Langmuir 1993,9,77-85
77
Redox-Active Monomeric and Polymeric Surfactants P. Anton, J. Heinze,t and A. Laschewsky’ Imtitut f i r Organische Chemie, Universitbt Mainz, J.J. Becherweg 18-20,0-65500 Mainz, Germany Received March 20,1992. In Final Form: Auguat 21,1992
A series of surfactant monomers and polymers bearing different redox-activemoieties were synthesized. Solubility, surface activity, aggregation behavior, and electrochemical properties of the compounds were inveatigated. Changes in the hydrophiliehydrophobic balance were achieved by controllingthe number of charges on the surfactanta via the redox moiety. Redox reactions of surfactanta bearing N-alkylated nicotinic acid or (ferrocenylmethy1)a”onium structures were not reversible in water. However, other ferrocene and viologen surfactanta showed good reversibility of the redox reactions, and drastic changes in their solubilitywere observed. Theaesurfactantaare consideredas promising systemsto trigger reversible changes in self-organization. Introduction Polymeric surfactants may offer many interesting potential applications, e.g., in emulsion polymerization or in tertiary oil recovery. T h e characteristic properties of polymeric surfactants result from formation of micellelike structures, as the side chains undergo intra- and intermolecular aggregation due to hydrophobic interactions. The aggregation behavior of both monomeric and polymeric surfactants depends on concentration, temperature, and pressure applied. In the case of surfactants bearing ionic groups, it can also be influenced by addition of electrolytes. The changes in self-organizationare based on physicochemicalvariation of hydrophilicity influencing the interaction between the surfactants themselves and the solvent.’ Altematively, modification of the self-organization by chemical reactions may be considered, keeping most physical parameters of the system constant. Furthermore, much wider variations of properties should be accessible by chemical reactions. To achieve chemical switching, suitable functional groups have to be incorporated into the surfactants which enable a marked change of the hydrophiliehydrophobic balance. For such a purpose, addition or removal of charge in the surfactants would be most efficient. In this context, exampleslike surfactants that undergo rearrangements, cleavable surfactants, or surfactanta bearing neutral ligands such as crown ethers forming complexea with ions have been reported.2 But mostly, such chemical modifications are irreversible,and reagenta have to be added to the system. In order to inducereversible changesof self-organization, other functional groups like photo- and electroactive groups have to be employed, with which charges can be created or removed without the addition of further reagenta, if desired. Only a few such examples exist for low molecular weight surfactants. Photochemical rearrangement of spiropyran moieties to merocyanines in surfactants was reported to reversiblyinfluencethe critical micelle concentration ( c ~ c ) .Analogous ~ changes were t
Prewnt addrew h t i t u tfor PhyeikalischeChemie,Universitiit
Freiburg, Albertatrm 21, D-7800Freiburg, Germany.
(1) L i n k , B.;Wennerstr6m, H. Top. Curr. Chem. 1980,87,1and referenom therein. (2) Haulm, M.; R i o r f , H. Angew. Chem., Znt.Ed. Engl. 1986,24, 882. Jaeger, D.A.; key, M.R. J. Org. Chem. 1982,47,311. Turro, N. J.; Kuo, P.-L. J. Phy8. Chem. 1986,90,837.PMUer, U.Mizellen- VesikelMiknwmulrionen; Springer-Verlq Berlin, Heidelberg,New York,1986; me alem referenw therein. (3)Tuuke, S.;Kurihara,5.;Yamaguchi, H.; Ikeda,T.J. Phya. Chem. 1987,91,249. DNmmond, C. J.; Albem, S.;Furlong, D.N.;Web, D. Lungmuir 1991, 7,2409.
achieved electrochemically with surfactants containing ferrocene or other metallocene~.~ For polymeric surfactants, corresponding investigationson reversible changes of aggregationare even more scarce,1althoughthe firation of surfactants to a polymeric backbone should provide severaladvantages. For example, demixing of differently charged surfactant species during the transformation process should be avoided, and cooperativeeffectsmay be possible. In this paper several monomeric and polymeric surfactants bearing different redox groups are investigated. Of particular interest was the influenceof such functional groups on parameters like solubility, surface tension, and self-organization behavior. Furthermore, the effect of polymer fixation was studied. For all systems, the redox process should result in the formation or disappearance of charge, in order to strongly modify the hydrophilicity of the compound. In the optimal case,the redox processes are reversible in water. This implies that all redox states are stable in thissolvent. Further, the redox group should be small, so that it does not interfere with the selforganization of the surfactants. Hence, we have c h w n surfactants based on N-alkylated nicotinamide, viologen, and ferrocene for initial studies. Their redox potentials are in the range of about -1 to +1.3 V, and thus accessible in water. The redox systems involved are outlined in Figure 1. N-alkylated nicotinamides show a one-step redox process. Reduction leads from a positively charged species (NA+)to an uncharged one (NAH). Such nicotinic acid derivativesare of particular interest because they aremodel compounds for biochemical redox processes which are mediated by NAD+ or NADP+. This redox process necessitatesproton transfer, and can be nonuniformwhen not performed enzymatically. Viologen undergoes a defined two-step redox process without proton transfer that can be induced both photo- and electrochemically. On reduction, the doubly charged viologen (V+) is transformed via a semiquinoid,radical cationicstate (V+) to an uncharged bis(dihydropyridine1 (VO). Ferrocene shows a one-step redox process. On oxidation, the uncharged ferrocene(FCO,is converted into the ferrocinium cation (Fc+). The oxidized state is known to be less stable than the reduced one. Chart I shows the monomeric surfactanta that were synthesized. Hydrophilic groups like the sulfonate group (4) Saji, T.;HoehinoK.;AoyeguiS.J. Am. Chem. SOC.NU,107,6866. YokoyamaS.;KurataH.;Harima,Y.;YamashitaK.;Hoehino,K.;Kokado, H. Chem. Lett. 1991,441.Oknhata, Y.;Ariga, K.; Seki,T.J . Chem. Soc.,
Chem. Commun. 1986,73.
1993 American Chemical Society
70 Langmuir, Vol. 9, No. 1, 1993
Anton et al.
+
FCQ
FC.
R'
R'
V0
V+
R'
I
c=o
cj d
-a
NAH
-H' - 2e'
R'
R'
I
c-0 I
R
NA*
Figure 1. Redox systems employed in the study.
of compounds 1-6, the ammonium group of 7, and the hydroxyl group of 8 guarantee a basic hydrophilicity independent of the redox state. The nicotinic acid derivatives 1-4 should thus allow the change from zwitterionic speciesto anionic ones. The viologen derivatives 6 and 6 should enable the transformationof cationicspecies via zwitterionicto anionicones. The ferrocenederivatives 7 and 8 may be converted from a monocationic species to a dicationicone, or from an uncharged species to a cationic one, respectively. The majority of the compounds (1-3, 6, 6) bearing acrylate and methacrylate moieties can be homopolymerized by free radical polymerization. Compounds 4,7, and 8 cannot be homopolymerized, but polymers can be accessibleby alternating copolymerization,e.g., of 4 with electron-rich comonomers, or of 7 and 8 with SO2.
Experimental Section Materials. All solvents used were analytical grade. Acetonitrile and triethylaminewere dried over molecular sieves of 0.3 nm. All other solvents were dried by neutral A1203 (Merck, activity 1). All water was purified by a Milli-Qwater purification system. The synthesis of monomers 1-3 is described elsewhere.6 Polymers poly(l)-poly(3) were prepared by free radical polymerization of the monomers in ethanol (poly(l), poly(2)) or in ethanokwater = 4 1 (poly(3)) at concentrationsof about 2-5% by weight, using 1-2 mol % azobis(ieobutyronitrile) (AIBN)an initiator. They were worked up as described previously.6 The polymers are insoluble in water, brine, methanol, or ethanol, but soluble in formamide and CFCOOH. Standard methods of molecular weight determination failed. A lower limit of the degree of polymerization can be deduced from the lack of an absorption band of nitrile end groups (from the initiator AIBN) at v/cm-1 = 2260-2240 (v, -CN) in the FTIR spectra of the polymers. As 5 mol % nitrile which is deliberately added to the polymers is still clearly visible, the degree of polymerization should exceed 20. Anal. Calcd for poly(1) (CrH42N20sS+ 1H20;M = 528.72): C, 59.01; H, 8.39; N, 5.30; S, 6.06. Found C, 57.70; H, 8.37; N, 5.31; S, 6.01. Anal. Calcd for poly(2) ( C B H ~ O ~ 1H20; S M 487.62): C, 56.65; H, 7.65; N, 2.87; S,6.57. Found: C, 56.44; H, 7.42; N, 2.71; S,6.91. Anal. Calcd for poly(3) (CaH3sN30&
+
(6) h h e w k y , A.; Zerbe,
I. Polymer 1991,32,2070, 2081.
1H20;M 471.61): C, 56.03; H, 7.91; N, 8.M; S,6.80. Found C, 64.21; H, 7.91; N, 8.23; S, 6.83. Synthesis of Monomer 4. 3-[2-[(Decyl-N-methylaea)carbonyl]ethenyl]pyridine. An 8-g (0.053-mol) sample of pyridylacrylic acid and 8.5 g (0.049 mol) of N-decyl-N-methylamine were diseolved in 15 mL of CH&12. Equimolar amounts iodide and triethylaminewere of 2-chloro-N-methylpyridinium added. The deep red mixture was refluxed over night and then diluted with dichloromethane. The organic phase was washed seven times with aqueoue 2 N NaOH and three times with water and dried over MgSO,. The solvent was removed, and the remaining dark brown residue was purified by flaeh chromatography (silica gel, eluent ethyl acetate). Yield: 9.2 g (62%)of a brownish oil. n~ (19OC) 1.5283. 'H NMR (CDCq, 200 MHz): b/ppm = 8.74 (m), 8.55 (m), 7.79 (m), 7.28 (m, 1 H each, C&N+), 7.70-7.55 (m, 1 H, COWH-),7.006.82 (m, 1 H, COCH--C-), 3.50-3.32 (m, 2 H, -CH2N), 3.10 (8, 3 H, -N(CHs)CO trans), 3.03 (8, 3 H, -N(CH3)C0 cis), 1.58 (m, 2 H, -CH2CN), 1.30-1.05 (e, 14 H, -(CH2)7-), 0.8 (t, 3 H, CH3-). 3-[3-[2-[ (N-Decyl-N-methylaza)carbonyl]ethenyl]pyridinio]propanesulfonate (4). A 4-g (13-"01) sample of 3-[(2[decyl-N-methylaza)carbonyllethenyl]pyridineand 1.61 g (13 "01) of propylsultonee were dissolved in 50 mL of acetonitrile and stirred at 80 "C under nitrogen for 3 days. On coohg, a white solid precipitated from the solution. The solvent was filtered off. The residue was washed thoroughly with ether and recrystallid twice from acetonitrile. Yield 3.9 g (70%) of white crystals. Mp: 170 OC. lH NMR (D20/Nd, 400 MHz): Wppm = 9.42 (d), 9.10-8.85 (m), 8.20-8.05 (m,1 H, 2H, 1 H, C&N+), 7.55 (d,2 H,COCH-CH-),4.83 (2 H,C&J+CHa),3.65(m),3.40 (m, 1 H each, CHzN), 3.28 (e, 3 H, -N(CHs)CO trans), 3.01 (t, 2 H, CH&03-), 2.95 (8, 3 H, -N(CHa)CO cis), 2.50 (m, 2 H, CH2CS03-), 1.70-1.45 (m, 2 H, CHzCNCO), 1.25 (m, 14 H, -(CH2)r), 0.85 (t, 3 H, CH3-1. Synthesis of Monomers 5 and 6. N-[ll-(Acryloy1oxy)undecyl]-4-( 4'-pyridyl)pyridinium Bromide. The acrylate was synthesized starting from 11-bromoundecyl acrylate? in analogytothe methacrylatedescribedbelow. Yield: 28%(white crystals). 'H NMR (CD3OD, 400 MHz): blppm = 9.18 (d), 8.86 (d), 8.50 (d), 7.93 (d, 2 H each, bipyridinium), 6.41 (m, 1 H, C H 4 C O O cis), 6.14 (m, 1 H, C==CHCOO) 5.87 (m, 1 H, CH--CCOO trans), 4.73 (t,2 H, CH2N+),4.16 (t,2 H, COOCH2), 2.10 (m, 2 H, CH2CN+),1.69 (m, 2 H, COOCCH,), 1.5-1.3 (m, 14 H, -(CH2)7-). N-[ 1l-(Methacryloyloxy)undecyl]-4-(4'-pyridyl)pyridinium Bromide. A mixture of 3.1 g (20mmol)of 4,4'-bipyridine, 6.4 g (20 mmol) of 11-bromoundecylmethacrylate? and 1 mL of nitrobenzene in 40 mL of acetonitrile was stirred at room temperature under nitrogen for 4 days. The yellow precipitate formed was ieolated by filtration and thoroughly washed with ether. It contained both the mono- and the biealkylatedreaction product. A 1.6-g sampleof crude biealkylated bipyridinium ealt was obtained by recrystallization from methand The monoalkylated product was obtained from the mother liquor, efter purification by flash chromatography (silica gel, eluent CHCL MeOH = 2:l) and recrystallization from acetone. Yield 3.5 g (37% ) of colorless crystals. Mp: 102-104 *C. lH NMR (CDCb, 200 MHz): Uppm = 9.55 (d), 8.82 (d), 8.38 (d), 7.70 (d, 2 H each, bipyridinium), 6.08 (8, 1 H, CH--CCOO cis), 5.52 (e, 1 H, CH--COO t r ~ )4.99 , (t,2 H, CH2N+),4.10 (t,2 H, COOCHp), 2.05 (m, 2 H, CH&N+) 1.92 (e, 3 H, C(CHdCOO), 1.62 (m, 2 H, COOCCH2), 1.25 (m, 14 H, -(CH2),-). For an alternative procedure, see Tundo et al.8 N-[ 1l - ( A ~ ~ o y ~ ~ ) ~ d ~ ~ ~ c y I ] - N - ( 3 - ~ ~ l f ~ ~ t o ~ ~ ~ ~ bipyridinium Bromide (6). The acrylate was synthesized and purifiedinanal~tothemethacryletebelow. Yield: 0.8g (16%) of yellow crystals. 'H NMR (D20,400 MHz): b/ppm = 9.2-9.1 (m), 8.55 (m, 4 H each, bipyridinium),6.38 (m, 1 H, CH--CCOO cis), 6.16 (m,1H, C=CHCOO), 5.93 (m,1 H, C H 4 C O O trans), 4.88 (t, 2 H, CH2CCSO3-), 4.70 (t, 2 H, CH2N+),4.15 (t, 2 H, (6) Propybultone ia a potent carcinogen. For handling, nee: Monroy Y.M.; Galin, J. C. Polymer 1984,26,121. (7) Hamid, S.M.; Sherringbn, D. C. Br. Polym. J. 1984,16, 39. (8) Tundo, P.; Kippenberger, D. J.; Politi, M.J.; Klahn, P.; Fendler, J. H. J. Am. Chem. SOC.1982,104,6352. Soto,
Langmuir, Vol. 9, No. 1, 1993 79
Redox- Active Monomeric and Polymeric Surfactants
Chart I. Compounds Employed in the Study
8
5 10
-
Data for Poly(6). 'H N M R (DIO, 400MHz): b/ppm 9.90COOCHO),3.00 (t,2 H, CHaOs-), 2.53 (2 H, CH2CSOa-), 2.06 (m, 9.00,8.70-8.40 (2 br d, bipyridinium), 4.26-3.85 (br, COOCHO), 2 H, CH&N+), 1.64 (m, 2 H, COOCCHn), 1.3S-1.25 (m, 14 H, 3.70 (t,CH&CSOa-), 3.10-2.86 (br, CH$I+, CHdOa-), 2.60-2.40 -(CHO)r). Anal. Calcd for CnHspBrNSOsS + lH2O (M = (br, CHpCSOa-), 2.20-2.00 (m,CH&N+), 2.0-1.9 (br, CHCOO), 601.31): C, 63.88,H, 6.87; N, 4.66, S, 5.33. Found C, 51.51; H, 1.80.90 (br, CHECOO-, -(CHz)r). Anal. Calcd for C p l b 7.09; N, 4.70; S,6.22. Like for monomer 5, monomer 6 and the BrN@& + lH2O (M = 601.31): C, 53.88; H, 6.87; N, 4.66. polymers poly(6) and poly(6), Le., all viologen derivatives, Found C, 60.16; H, 6.62; N, 4.40. although CHN analysis are acceptable, sulfur analysis gives uneatiefectory,too high values, for unknown reasone. However, Data for Poly(6). 'H NMR (D20, 400MHz):b/ppm = 9.40according to the 'H and 'F NMR spectra and TLC (eluent 9.00, 8.60-8.30 (br, bipyridinium), 3.60 (t, CH&CSOa-), 3.10for"ide), the structures are correct, and the compounde are 2.70 (br, CH.rN+, CHfiOa-), 2.6-2.4 Ibr, CH&SOa-), 2.20-2.00 free of impurities. (m,CH&N+), 1.70-1.00 (br,-CH&(CHs)COO-,-(CHdd AnaL 11-(M 6 t h . c ~ ~ l O ~ ) ~ ~ y ~ ] - ~ - ( ~ S ~ f ~ ~ ~Calcd R r for o p C&1BrNzO& yl)+ lHlO (M= 616.31): C, 54.61; H, 4,4'-bipyridinium Bromide (6). A 2.78-g (5.8-mmd)sampleof 7.04, N, 4.65; S,5.21. Found C, 52.38; H, 7.21; N, 4.30; S,6.00. N-[ll-(metbacryloyloxy)und~ll-~~4'-pyridyl)pyridinium broNJV-Dimethyl-N-(ferrocdnylmethyl)-N-und~-l0anylof propyleultone? and 0.1 g of mide, 0.695 g (5.7 "01) ammonium Bromide (7). A 2.24-g (9.2-"01) sample of n i ~in 26~mLof eacetonitdewere refluxed under nitrogen N - C f e r r o c e n y l m e t h y l ) - N ~ - ~and ~ y3.00 ~ e g (12.9 "01) for 2 days. The yellow precipitate formed was isolated by of l-bromoundec-lO-eneOwere stirred for 3 h at 60 OC. The fdtmtion, washed with cold acetonitrile,and repeatedly recryeresulting brown precipitate was recrystallized twice from ethyl taUized from ethanol. Yield 0.66 g (60%) of yellow crystals acetate and once from ethyl acetateacetone = 41. Yield 2.66 (decomposition at 2% OC). 'H NMR (D20,400 MHz): blppm g (60%) of a light brown solid. Mp: 123-124 OC. 'H N M R = 9.2-9.1 (m), 8.56 (m, 4 H each, bipyridinium),6.07 (m, 1H, (CDCb, 400 MHz): blppm = 5.78 (m, 1H, W H - ) , 6.08-4.80 CH-CCOO, cis), 6.66 (m, 1H, C H 4 X . 0 0 , trans), 4.89 (t,2 H, (m, 2 H, C H d - ) , 4.49,4.31(2 t, 4 H, C a ) , 4.28 (e, 6 H, C a ) , CH&CSOa-), 4.71 (t,2 H, CH&P), 4.14 (t,2 H, COOCHa), 3.02 3.32 (m, 2 H, FcCN+CHO),3.23 (m, 6 H,
