16060
J. Phys. Chem. C 2008, 112, 16060–16069
Hofmeister Effects in Colloidal Systems: Influence of the Surface Nature Teresa López-León, Manuel J. Santander-Ortega, Juan L. Ortega-Vinuesa, and Delfi Bastos-González* Biocolloid and Fluid Physics Group, Department of Applied Physics, UniVersity of Granada, AVenida FuentenueVa S/N, 18071 Granada, Spain ReceiVed: April 30, 2008; ReVised Manuscript ReceiVed: July 10, 2008
The original Hofmeister series concerned the effectiveness of different salts in precipitating negatively charged proteins. Although the relative position of the species in the series has been demonstrated to be almost universal, reversed Hofmeister sequences have been observed in many systems. Most of them are directly related with the sign of charge (positive or negative) of the surface studied. In this paper, we show experimental evidence of alterations in the Hofmeister order induced not only by changing the sign of the surface charge, but also by varying the hydrophobic/hydrophilic character of the surface. We have worked with a wide variety of systems showing different hydrophilic/hydrophobic character and having a pH-dependent charge sign. Particularly, the effect of these two factors (the surface charge and the hydrophilicity/phobicity) on the ion ranking has been analyzed by studying the colloidal stability of the systems. The typical inversions induced by modifying the charge sign of the particle were observed; in addition and more noteworthy, the direct series observed with hydrophobic systems turned into reversed series as the surface became more hydrophilic for surfaces of identical sign of charge. The correlation found between these inversions and restabilization processes promoted by hydration forces at high salt concentrations evidenced the solvent-structural nature of such inversions. Useful conclusions about the origin of these specific ion effects have been drawn from a comparative analysis among our systems. 1. Introduction Hofmeister effects or lyotropic sequences refer to the relative effectiveness of anions and cations to produce different specificities on a wide range of phenomena (i.e., ion ability to change the CMC value in micellar systems,1 inhibition of coalescence bubbles in aqueous electrolyte solutions,2 influence of the ion type in multilayer formation,3 specific modifications in the ζ-potential of colloidal systems,4 etc.). In Hofmeister’s original works,5,6 ions were ordered according to their ability to stabilize protein solutions, providing the following rankings
Citrate3- > SO42- > PO4H2- > F- > CH3COO- > Cl- > Br- > I- > NO3- > ClO4- > SCNN(CH3)4+ > NH4+ > Cs+ > Rb+ > K+ > Na+ > H+ > Ca2+ > Mg2+ where kosmotropic anions (cations) are on the left (right) and chaotropic anions (cations) are on the right (left). The origin of these series has been traditionally attributed to what Hofmeister called the water-withdrawing power of ions, that is, the capability of ions to interact with water molecules in such a way as to disturb the hydrogen-bonded structure of bulk water. The influence exerted by ions on the neighboring water molecules has been experimentally determined by a number of techniques and methods ranging from entropies of solution to H-H correlation-function measurements.7 On this basis, ions have been classified into two main groups: (i) those promoting water structure, called structure makers or kosmotropes, and (ii) those disrupting such structure, called structure* To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +34 958 240016. Fax: +32 958 243214.
breakers or chaotropes. In this scenario, Hofmeister effects arise from the structural changes that ions provoke in water network, which in turn affects the physicochemical properties of the substances in solution. However, this idea has been called into question by recent experiments showing that ions have no effect on water structure beyond the first sphere of surrounding water molecules.8,9 Certainly, what is widely accepted is that Hofmeister phenomena arise when, in addition to ions and water, a surface is present in the medium. This makes evident the determinant role of interfacial phenomena in ion specificity.10,11 Thus, surface-water, surface-ion, and surface-surface interactions must be considered to achieve a satisfactory explanation of the Hofmeister effects. The great amount of interfacial processes where ion specificity has been observed and its potential importance in biology makes the searching of a general theory capable to explain them one of the current main challenges in Interfaces and Colloid Science.12 The significance of the surface in the ion specific effects is manifested by the fact that the relative ion order in the lyotropic series can be altered by the nature of the surface in solution.13 Investigations performed with a wide variety of systems have revealed the existence of reversed Hofmeister sequences, depending on the charge properties of the surface analyzed. For the sake of clarity, we will use the term “direct series” for the sequences found by Hofmeister, and “inverse series” for those ordered the other way around. For instance, oxide materials with a high isoelectric point (iep), such as alumina or rutile, are typified by inverse series, whereas direct series are characteristic of inorganic materials with a low iep, such as silica.13,14 Similar results are found with proteins below and above their corresponding iep. In particular, the relative effectiveness of different ions in crystallizing proteins gives a reversed Hofmeister sequence for pH < iep, whereas a direct Hofmeister sequence
10.1021/jp803796a CCC: $40.75 2008 American Chemical Society Published on Web 09/20/2008
Hofmeister Effects in Colloidal Systems arises for pH > iep.15,16 Likewise, direct and inverse series recently observed with hydrophobic colloidal particles have proven to be related to the cationic or anionic character of the particle surface.4,17 A quantitative explanation of the inversions induced by the surface charge sign in the Hofmeister ranking has not been provided until recently. This explanation is owed to Ninham and co-workers,18–23 who justify the Hofmeister effects by considering the universal electrodynamic fluctuation forces or dispersion forces in the ion-surface interaction. In this theory, the ion specificity is mainly determined by the polarizability of ions in water. In this manner, the fact that the Hofmeister effects are greater and usually dominated by anions can be explained by the larger polarizability values of anions with respect to cations. On the order hand, the role of the surface is introduced so that the accumulation or exclusion of ions at the interface depends on the difference between the refractive index of the surface (ns) and water (nw). When ns > nw, the accumulation of the more polarizable anions at the interface is expected, whereas if ns < nw, these same anions will tend to be excluded. Nevertheless, this theory has an important drawback, since it considers water as continuum, a risky approach for explaining Hofmeister effects, because in many cases they appear to be water-structure mediated.14 In the last times, the research area that has developed a bigger expansion explaining Hofmeister effects is the air-water interface. Experiments, computational simulations, and theory have been combined to give insight into the molecular mechanisms underlying ion specificity in this simple interface. As a result, a new picture of the air-electrolyte interface has emerged nowadays.10,24–28 In this, the air-water interface is enriched by ions with high polarizability values following a nonmonotonic ionic profile. This behavior has been experimentally observed with halides anions10,27 and also with the thiocyanate anion.29 These last findings reinforce the dispersion forces theory, since the crucial role of the polarizability of ions in the observed ion specificity is verified. However, the commented Ninham’s theory would fail to predict the accumulation of more polarizable anions at air-water interface since nair is lower than nw.30–32 In the current work, we analyze solid/liquid interfaces, providing experimental results that cannot be explained with the Ninham’s theory and that highlight the key role of the surface water interactions in the Hofmeister effects. In particular, we observe direct and reverse series that cannot be attributed only to the charge sign of the surface. Specifically, it is reported how systematic alterations in the lyotropic series arise when modifying the hydrophobic/hydrophilic character of them while keeping their surface charge sign constant. To reach this purpose, we have studied the colloidal stability of a large variety of colloidal systems. Twelve different surfaces (six positive and six negative ones) and six different ions belonging to Hofmeister series (Cl-, NO3- and SCN- for anions, and Ca2+, Na+ and NH4+ for cations) were analyzed. Systems as diverse as polystyrene particles, protein-covered polystyrene particles, silica particles, and chitosan particles were considered in this comparative analysis. In each case, the Hofmeister effects were quantified by means of measuring the critical coagulation concentrations (CCC) associated with the different electrolytes. For clarity, in the paper we simply indicate these CCC values, while the whole stability curves and the electrokinetic characterization for the different systems are provided in references 4 and 33 (for previous works) or in the Supporting Information attached to this paper (for recent results). The discussion derived from these stability results has been focused on three aspects:
J. Phys. Chem. C, Vol. 112, No. 41, 2008 16061 (i) Hydrophobic systems. Comparison between positive and negative surfaces confirmed the existence of Hofmeister inversions induced by changing the surface charge sign. In these cases, it is shown that dispersion forces can account for these inversions. (ii) Systems with the same charge sign. Comparisons of the diverse anionic (or cationic) systems revealed the existence of partial inversions induced by the hydrophilic/phobic character of the surface. These results cannot be explained by dispersion forces theory. (iii) Correlations observed between partial inversions and restabilization processes originated by hydration forces at high salt concentrations support the existence of water-structuremediated mechanisms in the Hofmeister effects. 2. Experimental Methods 2.1. Colloidal Particles. The systems considered for this study are presented below; information on the synthesis procedures and the physicochemical characterization of the particles can be found in the denoted references. -Polystyrene latex particles. Four pools were synthesized with different surface groups. They were named as follows: AMJ5 (carboxyl groups),4 ABJ2 (sulfonate groups),4 AMJ10 (amine groups),4 and AJ25 (carboxyl + amine groups) (see Supporting Information). -Protein-coated polystyrene particles. Immuno-γ-globuline-G (IgG) was adsorbed onto polystyrene particles to change the hydrophobic character of these surfaces into partially hydrophilic. The amine and the sulfonate polystyrene latexes were used as a substrate to give the IgG-AMJ1033 and the IgG-ABJ2 complexes, respectively (see Supporting Information). -Silica particles under the commercial name of Nyacol were used as a negative hydrophilic sample (see Supporting Information). -Chitosan particles. Two different kinds of chitosan particles were used as positive hydrophilic systems: (i) chitosan nanogels, in which the chitosan chains were cross-linked by tripolyphosphate (TPP) polyanions,34 and (ii) nanocapsules, with a triglyceride (TG) core stabilized by a chitosan shell, which ultimately determines the surface properties of the particles (see Supporting Information). This last system will be referred to as TG-chitosan. Each of these surfaces has a generally dissimilar hydrophilic/ phobic character and a pH-dependent charge sign. To analyze concomitantly the effects of surface charge and hydrophilia, we studied colloidal stability at pH 4 and pH 10. Nonbuffered solutions were used throughout to avoid interferences between the Hofmeister ions and other electrolytes. Five salts were used in this study: (i) NaCl, NaNO3, NaSCN to compare differences between anions, and (ii) NaNO3, NH4NO3, Ca(NO3)2 to compare cations. 2.2. Colloidal Stability. The aggregation studies corresponding to the polystyrene, IgG-polystyrene, silica, and TG-chitosan particles were performed using a low-angle light-scattering instrument (a nephelometer). This instrument enabled us to monitor the temporal evolution of the light scattered at a fixed angle by an aggregating sample. The light scattered by the sample was collected at 10° for 120 s. The scattering cell was rectangular with a 2 mm path length. Equal volumes (1 mL) of salt and particle solutions were mixed and introduced into the cell by an automatic mixing device. The dispersions used for such coagulation experiments were diluted to minimize as much as possible the multiple scattering effects.
16062 J. Phys. Chem. C, Vol. 112, No. 41, 2008
López-León et al. 10°) curves registered by the nephelometer. Figure 1a shows a typical experiment in which the I(t, 10°) curves corresponding to the aggregation process of a polystyrene latex have been obtained at different salt concentrations. The initial slope, and thus the kinetics of the aggregation process, increases progressively with the ionic strength up to a critical salt concentration called CCC, which indicates the onset of the diffusive or rapid aggregation regime. At the CCC, the rate constant k reaches its maximum value (k ) kr). The stability ratio (W), also called Fuchs factor, is broadly used to study the stability of colloidal systems. It is defined by
W)
kr ks
(2)
in which the rate constant kr corresponds to the rapidest coagulation kinetics, and ks is the rate constant for a slow coagulation regime. The stability ratios derived from the curves in Figure 1a are plotted in Figure 1b versus the salt concentration. The log-log representation makes easy locate the critical coagulation concentration as that point where log W reduces to zero. When restabilization processes occurs at high salt concentrations, the critical stabilization concentration or CSC (defined as the minimum electrolyte concentration at which restabilization starts) can be similarly calculated.37 Finally, the destabilization of chitosan-TPP nanogels was studied by static light scattering using a commercial light scattering setup, 4700C (Malvern Instruments), working with an argon laser of wavelength equal to λ0 ) 488 nm. In this sample, the addition of salt did not provoke aggregation but disintegration of the nanogel particles, and thus, referring strictly to CCC values makes no sense for this system. (See the Supporting Information for more details). Figure 1. (a) Aggregation kinetics of a negative polystyrene latex for increasing Ca(NO3)2 concentrations: (0) 15 mM; (O) 20 mM; (4) 25 mM; (3) 30 mM; (f) 37 mM; (triangle pointing left) 50 mM; (triangle pointing right) 75 mM; and (]) 100 mM. (b) Stability ratio (W) versus Ca(NO3)2 concentration. The W values have been obtained from the data shown in (a) as explained in the main text.
For narrow angles and for the first steps of the aggregation process the light scattered by the sample increased linearly with time 35,36
I(t, θ) ) 1 + 2kn0t I(0, θ)
(1)
where I(t, θ) is the light scattered by the sample at the time t at the angle θ, k is the rate constant of the aggregation process, and n0 is the initial particle concentration. In this way, the k constant can be easily determined from the slopes of the I(t, TABLE 1: Representative Parameters for the Different Ionsa ions
R (Å)
R (Å3)
H ((1)
B
hν (1012 erg)
SCNNO3ClNH4+ Na+ Ca2+
2.13 2.06 1.82 1.48 0.97 0.99
6.74 4.30 3.59 1.86 0.15 0.47
0 0 1 2 4-5 6-12
-0.103 -0.046 -0.007 -0.007 0.086 0.285
8.7b 9.0 9.2 50.3b 54.7 41.0
a
Radius (R),38,39 polarizability (R),39,40 hydration number (H),41,42 Jones-Dole coefficient (B),43 and ionization energy in water (hν)39,40 for some ions. b Assumption deduced from similar molecules.39
3. Results and Discussion Since this work aims to analyze the influence of the surface nature in the Hofmeister effects, we have maximized the number of surfaces studied and concentrated our analysis on a few but representative ions rather than considering the complete Hofmeister series. Three different anions, SCN-, NO3-, and Cl-, and three different cations, NH4+, Na+, and Ca2+, were chosen to cover the gamut from chaotropic to kosmotropic character. Typical parameters characterizing this ability are ion size, hydration number, ion polarizability, or viscosity Jones-Dole coefficient B. The values of these parameters for the above ions are listed in Table 1. In general, large singly charged ions with low charge density, such as SCN-, NO3- or NH4+, are chaotropes. Conversely, small or multiply charged ions with high charge density, such as Ca2+, are kosmotropes. Na+ and Cl- ions are frequently considered as indifferent ions or as reference points in the Hofmeister series. Such parameters as hydration numbers or Jone-Dole coefficients give a specific idea of the chaotropic/kosmotropic degree of the mentioned ions. According to the values shown in Table 1, the ions used in this study can be ordered from the most chaotropic to the most kosmotropic as follows:
Anions: SCN- > NO3- > ClChaotropic f Kosmotropic Cations: NH4+ > Na+ > Ca2+ Colloidal stability is especially useful to test the Hofmeister effects since it is very sensitive to changes in the particlesurface potential, and it can also be affected by structural
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Figure 2. Stability ratio (W) versus Ca(NO3)2 concentration for different negative colloidal systems: (9) AMJ5 latex at pH 4; (O) silica particles at pH 4; (3) IgG-AMJ10 complex at pH 10; and (4) IgG-ABJ2 complex at pH 10. Solid lines serve to guide the eye in order to determine CCC values. Dotted lines determine CSC values.
modifications in the solvent. In a typical aggregation process, an increase in the ionic strength of the medium entails the system destabilization. This is basically due to a reduction in the interparticle electrostatic potential responsible for the stability of the system, as explained by the well-known Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.44 However, when the system has a certain hydrophilic character, it is possible to achieve a stable state with further salt addition, as shown in Figure 2. These restabilization processes at high salt concentrations have their origin in the so-called “hydration forces”.41,45,46 They are repulsive structural forces arising from: (i) water molecules strongly bounded to surfaces containing hydrophilic groups, and (ii) hydrated ions located at the proximities of the surface. The strength of these forces depends on the energy needed to disrupt the hydrogenbonding network close to the hydrophilic surface surrounded by hydrated ions when two of these surfaces approach each other.47 Empirically, the hydration repulsion between two hydrophilic surfaces (U) decays exponentially with the separation distance between surfaces (D):41,48,49
U ) U0e-D⁄λ0
(3)
where λ0 depends on the electrolyte in solution and U0 depends on the hydration of the surface. Therefore, if a given electrolyte is used (that is, λ0 will be constant), the repulsive hydration forces undergone by two approaching particles can be used to quantify their hydrophilic character, since the hydration forces will be more intense when the hydrophilicity of the particle surface increases. Highly hydrated cations, such as calcium, are typically used to visualize restabilization processes in colloidal systems; they have a large λ0 value, and thus, enhance hydration repulsion between particles. Furthermore, the analysis of these hydration forces using a given colloidal system but different electrolytes provides information on the specific influence of ions in the water structure. Therefore, these water-ion specificities, which are manifested in experiments where hydration forces play a role, can be correlated with Hofmeister effects. Since it is a method suitable for all the systems used in this study, the evaluation of the strength of hydration forces by means of CSC data was employed to measure the hydrophilic
character of our colloidal surfaces. Positive and negative surfaces were analyzed separately. Figure 2 shows the stability curves for four anionic surfaces in the presence of Ca(NO3)2. In most cases, there is a CCC beyond which the system is completely destabilized, and a CSC whereupon the system begins restabilizing by means of hydration forces. Similar experiments to those shown in Figure 2 were performed with the rest of salts and both positive and negative surfaces. Results concerning CCC and CSC values are displayed in Table 2. The intensity of these hydration forces in every case can be quantified through the corresponding CSC values (see Table 2): the lower the CSC, the higher the hydrophilicity. Of course, the most hydrophobic systems, as those formed by polystyrene particles (i.e., the latex AMJ5), do not show any restabilization. Therefore, according to the CSC data obtained with Ca(NO3)2, the anionic surfaces can be ordered from the most hydrophobic to the most hydrophilic as follows (note that the same order would result using any other electrolyte)
AMJ5 (pH10) < Silica (pH4)< IgG-AMJ10 (pH10) < IgG-ABJ2 (pH10) As mentioned, no restabilization processes were observed with the polystyrene latexes, AMJ5 and ABJ2 at pH4, and AMJ5 at pH10, since all of them were very hydrophobic surfaces. These surfaces would then be placed at the left side of the previous ranking without specifying their relative positions. However, they present different surface charge densities (σ0 AMJ5 at pH4 ) -6.1 µC/cm2, σ0 ABJ2 ) -9.6 µC/cm2, and σ0 AMJ5 at pH10 ) -20.5 µC/cm2)4 which should slightly alter the hydrophobic character of the surface: the higher the σ0, the lower the hydrophobicity. If this reasoning is taken into account, the ranking of anionic surfaces would remain as
AMJ5 (pH4) e ABJ2 (pH4)e AMJ5 (pH10) < Silica (pH4) < . . . The same criteria were used to order the cationic surfaces (see the CSC values listed in Table 2) in the following ranking:
AMJ10 (pH4) < AJ25 (pH4) < IgG-AMJ10 (pH4)< 0 and hνi > 0 (see Table 1). Therefore, the dispersion forces felt by our ions near the hydrophobic surfaces are attractive, being more intense in the case where the ion is more polarizable; (note that ionization energies in water are similar for all the anions, and similar among cations). According to the R*(0) values displayed in Table 1, for monovalent ions, attractive dispersion forces increase in the sequence
Cl- < NO3- < SCN- and Na+ < NH4+ From that, it can be inferred that the specific accumulation of SCN- ions due to the action of dispersion forces at the PSwater interface is more important than that of NO3- ions which, in turn, would approach more effectively than the Cl- ions. An accumulation of anions in the vicinity of a negative surface increases the charge density at the surface proximities, enhancing the stability of the system. That is, the most polarizable anion (SCN-) should give the most stabilizing effect, as reflected in our experiments. In the case of cations, on the contrary, the more effective accumulation of NH4+ cations at the interface would diminish the negative potential of the particles and, consequently, it would provoke a higher destabilizing effect with regard to the Na+ ions, which is also experimentally observed. The specific ion accumulation predicted by the dispersion forces theory, capable of explaining our stability results, was also reflected in electrophoretic mobility measurements (see reference 4 and Supporting Information). Let us now consider the most hydrophobic positive systems, that is: AMJ10 (pH 4) and AJ25 (pH 4). The sequences resulting
from the CCC values for monovalent ions are exactly opposite to those found with hydrophobic negative surfaces:
SCN- < NO3- < Cl(more unstable) (more stable) Na+ < NH4+ These results support the assumption that surfaces with different charge sign lead to reversed lyotropic series. According to the Ninham’s model, this can be readily explained considering that the accumulation of negative ions (due to dispersion forces) at the proximities of positive surfaces involves a reduction in the surface potential, and thus, it causes a destabilizing effect, conversely to what happens in negative particles where the accumulation of anions stabilizes. The same argument can be extended to cations. Therefore, we conclude that dispersion forces theory can account for the ion specificities observed in surfaces with similar hydrophobic character. Calcium, a divalent ion, always induces the lower colloidal stability irrespective of the surface charge sign or hydrophobic/ hydrophilic character. In this case, the double valence of the ion governs the stability of all the systems at moderate salt concentrations, so that any possible modifications of the particle surface potential due to specific accumulation/exclusion mechanisms is masked by the electrostatic interaction. 3.2. Hofmeister Sequences in More Hydrophilic Systems: Alterations Induced by the Hydrophilic Character of the Surface. According to Ninham’s model, systems with identical charge sign would lead to the same lyotropic series, provided that the refractive index of the surface is larger than that of water. However, in contrast with Ninham’s predictions, we have observed alterations in the anion series when the surface turns from hydrophobic to hydrophilic without changing the relative refractive index values. Table 3 presents the results concerning negative surfaces. The arrows in this table indicate alterations with regard to the Hofmeister series observed on a hydrophobic surface. With hydrophobic polystyrene particles, the Cl- < NO3- < SCN- order, discussed in the previous section, is found. However, the SCN- position becomes altered when the hydrophilic character of the surface increases and the restabilization
TABLE 4: Positions of Hofmeister Ions Ordered According to the CCC Values Obtained with Positively Charged Systems Differing in the Hydrophobic/Hydrophilic Charactera
a
Changes with regard to usual positive hydrophobic surfaces have been highlighted in bold. The arrows serve to guide the eye.
16066 J. Phys. Chem. C, Vol. 112, No. 41, 2008 phenomena arise. With an intermediate hydrophilic surface (see the AMJ5 (pH10) latex in Table 3), this chaotropic anion changes only one position. However, for higher hydrophilic systems, SCN- moves two positions, reversing its original place in “hydrophobic surfaces”, whereas the rest of the anions manifest the same order. Only with the system displaying the strongest restabilization processes, that is, the most hydrophilic system, the ABJ2-IgG (pH10), we find a completely reversed anion order, since the NO3- ion also inverts its position in the series. The anomalous behavior of SCN- and occasionally of NO3implies that these chaotropic anions, instead of stabilizing negative surfaces by dispersion forces, favor the particle aggregation. In terms of specific ion accumulation processes on surfaces, this result would imply that the concentration of SCN- and NO3- anions (acting as coions) at the particle-water interface must be lower than that of Cl-. Therefore, the local concentration of Na+ (acting as counterions) predicted by the usual Poisson-Boltzmann equation used in electric double layers becomes even higher due to the specific exclusion of chaotropic anions. Consequently, the potential at the proximities of the particle surface is reduced and the colloidal aggregation is favored. The “negative adsorption” or exclusion of these anions cannot be explained by considering their respective ion excess polarizabilities, and then Ninham’s model would fail to explain the results with hydrophilic surfaces. Therefore, the explanation must be sought in entropic mechanisms associated with rearrangements in the water structure, as suggested by Lyklema and Wiggins.14,50 Indeed, the fact that the strong destabilizing effects of SCN- and NO3- are also associated with important restabilization phenomena in hydrophilic systems argues for a mechanism that involves solvent-structural forces. The water structure at the surface proximities and around the ions (not considered in the Ninham’s theory) appears to be crucial to understand these alterations in the Hofmeister series. The chaotropic/kosmotropic concept can be readily extended from ions to any type of surface. Hydrophobic colloidal particles behave as chaotropic systems, inasmuch as the surface water interactions are weaker than the water-water interactions. Analogously, hydrophilic surfaces interact strongly with water and cause similar arrangements in the neighboring solvent molecules as kosmotropic ions. The general (and entropic) “like seeks like” rule was adapted by Be´rube´ and De Bruyn for associating ions with surfaces within this context.51 When a surface is immersed in a saline solution, the ions promoting water rearrangements similar to those induced by the surface on the adjacent water layers can be easily accommodated at the interface. Conversely, ions causing dissimilar solvent environments tend to be excluded from the interfacial zone.13,14,51–54 These entropically controlled processes would explain why the adsorption of chaotropic ions is favored in hydrophobic surfaces and precluded in hydrophilic ones. Figure 3a,b schematically represents the distribution of chaotropic anions near a hydrophobic and a hydrophilic surface, respectively. In the first case, there are no solvent-structural incompatibilities and the ion distribution can be modulated by dispersion forces. In the second case, the steric hindrance caused by the water structure adjacent to a hydrophilic surface determines the final ion distribution near the surface: the chaotropic ions being excluded from it, while kosmotropic ions accumulate in this region, as a consequence of the “like seeks like” rule. The inversions explained by this entropic mechanism were also detected on positive surfaces (Table 4), principally with chaotropic anions. In this case, however, systems with an
López-León et al. extreme hydrophilic character were necessary to detect any alteration in the lyotropic series, because in cationic systems anions act as counterions, and thus not only dispersion forces but also attractive electrostatic interactions favor the approach of anions toward the surface. Therefore, alterations in the lyotropic series for anions can be found only on surfaces where the repulsive anion-surface structural forces (based on the solvent conformation, see Figure 3b) are important enough to counterbalance the other two attractive anion-surface contributions (that is, electrostatic and dispersion forces). In our case, only the chitosan-TPP and TG-chitosan particles were hydrophilic enough to generate a structured water layer capable of excluding chaotropic anions from the water/particle interface, and thus, of inverting the series. Otherwise, typical Hofmeister series resulted. Again, inversions were more accentuated in the case of the SCN- ion (which alters its behavior in both chitosan systems) followed by the NO3- ion (which changes its position only in the TG-chitosan particles). As discussed in the next section, the unusual stabilizing properties displayed by these anions when added to the chitosan suspensions evidence the existence of structural/entropic forces that exclude these chaotropic anions from the particle/water interface. Finally, inversions induced by the hydrophilic character of the surface were hardly observed with cations. Only the most chaotropic cation (NH4+) acting as a coion tended to reverse its behavior in the presence of the most hydrophilic surface (chitosan). 3.3. Ion Specificity in Restabilization Phenomena. Restabilization in colloidal systems has commonly been ascribed to the presence of highly hydrated cations in solution.28 These cations, normally acting as counterions, are thought to reinforce the water structure near the hydrophilic surface, creating a steric barrier that precludes aggregation. Molina-Bolívar et al. have reported the conditions under which polystyrene latices and protein-latex complexes restabilize.55–58 These authors conclude that hydration forces responsible for such phenomena become more intense with the hydration number of the cation. Hydration forces have also been used to explain vesicle or liposome aggregation in the presence of divalent or trivalent cations, such as La3+.59–61 The relevance of studying these cations is because besides aggregation they can also act as fusogenic ions and membrane fusion is an essential molecular event in many molecular processes.57 These highly hydrated cations are supposed to strongly interact with the headgroup of the membrane surface via two possible mechanisms: (i) they interact with specific polar sites situated directly on the membrane surface and (ii) they are dehydrating the membrane region.61,62 In this paper, we show experimental evidence of restabilization processes induced not only by cations but also by anions and more notably, restabilization processes provoked by weakly hydrated ions (as SCN- and NO3-) where specific ion binding or dehydration of polar groups mechanisms are hardly plausible. The general trends characterizing the restabilization phenomena observed in our systems are enumerated below. (i) As expected, restabilization processes are more important as the surface becomes more hydrophilic. This can be inferred from the increase in the number of ions that can induce restabilization, and from the lower CSC values obtained in the most hydrophilic systems. (ii) Calcium is the cation that induces restabilization patterns in more occasions, acting as both counter- or coion. This is consistent with the assumption that these phenomena are more intense when the hydration number of the cations increases. (iii) Anions also play a key role in restabilization phenomena. Whereas the NaCl salt never provoked restabili-
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Figure 3. (a) Schematic distribution of chaotropic ions (big green spheres) and kosmotropic ions (small red spheres) at a hydrophobic surface/ water interface. Clear cyan areas near the surface represent low density water.50 (b) Schematic distribution of chaotropic ions (big green spheres) and kosmotropic ions (small red spheres) at a hydrophilic surface/water interface. Clear cyan areas near chaotropic ions represent low density water, whereas dark blue areas near the hydrophilic surface refer to as high density water.50 (c) Hypothetical ion distribution when two hydrophilic surfaces approach each other. Chaotropic ions (big green spheres) are excluded from high density water zones, whereas kosmotropic ions (small red spheres) tend to accumulate in them. This spatial distribution of ions and water near hydrophilic surfaces hinders the direct contact among both surfaces. This steric hindrance does not appear in hydrophobic surfaces (panel d). (d) The lack of high density water layers at the proximities of hydrophobic materials permits a significant approximation of two approaching surfaces, even in presence of a high number of ionic species, regardless its chaotropic (big green spheres) or kosmotropic (small red spheres) nature. This high approximation makes the particles fall into a primary minimum (with regard to its interaction energy curve), and this is why restabilization phenomena do not exist at high ionic strengths when working with hydrophobic systems.
zation except for the TG-Chitosan particles, important restabilization processes were observed when the NaNO3 or NaSCN salts were added to hydrophilic surfaces, evidencing the direct influence of the anion. In addition, opposite tendencies were observed in positive and negative surfaces. According to the CSC values, the most chaotropic anions have a stronger restabilizing effect on negative surfaces, and the inverse series is found in positive surfaces. iv) Restabilization can be induced by both chaotropic (weakely hydrated) and kosmotropic (highly hydrated) ions. These novel results can be understood within the scenario proposed to explain Hofmeister inversions of CCC in hydrophilic systems, in which the water structure around ions and surfaces determines the interparticle interactions. Generally, in the proximities of a hydrophilic interface, water molecules are
orientated to maximize the number of surface water bonds creating a high-density solvent layer. Such a solvent structural layer precludes chaotropic ions, which are presumably surrounded by another type of water structure, from approaching to the surface. All this eventually results in a steric hindrance mediated by the molecular order of water and ions near the surface that prevents particles from mutual approaching at very short distances. This in turn blocks the aggregation of hydrophilic particles when the number of ions is high enough, as experimentally observed. A scheme of this qualitative explanation is shown in Figure 3c. In the case of kosmotropic ions, such as calcium, both hydrophilic surfaces and ions have a strong tendency to be hydrated, forming a dense structure of water molecules and ions, which also hinders particle aggregation at high salt concentrations. Whatever the case, the solvent
16068 J. Phys. Chem. C, Vol. 112, No. 41, 2008 structures around hydrophilic particles and ions are translated into a steric barrier when two of these particles approach each other at high electrolyte concentrations. For hydrophobic surfaces, however, these protective water and ion shells do not exit and thus, two approaching particles can practically make contact (see Figure 3d), and they will inevitably collapse once the CCC has been surpassed. In this manner, the traditional concept of colloidal restabilization can be extended from highly hydrated cations to any structure-breaker/maker ion. On the basis of this model, we propose that restabilization processes observed in our systems can be understood in terms of the relative concentration of counterions (irrespective if they are cations or anions) at the proximities of the particle surface. Note that this type of repulsive structural forces is directly dependent on the local concentration of ions near the surface;41 in this sense, they must be dependent on the concentration of counterions, since they are much more numerous than the coions at the interfacial region. In addition, this local concentration of counterions is modulated by the specific accumulation/exclusion of chaotropic anions to/from the surface. In other respects, the repulsive hydration forces will be more intense when the hydration number of the counterion is higher. We will discuss first the negative surfaces in terms of this model. We have shown that the exclusion of chaotropic anions increases when the negative surfaces become more hydrophilic. This implies a local enrichment of Na+ ions (now acting as counterions), which means a more efficient screening of the particle surface potential, and then lower CCC values. Moreover, in these cases the enrichment of hydrated Na+ near the surface would also reinforce the structural-hydration forces that prevent aggregation. The CSC values for the IgG-AMJ10 and IgG-ABJ2 systems support this conclusion. On the one hand, the lowest CSC values were observed with the most chaotropic anion, SCN-, followed by the NO3-. On the other hand, the respective CSC values became clearly lower when the surface increased its hydrophilicity, since the exclusion of chaotropic anions by means of entropic mechanisms becomes also more effective. As these exclusion mechanisms mainly affect to anions, the data (both CCC and CSC) reflect a higher ion specificity for the SCN- and NO3- anions than for the NH4+ and Na+ cations. It must be emphasized that no restabilization was observed with NaCl in our systems. Since Cl- is the most kosmotropic anion employed and, hence, it is less affected by the entropic exclusion process, it would be necessary a more hydrophilic surface than that of the IgG-ABJ2 (pH10) system to observe restabilization by the local enrichment of the Na+ ion (counterion). It should be noted that restabilization induced by NaCl has indeed been observed by other authors working with highly hydrophilic negative systems.58 When the surfaces are positive, anions act as counterions, and according to our hypothesis they (but not cations) are responsible for the CSC values. Let us first consider the most hydrophilic positive surface (TG-chitosan), where the series from CCC data were inversed to those characteristic of hydrophobic surfaces and where all the salts induced restabilization. In this case, the lowest CSC value was found with Cl-, that is the anion with less chaotropic character, and consequently, the most hydrated anion. Higher CSC values were obtained with SCN- and NO3- (in the order SCN- > NO3-) because, given their more chaotropic nature, they tend to be excluded from the hydrophilic interface. Conversely, in less hydrophilicsurfacessuchasIgG-AMJ10(pH4)orIgG-ABJ2(pH4) the SCN- and NO3- showed a stronger tendency than Cl- to restabilize the system. It should be noted that the most chaotropic
López-León et al. anions have the most destabilizing effects on these moderately hydrophilic systems. This result indicates that both systems are still partially hydrophobic, and thus an accumulation (instead of an exclusion) of chaotropic anions takes place at the surface. In fact, for the less hydrophilic sample the IgG-AMJ10(pH4) surface, the accumulation of SCN- was so important that even made possible the restabilization of the system at high salt concentrations. The IgG-ABJ2(pH4) surface, however, seemed to be more hydrophilic, since more similar CCC values were obtained with the three anions. Nevertheless, the slightly lower CCC values obtained with NO3- and SCN- with respect to Clstill reflect a higher accumulation of these two anions at the interface. As a consequence, NO3-, that is a more hydrated ion than SCN-, shows restabilization at a salt concentration lower than SCN-. 4. Conclusions This paper provides experimental results about inversions in Hofmeister series when working with stable/unstable colloidal systems. Inversions in lyotropic sequences have been found when changing the surface charge sign (keeping constant the hydrophobic/hydrophilic degree), and when modifying the hydrophobic/hydrophilic character (without changing the charge sign of the particle). The Ninham’s model based on ion-surface dispersion forces works well in hydrophobic systems, but it clearly fails with hydrophilic surfaces. This result was expected, since this model considers water as a continuum. However, water molecules are structured at interfaces, and according to the experimental results shown in this paper this structured layer of solvent exerts a powerful influence in the ion specificity. Despite that dispersion forces are always present, and thus they should be taken into account in any colloidal or surface theory, an extension of the theory including at least the role of water structure at interfaces is mandatory to elucidate Hofmeister effects in colloidal systems. The plethora of results considered for this analysis has allowed us to suggest a plausible hypothesis capable to explain the ionic specificity in colloidal systems and that generalizes the concept of restabilization forces. The cornerstone of this hypothesis is the specific exclusion/accumulation of ions near the particle surface due to the entropic forces that stem from the structural changes that surfaces and ions provoke in water. In particular, the exclusion/accumulation processes of chaotropic anions in hydrophilic surfaces have been revealed to be crucial to understand the results showed in this paper. We have demonstrated that counterions (independently of their charge sign) are responsible for the restabilization phenomena. That is, not only highly hydrated cations, but also all type of ions (even chaotropic anions) can cause restabilization acting as counterions, provided that their local concentrations is high enough. The capacity of inducing restabilization in colloidal systems is mediated by three interrelated factors: (a) the hydrophilicity degree of the particle surface (restabilization phenomena are more important in more hydrophilic systems); (b) the hydration degree of the ions (the ability of inducing restabilization increases with the hydration number of the ion); and (c) the entropic exclusion/accumulation of ions around the surface. Finally, it is important to highlight that the inversions in the stability behavior as a function of the surface nature (hydrophobic/ philic) have been observed with few but representative ions. The use of many different surfaces has helped to observe the evolution of the results and hence to reach the above conclusions. In order to obtain a more extended representation of the
Hofmeister Effects in Colloidal Systems Hofmeister series, we have recently performed new experiments employing seven different anions (IO3-, F-, Cl-, Br-, I-, NO3-, and SCN-) and four representative colloidal surfaces (two positives and two negatives which differed in their hydrophobic/ hydrophilic nature). The results support the conclusions drawn from the present work. A paper with these new data is in preparation. Acknowledgment. Financial support from the “Comisio´n Interministerial de Ciencia y Tecnologı´a” Projects MAT200613646-C03-03 and MAT2007-66662-C02-01 (European FEDER support included) and from the “Consejerı´a de Innovacio´n, Ciencia y Tecnologı´a de la Junta de Andalucı´a” Projects P07FQM-2496 and P07-FQM03099 are gratefully acknowledged. We would like to thanks to Juan J. Valle-Delgado for the synthesis and characterization of the amphoteric AJ25 latex. Supporting Information Available: Experimental details and physicochemical characterization of the new systems. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Benrraou, M.; Bales, B. L.; Zana, R. J. Phys. Chem. B 2003, 107, 13432. (2) Henry, C. L.; Dalton, C. N.; Scruton, L.; Craig, V. S. J. J. Phys. Chem. C 2007, 111, 1015. (3) Klitzing, R. v. Phys. Chem. Chem. Phys. 2006, 8, 5012. (4) Lo´pez-Leo´n, T.; Jo´dar-Reyes, A. B.; Bastos-Gonza´lez, D.; OrtegaVinuesa, J. L. J. Phys. Chem. B 2003, 107, 5696. (5) Hofmeister, F. Arch. Exp. Pathol. Pharmakol 1888, 24, 247. (6) Kunz, W; Henle, J; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9, 19 (English translation Hofmeister works). (7) Cacace, M. G.; Landau, E. M.; Ramsden, J. J. Q. ReV. Biophys. 1997, 30, 241. (8) Omta, A. W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J. Science 2003, 301, 347. (9) Wachter, W.; Kunz, W.; Buchner, R.; Hefter, G. J. Phys. Chem. B 2005, 109, 8675. (10) Padmanabhan, V.; Daillant, J.; Belloni, L.; Mora, S.; Alba, M.; Konovalov, O. Phys. ReV. Lett. 2007, 99, 086105. (11) Salis, A.; Bilanicˇova´, D.; Ninham, B. W.; Monduzzi, M. J. Phys. Chem. B 2007, 111, 1155. (12) Koelsch, P.; Viswanath, P.; Motschmann, H.; Shapovalov, V. L.; Brezesinski, G.; Mo¨hwald, H.; Horinek, D.; Netz, R. R.; Giewekemeyer, K.; Salditt, T.; Schollmeyer, H.; Klitzing, R. v.; Daillant, J.; Guenoun, P. Colloids Surf., A 2007, 303, 110. (13) Lyklema, J. Mol. Phys. 2002, 100, 3177. (14) Lyklema, J. AdV. Colloid Interface Sci. 2003, 100–102, 1. (15) Rie´s-Kautt, M. M.; Ducruix, A. F. J. Biol. Chem. 1989, 264, 745. (16) Carbonnaux, C.; Rie´s-Kautt, M. M.; Ducruix, A. F. Protein Sci. 1995, 5, 2123. (17) Lo´pez-Leo´n, T.; Elaı¨ssari, A.; Ortega-Vinuesa, J. L.; BastosGonza´lez, D. ChemPhysChem. 2007, 8, 148. (18) Bostro¨m, M.; Williams, D. R. M.; Ninham, B. W. Phys. ReV. Lett. 2001, 87, 168103. (19) Kunz, W.; Lo Nostro, P.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9, 1. (20) Bostro¨m, M.; Tavares, F. W.; Finet, S.; Skouri-Panet, F.; Tardieu, A.; Ninham, B. W. Biophys. Chem. 2005, 117, 217. (21) Ninham, B. W.; Yaminski, V. Langmuir 1997, 13, 2097. (22) Bostro¨m, M.; Deniz, V.; Franks, G. V.; Ninham, B. W. AdV. Colloid Interface Sci. 2006, 123-126, 5.
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