Langmuir 2001, 17, 5657-5665
5657
Structure of the Complexes Formed between Sodium Dodecyl Sulfate and a Charged and Uncharged Ethoxylated Polyethyleneimine: Small-Angle Neutron Scattering, Electromotive Force, and Isothermal Titration Calorimetry Measurements Y. Li,† R. Xu,‡ S. Couderc,† D. M. Bloor,‡ J. Warr,| J. Penfold,§ J. F. Holzwarth,*,† and E. Wyn-Jones*,†,‡ Fritz-Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany, School of Sciences, Chemistry, University of Salford, Salford M5 4WT, United Kingdom, ISIS Facility, Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire OX11 O QX, United Kingdom, and Lever Brothers Limited, 3 St. James’s Road, Kingston upon Thames, Surrey KT1 2BA, United Kingdom Received March 22, 2001. In Final Form: June 8, 2001 A poly(ethylene oxide) derivative of polyethyleneimine behaves like a strong polyelectrolyte at pH ) 2.5 and a neutral polymer at pH 10. Both the charged and uncharged versions of this polymer bind strongly to the surfactant sodium dodecyl sulfate (SDS) with no phase separation taking place. Binding isotherms were measured using a dodecyl sulfate electrode, and these data were complemented with isothermal titration calorimetry (ITC) measurements. Small-angle neutron scattering measurements were also carried out at some specific concentrations in the binding region at pHs 2.5, 5.5, and 10. With the exception of one measurement, bound micelles were detected and their aggregation numbers could be evaluated. For the SDS/polymer system at pH 10, the polymer/surfactant complex contains 6-8 bound SDS micelles per polymer molecule at the binding limit. In a solution of 6.5 mM SDS/0.5% w/v polymer at pH 10, bound SDS exists in a nonaggregated form. A detailed examination of the ITC data for the SDS/0.5% w/v polymer system shows at pH 10 that this spot solution occurs in a narrow SDS concentration range immediately following the onset of binding and proceeding until the formation of proper bound micellar aggregates was detectable, whose presence and growth were characterized in the ITC experiments by a steplike decrease in the enthalpy per injection as a function of increasing SDS concentration. These data suggest that the absence of proper micellar aggregates is an inherent consequence of the binding mechanism in the early stages of SDS binding to ethoxylated polyethyleneimines.
Introduction The interaction between charged and uncharged polymers and ionic surfactants has been extensively studied and discussed in many review articles.1-6 These systems have been investigated by techniques which can probe, directly or indirectly, the behavior of specific components in the system, for example, surfactant monomers, micellar aggregates, polymer/surfactant complexes, and single polymers, as well as different macroscopic parameters * Corresponding authors. Professor J. F. Holzwarth and Professor E. Wyn-Jones, Physical Chemistry, Fritz-Haber-Institut, Faradayweg 4-6, D-14195 Berlin, Germany. Tel: +49 30 8413 55 16. Fax: +49 30 84 13 53 85. E-mail:
[email protected]. † Fritz-Haber Institut der Max-Planck Gesellschaft. ‡ University of Salford. § ISIS Facility. | Lever Brothers Limited. (1) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22, 85. (3) Robb, I. D. In Anionic Surfactants. Surfactant Sci. Ser. 1981, 11, 109. (4) Hayawaka, K.; Kwak, J. C. T. In Cationic Surfactants. Surfactant Sci. Ser. 1991, 37, 189. (5) Hansson, P.; Lindmann, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604. (6) (a) Linse, P.; Piculell, L.; Hansson, P. In Polymer-Surfactant systems; Kwak, J. C. T., Ed.; Surfactant Science Series Vol. 77; Marcel Dekker: New York 1998; pp 183-238. (b) Holzwarth, J. F.; Jobe, D.; Dunford, H. B. Czech. J. Phys. 1991, 41, 293.
characterizing the physical properties of the solution (for example, viscosity and surface tension). One of the fundamental prerequisites for the understanding of these systems is a reliable binding isotherm which can be used to underpin most of the above studies.3 Unfortunately, binding data are often not available. To avoid speculations that inevitably arise at the molecular level from many of the measurements dealing with these systems, we have developed surfactant selective electrodes7-10 which can provide rapid, accurate, and reliable binding data. Binding isotherms of this kind give direct information on the amount of surfactant bound per mole of the polymer. Unfortunately, bound surfactants in polymer/surfactant complexes are almost always aggregated and a molecular understanding of these complexes can emerge only when the structure and aggregation numbers are also known. We have recently shown that this can be achieved when small-angle neutron scattering (SANS) experiments are complemented with electromotive force (EMF) and isothermal titration calorimetry (ITC) measurements.11,12 (7) Davidson, C. J. Ph.D. Thesis, University of Aberdeen, Aberdeen, U.K., 1983. (8) Painter, D. M.; Bloor, D. M.; Takisawa, N.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2087. (9) Takisawa, N.; Brown, P.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2099. (10) Xu, R.; Bloor, D. M. Langmuir 2000, 16, 9555. (11) (a) Li, Y.; McMillan, C. A.; Bloor, D. M.; Penfold, J.; Warr, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 7999. (b) Li, Y.; Xu, R.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 10515.
10.1021/la010427z CCC: $20.00 © 2001 American Chemical Society Published on Web 08/02/2001
5658
Langmuir, Vol. 17, No. 18, 2001
Li et al.
EMF Measurements Surfactant membrane electrodes, selective to SDS, were constructed in the laboratory and used to determine monomer surfactant concentrations by measuring their EMF relative to a commercial bromide ion reference electrode.7-10 In practice, the EMF data monitor the monomer surfactant concentration (m1) as a function of total added surfactant (C) and the difference (C - m1) represents the amount of bound surfactant. The cells used for these measurements and the procedures to calculate the respective monomer surfactant concentrations have been described in detail elsewhere.7-10 Figure 1. Scheme showing the structure of the ethoxylated polyethyleneimine PEI(2000)EO20.
In this article, we extended the application of such an approach to investigate the structure of complexes formed between an ethoxylated derivative of polyethyleneimine (PEI)13a and sodium dodecyl sulfate13b (SDS). We have chosen this polymer because it is a strong polyelectrolyte at low pH and to all intents and purposes uncharged at high pH. In addition, it binds strongly to SDS and the presence of ethoxylated groups avoids phase separation when its strong polyelectrolyte form interacts with SDS.13b A recent report14 describing the interaction of the base polymer PEI with SDS was published shortly after our first article concerning the above system.13b The PEIs are polymers with a large number of amine groups, and their chemical structure is simple in the sense that each nitrogen atom is joined to another nitrogen via the ethylene (-CH2-CH2-) linkage13a and they contain primary (P), secondary (S), and tertiary (T) nitrogens in the ratio P/S/T of 1:2:1.13a The polymer used in the present work originates from a PEI polymer of molecular weight 2000, in which all the substitutable hydrogens on the primary and secondary nitrogens are replaced by ethoxylated chains which carry, on average, 20 repeat units. The polymer is pure in the sense that it does not contain excessive amounts of impurities (9 mM) to 0.5%
Table 6. Comparison of SDS Bound Concentration from EMF/ITC and SANS Measurements polymer concn (% w/v)
pH
0.5
10.0
2.5
2.5
2.5
5.5
2.5
10.0
SDS concn (mM)
amount of SDS bound (mM) from EMF
amount of SDS bound (mM) from SANS
6.5 9.0 12.0 10.0 19.0 25.0 6.0 10.0 6.0 10.0
3.6 6.0 10.0 9.4 17.0 23.0 5.8 8.8 3.7 8.0
6.0 11.0 8.0 17.0 22.0 6.0 9.0 5.5 9.0
polymer (uncharged) at the natural pH of ∼10 has a significant effect on SANS and is consistent with the observations of the EMF and ITC measurements of bound SDS micelles (see Figure 9). At 6.5 mM SDS, the SANS profile is only slightly different from the scattering of the polymer only and is indicative of monomeric SDS binding (that is, no bound micelles). Both profiles can be described as a Gaussian coil with similar parameters (as discussed earlier). The Rg values are almost identical (see Table 4), and the change in I(0) can be explained by a small change in contrast (∆F) due to interaction of SDS in its monomeric form with the polymer. The SANS data for 9 and 12 mM SDS are well described by the scattering law presented earlier (eqs 1 and 6). The bound SDS is aggregated as small spherical micelles with an aggregation number of ∼65, similar to that for free SDS micelles at low surfactant concentrations.23 The important model parameters are summarized in Table 5. At the higher polymer concentration (2.5% w/v), there is evidence from the SANS data for adsorbed micelles at both SDS concentrations, 6 and 9 mM (see Figure 10), and the micellar sizes are similar to those at the lower polymer concentration and are also listed in Table 5. From the absolute intensity (as discussed earlier), it is possible to extract the surfactant concentration associated with micelles. These values are in good agreement with those obtained from the electrode measurements, as shown in Table 6. Discussion All the SANS measurements were made in the respective binding regions, and with the exception of 6.5 mM SDS/0.5% polymer at pH 10 (soln I, Figure 8) the bound SDS was always found to exist as micellar aggregates. In the SANS data for solution I (soln I), there was no evidence for micellar aggregates and the bound SDS must be in a nonaggregated form. At first sight, we could be tempted
5664
Langmuir, Vol. 17, No. 18, 2001
Li et al.
Figure 9. Scattered intensity, I(Q) (in cm-1), for 0.5% w/v polymer at pH 10.0: (2) polymer only and with (4) 6.5 mM SDS, (b) 9.0 mM SDS, and (O) 12.0 mM SDS upper curve. The solid lines are calculated curves for the parameters in Table 3 and a Gaussian chain scattering law (2, 4) and for parameters in Table 4 and the model described in eqs 1 and 6 (b, O); experimental errors are indicated.
Figure 10. Scattered intensity, I(Q) (in cm-1), for 2.5% w/v polymer and 6 mM SDS (b) upper curve and 10 mM SDS (O). The solid lines are calculated curves for the parameters in Table 5 and the model described by eqs 1, 6, and 8.
to argue that this measurement was taken at the limit of sensitivity of the SANS technique especially since only 3.6 mM of SDS is bound and under these conditions any bound micelles, if they exist, probably cannot be detected. This argument, however, is incorrect because bound micelles with aggregation numbers of 65 were easily detected in the system 6.0 mM SDS/PEI 2.5% w/v polymer (soln II, Figure 8) at the same pH. In solution II, the amount of bound SDS is 3.7 mmol dm-3. Furthermore, if we assume that in both solutions I and II the initial stages of binding involve charge neutralization of the residual 4% of positively charged nitrogens in the PEI base by SDS monomers, this leaves more bound SDS in (I) (3.2 mmol) than in (II) (2.2 mmol), yet the bound micelles were observed only in (II). Therefore, we are left with the
conclusion that the absence of micellar aggregates in (I) is a real effect and therefore an inherent consequence of the binding mechanism during the early stages of binding. This conclusion is supported by the observation of subtle changes in the ITC enthalpy profile when SDS is titrated into 0.5% w/v polymer and 2.5% w/v polymer at pH ∼ 10. These data are recorded in Figure 8. The ITC method is extremely sensitive and can detect and monitor binding via an enthalpy change in systems where the concentrations are well below the sensitivity of conventional techniques. In addition, ITC can accurately and reliably record small changes in the ∆Hi’s during the titration process. Unfortunately, the stage of knowledge has not yet been reached16,17,35 where the enthalpy profile for surfactant binding can be directly linked to a binding
Complexes of SDS and Ethoxylated Polyethyleneimine
mechanism or isotherm. In the ITC enthalpy profile, the onset of binding is followed by a very pronounced maximum in the ∆Hi’s (Figure 8). Following this maximum, there is a steplike decrease in the ∆Hi’s which is a consequence of the formation of bound surfactant aggregates on the polymer. In the ITC profile for SDS/2.5% w/v polymer, the ∆Hi’s for solution II are located at the right-hand side of this maximum, that is, in the region where bound micelles are formed, a result confirmed by SANS. On the other hand, for SDS/0.5% w/v polymer the ∆Hi’s for solution I occur at the left-hand side of the ∆Hi maximum, that is, before the steplike change associated with bound micelles, again consistent with SANS. These data at the “spot” concentrations I + II are shown in Figure 8. The combined SANS/ITC data suggest that following the onset of binding, SDS exists on the polymer in a nonaggregated form until the maximum in ∆Hi is reached, after which micelles are formed. Indeed, it has often been suggested that dramatic changes in a thermodynamic quantity such as a maximum in ∆Hi can be correlated with structural changes in solution.36 At this stage, we recognize that the generally accepted view associated with the binding of SDS to neutral and weakly charged polymers is that when binding starts at concentration T1 this signals the formation of bound micelles on the polymer; very often this onset of binding is termed the critical aggregation concentration (cac). Such conventional wisdom has been produced by a consensus of opinion which has emerged from the data collected from a large volume of diverse experiments37 carried out on different polymer/surfactant systems. This view, which we subscribed to, certainly needs to be carefully reexamined especially with the emergence of more sensitive and direct experimental techniques such as SANS and ITC. The aggregation numbers found from the SANS data for the polymer solutions at pH 10 are the same for the (35) Torn, L. H.; de Keizer, A.; Koopal, L. K.; Lyklema, J. Colloids Surf., A 1999, 160, 237. (36) Wadso, I. Chem. Soc. Rev. 1997, 26, 79. (37) Lu, J. R.; Thomas, R. K. J. Chem. Soc., Faraday Trans. 1998, 94, 995.
Langmuir, Vol. 17, No. 18, 2001 5665 Table 7. Stoichiometry of SDS/Polymer Complexes at pH 10 polymer concn [P] (mM)
concn of bound SDS at T2 [S] (mM)a
Nagg
[S]/([P] Nagg)
0.058 0.116
31.8 47.6
65 65
8 6
a
Reference 13b.
five solutions that were studied. Since these numbers were determined at different polymer and SDS concentrations, we assume that these numbers will remain constant at 65 until the binding limit at concentration T2 is reached, where the polymer is saturated with bound SDS aggregates. The concentrations of bound SDS which were measured at this limit for PEI(2000)EO2013b are shown in Table 7. By dividing this number by 65, we can get an estimate of the “average” stoichiometry of the polymer/ surfactant complex which reduces to one polymer containing 6-8 bound micelles. We are not able to carry out similar calculations for the data on the polyelectrolyte system at pH 2.5 and 5.5 because the SANS data show that micellar growth takes place during the binding and no measurements are available at the saturation concentration T2. As explained previously, the micellar growth is promoted by the increased stability of the bound micelles resulting from electrostatic interactions with the polymer. This electrostatic effect also explains the increase in aggregation number from 44 to 95 for a SDS concentration of 10 mM at pH 5.5 and 2.5, respectively. Finally, the agreement found between the measured amount of bound SDS using the electrode and the calculated bound micellar concentrations via the SANS modeling procedures in Table 6 is on the whole excellent and is testament to the philosophy that the use of more than one technique38 is a distinct advantage in the study of polymer/surfactant systems. LA010427Z (38) Holzwarth, J. F. Langmuir 2000, 16, 8549.
