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Langmuir 1998, 14, 1026-1030
Use of Isothermal Titration Microcalorimetry To Monitor the Adsorption/Desorption Processes of Sodium Dodecyl Sulfate with Neutral Polymers G. J. Fox, D. M. Bloor, J. F. Holzwarth,† and E. Wyn-Jones* Division of Chemical Sciences, University of Salford, Salford M5 4WT, U.K. Received June 30, 1997. In Final Form: October 16, 1997 The optimization of neutral polymer/anionic surfactant mixtures is a critical process in the development of such systems for many various applications. In this report, we describe how systematic isothermal titration microcalorimetry experiments may be designed in such a way that both the adsorption and desorption processes of sodium dodecyl sulfate with two neutral polymers, PAPR* (a copolymer of N-(vinylacryloyl)pyrrolidine containing a covalently bonded 4-vinylpyridine dicyanomethylide chromophore) and PVP (poly(N-vinylpyrrolidone)), can be directly monitored. The desorption of bound SDS from the polymers is achieved using the nonionic surfactant hexaethylene glycol mono-n-dodecyl ether and the possible mechanism via which this process takes place is qualitatively discussed. Finally, one of the beneficial outcomes stemming from this work is that different stable combinations of polymer or polymer/ surfactant complex with mixed micelles and monomer surfactant can be formulated.
Introduction Formulations containing polymers and surfactants have found a wide variety of applications as industrial, cosmetic, pharmaceutical, agricultural, and domestic products. As a result, they have been the subject of a multitude of fundamental studies, which have been well documented in recent review articles.1-3 Most studies on neutral polymer/ionic surfactant systems have been carried out by adding known amounts of the surfactant to a polymer solution of constant concentration. The surfactant concentration corresponding to the onset of binding is usually denoted by T1 (sometimes called the critical aggregation concentration or cac). As further surfactant in excess of T1 is added, binding proceeds in such a way that a dynamic equilibrium exists between monomer surfactant in solution and bound surfactant in the form of micellar type aggregates. When the polymer is fully saturated with bound surfactant, free micelles occur in solution. This concentration is usually denoted T2. In some systems free micelles start occurring between T1 and T2.4,5 It is the varied and often dramatic changes in the physicochemical properties of the polymer/surfactant complex in the T1 f T2 binding region that render these systems amenable to form the basis of commercial products. In other areas it is the reduction or avoidance of binding that is desirable.6 According to Goddard,6 the creation of a formulation of commercial potential is assembled very much on an empirical trial and error basis, * To whom correspondence should be addressed. † Fritz Haber Institut der Max Planck Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. (1) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabham, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 123. (2) Robb, I. P. Anionic Surfactants, Surfactant Sci. Ser. 1981, 11, 109. (3) Hayawaka, K.; Kwak, J. C. T. Cationic Surfactants, Surfactant Sci. Ser. 1991, 37, 189. (4) Bloor, D. M.; Wan-Yunus, W. M. Z.; Wan Badhi, W. A.; Li, Y.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (5) Bloor, D. M.; Li, Y.; Wyn-Jones, E. Langmuir 1995, 11, 3778. (6) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabham, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 395.
and he also emphasizes that the optimization of mixtures could be considerably enhanced and much better understood if the interaction and binding characterization of the particular polymer and surfactant system could be established. Unfortunately, the interaction characteristics of neutral polymer/surfactant systems in the form of measurable parameters such as T1, T2, the onset of the formation of ‘free’ micelles, and the amount of surfactant and counterions bound to the polymer are only known for a handful of systems. Furthermore,9 the fundamental basis for the methodology of the control and manipulation of these systems for optimization is almost nonexistent. As a result we have embarked on a twofold program of fundamental studies of neutral polymer/ionic surfactant systems aimed at (1) characterizing the complete binding behavior in the mixture, by determining the measurable parameters described above,4,5,7-9 and (2) investigating methods to monitor attempts to control and manipulate binding in these systems.10 In relation to (1), we have found that a combination of emf measurements using surfactant-selective electrodes and isothermal titration microcalorimetry (ITC) provides rapid and reliable information on the binding behavior. In relation to (2), we have recently demonstrated10 that the binding of sodium dodecylsulfate (SDS) to the polymer PAPR* can be systematically controlled by addition of a nonionic cosurfactant, hexaethylene glycol mono-n-dodecyl ether (C12EO6). PAPR* is a copolymer of N-(vinylacryloyl)pyrrolidine (98.6%) and the covalently bonded chromophore 4-vinylpyridine dicyanomethylide (1.4%) that shows a strong solvatochromic visible shift in the presence of bound aggregated SDS. In practice, the neutral polymers that are of interest in relation to surfactant binding studies have no in-built solvatochromic probes to monitor binding. Consequently, an alternative methodology involving experiments that (7) Painter, D. M.; Bloor, D. M.; Takisawa, N.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2087. (8) Wan Badhi, W. A.; Wan-Yunus, W. M. Z.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1993, 89, 2737. (9) Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 2312. (10) Li, Y.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1996, 12, 4476.
S0743-7463(97)00695-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/07/1998
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are capable of monitoring attempts to control the binding process has to be established. In this article we have successfully demonstrated that ITC, which has recently emerged as a powerful technique to study these systems,4,5,9,11-15 can be used for these purposes. Experimental Section Sodium dodecyl sulfate (SDS) was prepared in the laboratory using the method described by Davidson.16 C12EO6 was a Nikkol product, used as received. The probe-labeled copolymer PAPR* was synthesized according to the method described by Velasques and Galin.17 The structure of PAPR* is shown below with the numbers indicating the mole fraction of each monomer. The molecular weight of the copolymer was determined to be 10 000 by gel permeation chromatography. Poly(N-vinylpyrrolidone) (PVP) was a product of Sigma, of MW 40 000, and was used without further purification.
Figure 1. Plot of the enthalpy change per injection (∆Hi) as a function of total SDS concentration for the SDS/PVP system, in 0.2 mol dm-3 NaBr: (0) 0% PVP; (2) 1% PVP.
Isothermal Titration Microcalorimetry. The microcalorimeter used in this work was the Microcal ITC instrument. In the ITC experiment, one measures directly the energetics (enthalpy changes) associated with processes occurring at constant temperature. Experiments were carried out by titrating micellar surfactant (adsorption, SDS; desorption, C12EO6) from a syringe into a sample solution containing different combinations of polymer, cosurfactant, and NaBr. An injection schedule (number of injections, volume of injection, and time between injection) is set up using interactive computer software, and this schedule is automatically carried out with all data stored to disk. After each addition, the heat released or absorbed as a result of the various processes occurring in the solution is monitored by the calorimeter. In the present work, we present the results of the ITC experiments in terms of the enthalpy per injection (∆Hi) as a function of injected surfactant concentration. The measurements were taken at 298 K.
Figure 2. Plot of the enthalpy change per injection (∆Hi) as a function of total SDS concentration for the SDS/PAPR* system, in 0.2 mol dm-3 NaBr: (0) 0% PAPR*; (b) 0.5% PAPR*. Table 1. surfactant/polymer system
Results The ITC “enthalpy profiles” as SDS systematically binds to the polymers PAPR* and PVP, in 10-4 and 0.2 mol dm-3 NaBr, have been carried out. The plots for both polymers in 0.2 mol dm-3 NaBr are shown in Figures 1 and 2. The plots in 10-4 mol dm-3 have been reported5,9 previously. Each plot shows the enthalpy per injection (∆Hi) as a function of the total SDS concentration in the presence and absence of the polymer at SDS concentrations spanning the binding region T1-T2. As has been reported previously,4,5,9,11-15 when surfactants bind to neutral polymers, their enthalpy profiles have distinguishing characteristic features-usually a well-defined maximum, which is followed by a minimum, and finally a point at which the ∆Hi data sets in the presence and absence of (11) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588. (12) Thuresson, K.; Nystrom, B.; Wang, G.; Lindmann, B. Langmuir 1995, 11, 3730. (13) Olofsson, G.; Wang, G. Pure Appl. Chem. 1994, 66, 257. (14) Brackman, J. C.; Van Os, N. M.; Engberts, J. B. F. N. Langmuir 1988, 4, 1266. (15) (a) Perron, G.; Francouer, J.; Desnoyers, J. E.; Kwak, J. C. T. Can. J. Chem. 1987, 65, 990. (b) Fox, G. J.; Li, Y.; Bloor, D. M.; WynJones, E. Unpublished results. (16) Davidson, C. J. Ph.D. Thesis, University of Aberdeen, 1983. (17) Velasques, D. L.; Galin, J. C. Macromolecules 1986, 19, 1096.
SDS/PAPR* (0.5%) in 0.2 mol dm-3 NaBr SDS/PAPR* (0.5%) in 10-4 mol dm-4 NaBr SDS/PVP (1.0%) in 0.2 mol dm-3 NaBr SDS/PVP (1.0%) in 10-4 mol dm-3 NaBr
T1 (mol dm-3)
T2 (mol dm-3)
0.42 × 10-3 60 × 10-3 2 × 10-3 50 × 10-3 0.25 × 10-3 105 × 10-3 1 × 10-3 90 × 10-3
polymer merge. In a comprehensive comparison of EMF and ITC data on a number of polymer surfactant systems15b we have shown that T1 is located at the point immediately preceding the maximum in the ITC enthalpy profile, as is indicated in Figures 1 and 2. On the other hand T2 is located at the point where the enthalpies per injection with and without the polymer merge following binding. The minimum observed in the enthalpy profiles signals that binding is almost complete. Table 1 lists the values for T1 and T2 determined from the above data. In 0.2 mol dm-3 NaBr, there are some notable differences in comparison with the previously reported work carried out in 10-4 mol dm-3 NaBr.5,9 For the SDS/PVP system (Figure 1), there no longer exists a broad endothermic peak but instead a sudden drop in the value of the enthalpy (∆Hi), taken to be the onset of binding T1. The profile takes on a similar shape to that of the micellization process for pure surfactant alone, before descending into a fairly sharp minimum, finally reaching a point, as expected, at which the ∆Hi data sets in the presence and absence of polymer merge. The shape of the profile for the SDS/
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Figure 3. Plot of the enthalpy change per injection (∆Hi) as a function of total C12EO6 concentration for the C12EO6/polymer systems, in 0.2 mol dm-3 NaBr: (0) 0% polymer; (b) 0.5% PAPR*; (2) 1% PVP.
PAPR* system in 0.2 mol dm-3 NaBr has the predicted characteristics, although the endothermic peak is less pronounced in the higher salt concentration. As expected the T1’s are lower for the solutions with the higher salt concentration because the presence of more bound sodium counterions reduces surfactant head group repulsions, thus further stabilizing the bound aggregate, which in turn means that the aggregate can be formed at lower added SDS concentrations. At the other end of the binding region, the T2’s are higher for the solutions containing 0.2 mol dm-3 salt. The effect of the added salt is also to increase the aggregation numbers of the bound micelles, thus increasing T2. Presumably on a mole to mole ratio, the number of the bound micelles per polymer micelles remains the same for the different salt concentrations, the only difference being that there are more surfactant monomers in the bound micelles of the solutions containing more salt. In the concentration range 0-70 × 10-3 mol dm-3, C12EO6 does not interact with either polymer. This is shown for the experiments carried out in 0.2 mol dm-3 NaBr in Figure 3. In 10-4 mol dm-3 NaBr, the same result was obtained. The enthalpy profiles in this diagram clearly show that the injection enthalpies observed during the C12EO6 titration are the same, with and without the polymer. The enthalpy profiles were determined as micellar C12EO6 was successively injected into 16 × 10-3 mol dm-3 SDS at two concentrations of NaBr (10-4 and 0.2 mol dm-3) with and without the polymers (PAPR*, 0.5%; PVP, 1%). A typical plot is shown in Figure 4. The choice of the SDS concentration was deliberate in the sense that, for both polymers, all the aggregated surfactant is bound to the polymer and is in equilibrium with the free monomer SDS in solution (see Figures 1 and 2), with no ‘free’ SDS micelles present. The noteworthy feature in each experiment is that the enthalpies per titration for the solution, with and without polymer, are different when the same amount of C12EO6 is successively added to each starting solution. Eventually, as more C12EO6 is added, the two enthalpy curves merge, and after this point the enthalpies per C12EO6 injection with and without polymer remain the same. This behavior is the same for all the systems studied. Once the injection enthalpies, with and without polymer, merge, the addition of C12EO6 has exactly the same effect on the SDS whether the polymer is present or not. The only explanation for
Fox et al.
Figure 4. Plot of the enthalpy change per injection (∆Hi) as a function of total C12EO6 concentration for the C12EO6/SDS (16 mM)/PAPR* system, in 10-4 mol dm-3 NaBr: (0) 0% PAPR*; (b) 0.5% PAPR*. Table 2. cosurfactant/surfactant/ polymer system
C12EO6 conc (Cdes) (mol dm-3)
C12EO6/SDS (16 mM)/PAPR* (0.5%) in 0.2 mol dm-3 NaBr C12EO6/SDS (16 mM)/PAPR* (0.5%) in 10-4 mol dm-3 NaBr C12EO6/SDS (16 mM)/PVP (1.0%) in 0.2 mol dm-3 NaBr C12EO6/SDS (16 mM)/PVP (1.0%) in 10-4 mol dm-3 NaBr
16 × 10-3 24 × 10-3 16 × 10-3 24 × 10-3
Figure 5. Plot of the difference in enthalpy change per injection ((∆Hi)D), as a function of total C12EO6 concentration for the C12EO6/SDS (16 mM)/polymer systems: (b) 0.5% PAPR*, (2) 1% PVP, in 0.2 mol dm-3; (O) 0.5% PAPR*, (4) 1% PVP, in 10-4 mol dm-3.
this is that, at the merger point, all the bound SDS has been stripped from the polymer and consequently behaves as the SDS-only solution. Furthermore, for C12EO6/PAPR* in 10-4 mol dm-3 NaBr, the C12EO6 concentration corresponding to complete desorption of SDS from the polymer corresponds to that found from the independent spectroscopy experiment, using the solvatochromatic probe on PAPR* to monitor the resorption of bound SDS.10 For each experiment, the C12EO6 concentration corresponding to the two curves merging is listed in Table 2. The desorption process is illustrated more clearly in Figure 5, where the difference in enthalpy per injection ((∆Hi)D), with and without polymer, is plotted as a function of C12EO6 concentration. For the purpose of this work, the
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Langmuir, Vol. 14, No. 5, 1998 1029 Table 3.
Figure 6. Plot of the difference in enthalpy change per injection ((∆Hi)D), as a function of total SDS concentration for the SDS/ C12EO6 (20 mM)/PAPR* system, in 10-4 mol dm-3 NaBr.
concentration of C12EO6 at which all of the SDS is completely stripped from the polymer has been given the label Cdes. It is clear from this work that C12EO6 has the ability to strip bound SDS from these polymers. Another key observation is that the efficiency of C12EO6 as an SDSdesorping agent is increased by a factor of 1.5 when the salt concentration is increased from 10-4 to 0.2 mol dm-3, also giving a sharper, more clearly defined desorption profile. The clue to the combined efficiency of C12EO6 and salt must be their ability to dramatically lower the “free” monomer SDS concentration in solution to a level well below the threshold that is required for SDS to bind to the polymers. This is achieved in two ways: (1) In free “mixed” micelles between the C12EO6 and SDS, by inference to cmc measurements18 and recently using EMF data,19 it is known that nonionic surfactants in their own right lower the SDS monomer concentration. Indeed19 a small amount of C12EO6 is known to reduce the monomer concentration of SDS below T1, the onset of binding in both SDS/PVP and SDS/PAPR*. (2) The addition of salt significantly lowers the cmc and hence the monomer concentration of an ionic surfactant in equilibrium with the surfactant monomer in a ‘pure’ or mixed micelle. We have also measured the enthalpy profiles as micellar SDS is successively injected into 20 × 10-3 mol dm-3 C12EO6 at two concentrations of NaBr (10-4 and 0.2 mol dm-3) with and without the polymers (PAPR*, 0.5%; PVP, 1%). A typical plot is shown in Figure 6. In this diagram the value 0 (zero) on the ordinate axis corresponds to the situation where the enthalpies per injection, with and without polymer, are the same. When the graph deviates from zero this indicates that SDS is binding to the polymer. The noteworthy feature for each measurement is that the injection enthalpies with and without polymer have been found to be initially the same. However, as more SDS is added, the enthalpies diverge in the sense that the enthalpy-driven processes in the polymer solution become less exothermic. The enthalpies of the polymer solution then reach a maximum followed by a shallow minimum, which is slightly more exothermic than the enthalpies for the solution with no polymer. Eventually, the enthalpies (18) Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992; p 501. (19) Hall, D. G.; Meares, P.; Davidson, C.; Wyn-Jones, E.; Taylor, J. ACS Symp. Ser. 1992, 501, 128.
cosurfactant/surfactant/ polymer system
SDS conc (T1) (mol dm-3)
C12EO6 (20 mM)/SDS/PAPR* (0.5%) in 0.2 mol dm-3 NaBr C12EO6 (20 mM)/SDS/PAPR* (0.5%) in 10-4 mol dm-3 NaBr C12EO6 (20 mM)/SDS/PVP (1.0%) in 0.2 mol dm-3 NaBr C12EO6 (20 mM)/SDS/PVP (1.0%) in 10-4 mol dm-3 NaBr
25 × 10-3 8 × 10-3 25 × 10-3 10 × 10-3
with and without polymer, will merge again and remain the same as further SDS is added. It is very interesting to note that the onset of binding occurs at a much higher SDS concentration for the solutions containing the higher salt concentration (see Table 3). In the absence of the nonionic surfactant, the opposite effect occurs (see Table 1, T1 values), since it is well-known that excess salt reduces the head-group interaction in the surfactant aggregate, thus making binding more favorable at lower SDS concentrations. It is quite likely that, in these experiments, the presence of salt does actually stabilize the bound micelles to some extent. However, this effect is heavily outweighed by the drastic reduction in the SDS monomer concentration resulting from the combined effects involving the formation of mixed micelles with C12EO6 and also excess salt. As a result, a far larger amount of SDS has to be added to the polymer/C12EO6 system before the SDS monomer concentration reaches the threshold value of T1 and allows binding to take place. On this basis, in the 10-4 mol dm-3 salt solutions, it is easier for the monomer concentration of SDS to reach this binding threshold value at lower total SDS concentration. Clearly the main issues here relate to monomer SDS concentrations which can only be resolved by direct measurements using electrodes, a problem which we are currently addressing. This in turn raises the possibility of some type of mixed micelle occurring on the polymer when the nonionic surfactant is present. At this stage we must emphasize that the critical concentrations measured in these mixed surfactant systems are not absolute quantities but refer to those measured for specific starting solutions, which will vary from one solution to another. In previous studies4,5,9,11-15 the ITC method has been used successfully to monitor the binding of a surfactant to a polymer, and further examples are given in this work. In addition we have shown that the method can be used to monitor the controlled desorption of bound surfactant from a polymer and also identify the binding region in a binary starting solution of a fixed polymer/C12EO6 composition. We believe that these are the types of experiments that are required to underpin any comprehensive study on the optimization of polymer/surfactant complexes. Another beneficial outcome stemming from these studies is that for the present polymer/surfactant mixtures, under the limited experimental conditions that have been used, the following components can be formulated independently: (1) free polymer plus monomer SDS, (2) polymer/SDS complex plus monomer SDS, (3) polymer/ SDS complex plus monomer SDS plus micellar SDS, (4) polymer/surfactant complex plus mixed micelles plus monomer surfactant, and (5) free polymer plus mixed micelles plus monomer surfactants. This again demonstrates how control and manipulation of these systems can result from systematic experiments, leading to different morphologies in the system. According to Dubin20 and his co-workers, who have investigated the binding of mixed SDS/nonionic surfactants with polycat-
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ions, it is also possible that the flexible EO6 head group in the mixed micelle can actually screen the anionic head group, thus preventing its attractive interaction with the polymers and, therefore, making an additional contribution to the desorption process. (20) Dubin, P.; Zhang, H.; Li, Y.; Kato, T. J. Colloid Interface Sci. 1996, 183, 546.
Fox et al.
Acknowledgment. G.J.F. thanks the University of Salford for a maintenance award. He also would like to thank Professor J. F. Holzwarth for the opportunity to carry out the ITC experiments and the Deutscher Akademischer Austauschdienst (DAAD) for the provision of a short-term research grant. LA970695Q
