG, values would anyhow require additional manipulation of roughly estimated values. To bypass all these difficulties by focussing attention on the behavior of the polymers a t a fixed concentration (the concentration at which the dilatometric and calorimetric experiments were also carried out) we find it expedient to use as a common "reference" curve the titration curve of MAiB. This curve is only slightly affected by the nature of the titrating base and, when properly translated along the vertical axis (pK.J, it can be made to coincide with the small pKa-a portion of the titration curves of expanded MA3MPe and MAMPe in water as well as with the entire titration curves of both polymers in a globule destroying solvent (water-methanol, 1:l V/V).29 The extrapolated pKo values which result are 4.55 for MA3MPe and 3.9 for MAMPe. This roughly approximate procedure, exemplified in the plots of Figures 3 and 4 leads to an estimate for MAMPe that AG, should be about 0.3 kcal/monomol (K+)and 0.4 kcal/monomol (Bu4N+)while AG, for MA3MPe would be about 0.5 kcal/monomol (K+) and larger than 0.7 kcal/monomol (Bu4N+),always at 25 "C. The AGc value for the MA3MPe-K+ system happens to be rather close to the value (0.41 kcal/monomol) reported by Barone et al. in a detailed work on MA3MPe: although their data refer to a higher polymer concentration. ( c ) Calorimetric Data. (i) Dilution. The enthalpy of dilution data, at 25 "C, AH, (cal/monomol), as a function of the degree of dissociation (0 a < 1;K+ and Bu4N+as counterions), are reported in Figure 5. Although the measurements of M i l d i * were mainly performed to correct the heat of dissociation results and 5x are thus related to a single dilution step (monomol/L), they can yield useful information by themselves. M d i l are exothermic in practically the whole range of the first proton dissociation, tending in some cases to change in sign on further dissociation. All the curves
-
N
The Journal of Physical Chemistry. Vol. 8 1, No. 20, 1977
1904
Crescenzi et al.
'
7 -
-
+I
-:I 0 2
I
I
, I
i
E
1 V
5 VI
L" x V
a
-3
0
'
3
t I -0
- 2
4 I U v, Lo
I
04
06
08
10
a Figure 6. Enthalpy changes on proton dissociation (kcal/mol of H+) of MA3MPe (squares), MAMPe (circles), MAiB (hexagons), and MAP (triangles), partially neutralized with KOH, in water at 25 OC. Average final polymer concentration was 5 X monomol/L. The calorimetric data are reported at the mean a value in the actual dissociation interval of the experiment, which is typically of ca. 0.06 a units.
increase in exothermicity on increasing apolar character of the residues on the chain, in the order MAP 5 MAiB MAMPe < MA3MPe, and finally, on passing from K+ to Bu4N+as counterions, there is an increase in lLi&i]( by a factor of ca. 4 for MAP, MAiB, and MAMPe and of ca. 7 for MA3MPe. Neglecting speculations on sign changes for a > 1,which could be related to the presence of an uneven distribution of negative charges on the chains with possible, although difficult to predict, conformational consequences, one can say that negative m d i l values are quite common for synthetic polyelectrolytes in the concentration range herein i n ~ e s t i g a t e d . ~The ' ~ ~difference ~~~ in i w , i l values, on the other hand, would very likely reflect, in our opinion, differences in water structure-forming ability of the solutes. One additional and interesting feature is the sigmoidal shapes of the MA3MPe/Kt, MAMPe/Bu4N+, and MA3MPe/Bu4Nt curves. A tentative explanation can be given as follows. At very low a the conformational state of the macromolecules is a rather compact coil with the majority of the alkyl side groups removed from the outer water phase (intramolecular micellization); therefore the amount of hydrophobic residues which upon dilution are freed to contacts with the solvent is rather small, giving roughly the same overall heat effect as for the flexible analogues with shorter side chains which have by far more contact sites with the surrounding water phase. In the transition region dilution can lead to an increasing number of hydrophobic sites/solvent interactions through an exothermic "iceberg formation" process, up to the end of the transition for which there is a sort of leveling off of the effects. The overall phenomenon certainly deserves more data on a wider concentration scale, and experiments in this sense are underway in our laboratory. (ii) Dissociation. The enthalpy of dissociation data at 25 "C, (kcal/mol of H'), as a function of the degree of dissociation,a,(0 < a < 1)are reported in Figures 6 and 7. The curves for the copolymers partially neutralized with KOH clearly show that increasing alkyl substitution in the The Journal of Physical Chemistry, Vol. 8 1, No. 20, 1977
0
02
04
0.6
08
a
10
Figure 7. Enthalpy changes on proton dissociation (kcal/mol of added H+) of MA3MPe (squares), MAMPe (circles), MaiB (hexagons), and MAP (triangles), partially neutralized with Bu,NOH, in water at 25 OC. Average final polymer concentration was 5 X lo3 monomol/L. The calorimetric data are reported at the mean a value in the actual dissociation interval
of the experiment, which is typically of ca. 0.06 a units.
maleic acid comonomer units makes the enthalpy of dissociation more and more negative, for all a values less than ca. 0.85. In this a region, m d i s s attains its minimum value of -4.7 kcal/mol of H+ around a = 0.7 for MA3MPe. Data collected using the copolymers partially neutralized with Bu4NOH show, in particular, that dissociation of MA3MPe is strikingly anomalous; the m d i S s goes in fact from positive ( m d i S s = 6 kcal/mol of H+ for a = 0.45) to negative values (A",,= -16 kcal/mol for a = 0.85):' The trends in the vs. a plots as well as the abnormally large enthalpy values indicate that both MAMPe and MA3MPe undergo a conformational change in the a range investigated. This conclusion is clearly beyond doubt for the copolymers partially neutralized with Bu4NOH. The calorimetric data do not appear however to lend themselves to an unambiguous deduction of the enthalpy associated with the conformational changes (AH,), contrary to what was experienced with analogous data for poly(methacrylic acid) and the maleic acid-butyl vinyl ether copolymer.1°-12 One can only deduce that AHc for both copolymers should be rather large and negative, in particular for MA3MPe with Bu4N+counterions. Taking into account the approximate AG, values given in section b, the conclusion is safely drawn that the entropy change for the passage from globular to open states of the uncharged copolymer chains is negative. This is in line with the hypothesis that the polymeric globules are stabilized by hydrophobic interactions and in qualitative agreement with the excess negative volume changes on transition (section a). Data of Figures 6 and 7 also suggest that, for all copolymers considered, the second dissociation reaction is endothermic (we report here only a few data for a > 1,as we are presently mainly interested in the conformational transition regions). The often marked difference in the pK, values for first and second ionization of such copolymers is thus mainly attributable to an enthalpy effect, but with both enthalpy
1905
Micellar Catalysis of Radical Reactions
and entropy being particularly unfavorable in the second ionization stage. As a final remark, the striking borderline behavior of the MAMPe copolymer (especially with Kf counterions) with respect to the conformational transition is to be underlined. Although dilatometry and f l u o r e ~ c e n c equite ~ ~ undoubtedly lend us to propose the occurrence of such a transition, potentiometry and calorimetry alone give weak support to it. This should once more induce one to be very cautious on relying upon one single technique for asserting the presence of a conformational transition for synthetic polyelectrolyte solutions.
Acknowledgment. This work has been sponsored by the Italian Consiglio Nazionale delle Ricerche, Rome. One of the authors (J.C.F.) is grateful to NATO for its financial support. References and Notes (1) On leave of absence from the Laboratoire de Chimie MacromolBculaire, UniversitB de Rouen, Mont-Saint-Aignan, France. (2) Present address: Istituto di Chimica-Fisica, UniversW di Roma, 00185 Roma, Italy. (3) V. Crescenzi, F. Delben, F. Quadrifoglio, and D. Dolar, J. Phys. Chem., 77, 539 (1973). (4) F. Quadrifoglio, V. Crescenzi, and F. Delben, Macromolecules,6, 301 (1973). (5) V. Crescenzi, F. Delben, S. Paoletti, and J. Skerjanc, J. Phys. Chem., 78. 607 (19741. (6) F. Delben, S. Paoletti, V. Crescenzi, and F. Quadrifoglio, Macromolecules, 7, 538 (1974). (7) E. Bianchi, A. Ciferri, R. P a r d , R. Rampone, and T. Tealdi, J. Phys. Chem., 74, 1050 (1970). (8) G. Barone, N. Di Virgilio, V. Elia, and E. Rizzo, J. Polym. Scl., Symp. No. 44, 1 (1974). (9) S. Katz and T. G. Ferris, Biochemistry, 5, 3246 (1966). (10) V. Crescenzi, F. Quadrifoglio, and F. Delben, J . Polym. Sci., Part A - 2 , 10, 357 (1972). (11) V. Crescenzi, F. Quadrifoglio, and F. Delben, J . Polym. Scl., Part C , 39, 241 (1972). (12) F. Delben, V. Crescenzi, and F. Quadrifoglio, Eur. Polym. J., 8, 933 (1972). (13) A. J. Begala and U. P. Strauss, J . Pbys. Chem., 76, 254 (1972).
(14) Similar data for MAE, MAP, MAiB,‘ as well for MA-alkyl vinyl ether copolymersi3 (0 C a C 1) fall in the range -6 to -7 mL/mol of H:’ these data, however, were obtained in the presence of added simple salts. (15) N. Ohno, K. Nitta, S. Makino, and S. Sugai, J. Polym. Scl., Polym. Phys. Ed., 11, 413 (1973). (16) A. Ikegami, Siopolymers, 6, 431 (1968). (17) J. Komiyama, M. Ando, Y. Takeda, and T. Iijima, Eur. Polym. J., 12, 201 (1975). (18) M. Mandel, J. C. Leyte, and M. G. Stadhouder, J. Phys. Chem., 76, 603 (1967). (19) P. L. Dubin and U. P. Strauss, J. Phys. Chem., 74. 2842 (1970). (20) P. L. Dubin and U. P. Strauss; “Hypercoiling in Hydrophobic Polyacids” in “Polyelectrolytes and Their Applications”, A. Rembaum and E. SB!&jgny, Ed., Reidel Publishing Co., Dordrecht,Holland, 1975, pp 3-13. (21) Addition of proper amounts of CU(CIO~)~ to the partially neutralized polyelectrolyte solutions makes it feasible to titrate both carboxyl functions of MAMPe and MA3MPe, as already discussed in the case of MAiB.“ (22) S. Paoletti, F. Delben, and V. Crescenzi, J. Phys. Chem., 80, 2564 (1976). (23) J. C. Fenyo, Eur. Polym. J., 10, 233 (1974). (24) J. C. Fenyo, J. Beaumais, and E. SBIBgny, J . Polym. Sci., Polym. Chem. Ed., 12, 2659 (1974). (25) H. Maeda and F. Oosawa, J. Pbys. Chem., 76, 3445 (1972). (26) G. S. Manning and H. Holtzer, J . Phys. Chem., 77, 2206 (1973). (27) K. Nitta and S. Sugai, J. Phys. Chem., 78, 1189 (1974). (28) K. N b , M. Yoneyama, and N. Ohno, Biophys. Chem., 3,323 (1975). (29) Unreported in Figure 1 to avoid too crowded plots. (30) J. Skerjanc, D. Dohr, and D. LeskovSek, 2.Phys. Chem. (Frankfurt a q Main), 56, 207 (1967). (31) J. Skerjanc, D. Dolar, and D. LeskovSek, Z. Phys. Chem. (Frankfurt am Main). 56. 218 119671. (32) J. SkerJanc,D: Dolar: and 6. LeskovSek, Z. Phys. Chem. (Frankfurt am Main), 70, 31 (1970). (33) N. Ise, K. Mita, and T. Okubo, J. Chem. SOC.,Faraday Trans. 7 , 69,, 106 (1973). (34) J. Skerjanc, Siophys. Chem., 1, 376 (1974). (35) K. Mita and T. Okubo, J. Chem. Soc., Faraday Trans. 7 , 70, 1546 (1974). (36) K. Mita, T. Okubo, and N. Ise, J. Chem. Soc., Faraday Trans. 7 , 71, 1932 (1975). (37) K. Mita, T. Okubo, and N. Ise, J . Chem. Soc., Faraday Trans. 1, 72, 504 (1976). (38) H. Daoust and A. Lajoie, Can. J . Chem., 54, 1853 (1976). (39) The rekheiy hrge AHb of MA3MPe for Q I0.6, a rather u n c o m m feature for the first dissociation of a polycarboxylic acid, might stem at least in part from Bu4N’ counterions binding and/or on Q dependent slightly on the aggregation of MA3MPe. (40) J. C. Fenyo, S. Paoletti, F. Delben, and V. Crescenzi, manuscript in preparation.
Micellar Catalysis of Radical Reactions. A Spin Trapping Study Dennis P. Bakallk and J. K. Thornas* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received November 15, 1976; Revised Manuscript Received June 1, 1977) Publication costs assisted by the US. Energy Research and Development Administration
The reaction of surfactant radicals and the spin trap 2-methyl-2-nitrosopropanehas been investigated. Sodium dodecyl sulfate and sodium octyl sulfate show no spin adduct when surfactant and spin trap are irradiated with 6oCoat concentrationsbelow their critical micelle concentration. At concentrations above the cmc a nitroxide is formed from the reaction of the trap and a secondary surfactant radical produced via hydroxyl radical attack on the hydrocarbon chain. The micellar catalytic effect and the site of hydroxyl radical attack are discussed. Introduction Radiation induced reactions in micellar systemshave received detailed Over the last few years,i A lively as interest exists in applying such simple systems +Theresearch described herein was supported by the Division of Physical Research of the U.S.Energy Research and Development Administration. This is Radiation Laboratory Document No. NDRL-1721.
for more complex systems such as membranes. However, these systems exhibit many interesting kinetic properties such as catalysis,2which are of direct interest to physical chemists. Several studies (reviewed in ref 1)already exist to show that a sharp change in kinetic properties occurs on micellization of the surfactant molecules. The present work is an to locate the nature and Of reaction of hydroxyl radicals, OH, with anionic surfactants; it has already been established that OH radicals show quite The Journal of Physical Chemistry, Vol. 61, No. 20, 1977
