Binding of Sodium Dodecyl Sulfate to Linear and Star Homopolymers

6458

Langmuir 2004, 20, 6458-6469

Binding of Sodium Dodecyl Sulfate to Linear and Star Homopolymers of the Nonionic Poly(methoxyhexa(ethylene glycol) methacrylate) and the Polycation Poly(2-(dimethylamino)ethyl methacrylate): Electromotive Force, Isothermal Titration Calorimetry, Surface Tension, and Small-Angle Neutron Scattering Measurements S. Couderc-Azouani,† J. Sidhu,‡ T. K. Georgiou,§ D. C. Charalambous,§ M. Vamvakaki,§ C. S. Patrickios,§ D. M. Bloor,‡,| J. Penfold,⊥ J. F. Holzwarth,*,† and E. Wyn-Jones*,†,‡ School of Chemical Sciences, Science Research Institute, University of Salford, Salford, M5 4WT, U.K., Fritz-Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany, Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus, and ISIS Facility, Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire, OX11 O QX, U.K. Received March 2, 2004. In Final Form: May 7, 2004 We investigated the binding of sodium dodecyl sulfate (SDS) to various linear and star polymers of the nonionic methoxyhexa(ethylene glycol) methacrylate (PMHEGMA) and the ionic 2-(dimethylamino)ethyl methacrylate (PDMAEMA), the latter being a polycation at low pH. The dodecyl sulfate ion selective electrode (EMF), isothermal titration calorimetry (ITC), and surface tension (ST) were applied to gain detailed information about interactions. In all cases there is evidence of significant binding of SDS over an extensive SDS concentration range spanning from ca. 10-6 to 0.1 mol dm-3. At pH 3, the polymer PDMAEMA is a strong polycation and here the binding is dominated by electrostatic 1:1 charge neutralization with the anionic surfactant. At their natural pH of 8.6, PMHEGMA and PDMAEMA polymers are essentially nonionic and bind SDS in the form of polymer-bound aggregates in the concentration range of ca. 1 × 10-3 to 3 × 10-2 mol dm-3. All the polymers also bind SDS to a lesser extent at concentrations below 1 × 10-3 mol dm-3 reaching as low as 10-7 mol dm-3. This low concentration binding process involves the polymer and nonassociated SDS monomers. As far as we are aware, this is the first example that such a low concentration noncooperative binding process could be observed in SDS/neutral polymer systems by EMF and ST. We also showed that the nonionic surfactant hexa(ethylene glycol) mono-n-dodecyl ether (C12EO6) and the cationic cetyltrimethylammonium bromide (C16TAB) interact with star PDMAEMA. We believe that the interaction of C12EO6 and CTAB is of similar noncooperative type as the first SDS binding process in the range from ca. 10-5 to 0.3 × 10-3 mol dm-3. At the high concentration binding limit Csat of SDS, the above polymers become fully saturated with bound SDS micelles. We applied small angle neutron scattering (SANS) to determine the structure and aggregation numbers of the star polymer/bound SDS micelles and calculated the stoichiometry of such supramolecular complexes. The SANS data on PDMAEMA star polymers in the presence of C12EO6 showed only a limited monomer binding in contrast to linear PDMAEMA, which showed monomer C12EO6 binding at low concentrations but micellar aggregates at 6 × 10-3 mol dm-3.

Introduction Mixtures of polymers and surfactants are applied in a diverse range of applications using various formulations and colloidal dispersions of industrial importance.1 As a result, a great deal of fundamental investigations have been generated to study the interaction between different * To whom correspondence may be addressed at Physical Chemistry, Fritz-Haber-Institut, Faradayweg 4-6, D-14195 Berlin, Germany. Tel: +49 30 8413 55 16. Fax: +49 30 8413 53 85. E-mail: [email protected]. † Fritz-Haber Institut der Max-Planck Gesellschaft. ‡ University of Salford. § University of Cyprus. | Deceased. ⊥ ISIS Facility, Rutherford Appleton Laboratory. (1) Goddard, E. D. Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993.

polymers and surfactants.1-5 In these studies many different types of experimental approaches were used. A key prerequisite to any experimental study is a binding isotherm which provides precise analytical information on the amount of free and bound surfactant during various stages in the binding process. This type of information is vital in underpinning many other experiments which monitor, directly or indirectly, various solution properties and physical parameters which are affected during the binding. For the most frequently studied systems, ionic (2) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22, 85. (3) Polymer-Surfactant systems; Kwak, J. C. T., Ed.; Surfactant Science Series vol. 77; Marcel Dekker: New York, 1998. (4) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604. (5) 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.

10.1021/la049450l CCC: $27.50 © 2004 American Chemical Society Published on Web 06/25/2004

Binding Surfactant to Linear and Star Polymers

Langmuir, Vol. 20, No. 15, 2004 6459

Table 1. Molecular Mass (MM) and Number of Arms of Star and Linear Homopolymers of 2-(Dimethylamino)ethyl Methacrylate (DMAEMA) and of Methoxyhexa(ethylene glycol) Methacrylate (MHEGMA)

polymer

structure

ΜM from gel permeation chromatography

PDMAEMA-30

linear star linear star linear star linear star linear linear linear star linear

5 500 70 000 12 000 150 000 23 200 780 000 28 200 800 000 57 300 91 000 6 200 14 000 6 700

PDMAEMA-50 PDMAEMA-70 PDMAEMA-100 PDMAEMA-200 PDMAEMA-500 PMHEGMA-9 PMHEGMA-15

surfactants, notably sodium dodecyl sulfate (SDS), and various polymers, we developed the use of the surfactant selective electrode (EMF) to measure binding isotherms.6-16 We also applied these binding data to complement other experiments, notably isothermal titration calorimetry (ITC) and surface tension17 (ST) which also provide important information concerning surfactant binding. From the combined data we can construct a profile of binding and focus on various transitions and critical features that occur during the binding process, e.g., (i) binding modes such as charge neutralization of a polyion with an oppositely charged surfactant, (ii) hydrophobically driven cooperative formation of bound aggregates below the critical micelle concentration (cmc) of the pure surfactant, (iii) the binding capacity of a polymer expressed in moles of bound surfactant per gram of polymer, and (iv) the onset of the formation of “free” surfactant micelles. Furthermore, changes in pH, where appropriate, and the addition of electrolyte to screen electrostatic interactions can help to provide additional detailed information on the system. It is now generally accepted that bound surfactants exist as micellar aggregates during interaction with all neutral polymers and also in the later cooperative stages of binding after charge compensation when an ionic surfactant binds to an oppositely charged polyion. When bound micelles are formed on the polymer chain, small(6) (a) Painter, D. M.; Bloor, D. M.; Takisawa, N.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2087. (b) WanBadhi, W. A.; Wan-Yunus, W. M. Z.; Bloor, D. M.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1993, 89, 2737. (7) Li, Y.; McMillan, C. A.; Bloor, D. M.; Penfold, J.; Warr, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 7999. (8) Li, Y.; Xu, R.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 8677. (9) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 3093. (10) Ghoreishi, S. M.; Li, Y.; Holzwarth, J. F.; Khoshdel, E.; Warr, J.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1999, 15, 1938. (11) Ghoreishi, S. M.; Fox, G. A.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 1999, 15, 5474 and references quoted therein. (12) Ghoreishi, S. M.; Li, Y.; Bloor, D. M.; Warr, J.; Wyn-Jones, E. Langmuir 1999, 15, 4380. (13) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 6326. (14) Li, Y.; Xu, R.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 10515. (15) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Wyn-Jones, E.; Holzwarth, J. F. Langmuir 2001, 17, 183. (16) (a) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2001, 17, 5742. (b) Li, Y.; Xu, R.; Couderc, S.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2003, 19, 2026. (17) (a) Thurn, T.; Couderc, S.; Sidhu, J.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2002, 18, 9267. (b) Kositza, M. J.; Rees, G. D.; Holzwarth, A.; Holzwarth, J. F. Langmuir 2000, 16, 9035. (c) Thurn, T.; Couderc-Azouani, S.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2003, 19, 436. (d) Petit, C.; Holzwarth, J. F.; Pile´ni, M. P. Langmuir 1995, 11, 2405.

ΜM from static light scattering

no. of arms

pKa

∼100 000

13-17

6.7

∼200 000

11-20

6.9

∼1 000 000

30-40

7.0

∼1 200 000

35-45

7.0

angle neutron scattering (SANS) can provide information concerning the structure of the surfactant aggregates. When SANS is combined with EMF data from surfactant selective electrodes near the binding limit, it is possible to determine the “average” stoichiometry of some supramolecular polymer/surfactant complexes.7,18 In favorable cases, SANS measurements can also detect noncooperative surfactant binding, as will be shown for C12EO6. In this work we report EMF, ITC, ST, and SANS measurements on systems involving SDS binding to linear and star homopolymers of methoxyhexa(ethylene glycol) methacrylate (PMHEGMA) and linear as well as star homopolymers of 2-(dimethylamino)ethyl methacrylate (PDMAEMA). The PMHEGMA polymers are nonionic whereas PDMAEMA is cationic below pH 7, but at its natural pH of ∼8, it has a negligible charge density. The PDMAEMA polymers studied have different molecular mass, and the various star polymers studied carry different numbers of arms which allows selective studies to be performed on their binding characteristics. Spot measurements were carried out on the cationic cetyltrimethylammonium bromide (C16TAB) and the nonionic hexa(ethylene glycol) mono-n-dodecyl ether (C12EO6) surfactants to examine their affinity for the polymers. The systematic variation of the polymer type from linear to star and from neutral to charged should result in a detailed binding picture. Experimental Section The linear and star polymers used were synthesized and supplied by one of us (Patrickios et al.) according to the procedures described in recent reports.19-23 The polymers investigated and their essential characteristics are listed in Table 1. We studied linear and star polymers of methoxyhexa(ethylene glycol) methacrylate (MHEGMA) and 2-(dimethylamino)ethyl methacrylate (DMAEMA). The linear polymers were synthesized and supplied with various degrees of polymerization (DP) and denoted, e.g., PDMAEMA-100 linear (DP ) 100). The star polymers contained an ethylene glycol dimethacrylate (EGDMA) core which was used to covalently link one end of several linear polymers of MHEGMA (18) (a) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Warr, J.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2001, 17, 5657. (b) Sidhu, J.; Bloor, D. M.; Couderc-Azouani, S.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Submitted for publication in Langmuir. (19) Vamvakaki, M.; Patrickios, C. S. Chem. Mater. 2002, 14, 1630. (20) Vamvakaki, M.; Hadjiyannakou, S. C.; Loizidou, E.; Patrickios, C. S.; Armes, S. P.; Billingham, N. C. Chem. Mater. 2001, 13, 4738. (21) Simmons, M. R.; Yamasaki, E. N.; Patrickios, C. S. Polymer 2000, 41, 8523. (22) Georgiades, S. N.; Vamvakaki, M.; Patrickios, C. S. Macromolecules 2002, 35, 4903. (23) Vamvakaki, M.; Yamasaki, E. N.; Hadjiyannakou, S. C.; Patrickios, C. S. Macromol. Symp. 2001, 171, 209.

6460

Langmuir, Vol. 20, No. 15, 2004

Couderc-Azouani et al.

Chart 1. Molecular Structure of Some Monomers and a Nonionic Star Polymer

or DMAEMA all having the same DP and arranged in a starlike configuration as illustrated in Chart 1 which also lists structures of the above monomers. The star polymers are characterized by the DP of the arms and by the number of arms. The surfactants used were sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (C16TAB) purchased from Sigma and hexa(ethylene glycol) mono-n-dodecyl ether (C12EO6) purchased from Fluka. SDS was recrystallized three times in pure ethanol before application. The binding data for the star and linear PDMAEMA were measured at pH 3 and at their natural pH. All the experiments were performed at 298 K. The experimental methods applied for the binding studies were as follows: Electromotive Force (EMF) Measurements. A new coated wire surfactant membrane electrode24 selective to DS- was constructed at Salford and used to determine monomer surfactant concentrations below and above the surfactant cmc by measuring the EMF relative to a commercial silver/bromide ion reference electrode. The EMF data monitor the monomer surfactant concentration (m1) as a function of total surfactant concentration (C); when aggregation occurs the difference (C - m1) represents the amount of associated surfactant. The EMF measurements were carried out over an extensive SDS concentration range from 5 × 10-7 to 10-1 mol dm-3. We would like to emphasize that in the concentration range 10-7-10-6 mol dm-3 SDS, the response of the electrode is non-Nernstian and here it is used very close to its lower limit of detection. Despite this limitation, we have included the EMF data as low as 10-7 mol dm-3 SDS in the diagrams even though confusion sometimes arises because of a cross over between the EMF with and without the polymer. The main reason for showing these data is to establish the existence of a fairly significant binding process at these very low concentrations. Isothermal Titration Calorimetry (ITC). The isothermal titration microcalorimeter applied was the MicroCal Omega ITC instrument (MicroCal Inc., Northampton, MA). In ITC experiments, one measures directly the enthalpy changes associated with processes occurring at constant temperature. Experiments were carried out by titrating SDS into linear and star PDMAEMA homopolymers in water. An injection schedule (number of injections (25), volume of injection (10 µL with injection duration of 20 s), and time between injections (4 min at 400 rpm)) was set up using interactive software, and all data were stored on a hard disk. After each addition, the heat released or absorbed as a result of the various processes occurring in the investigated solutions was monitored by the ITC microcalorimeter.13-18 Surface Tension (ST). The axis-symmetric drop shape analysis technique (ADSA)25 (constructed in Salford) was used to determine the surface tension of liquids from the shape of a pendant drop. The basic principle is to capture the drop image and detect the edge of the drop profile to find the initial parameters of Laplace’s equation which are then used in a (24) Xu, R.; Bloor, D. M. Langmuir 2000, 16, 9555. (25) Susnar, S. S.; Neumann, A. W. Trans. Can. Soc. Mech. Eng. 2000, 24, 215.

minimization procedure26 to evaluate the surface tension. The temperature-controlled chamber allows experiments to be performed with high accuracy thermostated at (0.1K. The system was tested with an ethanol-water mixture standard, established by Bircumshaw,27 and the surface tension results agreed very well in a range of (0.1 mN/m with the standard data. Small-Angle Neutron Scattering (SANS). 1. Experimental Procedures. The SANS measurements were made on the LOQ diffractometer28 at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory. The measurements were made using the white-beam time-of-flight method to give a Q range of 0.02-0.15 Å-1. A beam aperture of 12 mm and a sample path length of 5 mm were used, and the samples were measured at a temperature of 298 K. Both, the instrument configuration and sample geometry were optimized to give maximum sensitivity to low surfactant concentrations, over a limited Q range. The data were corrected for background scattering and detector response and converted to the scattering cross section (in absolute units of cm-1) using standard procedures.29 2. Theory. The SANS data were evaluated using established models for the micellar and polymer contributions to the scattering. The total scattering is approximated as

dσ/dΩ ) Ip(Q) + Im(Q)

(1)

This assumes that the cross term (correlations) between micelles and polymer make a negligible contribution to the scattering curves and that it is accounted for by the polymer term,8,18,30 which has no strong Q dependence compared to the micelle scattering. The model was convoluted with the known instrument resolution and compared with the data on an absolute intensity scale on a least-squares basis. Acceptable model fits require not only that the shape of the scattering is reproduced but also that the absolute value of the scattering cross section is in agreement, and this is reflected in the value of the scale factor, sf (data/theory), where an acceptable variation is ca. (10%. The SANS from star polymers is of a similar type to that recently reported by Cosgrove et al.31a and is described using the Gaussian star form factor developed by Benoit et al.,32 such (26) del Rio, O. I.; Neumann, A. W. J. Colloid Interface Sci. 1997, 196, 136. (27) Bircumshaw, L. L. J. Chem. Soc., Faraday Trans. 1922, 1, 887. (28) Heenan, R. K.; King, S. M.; Penfold, J. J. Appl. Crystallogr. 1997, 30, 1140. (29) Heenan, R. K.; King, S. M.; Osborn, R.; Stanley, H. B. RAL Int. Rep. 1989, RAL-89-128. (30) (a) Hayter, J. B. In Physics of Amphiphiles, Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, 1992. (b) Griffiths, P. C.; Fallis, I. A.; Teeraponchaisit, P.; Grillo, I. Langmuir 2001, 17, 2594. (31) (a) Wesley, R. D.; Cosgrove, T.; Thompson, L. Langmuir 1999, 15, 8376. (b) Wesley, R. D.; Cosgrove, T.; Thompson, L.; Armes, S. P.; Baines, F. L. Langmuir 2002, 18, 5704. (32) Benoit, H. J. Polym. Sci. 1953, 11, 507.


Langmuir, Vol. 20, No. 15, 2004 6461

that the form factor P(Q) is

P(Q) )

2 2 f-1 (1 - exp(-υ2))2 υ - (1 - exp(-υ)) + 2 2 fυ

[

]

(2)

where f is the number of arms and

υ)

(3f -f 2)

1/2

Q〈Rg〉star

(3)

〈Rg〉 is the star polymer radius of gyration, such that

IP(Q) ) I(0)PP(Q)

(4)

The SANS from the linear polymer is described using the usual Gaussian coil description33 where

(

Pp(Q) ) NV2(∆F)2 exp -

)

(QRg)2 3

(5)

The micelle structure was established by analyzing the scattering data using a standard and now well-established model for micelles.34a For a solution of globular polydisperse interacting particles, the coherent scattering cross section can be written by the so-called “decoupling approximation” (assuming that there are no correlations between position, orientation, and size)34 as given in eq 6

Im(Q) ) NF[S(Q)〈|F(Q)|2〉 + |〈F(Q)〉|2 - 〈|F(Q)|〉2]

(6)

here the averages denoted by 〈 〉 are averages over particle size and orientation, NF is the particle number density, S(Q) is the structure factor, and F(Q) is the particle form factor. The micelles are modeled as “core + shell”,8,30a and hence the form factor is given by eq 7

F(Q) ) V1(F1 - F2)Fo(QR1) + V2(F2 - Fs)Fo(QR2)

(7)

where Vi ) (4πRi3)/3, Fo(QR) ) 3j1(QR)/(QR), F1, F2, and Fs are the scattering length densities of the micelle core and shell, and of the solvent, respectively, and j1(x) is the first-order Bessel function. The inner core, made up of the alkyl chains only, is constrained to space fill a volume limited by a radius, R1, and defined by the fully extended chain length of the surfactant. Any remaining alkyl chains, headgroups, and corresponding hydration defines the radius of the outer shell, R2. Polydispersity is included using a Schultz size distribution of micelle sizes,34 which is a convenient distribution whose form is closely associated with the accepted theoretical forms of micelle polydispersity. The interparticle interactions are included using the rescaled mean spherical approximation, RMSA, calculation34b,35 for a repulsive (or attractive) Yukawa potential where the surface potential is defined by the surface charge and the Debye screening length, κ-1. κ is given by the usual form

κ)

(

)

8πm1e2 kBT

1/2

(8)

and m1 is taken as the surfactant monomer concentration. The adjustable model parameters are then the aggregation number (Nagg), surface charge (Zed), and polydispersity (poly).

Results and Discussion (i) Star and Linear Nonionic PMHEGMA. The binding of SDS to both the nonionic linear and star PMHEGMA was studied using a SDS selective electrode (EMF) and ITC. Typical binding data are shown in Figures 1 and 2. (33) Debye, P. J. Phys. Colloid Chem. 1947, 51, 18. (34) (a) Hayter, J. B.; Penfold, J. J. Colloid Polym. Sci. 1983, 261, 1022. (b) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (35) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 651.

Figure 1. (a) ITC graph of the dependence of ∆Hi on total SDS concentration for (4, 0, O) pure SDS and (2, 9, b) SDS 0.5% w/v linear PEGMA-9. T ) 298 ( 0.1 K for three different injected concentrations of SDS: 2, 5 mM; 9, 50 mM; b, 500 mM. (b) Graph of the EMF of the SDS-selective electrode (reference bromide ion Br- electrode) as a function of total SDS concentration in 10-4 mol dm-3 NaBr for (0) pure SDS and (9) SDS 0.5% w/v linear PEGMA-9. T ) 298 ( 0.1 K.

(a) Linear PMHEGMA-9. The ITC data, plotted as enthalpy per injection, ∆Hi, vs SDS concentration for the linear PMHEGMA-9 in Figure 1a display the characteristic features of the type of binding which was previously recorded involving SDS and many other nonionic polymers notably poly(ethylene oxide)36 and poly(n-vinylpyrrolidone),37 but some additional features are observed. Specifically, at the onset of binding involving SDS aggregates, the ∆Hi values become more and more endothermic and reach a maximum value where the binding process starts to become more exothermic with a distinctive steplike decrease in ∆Hi, which is usually associated with the cooperative process of the aggregation of SDS as polymer bound surfactant aggregates. When the binding nears completion the ∆Hi values go through a minimum before merging in an asymptotic fashion with the corresponding values measured in the absence of polymer at Csat (or T2) which signals the saturation of the polymer with bound SDS micelles and later the formation of free SDS micelles in the bulk. For this type of binding enthalpy profiles, the critical aggregation concentration Conset (or T1)sthe SDS concentration corresponding to the start of micellar type bindingscan in favorable cases be estimated from the onset of the maximum in the ∆Hi curves. In the (36) (a) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (b) Patterson, R. L.; Little, R. C. Nature 1975, 253, 36. (37) (a) Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 2312. (b) Wan-Badhi, W. A.; Wan-Yunus, W. M. Z.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1993, 89, 2737. (c) Hoffmann, H. Tenside, Surfactants, Deterg. 1995, 32, 462. (d) Murata, M.; Arai, H. J. Colloid Interface Sci. 1973, 44, 475.

6462


Figure 2. (a) ITC graph of the dependence of ∆Hi on total SDS concentration for (4, 0, O) pure SDS and (2, 9, b) SDS/0.5% w/v star PEGMA-15. T ) 298 ( 0.1 K for three different injected concentrations of SDS: 2, 5 mM; 9, 50 mM; b, 500 mM. (b) Graph of the EMF of the SDS-selective electrode (reference bromide ion Br- electrode) as a function of total SDS concentration in 10-4 mol dm-3 NaBr for (0) pure SDS and (9) SDS/ 0.5% w/v star PEGMA-15. T ) 298 ( 0.1 K.

present data, the leading edge of the maximum in ∆Hi is not well defined and we therefore estimate that the onset occurs at ca. 2 × 10-4 mol dm-3 SDS. Furthermore at SDS concentrations below Conset (or T1), the ∆Hi values level off at a much higher value than the corresponding ∆Hi values for cooperative SDS binding. This implies that the data at these low concentrations are characterized by another binding region which extends to much lower concentrations than the first ITC data point at 3 × 10-5 mol dm-3 SDS. The corresponding EMF binding data for SDS/0.5% w/v linear PMHEGMA-9 are displayed in Figure 1b. In the EMF experiment, binding occurs when the EMFs of the electrode with and without polymer are different. Despite the limitations in the electrode measurements at 10-6-10-7 mol dm-3 SDS, we are confident that as the SDS concentration is increased, the binding proceeds in two distinct stages until the EMFs merge again at ∼0.04 mol dm-3 SDS. The first region of binding up to ∼(1-2) × 10-4 mol dm-3 SDS is a weak noncooperative process involving ∼6 × 10-5 mol of bound SDS at the upper limit (2 × 10-4 mol dm-3 SDS) which corresponds to the transition between the two binding regions. The second binding process which is strongly cooperative is associated with the formation of bound SDS aggregates as is usually observed with all SDS/nonionic polymers. The transition between these two binding regions agrees well with the SDS concentration denoted Conset (or T1 or cac for critical aggregation concentration) relating to the onset of bound micelles as estimated from ITC. The binding is complete at 0.04 mol dm-3 SDS as shown when the EMFs and ∆Hi


values with and without the polymer mergesthis corresponds to Csat (or T2) ca. 0.032 mol of bound SDS on polymer. Small differences between EMF and ITC have to be expected, because both techniques monitor different quantities (see Experimental Section). (b) Star Nonionic PMHEGMA-15. The ITC binding enthalpy profile of SDS with nonionic star PMHEGMA15 (Figure 2a) shows similar trends to the data for the corresponding linear polymer, but in this case the characteristic features described above are not as well defined. Nevertheless the leading edge of the maximum in ∆Hi is estimated to occur at ∼2 × 10-4 mol dm-3 SDS, and we consider this to be the onset of cooperative micelle formation on the polymer. Unlike the linear polymer, the ∆Hi values at lower concentrations than 0.3 × 10-4 mol dm-3 are now negative and well below the reference SDS ∆Hi values. This suggests that in this region there is another binding process, caused by electrostatic attraction. Unfortunately at these low SDS concentrations, the ∆Hi values are near the sensitivity limit of the microcalorimeter especially below 10-4 mol dm-3 SDS. The EMF data for the PHEGMA-15 star follow almost exactly the same trends as for the corresponding linear polymer described under (a) and are consistent with the conclusions derived from the respective ITC data. The transition between the two binding processes in EMF (Figure 2b) is fairly clear and coincides with the onset, Conset (or T1), of the second binding process in ITC which starts around 2 × 10-4 mol dm-3 SDS. We also notice that the first low concentration binding region in the EMF data for the nonionic linear and star polymers are almost identical as are the Csat (or T2) values from ITC and EMF which are in both cases (30-40) × 10-3 mol dm-3 SDS. We now turn our attention to the star and linear PDMAEMA. Both EMF and ITC data for the same polymer with different molecular mass and DPs and also for star polymers with different DPs are very similar, and thus only a selection of the data is presented in the diagrams. (ii) Star and Linear PDMAEMA at pH 8.6 (Natural pH). The binding data measured using EMF, ITC, and surface tension for 0.2% w/v solutions of linear PDMAEMA-100 and star PDMAEMA-30 at natural pH of 8.6 are shown in Figures 3 and 4. To all extent and purposes, the PDMAEMAs can be regarded as neutral uncharged polymers at this pH. (a) Linear PDMAEMA-100. For linear PDMAEMA100, the EMF data cover the SDS concentration range ∼10-7-10-1 mol dm-3 (Figure 3a). The data show that binding occurs from at least ca. 2 × 10-6 to 3 × 10-2 mol dm-3 SDS; although it must be emphasized that at the initial concentration of 10-7 mol dm-3 we are very close to the lower SDS limit at which the electrode responds. The ITC data in Figure 3b show that at the lowest SDS concentration which could be measured, i.e., 3 × 10-5 mol dm-3, there is some difference between the ∆Hi values with and without polymer confirming the existence of binding, which already takes place below the first data point. For the linear polymer studied, PDMAEMA-100, the ∆Hi values at the lowest SDS concentrations are negative and below the calibration SDS data. As SDS binding proceeds, the ∆Hi values for the polymer/surfactant system undergo a very distinctive minimum at (0.81.1) × 10-3 mol dm-3 SDS and eventually merge with the ∆Hi values for pure SDS at 0.025 mol dm-3 SDS confirming the Csat (or T2) value found from EMF of ca. 0.025 mol dm-3. (b) Star PDMAEMA-30. The EMF data for PDMAEMA-30 star polymer in Figure 4a show that binding apparently takes place at even the lowest mea-


Langmuir, Vol. 20, No. 15, 2004 6463

Figure 3. (a) Graph of the EMF of the SDS-selective electrode (reference bromide ion Br- electrode) as a function of total SDS concentration in 10-4 mol dm-3 NaBr for (0) pure SDS and (9) SDS/0.2% w/v linear PDMAEMA-100 at natural pH (8.6). T ) 298 ( 0.1 K. (b) ITC graph of the dependence of ∆Hi on total SDS concentration for (4, 0, O) pure SDS and (2, 9, b) SDS/ 0.2% w/v linear PDMAEMA-100 at natural pH (8.6). T ) 298 ( 0.1 K for three different injected concentrations of SDS: 2, 5 mM; 9, 50 mM; b, 500 mM.

sured SDS concentration (0.3 × 10-6 mol dm-3 SDS) and the binding region extends to 2 × 10-2 mol dm-3 SDS when Csat (or T2) is reached. There is a very distinctive kink in the EMF data at (3-4) × 10-4 mol dm-3 SDS. The shape of ITC data (Figure 4b) for PDMAEMA-30 star polymer is not very different to that for the linear polymer. There are some larger differences compared to the linear polymer PDMAEMA-100 between the corresponding ∆Hi values with and without polymer at the lowest SDS concentrations confirming that the first binding region extends to very low SDS concentrations. However the Csat (or T2) values agree well between EMF and ITC experiments. In summary, there is no question that binding of SDS to the star and linear PDMAEMA polymers extends from below 10-6 to ∼3 × 10-2 mol dm-3 SDS. It is also clear that the amount of bound SDS in the polymer complexes varies from 1 or 2 SDS monomers per polymer at low SDS concentrations to a few micelles per polymer around Csat (or T2). This suggests two binding mechanismssa noncooperative process at low SDS concentrations and a highly cooperative process at higher SDS concentrations. Unfortunately neither the EMF data nor the ITC data show any distinctive transition between these two binding modes. In an attempt to investigate this aspect of binding, we carried out surface tension measurements on a selected number of star and linear PDMAEMA polymers in the presence of SDS. (c) Surface Tension (ST) at Natural pH. The ST data which show similar behavior for the linear and star

Figure 4. (a) Graph of the EMF of the SDS-selective electrode (reference bromide ion Br- electrode) as a function of total SDS concentration in 10-4 mol dm-3 NaBr for (0) pure SDS and (9) SDS/0.2% w/v star PDMAEMA-30 at natural pH (8.6). T ) 298 ( 0.1 K. (b) ITC graph of the dependence of ∆Hi on total SDS concentration for (4, 0, O) pure SDS and (2, 9, b) SDS/0.2% w/v star PDMAEMA-30 at natural pH. T ) 298 ( 0.1 K for three different injected concentrations of SDS: 2, 5 mM; 9, 50 mM; b, 500 mM. (c) Graph of the surface tension of 0.2% w/v star PDMAEMA-30 (b) and 0.2% w/v linear PDMAEMA-100 ([) as a function of increasing SDS concentration at natural pH and 298 K. The values of SDS alone (O) are also included for pH 7 and 298 K.

polymers have been measured over the SDS concentration range 10-5 to 0.05 mol dm-3. When SDS is initially added to 0.2% w/v of the linear PDMAEMA-100 and star PDMAEMA-30 (Figure 4c) polymer, the surface tension decreases in the range 10-5-10-3 mol dm-3 SDS. In this region, the corresponding EMF and ITC data show very conclusively that binding occurs with the formation of a polymer/surfactant complex in which the SDS is not aggregated until ∼(2-3) × 10-4 mol dm-3. One of the consequences of the formation of this type of complex is that the monomer surfactant also increases as the total

6464


SDS concentration increases. This factor together with the adsorption of the polymer/SDS complex at the surface38,39 accounts for this increase in surface activity. At ∼10-3 mol dm-3 SDS, the ST goes through a minimum and then increases sharply showing that the surface activity of the solution decreases as a result of the formation of a polymer/SDS micelle complex which displays polyelectrolyte type behavior. The depletion of the surface adsorption which has been previously observed39 occurs as a result of the removal of the polymer/ non aggregated SDS complex from the surface to form the hydrophilic polymer/SDS micelle complex. When the ST reaches a maximum at 0.006 mol dm-3 added SDS, all the available polymer in solution was converted into a polymer/SDS micelle complex, and the polymer is almost saturated with bound micelles. As further SDS is added, a small fraction is bound and the rest is taken up to increase the monomer SDS concentration in the bulk solution (and therefore at the surface). This results in increased surface activity until SDS free micelles are formed and the polymer becomes fully saturated at Csat (or T2) at which point the ST shows the steplike break point characteristic for cmc determinations. This is similar to what was observed for a range of ionic surfactant/polyelectrolyte mixtures.39 The break in surface tension where the formation of free micelles takes place corresponds well with the Csat (or T2) values measured in EMF or ITC. At this stage, we wish to summarize all the results from the binding studies on uncharged linear and star PMHEGMA and PDMAEMA polymers. It is clear from ITC and EMF data that the binding occurs over an extensive SDS concentration range and certainly from ca. 10-6 to 2.5 × 10-2 mol dm-3 SDS. Furthermore the surface tension data confirm that binding is taking place and clearly identify the transition between two binding processes which is manifested as a minimum in the ST measurements. (i) SDS Concentrations 10-7 to 1 × 10-3 mol dm-3. In the binding region, 10-7 to 0.3 × 10-3 mol dm-3 the binding process is rather weak and the interaction leads to a polymer surfactant complex containing unassociated DS- ions attached to the polymer chain. We can also look at the numbers involved here. For example at 3 × 10-4 mol dm-3 SDS, the maximum amount of bound SDS at the upper limit of the first binding process is ∼2.7 × 10-4 mol dm-3 SDS. If all the polymers are involved in binding, then this translates to ∼4 SDS monomers per PDMAEMA linear polymer with DP ) 100. For the PDMAEMA-30 star polymer, a similar calculation gives ∼9 SDS monomers bound per star polymer which contains ∼15 arms. The purpose of this type of calculation is not to claim that the data can be used for estimating the stoichiometry of complexes, rather it is to see if the numbers which emerge are reasonable and in the present case we believe this to be so. These numbers also show quite clearly that micelles are unlikely to be involved in binding to the polymer below 3 × 10-4 mol dm-3 and that only 4 of 100 (linear PDMAEMA-100) and 9 of 400 (star PDMAEMA-30) nitrogen groups are involved in the noncooperative binding process. This excludes a charge-charge interaction and suggests that charge dipole interactions together with entropic effects are involved. We believe that the existence of this first binding process is the most interesting observation in the present study. The process which (38) Taylor, D. J. F.; Thomas, R. K.; Li, P. X.; Penfold, J. Langmuir 2003, 19, 3712. (39) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K.; Taylor, D. J. F. Langmuir 2002, 18, 5147.


Figure 5. (a) Binding isotherm from EMF data for the linear PDMAEMA-100 polymer 0.2% w/v/SDS system at natural pH and 298 K. (b) Binding isotherm from EMF data for the star PDMAEMA-30 polymer 0.2% w/v/SDS system at natural pH and 298 K.

extends from 10-7 to ca. 0.3 × 10-3 mol dm-3 SDS has rarely been observed for SDS/neutral polymer systems with the exception of the highly surface active micellar Pluronics (PEO-PPO-PEO) block copolymers (EO and PO are ethylene oxide groups and propylene oxide groups, respectively) which can form mixed micelles with SDS.17 This is the first time that we could confidently identify this mode of binding for nonassociated neutral polymers. We are however aware that in one of the “classical” and much studied neutral polymers/surfactant system, namely, poly(vinylpyrrolidone) and SDS, binding has been detected below the critical aggregation concentration (cac) or Conset (or T1) denoting the onset of bound SDS micelles in the system. These observations were made using different specialized techniques not normally associated with conventional polymer/surfactant binding measurements, namely, an automatic continuous mixing technique to measure simultaneously light scattering and viscosity39 and also involving surface neutron reflectivity measurements.40 This binding process was not observed in our early EMF measurements on this system which means that the amount of SDS bound to PVP below Conset (or T1) is within the experimental limitations of the electrode as suggested in ref 37a. Although the cac is well-defined in the ITC experiment on PVP/SDS as the leading edge of the maximum in ∆Hi, it is clear that the ∆Hi values with or without the polymer below Conset (or T1) do not meets this observation can be interpreted as binding and the ITC technique is in this case more sensitive than the electrode in the detection of binding. Finally, a typical example in which binding below the critical aggregation concentration could be interpreted from electrode measurements is shown in Figure 2 of ref 12 of the hydroxy(40) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637.


Figure 6. (a) ITC graph of the dependence of ∆Hi on total SDS concentration for (4, 0, O) pure SDS + NaCl 0.1 M and (2, 9, b) SDS/0.2% w/v linear PDMAEMA-100 in 0.1 M NaCl at natural pH. T ) 298 ( 0.1 K for three different injected concentrations of SDS: 2, 5 mM; 9, 50 mM; b, 500 mM. (b) ITC graph of the dependence of ∆Hi on total SDS concentration for (4, 0, O) pure SDS + 0.1 M NaCl and (2, 9, b) SDS/0.2% w/v star PDMAEMA-30 in 0.1 M NaCl at natural pH. T ) 298 ( 0.1 K for three different injected concentrations of SDS: 2, 5 mM; 9, 50 mM; b, 500 mM.

propylcellulose/SDS system. There is no question that the first mode of binding in this system which extends to ca. 10-3 mol dm-3 is a noncooperative process involving the binding of a small number of nonaggregated SDS monomers to the polymer (ii) SDS Concentrations in Excess of 1 × 10-3 mol dm-3. At SDS concentrations in excess of 1 × 10-3 mol dm-3 the binding mechanism involves the cooperative formation of polymer bound surfactant aggregates. Indeed, here the binding process itself is driven by the hydrophobic mechanism which favors the aggregation of SDS monomers to form bound micelles. For example, the ITC data for PDMAEMA-30 star and PDMAEMA-100 linear are slightly different below 1 × 10-3 mol dm-3 SDS and almost identical above this concentration providing supporting evidence for the same binding mechanism. In addition the change of ∆Hi with SDS in this region is characteristic of the almost steplike shape expected for micelle formation on polymers. Finally, as we have previously reported all the Csat (or T2) values of these polymers are very similar. The main differences between the star and linear polymers during the binding process are that at corresponding SDS concentrations the linear polymer binds more SDS than the star polymer but the numbers merge at Csat (or T2). The binding isotherms in Figure 5 for the star and linear PDMAEMA polymers can be taken as an example.

Langmuir, Vol. 20, No. 15, 2004 6465

Figure 7. (a) Graph of the EMF of the SDS-selective electrode (reference bromide ion Br- electrode) as a function of total SDS concentration in 10-4 mol dm-3 NaBr for (0) pure SDS and (9) SDS/0.2% w/v linear PDMAEMA-100 at pH 3. T ) 298 ( 0.1 K. (b) ITC graph of the dependence of ∆Hi on total SDS concentration for (4, 0, O) pure SDS and (2, 9, b) SDS/0.2% w/v linear PDMAEMA-100 at pH 3. T ) 298 ( 0.1 K for three different injected concentrations of SDS: 2, 5 mM; 9, 50 mM; b, 500 mM.

In an attempt to assess the contribution of electrostatic attraction as a binding factor in these complexes, we have carried out ITC measurements in the presence of salt. The ITC measurements on the SDS/PDMAEMA-30 star polymer systems in the presence of 0.1 mol dm-3 NaCl are presented in Figure 6. Unfortunately, due to experimental limitations, major parts of the measurements span over the second binding region where micellar aggregates are bound on the polymer chain. For this region the ∆Hi values for the star and linear polymers are very similar in shape. It is noticeable however that at the lowest SDS concentrations (below 10-4 mol dm-3), the ∆Hi values are different as compared to the absence of salt. The most significant feature of these data is that the ∆Hi values are more positive than those obtained at the corresponding concentration in the absence of salt suggesting that here for SDS an electrostatic attraction is important. It is wellknown that addition of salt decreases electrostatic attraction which is reflected in the more positive ∆Hi values. This confirms again that electrostatic attractions (probably charge - dipole) takes place in the low SDS concentration range, but it also shows that entropic effects causing binding are present as well. ∆G must be negative for binding, and this can only occur via a marked entropy gain. We are unable to obtain reliable EMF data with such high salt additions because of salt interference with the membrane of the electrode. (iii) Star and Linear Polycations PDMAEMA at pH 3. The EMF and ITC for SDS binding to 0.2% w/v of the linear PDMAEMA-100 and star polycation

6466



Figure 9. SANS scattered intensity for 0.5% w/v star PDMAEMA-30 (O), -50 (4), -70 (3), and -100 (0) in D2O. The solid lines are model fits to the data using the Gaussian star form factor32 and the parameters in Table 2. T ) 298 K. Table 2. Model Parameters from SANS Analysis for 0.5% w/v Star PDMAEMA-30, PDMAEMA-50, PDMAEMA-70, and PDMAEMA-100 Polymers at 298 K

Figure 8. (a) Graph of the EMF of the SDS-selective electrode (reference bromide ion Br- electrode) as a function of total SDS concentration in 10-4 mol dm-3 NaBr for (0) pure SDS and (9) SDS/0.2% w/v star PDMAEMA-30 at pH 3. T ) 298 ( 0.1 K. (b) ITC graph of the dependence of ∆Hi on total SDS concentration for (4, 0, O) pure SDS and (2, 9, b) SDS/0.2% w/v star PDMAEMA-30 at pH 3. T ) 298 ( 0.1 K for three different injected concentrations of SDS: 2, 5 mM; 9, 50 mM; b, 500 mM.

PDMAEMA-30 at pH 3 are shown in Figures 7 and 8, respectively. During the binding process, precipitation occurs because at this pH almost all the N atoms in the polymer are positively charged. This is the typical binding behavior observed between a high charge density polycation and an oppositely charged surfactant. Precipitation prevented ST data from being taken. The EMF values of the electrode in the presence and absence of polymer are very different even at the lowest SDS concentration which indicates very strong binding (see Figures 7a and 8a). As SDS is added to the polymer (linear or star), the EMF value remains almost constant indicating that the SDS monomer concentration remains constant and that all the added SDS is used up to bind to the polymer. This process continues until precipitation occurs. In this SDS range, almost all the added SDS is taken up by the polymer in the form of a 1:1 charge interaction where the N+ groups on the polymer are neutralized by individual anionic surfactant monomers. This process is highly exothermic in the ITC experiments (see Figures 7b and 8b) as expected for electrostatic attractions. The same shape for the ITC curve at low pH was already observed for the system SDS/ polyethyleneimine9 confirming the charge neutralization. As the binding process proceeds to higher SDS concentration values, the polymer/surfactant complex becomes more hydrophobic in the sense that the hydrocarbon tails of all the 1:1 bound surfactants are exposed to the bulk solution. This process continues until the complex precipitates from solution. At this stage the concentration of bound SDS is equal to the concentration of the positive charges on the N groups of the polymers. This concentration equals 0.01

system

I(0)

〈Rg〉 (Å)

f arms

0.5% w/v star PDMAEMA-30 0.5% w/v star PDMAEMA-50 0.5% w/v star PDMAEMA-70 0.5% w/v star PDMAEMA-100

1.57 3.9 3.93 12.1

57.3 89.0 94.3 97.1

13 31 34 34

mol dm-3 (see EMF in Figures 7a and 8a) as compared to a value of 0.0125 mol dm-3 calculated from the weighed in polymer in aqueous solution (0.2% w/v) assuming that all the N groups are positively charged. The above numbers which emerge from this consideration are reasonably close. Precipitation is accompanied by a complete reversal of the initial EMF behavior, i.e., the EMF almost drops vertically with very little binding going on and all added SDS exists in the bulk solution in monomeric form. This distinctive steplike behavior occurs in EMF (Figures 7a and 8a) because the polymer at this stage is fully saturated with electrostatically bound surfactant. The corresponding monomer SDS concentration at full precipitation (0.01 mol dm-3) is extremely low (10-7 mol dm-3) and all the SDS that is further titrated into solution is used to build up the bulk SDS monomer concentration. Finally, SDS micelles are formed and Csat (or T2) is reached when the EMF of the SDS electrode with and without the polymer merge. The SDS micelles which are formed must interact with the polymer because the precipitate is being resolubilized. During the buildup of the monomer concentration following the initial precipitation, an additional extremely small amount of SDS does presumably bind to the precipitate. In the present experiments for PDMAEMA polymers, we were unable to find any differences between the binding behavior of the star and linear polymer. Small-Angle Neutron Scattering (SANS at Natural pH). The PDMAEMA star polymers (PDMAEMA-30, -50, -70, and -100) were measured in D2O in the absence of surfactant (see Figure 9) at 0.5% w/v. The scattering is well described by the Gaussian star form factor,32 given by eqs 2 and 3, and the model parameters are summarized in Table 2. The values of f were fixed from the known architecture, and I(0) and 〈Rg〉 fitted with scattering. Going from star PDMAEMA-30 to star PDMAEMA-100, the values of I(0) and 〈Rg〉 increase, consistent with the increase in molecular


Langmuir, Vol. 20, No. 15, 2004 6467

Table 3. Model Parameters from SANS Analysis for 0.5% w/v Star PDMAEMA-30, PDMAEMA-70, and PDMAEMA-100 Polymers in the Presence of 25 mM SDS (T ) 298 K) system

I(0)

〈Rg〉 (Å)

f arms

Nagg

Zed

R1 (Å)

R2 (Å)

Ceff (mM)

star PDMAEMA-30 + 25 mM SDS star PDMAEMA-70 + 25 mM SDS star PDMAEMA-100 + 25 mM SDS

3.7 8.0 6.0

60.0 120.0 135.0

13 32 32

28 80 80

19 20 20

12.7 16.7 16.7

15.0 21.8 21.8

60 35 20

Figure 10. SANS scattered intensity for 0.5% w/v star PDMAEMA-30 (O), -70 (4), and -100 (3), and 25 mM SDS in D2O. The solid lines are model fits to the data using the model described in the text and the parameters in Table 3. T ) 298 K.

mass and the spatial extent of the polymer. These results are broadly consistent with those reported for a different but related star polymer, with PEG (poly(ethylene glycol)) arms, by Cosgrove et al.31a The form of the scattering for three of the above star polymers with SDS is markedly different (see Figure 10). The data are consistent with the Gaussian form factor and bound interacting SDS micelles. This was modeled using eqs 1, 2 and 4, and the key model parameters are summarized in Table 3. The model parameters for the PDMAEMA star-70 and star-100 polymers provide micelle parameters which are consistent with free and polymer bound micelles.7,18 In the present work, the EMF, ST, and ITC data clearly indicated that aggregated SDS is bound to the polymers. The values I(0), 〈Rg〉, and f describing the contribution of the scattering from the polymer are consistent with earlier values and reflect the changes in conformation due to the binding of the SDS micelles. The parameter Ceff is the effective local surfactant concentration (not the real concentration) used to fit the data; it is varied in this case to principally match the peak in S(Q) (determined predominantly by the effective volume fraction) and the absolute values of the data. However, here it merely reflects an increase in the effective nearest neighbor packing of the micelles and the arms of the polymer and does not correspond to a real concentration increase. Apart from the increase in 〈Rg〉 there is no evidence for a major change in the conformation of the stars, in that a change in the number of arms was found not to affect the data. This is in contrast to what was found by Cosgrove et al31a for the binding of SDS onto PEG stars. The aggregation numbers for the bound micelles in the star PDMAEMA-30 are abnormally low compared to those found in the star PDMAEMA-70 and star PDMAEMA100 polymers and also those reported for the linear PDMAEMA of various molecular masses. The molecular mass of each arm in the star PDMAEMA-30 polymer is

ca. 5500. In the linear version of PDMAEMA-50 with chains of similar molecular mass, it can be inferred from the data of Cosgrove et al31b that the bound micelles have aggregation numbers similar to those found in the present work for star PDMAEMA-70 and star PDMAEMA-100. The large difference between SDS micelle aggregation numbers on the star PDMAEMA-30 of 28 and linear PDMAEMA-50 of 80 can be explained as follows. When SDS forms bound aggregates on a polymer chain, the onset of binding, Conset (or T1) or the critical aggregation concentration, is essentially a measure of the cmc of SDS in the presence of the polymer. Since the cac is considerably less than the cmc of free SDS micelles, the resulting increased stability of the bound SDS micelles is attributed to a reduction in the headgroup repulsion in the micelles as a result of polymer binding. In the case of short linear PDMAEMA-50, Cosgrove et al.31b showed that three to four polymer molecules bind to one micelle in order to achieve the stability equivalent to an aggregation number of 80. In the star PDMAEMA-30 however there are 1317 arms and 10-15 bound micelles. The restriction of movement of each arm imposed by the attachment to the core prevents the polymer chains from achieving equivalent surface coverage as if they were free. Therefore in this short arm star polymer the amount of polymer on the micellar surface is restricted, which results in less coverage and hence smaller micelles. The star PDMAEMA-70 contains 30-40 arms, and the supramolecular star/micelle complex contains ∼10 bound SDS micelles. On the other hand the star PDMAEMA-100 polymer contains 35-45 arms and ∼40 bound SDS micelles. We now turn our attention to the first mode of binding which involves nonassociated SDS bound to the polymer which is clearly a significant process in the present work. An interesting question is the mechanism of attraction between SDS and the investigated star and linear polymers. For SDS/PVP both Sorci and Reed41 and Purcell et al.40 hint an electrostatic effect. Indeed a feature concerning the first low concentration binding region is that the ∆Hi values are negative for the star polymer but less for the linear ones. A negative ∆Hi is normally associated with electrostatic attraction. We have also carried out ITC measurements of cetyltrimethylammonium bromide (C16TAB) and hexa(ethylene glycol) monon-dodecyl ether (C12EO6) with these polymers shown in Figure 11. In the case of cationic surfactant, only small differences between the respective ∆Hi values with and without the polymer occur over a very narrow C16TAB concentration range, and the binding capacity would appear to be minimal. On the other hand the ITC data for the nonionic C12EO6 shows that a significant difference in the ∆Hi values with and without the polymer occurs at low C12EO6 concentrations. Since it is generally regarded that the interaction between nonionic surfactants and nonassociating polymers are negligible, we feel that the changes observed in Figure 11b are noteworthy and deserve further investigation. Accordingly we have carried out some SANS measurements. The scattering from the linear PDMAEMA-200 polymer is consistent with the form factor for a simple Gaussian (41) Sorci, G. A.; Reed, W. F. Langmuir 2002, 18, 353.

6468



Figure 13. SANS scattered intensity for 0.5% w/v star PDMAEMA-100 (O), + 0.3 mM h-C12EO6 (4), and + 6 mM h-C12EO6 (3) in D2O. The solid lines are model fits to the data using the Gaussian star form factor32 and the parameters in Tables 2 and 5. T ) 298 K. Table 4. SANS Data Analysis (a) For Linear PDMAEMA-200 Polymer with or without C12EO6 system

I(0)

0.5% w/v linear PDMAEMA-200 1.14 0.5% w/v linear PDMAEMA-200 + 0.3 mM C12EO6 1.25

Rg (Å) 48.5 48.6

(b) For 0.5% w/v Linear PDMAEMA-200 and 6 mM C12EO6

Figure 11. (a) ITC graph of the dependence of ∆Hi on total C16TAB concentration for (4) pure C16TAB and (2) C16TAB/ 0.2% w/v star PDMAEMA-30 at natural pH. T ) 298 ( 0.1 K. (b) ITC graph of the dependence of ∆Hi in the ITC experiments as a function of total C12EO6 concentration for (4, 0, O) pure C12EO6 and (2, 9, b) C12EO6/0.2% w/v star PDMAEMA-30 at natural pH. T ) 298 ( 0.1 K for three different injected concentrations of C12EO6: 2, 5 mM; 9, 50 mM; b, 500 mM.

Figure 12. SANS scattered intensity for 0.5% w/v linear PDMAEMA-200 in D2O (O), + 0.3 mM h-C12EO6 (4), and + 6 mM h-C12EO6 (3). The solid lines are model fits to the data using the model described in the text and the parameters in Table 4. T ) 298 K.

coil (eq 4), as shown in Figure 12. The data for 0.5% w/v linear PDMAEMA-200 with 0.3 mM C12EO6 in D2O are also included in Figure 12 and are indistinguishable from those from the linear PDMAEMA-200 alone. The param-

Nagg

Zed

R1 (Å)

R2 (Å)

I(0)

Rg (Å)

Ceff (mM)

sf

120

8.0

16.7

28.6

2.8

64.4

6.0

0.98

eters for both sets of data analyzed using the Gaussian coil form factor are summarized in Table 4a. The data and fit for the linear PDMAEMA-200 polymer and 0.3 mM C12EO6 are consistent with the binding of C12EO6 monomers to the polymer, where the slight increase in I(0) (reflecting a change in the “contrast” between the polymer and solvent) and the invariance in Rg are indicative of a small but finite monomer binding. The data for the linear PDMAEMA-200 and 6 mM C12EO6 (see also Figure 12) are consistent with the scattering from the polymer and polymer bound C12EO6 micelles. The data were analyzed using eqs 1, 4, and 6 to give the parameters summarized in Table 4b. The micelle aggregation number is similar to that previously reported for C12EO6 micelles42 and polymer bound C12EO6 micelles7,18 (Nagg ∼ 70). The increase in Rg due to the binding of the micelles is consistent with that previously reported for polymer/surfactant mixtures.7,18 Similar measurements were made for the mixture of 0.5% w/v star PDMAEMA-100 polymer and C12EO6. At 0.3 mM C12EO6 the scattering data are indistinguishable from the polymer alone (see Figure 13), and no further analysis (other than reported for the data shown in Figure 9 and summarized in Table 2) was attempted. The data for 6 mM C12EO6 added to the star PDMAEMA 100 polymer are significantly different (see also Figure 13) and are best described using the star Gaussian form factor (with I(0) and Rg increasing from 3.9 to 22.3 and 94 to 124.0 Å, respectively). This implies that the C12EO6 at this concentration is bound as monomer and not as micelle (in contrast to the situation found for the linear polymer); the increases in I(0) and Rg are reflecting such binding (42) Penfold, J.; Staples, E.; Tucker, I.; Cummins, P. J. Colloid Interface Sci. 1997, 185, 424.


Langmuir, Vol. 20, No. 15, 2004 6469

Table 5. SANS Data Analysis for Star PDMAEMA-100 Polymer with or without C12EO6 (T ) 298 K) system

I(0)

Rg (Å)

0.5% w/v star PDMAEMA-100 0.5% w/v star PDMAEMA-100/0.3 mM C12EO6 0.5% w/v star PDMAEMA-100/6 mM C12EO6

3.9 3.9 22.3

94 94 124

and represent a change in contrast due to C12EO6 incorporation and an expansion of star PDMAEMA-100, respectively. All this is summarized in Table 5. This definitely confirms the earlier ITC data and notwithstanding the SANS conclusion for linear PDMAEMA-200 and 6 mM C12EO6; the consensus of the evidence seems to indicate that the bound C12EO6 is not associated. This may be a similar process to the results obtained for the first binding mode for SDS and these polymers. At present we are able to confirm that such binding exists for SDS and C12EO6, but our data for the latter do not justify further speculation concerning the nature of the attractive forces between these surfactant monomers and such polymers. Conclusion Nonionic linear and star homopolymers of the methoxyhexa(ethylene glycol) methacrylate (PMHEGMA) and the ionic 2-(dimethylamino)ethyl methacrylate (PDMAEMA) of different molecular mass showed strong interactions with SDS: (i) At very low concentrations a noncooperative type of monomer SDS binding was observed, which sometimes continued even below the detection limits of 10-5-10-7 mol dm-3 for SDS. (ii) At SDS concentrations of ∼10-3 mol dm-3, well below the cmc of pure SDS, cooperative binding of micellar SDS started on the polymer chains.

(iii) At concentrations above (2-3) × 10-2 mol dm-3 SDS (Csat), the polymers were saturated with SDS micelles and free micelles occurred, when more SDS was added. Surface tension measurements in combination with ITC and EMF showed further details about SDS binding during the transition between noncooperative and cooperative binding. ITC, EMF, and ST showed very similar behavior for binding, and the Csat values for fully SDS saturated polymers were in good agreement in the range of (2-3) × 10-2 mol dm-3 SDS. Star and linear PDMAEMA polymers were also investigated at pH of 3, showing polycationic character, which resulted in a 1:1 binding of anionic DS- accompanied by precipitation and then followed by resolubilization at higher SDS concentrations. SANS experiments proved the binding of micellar SDS to the star polymers and an “average” stoichiometry was evaluated for the supramolecular complexes. Both the cationic C16TAB as well as nonionic C12EO6 were also found to interact with the star polymers in a noncooperative fashion at low surfactant concentrations. In summary we can state, applying EMF, ITC, ST, and SANS, that linear and star polymers of PMHEGMA and PDMAEMA behaved similarly in SDS binding, with the respective linear polymers showing an increased binding capacity. All polymers exhibited a noncooperative monomer binding of SDS at very low concentrations followed by cooperative micellar SDS binding and after saturation the occurrence of free SDS micelles. LA049450L

Binding of Sodium Dodecyl Sulfate to Linear and Star Homopolymers

Recommend Documents