Langmuir 1989,5, 1250-1252
1250
Polymer-Surfactant Interactions. Binding of N-Tetradecylpyridinium Bromide to Ethyl(hydroxyethyl)cellulose Anders Carlsson and Bjorn Lindman* Physical Chemistry 1, Chemical Center, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
Tetsuya Watanabe and Keishiro Shirahama* Department of Chemistry, Faculty of Science and Engineering, Saga University, Saga 840, Japan Received January 5, 1989. I n Final Form: April 27, 1989 Binding isotherms of a cationic surfactant (N-tetradecylpyridinium bromide) to a nonionic polymer (ethyl(hydroxyethy1)eellulose)in water are presented. Surfactant binding was obtained by measurements of the surfactant ion activity in the presence and absence of polymer using a selective electrode. A strong cooperative binding to the polymer was detected at a surfactant concentration lower than the critical micelle concentration. Both an increase in salt concentration and a rise in temperature led to a more pronounced binding. The unusual temperature dependence is explained as an effect of conformational changes of the substituents and the backbone in ethyl(hydroxyethy1)cellulose. Thus, at elevated temperatures the polymer becomes more hydrophobic, which results in increased surfactant binding. As a consequence of this, the solubility of the polymer-surfactant complexes decreases when small amounts of salt are present. This was observed as a lowering of the cloud point of the system.
Introduction Surfactant ion specific electrodes' have been proved useful for the characterization of polymer-surfactant interaction in aqueous solution.2 The activity of the surfactant ion can be probed by simple means, and both anionic and cationic species may be studied in the presence of a charged or a neutral p01ymer.~~ We have applied this technique to a system consisting of ethyl(hydroxyethy1)cellulose (a nonionic cellulose ether) and N-tetradecylpyridinium bromide (a cationic surfactant) in water. Ethyl(hydroxyethyl)cellulose68 and other hydrophobic cellulose ethers such as hydroxypropylcellulose9 interact very strongly with ionic surfactants, especially in the presence of small amounts of electrolytes. This results in a dramatically changed solubility of the formed polymer-surfactant complexes, observed as strong nonmonotonic changes in the cloud In order to get a better understanding of these interactions, we performed measurements of surfactant ion activity using an electrode developed a t Saga University. Binding isotherms obtained a t various temperatures and salt contents (NaBr) will be presented. In addition, we present cloud point (CP) data which yield direct information on the polymer-surfactant interaction and which also are easily obtainable for our particular system (CP < 75 "C). Experimental S e c t i o n Materials. Ethyl(hydroxyethy1)cellulose (EHEC)was kindly provided by Berol Kemi AB, Stenungsund, Sweden (from the same batch previously used-). The viscosity-average molecular weight for this sample was 146000. The degree of substitution of ethyl groups was 0.9 per anhydroglucose unit, and the molar substitution of ethylene oxide groups was 2.1 per anhydroglucose unit. N Tetradecylpyridinium bromide (TDPB) was synthesized from 1-bromotetradecaneand dry pyridine. The crude substance was decolored by activated charcoal in methanol solution and recrystallized 3 times from acetone. NaBr was of extra pure grade. Procedures. Desalted and purified EHEC solutions were prepared by dialysis of a 1wt % solution against distilled water
* Author
to whom correspondence should be addressed.
for 5 days (EHEC powder of technical quality contains about 4 wt % NaCl). After freeze-drying,the polymer was redissolved in doubly distilled H20. The final concentration was 0.50 wt %. To one portion of the EHEC solution 5 mM NaBr was added and to another 50 mM. The solutions were equilibrated at room temperature for at least 1day. TDPB and NaBr concentrations are expressed in mmoles per liter of polymer solution (mM). Potentiometric titration was performed as described in previous p a p e r ~ . ~AJ ~concentration cell according to Figure 1was constructed, where M is the surfactant selective membrane and C1 and C2 are the outer and inner surfactant concentrations, respectively. The outer and inner solutions contained either 5 or 50 mM NaBr. The electromotive force (EMF)was measured with a digital voltmeter (Advantest TR-6843). The potentiometric titration was carried out in a small thermostated cell (20 mL). The experiment is limited to relatively low surfactant concentrations (roughly of the order of the cmc) due to an unfavorable interaction between the surfactant and the electrode membrane, which affects the reproducibility. A membrane containing 20% poly(viny1chloride) (PVC) and 80% bis(2-ethylhexyl)phthalate was prepared as follows: PVC and bis(2-ethylhexyl) phthalate were mixed and dissolved in 5 mL of tetrahydrofuran. T h e slurry was heated at 60 "C for about 10 min during gentle stirring to obtain a clear and viscous solution. This was then cast on a glass plate, and after slow evaporation of the solvent in dry atmosphere the resultant gel membrane was (1) Zana, R. In Surfactant Solutions: New Methods of Investigation; Zana, R., Ed.,; Marcel Dekker: New York, 1987;Surfactant Science Series, Vol. 22, Chapter 9. (2) Goddard, E. D. Colloids Surf. 1986, 19, 255. (3) Shirahama, K.; Yuasa, H.; Sugimoto, S. Bull. Chem. SOC. Jpn. 1981, 54, 375. Jpn. (4)Shirahama, K.;Takashma, K.; Takisawa, N. Bull. Chem. SOC. 1987, 60, 43. (5)Shirahama, K.;Himuro, A.; Takisawa, N. Colloid Polym. Sci. 1987, 265, 96. (6)Carlsson, A.; Karlstrom, G.; Lindman, B. Langmuir 1986,2, 536. (7) Carlsson, A,; Karlstrom, G.; Lindman, B.; Stenberg, 0. Colloid Polym. Sci. 1988, 266, 1031. (8) Carlsson, A.; Karlstrom, G.; Lindman, B. J. Phys. Chem. 1989,93, 3673. (9)Winnik, F.M.;Winnik, M. A.; Tazuke, S. J. Phys. Chem. 1987,91, 594. (10) Shirahama, K.; Nishiyama, Y.; Takisawa, N. J.Phys. Chem. 1987, 91, 5928.
0743-7463/89/2405-1250$01.50/0 0 1989 American Chemical Society
Polymer-Surfactant Interactions B
I I
E
Langmuir, Vol. 5, No. 5, 1989 1251
-
t
a
/
50
3
>
25OC
0 -
. E
U
5 -50
0
AB/iPC1
hC1 b r i d g e
I C,
Y
I C2 I KC1 bridge
-
100
0
ig/4gc1
Figure 1. Experimental setup for the potentiometric titration including a schematic representation of the concentration cell: a, potentiometer;b, Ag/AgCl electrode; c, KCl bridge; d, electrode membrane; e, stirring bar; f, saturated KCl; g, stirrer.
cut out and glued on a PVC tube (9-mm inner diameter and 11 cm long). The electrode membrane was annealed at 50 "C under reduced pressure for several hours. Determination of cloud point (CP)was carried out as described previously.w After the samples were heated above the clouding temperature, the CP was taken as the temperature when the last visible sign of clouds disappeared on cooling.
0.0 1
0.1
Ctot
50
t
'
1
10
1
10
mM
3 5 "c
Results and Discussion In Figure 2, the EMF of the concentration cell is plotted as a function of the total TDPB concentration. At 25 "C and without polymer, a Nernstian response was obtained with a slope of 59.1 mV/l0-fold change in surfactant concentration. At the critical micelle concentration (cmc), the EMF levels off to a more or less constant value, and the cmc of TDPB in 5 mM NaBr is determined to be 1.1 mM. In the presence of 0.5 wt 5% EHEC, again an EMF according to the Nernstian theory was obtained at low surfactant concentrations. However, a break point is observed already a t a surfactant concentration of 0.70 mM, with a large deviation from the polymer-free system. This implies that binding of TDP+ to EHEC occurs. At 35 "C a Nernstian slope in good agreement with theory was obtained (60.7 mV), and at this temperature the cmc was not significantly altered compared with that at 25 "C. However, and very interestingly, the break point in the presence of EHEC was now located at a lower surfactant concentration, i.e., at 0.60 mM, and a more pronounced deviation from the EHEC-free system was detected. A t 15 "C only a minor deviation from the polymer-free system could be observed. The effect of salt concentration was investigated at 35 "C. An increase of the NaBr concentration to 50 mM led as expected to a lower cmc (0.40 mM) and an onset of binding to EHEC already at about 0.2 mM TDPB. The binding isotherms in Figure 3 were constructed as described in ref 4 and 5. The electrode is obviously sensing a lower TDPB activity when EHEC is present. At a fixed EMF value, the concentration of bound surfactant ( c b ) is obtained from the relation Cb = c, - c,, where C, is the total surfactant concentration in the system and Cf is the free surfactant concentration sensed by the electrode. The amount of binding (X) is expressed as X = Cb/Cp, where C , is the actual EHEC concentration (the dilution during the titration has been taken into account), and is then plotted against Cf. The following can be deduced from the binding isotherms. First, the binding of TDP+ to EHEC is a strongly cooperative process (in a manner similar to micelle for-
-100
1 0.01
O
0.1
Ctot
'
mM
Figure 2. Electromotive force (EMF) versus the total TDPB concentration in systems containing 5 mM NaBr at 25 and 35 "C. Solid circles are for a solution without polymer, and open circles are for a 0.5 wt % EHEC-water solution.
mation), and around a critical concentration the binding increases dramatically with the surfactant concentration. A strong cooperativity has also been observed for the binding of hexadecylammonium surfactants to partly hydrolyzed poly(viny1 alcohol) polymers by using the same technique5 and various techniques for many other polymer-surfactant systems.2 Second, the critical surfactant concentration at which binding starts (often referred as TI2)is lower than the cmc. Third, the binding increases rapidly when increasing the temperature. This result, where an increase in temperature leads to a larger attraction between surfactant and polymer, is nontrivial but in agreement with previous finding^;^?^ we note that, in general, a weakly increasing dissociation of the polymersurfactant complexes with increasing temperature should be expected. In the case of EHEC, the interpretation of this behavior is that the polymer becomes more hydrophobic at elevated temperatures, which leads to an increased hydrophobic attraction of the hydrocarbon portion of the surfactant. This behavior is consistent with demonstrated conformational changes of the ethylene oxide
1252 Langmuir, Vol. 5, No. 5, 1989
Carlsson et al. Table I. N-Tetradecylpyridinium Bromide Concentrations at Which the Onset of Binding to EHEC Is Observed (T,) at 35 "C NaBr concn/mM T,/ m M 5 0.60 50 0.2
35OC
Y
-m 0 E E
. X
CMC j/
L,,'
n
0
0
0.5
I
1.o
1.5
Cf I mM Figure 3. Binding isotherme of TDPB to EHEC at 15,25, and 35 O C in the presence of 5 mM NaBr. The cmc of TDPB is actually unaffected in this temperature range. 100 5 mM Na8r
a
,LM
i
80
i
0
. n 0
60
?ne-phase region
40
0
20
40
60
TDPB conc I mM
Figure 4. Cloud point of a 0.50 w t % EHEC-water solution
versus the TDPB concentration at different NaBr concentrations: (0) 5, (A)50 mM. groups and the polymer backbone in EHEC.7s8 Presented in Figure 4 are the CP measurements for 0.50 wt 5% EHEC-H20 solutions as a function of the TDPB concentration. The upper curve represents an EHEC solution containing 5 mM NaBr, and the lower one represents an EHEC solution containing 50 mM NaBr. The curves reflect very well the synergistic effect observed for nonionic cellulose ether solutions:&* When a relatively small amount of salt is present, the addition of ionic
surfactant (cationic or anionic) initially causes a considerable CP depression. If the salt concentration is increased, the depth and width of the CP minimum will be more pronounced. Without surfactant, the addition of NaBr up to a concentration of 50 mM reduces the CP by only 1 OC. The CP measurements cover a much wider surfactant concentration range than the potentiometric titrations. However, the binding isotherms provide insight into the complicated phase behavior exhibited by the four-component system EHEC-ionic surfactant-salt-water. Recently, a model has been put forward to explain this beh a ~ i o r and , ~ the results from the present investigation conform to this explanation. Thus, when TDPB (or any other ionic surfactant) is added to an aqueous EHEC solution, the surfactant ions form complexes with the EHEC polymer. This starts at a TDPB concentration (TI) somewhat lower than the cmc as evident from the potentiometric titration data. As a consequence of the binding, the polymer becomes charged. Without salt, the charge repulsion between bound surfactant head groups causes an expansion of the polymer and is also strong enough to overcome the attraction between stretched-out polymer chains. Thus, the increase in charge leads to an increase in solubility, and a more or less immediate increase in CP is observed. With salt, the importance of the electrostatic forces is reduced, which is reflected both in the lower Tlwhen increasing the salt content of (cf. Table I) and the CP decrease at low TDPB concentrations.H At temperatures close to the CP of the polymer-surfactant system, a small amount of TDPB is sufficient to induce phase separation; here the solubility of the surfactant in the precipitated EHEC-rich phase is larger than that in the original EHEC-water solution. At lower temperatures, more TDP+ is needed for stabilizing the phase separation, which means that the formed complexes now contain more surfactant ions and fewer EHEC chains compared with those formed at higher temperatures. By this reasoning, it is expected that Tlshould decrease with increasing temperature. This is in agreement with the present results and previous results from NMR self-diffusion measurements.8 A t higher TDPB concentrations, the electrostatic effects start to dominate even in the presence of salt, and an increase in CP is observed.
Acknowledgment. We thank Minoru Nagao for his kind help with the potentiometric titrations and Margareta Arwidson at Berol Kemi AB for providing the EHEC samples. This work was supported by grants from the Swedish National Board for Technical Development and Berol Kemi AB. Registry No. TDPB, 1155-74-4;EHEC,9004-58-4; NaBr, 7647-15-6.
