Effects of Aggregates on Mixed Adsorption Layers of Poly(ethylene

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Langmuir 2009, 25, 4036-4046

Effects of Aggregates on Mixed Adsorption Layers of Poly(ethylene imine) and Sodium Dodecyl Sulfate at the Air/Liquid Interface† Katrin Tonigold,‡ Imre Varga,*,§,| Tommy Nylander,‡ and Richard A. Campbell*,‡,⊥ Department of Physical Chemistry 1, Lund UniVersity, P.O. Box 124, S-221 00 Lund, Sweden, Institute of Chemistry, Eo¨tVo¨s Lora´nd UniVersity, Budapest 112, P.O. Box 32, H-1518 Hungary, Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas Va¨g 51, S-100 44 Stockholm, Sweden, and Institut Laue-LangeVin, BP 156, 38042 Grenoble Cedex 9, France ReceiVed August 29, 2008. ReVised Manuscript ReceiVed NoVember 27, 2008 We have exploited the spatial and kinetic resolution of ellipsometry to monitor the lateral movement of inhomogeneous patches of material in mixed adsorption layers of poly(ethylene imine) and sodium dodecyl sulfate at the air/liquid interface. We show that the choice of sample preparation methods can have a profound effect on the state of the interface for chemically equivalent samples. The extent of aggregation in the bulk solution on relevant time scales is affected by specific details of the polymer/surfactant mixing process, which produces varying numbers of aggregates that can become trapped in the interfacial layer, resulting in an enhanced and fluctuating ellipsometry signal. It can be beneficial to apply the surface-cleaning method of aspiration prior to physical measurements to remove trapped aggregates through the creation of a fresh interface. At low pH, the ellipsometry signal of samples prepared with surface cleaning is remarkably constant over a factor of >500 in the bulk composition below charge equivalence, which is discussed in terms of possible adsorption mechanisms. At high pH, through observing temporal fluctuations in the ellipsometry signal of samples prepared with surface cleaning, we reveal two important processes: there is the spontaneous adsorption of aggregates >0.2 µm in diameter into the interfacial layer, and with time there is the fusion of smaller aggregates to generate new large surface aggregates. We attribute the favorability of the adsorption and fusion processes at high pH to reduced electrostatic barriers resulting from the low surface charge density of the aggregates. It is inappropriate in this case to consider the interface to comprise a homogeneous adsorption layer that is in dynamic equilibrium with the bulk solution. Our work shows that it can be helpful to consider whether there are macroscopic particles embedded in molecular layers at the air/liquid interface for systems where there is prior knowledge of aggregation in the bulk phase.

Introduction Poly(ethylene imine) [PEI] has captured a lot of attention from the academic and industrial research communities in recent years. Four factors in particular make PEI an interesting class of polymer both from a fundamental and practical perspective. First, the polymer molecular architecture can be readily varied by changing the proportion of primary, secondary, and tertiary nitrogen atoms, and hence the structure can range from linear to hyperbranched, which affects its morphology in bulk solution and at interfaces.1,2 Second, the physical properties depend on the molecular mass of the polymer,3 which is commercially available over the range of 2-750 kDa. Third, the polyelectrolyte charge can be set by modifying the solution pH: effectively, PEI has a high positive charge density at low pH but a diminished charge density at high pH.1,2 Fourth, the physicochemical properties are affected by the ionic strength because the addition of electrolyte increases the surface activity of polymer/surfactant complexes.3 † Part of the Neutron Reflectivity special issue. * Corresponding authors. (I.V.) Tel: +36 1 372 2514. Fax: +36 1 372 2592. E-mail: [email protected]. (R.A.C.) Tel: +33 476 207 097. Fax: +33 476 207 120. E-mail: [email protected]. ‡ Lund University. § Eo¨tvo¨s Lora´nd University. | Royal Institute of Technology. ⊥ Institut Laue-Langevin.

(1) Penfold, J.; Tucker, I.; Thomas, R. K.; Zhang, J. Langmuir 2005, 21, 10061–10073. (2) Wang, H.; Wang, Y.; Yan, H.; Zhang, J.; Thomas, R. K. Langmuir 2006, 22, 1526–1533. (3) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, J.; Zhang, X. L. Langmuir 2007, 23, 3690–3698.

The main focus of research on PEI has been its diverse and rich interactions with anionic surfactants such as sodium dodecyl sulfate (SDS).1-4 A very flexible system results from the ability to tune a combination of electrostatic and hydrophobic interactions through properties such as the polymer architecture and molecular mass or the solution pH and ionic strength. There is also considerable industrial interest in understanding the nature of surface and bulk interactions between polymers and surfactants because such mixtures are used extensively in commercial products such as shampoos and fabric conditioners.5,6 Fundamental work on the interactions of PEI and SDS at the air/liquid interface was performed using neutron reflectometry (NR) by Thomas, Penfold, and co-workers.1,3 NR is a particularly powerful technique because neutron wavelengths are on the order of interfacial thicknesses and the scattering properties of the interface can be tuned to give several fittable reflectivity profiles through selective deuteration of the surfactant and subphase.7 Experiments can be designed to give an efficient and unique return of information about the structure and composition of adsorbed layers.8 (4) Me´sza´ros, R.; Thompson, L.; Bos, M.; Varga, I.; Gila´nyi, T. Langmuir 2003, 19, 609–615. (5) (a) Goddard, E. D. Colloids Surf. 1986, 19, 301–329. (b) Kwak, J. C. T., Ed. Polymer-Surfactant Systems; Marcel Dekker: New York, 1998; Vol. 77. (6) Thalberg, K.; Lindman, B. Polymer-surfactant Interactions - Recent Developments. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (7) Penfold, J.; Thomas, R. K. J. Phys.: Condens. Matter 1990, 2, 1369–1412. (8) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899–3917.

10.1021/la8028325 CCC: $40.75  2009 American Chemical Society Published on Web 01/13/2009

Aggregate Effects on PEI/SDS Adsorption Layers

The first application of NR to interfacial layers on free liquids was performed in the late 1980s on the CRISP beamline at ISIS in the UK.9 Over the following two decades, a substantial body of work on surfactant adsorption at the air/liquid interface was performed with the technique, often in combination with tensiometry.10 In the early 2000s, a series of articles was published on strongly interacting polymer/surfactant mixtures adsorbed at the air/liquid interface, including work on the systems poly(diallyldimethylammonium chloride) + SDS11 and poly(styrene sulfonate) + cationic surfactants.12 For both systems, the experimental results were interpreted in terms of a competition between the favorability of surface and bulk complexes. The combination of electrostatic and hydrophobic interactions in the system sodium poly(acrylic acid) + cationic surfactants was also studied using NR.13 The subject area of polymer/surfactant interactions at the air/liquid interface is covered comprehensively in two excellent reviews.14 Although a piece of work concerning ethoxylated PEI preceded it,15 the first article on the surface interactions of PEI and SDS was published by Penfold et al. in 2005.1 In this study, NR was used to show that SDS adsorption was “unexpectedly most pronounced when the solution pH is high” in the presence of hyperbranched PEI and the appearance of a Bragg diffraction peak indicated the presence of a multilayer structure normal to the interface.1 The multilayer formation at the surface at high pH was shown to be very dependent on the polymer molecular mass and ionic strength.3 These studies were complemented with isothermal titration calorimetry measurements to demonstrate the pH dependence of the surfactant binding.2 Additionally, NR has also been used to study the strong influence of PEI on the surface properties of a binary surfactant mixture.16 The issue of the surface tension “cliff edge” for strongly interacting polymer/surfactant mixtures,11 where a rise in surface tension occurs as the bulk concentration of surfactant is increased, was taken to a new level with two publications in 2007 discussing a theoretical model to describe the phenomenon.17 The model is based on the underlying assumption that the polyelectrolyte/ surfactant system is a homogeneous one-phase system that is in dynamic equilibrium. The surface properties are rationalized in terms of the competition between two different types of polyelectrolyte/surfactant complexes: one that is very surfaceactive and one that is not surface-active. The model describes the experimental features of measured surface tension data for polyelectrolyte/surfactant systems but has not yet been evaluated in terms of bulk binding studies. (9) Bradley, J. E.; Lee, E. M.; Thomas, R. K.; Willatt, A. J.; Penfold, J.; Ward, R. C.; Gregory, D. P.; Waschkowski, W. Langmuir 1988, 4, 821–826. (10) Lu, J. R.; Thomas, R. K.; Penfold, J. AdV. Colloid Interfac. 2000, 84, 143–304. (11) (a) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. J. Phys., Condens. Matter 2000, 12, 6023–6038. (b) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. Langmuir 2002, 18, 5139–5146. (c) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. Langmuir 2002, 18, 5147– 5153. (12) (a) Taylor, D. J. F.; Thomas, R. K.; Penfold, J. Langmuir 2002, 18, 4748– 4757. (b) Taylor, D. J. F.; Thomas, R. K.; Hines, J. D.; Humphreys, K.; Penfold, J. Langmuir 2002, 18, 9783–9791. (c) Taylor, D. J. F.; Thomas, R. K.; Li, P. X.; Penfold, J. Langmuir 2003, 19, 3712–3719. (13) Zhang, J.; Thomas, R. K.; Penfold, J. Soft Matter 2005, 1, 310–318. (14) (a) Penfold, J.; Thomas, R. K.; Taylor, D. J. F. Curr. Opin. Colloid Interface Sci. 2006, 11, 337–344. (b) Taylor, D. J. F.; Thomas, R. K.; Penfold, J. AdV. Colloid Interface Sci. 2007, 132, 69–110. (15) Penfold, J.; Taylor, D. J. F.; Thomas, R. K.; Tucker, I.; Thompson, L. J. Langmuir 2003, 19, 7740–7745. (16) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, J.; Bell, C. Langmuir 2006, 22, 8840–8849. (17) (a) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, X. L.; Bell, C.; Breward, C.; Howell, P. Langmuir 2007, 23, 3128–3136. (b) Bell, C. G.; Breward, C. J. W.; Howell, P. D.; Penfold, J.; Thomas, R. K. Langmuir 2007, 23, 6042–6052.

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Considerable effort over the last two decades has been devoted to gaining an understanding of the nature of the interaction between polyelectrolytes and oppositely charged surfactants in the bulk phase.18 Most of the work has been carried out on linear polyelectrolyte systems. These investigations showed that usually a one-phase transparent system forms in the presence of a small amount of surfactant. By increasing the bulk surfactant concentration, the solution becomes turbid, and an associative phase separation occurs (i.e., a concentrated phase enriched in both polymer and surfactant separates from a dilute aqueous phase containing mostly small ions). By increasing the bulk surfactant concentration further, the turbidity may decrease, and redissolution of the polyelectrolyte/surfactant complexes can occur. The surfactant binding in the bulk phase is usually interpreted in terms of a two-part equilibrium process.19 First, condensation of the surfactant ions takes place as an ion-exchange reaction, which is followed by the cooperative binding of the condensed surfactant ions to the polyelectrolyte. Because of the strong interactions, the cooperative binding process is typically characterized by a very small critical aggregation concentration. Phase separation occurs when the charges on the polyelectrolytes are compensated for by those of bound surfactant molecules at polyelectrolyteto-surfactant ratios close to charge equivalence. If the polyelectrolyte-to-surfactant ratio is imbalanced, however, a stable one-phase system can form because of the significant net charge of polyelectrolyte/surfactant complexes. Several studies over the past decade have addressed the nature of the bulk solution interaction between hyperbranched PEI and SDS. Most of these investigations used indirect methods, where physical properties were measured as a function of the total surfactant concentration at fixed polymer concentration. Winnik et al. investigated the PEI/SDS interaction at high pH (low charge density) and found that SDS binding to the polymer is accompanied by the release of OH- ions.20,21 Furthermore, they observed that PEI/SDS mixtures exhibit an unusually large conductivity compared to that of corresponding pure SDS solutions. Isothermal calorimetry measurements indicated that the initial surfactant binding at low concentrations is exothermic, but the measured enthalpy curves exhibited an endothermic peak with increasing SDS concentration. Li et al. investigated the effect of solution pH on the surfactant binding to PEI.22 They found that SDS has a large binding affinity to PEI independent of the solution pH. These results were interpreted in terms of a two-step process where initial monomer binding is followed by cooperative binding involving the formation of micelle-like surfactant aggregates. This interaction mechanism was not consistent with direct surfactant binding isotherm measurements performed by Me´sza´ros et al. on PEI samples with a large molecular mass (Mm ) 750 (18) (a) Bergfeldt, K.; Piculell, L.; Linse, P. J. Phys. Chem. B 1996, 100, 3680–3687. (b) Ilekti, P.; Piculell, L.; Tournilhac, F.; Cabane, B. J. Phys. Chem. B 1998, 102, 344–351. (c) Ranganathan, S.; Kwak, J. C. T. Langmuir 1996, 12, 1381–1390. (d) Thalberg, K.; Lindman, B.; Bergfeldt, K. Langmuir 1991, 7, 2893–2898. (e) Thalberg, K.; Lindman, B.; Karlstrom, G. J. Phys. Chem. 1991, 95, 3370–3376. (19) (a) Allen, R. J.; Warren, P. B. Langmuir 2004, 20, 1997–2009. (b) Hansson, P. Langmuir 2001, 17, 4167–4180. (c) Hansson, P.; Almgren, M. J. Phys. Chem. 1996, 100, 9038–9046. (d) Nguyen, T. T.; Shklovski, B. I. J. Chem. Phys. 2001, 114, 5905–5916. (e) Schiessel, H.; Bruinsma, R. F.; Gelbart, W. M. J. Chem. Phys. 2001, 115, 7245–7252. (f) Skepo, M.; Linse, P. Macromolecules 2003, 36, 508–519. (20) Bystryak, S. M.; Winnik, M. A.; Siddiqui, J. C. Langmuir 1999, 15, 3748–3751. (21) Winnik, M. A.; Bystryak, S. M.; Chassenieux, C. Langmuir 2000, 16, 4495–4510. (22) Li, Y.; Ghoreishi, S. M.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 3093–3100.

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kDa).4,23 They showed that, independent of the solution pH and ionic strength, SDS binds to PEI by two different mechanisms with increasing surfactant concentration. In the first process, the dodecyl sulfate ions bind to the protonated amine groups in monomer form. Once a sufficient amount of monomer binding has taken place on individual polymer molecules, the PEI/SDS complexes collapse, and aggregation/precipitation of the complex particles occurs with time. In the second process, SDS molecules adsorb onto the surface of the collapsed PEI/SDS particles, which leads to their charge reversal. Once a sufficient amount of surfactant binding has taken place on the collapsed particles, an electrostatically stabilized dispersion is formed, and the dynamic aggregation/precipitation process is suppressed. Because a cooperative binding regime was not observed in the case of the hyperbranched-PEI/SDS system, they concluded that the binding mechanism was different from the general characteristics of the linear polyelectrolyte/surfactant interaction. A thermodynamic model was proposed to describe quantitatively the change in pH that accompanies SDS binding to PEI while taking into account the positive feedback of the binding on the protonation equilibrium of the amine groups.4 This treatment explained the experimental observations, also noted by Winnik et al.,21 that the binding capacity of PEI at charge equivalence is almost as large at high pH as when the initial charge density of PEI is almost an order of magnitude higher at low pH. The dispersion formation is not an equilibrium process on relevant time scales, so the specific details of the polymer/ surfactant mixing process can determine whether a kinetically stable dispersion of small aggregates or phase separation and precipitation occur.24 These effects were demonstrated and exploited to generate an extension of the composition range of the formation of kinetically stable nanoparticles.25 Bastardo et al. performed a detailed study with small-angle neutron and X-ray scattering to characterize the solution structures of the hyperbranched PEI with and without SDS both at high and low pH.26 They found that PEI molecules adopt elongated, flat ellipsoid structures and that on complexation with SDS molecules the conformation of the PEI molecules remains unaffected over the entire surfactant concentration range measured. With increasing SDS concentration, the individual PEI/SDS complexes start to aggregate and stack on top of each other. At high pH, the stacked structures are not very ordered. The spacing of the PEI layers decreases with increasing SDS concentration within the aggregates until charge equivalence is achieved, where it becomes constant. At low pH, the internal structure of the PEI/ SDS aggregates is ordered, as evidenced by a Bragg-like peak in the scattering curves. The fact that the position of the Bragg peak remains constant but its intensity increases with increasing SDS concentration demonstrates the growth of PEI/SDS aggregates with a well-defined structure as surfactant binding proceeds. SAXS measurements indicated that the PEI/SDS aggregates have a lamellar structure in which two neighboring PEI molecules are separated by a surfactant bilayer. Solvent isotope effects on PEI/SDS interactions have also been investigated, where it was shown that the charge density of PEI can be significantly different in H2O and D2O at high pH/pD values.27 Several different aspects of PEI self-assembly have recently been reported.21,28 For example, work by O’Driscoll et al. on the (23) Mezei, A.; Me´sza´ros, R. Langmuir 2006, 22, 7148–7151. (24) Mezei, A.; Me´sza´ros, R.; Varga, I.; Gila´nyi, T. Langmuir 2007, 23, 4237– 4247. (25) Mezei, A.; Me´sza´ros, R. Soft Matter 2008, 4, 586–592. (26) Bastardo, L. A.; Garamus, V. M.; Bergstro¨m, M.; Claesson, P. M. J. Phys. Chem. B 2005, 109, 167–174. (27) Bastardo, L. A.; Me´sza´ros, R.; Varga, I.; Gila´nyi, T.; Cleasson, P. M. J. Phys. Chem. B 2005, 109, 16196–16202.

Tonigold et al.

complexation of PEI with a cationic surfactant revealed the presence of elliptical micelles in the bulk solution and the spontaneous formation of hexagonal mesostructures at the air/ liquid interface.29 Studies have been conducted at the liquid/ silica interface concerning the reversibility of PEI adsorption,30 the interaction of PEI/SDS complexes,31 and multilayer formation with microfibrillated cellulose.32 A field of research that is currently growing concerns PEI-based nanoparticles, which have potential in gene-delivery applications.33 A useful optical technique for the characterization of polymer/ surfactant adsorption layers at the air/liquid interface is ellipsometry, where the reflection of elliptically polarized light depends on the dielectric profile normal to the interface.34 The phase change of light upon reflection at the surface of a polymer/ surfactant solution, relative to that of a clean interface, is determined by the structure and amount of adsorbed material. Although NR is undoubtedly a very powerful and flexible technique, there are distinct characteristics of ellipsometry that make it a valuable complementary technique (e.g., the kinetic resolution is at least 2 orders of magnitude faster and the sampled area of the interface is typically 3 to 4 orders of magnitude smaller than in NR). We exploit these facets of ellipsometry in the present work by the interpretation of erratic temporal variations in the optical signal, features that would most likely not be detected in the relatively slow and macroscopic measurements made using NR. Furthermore, the comparison of ellipsometry measurements, both with changing bulk composition and evolving time, allows us to track the relative amount of adsorbed material. Our intention is to shed new light on the effects of large aggregates present in adsorbed PEI/SDS layers at the air/liquid interface. To achieve this goal, we divide the results and discussion section into two parts: the effects of different sample preparation methods and the nature of the interface with changing polyelectrolyte charge density.

Experimental Section Materials. Deionized water was passed through a purification system (Milli-Q; total organic content ) 4 ppb; resistivity ) 18 MΩ cm). All experiments were carried out in 100 ppm hyperbranched PEI (molecular mass ) 750 000 g/mol; ratio of primary, secondary, and tertiary amine groups ) 1:2:1; BASF) and 0.1 M NaCl (Merck, 99.99%) at 25 °C. Batches of a few hundred milligrams of PEI were dissolved in salt solution, and these stock polymer solutions (1000 ppm) were stirred overnight to ensure full dissolution. The next day, the solutions were passed through a fresh 0.2 µm sterile membrane (Anotop 25; Whatman) at a rate of ∼0.2 mL/s to remove aggregated impurities. SDS (Sigma, 99.9%) was recrystallized three times in ethanol, and each time the solutions were cooled for several hours to maximize the purity of the surfactant; a hot filtration process was carried out beforehand to remove insoluble impurities. Ellipsometry. Ellipsometry measurements are based on the change in the polarization of light reflected at a surface. Polarized light can be regarded as being composed of two polarized waves, where the (28) Griffiths, P. C.; Paul, A.; Fallis, I. A.; Wellappili, C.; Murphy, D. M.; Jenkins, R.; Waters, S. J.; Nilmini, R.; Heenan, R. K.; King, S. M. J. Colloid Interface Sci. 2007, 314, 460–469. (29) O’Driscoll, B. M. D.; Milsom, E.; Fernandez-Martin, C.; White, L.; Roser, S. J.; Edler, K. J. Macromolecules 2005, 38, 8785–8794. (30) Me´sza´ros, R.; Thompson, L.; Varga, I.; Gila´nyi, T. Langmuir 2003, 19, 9977–9980. (31) Me´sza´ros, R.; Varga, I.; Gila´nyi, T. Langmuir 2004, 20, 5026–5029. (32) Aulin, C.; Varga, I.; Claesson, P. M.; Wagberg, L.; Lindstro¨m, T. Langmuir 2008, 24, 2509–2518. (33) (a) de Wolf, H. K.; de Raad, M.; Snel, C.; van Steenbergen, M. J.; Fens, M.; Storm, G.; Hennink, W. E. Pharm. Res. 2007, 24, 1572–1580. (b) Burke, R. S.; Pun, S. H. Bioconjugate Chem. 2008, 19, 693–704. (c) Pearce, M. E.; Mai, H. Q.; Lee, N.; Larsen, S. C.; Salem, A. K. Nanotechnology 2008, 19, 175103. (34) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1977.

Aggregate Effects on PEI/SDS Adsorption Layers

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electric vector of the p polarization oscillates parallel to the plane of incidence and the electric vector of the s polarization oscillates perpendicular to the plane of incidence. Upon reflection at the interface, the relative amplitude and the phase of these p and s components change by different amounts. The attenuation, described by Ψ, and the phase shift, described by ∆, depend on the optical properties of the surface and on the angle of incidence φ. The ellipsometric angles are related to the overall Fresnel reflectivity coefficients of the parallel and perpendicular components, rp and rs, respectively,34

rp ) tan Ψ exp i∆ rs

(1)

Ellipsometry at transparent air/liquid interfaces offers a very different capability to that at air/solid or liquid/solid interfaces where the solid absorbs light (e.g., semiconductors or metals). One limitation at the air/liquid interface is that Ψ is very insensitive to the optical properties of the thin film. Therefore, in this work we simply present measurements of ∆. We define here the effect of the surface adsorption layer on the measured ellipsometric phase shift as ∆surf ) ∆PS - ∆0, where ∆PS is the measured parameter for a polymer/surfactant solution and ∆0 is the reference value for pure water. In the thin film limit, ∆surf is linearly proportional to the ellipsometric thickness η,35

∆surf )

g(φ) η λ

(2)

where λ is the wavelength, g(φ) is a function that depends only on bulk properties and the angle of incidence given by35

g(φ) )

4π√εr,ambεr,subcos φ sin2 φ [εr,sub - εr,amb][(εr,sub + εr,amb)cos2 φ - εr,amb]

(3)

εr,amb is the relative permittivity of the ambient medium (air), εr,sub is the relative permittivity of the substrate (liquid), and εr ) n2 (the square of the refractive index). For an optically isotropic interface, Drude showed that η can be written in terms of the relative permittivity profile across the interface εr(z),36

η)

∫

[εr(z) - εr,amb][εr(z) - εr,sub] dz εr(z)

(4)

where z is the distance normal to the interface. The effect of scattering by thermal capillary waves is neglected in this representation of η because of the minor influence of the changing surface tension.37 For a single uniform isotropic layer separated by sharp stratified interfaces, eqs 2 and 4 may be reduced to

ksurf ∆surf ) ksurfd ) Γ F

of nonionic surfactant molecules.38 However, sophisticated models are required to treat more complicated interfaces, such as monolayers of ionic surfactant molecules.39 The physical picture of the air/liquid interface for aqueous PEI/ SDS mixtures is a subject of discussion in the present work, in which we reveal the presence of surface aggregates and consider the possibility of an adsorption layer where PEI/SDS complexes are electrostatically bound to a surface layer of SDS molecules. We do not attempt to derive the surface excess directly from measurements of ∆surf using the assumption of a uniform layer model because we have good reason to believe its application might be inappropriate. Instead, we take the magnitude of ∆surf to represent an approximate measure of the average amount of adsorption in the area illuminated by the laser beam and monitor relative changes with changing composition and surface age to reveal novel features of the system. An Optrel Multiskop null ellipsometer, equipped with an Nd: YAG laser with a wavelength of λ ) 532 nm, was employed at an angle of incidence of φ ) 50°, close to the Brewster angle φB ) arctan(nsol/nair) ) 53.2°, where ∆ is very sensitive to the buildup of material at the air/liquid interface. The spot size of the laser beam on the interface was ∼1 mm2, and the maximum data acquisition rate was ∼0.5 Hz. The instrument was used in the following configuration: varying polarizer (Pn), fixed compensator, and varying analyzer (An). Each measured value of ∆ was recorded in two zones (n ) 1 and 3) with a fixed compensator oriented at -45° to satisfy the nulling criterion (i.e., minimum light intensity at the detector for each pair of ′P1 and A1′ and ′P3 and A3′. Multizone data acquisition reduces the systematic errors introduced by imperfections in the optical components, and ideally P3 ) P1 + 90° and A3 ) -A1 in two-zone analysis. Before each experiment, the values of Ψ ) (A1 - A3)/2 and ∆ ) P1 + P3 for pure water were recorded 10 times to confirm the satisfactory alignment of the system. Also, the mean value of the measurements of ∆ provided the reference value ∆0, which was subtracted from values of ∆PS recorded for polymer/ surfactant mixtures at intervals of up to 30 s to yield values of ∆surf. For the data presented with respect to the bulk SDS concentration, the values of ∆surf were averaged over 15-30 min after surface preparation, and the error bars show the extremes in the values. This representation of the temporal variations in the data was chosen to show clearly any fluctuations in the values. Surface Tensiometry. A surface tension balance (KSV Spot 1 surface potential meter, KSV Instrument Ltd.) was used to record kinetic measurements of solutions in a Petri dish at a rate of one reading every 10 s. A sand-blasted platinum Wilhelmy plate of width 20 mm, height 10 mm, and thickness 0.10 mm was immersed up to one-third into liquid contained in the Petri dish. Initially, the entire plate was dipped in the solution to ensure that the full wetting criterion was met. The overall force F that acts on the plate was used to calculate the surface tension, γ, as

γ) (5)

F cos θ 2pp

(7)

and where d is the thickness, F is the density, Γ is the surface excess, and εsurf the dielectric constant of the polymer/surfactant adsorption layer. Hence for this simple case of a uniform isotropic layer of constant density with zero roughness, ∆surf is linearly proportional to both the layer thickness and the surface excess. Such a model can be a reasonable approximation for a homogeneous adsorption layer

where pp is the perimeter of the plate and θ is the contact angle between the liquid and plate. The full wetting criterion cos θ ) 1 was assumed. The Wilhelmy plate was cleaned immediately before each measurement: first it was washed with pure water and 2-propanol (Merck KGaA, Germany; purity >99.9%), and then it was heated in the blue flame of a Bunsen burner to ensure that it was dry and that any residual organics were burned off. The plate was hung on the surface pressure balance and then inspected visually to ensure that the orientation was level and perpendicular to the liquid surface. The plate was then immersed in pure water to achieve a surface tension of γ ) 71.8 ( 0.5 mN m-1 to ensure the cleanliness of the

(35) Motschmann, H.; Teppner, R. Ellipsometry in Interface Science. In NoVel Methods to Study Interfacial Layers; Miller, R., Mœbius, D., Eds.; Elsevier: New York, 2001. (36) Drude, P. Ann. Phys. Chem. (Leipzig) 1891, 126, 43. (37) Meunier, J. Light Scattering by Liquid Surfaces and Complementary Techniques; Marcel Dekker: New York, 1992.

(38) Goates, S. R.; Schofield, D. A.; Bain, C. D. Langmuir 1999, 15, 1400– 1409. (39) (a) Pe´ron, N.; Campbell, R. A.; Nylander, T.; Vareikis, A.; Makuska, R.; Gila´nyi, T.; Me´sza´ros, R. J. Phys. Chem. B 2008, 112, 7410–7419. (b) Tyrode, E.; Johnson, C. M.; Rutland, M. W.; Day, J. P. R.; Bain, C. D. J. Phys. Chem. C 2007, 111, 316–329.

where

ksurf )

g(φ)[εsurf - εamb][εsurf - εsub] εsurfλ

(6)

4040 Langmuir, Vol. 25, No. 7, 2009 dish and the good performance of the plate. Afterward, the equipment was used to measure the surface tension of polymer/surfactant solutions in the same dish. The solution preparation is described below. In every case, the values of γ were recorded up to 30 min after surface preparation at intervals of 10 s. Sample Preparation Protocols. It has been shown previously for the PEI/SDS system that the choice of polymer/surfactant mixing protocol affects the extent of aggregation and therefore the compositional phase boundaries in the bulk phase.4,24 Three kinds of mixing protocols were applied for the preparation of the PEI/SDS solutions in the present work. In every case, a plentiful supply of 0.1 M NaCl solution was made in large volumetric flasks. Note that the measurements were performed with added electrolyte to enhance surface adsorption and the production of bulk aggregates;3 the precipitation region would have covered a smaller composition range had the measurements been carried out in the absence of added salt.24 A few milliliters of this solution was used to dissolve a few hundred milligrams of PEI to give a stock solution of 1000 ppm PEI in 0.1 M NaCl, as described previously. Every data point presented is derived from an individual fresh solution that was made to a volume of 100 mL (100 ppm PEI; 0.1 M NaCl; varying concentrations of SDS) according to the following protocols. Basic Mixing. The following approach was carried out to produce the highest amount of kinetically trapped aggregates. The PEI/NaCl stock solution was diluted with 0.1 M NaCl solution to give 100 ppm PEI in 0.1 M NaCl solution. Then the pH was adjusted through adding a small volume of concentrated HCl or NaOH. This solution was then poured onto an appropriate amount of dry SDS crystals in a 500 mL volumetric flask. The solution was then shaken vigorously to dissolve the SDS crystals and produce a PEI/SDS/NaCl stock solution with a bulk SDS concentration of 4 mM. Further mixed solutions at lower SDS concentrations were obtained by dilution of the PEI/SDS/NaCl stock solution with PEI/NaCl solution. Standard Mixing. The following approach was carried out to replicate polymer/surfactant mixing protocols typically carried out in literature studies.40 The PEI/NaCl stock solution was diluted with 0.1 M NaCl solution to give a 200 ppm PEI in 0.1 M NaCl solution. (The PEI was twice the intended concentration of the mixed solution as a result of subsequent dilution.) Then the pH was adjusted through adding a small volume of concentrated HCl or NaOH. In separate 50 mL volumetric flasks, SDS in 0.1 M NaCl solutions were made over a range of ∼20 different SDS concentrations. (The SDS was twice the intended concentration of the mixed solution as a result of subsequent dilution.) To make each mixed solution, 50 mL of SDS in 0.1 M NaCl solution was transferred to a 100 mL beaker, and then 50 mL of 200 ppm PEI in 0.1 M NaCl was poured into the same beaker. The solutions were transferred to volumetric flasks and mixed by turning the flasks upside down several times. AdVanced Mixing. The following approach was carried out to produce the smallest number of nonequilibrium trapped aggregates. The pH of the 1000 ppm PEI/NaCl stock solution was adjusted through adding a small volume of concentrated HCl or NaOH. Ten milliliters of this PEI/NaCl was poured into a burette, which was positioned above a beaker. Ten milliliters of SDS in 0.1 M NaCl solutions were made over a range of ∼20 different SDS concentrations. (The SDS was 10 times the intended concentration of the mixed solution as a result of subsequent dilution.) In turn, each SDS solution was transferred to a second burette positioned above the beaker. Eighty milliliters of the 0.1 M NaCl solution was transferred to the beaker. A PTFE-coated magnetic stir bar was placed in the beaker and was set to stir vigorously. Over a period of 10 min, the contents of the two burettes were dripped slowly into the highly stirred salt solution. Additionally, two ways of preparing the surfaces were applied. Without Surface Cleaning. The solution, in a volumetric flask, was shaken gently and then poured into the Petri dish. The solution was used in measurements without any further treatment. With Surface Cleaning. The solution, in a volumetric flask, was shaken gently and then poured into the Petri dish. The surface was (40) Penfold, J.; Me´sza´ros, R. Personal communications, 2008.

Tonigold et al. aspirated for ∼3 s using a clean pipet attached to a water suction pump. This process served three purposes. First, it removed any small amount of surface contamination such as dust or lumps of precipitate (and trapped air bubbles) formed during mixing. Second, the creation of a fresh interface allowed the study of repeatable adsorption time scales. Third, a known amount of liquid could be used through aspirating the surface until the position of the reflected ellipsometer beam hit the center of the pinhole adjacent to the analyzer. Typically, the surface was cleaned 20-30 s before the start of the measurements. Measurement Procedures. A clean Petri dish with a diameter of 100 mm and a depth of 15 mm was placed on a dual-direction tilt stage, which sat on a vertical translation stage. The tilt stage was leveled with a spirit level. (Minor misalignment of the tilt stage had a negligible effect on the measurements because gravity made the surface in the center of the dish horizontal.) Solutions were shaken and then poured into the dish, and the depth of solution was always more than 10 mm. A neutral density filter that had been cut at an incline was placed in the bottom of the dish to prevent reflections from the liquid/glass interface entering the pinhole adjacent to the analyzer. These two factors ensured that the measured reflectivity was solely from the air/liquid interface. The surface tension plate was then dipped into the solution, as described above. To maximize the precision of the measurements, a procedure was carried out beforehand where the height of the air/liquid interface was fine tuned. First, the pinhole adjacent to the analyzer was closed. Then the horizontal position of the dish on the tilt stage was adjusted until the laser beam reflected at the point midway between the Wilhelmy plate and the far side of the dish (to minimize edge effects), and the vertical translation stage was adjusted until the reflected laser beam hit the center of the pinhole. This procedure was repeated until both alignment criteria were met. The pinhole was then opened to a diameter of 4 mm in order to make the measurements. Data Presentation. The kinetic and spatial resolution of null ellipsometry allows the detection of inhomogeneity in the surface layer from the presence of fluctuations in the signal. For all mixed PEI/SDS solutions where this characteristic was not displayed, the value of ∆surf had reached steady state within 15 min. The presented data are the mean value over a period of 15-30 min. The error bars show the maximum and minimum values recorded within this period for each measurement to give the reader an immediate visual impression of the extent of fluctuations, and hence the level of inhomogeneity in the surface layer. The kinetic surface tension measurements, performed using a Wilhelmy plate, were not subject to fluctuations resulting from surface inhomogeneity. A data presentation analogous to that of ellipsometry would therefore have no physical meaning. Instead, to estimate the equilibrium surface tension values, the data were extrapolated to infinite time, t, utilizing an asymptotic solution of the Ward-Tordai equation,41 which predicts a t-1/2 dependence of the surface tension in the long-time limit assuming diffusion-controlled adsorption kinetics. Figure 1 shows kinetic surface tension curves for three different bulk compositions of mixed PEI/SDS solutions at pH 4, and the inset illustrates the Ward-Tordai analysis to extrapolate the data to infinite time (i.e., zero reciprocal time). Figure 2 shows the absolute surface tension readings at 10 and 30 min after surface cleaning plus the extrapolation to infinite time using the Ward-Tordai analysis. Together the Figures portray a clear message that the application of Ward-Tordai analysis becomes important with corrections of several mN m-1 at low bulk SDS concentrations for the PEI/SDS system. In fact, had the values been taken after only 30 min, then discrepancies in γ of >6 mM m-1 would have been made in the case of the lowest concentrations measured.

Results and Discussion Effects of Different Sample Preparation Methods. If adsorption layers of polymer/surfactant mixtures at the air/liquid (41) (a) Ward, A. F. H.; Tordai, L. J. Chem. Phys. 1946, 14, 453–461. (b) Fainerman, V. B.; Makievski, A. V.; Miller, R. Colloids Surf., A 1994, 87, 61–75. (42) Me´sza´ros, R.; Thompson, L.; Bos, M.; de Groot, P. Langmuir 2002, 18, 6164–6169.

Aggregate Effects on PEI/SDS Adsorption Layers

Figure 1. Kinetic surface tension curves of PEI/SDS mixtures at pH 4 prepared with advanced mixing and surface cleaning, where cPEI ) 100 ppm and csalt ) 0.1 M. The bulk SDS concentrations are 2.02 × 10-3 mM (red diamonds, a), 1.71 × 10-2 mM (green inverted triangles, b), and 1.45 × 10-1 mM (blue circles, c). The inset shows the Ward-Tordai analysis of these data to extrapolate the surface tension values to infinite time (i.e., zero reciprocal time) using an asymptotic fit.

Figure 2. Surface tension measurements of PEI/SDS mixtures at pH 4 prepared with advanced mixing and surface cleaning, where cPEI ) 100 ppm and csalt ) 0.1 M. For each solution, absolute values at 10 min (blue diamonds) and 30 min (green triangles) after surface cleaning and the result of Ward-Tordai analysis to extrapolate the data to infinite time (red squares) are shown. The three arrows identify the solutions corresponding to the kinetic curves in Figure 1.

interface are at chemical equilibrium with the solution, then the measured physical properties must be independent of the chosen preparation method. To test the applicability of this assumption for the PEI/SDS system at pH 4, we used two extreme polymer/ surfactant mixing procedures where we set out to maximize (basic mixing) and minimize (advanced mixing) the production of aggregates as well as an intermediate method (standard mixing); see the Experimental Section for further details. Figure 3A depicts visual observations of the physical nature of the samples over a wide range of bulk compositions. With increasing SDS concentration, there are four solution regimes: (i) homogeneous, transparent samples (open symbols); (ii) turbid samples (light shaded symbols); (iii) samples with two distinct phases of precipitate separated from a transparent liquid (dark shaded symbols); and (iv) turbid samples again (light shaded symbols). The turbidity below charge equivalence can be

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Figure 3. (A) Visual observations for the appearance of PEI/SDS mixtures at pH 4 prepared with basic mixing (blue circles), standard mixing (green squares), and advanced mixing (red triangles) methods, where cPEI ) 100 ppm and csalt ) 0.1 M. (B) Values of the ellipsometric parameter ∆surf with respect to the bulk SDS concentration cSDS for solutions prepared using the three different mixing methods (open symbols as described above) with surface cleaning. The vertical line at cSDS ) 1.6 mM marks the calculated point of charge equivalence.

rationalized by taking into account that high local surfactant concentrations lead to the formation of collapsed PEI/SDS particles, which can aggregate, during the mixing process. Regardless of the choice of mixing polymer/surfactant method, a greater number of larger aggregates are produced with increasing bulk surfactant concentration until finally associative phase separation occurs close to charge equivalence. These observations are typical of the behavior of polyelectrolytes and oppositely charged surfactants in general.6 When the total surfactant concentration is increased further, the surfactant provides a large enough surface charge density for colloidal stabilization, and thus the size of the aggregates and the turbidity of the system decrease. The choice of polymer/surfactant mixing protocol has an effect on the aggregation behavior in the PEI/SDS system at relevant time scales. In agreement with previous literature results,4 the range of the turbid/precipitated region decreases as the quality of the mixing process is improved. For the basic mixing method, there is the formation of large aggregates and the production of high turbidity in general. For the advanced mixing method, local concentration gradients during the mixing process are minimized, the effect of which depends on the solution composition. Below charge equivalence, the formation of collapsed PEI/SDS particles and the extent of their aggregation are minimized, thus higher bulk surfactant concentrations are required to produce turbid solutions. Above charge equivalence, charge reversal of the PEI/ SDS aggregates occurs, but even so none of the measured solutions are completely clear. Figure 4 shows the ellipsometric response ∆surf for two cases (basic and advanced mixing) over a wide range of PEI/SDS composition. Surface cleaning was not performed in the case of these measurements. We make three observations. First, the measured signal depends on the preparation method, particularly in the composition range approaching charge equivalence, where the solutions become cloudy as a result of the formation of large aggregates in the bulk phase. Second, the extent of the temporal fluctuations of individual data points (at a given surfactant concentration) also depends on the preparation method. Third, the variations in magnitude from one data point to the next are

4042 Langmuir, Vol. 25, No. 7, 2009

Figure 4. Values of ∆surf with respect to cSDS for PEI/SDS mixtures at pH 4 prepared using basic mixing (filled blue circles) and advanced mixing (filled red triangles) methods without surface cleaning, where cPEI ) 100 ppm and csalt ) 0.1 M. The meaning of the error bars is described in the Experimental Section. The vertical line at cSDS ) 1.6 mM marks the calculated point of charge equivalence. The adjoining lines between the data points act as a guide to the eye only.

Figure 5. Kinetic evolution of ∆surf for PEI/SDS mixtures at pH 4 prepared using basic mixing (blue circles) and advanced mixing (red triangles) methods both without (filled symbols, A) and with (open symbols, B) surface cleaning, where cSDS ) 0.19 mM, cPEI ) 100 ppm, and csalt ) 0.1 M.

larger than the fluctuations for a given sample. The first and second observations show that the interfacial layer is far from chemical equilibrium on relevant time scales, especially in the composition range approaching charge equivalence. The third observation indicates that the preparation method and the formation of the interfacial area that is probed affect the measured surface properties more than any smooth change in the adsorbed amount or interfacial composition. To demonstrate the randomness of the fluctuations more clearly, Figure 5A shows examples of the kinetic evolution of turbid PEI/SDS mixtures for cSDS ) 0.19 mM prepared using the basic and advanced mixing methods for solutions without surface cleaning. In the basic mixing case, there is a nonmonotonic change in the ellipsometric response, and in the advanced mixing case, the values change considerably over the course of the monitored time period. Furthermore, we noted that during the measurements

Tonigold et al.

there was visible scatter from the ellipsometry laser beam, which changed with time. With these pieces of information taken together, we conclude that the surface is inhomogeneous and that fluctuations are related to lateral movement of particles on the surface during the measurements. The ellipsometry beam has an area on the liquid surface of ∼1 mm2, so for such pronounced changes in the optical response we estimate that some of the particles are macroscopic, on the order of at least several micrometers. An important question is whether the surface aggregates are in dynamic equilibrium with the bulk solution phase (i.e., whether new aggregates can enter the surface while others leave or whether the aggregates are trapped at the surface when the solution is transferred to the measurement dish). To explore this issue, we removed the floating aggregates by cleaning the surface: a clean pipet was attached to a suction pump, and the surface layer was removed by aspiration for ∼3 s. We would not expect measurable depletion of material from the bulk solution that remained in the dish. The amount of polymer and surfactant in solution typically exceeded the amount adsorbed to the surface layer by 4 to 5 orders of magnitude, and the removal of additional surfaceactive material during a few seconds of aspiration was limited by the diffusion of material to the subsurface of the liquid. Figure 3B shows ellipsometry data for three types of polymer/ surfactant mixing methods, all prepared with surface cleaning. The most notable observation is that the pronounced fluctuations disappear, which demonstrates that larger aggregates formed during the mixing process were trapped at the interface. Furthermore, the measured ellipsometry curves practically coincide with each other, regardless of the preparation method. This observation implies that the large trapped aggregates, which are more abundant in the basic mixing procedure and cause the observed differences in solutions below charge equivalence prepared without surface cleaning, are removed very effectively from the interfacial layer by the surface-cleaning method of aspiration. Lastly, the constancy of the data in the kinetic curves in Figure 5B for the two extreme mixing methods prepared with surface cleaning demonstrates the lateral homogeneity of the surface, at least on the micrometer scale, once any floating aggregates are removed by aspiration. It also shows that on the time scale of the measurements there is not a measurable effect of aggregates rising or adsorbing to the interfacial layer for PEI/ SDS mixtures at pH 4. To summarize, we have shown that ellipsometry can be used to detect aggregates at the air/liquid interface, and we believe that it is helpful to consider two practical issues when studying the physical properties of mixed adsorption layers from polyelectrolyte/surfactant solutions. First, the formation of nonequilibrium trapped aggregates may be minimized through the choice of a careful polymer/surfactant mixing method. Second, any trapped aggregates at the interface can be removed by surface cleaning prior to measurements. In the subsequent part of the Results and Discussion section, we consider only solutions prepared with the advanced mixing method and surface cleaning. Nature of the Interface with Changing Polyelectrolyte Charge Density. Figure 6 shows a comparison of the measurements of ∆surf for PEI/SDS mixtures at pH 4 with bulk binding isotherm data adapted from ref 42. In the latter case, the total amount of SDS bound to fixed amounts of polymer was determined by equilibrium dialysis. (Further details can be found in ref 42.) The constancy of the ellipsometry signal below charge equivalence is striking: the values of ∆surf have a standard deviation of just 7% coverage over a factor of >500 in the bulk composition (cSDS ) 2.0 × 10-3 to 1.1 mM). Because charge equivalence for

Aggregate Effects on PEI/SDS Adsorption Layers

Figure 6. Values of ∆surf for PEI/SDS mixtures at pH 4 (open red triangles) and the concentration of SDS bound to PEI from equilibrium dialysis measurements42 (filled blue circles), where cPEI ) 100 ppm and csalt ) 0.1 M. The solutions were prepared using the advanced mixing method with surface cleaning. The vertical line at cSDS ) 1.6 mM marks the calculated point of charge equivalence. The horizontal line at ∆surf ) 2.8 emphasizes the constant values measured below charge equivalence.

the system occurs at 1.6 mM, the composition of the bulk PEI/ SDS complexes in this composition range varies from close to zero to 0.4 dodecyl sulfate ions per monomer unit of PEI. If the interface were dominated solely by PEI/SDS complexes adsorbed from the bulk solution with a composition as predicted by the binding isotherm results, then with increasing cSDS the constant ellipsometry signal would necessitate a reduction in the number of adsorbed complexes to accompany their increase in mass. Indeed, the molecular mass of the largest complex (with 40% bound surfactant on a charge-to-charge basis) is 3 times the mass of the smallest complex, so a substantial reduction in the adsorbed amount of polymer would be required to rationalize the data, regardless of any assumptions made about the adsorption mechanism. One possible scenario below charge equivalence could be that free dodecyl sulfate ions adsorb to the air/liquid interface and electrostatically attract PEI/SDS complexes with a net positive charge. In such a case, the constant ellipsometry data might be explained qualitatively as follows: with increasing cSDS, the net negative charge of the SDS surface layer increases, so the electrostatic attraction of the disk-shaped complexes increases, which leads to a flatter configuration in the interfacial layer and hence a decreasing number of bound complexes. Such a mechanism would provide the required balance between the increasing mass and decreasing number of adsorbed complexes. However, it would not be consistent with the general description of the interface formed by NR experiments on lower-molecularmass samples by Penfold et al.,1 where below charge equivalence the adsorbed amount and composition of adsorption layers at pH 4 did not change substantially over a broad range of bulk composition. In that respect, our ellipsometry data are most simply related to the rather constant adsorbed amount and composition shown by NR rather than a more complicated mechanistic picture. From molecular adsorption arguments alone, however, it is not clear why the proportion of surfactant at the interface exceeds the amount of polymer even when the amount of polymer in the bulk exceeds the surfactant by >2 orders of magnitude or why the proportion of surfactant at the interface does not mirror the changing composition of the bulk solution in general. We conclude

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Figure 7. Values of ∆surf for PEI/SDS mixtures at pH 4 (red triangles), pH 7 (purple diamonds), and pH 10 (orange inverted triangles), where cPEI ) 100 ppm and csalt ) 0.1 M. Values for pure SDS solutions (black crosses) are shown for comparison, where csalt ) 0.1 M. The solutions were prepared using the advanced mixing method with surface cleaning.

that further work is required to develop a physical adsorption model to rationalize the combined experimental surface and bulk solution data. To complete our interpretation of the ellipsometry trends at low pH, in a narrow concentration range as charge equivalence is approached, the data decrease suddenly to half of the original values. The depletion of material at the interface coincides with the pronounced aggregation/precipitation behavior of the bulk solution (red triangles in Figure 3a). Above charge equivalence, the ellipsometry signal becomes roughly constant at a value that exceeds the ellipsometry signal observed for a full SDS monolayer. This observation shows that some polymer remains in the adsorption layer even when there are excess dodecyl sulfate ions over the total number of amine groups in the system. The interesting trends in the ellipsometry data at low pH prompted us to probe the physical nature of the interface with changing polyelectrolyte charge density. Figure 7 shows the ellipsometric response for PEI/SDS mixtures at pH 4 (68% of amine groups charged in the absence of SDS), pH 7 (47%), and pH 10 (6%);42 pure SDS data are shown for comparison. An increase in pH from 4 to 7 for the polymer/surfactant mixtures results in a negligible change in the measured ellipsometry signal, which can be related to the relatively small change in the binding characteristics of the mixture. However, two significant differences are observed at pH 10. First, the measured mean signal is greater, which is an observation similar to that made in an NR study on lower-molecular-mass samples by Penfold et al., who found an unexpectedly high coverage of SDS at high pH.1 Second, despite the fact that the same surface-cleaning procedure was applied as with the data recorded for lower pH values, random variations appear in the signal that are similar to those observed previously for samples at pH 4 prepared without surface cleaning. The observed fluctuations both with respect to changing bulk composition and evolving time remain, even above charge equivalence albeit to a lesser extent. The kinetic data of PEI/SDS mixtures at pH 10 reveal sharp changes that are not monotonically related to the surface age. Figure 8 shows an example of these fluctuations for a single composition well below charge equivalence (cSDS ) 0.016 mM), where ∆surf varies in the range of 3.3-5.2. In general, the measured

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Figure 8. Kinetic evolution of ∆surf for PEI/SDS mixtures at pH 4 (red triangles, a), pH 7 (violet diamonds, b), and pH 10 (orange inverted triangles, c), where cSDS ) 0.016 mM, cPEI ) 100 ppm, and csalt ) 0.1 M. Values for pure SDS solutions (black crosses, d) are shown for comparison, also with csalt ) 0.1 M. The inset shows simultaneous measurements of the surface tension γ using the same notation for the symbols and labels. The solutions were prepared using the advanced mixing method with surface cleaning.

fluctuations in ∆surf fall in the range of 10-100% in excess of the corresponding constant signal recorded at pH 4 and 7. These observations suggest that a significant factor in the increased mean adsorbed amount at high pH is the presence of discrete patches of extended PEI/SDS structures present in the interfacial layer. The laser beam of the ellipsometer has an area of about 1 mm2, so for the signal to fluctuate wildly we speculate that many of the patches present at the interface are on the order of several micrometers, as opposed to a more nanoscopic size distribution. For consistency with NR measurements of a Bragg diffraction peak on lower-molecular-mass samples by Penfold et al., we speculate that either lamellar-phase aggregates (as observed by Bastardo et al.26) adsorb from solution or lamellarphase patches form at the interface on a time scale of 0.2 µm in size, which were not in dynamic equilibrium with the bulk solution on relevant time scales. Measurable depletion from a solution of surface-active species such as surfactant molecules or polymer/surfactant complexes was not expected. It follows that any differences in the experimental data observed between this control experiment and standard measurements can be attributed to elimination from the system of macroscopic aggregates that in standard measurements must adsorb spontaneously to the interfacial layer after the creation of a fresh surface by aspiration. Figure 10 shows the ellipsometry data from these two measurements. As expected, the signal measured for the unfiltered sample displays temporal fluctuations and a larger mean ∆surf value than those of solutions at low pH. However, in the case of the filtered sample, the very first data points recorded (at a

Aggregate Effects on PEI/SDS Adsorption Layers

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remarkably constant over a factor of 500 in the bulk SDS concentration, and lateral inhomogeneity on the micrometer scale was not detected using ellipsometry. At pH 10, there are inhomogenous patches of extended structures in the interfacial layer, as shown by pronounced temporal fluctuations in the optical signal. We infer that PEI/SDS aggregates originating from the bulk solution adsorb to the interfacial layer at high pH and that this process contributes significantly to a higher measured adsorbed amount, which is neither steady nor uniform on the short time scales (0.2 µm in diameter, as confirmed by a control experiment involving a filtered solution. In this case, the observed instability in the measurements and higher adsorbed amount returned in