Specific Anion Effects for Aggregation of Colloidal Minerals: A Joint

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Specific Anion Effects for the Aggregation of Colloidal Minerals: A Joint Experimental and Theoretical Study Rui Tian, Gang Yang, Chang Zhu, Xinmin Liu, and Hang Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512078v • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Specific Anion Effects for the Aggregation of Colloidal Minerals: A Joint Experimental and Theoretical Study Rui Tiana, b, Gang Yanga,*, Chang Zhua, Xinmin Liua,b, Hang Lia,* a

College of Resources and Environment & Chongqing Key Laboratory of Soil

Multi-scale Interfacial Process, Southwest University, Chongqing 400715, China b

College of Chemistry and Chemical Engineering, Southwest University, Chongqing

400715, China * To whom correspondence should be addressed: E-mails: [email protected]; [email protected]; Phone: 086-023-68251504; Fax: 086-023-68250444.

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Abstract: In this work, dynamic light scattering experiments and density functional calculations were combined to demonstrate the specific anion effects for the aggregation of negatively charged colloidal minerals. Although the aggregation kinetics is dominated by electrolyte cations, anions also play a significant role. The critical coagulation concentrations and activation energies indicated a clear Hofmeister series for the various anions, as H2PO4- < Cl- < NO3- < SO42- < HPO42- < PO43-. Moreover, interaction energies of anions with cations and proton affinities of anions were explored as the influencing factors for anion specificities, which were corroborated by measurement of surface charge densities. Owing to the largest interactions with cations, PO43- causes the most pronounced inhibition effect to the aggregation kinetics and corresponds to the strongest anion specificity. Proton exchange from H2PO4- reduces the negative charges of minerals and accelerates the aggregation process, thus resulting in the inferior anion specificity than NO3-. Density functional calculations indicated that proton transfer from minerals to OH- can occur facilely and increase the negative charges of minerals, as confirmed by charge density measurements and dilution experiments. This further adds the aggregation difficulty and causes OH- to show distinctly stronger anion specificity than other univalent anions. Keywords: Hofmeister series; aggregation kinetics; activation energy; density functional calculations; proton transfer


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1. Introduction Aggregation and dissociation of colloidal and mineral particles are a common occurrence while attract the attention not only from chemistry but also from a variety of other disciplines.1-6 For example, the transport of metal ions by colloidal minerals can result in serious environmental concerns, such as alkaline soils and heavy metal pollutions. Extensive studies have been performed on their aggregation kinetics, which is known to be affected by electrolyte concentration, ion type, pH, surface charge, hydration or/and interaction forces.5,7-11 Among these factors, interaction forces play a significant role and have been described within the framework of the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory.12 Although the DLVO theory that underpins colloidal science remained unchallenged for nearly half a century, more and more researchers indicated that it is flawed and often conflicts with the experimental observations13-15, and such conflictions were assumed to be due to the improper account of Hofmeister (or specific ion) effects.14,16 Specific ion effects, whose significance is believed to be no less than Gregor Mendel’s work to genetics17, are closely associated with a variety of chemical and biological processes, such as surface tension, colloidal stability, chromatographic selectivity, protein folding and enzyme activity.14,17-21 To date, substantial progresses have been made on the understanding of ion specificities implicated during the aggregation of charged particles, such as proteins, membranes, minerals and colloids.20-25 A systematic study of the effects of different anions on protein interaction revealed the presence of Hofmeister series26, which was supported by the results of Gouvea et al.27 that the activation efficiency of protease follows the order of citrate3- > SO42- > Cl-. Inconsistent Hofmeister series were reported by other groups, however. The size exclusion chromatography experiments showed that SCN3 ACS Paragon Plus Environment


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accelerates the aggregation of monoclonal antibodies while SO42- has an opposite action.28 As demonstrated by Merenbloom and collaborators29, divalent anions may be more efficient to alter the protein conformations than univalent anions, while Möller et al.30 stated that SO42- rather than PO43- has a more pronounced effect on protein interaction. In the previous work, we demonstrated that electrolyte cations play a major role during the aggregation processes of negatively charged colloidal minerals.10 Do electrolyte anions also participate in the aggregation processes? If so, are there specific anion effects for the various anions, and how to characterize such anion specificities? In this work, these issues will be tackled by use of dynamic light scattering (DLS) technique10,31,32, and a clear Hofmeister series was found for the various electrolyte anions. Although specific ion effects are significant and ubiquitous, related mechanisms remain largely unclear.13,17,19,33,34 Density functional calculations were then combined with experiments with aim to unravel the mechanism of specific anion effects as well as the influencing factors. Meanwhile, we discussed the specific anion effects for H2PO4--like anions that may exchange protons with colloidal minerals and cause disorder to Hofmeister series. Finally, the distinct anion specificity of OH- that was detected for the first time was demonstrated by the conjunction of density functional calculations, surface charge density measurements and dilution experiments.

2. Materials and methods 2.1. Preparation of colloidal minerals Particles of colloidal minerals were extracted from yellow earth soils. According to the combined method proposed by Li et al.35, the surface charge density at neutral 4 ACS Paragon Plus Environment

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pH is around 0.289 C/m2. The colloidal minerals were prepared following the procedures32,36: Firstly, the samples were air-dried, screened through the 0.25 mm sieves, and 50 g were then transferred into the beaker. Secondly, 500 mL ultrapure water was added to the samples, and the pH was adjusted to pH = 8.5 by 10 mmol/L KOH solutions. Thirdly, colloidal minerals were dispersed by the probe-type ultrasonic homogenizer (Scientz-IID, Ningbo, China), and then diluted to 5000 mL with ultrapure water. Fourthly, colloidal minerals with diameter < 200 nm were collected using the static sedimentation means.36,37 As determined by the oven drying method, the concentration of colloidal minerals equals 0.88 g/L. Finally, the colloidal minerals were further diluted 10 times and adjusted to pH = 8.5 with 10 mmol/L KOH solutions, which are now ready for dynamic light scattering (DLS) experiments. The choice of pH = 8.5 was validated in the Supporting Information (S1. Choice of pH values for colloidal minerals). 2.2. Dynamic light scattering (DLS) measurements DLS measurements were performed with the BI-200SM multi-angle laser light scattering instrument (Brookhaven Instruments Corporation, New York, USA) that employs a BI-9000AT auto-correlator operating at a vertically polarized wavelength of 532 nm.10, 32 The typical volume of colloidal minerals was 5 mL per sample. The samples were each pretreated with a 2 min sonication and then electrolyte solutions with different concentrations and anions were respectively added. The electrolyte solutions containing colloidal minerals range within 10 ~ 150 mmol/L for KNO3, 10 ~ 150 mmol/L for KCl, 5 ~ 200 mmol/L for K2SO4, 50 ~ 250 mmol/L for K3PO4, 5 ~ 100 mmol/L for KH2PO4, 80 ~ 500 mmol/L for K2HPO4, 20 ~ 200 mmol/L for NaNO3, 20 ~ 200 mmol/L for NaCl, 10 ~ 200 mmol/L for Na2SO4, and 50 ~ 500 mmol/L for Na3PO4, respectively. The hydrodynamic diameters of colloidal minerals were 5 ACS Paragon Plus Environment


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recorded every 30 sec with the DLS technique, at a scattering angle of 90° and temperate of 298 ± 0.5 K. Three parallel DLS experiments were performed for each sample, and the reproducibility of obtained data was stated in the Supporting Information (S2. Reproducibility of experimental results). 2.3. Aggregation kinetics For the aggregation of colloidal minerals, the total average aggregation (TAA) ~

rate (vT(c0), nm/min) from the beginning (t = 0) to a given time t = t0 (t0 > 0) can be expressed as,32

1 t0 1 t0 D(t ) − D0 v~T (c0 ) = ∫ v~(t, c0 )dt = ∫ dt t0 0 t0 0 t

(1)

where c0 (mmol/L) is the electrolyte concentration, v~(t, c0) (nm/min) is the TAA rate from t = 0 to an arbitrary time t (t > 0), whereas D(t) and D0 (nm) are hydrodynamic diameters at time t (t > 0) and at the beginning (t = 0), respectively. According to the previous results10,32, the TAA rates increase linearly with electrolyte concentrations at first and then reach the platform; that is, the TAA rates at low and high concentrations are respectively represented by two lines. The point of intersection at these two lines is critical coagulation concentration (CCC), which stands for a quantitative indicator for colloidal stability.32 The TAA rates are further correlated with the activation energies ∆E(c0) (J/mol) for the aggregation processes,10 − v~T (c0 ) = K ⋅ c0 ⋅ e

v~T (c0 ) = K ⋅ e with

K=

∆E ( c0 ) RT

∆E ( c0 ) − RT

~v (CCC )/CCC T ~v (CCC ) T

(c0 ≤ CCC ) (c0 ≥ CCC )

(2)

(c0 ≤ CCC ) (c0 ≥ CCC )

(3)

where R (8.314 J/mol/K) and T (298 K) are gas constant and absolute temperature, 6 ACS Paragon Plus Environment

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respectively. K can be regarded as a constant, whether the electrolyte concentrations are below or above CCC.

3. Computational details Cluster models of minerals were constructed from kaolinite, one of the constituents of yellow earth soils. Kaolinite is a layered mineral consisting of alternative alumina octahedral sheet and silicate tetrahedral sheet, and its chemical formula in the neutral form is Al2Si2O5(OH)4. In this work, the cluster models of kaolinite mineral contain 6 Si and 6 Al atoms, with the Si/Al ratio of 1 (Figure 1). Generally, the deprotonation/protonation processes in minerals such as kaolinite can occur at two sites: (1) The hexagonal ring of silica surface that has been assumed to adsorb cations including proton (H+)38, see Figure 1a; (2) The O atom of the Al(O2H)Al link whose deprotonation/protonation behavior is dependent on the pH values of aqueous solutions, see Figure 1b.39-41 The cluster models of kaolinite mineral were divided into two regions and the high-level regions were displayed as ball

and

stick,

which

are

consistent

with

the

previous

treatments

of

aluminosilicates.42-46

Si3 Al4

Si4 Si5

Si2 O1 Al2 Si1 Al1

O2 Si6

Al2

Al6

(a) The hexagonal ring of silica surface

Al1

(b) Al(O2H)Al

Figure 1. Cluster models representing the local structures of kaolinite mineral. The 7 ACS Paragon Plus Environment


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high-level regions are shown as ball and sticks while the low-level regions as stick. All calculations were carried out by use of Gaussian09 software packages.47 In agreement with our previous studies42-43,48, the high- and low-level regions of kaolinite mineral were respectively described by B3LYP/6-31+G(d,p) and B3LYP/ 3-21G methods. The B3LYP/6-31+G(d,p) method was also used for adsorbents on kaolinite minerals as well as acidic, basic, water molecules and their interaction structures with metal ions (e.g., H3PO4, H2PO4- and NaH2PO4). The vibrational analyses were performed at the same theoretical level of geometry optimizations, and transition states were verified by a single imaginary frequency corresponding to the eigenvector along the reaction path. The interaction energy of metal ions (M) with anionic speices (X, which can be basic molecules or minerals) is written as, Eint = E(M-X) – E(M) – E(X)

(4)

where E(M), E(X) and E(M-X) are energies of metal ion, anionic species and their interacted structures, respectively. Different molecules or ions (H-X) have diverse abilities to donate the protons and this can be assesed by the use of proton affinity (PA), PA = E(X) – E(H-X)

(5)

where X is the deprotonated form of H-X. The lower PA, the higher acidity for H-X.

4. Results and discussions 4.1. Observation of specific anion effects The time-evolution hydrodynamic diameters of colloidal minerals have been measured in NaNO3, NaCl, KNO3 and KCl solutions with a wide range of 8 ACS Paragon Plus Environment

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concentrations, see Figures S16 ~ S20. As stated in the supporting information (S3. Size distribution characteristics of colloidal aggregates), at any specified time (t), the colloidal particles in electrolyte solutions have a size distribution; nonetheless, the hydrodynamic diameters that respond promptly as the aggregation proceeds are sufficient to describe the aggregation kinetics.10,49-52 According to the results of ~

hydrodynamic diameters, the correlations between the TAA rate vT(c0) and electrolyte concentration c0 are established and shown in Figure 2 and Table S1. Error analyses are discussed in the Supporting Information (S2. Reproducibility of experimental results), and the error bars of TAA rates based on three parallel DLS experiments fall within the size of markers (Figure S9), indicating that the results are accurate and have good reproducibility. The CCC values for NaNO3 and NaCl solutions are rather close (71.1 vs. 69.3 mmol/L) while those of KNO3 and KCl solutions are consistent with each other (34.9 vs. 34.4 mmol/L). The similar aggregation behaviors of colloidal minerals in two K+ (or Na+) solutions are further verified by the close size distributions shown in Figure S20. Then we turn to the activation energies ∆E(c0) that can give an accurate and quantitative description of ion specificity,10 see Table 1 and Figure 3. The activation energies above CCC are approximately zero (i.e., ∆E(c0) ≈ 0 for c0 ≥ CCC), for the TAA rates have already reached the plateau. At 15.0 mmol/L, the activation energies are respectively equal to 1.66RT, 1.52RT, 0.30RT and 0.28RT for NaNO3, NaCl, KNO3 and KCl solutions, where the values of Na+ are approximately 4.5 times as those of K+. It explicitly indicates that ∆E(c0) values are not affected much by the choice of anions (KNO3 vs. KCl or NaNO3 vs. NaCl) but remarkably by the choice of cations (KNO3 vs. NaNO3 or KCl vs. NaCl). Consistent results can be obtained by analysis of any other concentration below CCC. Accordingly, Hofmeister series is Na+ < K+ and electrolyte cations rather anions play 9 ACS Paragon Plus Environment


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a major role during the aggregation of colloidal minerals, in line with the fact that negatively charged colloidal minerals prefer to interact with electrolyte cations instead of anions.10 Table 1. Activation energies ∆E(c0) for the aggregation of colloidal minerals in NaNO3, NaCl, KNO3 and KCl solutions a Electrolyte solutions (CCC)

a

∆E(c0) = (c0 ≤ CCC)

KNO3 (34.9)

−RTln(−6.919/c0+1.198)

KCl (34.4)

−RTln(−6.588/c0+1.192)

NaNO3 (71.1)

−RTln(−15.38/c0+1.216)

NaCl (69.3)

−RTln(−14.93/c0+1.215)

Units of ∆E(c0) and CCC are RT and mmol/L, respectively.

90

90 NaNO3

75 60

60

45

45

30

30

15 0

15 71.1 0

40 80 120 160 200

90

0

69.3 0

40 80 120 160 200

90 KNO3

75

60

45

45

30

30

15

15

34.9 0

KCl

75

60

0

NaCl

75

30 60 90 120 150

0

34.4 0

30 60 90 120 150

Electrolyte Concentration c0 (mmol/L) ~

Figure 2. Changes of the TAA rates vT(c0) for the aggregation of colloidal minerals in NaNO3, NaCl, KNO3 and KCl solutions. 10 ACS Paragon Plus Environment

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5

Activation energy ∆E(c0) (RT)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NaNO3 4

NaCl KNO3

3

KCl

2 1 0 15

30

45

60

75

Electrolyte concentration c0 (mmol/L) Figure 3. Activation energies ∆E(c0) for the aggregation of colloidal minerals in KNO3, KCl, NaNO3 and NaCl solutions.

The respective roles of electrolyte cations and anions during the aggregation of colloidal minerals are further investigated. Figures S21 ~ S23, Figure 4 and Table S1 show the hydrodynamic diameters and TAA rates in Na3PO4, K2HPO4 and K3PO4 solutions. The CCC values in these solutions amount to 203.1, 173.6 and 149.3 mmol/L, respectively. It demonstrates that Hofmeister effects decrease as K+ > Na+ and agrees with the above and previous results.10, 53 As indicated in Table S2 and Figure 5A, for a given electrolyte concentration c0 below CCC, the activation energies of K3PO4 are less than those of Na3PO4 and abide by the Hofmeister series (K+ > Na+). Unfortunately, when taking K2HPO4 into consideration, a dilemma arises because at c0 < 96.6 mmol/L, the activation energies of Na3PO4 rather than K2HPO4 are smaller and this suggests an opposite Hofmeister series. The emergence of two conflicting Hofmeister series is due to that the cation concentrations (f0) in these electrolyte solutions are not identical to each other. The cation concentrations (f0) in Na3PO4, 11 ACS Paragon Plus Environment


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K2HPO4 and K3PO4 solutions are three, two and three molar equivalents as the electrolyte concentrations (c0). As electrolyte cations dominate the aggregation kinetics, the activation energies are also calculated against the cation concentrations (f0) as shown in Table 2 and Figure 5B. Based on the cation concentrations (f0), the activation energies ∆E(f0) below CCCC (critical coagulation concentration for electrolyte cations) always increase in the order of K3PO4 < K2HPO4 < Na3PO4 and thus we arrive to a consistent Hofmeister series (K+ > Na+).10,53 Meanwhile, the major role of electrolyte cations during the aggregation of colloidal minerals has been corroborated. The cation concentrations (f0) instead of electrolyte concentrations (c0) will be discussed afterwards unless otherwise noted.

Table 2. Expressions of activation energies ∆E(f0) for the aggregation of colloidal minerals in Na3PO4, K2HPO4, K3PO4 and KH2HPO4 solutionsa

a

Electrolyte solutions (CCCC)

∆E(f0) = (f0 ≤ CCCC)

Na3PO4 (609.3)

−RTln(−254.3/f0+1.417)

K2HPO4 (347.2)

−RTln(−199.1/f0+1.573)

K3PO4 (447.9)

−RTln(−277.3/f0+1.619)

KH2PO4 (22.6)

−RTln(−3.201/f0+1.142)

Units of ∆E(f0) and CCCC are RT and mmol/L, respectively.


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90

90

Na3PO4

75

K2HPO4

75

60

60

45

45

30

30

15

15

203.1

173.6

0

0 0

100

200

300

400

500

90

0

100

200

300

400

500

90

K3PO4

75

KH2PO4

75

60

60

45

45

30

30

15

15

149.3

22.6

0

0 0

50

100

150

200

250

0

20

40

60

80

100

Electrolyte concentration c0 (mmol/L) ~

Figure 4. Changes of the TAA rate vT(c0) along with electrolyte concentration c0 for aggregation of colloidal minerals in Na3PO4, K2HPO4 and K3PO4 solutions. 5

(A)

4

Na3PO4 K2HPO4

3

K3PO4

2 1

96.6

0 60

90

120

150

180

210

Electrolyte concentration c0 (mmol/L)

Activation energy ∆E(f0) (RT)

5

Activation energy ∆E(c0) (RT)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(B)

4

Na3PO4 K2HPO4

3

K3PO4

2 1 0 100

200

300

400

500

600

Cation concentration f0 (mmol/L) 13

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Figure 5. Activation energies vs. the concentrations of electrolytes (A) and cations (B) during the aggregation of colloidal minerals in Na3PO4, K2HPO4 and K3PO4 solutions.

Albeit electrolyte cations dominate the aggregation of colloidal minerals, it is apparent that the choice of different anions can affect the CCCC and ∆E(f0) values, which further alters the specific ion effects. As stated above, the activation energies are respectively equal to 1.66RT and 1.52RT for 15.0 mmol/L NaNO3 and NaCl solutions, and their difference (0.14RT) should be caused by the two different univalent anions. Electrolyte anions of various valences may exert a more pronounced effect. For example, at 180.0 mmol/L, the ∆E(f0) values corresponding to KNO3, K2HPO4 and K3PO4 solutions are calculated to be 0, 0.77RT and 2.54RT, respectively. That is, the specific anion effects that have been observed in other systems should also exist in the aggregation processes of negatively charged colloidal minerals that are dominated by electrolyte cations. 4.2. Hofmeister series and influencing factors Firstly, the Hofmeister series and specific anion effects are demonstrated regarding to the aggregation of colloidal minerals. The time-variation hydrodynamic diameters and TAA rates of colloidal minerals in Na2SO4 and K2SO4 solutions are shown in Figures S24 ∼ S26 and Table S1. The CCCC values in Na2SO4 and K2SO4 solutions are equal to 98.2 and 65.2 mmol/L, respectively (Table 3). The CCCC values of K2SO4 and Na2SO4 are approximately 1.9 and 1.4 times as those of electrolytes with univalent anions (KCl, KNO3 and NaCl, NaNO3), see Table 1. Accordingly, as compared to the univalent anions (Cl- and NO3-), SO42- exhibits a more efficient retardation to the aggregation kinetics. PO43- is trivalent and the CCCC values of K3PO4 and Na3PO4 are 12.9 and 8.7 times as those of KCl (KNO3) and NaCl (NaNO3), respectively (Tables 1 and 2). That is, an even more pronounced inhibition effect to 14 ACS Paragon Plus Environment

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the aggregation progress has been detected in the case of PO43-, and it is speculated that higher-valent anions result in stronger ion specificities. Table 3. Expressions of activation energy ∆E(f0) for the aggregation of colloidal minerals in K2SO4 and Na2SO4 solutionsa

a

Electrolyte solutions (CCCC)

∆E(f0) = (f0 ≤ CCCC)

Na2SO4 (98.2)

−RTln(−17.27/f0+1.176)

K2SO4 (65.2)

−RTln(−9.053/c0+1.139)

Units of ∆E(f0) and CCCC are RT and mmol/L, respectively.

The activation energies ∆E(f0) for the aggregation of colloidal minerals in Table 3 and Figure 6 indicate that for a given electrolyte cation, PO43- has a much larger ∆E(f0) value than SO42- and therefore corresponds to obviously stronger specific anion effects, which is consistent with the CCCC results. The activation energies ∆E(f0) below CCCC change in the order of Cl- < NO3- < SO42- < PO43- (Figures 3 and 6). Both anionic species and negatively charged colloidal minerals can interact with metal ions, and so they should compete with each other in this regard. Metal ions can be anchored more tightly by higher-valent anions because of stronger electrostatic and polarization interactions, which are substantialized by density functional calculations. The calculated interaction energies with Na+ (Eint) increase as NO3- (−546.7 kJ mol-1) ≈ Cl- (−546.8 kJ mol-1)

Specific Anion Effects for Aggregation of Colloidal Minerals: A Joint

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