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The Hofmeister Anion Effect and the Growth of Polyelectrolyte Multilayers Mikko Saloma¨ki,*,†,§ Piia Tervasma¨ki,† Sami Areva,‡ and Jouko Kankare† Department of Chemistry, University of Turku, FIN-20014 Turku, Finland, Graduate School of Chemical Sensors and Microanalytical Systems and Department of Physical Chemistry, University of Turku, FIN-20500 Turku, Finland Received December 10, 2003. In Final Form: February 18, 2004 The influence of a variety of counteranions on the properties of polyelectrolyte multilayers deposited by layer-by-layer technique is studied by using ellipsometry and AFM. We found out that in thin dry multilayers (20-90 nm) of poly(4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA), the thickness follows reasonably well the position of the counteranion in the Hofmeister series. The polyelectrolyte-counteranion interaction is studied by means of viscosity measurements of semidilute solutions of PDADMA in the presence of different anions. The dynamic viscosities follow the Hofmeister series of anions and correlate with the thickness of multilayers. Two parameters describing the interaction of ions with water, the Jones-Dole viscosity B coefficient and the hydration entropy, are used to explain the anion effect on the developing multilayer thickness. Reasonably smooth and monotonic functional dependence is observed between the layer thickness and these two parameters.
Introduction Sequential layer-by-layer (LbL) assembly of polyelectrolytes has proven to be a very competitive procedure of making thin and well-defined organic films.1 The deposition of polyelectrolyte onto an oppositely charged surface involves at all times a full surface charge reversal leaving the surface ready for the next polyelectrolyte deposition step.2 The amount of flexible polyelectrolyte deposited on to a surface depends to a great extent on the ionic strength of the polyelectrolyte solution.3 Also the electrolyte species is an important4 but a far less studied factor affecting the film properties and the overall constitution. Polyelectrolyte charges are, at least to some degree, compensated by the oppositely charged counterions in the solution. This compensation usually increases the hydrophobicity of the polymer, thus leading into a situation where the polyelectrolyte is deposited in loopy conformation5 and more polyelectrolyte is needed in the surface charge compensation which eventually results in thicker deposited layer.6 The charge screening increases if the ionic strength of the solution is increased, but in addition, the charge screening is also specifically depending on the polyelectrolytecounterion pair. Specific interaction of counterions with macromolecules has already been observed more than 100 years ago, and the phenomenon has been named the Hofmeister effect after its inventor.7,8 * Corresponding author. E-mail:
[email protected]. † Department of Chemistry, University of Turku. ‡ Graduate School of Chemical Sensors and Microanalytical Systems and Department of Physical Chemistry, University of Turku. § Graduate School, A ° bo Akademi University. (1) Decher, G. Science 1997, 277, 1232-1237. (2) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J.; Bo¨hmer, M. R. Langmuir 1996, 12, 3675-3681. (3) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893-8906. (4) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 81538160. (5) van der Schee, H. A.; Lyklema, J. J. Phys. Chem. 1984, 88, 66616667. (6) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160164. (7) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247260.
The phenomenon was originally found in the experiments concerning the precipitation of egg white proteins in the presence of different salts. Presently the Hofmeister effect is known to play an important role in several biological phenomena,9 but for some reason the effect is rarely mentioned in connection with synthetic polyelectrolytes. The Hofmeister effect refers to an ordered sequence of ions, the Hofmeister series, also called the lyotropic series. The order in this series depends on the ability of salts containing these ions to precipitate a given protein. From this original, rather narrow range the use of this series has been extended into numerous other fields, but still mainly within the biochemical domain. Irrespective of common usage, the underlying principles of the Hofmeister effect are not completely understood yet. The Hofmeister effect of anions has been generally noted to be greater than the effect of cations.10 Starting from the anion that has the greatest ability of salting-in some hydrophobic proteins, the Hofmeister series goes as follows: ClO4- > SCN- > I- >NO3- > Br- > Cl- > CH3COO- > HCOO- > F- > OH- > HPO42- > SO42-. The positions of some anions in the series vary depending on the method used to determine their properties.10,11 The anions in the series can be divided into two classes defined by the location of chloride, which can be treated as a median. The anions on the left of chloride in the abovementioned series are chaotropic ions (water structure breaking), which exhibit weaker interactions with water than water itself. The ions on the right of chloride are cosmotropic (water structure making), which exhibit strong interactions with water. A number of attempts have been made to quantify the Hofmeister series, i.e., to find a numerical parameter which could be universally used in the linear correlations. Although no such universal parameter has been found, the cosmotropic and chaotropic properties of the ions can be described by the viscosity B coefficient of the Jones(8) Hofmeister, F. Arch Exp. Pathol. Pharmakol. 1890, 27, 395-413. (9) Cacace, M. G.; Landau, E. M.; Ramsden, J. J. Q. Rev. Biophys. 1997, 30, 241-277. (10) Leontidis, E. Curr. Opin. Colloid Interface Sci. 2002, 7, 81-91. (11) Lo Nostro, P.; Fratoni, L.; Ninham, B. W.; Baglioni, P. Biomacromolecules 2002, 3, 1217-1224.
10.1021/la036328y CCC: $27.50 © 2004 American Chemical Society Published on Web 04/02/2004
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Dole12 empirical expression,13 where the viscosity (η) of the salt solution depends on the ion concentration (c).
η/η0 ) 1 + Ac1/2 + Bc
(1)
where η0 is viscosity of water at the same temperature. The values of A coefficients are rather small compared to the B coefficient values. Therefore, the A term affects the viscosity values in the concentration range c < 0.05 mol dm-3, and above that, the B term is dominant.13 The B coefficient of an ion is constant in a specified solvent at a given temperature. It is an additive quantity meaning that the B coefficient of an electrolyte is a sum of individual ionic B coefficients of the ions present in the solution. There are some important correlations between the B coefficient and ion properties (e.g., entropy of hydration).13 Roughly, for a cosmotropic ion, the B coefficient is positive and for a chaotropic ion negative in water at 25 °C. If the anions are arranged according to their B coefficient or hydration entropy, the resulting series of ions is in most cases fully compatible with the Hofmeister series. The aim of the study is to, for the first time, qualitatively incorporate the Hofmeister anion effects into the properties of the polyelectrolyte multilayers. Experimental Section Materials. Poly(sodium 4-styrenesulfonate) (PSS, 70 kDa, from Aldrich), 2-mercaptoethanesulfonic acid, sodium salt (MESA, from Aldrich), and (3-aminopropyl)triethoxysilane (Fluka) were used as received. Poly(diallyldimethylammonium chloride (PDADMA, 100-200 kDa, from Aldrich) was dialyzed against electrolyte solutions, in a membrane with a nominal Mw cutoff of 3500 (“SnakeSkin” dialysis tubing, Pierce Biotechnology, Inc.), to change the counteranions. The 2% (w/w) water solution of PDADMA (150 mL) was dialyzed twice against 5 L of 0.1 M electrolyte solution containing sodium salt of NO3-, Br-, ClO3-, BrO3-, HCOO-, or F-. Dialysis was carried out in a continuousflow system for 48 h. After that, the polymer was dialyzed against water to remove the excess electrolyte. The chloride form of PDADMA was purified by dialyzing against water. Polymer content was determined by evaporating a known portion of solution and drying to constant weight by first evaporating the excess water in a rotary evaporator and then heating to 110 °C for 24 h, which was enough to reach a plateau in mass decrease. Multilayer Preparation for Ellipsometry and AFM Measurements. The silicon (100) wafers were soaked in hot “piranha” solution (3:1 concentrated H2SO4:30% H2O2. Caution! piranha solution is very corrosive and must be treated with extreme caution; it reacts violently with organic material and must not be stored in tightly closed vessels) for 1 h, rinsed with water and dried. The silicon substrates were then silanized with (3-aminopropyl)triethoxysilane in dry toluene solution (1% v/v) for 4 min at 60 °C. The sequential dipping of the substrate was carried out in the following way. The substrate was soaked in a polyelectrolyte solution (10 mM PSS or PDADMA), with a base electrolyte concentration of 0.1 M, and kept there for 15 min. After that, the substrate was rinsed with pure water and soaked in water for three times for 1 min each. Then the substrate was blown dry with air and subsequently measured on an ellipsometer (Sentech SE 400) operating at 632.8 nm using reflection angles from 40 to 70° with an interval of 5°. The refractive index of the polyelectrolyte multilayer used in calculations was 1.54.14 The dipping cycle was carried out until there were 20 layers (10 bilayers) of polyelectrolytes on the surface of the silicon substrate. The surface roughness and film thickness were determined by the noncontact tapping mode atomic force microscopy (AFM) using a NanoScope III multimode AFM (Digital Instruments, (12) Jones, G.; Dole, M. J. Am. Chem. Soc. 1929, 51, 2950-2964. (13) Jenkins, H. D.; Marcus, Y. Chem. Rev. 1995, 95, 2695-2724. (14) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592598.
Santa Barbara, CA) apparatus. The used ultrasharp silicon cantilever was 125 µm in length with a resonance frequency of approximately 325 kHz. The tip height was 15-20 µm having a nominal radius of curvature less than 10 nm. All the measurements were carried out in ambient air conditions. The relative humidity was between 50 and 55% during the measurements. Quantitative measurements of the local root-mean-square (RMS) surface roughness, which defines the height fluctuations in a given area, were determined from the 10 × 10 µm2 scans using the software provided by the manufacturer. The film thickness was determined by removing a portion of the film by a razor blade and analyzing the cross sections of the scanned image. Polyelectrolyte Preparation for Viscosity Measurements. The PDADMA solutions were prepared using 10 mM (with respect to monomer) dialyzed PDADMA-Cl and adding the 0.1 M electrolyte in that solution. The solutions prepared in this manner do contain a small amount chloride (ca. 10% of the total counteranions), but this is acceptable in semiquantitative measurements. The viscosities of the solutions in 25 °C were measured on an Anton Paar AMVn automated microviscometer. The densities of the polyelectrolyte solutions were determined on an Anton Paar DMA 45 digital density meter.
Results and Discussion Sodium salts of ClO4-, SCN-, I-, NO3-, Br-, ClO3-, Cl-, BrO3-, HCOO-, and F- were selected for a PSS/ PDADMA-multilayer deposition, because they represent a wide-ranging Hofmeister series of monovalent anions. However, the first three in the series were found to be unusable in deposition because the 0.1 M solutions of NaClO4, NaSCN and NaI precipitated PDADMA from its 10 mM (with respect to monomer) solution. The abovementioned three anions are located far in the chaotropic side of the Hofmeister series. To quantify the precipitation phenomenon some suggestions based on anion hydration can be made. The viscosity B coefficient describes the ionwater interaction. The anions having a smaller B coefficient have weaker interactions with water, thus they are more hydrophobic. The anions that produce watersoluble PDADMA, from fluoride to nitrate in the series, represent a wide range of B coefficient values from +0.107 to -0.043 dm3 mol-1. If the values are compared to the values of precipitating anions (Table 1.), it can be noted that there probably exists a limiting value of the B coefficient for the total precipitation of PDADMA under these conditions. The employed B coefficient values are averages collected from the review of Jenkins and Marcus13 and it is noteworthy that the B coefficient value for thiocyanate is somewhat uncertain. The similar correspondence can be found in the hydration entropy values of anions. The hydration entropy can be treated as a measure of the structural effect of the ions in water. The anions that keep PDADMA soluble have entropy values from -150 to -70 J K-1 mol-1 and the precipitating anions have values from -66 to -44 J K-1 mol-1.15 Growth of Multilayers. The ellipsometry is a widely used tool for studying thin organic films with no absorbance at the operating laser light wavelength. The deposition of 20 layers of PSS/PDADMA system was carried out in the presence of different electrolytes and the layers measured step by step by using ellipsometry (Figure 1). The thickness increase per layer is nearly constant after the precursor layers are formed as reported by Ladam et al.16 The thickness order of the 20 layer (15) Marcus, Y. Ion solvation; Wiley: Chichester, England, 1985. (16) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F.; Langmuir 2000, 16, 1249-1255.
The Hofmeister Anion Effect
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Table 1. Anion Properties and the Effect on PSS/PDADMA Multilayers Deposited in 0.1 M Sodium Salt of Corresponding Anions
anion
hydration entropy of the anion (J K-1 mol-1)15
Jones-Dole viscosity B coeff of the anion (dm3 mol-1)13
rms roughness of 10 bilayers (nm)
thickness of 10 bilayers measured with ellipsometry (nm)
thickness of 10 bilayers measured with AFM (nm)
viscosity of the 10 mM PDADMA + 0.1 M counteranion solution (mPa s)
FHCOOBrO3ClClO3NO3BrSCNClO4I-
-150 -124 -95 -87 -80 -77 -70 -66 -59 -47
0.107 0.052 0.009 -0.005 -0.022 -0.043 -0.033 uncertain -0.058 -0.073
1.8 ( 0.2 1.7 ( 1.0 1.3 ( 0.3 2.6 ( 0.9 4.0 ( 1.0 4.6 ( 0.7 5.8 ( 0.1 a a a
23.2 26.9 39.1 43.6 64.7 74.3 87.0 a a a
18 ( 3 23 ( 1 32 ( 2 37 ( 2 48 ( 3 66 ( 2 77 ( 5 a a a
1.2547 1.1897 1.1396 1.1138 1.0780 1.0612 1.0609 a a a
a
Precipitates PDADMA in solution.
Figure 1. PSS/PDADMA film thickness determined with ellipsometry. Multilayer deposited in 0.1 M sodium salt of the corresponding anions.
polyelectrolyte multilayers ranked by the depositing anion, starting from the thickest, is the following: Br- > NO3> ClO3- > Cl- > BrO3- > HCOO- > F-. The positions of BrO3- and ClO3- in the Hofmeister series are uncertain, although they should be located near chloride. As a result, by taking the precipitating anions into account, this series is definitely in line with the classical Hofmeister series. The AFM thickness determination gives slightly smaller thickness values than those measured with ellipsometry (Table 1). Decent calibration of the AFM equipment plays a crucial role in the film thickness determination by the scraping method. The AFM measures the physical height of the layer ignoring the possible material defects inside the film, whereas in ellipsometry the film homogeneity is required for reliable results. The order of thickness measured by AFM is in line with the ellipsometric results, although the thickness values are about 15% smaller. This might raise a question about the refractive index used for fitting of the ellipsometric results. Film Morphology. The root-mean-square roughness of the 20 layer PSS/PDADMA films is shown in Figure 2. It can be seen that roughness remains almost constant in the case of three thinnest multilayers deposited in NaF, HCOONa, and NaBrO3. Their rms roughnesses are between 1 and 2 nm, obviously being indistinguishable from the roughness of the substrate. In thicker multilayers the roughness correlates with the thickness of the multilayer. The rms roughness values are in a range of 5-10% of the multilayer thickness in all of the measured multilayer films. The roughness values are in agreement with the reported roughness values for similar multilayer
Figure 2. AFM measured rms roughness values of 10 bilayer PSS/PDADMA films deposited in 0.1 M sodium salt of the corresponding anion.
systems.17,18 There was no “vermiculate” pattern17 visible even in the thickest multilayers. Therefore, it is assumed that the growth mechanism is identical in the presence of each monovalent electrolytes used in this study. Viscosity B Coefficient and Hydration Entropy. The connection between the parameters of ion-water interaction and the multilayer film thickness seems obvious on the basis of the experimental data. Multilayer thickness vs B coefficient and hydration entropy of counteranion are presented in Figure 3. Multilayer thickness vs B coefficient of the depositing anion shows an exponential decrease whereas thickness vs hydration entropy shows an exponential growth. The graph shows that the hydration entropy of anions follows the same order as the thicknesses of the corresponding multilayers. The B coefficient correlation has a minor defect due to the B coefficient values of nitrate and bromide, which are in contradiction with the experimental thickness order. The hydration entropy of the anion is apparently more reliable in predicting the layer thickness. It is important to point out here that the employed B coefficient values are averages13 collected from a great number of experimental results, sometimes with considerable variation. The use of counteranion hydration entropy in describing the counteranion-polyelectrolyte interaction leads to a question about the themodynamical aspects and the exact (17) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655-6663. (18) Voigt, U.; Jaeger, W.; Findenegg, G. H.; Klitzing, R. v. J. Phys. Chem. B 2003, 107, 5273-5280.
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Figure 4. Ellipsometric thickness of 10 bilayer PSS/PDADMA multilayer vs dynamic viscosity of 10 mM PDADMA + 0.1 M counteranion solution. PDADMA solution without added electrolyte gave a viscosity value of 2.75 mPas. The line is added as a guide for the eye.
Figure 3. (a) Thickness of 10 bilayer PSS/PDADMA multilayer vs B coefficient of the counteranion. Symbols: squares and solid line ) ellipsometry; circles and dashed line ) AFM. The lines are only added as guide for eye. (b) Thickness of 10 bilayer PSS/PDADMA multilayer vs hydration entropy of the counteranion. Symbols: squares and solid line ) ellipsometry; circles and dashed line ) AFM. The lines are added only as a guide for the eye.
mechanism of the deposition process. This aspect, however, will not be discussed here. The hydration entropy and the B coefficient are used here only qualitatively for describing the anion atmosphere in solution and its effect on polyelectrolyte charge screening. The viscosity B coefficient and hydration entropy of the anion are certainly different type of parameters although they have a fairly good linear relationship with each other.19 Charge screening of polyelectrolytes by counteranions has been studied in detail by Ghimici and Dragan20 using viscosity and conductivity measurements. The viscosity of a polyelectrolyte solution decreases in increasing counterion concentration, which is due to polyion shrinking induced by charge screening. The least hydrated counteranion binds strongest to the polymer charges inducing a viscosity drop at lower counteranion concentration than more hydrated anions. Also the conductivity drop of the solution is highest in the case of the least hydrated counteranion, since the bound anion does not contribute to the conductivity. Our viscosity measurements showed a similar trend (Figure 4). The counteranion that deposits the thickest polyelectrolyte layer produces the lowest viscosity of polyelectrolyte solution. It would qualitatively (19) Nightingale, E. R. J. Phys. Chem. 1959, 63, 1381-1387. (20) Ghimici, L.; Dragan, S. Colloid Polym. Sci. 2002, 280, 130-134.
give the impression that the viscosity of the polyelectrolyte solution can be treated as a measure of the binding strength of the counteranion. The viscosity of the polyelectrolyte solution correlates well with the hydration entropy of the counteranion giving also the conclusion that the least hydrated counteranion binds strongest to the polyelectrolyte. In the case of monatomic ions as in the halide series (I-, Br-, and Cl-) the interaction between anion and polyelectrolyte increases with the decrease of the radius of hydrated anion.20 The viscosity B coefficient has also some ion size relationship. In different structural classes (X-, XO3-, and XO4-) the hydrated anion size has a linear correlation with the B coefficient of the anion.19 On the other hand, the crystal radius of ions plotted against the B coefficient forms a U-shaped graph.21 Since our experimental data included also other than monatomic anions, the use of the hydration radius of the anion in the characterization of polyelectrolyte binding is not relevant. More accurate interpretation would involve more precise consideration of anion hydration shell and the factors affecting that. As a conclusion the polyelectrolytecounteranion interaction needs a more dependable parameter for describing the phenomenon. The hydration entropy of the anion would presumably meet this demand. From the early days of Hofmeister’s work, it has been known that the anion effect is larger than the cation effect. The cation effect still presumably exists in connection with the polyelectrolyte multilayers. Dubas and Schlenoff4 have studied the effect of variety of cations on the multilayer thickness with the similar polyelectrolytes. Their thickness series is in accordance with the Hofmeister cation series, and their thickness values correlate also with the B coefficient and especially with the hydration entropy of the corresponding cation. In the present work, the cation was always sodium in order to keep the cation effect constant. The Hofmeister effect in complex biological macromolecules is probably a combination between ion size effects and some specific binding. With the use of simple synthetic polyelectrolytes, the effect could be studied in a more general case, without the interference brought by the specific features of the complex macromolecular structure. (21) Podolsky, R. J. J. Am. Chem. Soc. 1958, 80, 4442-4451.
The Hofmeister Anion Effect
Conclusions We have observed a clear Hofmeister trend in the growth process of polyelectrolyte multilayers. The Jones-Dole viscosity B coefficient and the hydration entropy of the anion used in the deposition process have a strong relation to the thickness of the dry multilayer. Both parameters describe reasonably well the charge screening of the polyelectrolyte by the anion. The chaotropic anions apparently screen strongly the polyelectrolyte charges inducing the deposition of polyelectrolyte in a loopy conformation onto a surface, yielding a thick layer. The cosmotropic anions do not screen charges with the same strength allowing the polyelectrolyte to deposit in a more planar form. The counteranion-induced charge screening of the polyelectrolyte can also be observed from
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the viscosity value of the polyelectrolyte solution. The strongest binding anion, which is also the least hydrated, produces a polymer solution with the lowest viscosity as a result of apparent shrinking of the polyelectrolyte. The thickest polyelectrolyte layer will deposit from a polyelectrolyte solution with the lowest viscosity. The effect of ion-water interaction on the multilayer film thickness is supported by the experimental evidence. The exact mechanism behind the phenomenon is still unknown and it is under investigation in our laboratory. Acknowledgment. Grant #102279 from the Academy of Finland is gratefully acknowledged. LA036328Y
