Conformational Transitions of Flexible Hydrophobic Polyelectrolytes in

Mar 13, 2012 - Departments of Chemistry and Biomolecular Science, Clarkson University, 8 Clarkson Avenue, Potsdam, New York 13699, United. States...
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Conformational Transitions of Flexible Hydrophobic Polyelectrolytes in Solutions of Monovalent and Multivalent Salts and Their Mixtures Oleksandr Trotsenko, Yuri Roiter, and Sergiy Minko* Departments of Chemistry and Biomolecular Science, Clarkson University, 8 Clarkson Avenue, Potsdam, New York 13699, United States ABSTRACT: Conformations of cationic polyelectrolytes (PEs), a weak poly(2-vinylpyridine) (P2VP) and a strong poly(N-methyl-2vinylpyridinium iodide) (qP2VP), adsorbed on mica from saline solutions in the presence of counterions of different valences are studied using in situ atomic force microscopy (AFM). Quantitative characteristics of chain conformations are analyzed using AFM images of the adsorbed molecules. The results of the statistical analysis of the chain contour reveal collapse of the PE coils when ionic strength is in a range from tens to hundreds of millimoles per kilogram and reexpansion of the coils with a further increase of ionic strength up to a region of the saturated saline solutions. The competition between monovalent and multivalent counterions simultaneously present in solutions strongly affects conformations of PE chains even at a very small fraction of multivalent counterions. Shrinkage of PE coils is steeper for multivalent counterions than for monovalent counterions. However, the re-expansion is only incremental in the presence of multivalent counterions. Extended adsorbed coils at low salt concentrations and at very high concentrations of monovalent salt exhibit conformation corresponding to a 2D coil with 0.95 fraction of bound segments (segments in “trains”) in the regime of diluted surface concentration of the PE. Shrunken coils in the intermediate range of ionic strength resemble 3Dglobules with 0.8 fraction of trains. The incrementally re-expanded PE coils at a high ionic strength remain unchanged at higher multivalent salt concentrations up to the solubility limit of the salt. The formation of a strong PE complex with multivalent counterions at high ionic strength is not well understood yet. A speculative explanation of the observed experimental result is based on possible stabilization of the complex due to hydrophobic interactions of the backbone.

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INTRODUCTION In this paper, we use in situ atomic force microscopy (AFM) to study the effects of the nature and valency of salt on the chain conformation of strong and weak polyelectrolytes in aqueous solutions. In the first case study, a strong polycation, poly(Nmethyl-2-vinylpyridinium iodide) (qP2VP), was adsorbed on mica from saline solutions with mono- and tetravalent anions of KCl and K4[Fe(CN)6] and their mixtures at different ionic strengths and salt ratios. In the second case study, a weak polyelectrolyte, poly(2-vinylpyridine) (P2VP), was adsorbed on mica in the presence of a mixture of mono-, bi-, and triphosphate counterions. The results of the P2VP conformation study are compared to those obtained in our previous work for monovalent chloride and bivalent sulfate counterions. In the AFM method, conformations of polymer molecules are studied by direct visualization and measurement of the adsorbed molecules, and thus, collection of quantitative characteristics of the chain conformations.1−4 Results of the statistical analysis of chain contours in these experiments are compared with theories,5−18 simulations,5,6,12,14,15,19−32 and experiments6,9,33−42 available in literature. Two major discussions in this work are related to the transitions of PE chain conformation due to the presence of counterions of different valencies in a broad range of ionic strengths from tens of © 2012 American Chemical Society

millimoles up to the saturated saline solution, and the competition between counterions of different valencies. PEs are polymers that carry positive or negative charges on their backbone, or pendant groups, due to ionization of the functional groups in polar liquids. Weak PEs carry weak acidic or basic functional groups whose ionization state undergoes charges in a pH range close to their pKa values, whereas charges carried by strong PE are not affected by pH.7,8,20 Ionized groups interact with each other via repulsive electrostatic forces. Dielectric permeability of medium and counterions affect the strength of intrachain Coulomb interactions. Hydrophobic PEs have hydrophobic fragments that attract each other, decreasing less favorable interactions with water. A balance of these intrachain interactions, thermal motion, and counterions in dilute PE solutions result in a nonlinear dependence of polymer chain dimensions on electrical charge density on the PE chain.5,6 If a hydrophobic PE is insufficiently charged, its most probable conformation is a compact globule, as it minimizes contact with water. As a fraction of charged groups increases, Coulomb repulsive forces become comparable with hydroReceived: June 15, 2011 Revised: March 13, 2012 Published: March 13, 2012 6037

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of electrostatic and hydrophobic forces on the intrachain level. Thus, an extended coil of a charged PE can collapse in saline solutions of moderate salt concentrations when electrostatic repulsion is suppressed because of counterions condensation and, in the case of multivalent ions, by strong short-range ionic correlations. Mixing counterions of different valencies results in more complex behavior.61 Kundagrami and Muthukumar62 predicted the distribution of mixed counterions and an effective charge on the PE chain by minimizing the free energy that includes the adsorbed and unadsorbed ions entropy, the fluctuations of the electrostatic interactions among unadsorbed ions, the electrostatic interactions between charged groups and counterions, the free energy of the polyelectrolyte, and the electrostatic correlations between neutral ion pairs and triplets. They showed that divalent counterions replaced monovalent ions in competitive interactions with PE chains. Klos and Pakula65 used Monte Carlo simulations based on pair correlation functions to analyze changes in dimensions of PE chains in a mixture of multivalent salts. Their results revealed an increasing effect of counterions on the chain conformation with an increase of the counterions’ charge. A high salt concentration can cause chains to re-expand, as demonstrated by theoretical analysis5,6,17,39,41,59,64,65 and simulations.30,32 This phenomenon is explained by an increased condensation of counterions58 and a complex effect of noncondensed multivalent ions on the formation of the PE complex.17 Simulations have predicted a stronger effect of multivalent ions on the chain re-expansion as compared with monovalent ions.30−32,66 Only a few experimental studies have examined re-expansion phenomena, due to the limitations of experimental methods in conditions of highly concentrated saline solutions. DNA redissolution by multivalent salts67 and re-entrant condensation phenomena for polystyrene sulfonate using fluorescence correlation spectroscopy,68 and microrheological studies of changes in viscosity of dilute solutions of polystyrene sulfonate in the presence of multivalent counterions69 represent rare examples of such studies reported in literature. The experimental results on the conformation of adsorbed P2VP in saline solutions with monovalent and divalent counterions were reported in our previous communication.70 The results confirmed a stronger effect of divalent counterions on contraction of P2VP chains as compared with monovalent counterions. At the same time, re-expansion of the polymer chain at higher salt concentrations was expressed more strongly for monovalent counterions. Those results demonstrated quite a complex response of the flexible hydrophobic PE chains to the presence of counterions of different valancies. The results were explained by the formation of a very stable PE complex with divalent ions. In this work, we extend the analysis of P2VP chain conformations in the presence of multicounterion systems (a mixture of monovalent and tetra- and three-valent counterions). It is noteworthy that, in the experiments, we examined the conformations of adsorbed polymer chains on the mica surface, which reflect the balance of intrachain interactions, and interactions of chains with water, dissolved salts, and mica. In the discussion, however, we frequently refer to theoretical works and simulations for PEs in solutions. In experiments with adsorbed molecules, it is impossible to separate the effects of solvent and solid substrate on the conformation of adsorbed chains. If the concentration of counterions in solution is changed, the formation of a complex between the polyelec-

phobic forces, and the globule swells due to the osmotic pressure caused by an increased counterion concentration near the chain backbone and due to electrostatic repulsion of charged monomer units. In a highly charged PE chain, repulsive forces prevail over attractive hydrophobic forces and the chain extends. The effect of ionic strength on the behavior of PE molecules is central in processes of polyelectrolyte−micelle coacervation and adsorption,43,44 which are directly related to electrostatic interactions and persistence length of polyelectrolyte chains. Sensitivity of chain conformation to pH and salt in solution is essential in the fabrication of complex nanostructured systems based on weak polyelectrolytes, for example, mineralization of PE molecules,45,46 nanospheres synthesized by salt-induced complex coacervation of cDNA and polycations (gelatin and chitosan)47 or citosan and PEs,48 and microcapsulation.47,49,50 Effects of ionic strength are important in biological systems on both molecular and cellular levels. For example, aggregation of zinc-free insulin is triggered by ionic strength and pH.51 Inorganic salts alter tissue permeability.52 Salt ions affect solubility of proteins and stability of their structures as systematized by Hofmeister series.53,54 Fast and reversible compaction of the DNA molecule in response to salt-induced condensation process provides an efficient way of gene delivery in genetic therapy.39 Interactions between counterions and ionized PE chains is important for water ultrafiltration by polyelectrolyte membranes.55 The coagulation efficiency of polyelectrolytes for water purification56,57 can be improved at low concentrations of salts due to the formation of micellelike structures of surfactants and polyelectrolytes, lowering critical aggregation concentration (CAC), while at higher concentrations screening leads to a larger CAC.58 Thus, understanding of how the behavior of PEs is modified in saline solutions is important for many applications. At very low salt concentrations, when the Debye screening length in solution is greater than the chain size, salt ions do not affect chain conformation. However, when Debye screening length is less than the chain size, the strength of electrostatic interactions between charged groups decreases (exponentially with distance between them) and the PE chains shrink. When the Debye length is comparable with the monomer unit size, the screening effect blocks long-range electrostatic repulsion between ionized groups and the conformation of the PE chain is determined by interaction with the solvent.6 However, the influence of counterions on the chain conformation is very complex and could not be explained only by Debye−Huckel theory.39,60 Contraction of flexible chains into compact nearly neutral clusters was explained in terms of ionic correlations by Gonzalez-Mozuelos and de la Cruz.18 Thus, oppositely charged multivalent counterions tend to condense on the PE chains and generate a dense 3D ionic structures of the collapsed chains.13 According to the existing theories9,10,12,9,15 and simulations,20,22,58 multivalent ions exhibit a stronger tendency to the formation of a PE complex and cause a stronger bridging of charged groups than monovalent ions, as they form a more stable and favorable correlated system with the charged chains with fewer ions. In addition, Solis and de la Cruz17 summarized predictions related to shrinkage of PE coils upon addition of multivalent ions and subsequent re-expansion of the coils with a further increase of the salt concentration by including the complex chemical potential of the noncondensed multivalent ions in solution. These effects dramatically change the balance 6038

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Figure 1. AFM topography images of molecules adsorbed on the mica surface: (a) 80% quaternized P2VP extended coils obtained from a solution at pH 5.5 with no added salt, (b) 60% quaternized P2VP globules obtained from a solution at pH 5.5 with no added salt, (c) protonated P2VP extended coils obtained from a solution at pH 3 with no added salt, (d) 80% quaternized P2VP globules from a K4[Fe(CN)6] saline solution with the ionic strength 0.5 mol/kg, and (e) P2VP globules obtained from a Na3PO4/H3PO4 solution at pH 3 and the ionic strength of 0.5 mol/kg. Methods. Experiments were conducted as described elsewhere.73,75,76 Briefly, V-1 grade muscovite mica discs (Structure Probe) were glued to metal supporting disks (required to secure the samples in an AFM instrument), freshly cleaved prior to an experiment, and placed in a fluid cell of a MultiMode scanning probe microscope (Veeco Instruments). Then, the cell was filled with a PE solution. After an incubation period, the sample was scanned in tapping mode using SNL-10 silicon nitride probes (Veeco Instruments) with a tip radius of 20 nm and a spring constant of 0.32 or 0.06 N/m. The tapping force was chosen to be slightly less (94−98%) than the amplitude set point to minimize impact of the tip on adsorbed molecules. We excluded glass tools and containers (mainly polyethylene materials were used) from all the preparatory steps to avoid the adsorption of polymer molecules, which could affect the polyelectrolyte concentration in solutions. AFM images with typical scan sizes 1 × 1 and 0.5 × 0.5 μm2 were postprocessed with WSxM77 and self-coded software78 in order to collect statistical information on dimensions and conformations of the single molecules such as the contour length, the radius of gyration, and the end-to-end distance. The methods used to process the AFM images and extract quantitative characteristics are described in detail in ref 76. Briefly, the self-developed software was used to process the images. Coordinates of about 150−200 chains were recorded by dragging a cursor along the chain contour. The recorded coordinates were used to estimate rms end-to-end distances ⟨r2⟩1/2 and rms radii of gyration ⟨s2⟩1/2 of the chains. The radius of gyration was calculated by drawing the most probable path (judged by an operator) accounting for visible fragments. The end-to-end distance was recorded only if both ends of the molecule were clearly distinguishable. Both methods are complementary because they use different calculation algorithms and different information that is acquired for the molecular contours. For zoomed up images of P2VP molecules, coordinates of the chain ends could be found for 90% of visualized chains in extended state and 5% for collapsed coils. However, the error in estimation of the chain ends for collapsed coils is insignificant for the evaluation of rms endto-end distance in the latter case. The error bars in Figures 2 and 3 stand for standard deviations of measured values and indicate the uncertainties associated with the aforesaid procedure of dragging the cursor to follow chain contours and, ultimately, with the measurements of chain dimensions. The fraction of adsorbed segments (segments in trains) and segments in loops and tails were estimated using the “flooding”

trolyte and counterions and electrostatic interactions of the polymer with mica are affected. However, even uncharged P2VP and qP2VP interacts strongly with mica at conditions of a high ionic strength. Thus, strong van der Waals interactions between pyridine rings and the mica substrate compete with electrostatic interactions of P2VP and qP2VP with mica. This property of the (q)P2VP-mica system is valuable for the study of effects of pH and ionic strength on the conformation of adsorbed P2VP molecules when the molecules’ interactions with a substrate are only incrementally modified by electrostatic interactions. Although the interactions with the mica surface can change the polymer chain conformations, the qualitative features of the coil-to-globule transitions in solution and at solid−liquid interface were shown in many reports to be in accord for P2VP−mica−water system.34,37,71−73

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EXPERIMENTAL SECTION

Materials. P2VP (Mn = 152 000 g/mol; the number average degree of polymerization Nn = 1368; Mw/Mn = 1.05; Sigma-Aldrich) was dissolved in acidic water to form aqueous solutions in a concentration range of 10−3−10−4 g/L (monomer units concentration of 2VP is in the range of 9 × 10−6−9 × 10−7 g·equiv/L). The P2VP solutions’ pH was adjusted to pH 3 using H3PO4 (85%, Fisher Scientific). qP2VP (Mn = 116 500 g/mol; Nn = 504; Mw/Mn = 1.06; the degree of quaternization is 80%; Polymer Source) was dissolved in Millipore water (18.3 MΩ·cm) to form aqueous solutions in a concentration range of 10−2−10−3 g/L (monomer units concentration of 2VPQ is in the range of 4 × 10−5−4 × 10−6 g·equiv/L). Saline solutions of sodium phosphate (Fluka, Germany), potassium chloride (J.T. Baker), and potassium ferrocyanide (J.T. Baker) were prepared in Millipore water in a broad range of concentrations (from diluted to highly concentrated solutions). All the solutions were filtered using MillexLCR 0.45 μm (Millipore) filters. Phosphoric acid has three dissociation steps characterized by pKa values of 2.148, 7.198, and 12.375. The ionic strength of the sodium phosphate solutions and the concentration of phosphoric acid to be added to adjust pH to targeted values were calculated using the pKa values of phosphoric acid. Dissociation of potassium ferrocyanide in water at pH > 3 yelds: K+ and [Fe(CN)6]4−.74 6039

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function of the WSxM software to extract surface coverage versus Zcoordinate. In some experiments, it was impossible to clearly visualize single molecules at a high salt concentration. In those cases, the saline solution after adsorption and equilibration in the fluid cell was replaced with a salt-free aqueous solution (with adjusted pH) by injection into the cell (without drying the sample). No desorption or changes in conformation of the adsorbed molecules were detected in these experiments owing to quasi-irreversible adsorption of the polymer. This procedure helped to improve the quality of scanned images and the quantitative analysis of adsorbed chains’ conformations.

attempts to reproduce the visualization of necklace-like structures. Initially, we discuss the contraction of qP2VP chains (Figure 2). The extended coils shrink with an increase of the

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RESULTS AND DISCUSSION The experiments were conducted to monitor changes in conformations of adsorbed qP2VP and P2VP chains in the presence of counterions of different valancies and to compare the obtained results with those obtained for mono- and divalent counterions in our previous work.70 In aqueous solutions at pH 5.5, qP2VP molecules generate polycations and negatively charged counterions. In a dilute regime, a Coulumb repulsion between monomers, solvatation of counterions, monomer−counterion interactions, and attractive forces between hydrophobic fragments of the chain, which tends to minimize contact with water, are balanced. This balance affects the conformation of PE chains in aqueous solutions. An extended conformation of PE chains in dilute salt-free aqueous solutions could be achieved at certain quaternization degrees of qP2VP when electrostatic repulsion overcomes hydrophobic collapse of the chains. Our experiments showed that the degree of quaternization of 80% (P2VP was quaternized with CH3I in this case) was sufficient to yield an extended coil conformation (Figure 1a). In contrast, a 60% quaternized P2VP was adsorded in a collapsed globule conformation (Figure 1b). The same extended coil conformation was approached for P2VP quaternized (protonated) by HCl at pH 3 (Figure 1c). Thus, 80% quaternized qP2VP and protonated P2VP adsorb on the mica surface as extended coils (Figure 1a,c). The coils tend to shrink and decrease their volume as salts are added in a solution (Figure 1d,e). Because of different chain sizes of P2VP and qP2VP, we observed a difference in the relative shrinkage for these two samples (compare Figures 2 and 3). Sarraguca and Pais79 observed in simulations a stronger compaction of longer PE chains. Our experiments showed that the protonated at pH 3 longer P2VP chains (Nn = 1368) could achieve a more compact conformation (34% decrease in the rms radius of gyration) at higher salt concentration than shorter qP2VP (Nn = 504) chains that exhibited only 17% decrease in the rms radius of gyration. Theories and simulations predict that the coil-to-globule transition passes through intermediate states; the formation of the necklace-like structures, when repulsive forces between ionized groups, screened by counterions, are insufficient to overcome hydrophobic attraction. Experiments have demonstrated that hydrophobic PE globules break into several charged beads34,37,80−85 when hydrophobic attraction between the beads is balanced by electrostatic repulsion between charges on the beads. Necklace-like structures were shown to be unstable.34 Visualization of necklace-like structures was achieved in special experiments with rapid deposition of necklaces on the mica surface.34 Here, we did not make

Figure 2. rms radius of gyration (a) and the rms end-to-end distance (b) of adsorbed quaternized qP2VP molecules in salt-free solutions (open triangle) and as a function of the ionic strength in saline solutions of KCl (solid triangles), K4[Fe(CN)6] (solid squares), and KCl/K4[Fe(CN)6] mixtures (half solid triangles) with the following ratios: *2.49:0.01 mol/kg; **2.4:0.1 mol/kg. Lines are given for convenience to guide the reader.

K4Fe(CN)6 concentration that can be quantitatively characterized by changes in the rms radius of gyration (Figure 2a) and the rms end-to-end distance (Figure 2b). For the reader’s convenience, the upper X-axis is labeled in the Debye length (in nanometers). The most compact conformation was approached at ionic strengths in a range of 1 mol/kg > ionic strength > 0.1 mol/kg, when the Debye length falls between 0.3 and 1 nm. The multivalent [Fe(CN)6]4‑ counterions are more efficient in the compaction of PE chains, as can be concluded from the coils’ dimensions and from the slope of the graphs. The compaction begins at a lower ionic strength for [Fe(CN)6]4‑ anions as compared with Cl− counterions. 6040

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SO42− > Cl− and [Fe(CN)6]4− > {[Fe(CN)6]4− + Cl−}mixture > Cl−. Overall, this trend is in accord with the theory and results of simulations. Presence of multivalent salts results in a more compact PE conformation because of the combination of a stronger screening effect and stronger correlations of counterions due to their higher valence and greater charge per unit volume. Hydrated radii of ions change in the series [Fe(CN)6]4− > PO43− = HPO42− = H2PO4− = SO42− > Cl− as 0.47, 0.4 and 0.3 nm, respectively, while their hydrated shells of equivalent charge decrease as [Fe(CN)6]4− < PO43− < HPO42− = SO42− < Cl < H2PO4−. Thus, multivalent ions occupy a much smaller volume per equivalent charge in collapsed coils, as their effective charge per unit volume is greater and, consequently, the percentage of a PE coil volume occupied by counterions is minimized. Progressively smaller coil dimensions (defined by the rms radius of gyration) of P2VP can be found at 0.43, 0.25, and 0.16 mol/kg ionic strength for NaCl, Na2SO4, and Na3PO4 solutions, respectively. At 0.43 mol/kg ionic strength, P2VP chains approach the same rms radius of gyration of 16 ± 3.2 nm for all investigated solution of salts. This radius coincides with the radius of P2VP coil in theta conditions.86 A further increase of ionic strength results in re-expansion of the polymer coils in solutions of monovalent salt.70 Clustering of counterions around the PE coil results in an alternation of the balance of electrostatic forces so that the bridging effect vanishes, the backbone turns into a more hydrophilic state, and the chain re-expands. The re-expansion is less pronounced for multivalent counterions (Figures 2 and 3). The competition between monovalent and multivalent counterions is observed in the experiment in which two salts KCl and K4Fe(CN)6 were mixed in qP2VP solutions (see the data points labeled with asterisk in Figure 2). The effect of [Fe(CN)6]4‑ ions was observed even at 0.01 M fraction of this salt in mixture with KCl. Thus, a large excess of counterions causes swelling of PE chains for all salts, but at different degrees. Sodium chloride demonstrates the largest re-expansion, and at a very high concentration the chains re-expand to the size of the extended coil. The simulations suggest that the greater the valence of counterions, the larger is the re-expansion. Our experiments demonstrate an opposite tendency: re-expansion degree diminishes in the series Cl− > SO42− > {PO43− + HPO42− + H2PO4−}mixture and Cl− > {[Fe(CN)6]4− + Cl−}mixture > [Fe(CN)6]4− for protonated and quaternized P2VP, respectively. The re-expansion is minimal in sodium phosphate solutions (Figure 3). For multivalent counterions, the reexpansion approaches the saturation and remains unchanged with a further increase in the salt concentration. The analysis of coil conformations demonstrated that, at a low ionic strength, protonated P2VP molecules adsorb in a flat (2D) conformation with a very high fraction of segments bound to the substrate (Figure 4), provided that the surface concentration of adsorbed chains is below the overlap concentration (interchain interactions among adsorbed molecules are negligibly small). Interactions with the oppositely charged substrate result in flattening of the molecule and an increase of the contact area. Only a small fraction of segments (20%).

The collapse of P2VP chains quaternized with protons at pH 3 is shown in Figure 3. The obtained in this work graph for

Figure 3. rms radius of gyration (a) and the rms end-to-end distance (b) of adsorbed protonated at pH 3 P2VP molecules as a function of the ionic strength in saline solutions of NaCl (circles), Na2SO4 (squares), and Na3PO4 (triangles). Lines are given for convenience to guide the reader.

sodium phosphate solutions is compared with the previously published graphs for sodium chloride and sodium sulfate solutions.70 The comparison of the transition between extended coil and collapsed conformations reveals a much stronger effect of sodium phosphate than it was obtained for chloride and sulfate salts. For the experiments in phosphate salt solutions, the equilibrium concentrations of PO43− ions and HPO42− ions (at pH 3 and the ionic strength of 0.55 mol/kg) are 10−14 mol/ kg and 3 × 10−5 mol/kg, while the concentration of H2PO4− ions is 0.5 mol/kg (note that the concentration of monomer units is around 10−5 M). However, P2VP chains experience a much stronger effect of counterions as could be expected from the presence of only monovalent H2PO4− ions. The sharp shrinkage of the PE coils is likely due to a mixture of monovalent and multivalent ions. Consequently, the competition between phosphate ions to form the most favorable complexes Hn(P2VP)+n−nPO43− and Hn(P2VP)+n−nHPO42− could explain the stronger tendency for the coils shrinkage in sodium phosphate solution. By comparing two series of measurements for protonated P2VP and qP2VP, we may conclude that the efficiency of counterions in terms of their ability to compact PE coils decreases as follows: {PO43− + HPO42− + H2PO4−}mixture > 6041

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these structures at very high salt concentrations up to the solubility of the salt. The high stability of the PE complex at high concentrations of multivalent salts and the discrepancy with simulations are hard to explain. It is likely that even at very high salt concentrations the stability of the PE complex is amplified by hydrophobic interactions within the polymer chain. This hypothesis could be verified in experiments with a less hydrophobic PE. In our discussion, we ignored possible effects of interactions with the substrate, although that may indeed affect the dependence of adsorbed chains’ conformation on the ionic strength.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Figure 4. Fraction of the bound segments for protonated at pH 3 P2VP molecules adsorbed from saline solutions of NaCl (circles), Na2SO4 (squares), and Na3PO4 (triangles). The dimensions are compared with those for adsorbed P2VP coils at pH 4.8 with no added salt (tilted triangle). Lines are given for convenience to guide the reader.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported in part by the U.S. Army Research Laboratory and the U.S. Army Research Office under grant number W911NF-05-1-0339 and in part the National Science Foundation Grant CBET-0756461.

This behavior is clearly observed for NaCl solutions. The extended coils adopt a flat 2D conformation at low and high salt concentrations. In the range of ionic strengths between 0.1 and 1 mol/kg, the adsorbed compact chains resemble a 3D globular conformation of adsorbed uncharged polymer chains at pH 4.8.76 The reference experiment for a salt-free solution at pH 4.8, where P2VP molecules are collapsed due to hydrophobic interactions in an aqueous solution, demonstrates the lowest fraction of the segments in the first monolayer, because hydrophobic interactions in this case are strong enough to prevent flattening of the polymer chains. Thus, shrinkage and then re-expansion of PE chains as the salt concentration progressively increases are accompanied by transition from 2D to 3D and then again to 2D conformations of adsorbed chains. This scheme works well for monovalent counterions. However, we did not observe the transition from 3D to 2D chains for multivalent counterions.

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REFERENCES

(1) Sheiko, S. S. Imaging of Polymers Using Scanning Force Microscopy: From Superstructures to Individual Molecules. In New Developments in Polymer Analytics II, Book Series: Advances in Polymer Science; Schmidt, M., Ed.; Springer: Berlin/Heidelberg, 2000; Vol. 151, pp 61−174. (2) Gallyamov, M. O. Scanning Force Microscopy as Applied to Conformational Studies in Macromolecular Research. Macromol. Rapid Commun. 2011, 32 (16), 1210−1246. (3) Magonov, S. N. AFM in Analysis of Polymers. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John Willey and Sons Ltd: Chichester, 2000; pp 7432−7491. (4) Nishimura, T.; Tsuchiya, K.; Ohsawa, S.; Maeda, K.; Yashima, E.; Nakamura, Y.; Nishimura, J. Macromolecular Helicity Induction on a Poly(phenylacetylene) with C2-Symmetric Chiral [60]FullereneBisadducts. J. Am. Chem. Soc. 2004, 126, 11711−11717. (5) Dobrynin, A. V. Theory and simulations of charged polymers: From solution properties to polymeric nanomaterials. Curr. Opin. Colloid Interface Sci. 2008, 13, 376−388. (6) Dobrynin, A. V.; Rubinstein, M. Theory of polyelectrolytes in solutions and at surfaces. Prog. Polym. Sci. 2005, 30, 1049−1118. (7) Holm, C.; Joanny, J. F.; Kremer, K.; Netz, R. R.; Reineker, P.; Seidel, C.; Vilgis, T. A.; Winkler, R. G. Polyelectrolyte theory. In Polyelectrolytes with Defined Molecular Architecture II; Book Series: Advances in Polymer Science; Schmidt, M., Ed.; Springer: Berlin/ Heidelberg, 2004; Vol. 166, pp 67−111. (8) Holm, C.; Rehahn, M.; Oppermann, W.; Ballauff, M. Stiff-chain polyelectrolytes. In Polyelectrolytes with Defined Molecular Architecture II; Book Series: Advances in Polymer Science; Schmidt, M., Ed.; Springer: Berlin/Heidelberg, 2004; Vol. 166, pp 1−27. (9) Drifford, M.; Polyelectrolyte, M. D. Solutions with Multivalent Added Salts: Stability, Structure, and Dynamics. In Physical Chemistry of Polyelectrolytes, 1st ed; Radeva, T., Ed.; Dekker: New York, 2001; p 135. (10) Ullner, M. Comments on the scaling behavior of flexible polyelectrolytes within the Debye-Huckel approximation. J. Phys. Chem. B 2003, 107, 8097−8110. (11) Uyaver, S.; Seidel, C. Effect of Varying Salt Concentration on the Behavior of Weak Polyelectrolytes in a Poor Solvent. Macromolecules 2009, 42, 1352−1361.

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CONCLUSIONS In an intermediate range of ionic strengths, screening of electrostatic interactions and counterion condensation result in collapsed of hydrophobic polyelectrolyte coils that is affected by hydrophobic forces. Longer PE chains undergo a greater relative compaction than shorter chains. We observed substantial differences in behavior of hydrophobic PEs in solutions of monovalent salt as compared with multivalent salts: (a) contraction of polymer coils was approached at lower ionic strengths for multivalent counterions; (b) re-expansion was less pronounced for multivalent salts; (c) in counterion mixtures, the presence of multivalent ions affects PE conformation even at a very small fraction of multivalent counterions; (d) the adsorbed PE chains retained a 3D conformation at very high concentrations of multivalent salts in contrast to monovalent salt solutions; (e) multivalent counterions interact with PE chains much stronger than monovalent ions. This behavior is brought about by ion correlations effect in the presence of multivalent counterions. Multivalent counterions form intramolecular clusters within charged chains. The most surprising result is a high stability of 6042

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