Langmuir 1993,9, 681-684
681
Poly(N-vinyl-2-pyrrolidone)-Monoalkyl Xanthates. 1. Adsorption and Chemical Reaction Ligia Gargallo,’ Jorge Phrez-Cotapos, Josh G. Santos, and Deodato Radi6 Departamento de Qufmica Fkica, Facultad de Qulmica, Pontificia Universidad Catblica de Chile, Casilla 306, Santiago 22, Chile Received June 8,1992. In Final Form: November 10, 1992 The interaction of poly(N-vinyl-2-pyrrolidone)(PVP)with potassium monoalkyl xanthateswas studied by W-vis spectrophotometry and ultrafiitration measurementa. A simple association model was applied in order to describe the resulta. Some characteristics of the aesociation process were analyzed. The fmdinga suggest the presence of a more complex process than one simple association or chemical reaction process. We have supposed that the absorption obtained corresponds to the adsorption of the product. From the second-order rate constant values of the chemical reaction, we have calculated the activation free energy AG*for the reaction of each xanthate studied with PVP and ita variation with the number of carbon atoms in the alkyl chain. The different contributions to AG*values are discussed in terms of the one specific ionic interaction between -0CSS- and one positive active center in the polymer chain and one hydrophobic aeeociation with the backbone of the PVP.
Introduction The interaction of poly(N-vinyl-2-pynolidone)(PVP) with small coeolutes in aqueous and nonaqueous media has been throughly 8tudied.l- This water-soluble polymer shows a number of interesting properties. One of them is the high capability to interact with different kind of molecules, suchas iodine:JlJ4 detergents,’”% druga,n-30 aromatic c ~ m p o u n d s , ~ carboxilic J ~ J ~ ~ac~~ ids,& and other compounds. The biological behavior of poly(N-vinyl-2-pyrrolidone)(PVP)and more specifically physiological and pharmacological aspects are very important, because of the use of the polymer pharmaceu(1)Molyneux, P. J. Water-Soluble Synthetic Polymers: Properties and Behavior, 5th ed.; CRC P r e s Inc.: Boca Raton, FL, 1980; Vol. 1, Chapters 1 and 4;Vol. 11, Chapter 2. (2)Flory, P. J. Principles of Polymer Chemistry; Comell University Pres: Ithaca, NY,1953; p 523. (3)Molyneux, P. J. In Water: A Comprehensiue Treatise; Franks, F., Ed.;Plenum Prese: London and New York, 1975;Vol. 4,Chapter 7. (4)Molyneux, P.; Frank, H. J. J. Am. Chem. SOC.1961,83,3169. (5)Molyneux, P.; Frank, H. J. J. Am. Chem. Soc. 1961,83,3175. (6)Bandyopadhyay, P.; Rodriguez, F. Polymer 1972,13, 119. (7) Gargallo, L.;Radic’, D. Polymer 1983,24,91. (8)Gargallo, L.;Radic’, D. Polym. Commun. 1985,26,149. (9)Gargallo, L.; Mufloz, M. I.; Rloe, H.; Radic’, D. J. Colloid Interface Sci. 1986,113,480. (10)Meza, R.; Gargallo, L. Eur. Polym. J. 1977,13,235. (11)Siggia, S.J . Am. Pharm. Assoc. Sci. Ed. 1957,44, 201. (12)Coumoyer, R. F.; Siggia, S. J. Polym. Sci., Polym. Chem. Ed. 1974,12,603. (13)Kireh, Yu. E.; Sow, T. A,; Karaputadze, T. M. Eur. Polym. J . 1979,15,223. (14)Kireh, Yu. E.; Soos, T. A.; Karaputadze, T. M. Eur. Polym. J . 1983,19 (71, 639. (15)Molyneux,P. J.;Ahmed,G. S.Kolloid Z . 2.Polym. 1973,251,310. (16)Saito. S.J. Colloid Sci. 1960.15. 283. (17)Saito; S.J. Colloid Interface Sci. 1967,24, 227. (18)Saito, S.Kolloid Z . Z . Polym. 1967,215,16. (19)Saito, S.Kolloid-2. 1955,143 (2),66. (20)Bloor, D.M.;Wyn-Jones, E. J. Chem. SOC.,Faraday Trans. 2 1982,78,657. (21)hung, R.; Shah,D. 0.J. ColloidInterfoce Sci. 1986,113(2), 484. (22)Aizawa, M.;Komatau, T.; Nakagawa, T. Bull. Chem. SOC.Jpn. 1988,20,39. (23)Garcfa-Mpez de Sa, T.;Garrido, L. M.;Allende, J. L. Bull. Chem. SOC.Jpn. 1988,20,39. (24)Cabane, B.; Duplesaix, R. Colloids Surf. 1986,13, 19. (25)Chibowski.. S.: .S z m _ a_. J.. Pol. J. Chem. (Formerly Rocz. Chem.) 1982,b6, 359. (26)Lisei, E. A.; Abuin, E. J. Colloid Interface Sci. 1986,105 (l), 1. (27)Higuchi, T.; Kuramoto, R. J . Am. Pharm. Assoc., Sci. Ed. 1954, 43,393,398. (28)Guttmann, D.;Higuchi, T. J . Am. Pharm. Assoc., Sci. Ed. 1956, 45,659.
tically and clinically in a variety of applications including that as a blood-plasma substitute,and because of ita use in hair sprays and other aerosol products, dropleta of which may be inhaled into the lungs.51 This polymer hae some similarities with proteins especially serum albumin. In particular, PVP also shows strong interactions with anionic organic cosolutes. However, there is still no widely accepted explanation for the cosolute binding capability of PVP. Because of amphiphilic character of this polymer, the nature of the interaction should be different depending on the hydrophobic/hydrophilic properties of the cosolute. In this context, the study of the interaction of PVP with aliphatic compounds containing hydrophilic groups as monoalkyl xanthates seem to be interesting from a t least two pointa of view. First, it is possible to study the hydrophilic contribution to interaction, and second, if we (29)Miyawaki, G . M.; Patel, N. K.; Kostenbauder, J. J . Am. Pharm. Assoc., Sci. Ed. 1959,48,315. (30)Bahal, C. K.; Kcetenbauder, H. B. J. Pharm. Sci. 1964,53,1027. (31)Scholtan, W. Makromol. Chem. 1953,11,131. (32)Oeter, G. J. Polym. Sci. 1955,16,235. (33)Frank, H. P.; Barkin, S.; Eirich, F. R. J. Phys. Chem. 1957,61, 1375. (34)Reeves, R.L.;Harkaway, S. A.; Sochor,A. R. J . Polym. Sci., Polym. Chem. Ed. 1981,19,2427. (35)Takagiehi, T.; Imajo, K.;Nakagami, K.;Kuroki, N. J. Polym. Sci., Polym. Chem. Ed. 1977,15,31. (36)Takagiehi, T.; Kuroki, N. J . Polym. Sci., Polym. Chem. Ed. 1973, 11, 1889. (37)Sardharwalla, I.; Lawton, J. B. Polymer 1985,26, 751. (38)Plaizier-Vercammen, J. A.; Nbve, R. E. J. Pharm. Sei. 1982,71 (5),552. (39)Plaizier-Vercammen, J. A. J . Pharm. Sci. 1983, 72 (S),1042. (40)Molyneux, P. J.; Vekavakayanondha. J. Chem. SOC.,Faraday Trans. 1 1979,257,855. (41)Sheth, G . N. J. Appl. Polym. Sci. 1986,31,1227. (42)Sheth, G . N. J . Appl. Polym. Sci. 1986,32,4333. (43)Molyneux, P. J.;Carnarakia-Lentzoe, M.Colloid Polym. Sci. 1979, 257,855. (44) Molyneux, P.; Frank, H. P. J . Am. Chem. Soc. 1964,86,4753. (45)Uty-Sietel, C.; Sbille, B.; Quivoron, C. J . Polym. Sci., Polym. Symp. 1975,52,311. (46)Hecht, G.; Weese, H. Muench. Med. Wachenschr. 1943,90,11. (47)Thrower, W. R.; Campbell, H. Lancet 1951,260, 1096. (48)Ravin, H. A.; Seligman, A. M.;Fine, J. N. Engl. J. Med. 1952,247, 921. (49)Reynolds, J. E. F., Prased, A. B., Eds. Martindale: The Extra Pharmacopeia, 28th ed; The Pharmaceutical Presa: London, 1982. (50) Levy, G. B.; Caldae, I.; Fergus, D. AM^. Chem. 1952,24,1799. (51)Tsunemitau, K.; Murakami, Y.; Toyoahima, K. In Polyvinyl Alcohol: Properties ond Applications, Finch, C. A,, Ed.;John Wiley and Sons: London, 1973.
0743-7463/93/2409-0681$04.00/00 1993 American Chemical Society
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682 Langmuir, Vol. 9, No. 3, 1993
~
Table I. Absorbance Values at 346 nm for Monoalkyl Xanthate Solutions at 3 X 10-6 mol L-l with Different PVP Concentrations (mol L-1). [PVP] X AAAAAAA102 (EX) (PX) (BX) (AX) (HX) (OX) (PVP) 0.9 0.024 0.027 0.028 0.029 0.028 0.027 0.003 1.8 2.7 3.6 4.5 5.4 6.3 7.2 8.1 9.0
0.041 0.062 0.078 0.100 0.113 0.125 0.136 0.152 0.158
0.051 0.074 0.095 0.117 0.131 0.150 0.158 0.168 0.180
0.055 0.008 0.100 0.124 0.141 0.158 0.166 0.176 0.184
0.057 0.083 0.105 0.126 0.149 0.164 0.179 0.185 0.195
0.054 0.079 0.101 0.123 0.141 0.156 0.172 0.186 0.189
0.051 0.070 0.093 0.104 0.125 0.141 0.147 0.161 0.169
0.007 0.008 0.010 0.012 0.013 0.015 0.016 0.018 0.020
Measurements were carried out at 298 K and 2 hours after the mixing. 0
introduce different chain lengths in the cwolute, it should be possible to study the hydrophobic contribution to interaction. In both cases it is necessary to take into account the structure of PVP itself. According to Molineux,21PVP can be seen as a polymer containing a dipole in the monomer unit with a charge distribution similar to that of N-methylpyrrolid0ne.2~*~5 For this reason it is necessary to take into account the environment of the dipole because these differences could probably be highly significant in determining the hydrophilic PVP-cosolute interactions. According to the proposed structure, PVP presents a negative oxygen end and a positive nitrogen end. The position of the nitrogen atom in the PVP structure ie indeed closely similar to that of tetraalkylammonium ions which are not hydrated in solution.26 Obviously, the hydrophobic contributions to the PW-xanthate interactions might be those produced between the alkyl chain of the xanthates and the backbone of the poly(N-vinyl2-pyrrolidone). In order to clarify the different contributions of these kinds of molecules to the interaction with PVP, we report in this work the study of the interaction of PVP with monoalkyl xanthates, which possesses both hydrophilic character due to the thiocarbonate group and hydrophobic character due to the alkyl chains.
Experimental Section Reagents. Poly(N-vinyl-2-pyrrolidone) (PVP) was obtained from a commercialsample of the General BiochemicalLaboratory Park, Chagrin Falls, OH, lot no. 85899,MW = 3.6 X 105. All the alcohols used in this work were analytical grade without previous treatment. The aqueous solutions were prepared daily in redistilled water. Ethyl (EX),n-propyl (PX), n-butyl (BX), n-amyl (AX), n-hexyl (HX), and n-octyl (OX) potassium xanthates were prepared by mixing the respective potassium alkoxides with CS2 according to the reported methods.27 All xanthates were purified by recrystallization from acetone, and their UV-vis spectra in water solutions were recorded in a Perkin-Elmer Lambda-3B spectrophotometer. The spectra were compared with those reported ~ ' agreement was found, with a pronounced peak by R ~ O . Good at 301 nm having an absorptivity close to 1.7 X lW. Monoalkyl xanthates were also identified by their IR and lH NMR spectra. IR (KBr) spectra were obtained on a PerkinElmer Model 567 spectrophotometer. lH NMR spectra were taken with a Varian XL-100 spectrometer in deuterium oxide with tetramethylsilane as internal standard. UV-Vis Measurements. Two types of experiments were performed. In some cases the spectra were recorded in the 2803 W n m range at different times, and in others only the absorbance changes at 364 nm were recorded or measured after a time under the experimental conditions of Tables I and 11. Solutions of PVP were thermostatized in the spectrophotometer cells for 30 min. After the thermostatization was reached, a stock solution
Table 11. Rate Constant Values (&) of the First Step of the Reaction between PVP (MW = 3.6 X 1V) with Xanthates of Different Alkyl Chains and for Various Polymer Conoentrations, at 298 K ( m i d ) , and the Activation Free Energy Values ( A h ' and At?"') for the Different Reactions
0.431 0.436 0.437 0.449 0.489 0.448 av 1.1 X [XI 1 ~ 3 10-3 325 407 kobd[XI 307 AGB' (J/mol) 68.620 68.490 67.950 AG,' (J/mol) 85.420 85.300 84.750
0.20 0.18 0.16 0.13 0.10
0.323 0.352 0.320 0.331 0.365 0.338 1.1 x
0.340 0.361 0.368 0.373 0.347 0.358 1.1 x
iw
0.456 0.428 0.447 0.441 0.468
0.455 0.472 0.495 0.492 0.513
0.738 0.640 0.645 0.638 0.678
0.448
0.485
9.8 X lrP 457 67.660 84.750
1.1
0.668 1.2 x
x1w
485
67.490 84.550
1
~
557 67.160 83.750
Table 111. Absorbance Valum of the Filtrating and Filtrate Solutions for the Reaction between PVP at 0.2 mol L-l with [BX]at 1.5 X lo-' mol L-I at 298 K filtrating solution filtrate solution absorbance time absorbance time/min 301nm 346range/min 301nm 346nm 27 54 81 108
1.492 1.159 1.063 1.048
more xanthate
0.508 0.603 0.607 0.607 0.607
0-27 27-54 54-81 81-108
0.603 0.762
0.127 0.301
0.810
0.466 0.466 0.466
0.889
more xanthate
of xanthate was injected with vigorous stirring. All reactions were followed by at least three half-lives. Ultrafiltration Measurements. The ultrafiltration measurements were performed under the aame conditions as those of the kinetic measurements in an ultrafitrating cell, Amicon, of 50-mL volume equipped with a PM-10 membrane. As this process is very slow, it is not possible to refer the filtrate absorbance to a specific time, and the results in Table I11 correspond to time ranges of 27 min for each one. Kinetic Measurements. A stocksolutionof PVP, containing 0.2 mol L-l monomer units was prepared. Solutions of PVP containing0.18,0.16,0.13and 0.10mol L-*monomer units were prepared by dilution of the stock solution. Similarly for each mol L-l was prepared, and xanthate a stock solution of 2.4 X 1p2 its precise concentration was determined by absorbance measurements. Aliquots (3 mL) of PVP solutions were transferred to 1-cm quartz c e b (Hellma 111-QS)and placed into the thermoetated cell holder of a spectrophotometer (Perkin-Elmer Lambda-3B). After thermal equilibration and under stirring, 0.01 mL of the xanthate stock solution was injected into the cell. The reaction was followed by monitoring the changes in absorbance (A) with time at 346 nm; all the reactions were followed for no lees than three half-lives. The plots of log (A, - A) vs time were found to be linear. First-order rate constants (hob)were obtained in all cases. All the kinetic runs were performed in duplicate, and plots giving correlation coefficients worse than 0.99 were disregarded.
Results and Discussion All xanthates (X)here studied show a pronounced peak at 301 nm as has been reported.n When a solution of xanthate is added to a PVP solution, this band begins to decrease while a new band at 346 nm simultaneouslystarts to increase, presenting an isosbestic point at 318 nm,as we can see in Figure 1. Table I summarizes the absorbance (A) at 346 nm obtained for the fraction (molecular weight 3.6 X 105) of PVP solutions in the concentration range (0.9-9)X 1t2 mol L-'with different xanthates at a concentration of 3.0 X lo" mol L-1 at 298 K 2 hours after the mixing.
3
Poly(N-vinyl-2-pyrrolidone)-MonoalkylXanthates
Langmuir, Vol. 9, No.3, 1993 683
2
w
0
z U
m l
a
0 v)
m
4
1 / A ( EX)
Figure 2. l/[PVP]versua 1/A according to eq 9 for n-butyl xanthate [BX]at 298 K.
0
h (rim) Figure 1. PVP spectra of a 0.2 mol L-1 solution expressed in monomeric units with n-butyl xanthate (2X 10-4 mol L-9. The spectra are taken at different times (0, 5,10,15,20, 25,30,and 35 min) at 298 K.
Assuming an association process between PVP and X, a simple association model was applied in order to describe these results. This model supposes the following equilibrium: K.
PVP + x ?= PVP-x
(1)
K, = [PVP-X]/[X][PVP]
(2)
where
[XI and [PVP-XI are the xanthate concentrations in solution and associated with PVP, respectively. Considering the mass balance [XI,,,
[PVP],,,
= [XI + [PVP-XI
(3)
= [PVP] + [PVP-XI
(4)
and the relation
3
AM6= tpw-x[PVP-Xl
(5)
where A348 and epvp-x are the absorbance and molar extintion coefficient of PVP-X at X = 346 nm, we can obtain [XI
[XI,,
- A3&/epvp-x
(6)
By substituting eq 5 in eq 4 [PVPI [PVPl,d - A3*/epvp-x (7) As [PVPlt0d = 9.0 X 10-3 to 9.0 X 10-2 mol L-'monomer units and the AIepvp-x should be 0 to 3.0 X lP5,the second term of eq 7 is negligible. By substituting eqs 5-7, in eq 2, we obtain
1 Figure 3. l/[PVP]versus 1/A according to eq 9 for n-octyl xanthate [OX]at 298 K. 1 / A ( OX
A%pvp-x (8) {[XIT - A346/epvp-X) [PVPl By rearranging this equation, it is possible to obtain eq 9.
K, =
1/[PW,,
= -K,
+ {epvp-xK,[Xl,&AM
(9)
According to eq 9, a l/[PVP] vs l/A3&plot should be linear if Kais independent of [PVP]. As examples, Figures 2 and 3 show this type of plot for n-butyl and n-octyl
Gargallo et al.
684 Langmuir, Vol. 9, No. 3, 1993
xanthates, respectively. The same behavior was found for the other xanthates studied. From these figures it should be possible to infer that the association model is an adequate description of the experimental results. However, there are at least two questions about the characteristics of the association process. On one hand, the velocity of the process is too low for an adsorption step, and on the other, there is a considerable shift of the absorption band, from 301 to 346 nm, in the process. This observation should be more compatible with a Chemical reaction. In order to clarify this phenomenon, we have studied the kinetics of the process when PVP is added to a solution of xanthate. Figure 1shows the variation of the absorbance with time for n-butyl xanthate with PVP. Assuming a chemical reaction, in Table I1 we have summarized the rate constant values of the reaction betweenPVP (MW = 360 000) withxanthatesof different chain lengths and for various polymer concentrations at 298 K. There are at least two explanations for these results. First, we can assume that all the xanthate is adsorbed and a chemical reaction occurs over the adsorbed xanthate, and second, the pseudo-first order rate constants independent of [PVPI suggest that the polymer is the substrate and consequently the [XI is in excess over PVP; this is a contradictory result because from the experimental conditions (Table 11)it can be observed that [PVPI >> [XI; probably only a few sites in the polymer would be reactive, and [XI is greater than the active site concentration ([call. In order to clarify this behavior, some experiments by ultrafiltration were performed on the butyl xanthate-PVP system. Table I11 summarizes the absorbances of the filtrate (collected in a 27-min range) and those of the filtrating solution at 301 and 346 nm. Several conclusions can be obtained from these results: (1)The xanthate is, at least in part, initially adsorbed on PVP, evidenced as the difference in absorbance between the filtrate and filtrating solutions at 301 nm. (2) The decrease in absorbance at 301 nm of the filtrating solution is accompanied by an increase in that of the filtrate a t the same wavelength, showing that the product also presents an absorbance band near 301 nm. (3) The equilibrium is reachedafter about 1h, and the addition of more xanthate does not product changes in the filtrate nor filtrating solutions. This suggests that all the active centers in the polymer are not free. The difference in absorbance, under the equilibrium conditions, between the filtrate and filtrating solutions a t 301and 346 nm can be attributed to the adsorbed product, assuming equal molar absorption coefficients for free and adsorbed product, and using [PVPI = 0.2 mol L-l, an adsorption equilibrium constant of 1.5 is obtained which, considering the assumptions and approximations used, is in good agreement with that obtained from the intercept from Figure 2 for the same xanthate. Therefore, we can suppose that the results of adsorption indicated above correspond to the adsorption product. If we analyze the equilibrium constant obtained for the different xanthates from plots of eq 9, we can observe that all of them are closely similar ( - 5 ) , suggesting that there are no differences due to the nature of the chain. Whatever the model that we accept for the chemical reaction, the adsorbed xanthate reaction (unimolecular reaction) or one excess of xanthate on the active centers (bimolecularreaction), the results in Table I1indicate that both koba and kobs/[X] increase with an increase of the
2
4
6
8
1
0
TC
Figure 4. Activation free molar energies (AGB*and AGu*)versus the number of carbon atoms of the alkyl chain of the xanthates (n,)for the reactions of PVP (MW = 3.6 X lo6)and the different xanthates studied at 298 K.
number of carbon atoms of the alkyl chain; the reactivity enhancement can be attributed to the hydrophobic degree of the chain. If we now suppose that the bimolecular model is adequate to describe the results, and we calculate the activation free energy (AGB*),then the plot of AGB*values vs the number of carbon atoms (n,) in the alkyl chain is shown in Figure 4. This figure shows a linearity and obeys relation 10. AGB* = -260n,
+ 69.100
(10)
This behavior is indicative of a stabilization of the transition state with a contribution of each methyl and methylene group of -260 J mol-' to the activation free energy. The xanthate contribution to AGB' is about 59 kJ mol-l. On the other hand, if we now accept the unimolecular model as one appropriate approach, we can calculate the activation free energy (AGu*) from the kobs values and we can establish relation 11. AGu* = -270n,
+ 86.000
(11)
This relation also shows a stabilization of the transition state. In the latter case, each methyl and methylenegroup contributes -270 J mol-' to the activation free energy (AGu*). The xanthate contribution is about 76 kJ mol-'. The high values of the xanthate contribution should be explained in terms of a specific interaction between the xanthate and a charged active center in the polymer chain. The negative contribution to AG* (AGB* or AGu*) by the alkyl chain can be a consequence of a hydrophobic association with the backbone of the polymer. We can observe that the PVP-xanthate system is not a simple one; initial adsorption of xanthate, chemical reaction, and adsorption of productson PVP are involved. More work is in progress to clarify this aspect.
Acknowledgment. We thank the Fondo Nacional de Ciencia y Tecnologia (FONDECYT) for financial support.
