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Langmuir 1999, 15, 1938-1944
The Interaction between Nonionic Dendrimers and SurfactantssElectromotive Force and Microcalorimetry Studies S. M. Ghoreishi, Y. Li, J. F. Holzwarth,† E. Khoshdel,‡ J. Warr,‡ D. M. Bloor, and E. Wyn-Jones* Division of Chemical Sciences, Science Research Institute, University of Salford, Salford, M5 4WT, U.K. Received August 13, 1998. In Final Form: December 31, 1998 The interaction between the dendrimers poly(1,4-diaminobutane) DABn (n ) 8, 16, 32, 64) and poly(pyrrolidone-1,4-diaminobutane) DABnPy (n ) 8, 16, 32) with the surfactants sodium dodecyl sulfate (SDS), tetradecyltrimethylammonium bromide (TTAB), and hexaethyleneglycol monododecyl ether (C12EO6) have been studied using electromotive force (EMF) measurements involving a surfactant-selective electrode and also isothermal titration microcalorimetry (ITC). The EMF data shows apparent binding of SDS to all the dendrimers at SDS concentrations lower than 10-5 mol dm-3. As more SDS is added, the binding process continues until the dendrimer becomes fully saturated with bound SDS, at which point the EMF and ITC data for SDS solutions with and without the dendrimers merge. At low SDS concentrations the binding mechanism is a noncooperative process driven by hydrophobic interactions between the hydrocarbon chains in the cavity of the dendrimer and also electrostatic attraction between the surfactant headgroups and the mildly cationic nitrogen atoms in the inner core of the dendrimer. As binding proceeds, there is a gradual transition to a cooperative binding process in which micellar-type bound SDS aggregates are formed on the dendrimers. This continues until the dendrimer can no longer bind any further surfactant which signals the occurrence of free regular SDS micelles in solution. The binding between SDS micelles and the dendrimers is driven by primary electrostatic interactions which also promote stable micellar bound SDS aggregates. C12EO6 shows a limited amount of binding to some of the dendrimers, although the binding is noncooperative, driven by hydrophobic interactions between the surfactant and the internal cavity of the dendrimer. TTABr was only found to show a very small interaction with DAB16 at a high dendrimer concentration.
Introduction Dendrimer is the name of a new family of highly branched polymers that are synthesized in successive generations from a central core. This results in macromolecules with well-defined branches, very specific molecular masses, and uniform sizes.1,2 An important difference between linear polymers and dendrimers is that a linear polymer consists of an entanglement of random coil single molecular chains. A dendrimer, by contrast, is better defined in possessing a central core with branches that radiate out to a peripheral surface, giving rise to a very high number of terminal functional groups in each molecule. Among the most remarkable features of the higher generation dendrimers are that they are nanoscale in size and shape. They closely match the sizes and contours of many important proteins and bioassemblies, and as such one of their most important applications relates to molecular recognition and their guest-host properties. The 2-fold nature of the properties of dendrimers involves an external periphery containing multiple functional groups that account for their solubility in aqueous media and an internal hydrophobic core which is large enough to contain small molecules.3-12 Indeed, * To whom correspondence should be addressed. † Fritz-Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany. ‡ Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral L63 3JW, U.K. (1) Tomalia, D. A.; Esfand, R. Chem. Ind. 1997, 416-420 (and references therein). (2) Zang, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681-1712. (3) Jansen, J. F. G. A.; de Brabander, E. M. M.; Meijer, E. W. Science 1994, 266, 1226-1229.
host-guest chemistry can take place either at the interior or at the periphery of the dendrimer.12,14 Surfactants make ideal guests for the host dendrimer because of the wide variety available in their chemical structures, allowing a systematic investigation. As far as we are aware, there is a paucity of data concerning the capacity of dendrimers to host surfactants with few reports available on the interaction of dendrimers and surfactants5,8-12 of opposite charges. In addition, a recent report describes the use of electron spin resonance (ESR)15 and fluorescence probes11 used to study interactions with neutral dendrimers. (4) Matter, S.; Seiler, P.; Drederick, F. Helv. Chim. Acta 1995, 78, 19014-19122. (5) Turro, C.; Niu, S.; Bossman, S. H.; Tomalia, D. A.; Turro, N. J. J. Phys. Chem. 1995, 99, 5512-5517. (6) Ottaviani, M. F.; Bossmann, S.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1994, 116, 661. (7) Ottaviani, M. F.; Montalti, F.; Turro, N. J.; Tomalia, D. A. J. Phys. Chem. 1996, 100, 11033. (8) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515. (9) Ottaviani, M. F.; Turro, N. J.; Jockusch, S.; Tomalia, D. A. J. Phys. Chem. 1996, 100, 13675. (10) Ottaviani, M. F.; Turro, N. J.; Jockusch, S.; Tomalia, D. A. Colloids Surf. 1996, 115, 9. (11) Ottaviani, M. F.; Andechaga, P.; Turro, N. J.; Tomalia, D. A. J. Phys. Chem. 1997, 101, 6057. (12) Watkins, D. M.; Sayed-Sweet, Y.; Klimash, J. W.; Turro, N. J.; Tomalia, D. A. Langmuir 1997, 13, 3136-3141. (13) Jansen, J. F. G. A.; de Brabander-van den Berg, E. M. M.; Meijer, E. W. Science 1994, 266, 1226. (14) Newkome, G. R.; Moorefield, C. N.; Baker, G. R.; Saunders: M. J.; Grossman, S. H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1178-1180. (15) Ottaviani, M. F.; Daddi, R.; Brustolon, M.; Turro, N. J.; Tomalia, D. A. Appl. Magn. Res. 1997, 13, 347.
10.1021/la981028f CCC: $18.00 © 1999 American Chemical Society Published on Web 03/16/1999
EMF and Microcalorimetry Studies Scheme 1
Langmuir, Vol. 15, No. 6, 1999 1939 evaporator, yielding a white gelatinous solid. This crude product is continually triturated with ether, extracted, and finally filtered to yield a white homogeneous free-flowing solid (isolated yield ∼ 80%). The purity of dendrimers were checked using mass spectrometry and 1H and 13C NMR spectroscopy. EMF Measurements. Surfactant membrane electrodes selective to sodium dodecylsulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) were constructed in the laboratory and used to determine monomer-surfactant concentrations by measuring their EMF relative to a commercial bromide, or chloride ion reference electrode as appropriate. The cells used for these measurements and the procedures to calculate the respective monomer concentrations have been described elsewhere.17-20 Isothermal Titration Calorimetry (ITC). 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 first titrating the micellar surfactant into water and then into an aqueous solution containing a known amount of dendrimer. An injection schedule (number of injections, volume of injection, and time between injections) is set up using interactive software and this schedule 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 surfactant concentration.21-23 Each ∆Hi value represents the sum of many contributions including heats of binding, heat of micelle formation, enthalpy of dilution, conformational or structural change in the macromolecule, and so forth. In practice, the major and dominating contributions arise from binding and to a lesser extent the formation of free micelles. In some cases it is preferable to plot out differential enthalpies.23a The measurements were taken at 298 K.
Results
In this work we have used EMF measurements using surfactant-selective electrodes and isothermal titration calorimetry to study the interaction of an anionic, cationic, and nonionic surfactant with poly(1,4-diaminobutane) (DABn) (n ) 8, 16, 32, 64, and poly(pyrrolidone-1,4diaminobutane) (DABnPy) (n ) 8, 16, 32). Typical structures of DAB16 and DAB16Py are shown in Scheme 1. The surfactants used were tetradecyltrimethylammonium bromide (TTAB), sodium dodecyl sulfate (SDS), and hexaethyleneglycol monododecyl ether (C12EO6). Experimental Section TTAB was a Sigma product purified by repeated recrystallization. SDS was synthesized according to the procedure described by Davidson17 and C12EO6 was a Nikkol product used without further purification. The generation 2-5 poly(1,4diaminobutane), DABn, dendrimers were commercial samples supplied by DSM16 and used without further purification. Each generation of the poly(pyrrolidone-1,4-diaminobutane) dendrimers were synthesized from the corresponding generation of DABn dendrimers using the following procedure. L-Pyroglutamic acid (6 g) is dissolved in dimethylformamide (DMF) (25 mL), and to this solution is added 1,3-dicyclohexylcarbodiimide (9.5 g) in DMF (25 mL) at room temperature. The reaction mixture is stirred for 2 h at room temperature and then filtered. The filtrate is added slowly and dropwise to the DABn dendrimers (n ) 8, 16, 32) (5 g) and stirred vigorously for 24 h at room temperature. All solvents are removed using a rotary (16) O’Sullivan, D. A. Chem. Eng. News 1993, Aug 16, 20-22. (17) Davidson, C. S. Ph.D. Thesis, University of Aberdeen, 1983.
Both the EMF and ITC experiments were carried out in a similar way such that concentrated surfactant was titrated into an aqueous solution containing a constant amount of dendrimer. After each addition the cell EMF was measured in the electrode experiment and the enthalpy per injection (∆Hi) in the ITC experiment. The respective EMF and ∆Hi values are then plotted as a function of SDS concentration for the solutions with and without the dendrimer. A selection of EMF and ITC data are displayed in Figures 1-3 and 5-10 for the dendrimers DABnPy (n ) 8, 16, 32) and DABn (n ) 8, 16, 32) with the surfactants SDS, C12EO6, and TTAB (no EMF data can be taken with the nonionic C12EO6). Binding is taking place when the EMF and ∆Hi values are different for each corresponding titration for the solutions with and without dendrimer. All measurements were carried out in 10-1 and 10-4 mol dm-3 of NaCl with the exception of the EMF measurements for TTAB which were carried out in 10-4 mol dm-3 of NaBr. (a) SDS/Dendrimer Interaction. EMF Data (0.1 mol dm-3 of NaCl). The EMF data for pure SDS using the surfactant electrode measured relative to a commercial chloride ion-selective electrode is shown in Figure 1. The (18) Wan Badhi, W. A.; Wan Junus, W. M. Z.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1993, 89, 2737-2742. (19) Painter, D. M.; Bloor, D. M.; Takasawa, N.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2087. (20) Mwakibete, H. K. O.; Bloor, D. M.; Wyn-Jones, E. J. Colloid Interface Sci. 1996, 178, 335. (21) Bloor, D. M.; Wan Yunus, W. M. Z.; Wan Badhi, W. A.; Li, Y.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (22) Bloor, D. M.; Li, Y.; Wyn-Jones, E. Langmuir 1995, 11, 3778. (23) Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 2312.(a) Fox, G. J.; Bloor, D. M.; Holzwarth, J.; Wyn-Jones, E. Langmuir 1998, 14, 1026.
1940 Langmuir, Vol. 15, No. 6, 1999
Figure 1. Plot of the EMF of the SDS electrode (reference Cl-) as a function of total SDS concentration in NaCl (0.1 mol dm-3) for (9) pure SDS, (2) SDS + DAB8Py (0.05% w/v), and (b) SDS + DAB32 (0.05% w/v).
EMF is plotted as a function of measured SDS concentration in the presence of NaCl (0.1 mol dm-3). The EMF data can be divided into three regions. Region 1 is below an SDS concentration of 3 × 10-6 mol dm-3. The EMF is constant, showing that the electrode does not respond to changes in SDS concentration in this region. Region 2 is above an SDS concentration of 1 × 10-5 mol dm-3 . The electrode response is Nernstian with a slope of -58 mV/decade. Region 3 is at the end of the Nernstian region following a sharp break in the EMF corresponding to the formation of SDS micelles. As further SDS is added in the micellar region, the EMF of the electrode remains almost constant. The SDS concentration of 1 × 10-5 mol dm-3 at the lower end of region 2 corresponds to a cutoff in the Nernstian electrode response and the useful working range of the electrode. The almost constant value for the EMF in the micellar region 3 shows that the monomer SDS concentration is also constant in this region as is to be expected in the presence of 0.1 mol dm-3 of salt. The EMF response of the electrode in the presence of a constant amount of the dendrimers DAB8Py (0.05% w/v) and DAB32 (0.05% w/v) are also shown in Figure 1. In region 1 the EMF data with and without the dendrimer overlap as expected. However, once the cutoff point is reached and further SDS is added, the EMF of the dendrimer/SDS solution starts diverging from the EMF of SDS alone, showing that binding of SDS to the dendrimer takes place. As further SDS is added, the binding process proceeds until eventually the dendrimer is unable to bind any more SDS at which point the EMF data for the electrode with and without the dendrimer merge again and remain the same as more SDS is added. During the initial stages of binding the EMF data for the dendrimer/SDS system changes gradually with increasing SDS concentration. This is then followed by a very distinctive increase in the slope of the EMF/log[SDS] plot as the EMF data approach and merge with the corresponding measurements in the absence of the dendrimer. This critical concentration (denoted T2) is accompanied by a sharp break in the EMF as shown in Figure 1. EMF Data (1 × 10-4 mol dm-3 of NaCl). The above series of measurements have also been carried out for the dendrimers in the presence of 1 × 10-4 mol dm-3 of NaCl as shown in Figure 2. Note here that the response limit
Ghoreishi et al.
Figure 2. Plot of the EMF of the SDS electrode (reference Cl-) as a function of total SDS concentration in NaCl (1 × 10-4 mol dm-3) for (9) pure SDS and ([) SDS + DAB64 (0.05% w/v).
(region 1) for the electrode also occurs at an SDS concentration of 1 × 10-5 mol dm-3 (not shown). The EMF data for the DAB series in 10-4 mol dm-3 of NaCl initially shows anomalous behavior at SDS concentrations above the response limit where the EMF data for the dendrimer solutions are surprisingly found to be less than the corresponding values for the dendrimer-free solution. The EMF data eventually cross over after which they appear to display a normal type of behavior. We consider that the electrode data for these DAB dendrimers just above the crossover point are unreliable because at low SDS concentrations the DAB dendrimers adsorb onto the poly(vinyl chloride) (PVC) membrane of the electrode, thus affecting its performance. As the solution becomes richer in SDS, the surfactant will eventually solubilize the PVCadsorbed dendrimers, causing them to desorb into bulk solution, giving rise to normal electrode behavior as is observed at the higher SDS concentrations. This type of behavior has also been observed in previous studies in this laboratory for some linear chain polymers.24 These restrictions in the behavior of the electrode place different limits on this low SDS working range; however, in the reliable working range of the electrode close to T2, the EMF data with and without dendrimer are always different until they merge in the micellar range. We therefore consider the values for T2 to be reliable at the expense of the data in the lower SDS concentration range. ITC Data (0.1 mol dm-3 of NaCl). In the ITC experiment the enthalpy per injection (∆Hi) data (Figure 3) for SDS in the absence of dendrimer shows the expected profile at high ionic strength with a characteristic step change at the critical micelle concentation (cmc). In the presence of dendrimer ∆Hi decreases, showing considerable interaction between the surfactant and polymer. In addition, differing ∆Hi profiles are obtained, depending on the type of dendrimer used in the experiment: (a) DAB32 Dendrimer. A profile is observed which shows similar characteristics to that for pure SDS. A step change is observed corresponding to an cooperative aggregation phenomenon similar to micellization occurring at a SDS concentration slightly in excess of the cmc of the pure surfactant. (b) DAB8Py Dendrimer. A profile is observed which shows little similarity to pure SDS. Although a minimum in ∆Hi is observed, it displays a less severe curvature when compared to SDS or SDS/DAB32. (24) Ghoreishi, S. M. Ph.D. Thesis, University of Salford, Salford U.K., 1998.
EMF and Microcalorimetry Studies
Figure 3. Plot of ∆Hi in the ITC experiment as a function of total SDS concentration in NaCl (0.1 mol dm-3) for (9) pure SDS, (2) SDS + DAB8Py (0.05% w/v), and (b) SDS + DAB32 (0.05% w/v).
As expected for both types of dendrimers, at high concentrations of SDS above T2, the ∆Hi values become equal to that for the pure surfactant. Critical Concentrations in the Binding Process (0.1 mol dm-3 of NaCl). (a) Low SDS Concentration Binding Limit. The EMF data show the DABn dendrimers to have a lower binding limit for SDS (T1) in region 2 of the order 10-5 f 10-4 mol dm-3. However, it would appear that binding in the case of the DABnPy dendrimers extends into region 1 below the response limit of the electrode. At these initial stages of binding it is clear from the weighed-in concentrations and EMF data that the molar dendrimer concentration far exceeds the number of moles of bound SDS despite the fact that over these concentrations the dendrimer shows some adsorption onto the PVC membrane of the electrode. (b) T2, the SDS Concentration at Which the Dendrimers Are Saturated with Bound Surfactant. This point is reached when the ITC and the EMF data for the solutions with and without dendrimer merge. At SDS concentrations in excess of T2, the dendrimer is no longer involved in any further interaction with SDS. This is not an easy concentration to pinpoint accurately from the experimental data since ITC and EMF data for the solutions containing dendrimer become asymptotic to the data in the absence of polymer. As a result, T2 must be regarded very much in the same way as the cmc of a surfactant in that it takes place over a narrow range of surfactant concentrations. The values of T2 estimated from EMF and ITC data are in Tables 1 and 2, showing reasonably good agreement. (c) Tf, the SDS Concentration at Which Free Micelles Occur in Solution. The variation of the monomer SDS concentration with the overall SDS concentration is a very useful guide which may be used to distinguish between different mechanisms of binding and/or SDS aggregation. In the present work the monomer concentration of SDS, denoted m1, has been evaluated from the electrode data and is shown as a function of total SDS concentration, C1, in Figure 4. An increase in m1 with increasing C1 is the behavior expected when SDS exclusively binds to a macromolecule. When m1 decreases (in 10-4 mol dm-3 of NaCl) or remains almost constant (in 0.1 mol dm-3 of NaCl) with increasing C1, it signifies the onset of the formation of free micelles. Furthermore, when the maximum in m1 (denoted Tf) and T2 occur at the same total SDS concentration, then the polymer becomes fully saturated with bound SDS before free micelles occur in solution. At SDS
Langmuir, Vol. 15, No. 6, 1999 1941
concentrations exceeding T2 the EMFs and monomer concentrations for solutions with and without the dendrimer are the same. The values of T2 and Tf have been found to be the same for all systems studied (Tables 1 and 2), showing that all the dendrimers become fully saturated with bound SDS before free micelles occur in solution. (d) Maximum Binding Capacity of the Dendrimers. The values of T2-m1 where m1 is the monomer SDS concentration at T2 are listed in Tables 1 and 2. These values represent the maximum amount of SDS that the dendrimers can bind. At SDS concentrations approaching T2 the complexes involve one dendrimer containing 20-190 bound monomers of SDS, depending on the generation and type of dendrimer. (b) Cationic/Surfactant Dendrimer Interactions. The EMF and ITC data for solutions of TTAB with 0.05% w/v DAB16Py dendrimers are displayed in Figure 5 and clearly indicate that no interaction occurs between the dendrimer and this cationic surfactant. Only when additional EMF measurements were carried out between the DAB16 dendrimer at a much higher concentration of 0.25% w/v and TTAB that a very limited amount of binding was observed (Figure 6). At this concentration of dendrimer the onset of binding occurs at a total concentration of TTAB, approximately 5 × 10-4 mol dm-3 with no observable T2. The EMF data also show that the maximum in m1 occurs at 3 × 10-3 mol dm-3 of TTAB, indicating that free micelles start occurring in solution. When TTAB in excess of 3 × 10-3 mol dm-3 is added to the solution, it is likely that both free micelles are formed with possibly a limited amount of further binding taking place. At the onset of binding the molar ratio of dendrimer to bound TTAB monomers is 15:1 and at the onset of the formation of free micelles this ratio is approximately 1.5:1, clearly showing that only a very limited amount of binding is taking place which can be regarded as negligible in comparison to SDS. (c) Nonionic Surfactant/Dendrimer Interaction. The ITC data (Figure 7) clearly show that no interaction takes place between C12EO6 and the higher generation dendrimers DAB32 and DAB64 or over all generations of the DABnPy dendrimers. However, the data for the lowgeneration DABn dendrimers show a small amount of binding taking place, increasing from DAB8 to DAB16. Evidence has also been found from competitive binding studies using dye-nonionic surfactant systems that some degree of binding takes place between nonionic surfactants and the DAB low-generation dendrimers. Discussion The EMF (SDS monomer) and ITC data for the SDS/ dendrimer systems are summarized in Figures 8-10. Comparison of the data in this way provides further insight into the SDS/dendrimer binding process. Comparison of the ITC data for all dendrimers shows a decrease in the total SDS concentration at which the onset of a cooperative aggregation process (as shown by the position of the initial minimum in the data) involving the SDS occurs as the dendrimer generation increases. A comparable study of starburst dendrimers and SDS also report the existence of bound micellar aggregates.15 The EMF data over this range however in some cases show no observable change. This may be attributed to dendrimer adsorbed onto the PVC membrane of the electrode, preventing its correct operation. In fact, little change is observed in the dependency of the SDS monomer on total SDS concentration as the dendrimer generation is increased. When T2 is reached and the dendrimer is saturated with surfactant,
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Table 1. Summary of the Data for the Interaction between SDS and Dendrimers in 0.1 mol dm-3 of NaCl (m1 ) 1.3 × 10-3 mol dm-3 at T2 for All Systems)a dendrimer (D)
MW (D)
DAB8 DAB16
762 1684
DAB32 DAB64 DAB8Py DAB16Py DAB32Py
3508 7220 2106 3460 7060
conc (D) T2 [mol dm-3] T2 [mol dm-3] (T2-m1) [mol dm-3] [SDS]/[D] at T2 [SDS]/[D] at T2 [% w/v] ITC data EMF data EMF data [mol/g] [mol/mol] [N]/[D] [SDS]/[N] 0.05 0.03 0.05 0.1 0.05 0.05 0.05 0.03 0.05 0.1 0.05
0.023 0.020 0.030 0.011 0.008 0.01 0.006 0.025 0.007
0.018 0.007 0.013 0.028 0.007 0.008 0.007 0.004 0.008 0.013 0.007
0.0167 0.0057 0.0117 0.0267 0.0057 0.0067 0.0057 0.0027 0.0067 0.0117 0.0057
0.0334 0.019 0.0234 0.0267 0.0114 0.0134 0.0114 0.009 0.0134 0.0117 0.0114
25 32 39 45 40 97 24 31 46 40 80
14 30 30 30 62 126 14 30 30 30 62
1.79 1.07 1.3 1.5 0.65 0.77 1.71 1.03 1.53 1.33 1.29
[N] refers to the concentration of repeat units CH2CH2CH2N. [SDS] refers to the concentration of bound SDS ()T2-m1) from EMF data. a
Table 2. Summary of the Interaction between SDS and Dendrimers in 0.0001 mol dm-3 of NaCl (m1 ) 6.0 × 10-3 mol dm-3 at T2 for All Systems)a dendrimer MW conc (D) T2 [mol dm-3 ] T2 ()Tf) [mol dm-3] (T2-m1) [mol dm-3] [SDS]/[D] at T2 [SDS]/[D] at T2 (D) (D) [% w/v] ITC data EMF data EMF data [mol/gm] [mol/mol] [N]/[D] [SDS]/[N] DAB8 DAB16 DAB32 DAB64 DAB8Py DAB16Py DAB32Py
762 1684 3508 7220 2106 3460 7060
0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.025 0.023 0.018 0.016
0.019 0.019 0.017 0.019 0.013 0.013 0.012
0.013 0.013 0.012 0.013 0.007 0.007 0.006
0.026 0.026 0.024 0.026 0.014 0.014 0.012
20 44 84 188 29 48 85
14 30 62 126 14 30 62
1.43 1.47 1.35 1.49 2.07 1.6 1.37
a [N] refers to the concentration of of repeat units CH CH CH N. [SDS] refers to the concentration of bound SDS ()T -m ) from EMF 2 2 2 2 1 data.
Figure 4. Plot of SDS monomer concentration (m1) as a function of total SDS concentration for the (9) SDS/DAB64 (0.05% w/v) + NaCl (1 × 10-4 mol dm-3) system and the (2) SDS/DAB16Py (0.05% w/v) + NaCl (0.1 mol dm-3) system.
the measured monomer concentration may be considered to be reliable. The high binding ratios found for SDS concentrations approaching T2 clearly indicate that exceptional binding occurs between the dendrimers and SDS. In addition, the highly cooperative nature of the binding process at these concentrations suggests that the complexes involve dendrimers bound to SDS micelles. Under these conditions, geometric considerations involving such a high concentration of bound SDS must dictate that the bound surfactant exists as micellar-type aggregates. Furthermore, if the aggregation numbers are of the usual order for SDS micelles (∼60), by reference to Table 2 the complexes between SDS and the dendrimers DAB8 and DAB8Py at the saturation limit (T2), involve three dendrimers bound to one SDS micelle. On the other hand, the DAB64/SDS complexes at T2 involve two to three SDS micelles associated with one dendrimer. It may very well
Figure 5. Plot of the EMF data (in 1 × 10-4 mol dm-3 of NaBr) and ITC data (in 0.1 mol dm-3 of NaCl) as a function of the total TTAB concentration for (s) pure TTAB and (9 EMF, 4 ITC) TTAB + DAB16Py (0.05% w/v).
be that the dendrimer-bound micelles have different aggregation numberssthis is a matter we are currently addressing using neutron scattering.25 All measurements were conducted at the natural pH of the solutions. For example, in the SDS/DAB64 system the pH at completion of the experiment is ∼7.5. As far as we are aware, detailed knowledge of the pKa values of the repeat nitrogens in the interior and those on the periphery are not known. It is clear from the data that the strong binding observed for SDS and the absence of binding for the cationic surfactant indicate that at a low salt concentration electrostatic attractions dominate in the mechanism binding for SDS. In these circumstances it is clear that some of the N atoms are positively charged. From a further examination of the data in Tables 1 and 2 more (25) Ghoreishi, S. M.; Bloor, D. M.; Penfold, J.; Wyn-Jones, E. (Experimental results currently being processed).
EMF and Microcalorimetry Studies
Langmuir, Vol. 15, No. 6, 1999 1943
Figure 6. Plot of the EMF of the TTAB electrode (reference Na+) as a function of total TTAB concentration for the TTAB/ DAB16 system: (9) pure TTAB and (2) TTAB + DAB16 (0.25% w/v).
Figure 8. Plot of ∆Hi in the ITC experiment and the monomer concentration m1 as a function of the total SDS concentration in NaCl (1 × 10-4 mol dm-3) for all the DABnn dendrimers studied. Arrows indicate T2 from ITC data.
Figure 7. Plot of ∆Hi in the ITC experiment as a function of the total C12EO6 concentration for (s) pure C12EO6, (b) C12EO6 + DAB8, (2) C12EO6 + DAB16, and (4) C12EO6 + DAB32. (O) C12EO6 + DAB64. Inset: (s) pure C12EO6, (b) C12EO6 + DAB8Py, and (9) C12EO6 + DAB32Py. Concentration of all dendrimers ) 0.05% w/v.
detailed information concerning the mode of SDS binding to the dendrimers may be obtained. It is straightforward to calculate the concentration of bound SDS ([SDS]) per mole repeat unit of the dendrimer ([N]), as shown in the final column of each table ([SDS]/[N]). This has a constant value (∼1.4) for the DABn dendrimers in 10-4 mol dm-3 of salt, showing that the binding is of an ionic nature where the cationic nitrogen in the dendrimer interior is easily accessible to the anionic surfactant. This is not the case with the DABnPy dendrimers because of the presence of the pyrrolidone structure on the dendrimer periphery. Indeed, preliminary molecular modeling computations involving a search for minimum energy conformations using MSI software Insight 11 suggest that the dendrimers DABn have a stable planar minimum energy conformation while the DABnPy dendrimers do not.26 In the presence of 10-1 mol dm-3 of salt, ionic interactions are reduced, allowing hydrophobic interactions to dominate. The increased ionic strength has little or no effect on the interaction of SDS with DABnPy dendrimers as shown by the consistency in the values of [SDS]/[N]. This we believe is due to the presence of the pyrrolidone terminal groups of the dendrimer, preventing penetration (26) Warr, J.; Paul, P. Unpublished results.
Figure 9. Plot of ∆Hi in the ITC experiment and the monomer concentration m1 as a function of the total SDS concentration in NaCl (0.1 mol dm-3) for all the DABnn dendrimers studied. Arrows indicate T2 from ITC data.
of the SDS into the dendrimer core. On the other hand, under such circumstances the SDS/DABn systems show a strong dependency on the dendrimer generation, where values of n ) 32 or 64 show a reduction by a factor of 2 in [SDS]/[N] at saturation compared with that found at low ionic strength. If we compare this result with that obtained for the nonionic C12EO6/DABn systems, we find a similar effect with only the lower generation dendrimers n ) 8 or 16, showing any interaction with the surfactant.
1944 Langmuir, Vol. 15, No. 6, 1999
Figure 10. Plot of ∆Hi in the ITC experiment and the monomer concentration m1 as a function of the total SDS concentration in NaCl (0.1 mol dm-3) for all the DABnnPy dendrimers studied. Arrows indicate T2 from ITC data.
It is therefore clear that the lower generation dendrimers have hydrophobic regions exposed to the aqueous solvent which cannot be avoided by conformational rearrangement. An anionic surfactant (in high salt) or nonionic surfactant is able to bind to these internal regions, resulting in a complex with a higher hydrophilic surface, interfacing with the aqueous solvent.12 As the generation number increases (n ) 32 or 64), the ability of the dendrimer to undergo conformational changes means that it can avoid exposing hydrophobic regions, and the binding of surfactant is reduced (in the anionic case) or lost completely (in the nonionic case). A similar interpretation has been made in the field of proteins27 where there seems to be a critical limit to the number of amino acids needed to form a globular protein. At present, we do not have sufficient data to make constructive comments on how the variation in dendrimer concentration affects the bindingsthe data available at present are in Table 1. Conclusions The high degree of selectivity shown by the dendrimers to SDS must be associated with a more favorable and different selective binding mechanism which exclusively promotes intermolecular attraction between SDS and the dendrimer as well as the formation of bound aggregates below the cmc of SDS. The recognition of the dendrimer by surfactant following the order anionic . nonionic . cationic surfactant suggests that this primary driving force (27) Matthews, B. W.; Craik, C. S.; Neurath, H. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 4103.
Ghoreishi et al.
is electrostatic and probably arises because of the moderate net positive charge on the nitrogen atom in the dendrimer. This situation is very similar to that found in comparable studies of SDS with poly(vinylpyrrolidone) (PVP), polyvinylimidazole, and also some vinyl copolymers. In this situation the primary force promoting binding is the electrostatic attraction between the negative charge on the surfactant and the “cationic” nature of the polymer side group. This leads to the formation of polymer-bound SDS aggregates in which the polymer, because of its conformational flexibility, can wrap itself around the surface of the SDS aggregates in such a way that a beadlike necklace structure is formed for the complex. In terms of their maximum binding capacity at T2 a dendrimer can bind more than twice the amount of SDS per gram than any linear polymer under the same conditions. A more interesting comparison can be made if we compare the binding data for SDS to the DABnPy dendrimers with that for PVP in 0.1 mol dm-3 of NaCl at T2. The terminal groups of the dendrimers and the side chains of the PVP both contain pyrrolidone rings, and in these circumstances we can compare the amount of SDS bound per pyrrolidone group. The EMF data show that the dendrimers have superior binding ability with two to four SDS monomers bound per pyrrolidone group compared to one to two for PVP. The PVP/SDS complex resembles a polyelectrolyte since the electrostatic repulsion between bound aggregates extend the chain, thus preventing the functional groups in the links between the aggregate being available for binding. On the other hand, the functional groups of the dendrimers are more amenable and available to the headgroup on the surface of the SDS aggregate. The binding process for SDS with the DABnPy dendrimers would therefore appear to be dominated by the presence of the terminal pyrrolidone rings, while in the case of the DABn we expect a much greater penetration into the dendrimer core. Indeed, a noncooperative binding process was recently observed in a study11 of poly(amidoamine) dendrimers generated from a diaminododecane core and anionic surfactants, using the dye nile red as a probe. The binding process was thought to occur in this case as a result of the dendrimer interior acting as a host to the surfactant. It was also concluded that the functional groups of the larger generation dendrimers are in such close proximity that they effectively block access to large guest molecules or ions. In the present work involving SDS and dendrimers the binding process having low cooperativity or noncooperativity occurs for all generations of dendrimers. Clearly, the mode of formation of the dendrimer SDS complexes and also their structure at these low concentrations need further investigation. This is a matter which we intend to turn our intention to in the future using neutronscattering studies. Acknowledgment. S.M.G. wishes to thank The Islamic Republic of Iran for a maintenance award and Y. Li is grateful to the Fritz-Haber Institute of the Max Planck Gesellschaft, Berlin for a visiting fellowship. LA981028F
