3748
Langmuir 1999, 15, 3748-3751
Unusual Conductivity Changes for Sodium Dodecyl Sulfate Solutions in the Presence of Polyethyleneimine and Polyvinylamine Simon M. Bystryak and Mitchell A. Winnik* Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario, Canada M5S 3H6
Junaid Siddiqui ICI Films,† P.O. Box 411, Hopewell, Virginia 23860 Received November 3, 1998. In Final Form: March 1, 1999 The interaction of two polyamines, hyperbranched polyethyleneimine (PEI) and linear poly(vinylamine) (PVAm), with the anionic surfactant sodium dodecyl sulfate (SDS) in aqueous solution was studied at high pH by conductometry and potentiometry. At high pH, these polymers are protonated to only a very small degree. As very weak polyelectrolytes, they should behave in a manner similar to that for nonionic polymers. Upon titration of the PEI and PVAm aqueous solutions with SDS, the conductivity increases over the whole range of SDS concentrations. The character of the conductivity changes, however, is different from that of typical nonionic polymers, such as poly(ethylene oxide) or poly(vinylpyrrolidone). Unlike these nonionic polymer-surfactant systems, the absolute values of the specific conductivity of aqueous SDS solutions in the presence of PEI and PVAm are higher than that for pure SDS over the whole range of SDS concentrations. Potentiometric measurements showed that binding of SDS to PEI and PVAm is accompanied by the consumption of protons and, as a consequence, by an increase in the pH of the solutions. It has been shown that the contribution of OH- ions to the increase in the conductivity of PEI-SDS and PVAm-SDS systems should be taken into account. Some other possible mechanisms of the unusual behavior of the conductivity versus [SDS] plots in the presence of PEI and PVAm have been also proposed.
Introduction Conductometric titrations have been extensively employed for the study of polymer-surfactant interactions.1-4 The most extensively studied systems involve nonionic polymers and ionic surfactants. For example, Jones1 and later Schwuger2 investigated the interaction of SDS with poly(ethylene oxide) (PEO) in aqueous solutions. They showed that upon addition of increasing amounts of SDS to solutions containing a constant concentration of PEO, the specific conductivity (k) initially increases linearly, but at a certain SDS concentration the incremental increase of k becomes smaller, and the k versus [SDS] plot deviates from a straight line. Upon further addition of SDS, the curve approaches a second straight line. This type of behavior for the k versus [SDS] plots for the PEOSDS system is interpreted in terms of two break points marking a transition in the dominant charge-carrying species. The first break point, the so-called critical aggregation concentration (cac), represents the surfactant concentration at which small micelle-like aggregates of SDS bound to the polymer begin to form. The second break point, above the cac, is taken to be the polymer saturation point (PSP), where the saturation of the polymer with surfactant occurs. Above this point, both polymer-bound aggregates and regular aqueous micelles coexist, and the * To whom correspondence should be addressed: E-mail:
[email protected]. † Now Dupont films. (1) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (2) Schwuger, M. J. J. Colloid Interface Sci. 1973, 43, 491. (3) Goddard, E. D. In Interactions of surfactants with polymers and proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: London, 1993; p 123. (4) Witte, F. M.; Engberts, J. B. F. N. J. Org. Chem. 1987, 52, 4767.
amount of the latter increases with increasing total concentration of SDS. Below the cac, the conductivities of the solutions in the absence and in the presence of PEO are the same, and as a consequence, no interaction between the polymer and surfactant can be detected. Between the cac and the PSP, the conductance k of the solution in the presence of PEO is less than that in its absence, because the bound fraction of SDS molecules has a much lower molecular mobility and, hence, a lower molar conductivity. Above the PSP, the conductivity curves for SDS in the presence of PEO merge with the curve for pure SDS. Other features of the conductivity curves for the different polymer-surfactant systems are described in a number of publications.5-9 The studies referred to above all involved linear nonionic polymers. In contrast, there are very few studies of polyelectrolytes by conductance. It is in this context that we report here a conductometric study on the interaction between weak polyelectrolytes, hyperbranched polyethyleneimine (PEI) and linear poly(vinylamine) (PVAm), and the anionic surfactant sodium dodecyl sulfate (SDS) in unbuffered aqueous solution. Experimental Section Hyperbranched polyethyleneimine (PEI) with nominal molecular weight 75 000 (manufacturer’s specification; Mw calcu(5) Francois, J.; Dayantis, J.; Sabbadin, J. Eur. Polym. J. 1985, 21, 165. (6) Fadnavis, N. W.; van den Berg, H. J.; Engberts, J. B. F. N. J. Org. Chem. 1985, 50, 48. (7) Minatti, E.; Zanette, D. Colloids Surf., A: Physicochem. Eng. Aspects 1996, 113, 237. (8) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785. (9) Zanette, D.; Ruzza, A. A.; Froehner, S. J.; Minatti, E. Colloids Surf., B 1996, 108, 91.
10.1021/la981552m CCC: $18.00 © 1999 American Chemical Society Published on Web 04/29/1999
Conductivity Changes for SDS Solutions
Figure 1. Changes of specific conductivity of SDS water solutions in the absence and in the presence of PEO (1 wt %), PEI (0.5 wt %), and PVAm (0.35 wt %): (a) The values of conductivities were plotted as obtained. (b) The values of conductivities were corrected for the contribution of OH- ions (see the text). lated from light scattering is 750 000) and poly(ethylene oxide) (PEO) with Mw ) 10 000 were purchased from Aldrich. Poly(vinylamine) (PVAm) with Mw ) 23 000 was obtained from Air Products Chemicals, Inc. (Allentown, PA). The polymers were used as received, although the purity, especially of PEI, was checked by careful 1H NMR measurements and by atomic absorption spectroscopy for the presence of inorganic impurities. NaOH standard solutions were purchased from Aldrich. SDS was obtained from Fisher Scientific. Its purity was assessed by determination of its cmc (see below), and it was used without further purification. Aqueous solutions were made up using distilled water, which was deionized in a Millipore Milli-Q water system. The solution concentrations of PEI, PEO, and PVAm are given as weight percents (grams per 100 mL) or as the molar concentration on a monomer basis (moles of monomer per liter of solution). SDS concentrations are given in moles per liter. Conductivity titration measurements were carried out at 20 °C using a Fisher Scientific conductometer. Simultaneous with the conductivity, the pH of each solution was measured using a pH meter purchased from EXTECH Instruments (Waltham, MA). All experiments were carried out with aqueous solutions in the cell under constant stirring. To avoid disturbance of the conductivity and pH measurements, the titrations never lasted longer than 30 min; and after each run, the electrodes were rinsed with water and calibrated using standard calibration solutions.
Results and Discussion The changes of specific conductivity (k) of unbuffered SDS solutions in the absence and in the presence of PEO, PEI, and PVAm are presented in Figure 1a. The k versus
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[SDS] plot for SDS alone is typical for anionic and cationic surfactants in aqueous solutions.1,2,10,11 The critical micelle concentration (cmc) of the surfactant is calculated as the intersection of lines drawn through the two linear parts of the conductivity curve. We refer to this intersection as the break point. The incremental increase of the specific conductivity (k) above the cmc is smaller than that below the cmc, that is, before micellization. This difference arises because the conductivity of SDS micelles and their bound counterions is less than the sum of the conductivities of the free surfactant ion and its counterion. The cmc value determined from the k versus [SDS] plot in Figure 1 is 8.1 mM and agrees with the literature data.10 For the PEO-SDS system, our data (Figure 1a) show that k initially increases almost linearly, as in the absence of the PEO. Then, the k versus [SDS] plot deviates from a straight line and at higher SDS concentrations merges with the conductivity curve for pure SDS. This type of plot for PEO + SDS is well documented.1,7 As was mentioned in the Introduction, the two break points in the plot are interpreted as the critical aggregation concentration (cac) and the polymer saturation point (PSP), respectively.1,7 The common feature of the k versus [SDS] plots for nonionic polymers such as PEO and poly(vinylpyrollidone) is that the absolute values of the specific conductivities of the solutions in the presence of the polymer are always less than or equal to those in its absence. As can be seen in Figure 1, the character of changes of the conductivity with the SDS concentration for the PEI-SDS and PVAmSDS systems is completely different than that for the PEO-SDS system. The distinguishing feature of the k versus [SDS] plots for the PEI-SDS and PVAm-SDS systems is that the absolute values of the specific conductivity of the SDS solutions in the presence of PEI and PVAm are higher than those in their absence over the whole range of SDS concentrations. Thus, the fundamental difference between the conductivity changes for the PEI-, PVAm-, and PEO-SDS systems is that, for PEI + SDS and PVAm + SDS, the conductivity of the solution increases upon binding of the SDS to the polymer macromolecules. The data obtained from the k versus [SDS] plots for polymer-surfactant systems are usually analyzed without taking into account the contribution of H+ and OH- ion conductivities. Most measurements are performed at neutral pH, where these contributions to k are insignificant. In unbuffered water, PEI and PVAm solutions are basic, and the pH of aqueous solutions containing PEI, at the concentrations used in this work, is higher than 9.0. Moreover, the addition of anionic surfactants to weak polyelectrolytes is known to result in an increase in the pH of the solution as a consequence of hydrogen ion consumption.12,13 Figure 2 depicts the pH changes of the aqueous solution containing 0.5 wt % (0.11 M) PEI and 0.35 wt % (0.08 M) PVAm upon addition of increasing concentrations of SDS. As can be seen in this figure, the pH increases with increasing SDS concentration, eventually approaching a plateau. The same trend for the increase in the pH of the SDS solutions in the presence of PEI was observed in an early report,12 where only very low concentrations of PEI (less than 0.05 wt %) and SDS (less than 1.5 mM) were examined. (10) Moroi, Y.; Matsuoka, K. Bull. Chem. Soc. Jpn. 1994, 67, 2057. (11) Bahadur, P.; Sastry, N. V.; Rao, Y. K. Colloids Surf. 1988, 29, 343. (12) Van den Berg, J. W. A.; Staverman, A. J. Rec. Trav. Chim. PaysBas 1972, 91, 1151. (13) Hattori, T.; Katai, K.; Hattori, S.; Kato, M. Bull. Chem. Soc. Jpn. 1997, 70, 359.
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Bystryak et al.
Figure 2. Variation of the pH of SDS solutions in the presence of 0.5 wt % PEI and 0.35 wt % PVAm with the total concentration of SDS.
Figure 3. Changes of pH (circles) and conductivities (squares) of aqueous sodium hydroxide solutions in the absence (open symbols) and in the presence (filled symbols) of 20 mM SDS as a function of the total concentration of NaOH.
To interpret this change in pH, one assumes that one or both of the following reactions between PEI and SDS occur.
PEI + H+ + DS- h PEI-H+/DS-
(1a)
PEI-H+/OH- + DS- h PEI-H+/DS- + OH- (1b) Under these conditions, the contribution of OH- to the cumulative conductivity of the solution in the presence of the PEI may be significant. The conductivity of the H+ and OH- ions may be expressed as follows:
1000k(H++OH-) ) ΛH+[H+] + ΛOH-[OH-]
(2)
where k(H++OH-) refers to the specific conductivity of H+ and OH-, ΛH+ is the equivalent conductivity of H+, and ΛOH- is the equivalent conductivity of OH-. Thus, k(H++OH-) can be evaluated using ΛH+ and ΛOH- values and the measured concentrations (activities) of the ions. In titration experiments of PEI and PVAm solutions with SDS, the pH of the solutions was measured, giving directly the H+ ion activity. Using the values of ΛH+ ) 349.82 S cm2 mol-1 and ΛOH- ) 198 S cm2 mol-1 at infinite dilution of the ions,14 and their activities determined from the measured pH, we calculated k(H++OH-) values. We subtracted these values from the values of k measured in the mixtures of PEI and PVAm with SDS. The conductivity changes, corrected for OH- conductivity as described above, for SDS solutions in the absence and in the presence of 0.5 wt % PEI or 0.35 wt % PVAm are shown in Figure 1b. As can be seen in this figure, the absolute values of the specific conductivity, corrected for OH- conductivity, are still significantly higher than those in the absence of the polymers over the whole range of SDS concentrations. The correction of the conductivity data for the contribution of OH- ions assumes that the conductivities of these ions in the absence and in the presence of SDS are the same. To check whether this is the case, we performed some additional model experiments. In the first experiment, we examined the conductivity and pH changes of NaOH aqueous solutions in the absence and in the (14) Owen, B. B. Physical Chemistry of Electrolytic Solution; Reinhold Publishing Corp.: New York, 1958.
Figure 4. Conductivity changes of aqueous SDS solutions at pH 6.0, 11.0, and 12.0 as a function of the concentration of SDS.
presence of 20 mM SDS. As can be seen in Figure 3, the conductivity versus [NaOH] plots with and without SDS are practically parallel (the initial conductivity of the solution with SDS is caused by the contribution of the SDS molecules). This result shows that the mobility of Na+ and OH- ions does not change significantly in the presence of 20 mM SDS. In the second experiment, we examined the conductivity changes of aqueous solutions of SDS at various pH values, that is in the presence of different concentrations of NaOH. The k versus [SDS] plots at pH ) 6.0 ([NaOH] ) 0), pH ) 11.0 ([NaOH] ) 10-3 M), and pH ) 12.0 ([NaOH] ) 10-2 M), are presented in Figure 4. For the sake of clarity, the curves are shifted to a common origin by subtracting the initial conductivity of solutions (at [SDS] ) 0) caused by the contribution of Na+ and OH- ions at pH 11 and pH 12. The effect of SDS on the pH of the solutions is negligible. The data in Figure 4 show that the k versus [SDS] plots at pH 6 and pH 11 are practically the same, and the absolute values of the conductivity at pH 12 are a little lower than those at pH 6 and 11 above 5 mM SDS. We conclude that one cannot explain the increase in conductivity on the basis of an
Conductivity Changes for SDS Solutions
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influence of Na+ or OH- ions on the intrinsic conductivity of SDS itself. The phenomena we observe are due to the interaction of SDS with the polymer. The candidates for contributors to the enhanced conductivity are Na+, OH-, and DS- and possibly low concentrations of protonated polymer. The effect persists in the presence of small added quantities of NaOH (pH > 11), so the polymer itself is unlikely to be an important charge-carrying species. On the other hand, when the PEI sample is dialyzed to remove low-molecular-weight components, the magnitude of the enhanced conductivity is smaller, indicating that the mobility of the polymer itself may play a role. Since PVAm and PEI exhibit similar behavior in the presence of SDS, we can conclude that the enhanced conductivity is not associated with the hyperbranched architecture of PEI. For Na+, OH-, or DS- ions to serve as the main charge carriers, some feature of the interaction of SDS with the polymer must increase the mobility or the free ion concentration of these ions in solution compared to that in solutions of SDS micelles. In global terms, one can imagine several different mechanisms that may be operating. First, as discussed in ref 10, the high conductivity of these ionic micelle solutions may contain a contribution from counterion mobility such that “some counterions can move freely from one micelle to the other micelle through overlapping of diffuse double layers around each micelle when micelles come together”. This so-called hopping reaction of counterions for SDS micelles can be described by the expression
Na+M1 + M2 f M1 + Na+M2
(3)
where Na+/M is the sodium ion associated with a given micelle and M1 and M2 are different micelles. It is possible that in the presence of PEI- and PVAm-SDS aggregates the above reaction (eq 3) is even more favorable and, hence, the conductivity of normal SDS micelles is higher in the presence of PEI or PVAM than that in their absence. Inasmuch as the local concentration of PEI- and PVAmSDS micelle-like aggregates is higher than that of free SDS micelles at the same SDS concentration, then the Na+, OH-, and DS- ions can move from one aggregate to another more rapidly inside the PEI and PVAm macromolecules. The PEI and PVAm macromolecules may also
promote the attractions between “interpolymer” micellelike aggregates, increasing the average amount of time that these aggregates are in contact in the process of collisions of the polymer macromolecules. Another factor, which might promote an increase in the mobility of Na+, OH-, and DS- ions in the presence of PEI- and PVAm-SDS micelle-like aggregates is that the shape of these aggregates may be different from that of normal SDS micelles. It is well-known15-17 that in SDS solutions, in addition to the normal cmc, there occurs a change in the properties of the surfactant solutions at a higher concentration, which is often referred to as the second cmc. This transition, promoted by elevated surfactant concentration or the presence of salt, involves a change in the shape of the micelles from spherical to wormlike cylindrical micelles. This change in shape is accompanied by an increase in the conductivity of the solution. In particular, Miura and Kodama15 showed that the addition of NaCl to solutions of SDS resulted in an increase in the conductivity of the solutions above the second cmc. This increase is likely associated with an increase in the concentration of free sodium ions in the solution accompanying the transition from spherical to cylindrical micelles. If we suppose that SDS forms rodlike micelles when bound to PVAm or PEI, this may lead to an increase in the conductivity of the solutions. At the present time there is no compelling evidence in favor of either explanation, and further information is needed. For example, information on the mobility of individual species in these solutions is in principle available through pulsed-gradient NMR experiments, and dynamic light scattering measurements should provide information on how SDS affects PEI mobility in solution. We hope to report on these aspects of the interaction between polyamines and SDS in a future publication. Acknowledgment. The authors thank ICI, ICIs Canada, and NSERC Canada for their support of this research. LA981552M (15) Miura, M.; Kodama, M. Bull. Chem. Soc. Jpn. 1972, 45, 428. (16) Nguyen, D.; Bertrand, G. L. J. Phys. Chem. 1992, 96, 1994. (17) Paul, B. C.; Ismail, K. Bull. Chem. Soc. Jpn. 1993, 66, 703. (18) Schick, M. J. J. Phys. Chem. 1964, 68, 3585. (19) Hayashi, S.; Ikeda, S. J. Phys. Chem. 1980, 84, 744.
