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Adsorption of Branched-Linear PolyethyleneimineEthylene Oxide Conjugate on Hydrophilic Silica Investigated by Ellipsometry and Monte Carlo Simulations Daniel George Angelescu,*,†,§ Tommy Nylander,† Lennart Piculell,† Per Linse,† Bj€orn Lindman,† J€urgen Tropsch,‡ and J€urgen Detering‡ †
Physical Chemistry 1, Lund University, Box 124, SE-221 00 Lund, Sweden BASF SE, D-67056 Ludwigshafen, Germany § Romanian Academy, Institute of Physical Chemistry I. G. Murgulescu, 060021 Bucharest, Romania ‡
bS Supporting Information ABSTRACT: The adsorption of and conformation adopted by a branched-linear polymer conjugate to the hydrophilic silica aqueous solution interface have been studied by in situ null ellipsometry and Monte Carlo simulations. The conjugate is a highly branched polyethyleneimine structure with ethyleneoxide chains grafted to its primary and secondary amino groups. In situ null ellipsometry demonstrated that the polymer conjugate adsorbs to the silica surface from water and aqueous solution of 1 mM asymmetric divalent salt (calcium and magnesium chloride to emulate hard water) over a large pH range. The adsorbed amount is hardly affected by pH and large charge reversal on the negatively charged silica surface occurred at pH = 4.0, due to the adsorption of the cationic polyelectrolyte. The Monte Carlo simulations using an appropriate coarse-grained model of the polymer in solution predicted a coreshell structure with no sharp boundary between the ethyleneimine and ethyleneoxide moieties. The structure at the interface is similar to that in solution when the polymer degree of protonation is low or moderate while at high degree of protonation the strong electrostatic attraction between the ethyleneimine core and oppositely charged silica surface distorts the ethyleneoxide shell so that an “anemone”-like configuration is adopted. The adsorption of alkyl benzene sulfonic acid (LAS) to a preadsorbed polymer layer was also investigated by null ellipsometry. The adsorption data brought additional support for the existence of a strong polymer adsorption and showed the presence of a binding which was further enhanced by the decreased solvency of the surfactant in the salt solution and confirmed the surface charge reversal by the polymer adsorption at pH = 4.0.
’ INTRODUCTION Polyethyleneimine (PEI)-based polymers are polyelectrolytes extensively used as adhesives, dispersion stabilizers, thickeners, flocculating agents, retention aids for pigments and dyes13 in a wide range of applications including pharmaceutical formulations, personal care products, food products, paper industry, tertiary oil recovery, and household detergents.4,5 The PEIs are polymers with a large number of primary, secondary, and tertiary amine groups where each nitrogen atom is joined to another via an ethylene linkage. In fact, PEI is the generic name for a large group of water-soluble polyamines with varying molecular weight, topology, and shape. Ring-opening polymerization of the ethyleneimine can yield linear, branched,6,7 or comb-like8 structures. The PEI derivatives were obtained by the ethoxylation of each hydrogen attached to primary or secondary nitrogen. At low pH, the amine groups in PEI are protonated resulting in a highly positively charged polyelectrolyte, whereas at high pH, r 2011 American Chemical Society
the polymer is essentially neutral. As a pH-responsive charged polymer, its adsorption on oppositely charged surfaces is a complex process. First, the adsorbed weak polyelectrolytes may adjust their charge density in the adsorbed layer due to the local electrostatic potential generated by the surface, and second, the structure of the branched polyelectrolytes makes the adsorption more complicated than that of the linear ones. The interfacial properties of PEI on mica and silica surfaces have been recently investigated.913 PEI adsorbs in a flat configuration at low pH, whereas at high pH, a more extended adsorbed layer and larger adsorbed amount was found. The PEI adsorption leads to strong overcompensation of the surface charge, which can be interpreted as a strong nonelectrostatic affinity of the polymer toward Received: May 9, 2011 Revised: July 12, 2011 Published: July 14, 2011 9961
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Langmuir the silica surface. Another reason for charge reversal of large polyelectrolytes can be a mismatch between the charges at the interface and the polyelectrolyte charges, as well as the fact that the adsorbed polyelectrolyte can be trapped in a nonequilibrium state due to slow conformational changes.14 Considerable attention was also paid to the PEI interaction with oppositely charged surfactants.7,12,1520 The general picture that has emerged from the studies investigating the PEI-sodium dodecyl sulfate (SDS) complexation in aqueous solutions1518,21 shows linear and branched branched PEI interacting differently with oppositely charged surfactants. Evidence for hydrophobic and electrostatic interactions between the PEI and anionic surfactant SDS15,16 and for high affinity binding of the surfactant to the PEI12,17,18 were reported. It was argued that, depending on surfactant concentration, the complexation between SDS and PEI occurred as follows: at low surfactant concentration, DS monomer binds noncooperatively to the protonated amine groups of PEI until the PEI-SDS complex collapses and precipitates, whereas at higher SDS concentration, the surfactant adsorbs on the surface layer of the collapsed aggregates.12 The PEISDS complexation at the solid interface has also been addressed. When the association of the polyelectrolyte with oppositely charged surfactant at interfaces in general was investigated, the results were considerably dependent on the sequence of polymer and surfactant addition. Among the studies, three cases were distinguished: (i) the polyelectrolyte and surfactant had associated first in bulk and then the complex adsorbed to the surface,22,23 (ii) the polyelectrolyte had been adsorbed first and then the surfactant was added in the presence of polyelectrolyte in bulk,22 and (iii) the polyelectrolyte had been adsorbed to the surface prior to the surfactant addition to the polyelectrolyte-free solution.12,13 The binding of SDS to the preadsorbed PEI involved a noncooperative mechanism leading to an increase of the amount of adsorbed surfactant at increasing overall surfactant concentration according to a saturation-type adsorption isotherm,12 and to formation of a DS layer on top of PEI substrate, with surfactant polar heads oriented toward the solution.13 The derivatization of PEI with, e.g., ethylene oxide (EO) or propylene oxide (PO) groups provides an opportunity to modify the surface properties as well as the interaction with oppositely charged surfactants.17,24 The idea with ethoxylation is to increase the solubility and to enhance the surface activity. Li et al.17 reported that anionic surfactant SDS bound with a high affinity to the ethoxylated PEI, in a similar manner as to the bare PEI. It was also shown that the associative phase separation between the oppositely charged SDS and ethoxylated (EO) PEI did not occur provided that the ethylene oxide chains contained more than 20 units. Zhang et al. shows that adding a single EO (or PO) groups does not significantly change the interaction with oppositely charged surfactant. On the other hand, addition of (EO)3 groups weakens the interaction with oppositely charged surfactants, which in turn tends to lessen the deposition at the airliquid interface. Interestingly, it was shown that under high pH conditions PEI and sodium dodecyl sulfate (SDS) can form mulitlayers at airliquid interface.25 This was also observed for the PEI with single EO and PO groups, but not for the derivates with (EO)3 groups.24 The adsorption characteristics of comb-type charged polymers, where PEO side chains were attached to a positively charged backbone, on silica-like surfaces has been recently investigated using quartz crystal microbalance, reflectometry, atomic force microscopy, dual polarization interferometry, and neutron
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reflectivity.2628 The salient features experimentally found were reproduced by a self-consistent lattice mean field theory.29,30 The PEO-grafted polyelectrolyte adsorption was found to be relatively fast and mainly driven by the electrostatic interactions. The PEO side chains also had an affinity for the silica-like surface, leading to a rather complex adsorption process.26 Thus, it was shown that the adsorption kinetics and properties of the adsorbed comb-type polyelectrolyte were all strongly dependent on the degree of grafting.28 The adsorption proceeded near the charge compensation and the polymer backbone adopted a flat configuration at low degree of grafting, whereas the PEO side chains limited the absorption and led to a more extended adsorbed configuration of the charged polymer moiety at high degree of grafting.27 The overall conformation was due to the interplay between the relatively few but strong electrostatic backbone moietysurface interaction points and the more numerous but weaker interaction points between the PEO side chains and the surface. In this work, we use the in situ null ellipsometry technique and Monte Carlo simulations to investigate the adsorption behavior of a branched-linear poly(ethyleneimine)poly(ethylene oxide) conjugate to negatively charged silica, and the association between the preadsorbed polyelectrolyte and alkyl benzene sulfonic anionic surfactant. To the best of our knowledge, this is the first time a PEI derivative is investigated at a solidliquid interface, and as will be shown, the ethoxylated chains have significant impact on the association process with the hydrophilic silica. The focus of this work is on elucidating the adsorption in terms of amount and layer thickness of the polymer by in situ ellipsometry and the adopted conformation at the interface by Monte Carlo simulations. The association between preadsorbed polymer and anionic surfactant performed by in situ ellipsometry emphasizes the role of the degree of protonation of the polyelectrolyte in the interaction with the silica substrate.
’ MATERIALS AND METHODS Materials. Ethoxylated polyethyleneimine (referred to as PEI20EO), with a molecular weight of about 13 000 g mol1, was obtained from BASF as a commercial sample in the form of 80 wt % aqueous solution. The polymer originated from a hyperbranched PEI of a molecular weight ∼600 g mol1 with a number of 15 primary, secondary, and tertiary amine groups in the ratio 2:2:1.31 The substitutable hydrogens on the primary and secondary nitrogens were replaced by ethoxylated chains containing on average 20 repeating units. The structure of the polymer is schematically drawn in Figure 1. The anionic surfactant, the sodium salt of alkyl benzene sulfonic acid, (referred to as LAS) was also a commercial sample from BASF in the form of 50 wt % aqueous solution. The alkyl chain of the surfactant contained 1013 methylene groups with an average of 12 units. CaCl2 and MgCl2 hexahydrated salts were obtained from Lancaster (purity >99%). The water used was deionized and further purified by a Milli-Q filtration system (Milipore Corporation, Bedford, MA). The polymer and surfactant were used without further purification. Polymer and surfactant stock solutions were prepared by weighing appropriate amounts of those two components and by solubilizing them in water. In order to mimic the hard water in the household cleaning, additional stock solutions were prepared in a mixture of 0.8 103 mol L1 CaCl2 and 0.2 103 mol L1 MgCl2 (ionic strength I = 3 mM and water hardness 5. 6 dH). The pH values of the stock solutions were adjusted to the desired value by titrating appropriate amounts of 0.1 mol L1 HCl or NaOH solutions. Silica surfaces used as substrate for adsorption studies were prepared from polished silicon test slides (p-type, boron doped, resistivity 120 Ω 3 cm) with dimensions of about 1 5 cm2. They were thermally 9962
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of the concentrated polymer (105 ppm) and surfactant (3 104 ppm and 3 103 ppm) solutions. After injecting the stock solution, the Ψ and Δ ellipsometric angles were recorded in one zone as a function of time until no further change in the ellipsometric angles was detected. Evaluation of the Ellipsometric Data. The recorded Ψ and Δ angles obtained after polymer or surfactant injection were interpreted within the framework of the optical four-layer model, where the adsorbing layer and the surrounding media were treated as being composed of four homogeneously and flat optically isotropic layers. The mean refractive index nf and the average thickness df of the adsorbed layer were determined numerically.3234 The adsorbed amount of the polymer, Γ, was calculated from nf and df according to the de Feijter equation35 Γ¼
Figure 1. Generic structure of PEI20EO. oxidized in oxygen atmosphere followed by annealing and cooling in argon flow in order to grow the oxide layer to a thickness of about 30 nm. This has been shown to increase the accuracy of the ellipsometry measurements of adsorbed layers.32,33 The thickness and the optical property of this oxide layer were obtained from ellipsometry measurements carried out in two ambient media, namely, air and aqueous solution.32,33 The complex refractive index of bulk Si was about 5.50.35i, whereas the real refractive index of the SiO2 was 1.48, all parameters varying somewhat from batch to batch. The substrates were carefully cleaned prior to performing any ellipsometry measurements. Thereafter, the slides were rinsed with plenty of water and stored in 99 wt % ethanol where they were stable for months. Just prior to the experiment, the silica surfaces were additionally dried and additionally cleaned in a plasma cleaner (Harrick Scientific Corp., model PDC-3XG) under vacuum at 0.002 mbar air pressure for 5 min. Methods. 1. In Situ Null Ellipsometry. Experimental Setup. The ellipsometry measurements were performed with a modified, automated Rudolph Research thin-film null ellipsometer type 43603200E equipped with a xenon lamp and filter for a wavelength of 401.5 nm. The amplitude change Ψ and phase change Δ of the light upon reflection on the substrate depend on the dielectric structure normal to the interface. The measurements were carried out at an incidence angle ϕ = 68. A detailed description of the experimental setup as well as the procedure for characterization of the films adsorbed on a layered substrate are given in refs 32,33. The time resolution of the measurements was 2 s and the resolution of the ellipsometry angles, Ψ and Δ, was 0.002 and 0.005, respectively. In order to eliminate the systematic errors given by the imperfection of the optical components, the recorded angles were corrected using the data obtained from four-zone measurement carried out prior to the experiment. The adsorption experiments were performed in situ using a 5 mL trapezoidal cuvette of optical glass thermostatted at 25 C. Agitation of the solution was performed with a magnetic stirrer at about 150 rpm, and after polymer adsorption occurred, the polymer excess in bulk solution was diluted by flushing the cuvette with appropriate polymer-free solutions. The solvent exchange was carried out at the continuous flow 0.06 mL s1 and the exchange corresponded to about 20 cuvette volumes. The polymer and surfactant were added by pipetting appropriate volumes
ðnf n0 Þdf dn=dc
ð1Þ
where n0 is the refractive index of the bulk solution (n0 = 1.3423 + Cp(dn/ dc)), Cp is the polymer concentration (mg L1) and dn/dc the corresponding refractive index increment. dn/dc = 1.359 mL g1, and it was calculated as the weighted average of the PEI (dn/dc = 0.210 mL g1)12 and EO (dn/dc = 0.133 mL g1)36 moieties. 2. Potentiometric Titration. The charge density of the PEI20EO was obtained by conventional acidbase potentiometric titration. The pH of the polymer-containing solution was adjusted stepwise by addition of either HCl (0.1 M) or NaOH (0.1 M) aliquots with the time lag of 10 min between consecutive changes of pH. The blank titration curve (Cp = 0) recorded at the same ionic strength was subtracted from the titration curve of the sample. The degree of protonation R was obtained from the electroneutrality condition is then given by R¼ ¼
½EIþ ½EItot ½HCl ½NaOH ð10pH 10ð14 pHÞ Þ ð10pHðCp ¼ 0Þ 10ð14 pHðCp ¼ 0ÞÞ Þ ½EItot
ð2Þ where [EI]tot is the polymer concentration expressed as amino group equivalent and [HCl] and [NaOH] are the concentrations of added acid and base, respectively. The accuracy of both the pH and R values over the pH range investigated (311) is (0.05. 3. Monte Carlo Simulations. Model System. The conformation adopted by a single polymer molecule in bulk as well as at the silica surface was investigated by carrying out Monte Carlo simulations and using the so-called primitive model. The atomistic structure of the polymer was coarse-grained and the water considered as a continuum dielectric of constant relative permittivity. The polyelectrolyte was modeled as branched chains of different beads, which emulates the two polymer moieties. Hence, linear EO side chains were grafted onto the branched PEI structure. The EO chains were modeled similarly to the modeling in ref 37 where every other oxygen atom of the EO chain was modeled as a neutral soft sphere bead. Beads were connected with harmonic bonds and the chain flexibility was regulated by an angular force term. The nonbonded interaction parameters were taken from ref 37, while appropriate angular force and bond constants were chosen to give rms beadbead separation and mean angle values similar to those reported in ref 37. Concerning the PEI moiety, every tertiary amine group was considered as a soft sphere, and the bond, angular, and nonbonded interactions were modeled in a similar manner to those of the EO part. The coarse-grained PEI20EO and the monovalent counterions added to ensure neutrality of the system were enclosed in a hard-boundary spherical box of radius Rcell = 150 Å for bulk studies and in a rectangular box with the lengths Lx = Ly = 250 Å and Lz = 600 Å for simulation of the polymer adsorption onto the hydrophilic silica. In the latter case, periodic boundary conditions were applied; two hard walls were placed at zw = (173 Å and the charged silica surface was located at zw = 173 Å. A cubic lattice arrangement for the surface charges was considered. 9963
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All interactions in the system were taken as pairwise additive. The total potential energy U of the system is given by U ¼ Unonbond þ Ubond þ Uangle þ Uext
Table 1. Data of the Simulated Systems Polymer Conjugate (EI moiety)
ð3Þ
The first term can be expressed as a sum of soft-sphere interactions and electrostatic potential according to Unonbond ¼
2
ZZe 1
∑ uij ðrij Þ ¼ i∑< j 4πεi j0 εr rij i jzw j 0, jzi j < jzw j
∼2.93.4 Å
mean angle ÆRæ
∼115
number of grafted chains
17
number of beads per chain
10
rms beadbead separation ÆRbb2æ1/2 mean angle ÆRæ
∼5.6 Å a ∼126 a
Hydrophilic Silica location
173 Å
charge density
2.56 μC/cm2b
number of sites
400
counterions charge
+1e
ð8Þ
Table 1 summarizes the parameters used for modeling of PEI20EO in bulk and at the silica surface. Simulation Details. The Monte Carlo simulations were performed in the canonical ensemble employing the standard Metropolis algorithm, and the long-range electrostatic interactions in rectangular box were calculated using Ewald summation with extension to slab geometry. Three kinds of trial displacement were employed for the polymer: (i) single particle move, (ii) pivot rotation of a randomly selected EO chain, and (iii) polymer translation. The first move was also applied to the counterions. The probability of a single-particle move was 100 times larger than those of the other types of trial displacements. Each simulation consisted of an equilibration run of 1 105 passes (attempted moves per particle) followed by a production run of 10 106 trial moves per particle. Simulations were all carried using the integrated Monte Carlo/molecular dynamics/Brownian dynamics simulation package MOLSIM.38
radius of the spherec
Rc = 150 Å
box size d
250 Å 250 Å 600 Å
external walls d
zw = (173 Å
a
Calculated for a linear chain of 20 beads and the values are similar to those reported in ref 37. b Charge of SiO2 at pH ≈ 8.0. c Spherical cell used for polymer in solution. d Rectangular cell used for polymer interacting with silica surface. Structural Characterization. The shape of the two moieties of the PEI20EO polymer was characterized by the asphericity A defined as A ¼ 13
E1 E2 þ E1 E3 þ E2 E3 ðE1 þ E2 þ E3 Þ2
ð9Þ
where E1, E2, and E3 are the eigenvalues of the moment of the inertia tensor defined as
∑
(
rms nodenode separation ÆRnn2æ1/2
ð5Þ
the where j is the particle type, N the number of particles of type j, distance between two connected beads with the equilibrium separation rj0, and the bond force constant kjbond. The angular potential energy Uangle in eq 3 is given by Uangle ¼
0e +1e
Simulation Cell
j j j 2 kbond ri, i þ 1 r0 j
15
charge
Polymer Conjugate (EO moiety)
ð4Þ
where rij is the distance between the centers of particles i and j, Zi, the valence or the degree of protonation of particle i, e the elementary charge, ε0 the permittivity of the vacuum, εr the relative permittivity of the solvent, and εij, σij, and k the parameters defining the Lennard-Jones potentials as follows: εij = 0.167 kJ/mol, σij = 5.5 Å, and k = 8 if both i and j refer to EO beads and εij = 1.65 kJ/mol, σij = 1.6 Å, and k = 12 otherwise. The second term in eq 3 represents the harmonic potential connecting two coarse-grained units (EO or EI beads) and is given by Ubond ¼
number of beads
E¼
1 Nb ðri rCM Þðri rCM Þ Nb i ¼ 1
∑
ð10Þ
ri is the position vector of bead i and rCM is the vector of the center of mass of the whole molecule. The A parameter ranges from 0 (attained for a sphere) to 1 (obtained for a rod).
’ RESULTS AND DISCUSSION 1. Properties of PEI20EO in Aqueous Solutions. 1.1. Potentiometric Titration. As a weak polyelectrolyte, the PEI20EO
charge acquired by the protonation of its amine groups depends on the pH and ionic strength. In Figure 2, the degree of protonation R is plotted as a function of the pH for two aqueous solutions, Milli-Q water and 1 mM divalent salt solution. The polymer is practically neutral at pH > 10, whereas it has a considerable charge density in the neutral and acidic pH range; R is about 0.05 at pH = 9.0, increases to 0.4 at pH = 6.0, and increases to 0.8 at pH = 3.0 in Milli-Q water. Addition of the divalent salt increased the ionic strength and facilitates protonation of the tertiary amine groups for the moderately charged polymer. No effect of adding 1 mM divalent salt is noticed at either a high or low degree of protonation. The functional form 9964
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Figure 2. Degree of protonation (R) of 5000 ppm PEI20EO as a function of pH in Milli-Q water (filled symbols) and hard water, I = 3 mM (open symbols).
Figure 4. (a) Snapshot illustrating the conformation adopted by PEI20EO at the hydrophilic silica. The degree of protonation is (a) 0.2 and (b) 1.0. The same color code for the polymer moieties as in Figure 3 was used. The blue spheres represent the polymer counterions and the green flat surface the hydrophilic silica.
Figure 3. (a) Snapshot illustrating the conformation adopted by PEI20EO with a degree of protonation 0.4 in Milli-Q water. The redconnected spheres represent one EI group and the yellow-connected spheres two EO units. The counterions were omitted for the sake of clarity; Rc = 150 Å. (b) Number density of the EI (filled curve) and EO moieties (dotted curves) calculated with respect to the center-of-mass of the whole molecule. The degree of protonation is 0.0 (squares), 0.4 (circles), and 1.0 (diamonds).
of the titration curve resembles the one reported for nonethoxylated PEI,10,17 in spite of the fact that the primary and secondary amines were replaced by tertiary ones as a result of grafting the ethoxylated chains. 1.2. Monte Carlo Simulation. Simulation snapshot for the modeled PEI20EO at R = 0.4 is shown in Figure 3a. Here, the polymer molecule appears as nearly spherical (asymmetry parameter is 0.02) with the EO moieties as a dilute shell. The radial number densities of the EI and EO moieties are shown in Figure 3b for three degrees of protonation. The EI density
exhibits a maximum at the center-of-mass of the polymer and extends out to 10 Å, where a maximum in the number density of the EO moiety is found. Note that, while the density of the EO units decreases at radial distances shorter than 10 Å, it is still larger than zero at the polymer center-of-mass. Though Figure 3b supports a coreshell structure for the polymer, the overlap of the two distributions at r < 10 Å and the finite EO density at the center-ofmass of the polymer imply that the grafted EO chains are flexible enough to fold back so that there is not a sharp border between the EI core and the EO corona. The absence of a sharp radial boundary between the different moieties of a polymer was also reported for dendrimers39 and dendrimerpolymer conjugates.40 The radial distribution of the two moieties at various degrees of protonation shows that the EI density at the center-of-mass decreases with the increase of R. The core expansion is due to the intrachain electrostatic repulsion, which in turn leads to a decrease of the EI density in the core region. No change was observed in the EO density profile moving toward its periphery, and it is thus concluded that the overall spatial extension of the polymer does not depend on the polymer charge. 2. Properties of PEI20EO at Hydrophilic Silica Surface. The detailed molecular information gained from the Monte Carlo simulations has important implications for the interpretation of the experimental ellipsometry results. Therefore, we start discussing the properties of PEI20EO adsorbed to a hydrophilic silica surface by presenting the simulation results. 2.1. Monte Carlo Simulations. Simulation snapshots exhibiting the equilibrium configuration of the polymers with R = 0.2 and 1.0 at the silica surface bearing a constant negative charge of 2.56 μC/cm2 (corresponding to pH ∼8.0) are displayed in Figure 4. We focus on the initial interaction of the polymer with the oppositely charged surface, and therefore, only monovalent counterions were added to the simulation model to keep the system globally electroneutral. At R = 0.2, only the EO shell stays in contact with the surface (Figure 4a) and the adopted confirmation is similar to that reported for the free polymer (Figure 3a). On the other hand, the EI core is located next to 9965
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Figure 6. Probability distribution of the asphericity, A, for EI and EO moieties at the degree of protonation 0.2 (continuous curve) and 1.0 (dotted curve).
Figure 5. Number density of (a) EI and (b) EO moieties as a function of z-coordinate at the degrees of protonation 0.2 (dotted curve) and 1.0 (continuous curve). The gray box represents the location of the hydrophilic silica.
the surface at R = 1.0, and consequently, the EO shell is deformed. The “sea urchin” morphology (Figure 4a) leaving the spherical shape almost intact and the “anemone” (Figure 4b) morphology, with the EO chains distributed uniformly within a hemisphere, were previously reported for Monte Carlo simulations of the star polyelectrolyte adsorbed onto oppositely charged surfaces.39 The number densities of both moieties in the direction perpendicular to the solid surface are shown in Figure 5. Note that the negative charge surface was placed at z = 173 Å (see Model System section) and the simulation cell encompassed the volume determined by 173 Å < z < +173 Å. In line with the snapshot displayed in Figure 4a, the polymer with the net charge of only +3e, i.e., R = 0.20, is accumulated near the charged planar surface. In more detail, one sees for R = 0.20 that (i) there are maxima in the densities of both the EI and the EO moieties reached at 40 Å away from the surface, which corresponds to the radial extension of the polymer in bulk (see Figure 3b) and (ii) the EI core density is zero at the surface. The polymer is not deformed at the surface since the gain in free energy due to the electrostatic interaction is lower than the loss of entropy resulting from the deformation of the EO shell. This feature is in line with the weak interaction of the polymer at low R and silica surface demonstrated by null ellipsometry (see next section). At R = 1.0, the polymer is also attached to the surface as the bead density at distances from the surface higher than 100 Å (roughly the diameter of the free polymer) is zero. In addition, the strong electrostatic interaction modified the conformation
adopted by the polymer at the surface. Thus, the maxima of the two bead densities were shifted toward the surface. It is also observed that, unlike the polymer accumulation at R = 0.2, the EI density at the surface is nonzero, then it increases steeply and reaches a maximum at 13 Å above the surface. In line with the EI moiety behavior, the bead density of the EO shell at the surface increased as compared with the case of the weakly charged polymer adsorption (R = 0.2). The EO density profile narrowed as the width at the half-maximum decreased from 42 Å at R = 0.2 to 29 Å at R = 1.0. Consequently, the EO chains are partially deprived of their three-dimensional freedom and the shape of the polymer is modified. To alternatively assess the deviation in the polymer shape upon R increasing, we plot in Figure 6 the probability distribution of the asphericity of the EI and EO moieties. At R = 0.2, the EO shell adopted a nearly spherical shape as the probability distribution of A parameter reached a pronounced maximum at A ≈ 0.025. On the other hand, the EO shell became deformed at R = 1.0 as the probability distribution broadened and the maximum shifted at ∼0.075. Concerning the EI core, it docks to the surface at R = 1.0 without a significant change in shape. The simulations at constant charge density of the silica surface gave insight into the entropic role played by the noninteracting EO shell in the adopted configuration of the polymer at the interface. One step further to accommodate the simulations with the ellipsometry experiment is to take into account the pHsensitive charge of both polymer and silica surface. While the polymer degree of protonation was given in Figure 2, the surface charge density of the silica at varying pH was evaluated using the method developed by Behrens and Grier in ref 41. The simulations were carried out at 4 < pH < 8.5 taking into account the pH variation of the polymer and silica surface charges. We found that the EI number density reached a maximum near the silica surface at 5.0 < pH < 8.5. The closest maximum to the silica surface was located at z = 133 Å and was obtained at pH = 6.5 (full density profiles of the EI core at various pH values are shown in Figure S1 in Supporting Information). The degree of protonation of the polymer, the surface charge density of the bare silica, and the EI number density at z = 133 Å are displayed in Figure 7. The polymer attachment to the surface exhibits a non-monotonic behavior, and the high EI core density nearby the silica surface was attained at intermediate pH values ranging from 5.5 to 7.5. This strong attractive interaction found at these pH intermediate values is simply given by the opposite effect of pH on the 9966
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Table 2. pH, Degree of Protonation, r, Adsorbed Amount, Γp, Charge Ratio at the SolidLiquid Interface, σp/σ0, Adsorbed Layer Thickness, dp, Polymer Surface Coverage, Σp, and Reduced Loss in the Adsorbed Amount upon Rinsing, ΔΓ/Γp ΔΓ/Γp
Figure 7. Number density of EI beads, F(z), at z = 133 Å (filled circles) calculated from MC simulations, degree of protonation, R, (open circles) and the charge density of the silica surface, σ, (open squares) vs pH.
Γp (mg m2) σp/|σ0|b
I (mM)
pH
Ra
dp (Å)
Σpc
(%)
0 mM
8.5 6.5
0.09 0.40
0.45 ( 0.01 0.43 ( 0.01
90 ( 20 70 ( 10
0.87 0.84
12 25
4.0
0.67
0.33 ( 0.01
3 mM
8.5
0.09
0
≈20
50 ( 20
0.64
2
-
0
6.5
0.45
0.43 ( 0.01
≈3
30 ( 10
0.84
4.0
0.71
0.41 ( 0.01
≈27
40 ( 20
0.80
0.25 ≈2
-
27 0.5
Calculated from Figure 2. b σp and σ0 evaluated from Figure 8 and according to the silica model in ref 41, respectively. c ∑p = (Γp/Mp)πR2ext, where Mp stands for polymer molecular mass and Rext is the polymer inplane radial extension. a
2.2. In Situ Ellipsometry of the PEI20EO Adsorption onto Hydrophilic Silica. 2.2.1. Adsorption Kinetics. The adsorbed
Figure 8. Adsorbed amount of PEI20EO from a 200 ppm bulk concentration onto hydrophilic silica as a function of time in (a) Milli-Q water and (b) hard water, I = 3 mM at pH = 8.5 (circles), pH = 6.5 (squares), and pH = 4.0 (diamonds).
magnitude of the silica surface charge and the polymer charge. One also notes that the polymer density at z = 133 Å decreases steeply at low pH so that the polymer is not present at the surface at pH < 4.5. This observation is in contrast with the null ellipsometry experiments that showed a strong polymer substrate interaction at pH = 4.0 (see next section). The lack of agreement between the simulations and the experimental results might be explained by the charge regulation of the silica surface or by a significant nonelectrostatic driving force for the polymer adsorption to the silica surface.42
amount, Γ, of PEI20EO on a silica surface versus time from a 200 ppm polymer solution is shown in Figure 8 at different pH and ionic strength conditions. The time evolution of Γ suggests that the adsorption process can be divided into three distinct parts. The initial kinetics features a high and constant adsorption rate after which the adsorption rate decreased with increasing time until a steady state was reached eventually. Note that, under acidic conditions, pH = 4.0, the second regime, of low adsorption rate, extends for hundred of seconds whereas the plateau adsorption is reached much faster, within tens of seconds, at neutral and high pH. The fact that steady state adsorption is reached faster at pH > 4 indicates that lateral rearrangements within polymer layer, required to accommodate additional molecules in the layer, occur rapidly at neutral and high pH. On the other hand, at low pH, when the polymer is highly charged and the silica surface is weakly charged, the intermediate adsorption process lasts for a significantly longer time. This is likely an effect of the stronger repulsive force within the adsorbed layer. In contrast with these findings, it has been reported that the adsorption rate of the nonethoxylated PEI decreases more gradually in the intermediate adsorption regime also at basic pH and low ionic strength or low pH and high ionic strength.10 The adsorbed amount in the steady-state regime is summarized in Table 2. It is weakly dependent on the degree of protonation in Milli-Q water and decreases from 0.45 mg m2 at pH = 8.5 to 0.33 mg m2 at pH = 6.5. In contrast, at I = 3 mM (hard water) the weakly charged polymer at pH = 8.5 did not adsorb onto the hydrophilic surface, whereas an adsorption was noticed at lower pH values. About the same steady state adsorbed amount, ∼0.4 mg m2, in hard water was reached at pH = 6.5 as well as at pH = 4.0, but the adsorption kinetics differed (Figure 8b). The lack of polymer binding at pH = 8.5 can be understood by the competitive interactions of the weak charged polymer and the divalent cations with the oppositely charged silica surface, suggesting that the adsorption is controlled by electrostatic forces. It is known that the long EO chain may adsorb onto hydrophilic silica in spite of a weak attractive interaction between ethylene oxide groups and the silica surface.43 It has also been shown that divalent cations may enhance the strength of the 9967
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Langmuir attraction between PEO and silica.44 To gain an understanding of the role of the EO moiety in the adsorption mechanism, adsorption studies with linear PEO were carried out. We chose a relatively short chain with an average number of 130 EO units, which corresponds to roughly half of the ethylene oxide content in PEI20EO. Our choice was based on the Monte Carlo simulations in section 2.1. According to the simulation results, not more than half of the grafted chain may come in contact with the silica surface owing to the coreshell structure of the conjugated polymer. The affinity of this PEO polymer toward silica substrate was very weak at pH = 4.0, as we found that the adsorbed amount was at the limit of detection, ∼0.05 mg/m2, and no adsorption was found at higher pH values even if the PEO bulk concentration was higher than 500 ppm. This fact is in line with previous findings that both PEO polymers and EO-containing block copolymers show a pH-dependent adsorption at the hydrophilic silica and a critical pH above which no adsorption takes place.45,46 On the basis of the very weak affinity of the low molecular weight PEO, one would conclude that the grafted EO chains are not long enough to cause the observed adsorption of PEI20EO under neutral and acidic conditions. Assuming that no change in the degree of protonation occurs for either the polymer or silica surface on polymer adsorption (this point is discussed further below), the ratio of the charge of the polymer in the adsorbed state, σp, and the charge of the silica, |σ0|, can be estimated and the results are shown in Table 2. At pH = 8.5, the surface charge density is only partially compensated, σp/|σ0| < 1, as the low charge of the polymer gives only a weak electrostatic attractive interaction between the polymer and silica surface. The charge of the adsorbed polymer increasingly compensates the surface charge when lowering the pH, so that eventually, high “overcompensation” of the surface charge was reached under acidic conditions. However, polyion adsorption on charged surfaces may generally induce an additional charge regulation of the surfaces sites and of the acidic/basic functionalities of the polyion, as demonstrated in the study by Samoshina et al.14 Hence, the overcompensation shown in Table 2 might be overestimated by our assumption regarding the surface chargeable groups as independent of the polyelectrolyte adsorption. A large overcompensation of the silica charge was also found for the adsorption of nonethoxylated PEI.10,11 Meszaros et al. proved by results from ζ potential measurements that there is a dramatic shift of the apparent isoelectric point of silica due to PEI adsorption, from about 4 of the bare silica to about 11 of the PEI-coated silica. As an explanation for the adsorption behavior, a strong nonelectrostatic affinity of the amine groups toward the silica surface was proposed,10,12 and the origin of this interaction might come from surface complexation and hydrogen bonding.47,48 The determination of thickness of the adsorbed layer, which was obtained from the ellipsometry measurement, is rather uncertain in spite of the relatively high surface coverage (see below).49 The thickness of the adsorbed polymer was less than 100 Å (see Table 2), but the possible change in the thickness of the adsorbed layer following a variation of pH or ionic strength was not significant enough to be estimated. However, one can infer that the polymer adsorbs as a single layer at the silica surface since the thickness of the adsorbed layer is comparable with the radial extension of the polymer in aqueous solution given by the Monte Carlo simulations to about 40 Å (see above on polymer simulations). For nonethoxylated PEI, it is known that the adsorbed layer thickness decreases by increasing the substrate charge density.12,50,51
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The surface coverage, Σp, defined as the number density of the adsorbed polymer molecules times the surface area covered by one adsorbed polymer molecule is displayed in Table 2. The latter parameter was assessed from the Monte Carlo simulations and is defined as πRext2, where Rext = 40 Å is the radial distance where the number density of the EO units has decayed to zero (see Figure 3b). It should be noted that this value represents the maximal possible extension of the molecule and the “real” value is likely to be smaller. Here, we use the spatial extension of the polymer in the bulk since the simulations also showed that the inplane extension of the polymer was not affected by the adsorption. This feature is rather general for highly branched structures, and Pfau et al.50 reported that the adsorbed PEI retained the bulk lateral diameter and rather contracted in its direction orthogonal to the surface. The estimated surface coverage is as high as between 0.8 and 0.9 in most instances, which implies (i) that the EO moieties do not overlap, (ii) the plateau adsorption is limited by the steric barrier of the previously adsorbed polymers rather than by the surface charge compensation, and (iii) the in-plane diffusion of the adsorbed polymer molecules may allow for the formation of closely packed polymers at the surface. As argued above, it seems that differences in the relaxation of the adsorbed polymer molecules through lateral diffusion would explain the differences in the adsorption kinetics. At neutral and high pH, a high in-plane diffusion of an adsorbed polymer molecule, as` is suggested by the “sea urchin” configuration, would enable fast reorganization and implicitly a fast approach to saturation adsorption. On the other hand, the “anemone” polymer morphology at pH = 4.0 would imply a slower in-plane diffusion and, consequently, a slow approach to saturation adsorption. 2.2.2. Reversibility of Polymer Adsorption. After the polymer adsorption had reached steady state, the bulk solution was diluted by flowing polymer-free solvent through the measuring cell (rinsing). It was found that at the beginning of rinsing, desorption proceeds relatively rapidly, but the rate of the process slowed down with time so that a plateau value representing the polymer that appeared to be irreversibly adsorbed was reached within tens of minutes. The total fraction of polymer desorbed after extensive rinsing, ΔΓ/Γp, is given in Table 2. It is noted that (i) the polymer was partly removed at pH g 6.5 and (ii) no desorption was observed for the highly charged adsorbed polymer (low pH). These features hint at electrostatic attractive interaction between the silica substrate and the adsorbed PEI20EO. The larger fraction removed upon rinsing at pH g 6.5 can be explained by poor attachment of the polymer to the substrate. At pH = 4.0, the polymer becomes highly charged and the simulations suggested that the charged EI moiety was in direct contact with the silica surface. As a consequence, the enhanced electrostatic attraction dominates over the steric repulsion and thus gives the irreversible polymer adsorption upon rising. 2.2.3. Characteristics of Adsorbed PEI20EO in the Presence of LAS. To obtain further insight into the interfacial silicapolymer interaction, the anionic surfactant LAS was added to the preadsorbed PEI20EO on the hydrophilic silica. Surfactant addition was carried out after extensive dilution of the polymer in order to ensure that the surfactant interacts exclusively with polymer irreversibly adsorbed on silica. After reaching the steady state (the plateau in the adsorbed amount) upon rinsing, appropriate volumes of LAS stock solutions were added sequentially. After each injection, the system was equilibrated until a plateau in the adsorbed amount was reached prior to the next surfactant addition was done. Figures 9 and 10 show the change in the 9968
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Figure 9. Effect on (a) the adsorbed amount, Γ, and (b) the layer thickness, d, of the LAS surfactant addition to preadsorbed PEI20EO with no added salt; the filled symbols stand for the values for the preadsorbed polymer layer. The pH values were 8.5 (circles), 6.5 (squares), and 4.0 (diamonds).
adsorbed amount and thickness of the preadsorbed polymer layer as a function of surfactant concentration. As in the previous section, the experiments were performed at three pHs and two different ionic strengths. The adsorbed amount and thickness of the adsorbed layer were obtained from the ellipsometry measurements using the four-layer model as used for the polymer adsorption. This simplified optical model only allows for determining the change in the overall adsorbed amount, and an additional error is introduced since the same refractive index increment was used for both adsorbed polymer and surfactant. However, the effect is small and will not affect the conclusion drawn. In Milli-Q water, the increase in the adsorbed amount at surfactant addition implies the presence of the surfactant at the interface (Figure 9a). We recall that LAS does not adsorb to the bare silica, which means that the surface is selective for the polyion. It should also be stressed that the surfactant adsorption occurred at high pH (low degree of protonation) where the surface charge is undercompensated by the preadsorbed polymer. This result is due to either a surfactant affinity for EO shell or a charge regulation of EI moiety. The former assumption is questionable as the affinity seems to be related to the PEO topology. Thus, while linear PEO bound substantial amounts of anionic surfactants,52 an almost suppressed SDS binding was reported when PEO chains were grafted onto a linear backbone.53 More intriguingly, the latter feature held irrespective of the degree of grafting. It can therefore be stated that the protonation equilibrium of EI moiety was likely shifted as a result of LAS binding, in a similar manner to the bulk PEI-SDS complexation at high pH.12
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Figure 10. Effect on (a) the adsorbed amount, Γ, and (b) the layer thickness, d, of the LAS surfactant addition to preadsorbed PEI20EO at I = 3 mM; the filled symbols stand for the values for the preadsorbed polymer layer. The pH values were 6.7 (circles), 6.0 (squares), and 4.0 (diamonds). The vertical dotted lines denote the boundaries of the region where the surfactant precipitated.
At pH 6.5 and 4.0, when the net charge of the surface, including the adsorbed polyions, has changed sign, the electrostatic interaction should be the dominating force driving the LAS toward the surface. Figure 9a also indicates that, independent of the polymer degree of protonation, LAS has a high affinity to the preadsorbed polymer, an increase in the adsorbed amount being observed already at very low LAS concentration (6 ppm). The largest amount of LAS adsorbed is found at pH = 6.5 where the silica surface is already overcharged by the preadsorbed polymer. It is also noted that the total adsorbed amount behaves nonmonotonically as a function of the overall LAS concentration. Initially, the adsorbed amount increases steadily on increasing the overall surfactant concentration, which is correlated to a noncooperative (unimeric) binding mechanism of the anionic surfactant to the polymer. This is in line with the interaction of the SDS and nonethoxylated PEI, where no critical aggregation concentration in the binding isotherm was found.20 A maximum in the adsorbed surfactant occurred at about 800 ppm, irrespective of the polymer degree of protonation. This concentration represents the cmc of the LAS in Milli-Q water, where one would expect that the adsorbed surfactant reach a plateau value as further increase of surfactant concentration does not increase the monomer concentration. However, since the adsorbed amount actually decreased steadily at surfactant concentrations larger than cmc and, except at pH = 4.0, eventually became lower than the initial film mass, we conclude that a polymersurfactant 9969
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Langmuir complex was formed at the interface and subsequently desorbed from the surface. It is generally observed for cationic polymers that the desorption of complexes between polyelectrolytes and oppositely charged surfactant from the surface occur when the overall surfactant concentration exceeds cmc. In addition to electrostatic attraction, an additional hydrophobic interaction between the surfactant aggregates and the polymer was inferred.5458 These findings are in contrast to the behavior of the adsorbed nonethoxylated PEI in the presence of sodium dodecyl sulfate. In this case, the increasing surfactant concentration led to an increased mass at the interface which was described by a saturation-type adsorption isotherm with no polymer or surfactant desorption at bulk surfactant concentrations higher than the cmc.12 The different adsorption properties found here for ethoxylated PEI likely arise from the low molecular mass of the PEI moiety. There were only few electrostatic EIsurface interaction points and the polymersurfactant complex detachment occurred easily above cmc due to the competition between the LAS micelles and silica surface for binding the oppositely charged polymer. The effect on the layer thickness of LAS addition is shown in Figure 9b. In spite of the gain in the film mass at surfactant concentrations lower than cmc (∼800 ppm), no pronounced swelling of the polymer layer was observed. On the other hand, the partial removal of the PEI20EO-LAS complex was associated with an increase in the adsorbed layer thickness. A similar behavior was reported for a range of other cationic polyelectrolyte and oppositely charged surfactant system.22,23,57,58 The layer swelling is attributed to increase electrostatic repulsion within the adsorbed complex. In addition, care must be taken regarding the determined thickness as absolute values as the fourlayer model used to interpret the data may overestimate the thickness at low surface coverage. The adsorptiondesorption process at increasing surfactant concentration also took place with the sequential addition of LAS from hard water (see Figure 10a). However, here a bulk phase separation of LAS occurred in the hard water, which precluded ellipsometry measurements at LAS concentrations in the range 1001500 ppm. The high affinity binding of the surfactant to the polymer resulted in an increase of the overall adsorbed amount for pH e 6.0. At pH = 6.7, the surfactant is presumably also bound to the polymer, yet due to the weaker interaction of the polymer with the silica substrate, the surfactant binding promotes polymersurfactant complex desorption at low surfactant concentration. The surfactant adsorption increases sharply on approaching the phase separation in the LAShard water binary system, i.e., at 100 ppm. Notably, the cmc is about 90 ppm and, therefore, the micellization process starts just prior to the surfactant precipitation. Both large adsorption and bulk phase separation of surfactant are, in general, promoted by the decrease in the surfactant solvency, which in this case is given by the presence of the divalent cations and is not related to the presence of the preadsorbed polymer conjugate. Significant depositions onto both hydrophilic and hydrophobic silica surfaces associated with phase separation in oppositely charged polyion and surfactant mixed solutions were recently reported.22,23,57,58 However, in these studies the two related phenomena were consequences of the direct interaction of the surfactant with the polyion. When the LAS concentration is above the resolubilization threshold, the desorption process occurs. At the first stage, it represents the surfactant redissolution alone because of its increased bulk solvency, and at the second stage, the PEI20EO-LAS complex
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was removed owing to the binding to LAS micelles. Compared to results obtained without adding divalent ions, the complex was removed to a greater extent from the silica surface. Again, the competitive binding of the divalent ions and the PEI20EO-LAS complex to the silica surface facilitates the dissolution of the complex. The layer thickness in the preprecipitation regime, i.e., for LAS concentrations less than 100 ppm, showed the same trend as in the studies carried out in Milli-Q water; namely, it did not change significantly. Once the phase separation is reached at surfactant concentrations higher than 100 ppm, the film thickness expands sharply as a result of the surfactant deposition. On the other hand, in the postprecipitation regime the variation of the thickness with the surfactant concentration depends on the solution pH. The thickness decreases steadily and approaches the value corresponding to the bare preadsorbed polymer at pH = 4.0, whereas there is a steady and sharp increase in thickness at pH > 4.0. As the assessed film swelling occurs at similar concentrations as surfactant and polymer desorption, one can conclude, as without the added salt, that the enhanced film thickness is a consequence of increasing the repulsion in the PEI20EO-LAS complex and decreasing the surface coverage when the complex detached from the silica surface.
’ CONCLUSIONS The properties of the ethoxylated polyethyleneimine adsorbed from aqueous and divalent salt solutions on the hydrophilic silica were investigated by Monte Carlo simulations and in situ null ellipsometry. The simulations revealed a radial coreshell organization of the polymer conjugate irrespective of its degree of protonation. The inner part contained the EI moiety and parts of EO chains, whereas the outer part was formed exclusively by the EO chains. A weakly charged polymer was accumulated at the silica surface retaining the bulk configuration, whereas at high degree of protonation, the polymer conjugate adhered strongly to the silica with significant deformation of the ethylene oxide corona. The adsorption configurations of the coreshell polymer are given by the balance of the polymer coresilica attractive electrostatic interaction and the polymer shellsilica repulsive steric interaction. In situ null ellipsometry demonstrated that the polymer conjugate adsorbed on the silica surface in a wide pH domain. Fast kinetics adsorption at pH g 6.5, implying fast lateral diffusion of the polymers at the surface, and slow kinetics adsorption at pH = 4.0 resulted in similar film mass at steady state. The difference in surface diffusion kinetics correlates with the different configurations of the polymer found in the Monte Carlo simulations. The resulting pronounced apparent overcompensation of the silica surface charge under acidic conditions is in reality lowered by an additional dissociation of the surface silanol sites occurring in the presence of adsorbed polymer conjugate. The electrostatic interaction driving the polymer conjugate adsorption was also reflected indirectly upon addition of the LAS anionic surfactant to the preadsorbed polymer. Initially, the surfactant binds noncooperatively with high affinity to the polymer, and at surfactant concentrations above the cmc, the polymersurfactant complex was partially removed because of its interaction with surfactant micelles. The electrostatically driven silicapolymer interaction is also reflected in the low efficiency of the polymersurfactant removal at pH = 4.0, where the polymer conjugate experiences a high degree of protonation. 9970
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Langmuir The divalent cations present in hard water compete with the polyion for accumulation at the silica surface, a process that has two main effects. First, the weakly charged polymer at pH = 8.5 did not adsorb, and second, the polymersurfactant complex desorption at surfactant concentration above cmc occurs to a greater extent. The divalent cations have also a direct effect on the surfactant by modifying its solvency, which in turn alters the surfactant adsorptionpolymersurfactant complex desorption sequence upon increasing the bulk surfactant concentration.
’ ASSOCIATED CONTENT
bS Supporting Information. Additional figure as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This project was funded by BASF. L.P. and T.N. gratefully acknowledge support from the Swedish Research Council (VR) through the Linnaeus grant Organizing Molecular Matter (OMM) (239-2009-6794). ’ REFERENCES (1) Lindquist, G. M.; Stratton, R. A. J. Colloid Interface Sci. 1976, 55, 45–59. (2) Horn, D.; Linhart, F. Paper Chemistry; Blackie Academic & Professional: Glasgow and London, 1996. (3) Alince, B.; Vanerek, A.; van de Ven, T. G. M. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 954–962. (4) Molyneux, P. Water-Soluble Synthetic Polymers: Properties and Behaviour; CRC Press: Boca Raton, 1983; Vol. 1. (5) Madkour, T. M. Poly(ethylene) imine, Polymer Data Handbook; Oxford University Press: New York, 1999. (6) Chen, Y.; Shen, Z.; Pastor-Perez, L.; Frey, H.; Stiriba, S.-E. Macromolecules 2005, 38, 227–229. (7) Bastardo, L. A.; Garamus, V. M.; Bergstr€om, M.; Claesson, P. M. J. Phys. Chem. B 2005, 109, 167–174. (8) Koper, G. J. M.; vanDuijvenbode, R. C.; Stam, D. D. P. W.; Steuerle, U.; Borkovec, M. Macromolecules 2003, 36, 2500–2507. (9) Claesson, P. M.; Paulson, O. E. H.; Blomberg, E.; Burns, N. L. Colloids Surf., A 2002, 123124, 341–353. (10) Meszaros, R.; Thompson, L.; Bos, M.; Groot, P. d. Langmuir 2002, 18, 6164–6169. (11) Poptoshev, E.; Rutland, M. W.; Claesson, P. M. Langmuir 2002, 18, 2590–2594. (12) Meszaros, R.; Thompson, L.; Varga, I.; Gilanyi, T. Langmuir 2003, 19, 9977–9980. (13) Dedinaite, A.; Meszaros, R.; Claesson, P. M. J. Phys. Chem. B 2004, 108, 11645–11653. (14) Samoshina, Y.; Nylander, T.; Shubin, V.; Bauer, R.; Eskilsson, K. Langmuir 2005, 21, 5872–5881. (15) Bystrak, S. M.; Winnik, M. A.; Siddiqui, J. Langmuir 1999, 15, 3748–3751. (16) Winnik, M. A.; Bystryak, S. M.; Chassenieux, C. Langmuir 2000, 16, 4495–4510. (17) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 3093–3100. (18) Wang, H.; Wang, Y.; Yan, H.; Zhang, J.; Thomas, R. K. Langmuir 2006, 22, 1526–1533.
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