Conditions Leading to Polyelectrolyte Complex Overcharging in

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Conditions Leading to Polyelectrolyte Complex Overcharging in Solution: Complexation of Poly(acrylate) Anion with Poly(allylammonium) Cation Tomislav Kremer, Davor Kovačević, Jasmina Salopek, and Josip Požar* Division of Physical Chemistry, Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia S Supporting Information *

ABSTRACT: Complexation between poly(acrylate) (PA) and poly(allylammonium) (PAH) macroions at pH = 7.0 was studied by means of electrokinetics, microcalorimetry, and DLS. At low polyelectrolyte concentrations and no electrolyte present strong overcharging of primary nanocomplexes occurred. In contrast, the increase in polyelectrolyte concentration led to flocculation taking place near the equivalence. The nanocomplex charge reversal was also achieved in the high polymer concentration regime by the abrupt instead of stepwise titrant addition. The procedure was successfully used in the case of several other polyion pairs. The presence of electrolyte affected the PAH−PA interpolyelectrolyte neutralization considerably, leading to ion-specific aggregation and extrinsic charge compensation. The complexation energetics was weakly influenced by ionic conditions. To the best of our knowledge, the reported results are the first direct evidence for primary complex overcharging. Their rationalization explains why interpolyelectrolyte neutralization in solution and at surfaces usually results in formation of neutral and charged products, respectively.

1. INTRODUCTION Polyelectrolyte complexes (PEC)1−6 and polyelectrolyte multilayers (PEM),7,8 have been attracting attention of colloid and polymer scientists for quite some time. The primary reason for such an interest nowadays lies in their potential use in biotechnology, medicine, and various branches of industry.9−13 However, the number of more fundamentally oriented studies has increased considerably during the past two decades.5,14−23 After all, the understanding of the factors that contribute to stability and dynamics of such nanoassemblies provides a path for their targeted design and fine-tuning of their chemical and physical properties. One of the still unresolved issues concerning interpolyelectrolyte neutralization is the relatedness of PEC formation in solution with the process of PEM deposition at surfaces, primarily in the context of overcharging (i.e., the reversal of surface charge upon its exposure to solutions containing oppositely charged polyions).24,25 The resulting charge overcompensation has been recognized as the main driving force of PEM formation from the very beginnings.7 In contrast, the insoluble precipitates containing approximately equimolar ratios of oppositely charged monomers were most often reported as products of corresponding reactions in solution3,6,22,26at least in the case when the distances between charged monomers were complementary (vinylic polyions containing one charged group per monomer), the electrolyte concentration low, and the neutralization carried out by stepwise addition of one reactant (titrant) into the solution of the other (titrand). However, investigations of such reactions © XXXX American Chemical Society

by means of light scattering techniques revealed that nanosized polyelectrolyte complexes were being formed prior to precipitation (flocculation).4,5,18,23 The sign of their considerable surface charge corresponded to that of the titrand (polymer present in excess).5,18,23 The obtained suspensions were stable for months.18,23 Further titrant addition to such systems hence must have led to neutralization of the primary complex corona charge. As a consequence, oppositely charged secondary complexes bearing the excess of titrant monomers at the surface were formed. Their reaction with the primary complexes eventually resulted in flocculation. After an approximately equivalent amount of titrant has been added, the number of free corona monomers diminished, and no further changes in solution conductivity,5 counterion activity,23,27 viscosity,5 or enthalpy changes5,18 could be observed, regardless of the reactant addition order. The interpolyelectrolyte neutralization of many polyelectrolyte pairs essentially proceeded as explained by Fuoss and Sadek in the very first reported study of interpolyelectrolyte neutralization.2 Unfortunately, during the decades that followed this straightforward explanation of the sequence of events which occur upon stepwise addition of one polyelectrolyte into the solution of the other was largely forgotten. Many authors who studied the reactions using light scattering techniques considered the molar ratio at which the flocculation begins to appear (usually at Received: August 30, 2016 Revised: October 21, 2016

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Macromolecules monomolar ratios (r) equal to or higher than r = 0.8) as the titration end point.4 After becoming aware that the reactions usually proceeded above this molar ratio, they introduced various titration end points (end point as determined by potentiometry, viscosimetry, turbidimetry, etc.)4 It should be noted that turbidimetry was and still is predominantly used in investigations of interpolyelectrolyte neutralization. As a consequence, quite limited information concerning the complexation of many polyion pairs is available. The fact that interpolyelectrolyte neutralization resulted in precipitates containing approximately equal amounts of oppositely charged monomers irrespectively of the reactant addition order sometimes also led to erroneous conclusions that equilibrium products are being formed, i.e., that the process is reversible. In the relatively recent calorimetric study of reaction between sodium poly(styrenesulfonate) and poly(allylamonium) chloride the dependence of measured enthalpy changes upon reactant concentration was even processed using a model that assumes the formation of soluble equilibrium products in solution.28 Crucial evidence against rapid equilibrium establishment has been ignored; charged nanocomplexes were formed prior to phase separation.18 Moreover, since the product of reaction was sparsely soluble, a different model had to be used for equilibrium description. The irreversible nature (i.e., the formation of metastable precipitates) of this reaction18,23 as well as several others involving vinylic polyelectrolytes5,29−31 was revealed by carrying out the reactions in concentrated electrolyte solutions. Such investigations clearly indicated that depending on the titration direction, insoluble precipitates of different composition (i.e., containing different amounts of oppositely charged monomers) were obtained. Namely, the increase in simple salt concentration led to ion-specific aggregation of primary complexes as well as their notable overcharging. As a consequence, the composition of corresponding metastable reaction products deviated from the 1:1 monomolar ratio considerably. The equilibrium establishment proceeded extremely slowly (up to several months) and only in the presence of a simple binary salt.5,23 These findings were in accord with the results of Tsuchida32 and Kabanov33−35 groups regarding soluble polyelectrolyte complexes (products of reactions involving polyions with notably different polymerization degrees). Despite the apparent reversibility of the interpolyelectrolyte neutralization, the arguments against rapid formation of equilibrium products in solutions containing oppositely charged polyelectrolytes of high charge density seem to be strong indeedparticularly once the formation of corresponding, metastable multilayers14,15 is taken into account. However, if the titration of one polyelectrolyte with the other leads to formation of metastable products in solution (as it does during the interpolyelectrolyte neutralization at surfaces), a question arises as to why their overcharging cannot be observed. That is, why, in the case when no supporting electrolyte is added, does the complexation proceed only up to approximately an equimolar ratio of oppositely charged monomers? After all, the multilayers are characterized by considerable surface charge and so are the primary polyelectrolyte complexes. The complete monomer pairing could therefore be realized by strong primary complex overcharging as well. As it turns out, the answer to this question lies in the way in which the interpolyelectrolyte neutralization in solutions is typically carried out, i.e., the stepwise addition of titrant to titrand solutions at relatively high monomer concentrations. In this

paper we provide the evidence for strong overcharging of primary poly(acrylate) anion (PA) and poly(allylammonium) cation (PAH) complexes at lower polymer concentrations. The overcharging of primary PA−PAH as well as of poly(styrenesulfonate)−poly(allylammonium) (PSS−PAH) and poly(styrenesulfonate)−poly(diallyldimethylammonium) (PSS−PDADMA) complexes was also induced by varying the amount of titrant introduced, even at relatively high monomer concentrationsmore precisely, at reactant mixture compositions close to those at which flocculation in “classic” titration experiments can be observed. In addition, the complexation energetics of PA−PAH interpolyelectrolyte neutralization was explored calorimetrically, and the size of complexes was determined from DLS experiments. The influence of the counterion type on the process of monomer pairing was explored in detail and compared with the results obtained in the case of PAH−PSS pair.18,23 For that purpose, binary salts, composed of monovalent cations and anions with quite different standard thermodynamic hydration parameters, namely LiCl, NaCl, KCl, CsCl, (CH3)4NCl, NaBr, NaI, NaNO3, and NaClO4, were used. The results are discussed in the light of polyion−counterion preferences and counterion hydration. The composition and structure of obtained metastable reaction products are addressed as well.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(allylamine) hydrochloride (PAHCl; Mw = 15 000 g mol−1) and poly(acryilic) acid (PAA; Mw = 230 000 g mol−1) were obtained from Aldrich and Fluka, respectively. The monomer functionalization degrees ( f) were determined by potentiometric titrations with standardized sodium hydroxide solutions purchased from Aldrich. NaOH(aq) concentration was determined by titration with potassium hydrogen phthalate (Aldrich) solution. The values obtained were f(PAA) = 0.89 and f(PAHCl) = 0.94. Monomer concentrations of all prepared polymer solutions were corrected accordingly. The solutions used for DLS, electrokinetic, and microcalorimetric investigations were prepared by diluting the polymer stock solutions (0.25 mol L−1). Deionized water was used in all cases. To ensure the maximum charge density of both polyions,36,37 3-(N-morpholino)propanesulfonic acid (MOPS) was added to solutions of polycation and polyanion. The pH of the stock buffer solution (c(MOPS) = 0.25 mol L−1) was adjusted with concentrated sodium hydroxide solution to pH = 7.0. Poly(diallyldimethylammonium) chloride (Mw ≤ 100 000 g mol−1) and sodium poly(styrenesulfonate) (Mw = 70 000 g mol−1), used for investigation of primary complex overcharging, were obtained from Aldrich. The salts (NaCl, NaBr, KCl, NaI, (CH3)4NCl, NaClO4, NaNO3, CsCl, and LiCl) were purchased from various manufacturers (Merck, Sigma, Aldrich) and were of analytical purity grade. The ionic strength of polymer solutions was adjusted by adding the appropriate volume of concentrated salt solutions (5 mol L−1). Barium chloride (Aldrich), 18-crown-6 ether (Fluka), and tris(hydoxymethyl)aminomethane (THAM) (Aldrich) were used for calorimeter calibration. 2.2. Dynamic Light Scattering and Electrophoretic Measurements. The PAH−PA, PAH−PSS, and PDADMAC−PSS complexes were prepared by mixing the solutions containing oppositely charged polyions. In order to avoid the dependence of polyelectrolyte complex size on stirring rate,4,18 the titrant was always added under vigorous agitation with a magnetic stirrer. The effective hydrodynamic diameters (Deff) of prepared complexes were determined from dynamic light scattering experiments. For this purpose a Brookhaven 90 Plus particle size analyzer (Brookhaven Instruments Corp.) was used. The detector was in all cases placed at 90° with respect to the incident beam. The diffusion coefficients of particles were calculated B

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Figure 1. (a) Mean effective hydrodynamic diameters of colloid complexes prepared by titrations of PA (V0 = 2.0 mL) with PAH in MOPS buffer (pH = 7.0) at 25 °C. Concentration of titrand is given in the figure; cm(titrant) = c(MOPS) = 10 × cm(titrand). (b) Mean effective hydrodynamic diameters of colloid complexes obtained by titrations in the opposite direction. Black circles indicate that flocculation occurred at higher monomer ratios. Error bars indicate the standard error of the mean of two experiments. from the autocorrelation function, and the effective hydrodynamic diameters (Deff) were obtained from the Einstein−Stokes equation.38,39 Electrophoretic mobilities of the complexes were determined by means of ZetaPlus (Brookhaven Instruments Corp.) from the Doppler shift frequency and the applied electric field.38,39 2.3. Microcalorimetric Investigations. Microcalorimetric titrations were carried out by means of an isothermal titration microcalorimeter, CSC 4200 ITC (Calorimetry Science Corp.), at 25.0 ± 0.1 °C. The calorimeter reaction cell was filled with polyelectrolyte solution (V = 1.3 mL; cm(titrand)/mol L−1 ≈ 1 × 10−3 or 5 × 10−3). The enthalpy changes were recorded upon stepwise additions (400 s time intervals) of oppositely charged polyelectrolyte (cm(titrant)/mol L−1 ≈ 1 × 10−2 or 5 × 10−2) from a 250 μL Hamilton syringe under the same ionic conditions as in the reaction cell. Heats of titrant dilution were obtained by blank experiments. The enthalpies of titrand dilution were negligible in all cases. To avoid the dependence of particle size upon stirring rate,4,18 the calorimetric cell content was mixed vigorously (300 rpm). All measurements were conducted three or more times. The calorimeter was calibrated chemically by carrying out the complexation of barium(II) by 18-crown-6 in aqueous medium at 25 °C. The results obtained (log K = 3.71, ΔrH = −31.1 kJ mol−1) were in good agreement with the literature values (log K = 3.75, ΔrH = −31.5 kJ mol−1).40 Its reliability was additionally checked by protonation of THAM(aq) with HCl(aq). The agreement between determined (ΔrH = −49.2 kJ mol−1) and the literature data (ΔrH = −47.6 kJ mol−1)41 was again satisfactory.

Macroscopic phase separation at higher reactant concentrations indicates that monomer pairing proceeded as described by Fuoss and Sadek.2,18,23 At first, polyelectrolyte complexes with excess titrand monomers were formed (primary complexes). The oppositely charged secondary complexes started to occur at monomer ratios at which the increase in particle size was noticed. Finally, their coalescence with smaller primary complexes resulted in flocculation. In contrast, no phase separation could be observed at lower polymer concentrations (cm(titrand)/mol L−1 ≤ 1 × 10−3). The particle size instead increased abruptly at a certain monomer ratio (r ≈ 0.75 for PAH → PA addition and r ≈ 1.0 or 1.2 in the case of PA → PAH addition), remaining constant afterward. The accordingly obtained suspensions showed no tendency toward phase separation in periods of several months. These findings suggest that the secondary complexes were formed at lower polymer concentrations (cm(titrand)/mol L−1 ≤ 1 × 10−3). In order to investigate such a possibility, the electrophoretic mobilities (μ) of obtained particles were measured (Figure 2).

3. RESULTS AND DISCUSSION 3.1. Complexation of Oppositely Charged Polyelectrolytes without Added Supporting Electrolyte. The interpolyelectrolyte neutralization of poly(acrylate) anion (PA) and poly(allylammonium) cation (PAH) in aqueous MOPS buffer solutions (pH = 7.0) was investigated by means of DLS at several reactant concentrations (1 × 10−4 ≤ cm(titrand)/mol L−1 ≤ 1 × 10−2; the concentration of titrant monomers was always 10 times larger than that of titrand). To maintain the pH of the reactant mixture constant, the buffer concentration was kept notably higher than that of titrand monomers (c(MOPS)/ cm(titrand) ≈ 10) and hence of the titrant, after its addition). The effective hydrodynamic diameters of complexes, prepared by stepwise titrant addition, are shown in Figure 1. As it can be seen, the average diameters of complexes formed were more or less constant up to flocculation (r ≈ 0.75 (r = nm(titrant)/nm(titrand)) in the high polymer concentration regime (cm(titrand)/mol L−1 ≥ 5 × 10−3), irrespectively of the reactant addition order.

Figure 2. Mean electrophoretic mobility of PAH−PA complexes obtained by titrations of the polycation with the polyanion and vice versa in MOPS buffer (pH = 7.0) at 25 °C (cm(titrant) = c(MOPS) = 1.0 × 10−2 mol L−1, cm(titrand) = 1.0 × 10−3 mol L−1; r = titrant-totitrand monomer ratio). Error bars indicate the standard error of the mean of two experiments.

As it could be expected, the sign of the primary complexes charge was equal to that of the titrand.5,18 Once the neutralization of the excess surface charge started to occur (more or less coinciding with the monomolar ratio at which the increase in particle size could be observed by means of DLS), the increase/decrease of electrophoretic mobility was noticed. C

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Figure 3. (a) Thermograms obtained by titration of PA with PAH and vice versa at θ = 25.0 ± 0.1 °C and pH = 7.0 (cm(titrant) = c(MOPS) = 1.3 × 10−2 mol L−1), cm(titrand) = 1.0 × 10−3 mol L−1, V(titrand) = 1.3 mL). (b) Corresponding mean enthalpy changes as a function of titrant to titrand molar ratio (r). Error bars indicate the standard error of the mean of three independent experiments.

This finally led to particle charge reversal. The secondary complexes were indeed the metastable reaction products in the low concentration regime. Upon closer examination of electrokinetic results, one can observe that the charge reversal first occurred at monomer ratios lower than unity. This can be explained by mismatches in kinetically controlled monomer pairing, especially during primary complex overcharging.2,29 Namely, the neutralization of primary complex corona monomers should be less efficient in comparison to that occurring at lower monomer ratios. The recent potentiometric measurements of counterion activities during PAHCl−NaPSS complexation23 are in agreement with this explanation. The other reason could be the lower charge of polyions than that anticipated on the basis of literature data (polyions should be fully charged at pH = 7.0).36,37 The real reactant monomer ratios can also be different than those given in Figure 2 due to experimental errors in polymer standardization. On the other hand, even if the functionalization degrees of both polymers are correctly determined, the nonfunctionalized (neutral) monomers are randomly distributed among the chains. This affects the value of the corrected molar ratio (taking into account only the concentration of charged monomers) at which charge reversal occurs, especially in the case of low functionalization degrees. The influence should not be substantial in the herein investigated case ( f(PAA) = 0.89, f(PAH) = 0.94). The absence of flocculation at lower polymer concentrations (cm(titrand)/mol L−1 ≤ 5 × 10−3) can be explained by differences in polyion chain length. Because of large polyanion polymerization degree, the concentration of primary complexes remains low even at relatively high monomer concentrations. This reduces the probability of primary and secondary complex coalescence, which is crucial for the onset of macroscopic phase separation. Apart from the investigations of PA−PAH complex size and their electrophoretic mobility, the corresponding reaction energetics was studied calorimetrically. As an example of the results obtained at lower polymer concentrations, cumulative enthalpy changes as a result of stepwise titrant to titrand addition and the corresponding thermograms are shown in Figure 3. One can see that no measurable heat effects above equivalence could be observed. Unlike in the case of several other studied polyion pairs,18,23 the reactant monomer ratios at which secondary complexes started to occur were not well visible (i.e., the slope of ΔH vs r is almost constant up to equivalence). As in our previous studies,18,23 the enthalpies of the primary complex formation were calculated by dividing the

cumulative enthalpy changes measured at lower molar ratios (r ≤ 0.8, Figure 3) with the total amount of added titrant monomers. The enthalpies of secondary complex formation were obtained as the quotient of the overall enthalpy change and the amount of the titrand monomers in the reaction cell. The accordingly determined values are enlisted in Table 1. It should be mentioned that the calorimetric cell content was checked after each experiment. The formation of precipitate was never observed at examined reactant concentrations. Table 1. Enthalpies of Primary and Secondary Complex Formation Obtained during Titration of PA with PAH and Vice Versa at pH = 7.0 and θ = 25.0 ± 0.1 °Ca PAH → PAA

ΔrH ± SE (kJ mol−1)

primary complexes secondary complexes PAA → PAH

−9.8 ± 0.2 −11.4 ± 0.2 ΔrH ± SE (kJ mol−1)

primary complexes secondary complexes

−12.2 ± 0.4 −10.9 ± 0.3

cm(titrand)/mol L−1 = 1.0 × 10−3, cm(titrant)/mol L−1 = 1.3 × 10−2 = c(MOPS)/mol L−1. SE = standard error of the mean (N = 3).

a

The differences between primary and secondary complex formation enthalpies were negligible from the thermodynamic point of view. As will be evident from the results of calorimetric experiments in the high concentration regime, this can be explained by the almost isoenthalpic overcharging of both complex types and by the fact that most of monomer pairing actually occurs during the process of primary complex formation. (The formation of secondary complexes could be noticed at ratios quite close to the equivalence, i.e., after considerable amount of oppositely charged monomers has been paired.) The interpolyelectrolyte neutralization of PA and PAH in the used buffer solution was relatively favorable in terms of enthalpy. The so far studied reactions involving strongly charged polyelectrolytes were, more or less, isoenthalpic.3,5,18,22,26,28 The lower complexation enthalpies in the case of herein investigated pair were most likely due to the relatively poor screening of the polyion charge with the zwitterionic form of buffer. Namely, the chloride anions, which neutralize the polycation charge in the solid state (PAH was obtained as a chloride salt), were, at least in part, expelled from the polyion domain with the buffer anions which were present in considerable excess.42,43 The polyanion charge is to a certain D

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ionic strengths. Their aggregation would otherwise occur with aging. This is in accord with the earliest theoretical investigations of overcharging phenomenon. Namely, the investigations of Netz and Joanny44 and Castelnovo and Joanny45 indicated that considerable overcharging of PEMs can occur (ΔG < 0) provided the unfavorable monomer interactions become sufficiently screened. This can be accomplished by increasing the polyion−polyion distances at the surface, which supports “dangling chain” rationale of the surface charge excess at surfaces and in solutions. The overcharging could also be induced by increasing the electrolyte concentration, which is in accord with the ionic strength dependence of PEM thickness,46,47 salt induced aggregation,20,48 and overcharging of primary PECs20,23 in solution. However, it should be noted that due to complexity of the studied system, only the essential polyelectrolyte and surface properties could be taken into account. The polyions and the surface were characterized by certain charge densities; solvent was modeled as a dielectric continuum and counterions as point charges, whereas all electrostatic interactions were treated on the Debye−Hückel level. This approximation is suitable only in the case of relatively low polyion charge densities or higher salt concentrations and therefore not capable of predicting the primary PECs or PEMs formation in the case of strongly charged vinylic polyelectrolytes. Later on, Panchagnula et al.49 and Patel et al.50 studied the process of multilayer formation on nanoparticles and surfaces by means of MD simulations using a more sophisticated “beads on a string” polyion model. They noticed considerable sequential overcharging in the case of strongly charged polyions in salt-free solutions provided the chains were sufficiently long (Pn ≥ 32). The phenomenon was predominantly due to long strands of polymer chains protruding from the surface. Although no comparison of surface and solution chain counterion affinities was given, the observed overcharging mechanism apparently did not lead to notable, further counterion condensation. The obtained nanocomposites displayed considerable interlayer mobility, particularly in the case of longer polyions, which the authors ascribed to nonequilibrium adsorption. The described behavior is in complete accord with the “dangling chain explanation” of PEM and primary PEC surface charge excess. On the other hand, the polyion (coarse grain chain) and solvent (dielectric continuum) models used were again rather crude. Further experimental and more sophisticated theoretical investigations are needed to explain whether the PEM and PEC overcharging leads to more pronounced counterion condensation to excess surface monomer. In contrast, the question whether the interpolyelectrolyte neutralization in solution and at surfaces leads to formation of metastable or equilibrium products can be easily answered by performing simple titration experiments, which will be described in the following section. 3.1.1. Polyelectrolyte Complex Overcharging in Solution. Quite a few review articles discussing the correlations between the process of PEC and that of PEM formation have been written over the past decade.18,19,25,51,52 Among many recognized similarities, a fundamental difference between the outcomes of interpolyelectrolyte neutralization at surfaces and in solution has occasionally been pointed out.24,25 No evidence of interpolyelectrolyte neutralization involving strongly charged vinylic polyelectrolytes above equivalence can be noticed in “classic” titration experiments whereas strong overcharging of each layer is achieved upon its exposure to solutions containing

extent compensated with the protonated buffer form as well. The sodium cation, introduced during pH adjustment of the stock solution, counterbalances the rest charge of the poly(acrylate) charge. The isoenthalpic complexation of PA and PAH in concentrated aqueous NaCl solutions (section 3.2) supports this claim. The complexation energetics was also studied in the high polymer concentration regime (Figure S3 in Supporting Information). The cumulative enthalpy changes again suggest quite efficient monomer pairing up to equivalence. The maximum amount of paired monomers within the flocculate corresponds to the amount of titrand monomers in the cell (titrant is present in excess above equivalence). The enthalpies of flocculate formation were hence calculated by dividing the cumulative enthalpy changes with the titrand monomer amount (Table 2). The enthalpies of primary complex formation were obtained as described earlier. Table 2. Enthalpies of Primary Complex and Flocculate Formation Deduced by Processing the Calorimetric Data Obtained during Titration of PA with PAH and Vice Versa at pH = 7.0 and θ = 25.0 ± 0.1 °Ca PAH → PAA

ΔrH ± SE (kJ mol−1)

primary complexes precipitate PAA → PAH

−3.8 ± 0.2 −3.6 ± 0.2 ΔrH ± SE (kJ mol−1)

primary complexes precipitate

−3.5 ± 0.3 −2.6 ± 0.1

cm(titrand)/mol L−1 = 5.0 × 10−3, cm(titrant)/mol L−1 = 5.0 × 10−2 = c(MOPS)/mol L−1. SE = standard error of the mean (N = 3).

a

Evidently, the formation of flocculate with considerably lower surface charge compared to that of primary complexes is not particularly enthalpically favored. The measured values are close to zero in both cases. Such finding is in accord with previously reported results regarding PSS−PAH interpolyelectrolyte neutralization.18,23 Interestingly, the PA−PAH interpolyelectrolyte neutralization was more favorable in the low concentration regime (Table 1). This could be due to less pronounced electrostatic repulsions in more dilute polyelectrolyte solutions. The enthalpy values measured at lower reactant concentrations are also little less reliable, which can also account for the observed differences. The isoenthalpic formation of primary complexes could be a consequence of the more pronounced counterion condensation to the exposed corona chains in comparison to those present in solution. Interestingly, the measurements of counterion activity during PAHCl-NaPSS interpolyelectrolyte neutralization do not support this rationale.23 As already mentioned, the amount of counterions released after the flocculation onset was somewhat lower than during primary complex formation. However, it should be noted that the observed behavior was, at least in part, caused by the less neat monomer pairing at higher than at lower monomer ratios. The two opposing effects could therefore cancel each other out. On the other hand, the strong overcharging could by large be the consequence of dangling polymer chains protruding out of the primary complex surface, which, from the point of counterion condensation, behave similarly as chains in solution. After all, the colloid stability of primary complexes indicates that the probability of finding several titrand chains in the vicinity of a certain titrant monomer sequence must be quite low, at least at moderate E

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Figure 4. Size (a) and electrophoretic mobility (b) of primary and secondary PAH−PA complexes obtained at pH = 7.0 and 25 °C. The primary complexes were prepared by stepwise titrant to titrand addition. The secondary complexes were obtained by the abrupt addition of titrant in excess at 0.6 monomer ratio (cm(titrant) = c(MOPS) = 5.0 × 10−2 mol L−1, cm(titrand) = 5.0 × 10−3 mol L−1, and V0(titrand) = 2.0 mL). Error bars indicate the standard error of the mean of five consecutive sample measurements.

flocculation could be observed in this case. The outcome of interpolyelectrolyte neutralization was different than in a titration experiment with a uniform increase in monomer molar ratio. The electrophoretic mobilities of accordingly prepared particles were measured as well (Figure 4b). As expected, the sign of the primary complex charge was equal to that of the titrand. The addition of the titrant in excess resulted in its reversal. Strong primary complex overcharging without flocculation must have occurred. The analogous experiments with two most often investigated polyelectrolyte pairs, namely PSS−PDADMAC and PSS−PAH in the high polymer concentration regime (cm(titrand) ≥ 1.0 × 10−3 mol L−1 for both pairs) were performed (Figures S1 and S2, Supporting Information). The outcome was the same as in the case of PAH−PA pair. In contrast, the titration with uniform and relatively small increase of titrant amount with each addition (Δr = 0.1) always resulted in flocculation. Strong overcharging of all examined primary complexes could be induced by abrupt addition of titrant monomers in excess for all three investigated pairs. The obtained results suggest that there are no fundamental differences between the processes of interpolyelectrolyte neutralization taking place in solution and at surfaces. The overcharging can be obtained in both cases, provided that one of the charge monomers is always present in large excess. The described results also clearly indicate that rapid establishment of equilibrium in solutions containing the investigated polyelectrolyte pairs can be ruled out. They are the direct proof of metastable complex formation. For the time being, a question remains whether the equilibrium establishment leads to formation of neutral or overcharged products. Our earlier investigations20,23 of PAH− PSS interpolyelectrolyte neutralization suggest that 1:1 monomer ratios favor the formation of neutral precipitates. The metastable overcharged products obtained during titration of PAH with PSS up to equivalence slowly transformed into precipitates containing equal amounts of oppositely charged monomers even in concentrated NaClO4 solutions (2 mol L−1). The increase in entropy due to the counterion release is, from the thermodynamic point of view, extremely favorable. Quantitative monomer pairing can occur even in the case of endothermic complexation. With this respect, the interpolyelectrolyte neutralization seems to be quite similar to dissolution of most simple salts (sparsely soluble salts with high lattice enthalpies, e.g., silver halides, excluded), albeit certainly less entropically favorable. As a consequence, the formation of

oppositely charged polyions. As mentioned in the Introduction, the highly similar composition of the practically insoluble metastable product obtained by titrations ran in both directions has occasionally led to erroneous conclusion that the complexation led to formation of equilibrium products. The fact that strong overcharging of primary complexes in solution occurs as well has often been overlooked. In sharp contrast, the metastability of PEMs was clearly demonstrated by the Schlenoff group in the 1990s.14,15 More recent experiments concerning the polymer mobility inside these nanostructures53 as well as corresponding MD simulations49,50 are in accord with these results. By considering the conditions leading to PEM formation, one can conclude that considerable overcharging is always achieved once the large surplus of oppositely charged monomers with respect to those protruding out or situated at the surface is introduced. Consequently, the ratio of oppositely charged monomers, available for neutralization in each sequential layer deposition, is quite similar to that during the process of primary complex formation in solution. These nanocomplexes are hence as strongly charged, as are the corresponding multilayer surfaces. In contrast, the free titrand to titrant monomer ratio in solution becomes similar near the flocculation point during stepwise titration experiments. Such conditions obviously do not favor strong overcharging, since no changes in measured properties of reaction mixtures can be observed above equivalence, at least in the case of equivalent polyion charge densities. Despite this fact, the flocculate can still bear some excess of titrant even at equivalence due to the less neat monomer pairing after the flocculation onset. A certain portion of primary complex corona monomers simply becomes buried within the precipitate. By considering the conditions leading to PEM deposition at surfaces and formation of charged nanocomplexes in solution, it follows that strong primary complex overcharging could be, at least in principle, achieved by the abrupt introduction of the titrant in large excess. The addition must be made at molar ratios close to those at which flocculation occurred in “classic” titration experiments. In order to test this hypothesis, positive and negative primary complexes PAH−PA were prepared by stepwise titrant to titrand addition up to r = 0.6 under vigorous stirring. (The flocculation onset occurred at r ≈ 0.7 in the high polymer concentration regime (cm(titrant) = 10 × cm(titrand) = 5 × 10−2 mol L−1.) This was followed by the addition of the equivalent amount of titrant. As can be seen in Figure 4a, no F

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Figure 5. (a) Mean effective hydrodynamic diameters of colloid complexes prepared by titrations of PA (cm/mol L−1 = 1.0 × 10−3, V0 = 2.0 mL) with PAH (cm/mol L−1 = 1.0 × 10−2) in 0.1 mol L salt solutions at pH = 7.0 (c(MOPS)/mol L−1 = 1.0 × 10−2) and 25 °C. (b) Mean effective hydrodynamic diameters of colloid complexes obtained by titrations in the opposite direction with equal titrant and titrand concentrations. Black circles correspond to monomer ratios above which flocculation occurred. Error bars indicate the standard error of the mean of two experiments. The results of corresponding salt free experiments are given for comparison (dashed line).

salt-free interpolyelectrolyte neutralization should be clearly distinguished from that taking place in highly concentrated electrolyte solutions. A more pronounced overcharging,20,23 as well as notable extrinsic compensation of charge,20,29−31 in metastable and, most likely, in certain equilibrium products can be expected with the increase in salt content. Furthermore, as demonstrated by Michaels3 and later by the Schlenoff group,55,56 certain PEMs and 1:1 PECs (for instance, PSS− PDADMAC) behave like ion exchangers, swelling and selectively binding certain simple ions at high electrolyte concentrations. 3.2. Complexation of Oppositely Charged Polyelectrolytes in Binary Electrolyte Solutions (Low Polymer Concentration regime). The PA−PAH interpolyelectrolyte neutralization in the low polymer concentration regime was also studied in NaCl, NaClO4, and (CH3)4NCl aqueous solutions. The sodium salts were chosen because of the remarkable anion specific effects observed in the case of PAH− PSS complexation in concentrated NaClO4(aq) and NaCl(aq).18,23 Tetramethylammonium chloride was used as supporting electrolyte due to low screening abilities of the bulky organic cation in comparison with sodium.57 In order to ensure the exchange of PA and PAH counterions which counterbalanced the macromolecular charge in corresponding MOPS buffer solutions with the ions of introduced electrolyte, a considerable excess of simple binary salt with respect to buffer was added (10:1 and 50:1) to reactant solutions; i.e., in situ counterion substitution was performed. The electrolyte concentration was also kept high since the influence of counterion type on the course of interpolyelectrolyte neutralization can be observed solely in concentrated salt solutions.18,29−31 As an example of obtained results, the effective hydrodynamic diameters the complexes prepared by titrations in both directions (cm(titrand)/mol L−1 = 1.0 × 10−3 and cm(titrand)/ mol L−1 = 1.0 × 10−2) in 0.1 mol L−1 salt solutions are shown in Figure 5. One can observe that phase separation could be occasionally noticed at molar ratios close to equivalence. The reactant concentrations were obviously quite close to their critical values at which flocculation always takes place during titration experiments. The presence of simple electrolyte at this ionic strength did not affect the interpolyelectrolyte neutralization significantly. The flocculation at r ≈ 0.8 occurred in some cases,

neutral product simply corresponds to maximum increase in translational entropy at 1:1 monomer composition. However, the predominantly entropy-driven complexation may not lead to formation of neutral equilibrium product if the composition of the reaction mixture deviates from 1:1 monomer ratio. Oskolov and Potemkin54 have shown that the overcharged product formation could be favored in that case since some additional counterions can be released from the polymer strands close to the neutral complex core. (The overcharged colloid PECs displayed a larger entropic stability in comparison to neutral precipitates.) Of course, such a scenario assumes that the overcharging is mainly due to long strands of only a few dangling chains. Quite strong monomer−monomer repulsions would otherwise occur, resulting in even more pronounced counterion condensation to polyions at the surface in comparison to those occurring in one-component polyelectrolyte solutions. As mentioned before, the overcharging of equilibrium products can “naturally” occur at asymmetric monomer ratios due to polymer structure of reactants. Namely, if the protruding polymer chains are more further apart (on the precipitate surface) or if the primary complex overcharging is due to only a few uncompensated long strand of titrand monomers, these excess sequences are from the point of view of counterion condensation, essentially the same as free polyions in solution. Consequently, no extra “overcharging forces” are needed to induce product overcharging provided the polymerization degree is high enough. Both the experimental and theoretical investigation of multilayers are in accord with this rationale. The multilayer formation49,50 and their thickness17 were found to be crucially dependent on the polyion length. The dependence of equilibrium product composition on reactant monomer ratio could perhaps be investigated spectrophotometrically by monitoring the UV spectra of supernatants once the equilibrium is reached. The equilibrium establishment can be in some cases (for instance, PAH−PSS or PSS−PDADMAC interpolyelectrolyte neutralization taking place in binary salt solutions) checked by determining the supernatant composition above precipitates prepared by stepwise titrant to titrand additions performed in both directions. However, it should be noted that these experiments are limited to UV active polyions and rather concentrated salt solutions. The equilibrium is reached extremely slowly (if ever) in the case of salt-free solutions.20,23,32−34 That being said, the G

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studies of PAH−PSS interpolyelectrolyte neutralization18 and can be rationalized in terms of the anion charge density and related hydration.18,22,58 Interestingly, the more pronounced condensation of this weakly hydrated anion to PAH in comparison with all halide anions was noticed.18 The critical electrolyte concentration required for the onset of primary positive complex aggregation PAH−PSS complexes was the lowest in the case of perchlorate salt. This could be due to less pronounced hydration of bound perchlorate anions or to their more pronounced condensation. A more detailed discussion on the subject will be given in the following section. Somewhat more favorable PAH−PA complexation enthalpies in (CH3)NCl(aq) than in NaCl(aq) can again be attributed to differences in cation hydration. The binding of more strongly hydrated sodium over tetramethylammonium cation is slightly more enthalpically favorable. (The complexation enthalpies are positive in NaCl and negative in (CH3)4NCl solutions.) The results of DLS measurements performed at higher polymer concentrations (section 3.3) indicate the more efficient screening of PA charge with sodium in comparison with bulky tetramethylammonium cation. The poly(acrylate) seems to prefer the binding of small ions of high charge density (section 3.3). Despite described variations in measured complexation enthalpies with the counterion type, the complexation was more or less isoenthalpic, that is, predominantly, and in some cases entirely (endothermic complexation), driven by the entropically favorable expulsion of counterions and their hydration water into the bulk.3,5,18,22,23 The only slight exceptions are the somewhat more enthalpically demanding processes of primary and secondary complex formation in NaClO4(aq). The electrolyte concentration influenced the binding energetics poorly. The enthalpies associated with removal of the ionic atmospheres (including both the directly and indirectly bound counterions and hydration water molecules) at higher and lower ionic strengths must therefore be similar, despite notable contraction of the double layer at higher salt concentrations. An analogous trend was reported in the case of several other investigated polyelectrolyte pairs.18,22,23 However, one must be aware that a substantial portion of monomers in obtained complexes and precipitates can remain unpaired in concentrated electrolyte solutions. This kind of behavior was also noticed in the case of PAH−PA pair, which will be discussed in the following section. 3.3. Complexation of Oppositely Charged Polyelectrolytes in Binary Electrolyte Solutions (High Polymer Concentration Regime). In order to investigate the influence of the counterion type on the course of PAH−PA interpolyelectrolyte neutralization in more detail, DLS and calorimetric experiments analogous to those described in previous section were performed at higher polymer concentrations. The effective hydrodynamic diameters of complexes prepared by stepwise titrant to titrand addition in NaClO4 and NaCl aqueous solutions are shown in Figure 6. A sudden increase in size of positively charged complexes at r ≈ 0.6 (NaClO4) and at r ≈ 0.8 (NaCl) can be observed in 0.1 mol L−1 salt solutions. The formation of large particles occurred at even lower monomer ratios in more concentrated salt solutions, particularly in NaClO4 (Figure 6b). This kind of behavior suggests the anion-specific aggregation of positive complexes.5,18,27,48,59 The influence of countercation type on the PA−PAH interpolyelectrolyte neutralization in the high polymer

indicating relatively high monomer pairing degree. In contrast, the DLS measurements performed in 1.0 mol L−1 salt solutions (Figure S4) bear evidence of direction-dependent complex size. As will be evident from investigations performed at higher polymer concentrations (section 3.3), the described results can be explained by electrolyte induced strong overcharging of primary complexes which in the end leads to direction dependent product composition and extrinsic charge compensation (asymmetric interpolyelectrolyte neutralization).18,23,29−31 The results of calorimetric titrations, obtained at lower polymer concentrations, support this claim (Figures S5−S8). As one can see, measurable enthalpy changes could be observed up to higher monomolar ratios when PAH was added to PA than in the case of titration carried out in the opposite direction, especially in concentrated electrolyte solutions. The described results are obviously a clear evidence of nonequilibrium interpolyelectrolyte neutralization.18,29 The calorimetric experiments always resulted in formation of secondary polyelectrolyte complexes. The collected data were hence processed according to procedure described in the previous chapter. By inspecting the deduced complexation enthalpies (Table 3), it can be concluded that the exchange of Table 3. Enthalpies of Primary and Secondary Complex Formation Deduced by Processing the Calorimetric Data Obtained during Titration of PA with PAH and Vice Versa in Binary Electrolyte Solution at pH = 7.0 and θ = 25.0 ± 0.1 °Ca PAH → PA (Primary Complexes)

c(electrolyte) 0.1 0.5

c(electrolyte) 0.1 0.5

c(electrolyte) 0.1 0.5

ΔrH ± SE (kJ mol−1) NaCl

ΔrH ± SE (kJ mol−1) (CH3)4NCl

1.1 ± 0.1 −2.2 ± 0.1 3.3 ± 0.2 −2.9 ± 0.2 PAH → PA (Secondary Complexes) ΔrH ± SE (kJ mol−1) NaCl

ΔrH ± SE (kJ mol−1 ) (CH3)4NCl

0.7 ± 0.1 −1.8 ± 0.2 2.3 ± 0.1 −0.6 ± 0.1 PAA → PAH (Primary Complexes) ΔrH ± SE (kJ mol−1) NaCl

ΔrH ± SE (kJ mol−1) (CH3)4NCl

1.9 ± 0.1 −2.8 ± 0.1 2.0 ± 0.1 −2.1 ± 0.1 PA → PAH (Secondary Complexes)

ΔrH ± SE (kJ mol−1) NaClO4 5.1 ± 0.2 7.1 ± 0.3 ΔrH ± SE (kJ mol−1) NaClO4 4.9 ± 0.3 6.0 ± 0.5 ΔrH ± SE (kJ mol−1) NaClO4 7.5 ± 0.3 7.4 ± 0.1

c(electrolyte)

ΔrH ± SE (kJ mol−1) NaCl

ΔrH ± SE (kJ mol−1) (CH3)4NCl

ΔrH ± SE (kJ mol−1) NaClO4

0.1 0.5

0.9 ± 0.1 1.2 ± 0.2

−2.2 ± 0.1 −1.2 ± 0.1

3.1 ± 0.3 1.8 ± 0.2

cm(titrand)/mol L−1 = 1.0 × 10−3, cm(titrant)/mol L−1 = 1.3 × 10−2 = c(MOPS)/mol L−1). SE = standard error of the mean (N = 3).

a

buffer anion with that of the added electrolyte affected the neutralization energetics considerably. The complexation was in all cases notably less favorable than in the pure buffer solution (Table 1). The neutralization was even endothermic in sodium chloride and perchlorate solutions (i.e., entirely driven by the favorable entropy changes as a result of the counterion and hydration water release). The polycation enthalpical preference for perchlorates over chlorides is in accord with our previous H

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Figure 6. Mean effective hydrodynamic diameters of colloid complexes obtained by titrations of PAH (cm = 5 × 10−3 mol L−1, V = 2 mL) with PA (cm = 5 × 10−2 mol L−1) in (a) 0.1 mol L−1 and (b) 0.5 mol L−1 sodium salt solutions; θ = 25 °C and pH = 7.0 (c(MOPS) = 5 × 10−2 mol L−1). Error bars indicate the standard error of the mean of two experiments. The results of corresponding salt-free experiments are given for comparison (dashed line).

Figure 7. Mean effective hydrodynamic diameters of colloid complexes obtained by titrations of PA (cm = 5 × 10−3 mol L−1, V = 2 mL) with PAH (cm = 5 × 10−2 mol L−1) in (a) 0.1 mol L−1 and (b) 0.5 mol L−1 chloride salt solutions; θ = 25 °C and pH = 7.0 (c(MOPS) = 5 × 10−2 mol L−1). Error bars indicate the standard error of the mean of two experiments. The results of corresponding salt free experiments are given for comparison (dashed line).

Figure 8. (a) Mean effective hydrodynamic diameters of particles obtained during titrations of negative complexes (n(PAH)/n(PA) = 0.5), prepared by addition of PAH·Cl (cm = 5 × 10−2 mol L−1) to solutions containing PA (cm = 5 × 10−3 mol L−1, V0 = 2 mL) with various chloride salts (c = 5 mol L−1). (b) Mean effective hydrodynamic diameters of particles obtained during titrations of positive complexes (n(PA)/n(PAH) = 0.5), prepared by addition of PA (cm = 5 × 10−2 mol L−1) to solutions containing PAH·Cl (cm = 5 × 10−3 mol L−1, V0 = 2 mL) with various sodium salts (c = 5 mol L−1); θ = 25 °C pH = 7.0 (c(MOPS) = 5 × 10−2 mol L−1). Error bars indicate the standard error of the mean of two experiments.

In order to investigate whether ion specific aggregation of primary PAH−PA complexes occurred, the corresponding negative and positive primary complex suspensions were titrated with simple electrolyte solutions and the particle size was monitored.18,23 The negatively charged complexes were prepared by titrating the PAHCl with PAA in MOPS buffer solution up to 0.5 monomolar ratio. The increase in titrant to titrand monomer ratio during each stepwise addition was maintained constant and equal to Δr = 0.1. To the aliquots of

concentration regime was again examined in NaCl(aq) and (CH3)4NCl(aq) (Figure 7). The results in 0.1 mol L−1 solutions of both electrolytes were relatively similar. The flocculation at relatively high titrant to titrand ratio indicates that monomer pairing proceeds as efficient as in MOPS buffer solutions. However, the observed increase in particle size at lower monomer ratios in 0.5 mol L−1 NaCl suggests the electrolyte-induced negative complex aggregation. I

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Figure 9. (a) Thermograms obtained during calorimetric titration of PA with PAH and vice versa in 0.1 mol L−1 NaClO4(aq), θ = 25.0 ± 0.1 °C, and pH = 7.0 (cm(titrant) = cm(MOPS) = 5.0 × 10−2 mol L−1, cm(titrand) = 5.0 × 10−3 mol L−1, V(titrand) = 1.3 mL). (b) Corresponding mean enthalpy changes as a function of titrant to titrand molar ratio (r). Error bars indicate the standard error of the mean of three titration experiments. The results of salt-free experiments are given for comparison (dashed line).

PAH monomers at their surface. The anion-specific aggregation of PAH containing positive complexes can therefore be expected irrespectively of the polyanion type. (The effect should be transferable.) On the other hand, in contrast with the herein reported results, solely the aggregation of positively charged PSS−PAH complexes was observed.18,23 This could be a consequence of much larger distance of charged groups from the PSS backbone or/and more favorable sulfonate hydration, eventually leading the less pronounced counterion condensation to this polyanion. It would be interesting to compare the osmotic coefficients of PAH, PSS, and PA salts. The corresponding data for alkali poly(styrenesulfonates) can be found in the literature,64 whereas, to the best of our knowledge, no such investigations of corresponding PA and simple PAH salts have been reported. A rather promising method for elucidating the possible average arrangement of counterions around strongly charged polyions is the all atom explicit water molecular dynamics (MD) simulation.65,66 Such studies could be particularly informative taking into account the recent results of Fu and Schlenoff,22 which clearly suggest the differences in the extent of counterion hydration and water structure in the vicinity of various PDADMAC salts. Namely, this fact, the herein and previously reported aggregation experiments,20,23 and the investigations of electrolyte type dependent PEM thickness67−69 indicate that considerable differences in strength of counterion−polyion interactions, or even in the extent of counterion binding, can be expected as well, at least at higher electrolyte concentrations. The often observed correlations of ion-specific effects with counterion size, charge density, and standard hydration parameters (ΔhydX; X = G, H, S) are hence reasonable. A more pronounced direct (contact) binding of larger counterions is favored from the point of view of water−counterion interactions.58 On the other hand, the realized electrostatic interactions are weaker when compared to those between contact ion−monomer pairs formed with smaller counterions. In addition, smaller counterions (larger charge density) screen the unfavorable monomer interactions more efficiently at larger distances from the polyion chain (solvent-shared and solvent-separated monomer− counterion pairs) when compared to larger counterions. Of course, when discussing the counterion−polyion preferences, the entropic effects of counterion condensation must be considered as well. With this respect the desolvation of smaller counterions is entropically favored.58 The situation is further complicated by the possible chelate counterion binding,62

accordingly prepared suspension, concentrated alkali-metal and tetramethylammonium chloride solutions (c/mol L−1 = 5) were added in 5 min intervals, and the particle size was monitored (Figure 8a). The anion-specific aggregation of positive complexes was explored analogously (Figure 8b). As in our previous study of the PAH−PSS pair,18 sodium halides, nitrates, and perchlorates were examined. Several important conclusions can be drawn from the results obtained. First, the electrolyte induced aggregation of both positively and negatively charged complexes can indeed be observed in concentrated salt solutions. Second, the surface charge of PA-coated complexes was higher than of those bearing the excess of PAH monomers. This is evident from the values of critical electrolyte concentrations needed for the onset of positive and negative PAH−PA complex aggregation. As already mentioned, the PAH chains prefer the binding of weakly hydrated oxyanions. In contrast, the binding of strongly hydrated sodium and, to a lesser extent, lithium seems to be favored by PA. Such findings are in agreement with the studies of PA protonation equilibrium in the presence of alkali metal and alkylammonium salts.57 The osmotic coefficient investigations of alkali metal poly(acrylates)60 also suggest the highest affinity of the polyanion toward Na+. Interestingly, quite similar osmotic coefficients of potassium and lithium PA were obtained,60 whereas the herein reported aggregation experiments suggest stronger binding of smaller Li+ cation. The more recent investigations of polyelectrolyte gel collapse in mixed solvents (water containing alcohols, DMSO, acetone, THF)61,62 are also in accord with osmometric results. However, the above experiments were made in mixed solvents and involve cross-linked polymers, which can affect the counterion−polyion preferences considerably. For instance, the higher affinity for Cs+ in comparison to Na+ in pure methanol was reported.63 The observed differences regarding the preference of polyanion for Li+ or K+ in salt-free and salt-added aqueous polymer solutions could be the consequence of counterion concentration. Namely, the extent of counterion condensation could be influenced by its concentration, especially at higher ionic strengths, where more pronounced desolvation of monomer functionalities and counterions occurs. The herein described aggregation results confirm the previously reported anion-specific aggregation of complexes PAH−PSS observed in concentrated solutions of investigated sodium salts.18,36 The agreement is reasonable; positively charged PAH−PSS and PAH−PA complexes bear an excess of J

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Figure 10. (a) Thermograms obtained during calorimetric titration of PA with PAH and vice versa in 0.5 mol L−1 NaClO4(aq), θ = 25.0 ± 0.1 °C and pH = 7.0 (cm(titrant) = cm(MOPS) = 5.0 × 10−2 mol L−1, cm(titrand) = 5.0 × 10−3 mol L−1, V(titrand) = 1.3 mL). (b) Corresponding mean enthalpy changes as a function of titrant to titrand molar ratio (r). Error bars indicate the standard error of the mean of three titration experiments. The results of salt-free experiments are given for comparison (dashed line).

described results and those previously reported for PAH−PSS pair18,23 allows certain conclusions to be made. As mentioned before, large, ion-specific increase in the particle size at lower monomer ration indicates the electrolyteinduced aggregation of like-charged PAH−PA primary complexes. The analogous behavior was noticed in the case of positive PAH−PSS complexes.18,23 Apart from that, their strong overcharging in concentrated NaClO4 solutions (c ≥ 0.5 mol L−1) was observed spectrophotometrically. (The composition of supernatants obtained by titrations carried out up to 1:1 monomer ratio was clearly direction dependent.) Because of this fact, more titrand monomers became involved in nonequilibrium neutralization of the titrant introduced, leaving much of the functional groups neutralized with counterions. The described sequence of events could also be observed calorimetrically. The enthalpy changes, obtained by titrating PAH with PSS in 0.5 mol L−1 NaClO4 and NaNO3 solutions, were noticed up to molar ratios considerably lower than unity. Such findings are in accord with calorimetric investigations of PAH−PA interpolyelectrolyte neutralization (Figure 9 and Figures S11 and S12). To summarize, the PAH−PA interpolyelectrolyte neutralization in concentrated solutions seems to result in both positive and negative primary complex aggregation as well as with their more pronounced overcharging. When combined, these effects lead to asymmetric interpolyelectrolyte neutralization, resulting in the excess of titrand monomers in obtained metastable precipitates. The complexation enthalpies (primary complexes and precipitate formation enthalpies) are enlisted in Table 4 (Supporting Information). The given values are very close to zero, regardless of the final product composition. This is in agreement with the results obtained at lower polymer concentrations (section 3.2).

which is strongly dependent on size and distribution of charged functionalities. As mentioned earlier, distance of charged groups from the backbone can affect the counterion binding as well. Consequently, no universal trend in counterion−polyion preferences can be expected. A combination of all mentioned contributions can lead to higher affinity toward more strongly (PA), weakly (PAH), or moderately charged counterions. The counterion affinities can in some cases be investigated calorimetrically, simply by performing titration experiments in the high polymer concentration regime. More importantly, these experiments often provide valuable information regarding the course of interpolyelectrolyte neutralization in concentrated salt solutions.20,23 With this in mind, calorimetrical titrations of PA with PAH and vice versa in solutions of several chosen electrolytes (NaCl, NaClO4, and (CH3)4NCl) at two salt concentrations (0.1 and 0.5 mol L−1) were performed in the high polymer concentration regime. The calorimetric experiments in 0.1 mol L−1 NaCl and (CH3)4NCl indicate that monomer pairing proceeded up to approximately 1:1 monomer ratio (Figures S9 and S10). Such behavior was also noticed in the low polymer concentration regime. In contrast, the measurable enthalpy changes in NaClO4(aq) (Figure 9) could be observed up to lower monomer ratios in the case of PA to PAH addition. The complexation was accompanied by measurable heat effects below r ≈ 0.5 in both 0.5 mol L−1 NaClO4 (Figure 10) and NaCl (Figure S11), irrespectively of titration direction. The interpolyelectrolyte neutralization was virtually isoenthalpic in 0.5 mol L−1 (CH3)4NCl (Figure S12) in the case of PA into PAH addition. The enthalpy changes collected during titration in the opposite direction could be observed up to r ≈ 0.7. The described calorimetric results strongly suggest that the complexation in concentrated salt solutions proceeded up to monomer ratios which were considerably lower than unity. Namely, the counteriondependent magnitude and sign of enthalpy changes, which were measured at lower monomolar ratios, suggest that certain heat effects could be expected up to equivalence, at least in some cases. Still, isoenthalpic processes (neutralization, aggregation, and flocculation), taking place at higher molar ratios, cannot be ruled out with certainty. Since both polymers absorb electromagnetic radiation in the far-UV, spectrophotometry does not provide useful information on the composition of precipitates obtained by experiments ran in both directions.18,23 However, the similarity between the herein

4. CONCLUSION The results obtained in this study suggest that there are no fundamental differences between the processes of PEC and PEM formation. Because of the extremely slow equilibrium establishment in systems containing strongly charged polyions, the overcharging can be obtained in both cases, provided a large surplus of one of the reactants is available. The monomers in solution are always introduced in considerable excess with respect to oppositely charged surface groups during PEM buildup. The kinetically controlled adsorption hence results in quite a few polyion sequences dangling from the surface. The K

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of a polyion toward particular counterion clearly depends on the functional group type and its hydration. That is on the balance between the strength of counterion−monomer interactions on one hand and their hydration on the other much as in simple ion pairing.70 The chelate counterion binding, depending on the compatibility of counterion size and monomer distribution, can occur as well. As a result, no universal trends in counterion−polyion preferences (i.e., monotonous variation of polyion affinity toward various counterions with their size and related charge density) can be expected. The calorimetric results indicate that the overcharging of PAH−PA primary complexes in not enthalpically demanding. The primary complex formation enthalpies were quite similar to those corresponding to flocculate formation. This is in accord with previous investigation of PAH−PSS complexation energetics.18,23 There are two main reasons which can lead to isoenthalpic charge reversal. The first of them could be the more pronounced condensation of counterions to monomers at the surface in comparison to the free chains in solution. This lowers the unfavorable interactions between like-charged surface monomers. The second is the polymer structure of the reactants themselves, resulting in long monomers strands at the surface of charged PECs which could, with respect to counterion condensation, be quite similar to those of free polyions in solution. The provided explanation can account for the universality of overcharging phenomenon in the case of PEM formation.8 After all, the herein presented results indicate common origin of this phenomenon in solutions and at surfaces. Apart from overcharging, other striking similarities between the processes of PEM and PEC formation can be found in the literature. For instance, the counterion specific effects affecting the composition of multilayers67−69 and complexes18,29−31 can be observed only at higher electrolyte concentrations. In addition, the influence of the counteranion type on the composition of the PAH−PSS neutralization products in solution18 and at surfaces69 was essentially the same. The stabilities of several different PECs in concentrated NaCl solutions could be clearly correlated to those of the corresponding multilayers.19 For the time being, further investigations are still needed to elucidate in which extent the results obtained in solution can be used for prediction of polyelectrolyte multilayers properties and vice versa. With this respect, the results regarding the time scale needed for equilibrium establishment in solution could be of particular importance, since the application of metastable PEMs and PECs presupposes their long-term stability.

adsorption of another layer is therefore enabled since now a surplus of oppositely charged monomers with respect to those available for neutralization is present. In the end, metastable superstructures containing several hundred individual layers can be obtained.8 The situation is remarkably similar in the case when polyelectrolyte complexation is performed by stepwise addition. The initially high molar ratio of titrand to titrant monomers again leads to overcharging. However, once the neutralization of the primary complex corona charge starts to occur (usually at r ≥ 0.7), the ratio of titrand to introduced titrant monomers becomes similar. Because of this fact, the stepwise titrant addition, and the less efficient neutralization of the dangling titrand chains, the reaction proceeds up to approximately monomolar ratios. That is no complexation (and hence notable overcharging) can be noticed by various experimental techniques above equivalence. However, the overcharging can be provoked by the abrupt addition of titrant in large excess to suspensions of oppositely charged primary complexes, prepared by “classic” titration experiments. The nanocomplex charge reversal can also occur during stepwise titrant to titrand addition provided that the polymer concentration is low. Such conditions simply reduce the probability of primary and secondary complex coalescence, which leads to flocculation in the high polymer concentration regime. The abrupt titrant addition to suspensions of primary complexes at monomolar ratios, r ≈ 0.6 or higher, can be used as an unambiguous complexation reversibility test. Namely, if the “classic”, stepwise titration experiment results in precipitation at monomolar ratios close to equivalence, whereas no flocculation following the abrupt titrant addition up to the same titrant to titrand monomer ratio (r > 1) is observed, the complexation can be proclaimed as irreversible. Previously used, indirect proof included the investigations (calorimetric, turbidimetric, spectrophotometric) of the complexation process at higher electrolyte concentrations,18,23,29−31 and evoking the results of kinetic studies,5,23,33,34 in which extremely slow equilibrium establishment, solely in the presence of electrolyte, was noticed. These arguments, and the Fuoss explanation of the sequence of events in titration experiments,2 were than used to rationalize the apparent reversibility in experiments in which no supporting electrolyte was added or at lower ionic strengths. The herein reported investigations of PA−PAH complexation suggest predominantly entropically driven monomer pairing. As in several reported studies,18,22 an interesting correlation of complexation energetics and polyion−counterion preferences with counterion size, charge density, and corresponding hydration enthalpies was observed. The critical electrolyte concentration required for the aggregation of positive PAH−PA complexes was the lowest in the case of NaClO4, confirming the previously observed preference of PAH monomers for condensation of this weakly hydrated anion. In fact, exactly the same trend in polycation− counteranion preferences as in the case of PAH−PSS pair was noticed.18 The agreement between the herein reported results and those obtained in the case of PAH−PSS pair is reasonable once the structure of metastable products of interpolyelectrolyte neutralization is taken into account. (PAH monomers are present at the surface of positive PAH−PSS and PAH−PA primary complexes.) Unlike poly(allylammonium) cation, the poly(acrylate) anion prefers the binding of highly solvated counterions. The affinity

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01892. Figures S1−S12 and Table 4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest. L

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ACKNOWLEDGMENTS This research was supported by the Croatian Science Foundation under the project IP-2014-09-6972.

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DOI: 10.1021/acs.macromol.6b01892 Macromolecules XXXX, XXX, XXX−XXX