Moderation of the Interactions between Sodium Dodecyl Sulfate and

P. C. Griffiths, A. Y. F. Cheung, C. Farley, I. A. Fallis, A. M. Howe, A. R. Pitt, R. K. Heenan, S. M. King, and I. .... Small-Molecule Self-Assembly ...
0 downloads 0 Views 148KB Size
Download PDF
Langmuir 2000, 16, 8677-8684

8677

Moderation of the Interactions between Sodium Dodecyl Sulfate and Poly(vinylpyrrolidone) Using the Nonionic Surfactant Hexaethyleneglycol Mono-n-dodecyl Ether C12EO6: an Electromotive Force, Microcalorimetry, and Small-Angle Neutron Scattering Study† Y. Li,‡ R. Xu,§ D. M. Bloor,§ J. Penfold,⊥ J. F. Holzwarth,*,‡ and E. Wyn-Jones*,‡,§ Fritz-Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195, Berlin-Dahlem, Germany; ChemistrysSchool of Sciences, University of Salford, Salford, M5 4WT, U.K.; and ISIS Facility, Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire, OX11 0QX, U.K. Received March 1, 2000. In Final Form: June 21, 2000 The polymer-surfactant complex formed between sodium dodecyl sulfate (SDS) and poly(vinylpyrrolidone) (PVP) contains SDS micelles bound to the polymer chain. When a PVP sample of MW 360 000 is saturated with SDS micelles, the complex is a beadlike structure with the PVP wrapped around 25 SDS micelles. The bound SDS was systematically removed from the polymer chain by the addition of the nonionic surfactant hexaethylene glycol mono-n-dodecyl ether, C12EO6, and this process was monitored using isothermal titration calorimetry (ITC). Electromotive force, emf, measurements carried out with a dodecyl sulfate electrode were also used to monitor the desorption of bound SDS from the polymer chain via the SDS monomer concentration. Furthermore, these measurements showed that during the removal of SDS from the polymer, mixed SDS/C12EO6 micelles were formed both on the polymer chain and in solution. The structure and composition of these mixed micelles were determined using small-angle neutron scattering (SANS). The data indicate that electrostatic interactions are the main factor influencing the binding.

Introduction Polymers and surfactants are frequently employed together in many industrial and pharmaceutical applications. As a result, this has prompted a large volume of fundamental studies directed at understanding the equilibrium, kinetic, structural, and rheological properties of many different systems of these types.1-14 Dubin and coworkers15,16 reported extensive studies on the reduction † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. * Address correspondence to: Professor E. Wyn-Jones, c/o Prof. Josef F. Holzwarth, Physical Chemistry, Fritz-Haber Institut, Faradayweg 4-6, D-14195 Berlin, Germany. Fax: +49 30 8413 5385. E-mail: [email protected]. ‡ Fritz-Haber Institut der Max-Planck Gesellschaft. § University of Salford. ⊥ Rutherford Appleton Laboratory.

(1) Goddard, E. D. Interactions of Surfactants with Polymers and Protein; Goddard, E. D., Ananthapadamanabhan, K. P. Eds.; CRC Press: Boca Raton, FL, 1993. (2) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22, 85-92. (3) Robb, I. D. Anionic Surfactants. Surfactant Sci. Ser. 1981, 11, 109. (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 Sciience Series; Marcel Dekker: New York, 1998; Vol. 77, pp 193-238. (6) Bloor, D. M.; Li, Y.; Wyn-Jones, E. Langmuir 1995, 11, 3778. (7) Li, Y.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1996, 12, 4476. (8) Fox, G. J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1998, 14, 1026. (9) Davidson, C. J. Ph.D. Thesis, University of Aberdeen, 1983. (10) Painter, D. M.; Bloor, D. M.; Takisawa, N.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2087. (11) Wan-Badhi, W. A.; Wan-Yunus, W. M. Z.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1993, 89, 2737. (12) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 6326 and references therein.

of electrostatic interactions between SDS micelles and oppositely charged polyelectrolytes. It is now well accepted that when an ionic surfactant binds to a nonionic polymer, the surfactants exist as micellar type aggregates which are bound to the polymer chain. In several of the above applications it is necessary to optimize the formulation of the blend so that a product having acceptable physicochemical properties and very often appearance can be processed. This is usually achieved by manipulating the polymer/surfactant interactions. It is well-known among formulation chemists that moderation of polymer/surfactant interactions can be achieved by addition of a nonionic surfactant or salt.1 Unfortunately, very little information is known about the mechanism of this process other than the important function of the additives to lower surfactant activity. We have recently shown that treating a solution of a polymer/SDS complex with the nonionic surfactant hexaethylene glycol mono-n-dodecyl ether (C12EO6) leads to the complete removal of the bound SDS to form “free” mixed SDS/C12EO6 micelles in solution,6,7 We have also shown that this process can be monitored using isothermal titration calorimetry (ITC).8 In order to gain more information about equilibrium and structural changes during surfactant/polymer and surfactant/surfactant interactions, we report here our ITC, emf (SDSselective electrode), and neutron scattering measurements on the poly(vinylpyrrolidone)/SDS system in the presence of C12EO6. (13) Ghoreishi, S. M.; Fox, G.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 1999, 15, 1938. (14) Wang, G.; Olofsson, G. J. J. Phys. Chem. B 1998, 102, 9276. (15) Yoshida, K.; Dubin, P. L. Colloid Surfaces A 1999, 147, 161. (16) Zhang, H.; Li, Y.; Dubin, P. L.; Kato, T. J. Colloid Interface Sci. 1996, 183, 546 and references quoted therein.

10.1021/la000292h CCC: $19.00 © 2000 American Chemical Society Published on Web 08/30/2000

8678

Langmuir, Vol. 16, No. 23, 2000

Li et al.

Chart 1. Bond Structure of Polyvinylpyrrolidone

extrapolated to Q ) 0, gives directly an estimate of the micellar composition. As an example (see ref 20), for the combination of h-SDS/hC12hEO6 and h-SDS/dC12hEO6 in D2O, the volume and mole fractions of C12EO6 in the micelles are given by

Vf )

Experimental Section SDS was synthesized according to the procedure described by Davidson9 and C12EO6 was a Nikkol product (>98%) used as supplied. The deuterated surfactants were kindly supplied by Dr. R. K. Thomas, Oxford. The PVP (MW 360 000) was an Aldrich product purified by freeze-drying. The bond structure of PVP is given in Chart 1. Emf Measurements. Surfactant membrane electrodes selective to sodium dodecyl sulfate (SDS) were constructed in the laboratory and used to determine monomer surfactant concentrations (m1) by measuring their emf relative to a commercial bromide reference electrode. The cells used for these measurements and the procedures to calculate the respective monomer concentrations have been described elsewhere.9-12 Isothermal Titration Calorimetry (ITC). The microcalorimeter used was the Microcal Omega ITC instrument. In the ITC experiment, one measures directly the energetics (enthalpy changes) associated with processes occurring at constant temperature. The experimental procedures have been described elsewhere.8,12-14 All measurements were carried out at 25 °C. Small-Angle Neutron Scattering. 1. Experimental Details. The SANS measurements were made on the LOQ diffractometer17 at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory, UK. The measurements were made using the whitebeam time-of-flight method with a limited wavelength range at a source frequency of 50 Hz (giving a Q range 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 25 °C. The instrument configuration and sample geometry was optimized to give maximum sensitivity to low surfactant concentrations, over a limited Q range. The data from all three diffractometers were corrected for background scattering and detector responses, and converted to scattering cross section (in absolute units of cm-1) using standard procedure.18 2. Evaluation of Mixed Micelle Compositions. The scattering cross section for colloidal particles in a solution can be written as19

∫(F (r) - F ) exp(iQ)‚r d r|

dσ ) N| dΩ

ν

3

F

s

2

(1)

where FF and Fs are the particles and solvent scattering length densities and N is the number of particles per unit volume. For spherical monodisperse micelles in solution20 then

dσ ≈ NP(Q) S(Q) dΩ

(2)

where N is the micelle number density, P(Q) the particle from factor (P(Q) ) 3(sin(QR) - QR cos(QR)/(QR)3/3) in which R is the particle radius and S(Q) the interparticle structure factor arising from the intermicellar interactions in the solution. For a dilute solution of mixed surfactant micelles in the limit of small Q, where P(Q) ≈ S(Q) ≈ 1.0, the scattered intensity can be written as

I(Q) R

∑N V i

2

i

(FiF - Fs)2

(3)

i

where N is the micelle number density, V the micelle “dry” volume, FF the micelle scattering length density, and Fs the solvent scattering length density, and the summation is over all micelle compositions for the binary surfactant mixture. By making two different measurements, one with both surfactants protonated, and a second with only one of the surfactants deuterated, the ratio of the two measured intensities,

(xR1 - 1)(Fhsds - FD2O) (FhC12E6 - Fhsds) - xR1(FdC12E6 - Fhsds) Mf )

Vf/VC12E6 Vf/VC12E6 + (1 - Vf)/Vsds

(4)

(5)

where Vf and Mf are the volume and mole fraction of C12EO6 in the micelles, R1 is the intensity ratio Ihsds/hC12E6/Ihsds/dC12E6 (at Q ) 0), Fi is the scattering length density of component i, Ii is the scattered intensity (at Q ) 0) for system i, and Vi is the molecular volume of component i. For such SANS measurements, instrumentally related systematic variations, such as detector response and calibration errors, introduce errors in the determination of the absolute scattering cross sections of typically ∼10%. However, in determining the micelle composition from the intensity ratio in eq 4, it is assumed that the composition distribution is narrow. This leads to many of these uncertainties canceling out and the error in the composition typically becomes ∼1%. Mixed micelles can, of course, have large composition fluctuations. From eq 3 it can be seen that the different isotopic labeling schemes used will reflect the different components of the micelle composition distribution, and this will result in a small error in the deduced micelle compositions. By including a distribution in the calculation of composition, we have shown20 that this error can be evaluated, as in the limit of high concentration the true micelle composition must approach that of the solution. In some of the data presented in this paper, S(Q) * 1.0, and so eq 4 is not strictly correct. However, as S(Q) should not vary with isotopic content and providing the data obtained can be effectively extrapolated to small Q (where P(Q) ∼ 1.0) the contribution from S(Q) should cancel in the ratio of intensities (R1). We verified this by making some of the measurements with the complementary combinations of hh/dh and hh/hd, which within error give the same composition, and by detailed modeling (including composition as an additional variable) of the different isotopically labeled combinations. 3. Analysis of Micellar Structure. The micelle structure was established by analyzing the scattering data using a standard and now well-established model for micelles.19 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)19,21

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

(6)

where the averages denoted by 〈 〉 are averages over particle size and orientation. Np is the particle number density, S(Q) the structure factor, and F(Q) the particle form factor. (Note that for spherical monodisperse particles 〈F(Q)〉2 ) 〈F(Q)2〉 ) P(Q), see also eq 2.) The micelles are modeled as a “core + shell”,19 and hence the form factor is

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

(7)

where, Vi ) (4/3)πRi3, Fo(QR) ) 3j1(QR)/(QR), j1 is a first-order (17) Heenan, R. K.; King, S. M.; Penfold, J. J Appl. Cryst. 1997, 30, 1140. (18) Heenan, R. K.; King, S. M.; Osborn, R.; Stanley, H. B. RAL Internal Report, RAL-89-128; Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire, OX11 0QX, UK, 1989. (19) Hayter, J. B.; Penfold, J. J Coll. Polym. Sci. 1983, 261, 1072. (20) Staples, E.; Tucker, I.; Hines, J.; Thompson, L. J.; Thomas, R. K.; Penfold, J. Langmuir 1995, 11, 2496. (21) Hayter, J. B. In Physics of Amphiphiles, Micelles, Vesicles, and Microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, 1992.

Interactions between SDS and Poly(vinylpyrrolidone)

Langmuir, Vol. 16, No. 23, 2000 8679

spherical Bessel function, and F1, F2, and Fs are respectively the scattering length densities of the micelle core, shell, and solvent. The inner core, made up of the alkyl chains only, is constrained to space fill a volume limited by a radius, R1, defined by the fully extended chain length of the surfactant, and containing a fraction, alf, of the alkyl chains. Any remaining alkyl chain (1 - alf), headgroups, and corresponding hydration defines the radius of the outer shell, R2. Polydispersity is included using a Schultz size distribution of micelles sizes,21 where σ2 is the variance of the distribution. This is a particularly 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, calculation22,23 for a repulsive Yukawa potential. The adjustable model parameters are then the aggregation number (agg), surface charge (z) (or potential), and polydispersity (σ). The micelle composition is fixed from the values obtained from eqs 4 and 5, and the model is adjusted according to that composition. Although most of the data presented in this paper are consistent with the polydisperse core and shell model of the spherical micelles, some of the data are best described using a modified description where the micelles are no longer spherical, but elliptical (prolate ellipses). A core and shell description is retained and the dimensions constrained in the same way as the spherical micelles,21 and the short dimension of the elliptic inner core now corresponds to R1. The ellipticity is characterized by the parameter ee such that the ellipse axes of the inner core are R1, R1, ee•R1 (two axes equal, one modified by the factor ee). Measurements of the PVP alone were modeled using the wellestablished scattering law for a Gaussian coil

I(Q) ) I(0)2(u - 1 + exp(-u))/u2

(8)

where u ) (QRg)2, and I(0) ) φV(∆F)2, with φ ) C/d, V ) M/dNa, and d is the polymer density, V the volume per coil, C the concentration, ∆F the scattering length density contrast, and Rg the radius of gyration such that

I(0) )

C 2 V Na∆F2 M

(9)

For the polymer/surfactant mixtures, free polymer is included in the modeling as a Gaussian coil scattering in addition to the micellar scattering, and polymer incorporated into the micelles (in the outer shell) is included as an extra parameter (nmol, number of polymer monomers per micelle). 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. This is reflected in the value of the scale factor, sf (data/theory), and the acceptable variation is ∼(10%. In the cases where different “contrasts”, different isotopically labeled combinations, are measured, there is an additional constraint that the model has to be consistent for all the “contrasts”. Given that the micelle composition is derived simply from the intensity ratios using eqs 4 and 5 above, the refinable parameters in the model fit of the micellar structure are the aggregation number, (agg), the surface charge (z), and the polydispersity, (σ). R1 is constrained by the model assumptions and R2 determined by the model constraints and aggregation number (and ee, in the case of elliptical micelles). The measurements with different “contrasts” and isotopically labeled combinations and the associated variable fitted parameters given the best estimate of the errors associated with those parameters. The error in agg is typically ∼5%, and for the micelle charge ∼10%. The data, although requiring finite polydispersity, are less sensitive to the absolute value. Furthermore, the other model parameters do not depend strongly on the polydispersity, σ. The typical error for σ is σ ) 0.1 ( 0.05; and is consistent with theories of micellization. (22) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (23) Hayter, J. B.; Hansen, J. P. Mol. Phys. 1982, 42, 651.

Figure 1. Graph of the emf of an SDS electrode (reference silver/Br- electrode) as a function of total SDS concentration with and without 1% (w/v) PVP at 25 °C: (b) pure SDS; (2) SDS + 1% (w/v) PVP. For the data involving polymer and micelles there are three additional model parameters which account for the “free” polymer and polymer “incorporated” into the micelle; their relative contributions are discussed in more detail in the discussion of the results. From the variation in the parameters for the different “contrast” data, and from repeated model fits with different starting parameters, the typical errors in Rg, I(O), and nmol are ∼(10%.

Results and Discussion 1. Emf and ITC Data. Both the emf and ITC experiments are carried out in a similar way such that concentrated surfactant containing polymer is titrated into an aqueous solution containing the same amount of PVP. After each addition, the cell emf was measured in the electrode experiment and the enthalpy per injection (∆Hi) in the ITC experiment. The binding isotherm and enthalpy profile of SDS in the presence of 1% w/v of PVP in 10-4 mol dm-3 NaBr was measured using the SDS electrode and ITC, respectively. The emf of the SDS electrode as a function of added SDS in the presence and absence of a 1% PVP sample are shown in Figure 1. Binding of SDS to the PVP takes place when the emf of the electrode with and without the polymer is different. As shown in Figure 1 the two emf’s diverge at an SDS concentration denoted T1 when SDS starts binding to the polymer and they remain different over the binding region. The binding isotherm is shown in Figure 2 and agrees well with previous work on PVP.11 The present isotherm was measured up to an SDS concentration of T2 and the amount of bound SDS refers to polymer bound surfactant only. It is now well established that the SDS exists on the polymer as micellar type aggregates which grow in size and number during the binding process.1,11,12 When the polymer is fully saturated with bound SDS it is no longer involved in any further binding and the emf’s merge again at an SDS concentration denoted T2. As the binding proceeds after T1 the monomer SDS concentration increases and it reaches a maximum at T2. After T2 any further SDS that is added is used to form free regular micelles which are accompanied by a decrease in m111,12 the SDS monomer concentration. In the absence of excess salt this decrease in m1 is caused by an increase in the ionic strength of the solution as more SDS is added. In the ITC enthalpy profile shown in Figure 3, T1 occurs at the onset of the maximum that is observed in ∆Hi. We believe that when a maximum in ∆Hi occurs this is a more realistic and acceptable way to define T1 from ITC; this

8680

Langmuir, Vol. 16, No. 23, 2000

Figure 2. Binding isotherm at 25 °C showing the amount of polymer-bound SDS plotted as a function of monomer SDS concentration m1. Binding data were terminated at T2 where free SDS micelles start to form. (2) SDS + 1% (w/v) PVP.

Figure 3. Graph of ∆Hi in the ITC experiment as a function of total SDS concentration for (b) SDS alone; (O) SDS + 1% (w/v) PVP measurements carried out at 25 °C.

Figure 4. Graph of ∆Hi in the ITC experiment as a function of total C12EO6 concentration for (b) C12EO6 alone; (O) C12EO6 + 1% (w/v) PVP; (0) C12EO6 + 16 mM SDS; (4) C12EO6 + 16 mM SDS + 1% (w/v) PVP measurements carried out at 25 °C.

is a procedure which fits in well with emf data for PVP and many other neutral polymers. Finally, T2 occurs when the ∆Hi with and without the polymer merge. There is excellent agreement between the positions of T1 and T2 in the corresponding emf and ITC experiments. ITC experiments have also shown that C12EO6 does not bind on PVP. This is illustrated in Figure 4 where the ∆Hi’s for the titration of C12EO6 into an aqueous solution with and

Li et al.

Figure 5. Graph of the emf of the SDS electrode (reference silver/Br- electrode) as a function of total C12EO6 concentration with and without 1% (w/v) PVP at 25 °C: (b) C12EO6 + 16 mM SDS; (2) C12EO6 + 16 mM SDS + 1% (w/v) PVP.

without the polymer coincide from 0 to 70 × 10-3 mol dm-3 C12EO6. We now consider the systematic desorption of bound SDS aggregates from the PVP chains using the non ionic surfactant, C12EO6, as an additive. The ITC procedure to monitor this process has been described elsewhere and is as follows:8 1. A “sample” solution is chosen containing 16 × 10-3 mol dm-3 of SDS in 1% PVP. In this solution the aggregated surfactant is bound to the polymer (see Figure 1) and is in equilibrium with free monomer of surfactant in solution and no “free” micelles are present. This follows from the emf data in Figure 1 in which the bound SDS concentration is estimated to be 12 × 10-3 mol dm-3, leaving the free SDS concentration just below the CMC of SDS. 2. A “control” solution of SDS only containing the same total SDS concentration as 1 (16 × 10-3 mol dm-3) in which ∼8 mol dm-3 monomer SDS is in equilibrium with ∼8 mol dm-3 micellar SDS (expressed in moles of monomer). In two separate ITC experiments C12EO6 is injected by titration into solutions 1 and 2 under exactly the same conditions. The results are shown in Figure 4. The noteworthy feature here is that the corresponding enthalpies per injection for the solutions are different when the same amount of C12EO6 is initially added. This is not an unexpected result because the SDS in solution 1 exists as polymer-bound micellar aggregates in equilibrium with monomers whereas in solution 2 the SDS exists as normal micelles and monomers. Eventually, as more C12EO6 is added, the two enthalpy curves become closer together and finally merge at 20 × 10-3 mol dm-3 C12EO6. When the enthalpies merge, further addition of C12EO6 has exactly the same effect on the SDS whether the polymer is present or not. At this point, all the bound SDS has been removed from the polymer and consequently behaves as the SDS only solution. This has been clearly demonstrated in earlier publications.7,8 A parallel series of experiments using the same starting solutions as in solutions 1 and 2 were then carried out using the SDS selective electrode to measure monomer SDS concentrations during the desorption process with C12EO6. The results are in Figure 5. Again the same pattern of behaviour as found in the above ITC experiment is observed. Initially the emf values are different and remain so as C12EO6 is initially added. Eventually the emf curves get closer together and finally merge at the

Interactions between SDS and Poly(vinylpyrrolidone)

same C12EO6 concentration measured in the corresponding ITC experiment of Figure 4. When the emf’s merge, the SDS in the polymer solution behaves in exactly the same way as the SDS-only solution. This clearly shows that at this point all the bound SDS has been removed from the polymer. Figure 4 also shows that during the addition of C12EO6, SDS is simultaneously removed from the solution (as monomer) and from the polymer to form free SDS/ C12EO6 mixed micelles in solution with a consequent decrease in the cmc as measure by m1. We now refer to the binding of SDS to PVP as shown in Figures 1 and 2. In these circumstances T1 corresponds to the onset of the formation of bound SDS in the form of micellar aggregates. At constant temperature and salt concentration T1 is independent of polymer concentration and molecular weight and varies with salt and temperature in a very similar way to the cmc of SDS.1 In reality T1 is the critical “micelle” concentration at which polymerbound micelles are formed and is sometimes referred to as the “critical aggregation concentration”. Alternatively, from a thermodynamic viewpoint the condition which must be satisfied before SDS can form pure single component bound micellar aggregates on PVP at constant temperature and salt concentration is that the monomer SDS concentration must reach T1. An examination of the data in Figure 5 referring to the mixed SDS/C12EO6 data shows that surfactant aggregates exist on the polymer even when the monomer SDS concentration is less than T1. Furthermore, these surfactant aggregates must contain SDS for binding purposes (C12EO6 does not bind the PVP). The only way that such surfactant aggregates can exist on the polymer at SDS concentrations when m1 is less than T1, measured in the absence of C12EO6, is that the bound aggregates are mixed C12EO6/SDS micelles. In addition, during this process free C12EO6/SDS mixed micelles must also form in solution. Neither the electrode or ITC data can tell us anything about the composition or structure of the mixed micelles; therefore, small-angle neutron scattering measurements, using h/d isotopic labeling of the surfactants, were used to determine both structure and composition. 2. SANS Data. SANS measurements were made for SDS/PVP mixtures (16 mM SDS/1% PVP), and SDS/ C12EO6/PVP mixtures (16 mM SDS/2 mM C12EO6/1% PVP, 16 mM SDS/10 mM C12EO6/1% PVP, and 16 mM SDS/20 mM C12EO6/1% PVP) in D2O. For comparison, and to determine more directly the role of the polymer, measurements were also made for 1% PVP/D2O, 16 mM SDS/ D2O, and 16 mM SDS/20 mM C12EO6/D2O. In the measurements with the surfactant mixtures, different isotopically labeled (deuterium labeled) combinations were used; h-SDS/h-C12EO6, d-SDS/h-C12EO6, h-SDS/d-C12EO6, and d-SDS/d-C12EO6. (a) 1% PVP/D2O. The scattering from 1% PVP in D2O (see Figure 6) is well described by the scattering law for a Gaussian coil (see eq 8), with a radius of gyration ∼80 Å (in reasonable agreement with a MW of 360 000), and an I(0) of ∼1.06. (b) 16 mM SDS/D2O. The scattering from 16 mM SDS in D2O was analyzed using the well-established model for micelles, described in the previous section. The data and model fits are shown in Figure 7. The model parameters are summarized in Table 1 and are consistent with those reported elsewhere for SDS at low surfactant concentrations.19,24,25 (24) Cabane, B.; Duplessix, R.; Zemb, T. Surfactants in Solution; Mittal, K., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 1, p 373. (25) Berr, S.; Jones, R. P. M. Langmuir 1988, 4, 1247.

Langmuir, Vol. 16, No. 23, 2000 8681

Figure 6. Scattered intensity I(Q), for 1% PVP in D2O (b): solid line is a fit to the scattering dependence for a Gaussian coil (eqs 8 and 9, for Rg ) 80 Å, I(0) ) 1.06).

Figure 7. Scattered intensity I(Q), for 16 mM SDS in D2O (b): solid line is a fit to the micellar model described in the text using eqs 6 and 7, and for the parameters in Table 1. Table 1. Model Parameters from Analysis of 16 mM SDS/D2O, from SANS Data aggregation no.

charge

R1 (Å)

R2 (Å)

polydisp σ

57

13.0

16.7

19.5

0.10

(c) 16 mM SDS/20 mM C12EO6/D2O. The scattering data for 16 mM SDS/20 mM C12EO6/D2O were analyzed using the micellar model described previously, but modified to include a binary mixture of surfactants (described by the volume fraction of the mixture, Vf, in the micelles). In this case, the micellar composition is assumed to be the same as the solution composition, as the isotopic combination measured, h-SDS/h-C12EO6, is not particularly sensitive to composition. The data and model parameters (see Figure 8 and Table 2) are consistent with previous measurements.20 The mixed micelles here are best described by ellipses, rather than polydisperse spheres. (d) 16 mM SDS/1% PVP/D2O. The addition of PVP results in a marked change in the scattering of the SDS micellar solution (see Figure 9). This is a regime where, judging from ITC and emf measurements, there are no “free” SDS micelles, and the solution consists of aggregated surfactant bound to the polymer in equilibrium with the surfactant monomer concentration in solution. Qualitatively the change in scattering is consistent with increased intermicellar interactions and a modest increase in aggregation number. There is no evidence for a contribution from “free” polymer (as illustrated in Figure 6). The data were analyzed (see solid line in Figure 9) using the micellar model described previously, but modified to incorporate some polymer in the outer shell (described by

8682

Langmuir, Vol. 16, No. 23, 2000

Li et al.

Figure 8. Scattered intensity I(Q), for 16 mM SDS/20 mM C12EO6 in D2O (b): solid line is a fit to the model described in the text using eqs 6 and 7, and for the parameters in Table 2.

Figure 9. Scattered intensity I(Q), for 16 mM SDS/1% PVP/ D2O (b): solid line is a fit to the model described in the text using eqs 6 and 7 for the micelles and eqs 8 and 9 for the “free” polymer contribution, and for the parameters in Table 3. Table 2. Model Parameters from Analysis of 16 mM SDS/20 mM C12EO6/D2O, from SANS Data aggregation no.

charge

R1 (Å)

R2 (Å)

polydisp σ

ee

94

40.0

16.7

21.0

0.0

1.56

Table 3. Model Parameters from Analysis of 16 mM SDS/1% PVP/D2O, from SANS Data aggregation no. charge 85

80

R1 (Å)

R2 (Å)

16.7 26.2

polydisp σ 0.1

ee

I(0)

Rg (Å)

nmol

0.0 0.45 12.6 150

nmol, the number of monomers per micelle), and with an additional contribution from the polymer between adsorbed micelles (described as a Gaussian coil, but with a different radius of gyration). The incorporation of polymer into the outer micellar shell is required to predict the correct shape of the scattering profile and its absolute value. The parameters obtained from this analysis are summarized in Table 3. Comparing the parameters in Table 3 with those for SDS/D2O in Table 1, the addition of PVP results in only a modest increase in micellar size. It is, however, necessary to include polymer in the outer shell of the micelle to fully describe the data, and it has an impact on both the absolute amount of scattering (it changes the scattering length density of the outer shell) and the shape of the scattering curve (R2 is significantly larger). The increase in intermicellar interactions was accommodated in the model by an increase in surface charge. This gives rise to a micellar charge that appears to be unrealistically high and is difficult to interpret. An

Figure 10. Scattered intensity I(Q), for 16 mM SDS/2 mM C12EO6/1% PVP in D2O: (b) h-SDS/h-C12EO6, (O) h-SDS/dC12EO6, (4) d-SDS/h-C12EO6, and (2) d-SDS/d-C12EO6; solid lines are fits to the micellar model described in the text using eqs 6 and 7 and eqs 8 and 9 for the “free” polymer contribution and for the parameters in Table 5. The micelle composition is obtained from the analysis of the intensity ratios using eq 4, and values in Table 4.

effective increase in micellar volume fraction, due to binding of micelles to the PVP, could also account for the increase in the intermicellar interactions, but it is not straightforward to include such an effect in the model. Although it is not possible from our data to distinguish between the two effects, the latter contribution is more likely and the charge should not in this case be interpreted too literally. The change in charge of the micelles should not necessarily be interpreted too directly. The modest increase in aggregation number is similar to that obtained for added electrolyte. The contribution from the polymer not incorporated into the micelles is described by a significantly smaller Rg. (e) SDS/C12EO6/PVP/D2O. Measurements were made for three different surfactant concentrations and compositions (16 mM SDS/2 mM C12EO6, 16 mM SDS/10 mM C12EO6, 16 mM SDS/20 mM C12EO6), corresponding to three different regimes of polymer/surfactant interaction, as defined by the emf and ITC measurements described earlier. At the lowest C12EO6 concentration it is expected that all the micelles are bound, whereas at the highest concentration the micelles are predominantly free. At intermediate concentrations both free and bound micelles should be present. Measurements were made for four different isotopically labeled combinations of the two surfactants (h-SDS/h-C12EO6, d-SDS/h-C12EO6, h-SDS/dC12EO6, and d-SDS/d-C12EO6), and the data for three different surfactant concentrations and compositions are shown in Figures 10-12. Superficially, the scattering is very similar to that for SDS/C12EO6 mixed micelles in the absence of PVP (see for example the dashed line in Figure 12). However, the different isotopic combinations provide sensitivity to the micellar structure and composition. As described earlier,20 the intensity ratios for the combinations h-SDS/h-C12EO6 and h-SDS/d-C12EO6, or d-SDS/h-C12EO6 provide a direct estimate of the micellar composition, and the data are summarized for the three compositions measured here in Table 4. Given an error ∼(0.05 in Mf, the values in Table 4 for the two different intensity ratios are in reasonable agreement and are consistent with the solution compositions for all three solutions. For the 16/20 combination a composition close to the solution composition is expected. However, for the 16/10 and 16/2 combinations this would also imply that the composition of the bound micelles are similar to the

Interactions between SDS and Poly(vinylpyrrolidone)

Figure 11. Scattered intensity I(Q), for 16 mM SDS/10 mM C12EO6/1% PVP in D2O: (b) H-SDS/h-C12EO6, (O) h-SDS/dC12EO6, (4) d-SDS/h-C12EO6, and (2) d-SDS/d-C12EO6; solid lines are fits to the micellar model described in the text using eqs 6 and 7 and eqs 8 and 9 for the “free” polymer contribution, and for the parameters in Table 5. The micelle composition is obtained from the analysis of intensity ratios using eq 4, and the values in Table 4.

Figure 12. Scattered intensity I(Q), for 16 mM SDS/20 mM C12EO6/1% PVP in D2O: (b) h-SDS/h-C12EO6, (O) h-SDS/dC12EO6, (4) d-SDS/h-C12EO6, and (2) d-SDS/d-C12EO6; solid lines are fits to the micellar model described in the text using eqs 6 and 7, and eqs 8 and 9 for the free polymer contribution, and for the parameters in Table 5. The micelle composition is obtained from the analysis of intensity ratios using eq 4, and values in Table 4. The dashed line is the scattered intensity for 16 mM h-SDS/20 mM h-C12EO6 in D2O in the absence of polymer.

free micelles and hence the solution. That is, the bound micelles do not appear to be substantially richer in SDS. The scattering for 16 mM SDS/2 mM C12EO6/1% PVP is similar to that for SDS/PVP (see Figures 9 and 10). The scattering for all four contrasts can be described using the same model (see Table 5 and the solid lines in Figure 10), as described earlier, i.e., polydisperse core + shell particles, with PVP incorporated into the outer shell of the micelles. However, there appears to be less polymer incorporated than for the SDS/PVP mixtures alone. In contrast, d-SDS/d-C12EO6 is particularly sensitive to the amount of polymer incorporated and to its location within the micelle. Without polymer being included in the micelle, the micelle should be closely “index matched” to the solvent and the scattering consequently much less intense. The measured intensity for this “contrast” can only be explained by incorporating polymer into the micelles. Furthermore, it is possible to distinguish between locating the polymer in the outer shell of the micelle and it being distributed throughout the micelle. In the latter case, significantly worse model fits are obtained. For this

Langmuir, Vol. 16, No. 23, 2000 8683

composition there is no evidence of “free” polymer in the scattering profiles, and we will return to this point later in the discussion of the data from the two other compositions measured. The analysis also shows that the addition of PVP does not induce micellar growth for the 16/2 mixture. The analysis is also consistent with the ITC and emf data in concluding that all the micelles are bound. The scattering data for the compositions 16 mM SDS/ 20 mM C12EO6, and 16 mM SDS/10 mM C12EO6 in 1% PVP/D2O show some interesting changes compared to the previous data for 16 mM SDS/2 mM C12EO6/1% PVP/D2O. The data for h-SDS/h-C12EO6 in D2O are similar to those in the absence of PVP (see dashed line Figure 12). However, the other contrasts show an increase in scattered intensity at low Q, which is attributed to the contribution from the “free” polymer in solution with an Rg similar to that of PVP alone. This is particularly visible for the “contrast” d-SDS/d-C12EO6, where the scattering is dominated by that from the “free” polymer; this allows a reliable estimate of I(0) and Rg to be made. The scattering data for both of these SDS/C12EO6 compositions are consistent with a micellar model described by core + shell ellipses mentioned earlier, and used for SDS/C12EO6 mixed micelles in the absence of PVP. The model parameters for the three contrasts, h-SDS/h-C12EO6, d-SDS/h-C12EO6, and h-SDS/ d-C12EO6, are summarized in Table 5 and are consistent with the scattering being dominated by “free” micelles (that is, no PVP incorporated into the mixed micelles). The up-turn in the scattering at low Q, as discussed earlier, is accounted for by the contribution from “free” polymer, modeled as a Gaussian coil with an Rg similar to that for 1% PVP alone. The micelle model parameters are similar to those for SDS/C12EO6 without PVP. This model does not, however, entirely explain the data for the d-SDS/dC12EO6 “contrast”. In this case the micelles should be effectively “index matched” to the solvent, and the scattering should arise from the “free” PVP only. Although the “free” polymer contribution is evident, there is additional micellar scattering that cannot be explained in term of “free” micelles. This implies that there is a contribution from micelles bound to PVP, and that the “contrast” and hence the scattering arises from PVP being incorporated into the outer shell of the bound fraction of micelles. The data for this “contrast” and for both compositions, 16 mM SDS/20 mM C12EO6 and 16 mM SDS/ 10 mM C12EO6, were modeled by assuming a fraction of free micelles and polymer and a fraction of bound micelles with PVP incorporated into the outer shell of the micelles. The bound micelles were assumed to have a structure and composition similar to those of the composition 16 mM SDS/2 mM C12E6/1% PVP solution (see Table 5). From the approximate weighting of these two model contributions it would imply that for 16 mM SDS/20 mM C12EO6, ∼20% of the micelles are still bound whereas for 16 mM SDS/10 mM C12EO6 ∼50% are bound. This is also reflected in the values of I(0) used to describe the free polymer component (see eq 8), compared to the value for 1% PVP in D2O, which implies that ∼30% of the polymer is still associated with micelles for the 16/20 mixture, whereas ∼60% is associated for 16/10 mixture. Due now to the number of parameters, and uncertainties associated with the modeling of such a complex mixture, these numbers should be considered as a rough guide rather than an accurate estimate. Nevertheless, they provide a good description of the SANS data and are broadly consistent with the ITC and emf results.

8684

Langmuir, Vol. 16, No. 23, 2000

Li et al.

Table 4. Micellar Compositions for SDS/C12EO6/PVP Mixtures, from SANS Data system

solution composition (mol %)

intensity ratio hh/hd

hh/dh

Vf

Mf

1% PVP/16 mM SDS/2 mM C12EO6 1% PVP/16 mM SDS/10 mM C12EO6 1% PVP/16 mM SDS/20 mM C12EO6

0.89 0.62 0.44

1.2 2.1 2.8

12.5 3.5 2.5

0.84, 0.74 0.47, 0.46 0.31, 0.37

0.92, 0.84 0.61, 0.60 0.45, 0.51

Table 5. Model Parameters from Analysis of 1% PVP/D2O/SDS/C12E6, from SANS Data system

contrast

agg no.

charge

R1 (Å)

R2 (Å)

polydisp σ

ee

I(0)

Rg (Å)

nmol

16 mM SDS/2 mM C12E6

hh hd dh dd hh dh hd hh dh hd

97 101 96 96 97 86 88 89 85 83

48 51 57 57 16 16 17 20 20 20

16.7 16.7 16.7 16.7 16.7 16.7 16.7 16.7 16.7 16.7

25.0 25.2 24.5 24.5 21.0 21 21 21.0 21.0 21.0

0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 1.62 1.62 1.62 1.5 1.4 1.4

0.60 0.52 0.62 0.62 0.6 0.7 0.6 0.4 0.4 0.4

10.0 10.0 10.0 10.0 80.0 80.0 80.0 80.0 80.0 80.0

42 40 40 40 0.0 0.0 0.0 0.0 0.0 0.0

16 mM SDS/10 mM C12E6 16 mM SDS/20 mM C12E6

Conclusion From a thermodynamic viewpoint the dominant parameter which determines whether SDS binds to PVP is the free monomer SDS concentration in the bulk solution. The continued addition of C12EO6 to an SDS/PVP complex in solution gradually reduces the monomer SDS concentration to a value lower than T1 but still allows mixed SDS/C12EO6 micelles to bind on the polymer. During this process the bound surfactant is continually being stripped off the polymer until the polymer becomes free of bound surfactant in C12EO6 rich solution. The aggregation numbers of bound micellar SDS and also SDS/C12EO6 mixed micelles are not very much different from the values measured in the absence of polymer. In addition, when bound mixed micelles are in equilibrium with free mixed micelles the micellar compositions are very similar. The concentration of the bound mixed micelles, however, decrease as the C12EO6 content is increased. The significant difference between the free and bound pure SDS and mixed micelles is that the polymer is associated with the surface of the bound micelles in the complex. Indeed this work has provided a strong basis to confirm earlier indirect evidence that the interaction between PVP and SDS micelles involves an attraction between polymer segments and the micellar surface. Although exact details are not known,26,27 it is most likely that this attractive interaction is dominated by electrostatic forces. This is based on the premise that the majority of cationic surfactants do not interact with PVP. Dubin and co-workers15,16 have extensively used mixed SDS/nonionic surfactant mixtures to moderate the very strong electrostatic interaction between SDS micelles and oppositely charged polycations In these systems the interactions involved are exclusively between SDS micelles and the oppositely charged polyelectrolyte and are dominated by Coulombic attraction between the opposite charges. This interaction often leads to the precipitation of the complex from solution. It is well-known that phase (26) Chari, K. J. Colloid Interface Sci. 1992, 151, 294. (27) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637.

separation of this kind can be avoided by reducing the surface charge density on either, or both, the polyelectrolyte and SDS micelles. Dubin et al. have achieved a moderation of the phase separation by reducing the charge density of the SDS micelles by forming mixed micelles with nonionic surfactants. These results suggest that in the present work the charge density of the micelle is the factor which also determines the extent of binding. This confirms the electrostatic nature of the interaction between SDS and PVP. Although other nonionic surfactants are known to desorb SDS from nonionic polymers, it could be that the bulky EO headgroups in the mixed micelle increase the average distance between bound PVP segments and the sulfate headgroup on the mixed micellar surface. In the 1% PVP/SDS system at 16 mM of SDS the amount of bound SDS estimated from the emf experiment is 12 mM. The aggregation number of the bound SDS micelles is 85 which means that on average there are 5 bound micelles per polymer chain. From the SANS analysis the number of polymer monomer units incorporated on the surface of each micelle is 150. This number is much less, ∼40, when mixed micelles are formed on the polymer at 16 mM SDS/2 mM C12EO6. The reason for this is that the EO headgroups in the mixed micelles shield many of the charged sulfate headgroups from the polymer and this leads to a more loose form of binding. The final result is complete desorption of SDS as the C12EO6 content of the micelle is increased. In conclusion, we have demonstrated that polymerbound SDS micellar aggregates are removed from the polymer PVP by the addition of the nonionic surfactant C12EO6. This desorption can be monitored using emf and ITC experiments. The addition of C12EO6 to a solution containing PVP/micellar SDS complex leads to the formation of mixed SDS/C12EO6 bound to the polymer followed by the simultaneous removal of SDS monomers from the bulk solution and also polymer bound SDS. The SDS is used to form free SDS/C12EO6 mixed micelles in solution. LA000292H