Ionic Specificity in Rapid Coagulation of Silica Nanoparticles

Jan 23, 2018 - (36, 45, 46) These data certainly inform us how cations accumulate on the silica surface, but no information about the thickness of ads...
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Cite This: Langmuir XXXX, XXX, XXX−XXX

Ionic Specificity in Rapid Coagulation of Silica Nanoparticles Ko Higashitani,*,† Kouta Nakamura,‡ Tomonori Fukasawa,§ Katsumi Tsuchiya,‡ and Yasushige Mori‡ †

Department of Chemical Engineering, Kyoto University-Katsura, Nishikyo-ku, Kyoto 615−8510, Japan Department of Chemical Engineering and Materials Science, Doshisha University, Kyotanabe, Kyoto 610−0321, Japan § Department of Chemical Engineering, Hiroshima University, Higashi Hiroshima, Hiroshima 739−8527, Japan ‡

ABSTRACT: The Smoluchowski theory has been widely accepted as the basic theory to estimate the rapid coagulation rate of colloidal particles in electrolyte solutions. However, because the size and specificity of molecules and ions are not taken into account, the theory is applicable only if the particle size is large enough to neglect the effects caused by the structured layers composed of water molecules, ions, and hydrated ions adsorbed on the colloidal surface. In the present study, the rapid coagulation rates of silica nanoparticles in concentrated chloride and potassium solutions were measured by using a low-angle light-scattering apparatus, and the dependence of the experimental value of rapid coagulation rate, KRE , on the particle diameter, Dp, and also on the Gibbs free energy of hydration of ions, ΔGhyd, was investigated extensively. The following were found. (1) When the particle size was small enough, the value of KRE reduced exponentially not only with the decreasing particle size but also with the increasing value of (−ΔGhyd) of cations (counterions) in the case of chloride solutions and with that of anions (coions) in the case of potassium solutions. (2) Silica nanoparticles of Dp ≲ 70 nm in 1 M KNO3 and KSCN solutions did not coagulate at all, although they coagulated at Dp ≳ 100 nm as in the other potassium solutions. These unexpected phenomena were explained by the proposed mechanisms.

1. INTRODUCTION This is the succeeding report based mostly on the data given in the previous paper,1 but our attention here is focused on the specificity of co- and counterions to the rapid coagulation rate of silica nanoparticles in solutions, which was observed but not discussed at all previously. A brief view of the previous paper is described below. The basic theory for the Brownian coagulation of monodispersed particles in the medium was initially developed by Smoluchowski.2 When it is applied to the rapid coagulation of particles in electrolyte solutions, the theory must be modified by including the contributions of static interactions and the hydrodynamic drag force of the squeezing flow between colliding particles.3,4 According to the modified Smoluchowski theory in which the contribution of the electrostatic repulsive force is regarded as negligible, the change of the total number concentration of particles Nt at the initial stage of coagulation of monodisperse spherical particles and the rate constant of 3−6 rapid coagulation KSM R are expressed as follows. dNt /dt = −KRSMNt 2

⎡ KRSM = 2kT /⎢3μ ⎣

∫0

separation distance between particle surfaces and a is the particle radius. Τhe coefficient β for the squeezing flow between colliding particles5 and the van der Waals attractive potential Vsph A for equal spheres are given by the following equations, respectively. 2

β exp(V Asph(h ̅ )/kT ) 2

(2 + h ̅ )

⎤ dh ̅ ⎥ ⎦

(4)

where A is the Hamaker constant. When β = 1 and = 0, KSM R reduces to the rate constant of the classical Smoluchowski theory, KSR = 4kT/3μ.2 If the viscosity of water is used for μ and the value of A is taken to be 8.3 × 10−21 J for silica particles in −18 water,4,7 KSR and KSM R at T = 25 °C are equal to 6.16 × 10 3 −18 3 S m /s and 3.49 × 10 m /s (=0.566KR), respectively. We note S that both KSM R and KR are independent of the particle size Dp(=2a). Numerous attempts have been made to check the adequacy of eq 2, and the experimental values of rapid coagulation rate KER were found to be scattering mostly around the value predicted by eq 2,7−17 although the recent extensive studies Vsph A

(2)

where t is the elapsed time, k is the Boltzmann constant, T is the temperature, and μ is the viscosity of medium. h̅ is a nondimensional separation distance, h/a, where h is the © XXXX American Chemical Society

(3)

⎡ ⎛ h ̅ 2 + 4h ̅ ⎞⎤ A 2 2 ⎜⎜ ⎟⎥ V Asph = − ⎢ 2 + + ln 2⎟ 6 ⎢⎣ h ̅ + 4h ̅ (h ̅ + 2)2 ⎝ (h ̅ + 2) ⎠⎥⎦

(1) ∞

2

β = (6h ̅ + 13h ̅ + 2)/(6h ̅ + 4h ̅ )

Received: November 30, 2017 Revised: January 15, 2018 Published: January 23, 2018 A

DOI: 10.1021/acs.langmuir.7b04081 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Table 1. Physicochemical Properties of Ions and Salts32−35 chloride ion 32

radius of ion (nm) hydration number32 Gibbs energy of hydration (kJ/mol)32 Jones−Dole viscosity B coefficient (dm3 /mol)33 shape of ion (anisotropy)34 melting temperature of salts (°C)35

Li+

Na+

potassium K+

Cs+

Cl−

Br−

I−

NO3−

0.069 5.2 −475 0.150

0.102 3.5 −365 0.086

0.138 2.6 −295 −0.007

0.170 2.1 −250 −0.045

0.181 2.0 −340 −0.007

0.196 1.9 −315 −0.032

0.220 1.6 −275 −0.068

0.179 2.0 −300 −0.046

0.213 1.7 −280 −0.103

spherical 605

spherical 801

spherical 770

spherical 645

spherical 770

spherical 730

spherical 680

discoidal 333

cylindrical 173

derived a little smaller value of KER: 0.24KSR ∼ 0.34KSR.18,19 Hence, eq 2 has been widely accepted for a long time, but a few studies, including our previous one,1 reported that the value of KER reduces by several orders of magnitude with a decreasing particle size.20−24 It is known that there exists a thin structured layer on the particle surface in solutions, which gives rise to the short-range repulsion between colliding particles. There are a few hypotheses for the origin of this repulsion: the layer composed of water molecules and counterions adsorbed on the surface, the structured layer made of hydrogen bonds between water molecules, the gelation layer of silica at the silica−solution interface, and the roughness of surfaces.24−29 Although the detailed structure of the layers on silica surfaces has not been clarified yet, the structured-layer repulsive potential between plates in solutions VplSL has been considered to be expressed by an exponential decay function. The simplest form of VplSL is given as follows.25 pl VSL = V0 exp( −h/λ)

prediction by eq 7 indicates that the orders of reduction of KER are originated from the short-range repulsion force because of the structured layers on the surface of silica nanoparticles. The careful observation of the data in previous papers indicates that the reduction of KER depends not only on the particle size but also on the Gibbs free energy of hydration of ions, especially at Dp ≲ 70 nm. Our attention in this study is focused on the dependence of KER on the free energy of hydration of co- and counterions and their specificity at Dp ≲ 70 nm. The molecular-scale structures of cations and anions adsorbed on the silica surface are speculated, and the characteristics of ionic specificity are discussed.

2. EXPERIMENTAL SECTION The detailed experimental conditions and procedures are given in the previous reports,1,21 but the framework is briefly described below. 2.1. Materials. Milli-Q water was used as pure water. Two series of electrolyte solutions were prepared: 4 M LiCl, NaCl, KCl, and CsCl solutions and 2 M KCl, KI, KNO3, KSCN, and KBr solutions (Wako, Japan). Physicochemical properties of ions and salts employed are given in Table 1. Spherical silica particles of 38(3.6), 55(4.8), 70(5.2), 100(5.6), 177(9.5), 287(14.6), and 410(19.8) nm in diameter (standard deviation), which were kindly supplied by JGC Catalysts and Chemicals Ltd, Japan, were employed. Their zeta potentials were measured in 0.1 mM LiCl solution of pH = 7.0 ± 0.2 by using the Zetasizer Nano ZS (Malvern Instruments, UK) and calculated to be 48.7 ± 2.0 mV by the Smoluchowski’s equation.36 The pH of the electrolyte and colloidal solutions was adjusted to be 7.0 ± 0.2 by using 0.1 M HCl solution (Wako, Japan), just before each experiment. All of the experiments were conducted in the room temperature of 25 ± 1 °C. 2.2. Apparatus and Experimental Procedure. The low-angle light-scattering apparatus with a He−Ne laser light of 632.8 nm wavelength and the scattering angle between 1.22 and 3.0° was used to detect the change of the scattered light with time. Colloidal and electrolyte solutions of equal amount were mixed instantaneously using a stopped flow system, just before entering into the scattering cell of the low-angle light-scattering apparatus. The change of intensity of the scattered light at time t, I(t), was detected by the phototube. The values of KER were determined by using the following equation derived by Lips and Willis.37

(5)

where V0 and λ are the arbitrary fitting parameters. In the previous paper, the values of V0 and λ were chosen, such that the force curve given by eq 5 satisfied the typical features of the total interaction forces between silica surfaces in highly concentrated electrolyte solutions measured by an atomic force microscope.28,30 The repulsive potential for equal spheres, 4,25 Vsph SL , is then given by using the Derjaguin approximation. sph VSL = πa

∫h

∞

pl VSL (h)dh = πaλV0 exp( −h/λ)

(6)

By taking account of the repulsion of eq 6, as well as the disaggregation probability of particles coagulated in a shallow minimum of the total interaction potential Vsph (≡Vsph T SL + sph 31 VA ), an analytical equation of the rapid coagulation rate * KSM for silica nanoparticles was derived as follows. R KRSM * = ⎡ ⎢2kT ⎢⎣

∫h

∞

3μa

sph (h))/kT ) β(h − hmin)exp((V Asph(h + 2Δ) + VSL

min

⎡ ⎛ V sph ⎞⎤ T × ⎢1 − exp⎜⎜ min ⎟⎟⎥ ⎢ kT ⎠⎥⎦ ⎝ ⎣

SCN−

(2a + h)2

⎤ dh ⎥ ⎥⎦

[I(t ) − I(0)]/I(0) = 2KREN0t

(8)

where N0 is the total number concentration of particles determined by the particle size and the dry weight of particles in suspensions. To confirm the reliability of the data, the light intensity at the scattering angle 90°, I90(t), was also measured by a dynamic light scattering spectrophotometer (DLS-8000, Otsuka Electronics), following the procedure reported elsewhere.15

(7)

where Δ is the gap between the solid surface of silica and the plane at h = 0, which is often assumed to be 0.5 nm per surface.25 The subscript, min, indicates the parameters corresponding to the point of potential minimum of Vsph T . It is found that eq 7 coincides with eq 2 at Dp ≳ 300 nm and, at the same time, predicts the orders of magnitude reduction of KER with a decreasing value of Dp at Dp ≲ 300 nm. This qualitative agreement between the experiment and the

3. RESULTS AND DISCUSSION 3.1. Effects of Counterions. Figure 1 shows the dependence of values of KER for silica nanoparticles in chloride B

DOI: 10.1021/acs.langmuir.7b04081 Langmuir XXXX, XXX, XXX−XXX

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order of Li+ > Na+ > K+ > Cs+.50 Mähler and Persson concluded by the measurements with large-angle X-ray scattering and double difference infrared spectroscopy that the lithium ion in aqueous solution is strongly hydrated, most probably with a second hydration shell present.51 DeWaltKerian et al. illustrated schematically the adsorption of Li+ and Cs+ ions on the silica surface, as shown in Figure 2a, from the

Figure 1. Dependence of the rapid coagulation rate (KER) on the Gibbs free energy of hydration (−ΔGhyd) of cations for silica nanoparticles of various sizes in 2 M chloride solutions.

solutions on the Gibbs free energy of hydration of cations (−ΔGhyd) given in Table 1. Because the negatively charged silica surface is neutralized by the adsorption of cations in a 2 M electrolyte solution, the coagulation rate must be rapid enough for the value of KER to coincide with the prediction by eq 2. The value of KER certainly agreed with that given by eq 2 at Dp ≳ 287 nm, but it decreased by the orders of magnitude with decreasing particle size at Dp ≲ 287 nm, as reported previously.1 It is important to note that this reduction of KER with (−ΔGhyd) is classified into two regions in terms of the particle size: the region of 287 ≳ Dp ≳ 100 nm where KER decreases with decreasing particle size, almost independently of (−ΔGhyd), and the region of Dp ≲ 55 nm where KER reduces by a few orders of magnitude not only with decreasing particle size but also with increasing value of (−ΔGhyd). Our interest in this study is in the reason why the exponential decay of KER with (−ΔGhyd) appears explicitly at Dp ≲ 55 nm. The ionic specificity at the solid−liquid interface, which is related with the Hofmeister sequence, has been attracting the curiosity of many scientists for a long time.38,39 Over the past decade, extensive investigations have been made using a wide variety of methodologies, but still no comprehensive mechanism to explain all of the phenomena systematically has been reported, as far as the authors know.18,34,40−43 In these reports, the specificity is considered to be closely related with “kosmotropic (structure-making)” and “chaotropic (structurebreaking)” properties of the ions in solutions. The kosmotropic and chaotropic properties are often correlated with the positive and negative values of the Jones−Dole viscosity B coefficient given in Table 1, respectively. As for the ability of the adsorption of alkaline ions on the silica surface, it follows usually the direct order of Hofmeister sequence, Cs+ > K+ > Na+ > Li+. The recent studies, however, clarified that the order of cations may be reversed by the solution pH,44 and the negative charge of silica surface may be even inversed by the excessive accumulation of chaotropic monovalent cations, such as Cs+.36,45,46 These data certainly inform us how cations accumulate on the silica surface, but no information about the thickness of adsorbed layer is given. The thickness of the structured layer at the solid−liquid interface has been speculated indirectly by macroscopic experiments.47−49 Recently, various spectroscopic methods were employed to know directly the detailed behaviors of ions in aqueous solutions. According to the study by Brown et al. with X-ray photoelectron spectroscopy, the thickness of the adsorbed layer of ions on the silica surface increases on the

Figure 2. Schematic drawings of the adsorption of structuredetermining ions. (a) Chaotropic Cs+ ions adsorb excessively on the silica surface, and the kosmotropic Li+ ions with the primary and secondary hydration shell make a thick adsorption layer to neutralize the silica surface. (b) Anions with different values of (−ΔGhyd) adsorb on the excessive potassium layer on the silica surface. (c) Nonspherical anions, which have the weak cohesive energy to potassium, make the fragile structure by adsorbing randomly on the excessive potassium layer on the silica surface.

investigation with the broadband vibrational sum frequency generation spectroscopy.52 The detailed analysis used in these studies is out of scope of this paper, but the results obtained allow us to image the correlation between the adsorption of ions and the stability of silica particles, as follows. The highly hydrated kosmotropic cation, Li+, adsorbs less, but it makes a thicker adsorbed layer than the poorly hydrated chaotropic cation, Cs+, as shown schematically in Figure 2a. The thicker the adsorbed layer is, the more stable the colloidal particles are, because the structured layer of cations generates the repulsive force between colliding particles, as speculated by eq 6. Hence, we believe that the above-mentioned reports support the trend of KER at Dp ≲ 55 nm in Figure 1: the thickened structured layer gives rise to the decrease of KER with (−ΔGhyd) of cations in the region where the particle size is small enough to be comparable with the thickness of the structured layer. 3.2. Effects of Coions. Figure 3 shows the dependence of the values of KER for the coagulation in potassium solutions on the value of (−ΔGhyd) of anions. The potassium ion is chaotropic, as shown in Table 1. Most likely, highly chaotropic cations in concentrated solutions tend to adsorb on the silica surface excessively and inverse the surface charge.36,46 Then, anions are the counterions against the silica surface coated with K+ ions. We believe this is what is happening in the present experiment. In the region of Dp ≳ 287 nm, the value of KER coincided with the value predicted by eq 2 and then reduces exponentially with the decreasing particle size at 287 nm ≳ Dp ≳ 100 nm, independently of the kind of anions, as in the case of cations. As for KI, KBr, and KCl solutions, the value of KER reduces further C

DOI: 10.1021/acs.langmuir.7b04081 Langmuir XXXX, XXX, XXX−XXX

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Figure 3. Dependence of the rapid coagulation rate (KER) on the Gibbs free energy of hydration (−ΔGhyd) of anions for silica nanoparticles of various sizes in 1 M potassium solutions. Dotted down arrows indicate that the values of KER are infinitely small.

not only with decreasing particle size but also with increasing value of (−ΔGhyd) of anions at Dp ≲ 70 nm. This trend is the same as in the case of cations, although anions are the structure-determining ions in this case. Hence, the more hydrated the anions are, the thicker the structured layer on the silica surface is, as illustrated schematically in Figure 2b. The effects of coions on the particle stability are extensively investigated by Bastos-González et al.34,53 According to their results, the negatively charged stable particles with hydrophilic surfaces were first coagulated at their critical coagulation concentration with the addition of sodium salts, but the coagulated particles were restabilized above the critical stabilization concentration. They claimed that this restabilization was caused by the structured layer on the particle surface. This restabilization because of the structured layer is essentially consistent with our mechanism for the reduction of KER because of the particle size as well as the free energy of hydration of ions, although the detailed correspondence cannot be clarified at present because of the different experimental conditions. The most striking result in the present study is that silica particles of Dp ≲ 70 nm did not coagulate at all in KNO3 and KSCN solutions, although their values of KER were nearly the same with others at Dp ≳ 100 nm, as shown in Figure 3. To confirm this puzzling result, the change of the light intensity scattered at 90°, I90(t), for 55 and 287 nm particles in 1 M solutions was measured by using the dynamic light scattering spectrophotometer. As shown in Figure 4a, all of the values of I90(t)/I90(0) for Dp = 287 nm coincide with each other, independently of the kind of anions, whereas those for Dp = 55 nm depend on the kind of anions, as shown in Figure 4b, and the order of the coagulation rate of particles in KI, KBr, and KCl solutions agrees with that in Figure 3. More importantly, no change of I90(t)/I90(0) with time is confirmed for NO3− and SCN− ions. Hence, it is certain that silica particles of Dp ≲ 70 nm are very stable in highly concentrated solutions of KNO3 and KSCN. This stabilization of colloidal particles in a concentrated KNO3 solution was already found by Healy et al. in 1978,54 and the special stability of colloids in a KSCN solution was also reported by Peula-Garcia,́ 53 although the dependence of their stability on the particle size was not discussed. We are now interested in how we can understand these results of KNO3 and KSCN solutions. According to the physicochemical data in Table 1, most properties of KNO3 and KSCN are not too different from the others, except that their

Figure 4. Time dependence of the light intensity scattered at 90°, I90(t), measured by the dynamic light scattering spectrophotometer. (a) 287 and (b) 55 nm particles in 1 M various potassium solutions.

melting temperatures are exceptionally lower than the others and the shapes of NO3− and SCN− ions are not spherical. The low melting temperature implies that the cohesive energy between potassium and anions is quite small, which may be originated from the nonsphericity of discoidal NO3− and cylindrical SCN− ions. This low cohesive energy between K+ ion and nonspherical ions is consistent with the concept of “like seeks like”55 and the Collins’ concept of matching water affinities.33 In addition, NO3− and SCN− ions tend to be strongly excluded from the bulk water toward the particle surface because of their highly chaotropic properties expected by their Jones−Dole viscosity B coefficients. This gives rise to the situation that the nonspherical ions accumulate around the particle surface, being weakly bound with the K+ layer adsorbed on the silica surface. Then, most likely the thick and fragile structure of nonspherical anions will be formed on the silica particle surface, as illustrated in Figure 2c.56 If this is the case, the fragile structures will be broken partly by the strong van der Waals attractive force in the case of large particles, whereas they will act as a strong repulsive force if particles are so small that the van der Waals force is too weak to destroy the structure. This hypothesis explains systematically the puzzling results obtained, but it must certainly be verified by further investigations.

4. CONCLUSIONS The dependence of the rapid coagulation rate of silica nanoparticles on the particle size and the kind of co- and counterions of 1:1 electrolytes were investigated by using a lowangle light-scattering apparatus, and the following conclusions are drawn. (1) The rapid coagulation rate KER of silica nanoparticles at Dp ≳ 287 nm coincides with the prediction by the modified Smoluchowski theory, but it reduces by orders of magnitude with the decreasing particle size at Dp ≲ 287 nm. This extraordinary reduction depends also on the kind of electrolytes at Dp ≲ 100 nm: the value of KER D

DOI: 10.1021/acs.langmuir.7b04081 Langmuir XXXX, XXX, XXX−XXX

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Simultaneous Static and Dynamic Light Scattering. Langmuir 1996, 12, 5541−5549. (16) Holthoff, H.; Schmitt, A.; Fernández-Barbero, A.; Borkovec, M.; Cabrerı ́zo-Vı ́lchez, M. á.; Schurtenberger, P.; Hidalgo-álvarez, R. Measurement of Absolute Coagulation Rate Constants for Colloidal Particles: Comparison of Single and Multiparticle Light Scattering Techniques. J. Colloid Interface Sci. 1997, 192, 463−470. (17) Zhou, H.; Xu, S.; Mi, L.; Sun, Z.; Qin, Y. A Study on Independently using Static and Dynamic Light Scattering Methods to Determine the Coagulation Rate. J. Chem. Phys. 2014, 141, 094302. (18) Oncsik, T.; Trefalt, G.; Borkovec, M.; Szilagyi, I. Specific Ion Effects on Particle Aggregation induced by Monovalent Salts within the Hofmeister Series. Langmuir 2015, 31, 3799−3807. (19) Ruiz-Cabello, F. J. M.; Trefalt, G.; Oncsik, T.; Szilagyi, I.; Maroni, P.; Borkovec, M. Interaction Forces and Aggregation Rates of Colloidal Latex Particles in the Presence of Monovalent Counterions. J. Phys. Chem. B 2015, 119, 8184−8193. (20) Ludwig, P.; Peschcl, G. Determination of the Coagulation Kinetics in Silica Hydrosols by Photon Correlation Spectroscopy. Prog. Colloid Polym. Sci. 1988, 76, 42−46. (21) Higashitani, K.; Kondo, M.; Hatade, S. Effect of Particle Size on Coagulation Rate of Ultrafine Colloidal Particles. J. Colloid Interface Sci. 1991, 142, 204−213. (22) Axford, S. D. T. Aggregation of Colloidal Silica: ReactionLimited Kernel, Stability Ratio and Distribution Moments. J. Chem. Soc., Faraday Trans. 1997, 93, 303−311. (23) Kobayashi, M.; Juillerat, F.; Galletto, P.; Bowen, P.; Borkovec, M. Aggregation and Charging of Colloidal Silica Particles: Effect of Particle Size. Langmuir 2005, 21, 5761−5769. (24) Škvarla, J. Quantitative Interpretation of Anomalous Coagulation Behavior of Colloidal Silica Using a Swellable Polyelectrolyte Gel Model of Electrical Double Layer. Langmuir 2013, 29, 8809− 8824. (25) Israelachvili, J. N. Intermolecular and Surface Forces; Elsevier: Amsterdam, 2011. (26) Parsons, D. F.; Boström, M.; Lo Nostroc, P.; Ninham, B. W. Hofmeister Effects: Interplay of Hydration, Nonelectrostatic Potentials and Ion Size. Phys. Chem. Chem. Phys. 2011, 13, 12352−12367. (27) Bitter, J. L.; Duncan, G. A.; Beltran-Villegas, D. J.; Fairbrother, D. H.; Bevan, M. A. Anomalous Silica Colloid Stability and Gel Layer Mediated Interactions. Langmuir 2013, 29, 8835−8844. (28) Donose, B. C.; Vakarelski, I. U.; Higashitani, K. Silica Surfaces Lubrication by Hydrated Cations Adsorption from Electrolyte Solutions. Langmuir 2005, 21, 1834−1839. (29) Parsons, D. F.; Walsh, R. B.; Craig, V. S. J. Surface Forces: Surface Roughness in Theory and Experiment. J. Chem. Phys. 2014, 140, 164701. (30) Valle-Delgado, J. J.; Molina-Bolívar, J. A.; Galisteo-González, F.; Gálvez-Ruiz, M. J.; Feiler, A.; Rutland, M. W. Hydration Forces between Silica Surfaces: Experimental Data and Predictions from Different Theories. J. Chem. Phys. 2005, 123, 034708. (31) Richmond, P.; Smith, A. L. Initial Rate Constants for Coagulation in the Presence of Energy Minima of Restricted Depth. J. Chem. Soc., Faraday Trans. 2 1975, 71, 468−473. (32) Marcus, Y. A Simple Empirical Model Describing the Thermodynamics of Hydration of Ions of Widely Varying Charges, Sizes, and Shapes. Biophys. Chem. 1994, 51, 111−127. (33) Collins, K. Ions from the Hofmeister Series and Osmolytes: Effects on Proteins in Solution and in the Crystallization Process. Methods 2004, 34, 300−311. (34) Bastos-González, D.; Pérez-Fuentes, L.; Drummond, C.; Faraudo, G. Ions at Interfaces: the Central Role of Hydration and Hydrophobicity. Curr. Opin. Colloid Interface Sci. 2016, 23, 19−28. (35) Iwanami Rikagaku Jiten. Dictionary of Physics and Chemistry, 5th ed; Iwanami: Tokyo, 1998. (36) Franks, G. V. Zeta Potentials and Yield Stresses of Silica Suspensions in Concentrated Monovalent Electrolytes: Isoelectric Point Shift and Additional Attraction. J. Colloid Interface Sci. 2002, 249, 44−51.

reduces with the increasing free energy of hydration of the structure-determining ions on the silica surface, where structure-determining ions are cations in chloride solutions and anions in potassium solutions. (2) Silica nanoparticles in 1 M KNO3 and KSCN solutions did not coagulate at all at Dp ≲ 70 nm, although they did coagulate at Dp ≳ 100 nm as in the other potassium solutions. This puzzling behavior was considered to be caused by the fragile structures formed by the weak adsorption of highly chaotropic nonspherical NO3− and SCN− ions on silica nanoparticles coated by K+ ions.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ko Higashitani: 0000-0003-4016-5320 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors thank the JGC Catalysts and Chemicals Ltd. for their kind supply of silica nanoparticles. REFERENCES

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DOI: 10.1021/acs.langmuir.7b04081 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.7b04081 Langmuir XXXX, XXX, XXX−XXX