Article pubs.acs.org/Langmuir
The Governing Role of Surface Hydration in Ion Specific Adsorption to Silica: An AFM-Based Account of the Hofmeister Universality and Its Reversal Jonathan Morag, Matan Dishon, and Uri Sivan* Department of Physics and the Russell Berrie Nanotechnology Institute, Technion − Israel Institute of Technology, Technion City, Haifa 32000, Israel ABSTRACT: AFM measurements of the force acting between silica surfaces in the presence of varied alkali chloride salts and pH’s elucidate the origin of the Hofmeister adsorption series and its reversal. At low pH, electrostatics is shown to be insignificant. The preferential adsorption of Cs+ to the silica surface is traced to the weak hydration of neutral silanols and the resulting hydrophobic expulsion of weakly hydrated ions from bulk solution to the interface. The same interactions keep the strongly hydrated Na+ and Li+ in solution. As pH is increased, a tightly bound hydration layer forms on deprotonating silanols. Cs+ is correspondingly expelled from the surface, and adsorption of small ions is encouraged. The deduced role of surface hydration is corroborated by hydration repulsion observed at high pH, surface overcharging at low pH, and data in other oxides.
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activity, and cell growth,17 exhibit the same Hofmeister series or its reverse, with only few exceptions. In the case of colloidal and protein suspensions, precipitation is associated with surface charge neutralization. 18 The appearance of the Hofmeister series and its reversal hence indicate a parallel universality in ion specific adsorption (ISA), whose underlying mechanism is the topic of the present article. ISA is also responsible for other ISE. Ions are conventionally classified according to their effect on the surrounding water structure. The small Li+ attracts adjacent water molecules more strongly than two neighboring water molecules do. It enhances water order and is considered a “structure maker” or kosmotrope. The large Cs+, on the other hand, interacts weakly with water and is correspondingly named in the literature a “structure breaker” or chaotrope. According to Gurney,19 the short-range hydration forces modify the interaction between oppositely charged ions. They enhance attraction between two kosmotropes or two chaotropes and induce short-range repulsion between two ions of opposite nature. This principle was generalized by Gierst et al.20 to surfaces and macromolecules. Similar ideas were later introduced by Collins21,22 in his formulation of the “like seeks like” principle to explain ISE in the context of proteins.1 A particularly insightful application of Gurney’s19 and Gierst’s20 ideas to explain the universal reversal of the Hofmeister series on going from pzc < 4 to pzc > 4 oxides23 is found in Bérubé and de Bruyn10 and Dumont and
INTRODUCTION The contact between protic surfaces and saline aqueous solutions is ubiquitous in nature and technology. Two prevailing examples include proteins with their acidic and basic residues and minerals with their metal hydroxide surface groups. Protic substances, brought into contact with water, acquire charge by proton exchange. The resulting surface field and the propensity to form hydrogen bonds lead to water and ion rearrangement near the interface. This surface dressing, taking place over one or two layers of water molecules, controls diverse phenomena from protein folding and enzyme activity to surface wetting and the stability of colloidal suspensions. Comparable strength of the often-competing interactions involved leads to pronounced ion specific effects (ISE),1 some of which were discovered over a century ago by Franz Hofmeister in his studies of protein precipitation by added salts.2 These studies ranked different ions according to their protein precipitation capacity and found, among other things, that the precipitation power of alkali cations correlates with their bare radius, regardless of the protein type.2 Remarkably, the same order of precipitation power, denoted direct Hofmeister series Cs+ > Rb+ > K+ > Na + > Li+
(1)
has been found in many other systems, such as colloidal suspensions of the metal oxides silica,3 tungsten trioxide,4 and manganite.5 At the same time, the precipitation of colloidal suspensions of other oxides including alumina,6−8 titania,9−13 and hematite14,15 was found to follow an exactly reverse order. In fact, a wealth of ostensibly independent phenomena, including protein denaturation and crystallization,16 enzyme © 2013 American Chemical Society
Received: February 6, 2013 Revised: April 22, 2013 Published: April 30, 2013 6317
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collaborators.11,15 Notwithstanding the significant predictive power of these principles, more recent studies, including the experiment presented below, point to additional key elements, most notably surface polarity and hydrophobicity,24−27 as well as potential dispersion forces associated with ionic polarizability.28,29 These new elements are naturally missing in early theories that originate from ion−ion interactions.19,20 The understanding of ISA in the protic case requires, hence, notions beyond hydration modification of Coulomb attraction between oppositely charged groups, a reminder of ISA found in neutral interfaces such as the air−electrolyte one.30,31 Indeed, we show below that the Hofmeister adsorption order, eq 1, is dictated by the strength of surface hydration rather than electrostatics. Motivated by the incompleteness of the Gurney−Gierst− Collins (GGC) framework, and the fact that most studies compare different materials, we have sought a single surface that shows the full crossover from the Hofmeister order to its reverse as a function of an external parameter, pH in our case. We found it in silica an archetype protic oxide whose intrinsic charge is rendered, similarly to other oxides, by the reactions
deduced by calibrating change in cantilever deflection versus the rise in the piezoelectric stage with the two in contact. All solutions were prepared with analytical grade salts, dissolved in 18 MΩ DI water. pH was set without buffer, using HCl or M−OH, where M was identical to the electrolyte cation. Dissolved CO2 was expelled from the solution using high-purity nitrogen gas immediately preceding the measurements. The solution pH was monitored in real time with the glass pH electrode sampling the solution in situ in a sealed holder, immediately “downstream” from the fluid cell. We note that pH electrode measurements are known to be ion-specific. However, at our salt concentrations, this effect is expected to be smaller than 0.1 pH unit,43 namely, smaller than our perceived error. Measurements in pure saline solutions titrated with hydrochloric acid to pH 5.5 were identical to those measured at the same pH in the presence of atmospheric CO2 buffering. The sample surfaces were soaked in 1 M NaCl solution for 30 min prior to the experiment. We have found that this procedure generally increases surface charge and improves sample stability and reproducibility. We attribute this behavior to irreversible adsorption of hydroxyls onto remaining exposed silicon atoms on the surface, as promoted by screening.44 Force curves were acquired with slow approach velocities (∼100 nm/s) to avoid hydrodynamic effects. At the end of every set of measurements, a reference force vs distance curve was measured in 1 mM NaCl solution at pH 5.5 and verified to coincide with an identical measurement taken at the beginning of the set.
MOH + H+ ⇌ MOH 2+ MO− + H+ ⇌ MOH
(2)
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with M = Si. Series reversal by pH has previously been observed on oxide surfaces, such as SnO232 and TiO2,11 as well as in coagulation of lysozyme protein.33−35 Zhang and Cramer also showed series reversal in lysozyme due to an increased electrolyte concentration.36 The silica surface is negatively charged in the entirety of our working pH range. Near the pzc, which is 2−3 for silica, neutral protonated surface sites dominate, but these deprotonate nearly completely at elevated pH, yielding an intrinsic surface charge. Note that the total or outer Helmholtz plane37 (OHP) charge mentioned below includes the adsorbed ions as well. Unlike most studies of ISA, we utilize direct force−distance measurements with an atomic force microscope (AFM), which provide the added details needed for deciphering the underlying ion adsorption mechanisms. Measurements using ζ-potential, yield stress, surface force apparatus, and AFM have established that ion adsorption to silica follows the direct Hofmeister series,38−41 as expected from its low pzc. We report here that silica exposed to oxygen plasma and salt treatment (see below) displays enhanced intrinsic surface charge and a full series reversal at pH ≈ 7 in agreement with indirect evidence from second harmonic generation (SHG) measurements.42 As we show, this series reversal, taking place in the same material, yields new insight into the physics underlying ISA.
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RESULTS The measured force, normalized to the colloid’s radius, is plotted in Figure 1 vs colloid−surface separation for three concentrations of LiCl, NaCl, and CsCl solutions and two pH values. At large separations, the Debye−Hückel approximation holds, and the force on the colloid decays exponentially with h according to a characteristic screening length λ, which shortens with concentration. At these low concentrations, the screening length is evidently independent of salt type. As a reference, we add to Figure 1a a best fit of the 1 mM LiCl curve to DLVO theory,18 assuming ε = 78 for the water dielectric constant and a Hamaker constant of H = 6.6 × 10−21 J.41 The experimental curve clearly rests between the constant charge (CC) and constant potential (CP) limiting cases, except at small separations, where short-range repulsion, attributed to hydration forces,45−47 masks the expected vdW attraction. In agreement with previous publications,38,48−50,40,41 the force magnitude at all concentrations near pH 5.5 (Figure 1a) decreases with increasing bare ionic radius, in accordance with the direct Hofmeister series, eq 1. The larger Cs+ adsorbs more readily than Na+, and the latter adsorbs to the slightly deprotonated silica better than Li+. A dramatic change takes place at high pH (Figure 1b). The force in the presence of Cs+ is now greater than in Li+, with Na+ fitting in between. The Hofmeister series is clearly reversed, indicating a corresponding reversal of cationic propensity to the fully deprotonated silica surface. As mentioned earlier, the same transformation from direct series to its reverse at high pH has been observed in SnO232 and partially so in TiO2 hydrosols.11 Both oxides exhibit a pzc near the phenomenological dividing line of 4. The insets to Figure 1 highlight the short-range interaction at different pH. In inset of Figure 1a, the force is measured at 3 mM LiCl near pH 5.5, 9.3, and 10. With increasing pH, the force increases slightly but more importantly, a clear shortrange repulsion evolves. A slightly less pronounced effect is seen with Cs+ (inset to Figure 1b).
EXPERIMENTAL DETAILS
Our experimental procedure was almost identical to ref 40. Briefly, a 5 μm diameter silica colloid (Bangs Laboratories) was glued onto a goldcoated, silicon nitride AFM cantilever (Bruker MLCT-O10). The cantilever, along with a pristine silicon substrate (Siltronix-100) underwent 30 s plasma oxidation (Plasma-Therm 790 RIE) at 50 W rf power and 50 mTorr of O2 gas. Measurements of the force between the colloid and the substrate in the presence of several solutions were performed under moderate, steady flow through a fluid cell (Bruker MTFML) using a commercial AFM (Bruker Multimode), modified to yield low noise data.40 Force was determined by multiplying the cantilevers’ deflection by their individual spring constant (∼.05 N/m) as measured by thermal fluctuations. Colloid−substrate separation was 6318
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Figure 2. Outer Helmholtz potential and total charge as a function of pH in the presence of 10 mM alkali salts. The lines serve a guide to the eye. The two insets show data from experiments performed on other colloid−substrate pairs at high and low pH. There is slight variation in charge between different samples, but the series order remains consistent with the picture described.
normal Hofmeister order is thus reversed, first between Na+ and Li+ and then at pH 7 between Cs+ and the other two cations. As further proof of series reversal, we present in the insets to Figure 2 consistent results from two additional experiments performed on different samples. In each inset, three measurement sets are chosen from the low- and high-pH regimes. One of these samples was oxidized in a Harrick plasma cleaner DC-326 for 1 min, as further validation that the reversal is not an artifact of the specific plasma process. There exists small variation in measurements, but the phenomenology is identical to that in the main figure. Ignoring pH values below 4.7, where the first reaction of eq 2 is operative, we find a strikingly different behavior of the structure making ions Na+ and Li+ compared with the structure breaker Cs+. Specifically, the data indicate an additional adsorbing mechanism of Cs+ whose effect diminishes gradually as pH approaches 7.5. This mechanism is clearly inoperative with the two kosmotropes. In Figure 3, we plot the OHP potential as a function of salt concentration with representative values from both high and low pH. Figure 3 was measured on the same sample as Figure 1.
Figure 1. AFM force curves in the presence of LiCl, NaCl, and CsCl salts at 1, 3, and 30 mM concentrations, presented near pH 5.5 (a) and pH 9.8 (b). All curves were measured with the same colloid−substrate pair. The direct Hofmeister series observed at low pH is reversed at high pH. For reference, the 1 mM LiCl pH 5.5 curve is fitted with the DLVO model at constant surface charge (CC) and constant potential (CP), respectively. Figure insets show the force at small separations in the presence of Li+ (a) and Cs+ (b) measured near pH 5.5, 9.3, and 10. Force in the insets increases with pH in both cases.
Potential and total charge near the surface are estimated by fitting force curves to the Poisson−Boltzmann equation40 at separations greater than several λ. In the limit that the bound layer is much thinner than λ, these quantities approximate the potential and the total charge up to the OHP. Figure 2 depicts OHP potential and total charge as a function of pH at 10 mM salt concentrations. Each plot point indicates an average of roughly ten consecutive measurements on the same sample. From pH 3 to 4.7 the negative OHP charge and potential increase monotonically for all salts, retaining the direct Hofmeister order. The steep increase of charge is attributed to full depletion of the doubly protonated silanols and deprotonation of 1−2% of the neutral silanols40,41 (eq 2). As pH is further increased, the OHP charge grows monotonically in the presence of Cs+, reaching a limiting value at pH 7.5. In contrast to Cs+, and despite the increasing deprotonation of surface groups,51,42 the OHP charge in the presence of Na+ and Li+ remains practically constant in the full pH range from 4.7 to 9.8. A slight decrease of total charge takes place in lithium between pH 5.5 and 6.2, positioning it above sodium. The
Figure 3. Outer Helmholtz plane potential plotted as a function of salt concentration, measured near pH 5.5 and 9.8. Potential decreases approximately logarithmically with increased concentration. 6319
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interface at low pH bears marked similarities with the air−water interface.30,31 The higher polarizability of cesium may add a dispersion force28,29 acting on this ion in the same direction as the hydrophobic force. When pH is increased above 4.7, more silanols deprotonate and the bare surface charge increases significantly.42 Yet, for Na+ and Li+ the OHP potential and total charge remain essentially constant (Figure 2), indicating full screening of the growing surface charge by the kosmotropic ions. At the same time, for Cs+ the initially low OHP charge and potential grow steadily up to pH 7.5, indicating a corresponding release of adsorbed Cs+ ions. The data show then that the mechanisms that drove Cs+ to the surface at acidic pH vanish above pH 7.5 and leave the stage for other forces that reverse the Hofmeister adsorption series. The hydrophobic−hydrophilic transition observed in our experiment coincides with the reorientation of water molecules observed by phase-sensitive sum-frequency vibrational spectroscopy of the quartz−water interface.57 At low pH, when the surface is dominated by neutral silanols, water tends to orient with its oxygen facing the surface. As pH is increased above 4.5, water molecules gradually reorient to form hydrogen bonds between water hydrogen and the growing number of deprotonated silanols. These bonds, having an electrostatic component, are expected to be stronger than those between water and neutral silanols. Indeed, the force curves depicted in the insets to Figure 1 disclose a corresponding hydration repulsion at elevated pH. Combining the AFM results with the optical data, we associate the gradual suppression of Cs+ adsorption with the growing number of strong hydrogen bonds between water and deprotonated silanols. As a result, the balance between the forces that drive Cs+ to the surface and disruption of the silica− water hydrogen bonds should gradually shift in favor of bulk solvation of ions as evident from the gradual decrease in excess Cs+ adsorption and the eventual flattening of its curve at pH 7.5 (Figure 2). The tight hydration of deprotonated silanols also explains why Cs+ ions adsorb at low pH to other sites, as deduced above from the observed overcharging40 and its reduction with pH. In a sense, silica is transformed from a neutral chaotrope at low pH to a charged kosmotrope at high pH. It comes as no surprise then that the Hofmeister series is accordingly reversed.10,11,20,21 Note, however, that unlike the GGC picture, binding of the chaotropic cation to the chaotropic neutral surface does not involve pair formation. In fact, it may not necessarily take place on the neutral silanols. Furthermore, the strengthening hydrogen bonds of charged silanols with water suppress Cs+ adsorption to its binding sites, which are shown by the charge reversal argument to be different from the charged silanols. Surface hydration emerges then as a global rather than local property, blocking the approach of chaotropes to their surface binding sites. The independence of the OHP charge and potential (Figure 2) on proton concentration above pH 7.5 implies full compensation of newly ionized silanols by adsorption of all three cations. The different OHP charge indicates closer approach of the Li+ ions to the SiO− groups compared with Cs+, with Na+ in between. Previously, Boström et al.58 were able to reproduce anionic series reversal on the surface of lysozyme using a modified stern model that accounts for dispersion forces.28,29 Though we do not rule out dispersion forces as a possible factor for ISA in our
The potential is negative and drops in magnitude roughly as the logarithm of the concentration. The decrease in potential is predominantly due to improved charge screening (shorter screening length) in the diffuse layer, though there are also marked differences between the salts due to ISA. We note again the direct (reversed) Hofmeister sequence at acidic (basic) pH and point to the similarity of the Li+ and Na+ curves between the two pH’s, primarily in magnitude but also in slope. Meanwhile, there is a profound reduction in the adsorption propensity of cesium in going from pH 5.5 to 9.8, as witnessed in the loss of its steep adsorption slope, and the greater overall magnitude.
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DISCUSSION AND CONCLUSIONS We turn now to discuss the key experimental features and their consequences. We start with the excess Cs+ adsorption observed at pH ≤ 7.5 (Figure 2) and its increase with bulk ion concentration (pH 5.5 curve in Figure 3). Extrapolation of the latter curve for cesium suggests that the total charge will become positive roughly at concentrations of 100 mM. These data are fully consistent with earlier experiments that show growing adsorption and eventually charge reversal at Cs+ bulk concentrations above 60−100 mM for the silica surface3,40,41 and at similar concentrations for kaolin and montmorillonite.52 Similar charge reversal has been observed with potassium40,52 and stronsium.41 The continuing adsorption of chaotropic cations to the already positive surfaces implies a driving force other than electrostatics. A similar conclusion is deduced from anionic adsorption to latex colloids.27 Moreover, we note that below pH 4.5 a decrease in pH has a minimal effect on excess cesium adsorption (compare with the nonadsorbing sodium and lithium). In addition, since during overcharging deprotonation is more than compensated by cesium adsorption, one is lead to conclude that Cs+ must adsorb also, and perhaps exclusively, to sites other than deprotonated silanols such as the hydrophobic pockets found in numerical simulations of the silica−water interface.53−55 This conclusion contrasts the popular assumption that cation adsorption is invariably driven by electrostatic attraction. The next clue to the origin of the adsorption mechanism is provided by the aforementioned Hofmeister order and its reversal observed in low and high pzc oxides, respectively. As shown by Healy and Fuerstenau,56 the heat of immersion of oxides grows linearly with pzc, associating normal Hofmeister order with weak surface water interaction and vice versa. Extensive Cs+ adsorption (most likely in its dehydrated form) is thus found to characterize weakly hydrated oxides. In a similar manner, chaotropic anion adsorption takes place in hydrophobic latex colloids but not in hydrophilic ones.27 Finally, note that bulk hydration of cesium is unfavorable compared with that of small kosmotropes due to both disruption of a larger number of water hydrogen bonds and a smaller gain of electrostatic energy by hydration. All these facts, along with the large heat of hydration for gaseous cesium,21 lead to the conclusion that at low pH electrostatics is negligible and Cs+ is expelled from solution to the water−silica interface by essentially hydrophobic forces. The free energy is apparently minimized by restoration of water structure in the evacuated ionic cavity in the bulk at the expense of breaking weaker hydrogen bonds on the surface between neutral silanol groups and neighboring water. The proposed mechanism also explains the retention of the strongly hydrated Li+ and Na+ in solution. In that respect, the silica−water 6320
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measurements, we see no clear mechanism that can account for the “turning off” of these interactions at basic pH. Expulsion of cesium at high pH and the resulting reversal of the Hofmeister series require then the incorporation of surface hydration. We also note that the reversal due to pH in the case of proteins was observed near the pzc by switching the sign of the surface charge.33−35 Our case, as is the case with other oxides,11,32 shows Hofmeister reversal that occurs due to excessive deprotonation of the protic sites far away from the pzc. According to the GGC picture, the strong binding of two kosmotropic counterions in solution is accompanied by dehydration and contact of the two small ions. The same could happen here with the charged silanol group playing the role of a kosmotropic anion. Alternatively, as proposed by Bérubé and de Bruyn for the case of rutile,10 cation binding to the anion may be mediated by water molecules oriented with one of their hydrogens facing the anion and oxygen facing the cation. The two ions thus share their hydration shell. Similar water bridging has been observed in simulations of alkali binding to the tightly hydrated carboxylate anion.59 These simulations show that the smaller the cation the stronger its tendency to share its hydration shell with the anion. The same trend shows at high pH in Figure 2. The smaller the cation, the closer it approaches the charged silanols and the better it screens them. Experimentally, we could not discern between the direct GGC ionic contact scenario and water mediated binding. We merely point out that the attribution of the gradually decreasing Cs+ adsorption observed at 4.7 ≤ pH ≤ 7.5 to its expulsion from the surface by the growing number of strong hydrogen bonds favors the picture of water-mediated adsorption at high pH. In summary, force microscopy reveals the origin of ISA to silica and potentially to other protic surfaces. The emerging picture highlights the governing role of surface hydration in ISA and the negligible role played by electrostatics in the regime where direct Hofmeister order is observed. The proposed ISA mechanism accounts for all facts known to us, including previously unexplained overcharging by chaotropic cations at low pH, appearance of short-range repulsion forces at high pH, water reorientation with pH,57 and ISA to oxides of different pzc. It is also consistent with the theoretical phase diagram proposed in refs 25 and 26 as well as the experiments in refs 24 and 27. Integrating all these facts, we point out then that the Hofmeister series and its reversal can be explained by a single microscopic conjecture: the hydrogen bond of a water molecule to its neighboring water is stronger than the bond to neutral silanol and weaker than its bond to deprotonated silanol. The rest follows naturally.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (U.S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The support by the German−Israeli Foundation under Grant I1045-82.14/2009 and the Israeli Science Foundation under Grant 1403/12 is gratefully acknowledged. We thank Roland R. Netz, Lucio C. Ciacchi, and David Andelman for very useful discussions. 6321
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dx.doi.org/10.1021/la400507n | Langmuir 2013, 29, 6317−6322
