JOURNAL OF POLY.MER SCIENCE: Polymer Chemistry Edition VOL. 12. 603-612 (1974)

Interaction of Polyvinylpyrrolidone and Iodine

ROBERT F. COURNOYER and SIDNEY SIGGIA, Department of Chemistry, University of Massachusetts, Amherst, Massachusetts, 01 002

Synopsis

A nonsolvent preparation of the polyvinylpyrrolidone (PVP) complex provides new insight into the nature of the polymer-iodine interaction. The preparation is obtained by simply mixing PVP with crystalline iodine and is of interest because it provides a system in which no interfering ions are present and only one type of iodine is initially present. The iodine is shown to undergo hydrolysis with moisture in the polymer to give iodide and hypoiodite. The ionic forms of iodine appear to associate with the molecular iodine, resulting in the final stable PVP-iodine complex.

INTRODUCTION

The number of studies involving polymer and iodine interactions are so vast that the specific citing of even a representative collection of work would have t.o be the subject of a review article. In the last few years i t has been generally accepted that the complexation of iodine with many polymers involves molecular as well as ionic iodine. In most cases the ionic iodine is believed to be the iodide ion. I t has also bwn reported that in many eases other foreign ions can have a pronounced effect. on the final polymer-iodine complex product.

Presented in this work is part of a ytudy of the interaction of polyvinyl- pyrrolidone (PVP) with iodine. Previous work on this complex has largely involved PV1'-iodine complexes prepared in solution, the solvent usually being water. These studies involvod introduction of iodine as t.he tri- iodide ion or employcd some in situ gcneration of iodine. The invttstiga- tion described herein involves a nonsolvent preparation of the complex, the product being denoted PVP-I. PVP-I is made by mixing powdered PVP with crystalline iodine. The remarkablc stability of PVP-I so prepared has been reported.' This nonsolvent complex preparation is in- teresting because the iodine is introduced as molecular iodine without any other typcs of iodine or other ions being present. This system therefore provides an excellent opportunity to investigate the nature of t.he PVP- iodine complex with particular regard to the role of ionic iodine in t.he complexation process.

603

@ 1974 by John Wtiey & Sons, Inc.

604 COURNOY ER AND SIGGIA

EXPERIMENTAL

Reagents The powdered PVP was kindly supplied by the General Aniline and

Film Corporation. All of the polymer used came from one batch of phar- maceutical grade PVP with a number average molecular weight of 40,000. The reagents used were of analyt.ica1 grade and were supplied by eit.her the Fisher Scientific Company or thc J. T . Haker Chemical Company. The starch indicator solution was prepared by Anderson Laboratories Inc. Reagents for the Karl Fischer titration were supplied by EIarleco.

Synthesis The PVP-I complex was prepared by adding powdered PVP and crystal-

line iodine in known amounts to a glass stoppered container. The total sample volume was from 0.2 to 0.4 times the container volume. The load- ing factors employed ranged from 0.04 t.0 0.43. The loading factor is the number of grams of iodine added per gram of dry polymer. Two series of preparations were investigated. Thc only difference between the two series was t.he water cont.ent of the PVP, 10.1% in series I, and 12.6’% in the series I1 preparation. The PVP-I samples were then tumbled for 24 hr a t 60 * 2°C.

The moisture content of the PVP was increased by intermittently ex- posing t.hc polymer to a moist atmosphere and then tumbling for several hours a t 60°C. This process was repeated until the desired moisture con- tent was achieved and analysis showed the water content to be homoge- neous.

The PVI’ film was prepared by evaporating 5-10y0 solutions of PVP in dichloromcthane on a covered mercury pool a t room temperature. Thc iodinc-containing film was prepared by placing the film in a saturated iodine atmosphere until a pronounced red color developed.

Titrations All titrations were carried out a t room temperature by using either a 50

ml or 5.0 ml precision buret. The amount. of analyte taken was so ad- justed that the minimum titer was 40y0 of the total buret volume. Dis- tilled deionized water was used.

The total iodine present was determined in order to show if all of t.he iodine added in the synthesis step was still present in the final PVP-I system. The procedure was to reduce all of the iodine to iodide with bi- sulfite and t.itrate with standard silver solution as described by Siggia.’

The possibility that t.he molecular iodine addcd in the synthesis step was undergoing somc change on complexation was irivest.igated with sodium thiosulfatc titrations of the iodine present in the PVP-1 complex. The amount of iodine not titratable with thiosulfate after complexation with PVP was compared to the amount of iodine predicted to be present from the synthesis condit.ions. The ratio of t,he iodine not available to thio-

POLY V 1 S Y LPY RROLIDON E-IODIN E INTEI3ACI'IOY 605

sulfate titration to the theoretical amount of iodine present is termed the fraction of iodine not titratable. Portions of the series I and series I1 PVP-I samples wcre weighed out, dissolved in water, and titrated with a thiosulfate standard solution. The procedure is that described by Siggia.' The homogencity of t.he thiosulfate titratable iodine was determined by sampling various port.ions of each of the series I arid I1 samples.

To determine if the nont,itrat.ablc iodine is dependent on whether the preparation is a nonsolvent or aqueous solvent method, the solvent-pre- pared PVP-iodine complex was also studied. Aquwus solutions of PVP were combined with aqut!ous triiodidc in varying amounts to form the complex. The amounts taken were such t.hat the loading factors ranged from 0.1 to 1.0. These solutions were titrated with standard thiosulfate. Triiodide blanks containing no PVP were also run and these titers were compared with the titers in the prcserice of PVP.

The possibility that the molecular iodine added to form the PVP-I com- plex might undergo some change to another type of iodine was investigated by analyzing for the common types of iodine. The analysis for periodate, iodate, hypoiodite, iodine, and iodide generally followed the procedures of Schulek and B a r ~ z a . ~ The titratable iodine in samples with various load- ing factors was reacted with thiosulfate, the solution acidified with per- chloric acid, and the subsequently liberated iodine was titrated. The reactions pertinent to this analysis are given in eqs. (1)-(3).

Iodide does not react with acid to give iodine. These analyses were re- peated in 0.2M potassium iodide. I t was necessary to t.itrate the PVP-I complex prior to acidificat.ion instead of acidifying and titrating the total iodine because the PVP-I complex precipitates in strong mineral acid. This init,ial iodine titration also eliminates the possible interpret.ation difficulties arising from any acid stability of the complex with regard to t.he liberation of iodine.

We observed that aqueous solutions of the nonsolverit PVP-I product had lower pH's than solutions of PVP alone. A comparison of the number of moles of acid produced per mole of iodirie not titratable wit.h thiosulfate was done with hopes of further elucidating the nature of the iodine complexa- tion. A quantitative determinat.ion of the number of moles of acid pro- d u c d by complexation was done for 1'VP-I samples of various loading factors. PVP-I samples were dissolved in water and titrated with 0.1N standard sodium hydroxide to a potentiometric endpoint. Duplicate an- alysis was done on five samples with the following loading factors: 0.041, 0.130, 0.201, 0.281, 0.354. Aqueous solutions of PVP were also titrated to provide an acid blank for the system.

606 COURNOYER AND SIGGIA

Water The effect of the water content of the PVP on the complex formation

\\-as studied. The infrared spect.rum of a PVP film was recorded and re- corded again after exposure t.o iodine vapor. The infrared absorpt,ion a t 2.S6 p att.ributsble to the hydroxyl group was monitored. Quantihtion of water in six PVP-I samples of various loading factors and PVP without iodine was done by infrared spectroscopy by the method described earlier.' A Perkin-Elmer 137 double-beam spc!ct.rophotometer was used to record the spectra.

The water content of the series I and I1 PVP prior to iodine addition was also determined by a Karl Fischer titration.

Visible Spectra Any differences in thc iodine complex that are dependent on the method

of preparation being a solvent or nonsolvent procedure might be indicated in t.he visible spectra of the samples. The visible spectra wcrc recorded at 30°C on a Beckman Acta 3 double-beam apcctrophotomctcr. The sample cells were quartz, with a 1-cm path length. PVP-I 1&50 mg, of various loading factors was dissolved in 100 ml of water, and the visible spectrum was recorded from 340 to 650 nm.

Sodium chloride solutions cells wwe used.

RESULTS AND DISCUSSION The results of the thiosulfatc titrations for the series I and I1 samples are

presented in Tables I and 11. Also included is the data resulting from a linear regression analysis of these results. The fraction o f iodine not titratable is the ordinate and the loading factor is the abscissa. On(. ob- serves that all of the iodine initially added is not in a thiosulfatc titratable

TABLE I Series I

Loading factor Fraction not titratable. Fraction reacted'

0.053 0.070 0.106 0.144 0.170 0.195 0.209 0.222 0.284 0.336 0.351 0.403

Slope and variance Intercept

0.364 0.350 0.344 0.318 0.307 0.265 0.255 0.2.54 0.256 0.207 0.231 0.188

-0.494 f 0.002 0.383

0.416 0.400 0.393 0.363 0.351 0.303 0.291 0.290 0.293 0.237 0.2M 0.21.5

-0,564 f 0.002 0.438

a Mean of duplicate analyses.

POI,YVINYI~€'YRROLIDOUE-IODINIS INTEHACTION 607

TABLE I1 Series I1

Loading factor

0.041 0.081 0.130 0.201 0.282 0.354

- - .. -. -

0.434

Fraction not titratable"

0.372 0.346 0 2!#5 0.257 0.2G7 0 , 18.5 0.147

- - . - Fraction reacted"

0.42.; 0 . :w5

0.294 0.237 G.211

. -

n. 337

n. 168

Slope and variance -0.570 f 0.001 -0.631 + 0.001 Intercept 0.382 0.437 - .. -. . - .- -. -_ - _- .- - - -

a Mean of duplicate analyses.

form arid that the fract.ion not. t.itratablcl decreases with an increased load- ing factor. Two linear curvos with differctnt nctgativc slopes arid a common intercept are observed. In thc limit of a zero loading factor one observcts a common int.crcctpt of 0.383 for both series indicating that %.3G/, of the t.otal iodine present is in some form not titratablc by thiosulfatc. A s the loading factor increased beyond 0.5, a lack of stability o f the PVP-I complex was observed that indicatctd supcrsaturation of the polymer with iodinct. AS the saturation point. is rcach(id, approximately 15% of the total iodinc. is not titratable 1vit.h thiosulfate. The titratablc iodine was found to be homogmcously distributed throughout the samplw.

Analysis for the total iodine prcsont indicated that for twtlvo analysw on twelve samples of varying loading factors 97.6% (standard deviation 0.4y0) of t.he total iodine added was still present in the W1'-I product. These analyses show that most of thc iodine initially added is still present in the polymer iodine system although somc of it is riot thiosulfat,c.-t.itratable.

Thc thiosulfatc t.it,rations of PVP-iodine comp1exc.s prcparc>d aqueously with triiodido showed no loss of titratablc iodine when comparctd wit,h solutions of triiodid(. in t,ho absence of PVP.

The comparison of the thiosulfate tit.ration data for the solvent arid nonsolvcrit comp1c.x prcparations indicates t.hat the PVP-I complrx under- goes somc unique chemistry not found in the aqucously prcyarc.d iodine complexes. I hcse results also s u g e s t that tho prcscnce of t.hc iodido ion in the aqueous preparation may prevent the change of thc iodine. to a non- titratablc form.

Siggia2 has ment,ionctd that the stable PVP-I complex can be prctparcd by thc nonsolvent procedure only if a minimum of 4.9% ivater is prescrit i n the polymw. Our present studies also rctvcal that, watt,r is intimatdy involved in the polymer iodintt reaction. Thc irifrarttd analysis of thc I'VI' film brfore and aft.cr exposure t.o iodine vapor shows a mnrkcd tlrcrcmc i n the watw absorption peak centered at. 2.S6 p after the iodintt trcat.mcnt. Wo do not rule out the displacement of watcr from the po1ymc.r by iodine

r 7

608 COURNOYER ASD SIGGIA

and feel that this to some extent may be the case because we have obscrvcd iodine to displace dichloromethanc in the PVP film.

Quantitation of water in six PVP-I samples of varying loading factors reveals 7.4 (standard deviation 3.8) moles of water lost for each mole of iodine not titratable. Because our method of analysis does not distinguish between reacted and displaced water and the possibi1it.y of cvaporativc losses, no definite conclusions can be drawn from the quantitativc data.

As mentioncd above, two different slopes of the curves for scriw I and I1 reported in Tables I and I1 result from using PVP with different water con tent .

These data and the literature suggest that. water is somehow intimately involved in the formation of t.he PVP-I complex.

The sodium hydroxide titrations indicate that 2.33 (standard deviation 0.16) moles of acid are liberated per mole of iodine riot t.hiosulfatc-t.itratable in the PVP-I samples. The release of acid and the involvement of water suggest the possibility that an iodine hydrolysis reaction may be occurring.

Of periodatc, iodate, hypoiodit.e, iodine, and iodide, only the hypoiodite and the iodine react with thiosulfate.2

The reactions are:

2sz03- + Iz S,Oa + 21- (4)

2so4- + 4 1 - + 2H+ ( 5 )

One observes a stoichiometric factor of one-eight.h for the hypoiodite- thiosulfate reaction when compared with the iodine-thiosulfate reaction ; that is, 8 moles of hypoiodite oxidize the same amount of thiosulfate as does 1 mole of iodine. If the molecular iodine used to prepare the complex was to change to any of the cited types of iodine, a decreasc in the titratable iodine would be observed.

In t.he analysis for the different types of iodine, no additional iodine was generated by the acidification step. This analysis indicates that, t,he non- thiosulfate t,itratable iodine is not in either the iodate or periodatc form. The possibility that hypoiodite is present in the PVP-I complex still exist.s because thiosulfate reacts with hyp0iodit.e by equat.ion (4). This reaction would consume the hypoiodite before the acidificatiori step and thus prcvent generation of any additional iodine. Under the conditions imposed by the PVP-I system we have not yet been able to find a suitable direct analysis of hypoiodite. These analyses indicate that the nontitrat.able iodine cx- sists as cither hypoiodite or iodide or as combination of these t x o species.

The formation of an iodo-PVP compound w d d also result in a loss of titratable iodine. Our own studies, notably the infrared invest,igations, do not indicate the formation of an iodo compound. There is no literature evidencct substantiating the possiblity that, the formation of an iodo-PVP compound is occurring.

The visible spectra normalized to the t.otal iodine content arc prcscnted in Figure 1. In thc presence of PVP, triiodide has an absorption maximum

s203 + 410- + HzO

POLY VINYI,PYRROLIDO.UE- IODISUE INTEHACTIOX 609

at approximately 360 therefore, wc may attribute the 360 nm ab- sorption maximum in Figure 1 to a triiodidelikc species. I’Iolccular iodine has an absorption maximum at 460 nm in aqueous solution ;7 thorc.forc tht: remaining peak may bc attributcd to a molecular iodinclikc species. Uc- cauw ~e arc not dealing with aqucous triiodidc or iodine and because a strong PVI’ iodine intcract.ion does occur the term “iodindilw” arid “tri- iodidclikc” arc believed to best describe the situation. Tht. peak at. 460 nm is not obvious \\hen thc iodine complex is prcparcd in aquc.ous solution where iodide is p r c ~ c . n t . ~ - ~ NCcl and SCbillc4 have reported a diffcrtmtial

l l l l l l . l l l l l . 400 500 600

nrn

Fig. I . Spectra of several aqueous solutions of PVP-I with loading factors of (1)0.041, (8) 0.201, arid ( 3 ) 0.3.54. Absorbance is normalized to total iodine versus wavelength.

spectrum of thc complex prepared with aqueous triiodidc that reveals peaks which suggest. that the molccular iodinclilx species may be present, but to a much less degree than in thc 1’VI’-I nonsolvent sample.

The visible spectra reflect the dccreasct in the fraction of titratablc iodine with decreased loading factor observed with the titratiori data, i f onr as- sumes that the triiodidcliltc and molecular iodiridikc: species on the polymer arc titratable with t.hiosulfatc: as they arc in aqucous solution. Thc loss of titratablc iodine implicts that thwe is an ionic form of iodinc. in tho complttx. The triiodidrlikc absorption peaks can be thought of as arising from t.hc as-

610 COURNOYER ASD SIGGIA

sociat.ion of ionic and molecular iodine to form t.ht: I’VP-I complex. The relativc amounts of each t,ypc of iodinc would be dependent. on how much of thc iodine changed to the nontitratablc state. A loss of iodine in the thio- sulfat.e-t,itrabable state would, on a relative scale, result in a dccrcasc in absorption at 460 nm and an increase in absorption a t 360 nm.

In Figure 1 t.he absorbance a t 360 nm clearly incrcascs with decreasing loading factor. The different magnitudes of variation in peak intensities are due to the absorptivity differences of the two absorbing spcxies. Bc- cause the absorptivities and the peak shapes are not. well characterized for this syst.em, quantitative interpretation of the visibla spectra has not been attempted .

The visible spectra substantiate the observation that tho titratable iodine is a function of t.he loading factor and indicate morcovc’r that t.he iodine exists as both thc triiodidelikc and molecular iodinelike spctcics or as a single species exhibiting visible spectral characteristics of bot.h typc>s of iodine. The presencct of the triiodidelike chromophore suggests that. somc of the nontitratablc iodine is associat.ing with the titratable iodine in the presence of the PVP.

The hydrolysis reaction (6) is consistent with and accounts for t.he ex- perimental observations.

I:, + H20 10- + I- + 2H+ (6)

As mentioned, the iodide product in cq. (6) and most probably its isoelw- tronic partncr, hypoiodite, appear to bc associatvd tvith iodine in thc com- plex. The apparent stability of the hypoioditct ion which is not normally stable under these conditions suggest that, this species is involved in the final PVP-I complex. The complex formation would removc the hy- drolysis reaction products and shift the reaction in eq. (6) to the right.

Mokhnachs-10 has also proposed the existence of positive and negative univalent oxidation states of iodinc in l’V1’ and other polymer complexes.

The realizat.ion that eq. (6) predict.s a mole of titratablc hypoiodite, eq. ( 5 ) , for each mole of iodine reacted and that there is a stoichiometric factor of one-eighth for hypoiodite when compared to the iodinclthiosulfate reac- tion allows the parameter of iodinc riot titratable to be converted to a more meaningful parameter, i.e., iodine reacted or hydrolyzed. The conversion relationship is that the iodinc not titratable is seven-eighths of t.hc iodine reacted. The computation of the moles of acid produced per mole of io- dine reacted gives a value of 2.03 (standard deviation 0.14). This value is in agreement. with the value predicted by cq. (ti), and this agreement fur- ther substantiates the validity of the described hydrolysis reaction.

Tho fraction of iodine reacted for various loading factors is presented in the third column of Tables I and 11. The linear regression analysis dat.a are also included. In the limit of zero loading factor the intcrcepts show that 0.437 mole of iodine reacts for each mole of iodine initially added to the I’VP. This intercept indicates that approximately 3 moles of ionic iodine

POLYVI?4YLPYRROLIDO?4FrIODINE INTERACTION 61 1

exist for cvcry 2 moles of iodine still in the molecular form. therefor(: represent thc PVP-I complcx by thc: structure I :

One can

PVP-(I?),(IO-, I-)v ?//x < 1.5 I

The valuc of y/x is a function of synth(:sis conditions. The PVI’, of course, must have sufficient moisture to allow the hydrolysis to procced.

Similarities betwwn our results and those reported by Barkin: Frank and Eirichll for aquctously prcparcd 1’VI’-iodinc can be observctd. They re- port that the ratio of ionic to molccular iodine in the PVP complex in- creases and approaches unity with increasing iodidc/iodine ratio in the iodirw rcagctnt usctd to prepare the complox. Our data can be intcrpretcd analogously if one assumes that. tho hydrolysis reaction is the ratct de- tcrmiriirig step. IJndcr these circumstanccts, the ionicimolecular iodine ratio of complexirig reactants .r\-ould bc incrt.ascd by docreasing the loading factor. Based on the work of Barkiri c:t al.ll one would expect an increasing ionic/molccular iodine ratio in th(. final I’Vl’-J complex for a decreasing loading factor; this is what is obsttrvcd. The maximum value of this ratio is 1.5 in our system rather than 1.0 reported for thv aquctously prcparctd complex.

CONCLUSIONS

This work indicat,es that the iodine in the PVP-I complex exist9 as three t.ypes: Thc ratio of the amounts of ionic to molecular iodine in the complex is a function of synthesis parametctrs. The formation of the complex is thought to involvct thtt production of iodide and hypoioditt: ionic species by iodinct hydrolysis and association of these ionic spccics with molecular iodine to give the final stable PVP-I compltx.

In solution preparations of the complex, ivhen the ionic iodine is already present, the driving force for thc hydrolysis is absent and the thiosulfatc- titratablc iodine does not. change its form. Our results indicate that t.hc \vater-iodint-PVI’ systcm will gcnwatc. its own ionic iodine by hydrolysis ivhm thwe is none available for complexation.

iodide, hypoiodite, and iodinv.

We would like to ac:kriowledKe the helpful suggestions of Marion Rhodes in the organi- zation of this manilscript.

References I . S. Siggia, J . Am. Phurm. Assoc., 46, 201 (1957). 2. S. Siggia, (General Aniline and Film Co.) U. S. Pat. 2,900,305 (Aug. 1959). 3. E. Schulek and L. Barcza, Talanla, 8, 281 (1961). 4 . S. Nee1 arid B. SCbille, J . Chim. f’hys., 58, 738 (1969). 5 . G. Oster rind E. H. Immergut, J . Amrr. C h m . SOC., 76 , 139.7 (l!354). 6. 1. .Moriguchi, Y. Araki, and N. Kaneniaa, Chcm. P h r m . Bull. (Tokyo), 17, 2088

7. A. 1). Autrey and lt. E. Connick, J . Amer. C h n . Soc., 73, 842 (1951). (1969).

612 COURNOYER AND SIGGIA

8. V. 0. Mokhnach and L. N. Propp, Dokl. A tad . Nauf . SSSR, 170,842 (1966). 9. V. 0. Mokhnach and N. M. Rusakova, Dofl. Akud. Nauk. SSSR, 146, 1290

(1962). 10. V. 0. Mokhnach and I. P. Zueva, Dotl. Akud. Nauk. SSSR, 136,832 (1961). 11. S. Barkin, H. P. Frank, and E. R. Eirich, Ricera Sci., 25A. 844 (1955).

Received December 5, 1973 Revised December 12, 1973