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Interaction between cationic surfactants and poly(vinyl alcohol)
Colloids and Surfaces, 40 (1989) 261-266 Elsevier Science Publishers B.V., Amsterdam -
261 Printed in The Netherlands
Interaction Between Cationic Surfactants and Poly(Viny1 Alcohol) KEISHIRO SHIRAHAMA*, MARIKO OH-ISHI and NOBORU TAKISAWA Department of Chemistry, Faculty of Science and Engineering, Saga University, Saga 840 (Japan) (Received 19 January 1989; accepted 21 March 1989)
ABSTRACT The binding of four cationic surfactants (tetradecylpyridinium chloride and bromide, dodecylpyridinium chloride and dodecylammonium chloride) to poly(viny1 alcohol) (10% acetylated) was measured in aqueous solution by potentiometry using a surfactant-selective electrode and complementarily by equilibrium dialysis. The amount of binding was very small at low equilibruim concentrations (C,), but increased markedly as the Cr approached the critical micelle concentration (c.m.c) of each surfactant. By comparing the effect of cationic head groups, counter-ion, and alkyl chain length at low Cr,it was inferred that the alkyl chain plays a minor role, while the polar head groups, especially a monoalkylammonium group, contribute very much to the binding affinity. As for the effect of added electrolyte, it was very little at low Cr, but at higher Cr, binding increased sharply near the c.m.c. The amount of binding was replotted against a reduced equilibrium concentration, Cr/c.m.c. and then all the binding isotherms came together on a single isotherm showing that ordinary micellization has an intimate connection with the binding at higher Cf.
INTRODUCTION
It is puzzling why cationic surfactants interact very little with uncharged water-soluble polymers, such as poly (N-vinylpyrrolidone) (PVP), poly (vinyl alcohol) (PVA), or poly (ethylene oxide) (PEO) [l-3], to which, in contrast, anionic surfactants bind very strongly [ 4-61. Saito made up a table showing how cationic surfactants lack the affinity toward the neutral polymers, based upon rather indirect evidence such as viscosity, solubilization and other experiments [ 731. In a previous report [ 91, we described the binding isotherms of two cationic surfactants to various neutral polymers obtained by means of the surfactantselective electrode. These polymers except for PVP do bind the cationic surfactants, but to a much lesser extent than anionic surfactants. In this present paper, four cationic surfactants, tetradecylpyridinium bromide and chloride (TDPX, X=Br, Cl), dodecylpyridinium chloride (DPCl)
0166-6622/89/$03.50
0 1989 Elsevier Science Publishers B.V.
262
and dodecylammonium chloride (DACl) were chosen to see how the head groups and alkyl chain length affect the binding behaviors. As for a neutral polymer, a selection has been made for 10% acetyl poly (vinyl alcohol) (PVASO) which was found to bind cationic surfactants the most in the previous work [ 91. Electrodes responsive to the cationic surfactants were employed to construct binding isotherms, which are a clear manifestation of the interaction between the surfactant and the polymer. Conventional equilibrium dialysis was also complementarily utilized to confirm the potentiometric results. EXPERIMENTAL
Materials
Tetradecylpyridinium bromide (TDPBr ) was synthesized by treating tetradecyl bromide with dried pyridine. The crude TDPBr was decolored with activated charcoal in methanol solution and then recrystallized three times from acetone. Tetradecylpyridinium chloride (TDPCl) was obtained by ion exchanging the bromide in a concentrated NaCl solution followed by recrystallization twice from acetone. Dodecylpyridinium chloride (Tokyo Kasei) was purified by a similar method to TDPBr as above. Dodecylammonium chloride (Tokyo Kasei) was recrystallized three times from ethanol. The c.m.c. of these surfactants were 2.82, 3.64, 17.4 and 15.0 mmol dmm3 at 25°C for TDPBr, TDPCl, DPCl and DACl, respectively, in agreement with the literature values [lo].The c.m.c. values of these surfactants in salt media were obtained as a breakpoint on an electromotive force (e.m.f. ) versus log (surfactant concentration) plot and they decrease reasonably, reflecting the shielding effect by the added electrolytes. A 5% solution of PVA (Iwai Chemical, 10% acetylated, average degree of polymerization = 500) was purified by dialysis against distilled water for three days. Methods
The surfactant-selective electrode consists of a membrane containing 25% PVC and 75% bis (2-ethylhexyl)phthalate and prepared as before [ 111. The electrode with this membrane responded equally well to the four cationic surfactants used in this work. Potentiometric titration was carried out in a small thermostated cocylindrical cell (5 cm”). The electromotive force was measured with a digital multimeter ( Advantest R6843). Equilibrium dialysis was carried out in five dialysis cells (2.5 cm3 ) engraved in tandem in methacrylate resin blocks, where cellulose membrane (Visking tubing) was inserted in between the blocks. Dialysis was continued for as long as a week to ensure equilibrium. Equilibrated concentrations of TDPCl were
263
measured photometrically, where the molar extinction coefficient at 259 nm, e=4.12*103 [cm mol-’ dme3] was used. All measurements were performed at 30’ C. RESULTS AND DISCUSSION
The e.m.f. of the TDPBr electrode in two NaBr solutions with and without polymer is plotted against log(tota1 TDPBr concentration) in Fig. 1, where the straight line has the Nernstian slope, 59.4 mV at 30°C. In the presence of 1.24% PVASO, however, the e.m.f. deviates from the Nerstian response showing that a part of TDPBr is taken up by the polymer. Following the arrows in Fig. 1, one can obtain the equilibrium concentration, Cf, and the amount of bound surfactant, LIC= C, - C, with C, being the total surfactant concentration. The amount of binding per unit polymer concentration, X=&/C,, is plotted against log Cf to obtain binding isotherms for TDPBr in 5 and 20 n-&f NaBr solutions as seen in Fig. 2, where the results of the dialysis experiment in 20 mA4 NaBr are also added. It is noted that dialysis results come along with the binding isotherm obtained potentiometrically. That the data obtained by different methods agree with each other, confirms a proper electrode performance in the present study. At low Cf, the amount of binding is very small and does not seem to depend on the NaBr concentration, but binding increases sharply as C, approaches the
. 0
2
T x
O-2 lOg(Cs/mM)
log(CflmM)
Fig. 1. (left) Potentiograms for TDPBr systems at 30°C: (m, 0) without polymer at 5 and 20 m&f NaBr, respectively; (0, 0 ) with 1.24% PVA90 at 5 and 20 m&f NaBr, respectively. Fig. 2. (right) Binding isotherms for TDPBr-PVASO system at 30°C: (0, 0 ) potentiometric results at 5 and 20 n-&f NaBr, respectively; (A ) dialysis results in 20 mM NaBr. Arrows show the c.m.c. in respective salt media.
264
--
2
0
-1
1
log(Cf/mM)
Fig. 3. Binding isotherms of three different surfactant-PVASO systems in 5 n&I NaCl at 30°C: (A ) TDPCI; (A ) DPCl and ( 0 ) DACl.
c.m.c. Binding isotherms for different kinds of surfactants are displayed in Fig. 3. The effect of alkyl chain length is seen by comparing the results of TDPCl and DPCl. At low Cf, the binding behaviors look alike, while at higher Cf near the c.m.c. of each surfactant, binding increases drastically. It is to be noted that DACl is bound more than DPCl. This difference must come from the different nature of the two head groups. To see the diversity in binding behaviors more clearly, an apparent distribution coefficient, K,, of surfactant between the aqueous bulk and polymer phases, defined by K n =X/C& is plotted against logarithm of C, in Fig. 4. The effect of the monoalkylammonium group is marked as compared with that of the pyridinium head group. A small radius of the head group and hydrogen bonding between the ammonium group and polymer may be responsible for its higher binding affinity. In contrast, it is seen that the effect of alkyl chain is relatively small at low Cf, when the Kn values of DTPCI and DPCl are compared. Replacement of the bromide counter-ion with chloride ion does not change the binding affinity at any Cf in the present system as seen in Fig. 4. The Kr, values for TDPBr in three NaBr concentrations are shown in Fig. 5. The binding affinity is almost the same irrespective of the NaBr concentration at low Cf, but it increases significantly as the Cf approaches the c.m.c. as pointed out above (Fig. 2). In Fig. 6, the KD values are replotted against C,/ c.m.c., a sort of reduced concentration. There appears to be a striking feature in that the three curves coincide with one another suggesting that the binding shares some similar mechanism with ordinary micellization with respect to the effect of added electrolyte. It can be imagined that surfactants are bound in clusters on polymer chains at the high C, near the c.m.c. and that the clusters are much like ordinary micelles. The surfactants are bound cooperatively to
265
iog(Cf/mMl
Fig. 4. Apparent distribution constants, Ko, of various systems in 5 m&f NaX at 30’ C: ( A ) DPCL; (0 ) DAC1; (0 ) TDPBr and ( A ) TDPCl.
.” 1.E
0
0 0
0 0
1.0
.
.
9
8 0.5
-2
-1
0
log(Cf/mM)
Fig. 5. (left) Effect of added NaBr on Ko for TDPBr system at 30” C. NaBr concentration: (0 ) 5mM, (0) 10mMand (@) 2OmM. Fig. 6. (right) Kn versus a reduced concentration plot for the system shown in Fig. 5. Circles refer to the same as in Fig. 5.
polymers, but a further increase in binding is eventually suppressed by the onset of micellization. Furthermore, detailed examination reveals that there is a slight tendency for the curve of higher NaBr concentration to increase more steeply than that of lower NaBr concentration at high Cf region. (Compare the closed circles with the half-closed ones.) This may suggest that the cluster size is larger at a higher NaBr concentration.
266
0
5.0
2.5
7.5
Xl(mM/v~
Fig. 7. Scatchard plot for DACI-PVASO system in 5 m&f NaCl at 30°C. The insert shows a more detailed plot without thining the data at low Cr.
In Fig. 7, a Scatchard plot for DACl-PVASO system is shown. It is noted that there is a straight line portion with a negative slope at very low C,, where surfactants are bound independently of each other, i.e. Langmuir type binding. The negative slope portion is subsequently followed by a positive slope portion indicative of a cooperative binding. This cooperativity seems very weak, until the Cf approaches the c.m.c. where the cooperativity seems nearly as strong as that of ordinary micellization. The polymer in this case is imagined to play the role of inducing micellization on it even below the c.m.c., just as long chain alcohols, for example, induce micelle formation, or reduce the c.m.c. drastically [3,111.
REFERENCES 1
2 3 4 5 6 7 8 9 10 11
I.D. Robb, in E.H. Lucassen-Reynders (Ed.), Anionic Surfactants, Surfactant Science Ser., Vol. 11, Marcel Dekker, New York, 1981, p. 109. S. Saito, J. Colloid Interface Sci., 24( 1967)227. E.D. Goddard, Colloids Surfaces, 19 (1986)255. H. Arai, M. Murata and K. Shinoda, J. Colloid Interface Sci., 37 (1971)223. K. Shirahama, Colloid Polym. Sci., 252 (1974)978. K. Shirahama and N. Ide, J. Colloid Interface Sci., 54( 1976)450. S. Saito and M. Yukawa, J. Colloid Interface Sci., 30 (1969)211. S. Saito, J. Polym. Sci. A-l, 8(1970)263. K. Shirahama, A. Hirumo and N. Takisawa, Colloid Polym. Sci., 265( 1987)96. P. Mukerjee and K.J. Mysels, Critical Micelle Concentrations of Aqueous Surfactant Solutions, U.S. Government Print. Office, Washington, DC, 1971. K. Shirahama and T. Kashiwabara, J. Colloid Interface Sci., 36(1971)65.
261 Printed in The Netherlands
Interaction Between Cationic Surfactants and Poly(Viny1 Alcohol) KEISHIRO SHIRAHAMA*, MARIKO OH-ISHI and NOBORU TAKISAWA Department of Chemistry, Faculty of Science and Engineering, Saga University, Saga 840 (Japan) (Received 19 January 1989; accepted 21 March 1989)
ABSTRACT The binding of four cationic surfactants (tetradecylpyridinium chloride and bromide, dodecylpyridinium chloride and dodecylammonium chloride) to poly(viny1 alcohol) (10% acetylated) was measured in aqueous solution by potentiometry using a surfactant-selective electrode and complementarily by equilibrium dialysis. The amount of binding was very small at low equilibruim concentrations (C,), but increased markedly as the Cr approached the critical micelle concentration (c.m.c) of each surfactant. By comparing the effect of cationic head groups, counter-ion, and alkyl chain length at low Cr,it was inferred that the alkyl chain plays a minor role, while the polar head groups, especially a monoalkylammonium group, contribute very much to the binding affinity. As for the effect of added electrolyte, it was very little at low Cr, but at higher Cr, binding increased sharply near the c.m.c. The amount of binding was replotted against a reduced equilibrium concentration, Cr/c.m.c. and then all the binding isotherms came together on a single isotherm showing that ordinary micellization has an intimate connection with the binding at higher Cf.
INTRODUCTION
It is puzzling why cationic surfactants interact very little with uncharged water-soluble polymers, such as poly (N-vinylpyrrolidone) (PVP), poly (vinyl alcohol) (PVA), or poly (ethylene oxide) (PEO) [l-3], to which, in contrast, anionic surfactants bind very strongly [ 4-61. Saito made up a table showing how cationic surfactants lack the affinity toward the neutral polymers, based upon rather indirect evidence such as viscosity, solubilization and other experiments [ 731. In a previous report [ 91, we described the binding isotherms of two cationic surfactants to various neutral polymers obtained by means of the surfactantselective electrode. These polymers except for PVP do bind the cationic surfactants, but to a much lesser extent than anionic surfactants. In this present paper, four cationic surfactants, tetradecylpyridinium bromide and chloride (TDPX, X=Br, Cl), dodecylpyridinium chloride (DPCl)
0166-6622/89/$03.50
0 1989 Elsevier Science Publishers B.V.
262
and dodecylammonium chloride (DACl) were chosen to see how the head groups and alkyl chain length affect the binding behaviors. As for a neutral polymer, a selection has been made for 10% acetyl poly (vinyl alcohol) (PVASO) which was found to bind cationic surfactants the most in the previous work [ 91. Electrodes responsive to the cationic surfactants were employed to construct binding isotherms, which are a clear manifestation of the interaction between the surfactant and the polymer. Conventional equilibrium dialysis was also complementarily utilized to confirm the potentiometric results. EXPERIMENTAL
Materials
Tetradecylpyridinium bromide (TDPBr ) was synthesized by treating tetradecyl bromide with dried pyridine. The crude TDPBr was decolored with activated charcoal in methanol solution and then recrystallized three times from acetone. Tetradecylpyridinium chloride (TDPCl) was obtained by ion exchanging the bromide in a concentrated NaCl solution followed by recrystallization twice from acetone. Dodecylpyridinium chloride (Tokyo Kasei) was purified by a similar method to TDPBr as above. Dodecylammonium chloride (Tokyo Kasei) was recrystallized three times from ethanol. The c.m.c. of these surfactants were 2.82, 3.64, 17.4 and 15.0 mmol dmm3 at 25°C for TDPBr, TDPCl, DPCl and DACl, respectively, in agreement with the literature values [lo].The c.m.c. values of these surfactants in salt media were obtained as a breakpoint on an electromotive force (e.m.f. ) versus log (surfactant concentration) plot and they decrease reasonably, reflecting the shielding effect by the added electrolytes. A 5% solution of PVA (Iwai Chemical, 10% acetylated, average degree of polymerization = 500) was purified by dialysis against distilled water for three days. Methods
The surfactant-selective electrode consists of a membrane containing 25% PVC and 75% bis (2-ethylhexyl)phthalate and prepared as before [ 111. The electrode with this membrane responded equally well to the four cationic surfactants used in this work. Potentiometric titration was carried out in a small thermostated cocylindrical cell (5 cm”). The electromotive force was measured with a digital multimeter ( Advantest R6843). Equilibrium dialysis was carried out in five dialysis cells (2.5 cm3 ) engraved in tandem in methacrylate resin blocks, where cellulose membrane (Visking tubing) was inserted in between the blocks. Dialysis was continued for as long as a week to ensure equilibrium. Equilibrated concentrations of TDPCl were
263
measured photometrically, where the molar extinction coefficient at 259 nm, e=4.12*103 [cm mol-’ dme3] was used. All measurements were performed at 30’ C. RESULTS AND DISCUSSION
The e.m.f. of the TDPBr electrode in two NaBr solutions with and without polymer is plotted against log(tota1 TDPBr concentration) in Fig. 1, where the straight line has the Nernstian slope, 59.4 mV at 30°C. In the presence of 1.24% PVASO, however, the e.m.f. deviates from the Nerstian response showing that a part of TDPBr is taken up by the polymer. Following the arrows in Fig. 1, one can obtain the equilibrium concentration, Cf, and the amount of bound surfactant, LIC= C, - C, with C, being the total surfactant concentration. The amount of binding per unit polymer concentration, X=&/C,, is plotted against log Cf to obtain binding isotherms for TDPBr in 5 and 20 n-&f NaBr solutions as seen in Fig. 2, where the results of the dialysis experiment in 20 mA4 NaBr are also added. It is noted that dialysis results come along with the binding isotherm obtained potentiometrically. That the data obtained by different methods agree with each other, confirms a proper electrode performance in the present study. At low Cf, the amount of binding is very small and does not seem to depend on the NaBr concentration, but binding increases sharply as C, approaches the
. 0
2
T x
O-2 lOg(Cs/mM)
log(CflmM)
Fig. 1. (left) Potentiograms for TDPBr systems at 30°C: (m, 0) without polymer at 5 and 20 m&f NaBr, respectively; (0, 0 ) with 1.24% PVA90 at 5 and 20 m&f NaBr, respectively. Fig. 2. (right) Binding isotherms for TDPBr-PVASO system at 30°C: (0, 0 ) potentiometric results at 5 and 20 n-&f NaBr, respectively; (A ) dialysis results in 20 mM NaBr. Arrows show the c.m.c. in respective salt media.
264
--
2
0
-1
1
log(Cf/mM)
Fig. 3. Binding isotherms of three different surfactant-PVASO systems in 5 n&I NaCl at 30°C: (A ) TDPCI; (A ) DPCl and ( 0 ) DACl.
c.m.c. Binding isotherms for different kinds of surfactants are displayed in Fig. 3. The effect of alkyl chain length is seen by comparing the results of TDPCl and DPCl. At low Cf, the binding behaviors look alike, while at higher Cf near the c.m.c. of each surfactant, binding increases drastically. It is to be noted that DACl is bound more than DPCl. This difference must come from the different nature of the two head groups. To see the diversity in binding behaviors more clearly, an apparent distribution coefficient, K,, of surfactant between the aqueous bulk and polymer phases, defined by K n =X/C& is plotted against logarithm of C, in Fig. 4. The effect of the monoalkylammonium group is marked as compared with that of the pyridinium head group. A small radius of the head group and hydrogen bonding between the ammonium group and polymer may be responsible for its higher binding affinity. In contrast, it is seen that the effect of alkyl chain is relatively small at low Cf, when the Kn values of DTPCI and DPCl are compared. Replacement of the bromide counter-ion with chloride ion does not change the binding affinity at any Cf in the present system as seen in Fig. 4. The Kr, values for TDPBr in three NaBr concentrations are shown in Fig. 5. The binding affinity is almost the same irrespective of the NaBr concentration at low Cf, but it increases significantly as the Cf approaches the c.m.c. as pointed out above (Fig. 2). In Fig. 6, the KD values are replotted against C,/ c.m.c., a sort of reduced concentration. There appears to be a striking feature in that the three curves coincide with one another suggesting that the binding shares some similar mechanism with ordinary micellization with respect to the effect of added electrolyte. It can be imagined that surfactants are bound in clusters on polymer chains at the high C, near the c.m.c. and that the clusters are much like ordinary micelles. The surfactants are bound cooperatively to
265
iog(Cf/mMl
Fig. 4. Apparent distribution constants, Ko, of various systems in 5 m&f NaX at 30’ C: ( A ) DPCL; (0 ) DAC1; (0 ) TDPBr and ( A ) TDPCl.
.” 1.E
0
0 0
0 0
1.0
.
.
9
8 0.5
-2
-1
0
log(Cf/mM)
Fig. 5. (left) Effect of added NaBr on Ko for TDPBr system at 30” C. NaBr concentration: (0 ) 5mM, (0) 10mMand (@) 2OmM. Fig. 6. (right) Kn versus a reduced concentration plot for the system shown in Fig. 5. Circles refer to the same as in Fig. 5.
polymers, but a further increase in binding is eventually suppressed by the onset of micellization. Furthermore, detailed examination reveals that there is a slight tendency for the curve of higher NaBr concentration to increase more steeply than that of lower NaBr concentration at high Cf region. (Compare the closed circles with the half-closed ones.) This may suggest that the cluster size is larger at a higher NaBr concentration.
266
0
5.0
2.5
7.5
Xl(mM/v~
Fig. 7. Scatchard plot for DACI-PVASO system in 5 m&f NaCl at 30°C. The insert shows a more detailed plot without thining the data at low Cr.
In Fig. 7, a Scatchard plot for DACl-PVASO system is shown. It is noted that there is a straight line portion with a negative slope at very low C,, where surfactants are bound independently of each other, i.e. Langmuir type binding. The negative slope portion is subsequently followed by a positive slope portion indicative of a cooperative binding. This cooperativity seems very weak, until the Cf approaches the c.m.c. where the cooperativity seems nearly as strong as that of ordinary micellization. The polymer in this case is imagined to play the role of inducing micellization on it even below the c.m.c., just as long chain alcohols, for example, induce micelle formation, or reduce the c.m.c. drastically [3,111.
REFERENCES 1
2 3 4 5 6 7 8 9 10 11
I.D. Robb, in E.H. Lucassen-Reynders (Ed.), Anionic Surfactants, Surfactant Science Ser., Vol. 11, Marcel Dekker, New York, 1981, p. 109. S. Saito, J. Colloid Interface Sci., 24( 1967)227. E.D. Goddard, Colloids Surfaces, 19 (1986)255. H. Arai, M. Murata and K. Shinoda, J. Colloid Interface Sci., 37 (1971)223. K. Shirahama, Colloid Polym. Sci., 252 (1974)978. K. Shirahama and N. Ide, J. Colloid Interface Sci., 54( 1976)450. S. Saito and M. Yukawa, J. Colloid Interface Sci., 30 (1969)211. S. Saito, J. Polym. Sci. A-l, 8(1970)263. K. Shirahama, A. Hirumo and N. Takisawa, Colloid Polym. Sci., 265( 1987)96. P. Mukerjee and K.J. Mysels, Critical Micelle Concentrations of Aqueous Surfactant Solutions, U.S. Government Print. Office, Washington, DC, 1971. K. Shirahama and T. Kashiwabara, J. Colloid Interface Sci., 36(1971)65.