Subscriber access provided by CORNELL UNIVERSITY LIBRARY
Article
Structures and Acidity Constants of Silver-Sulfide Complexes in Hydrothermal Fluids: A First Principles Molecular Dynamics Study Mengjia He, Xiandong Liu, Xiancai Lu, Chi Zhang, and Rucheng Wang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b08403 • Publication Date (Web): 06 Oct 2016 Downloaded from http://pubs.acs.org on October 7, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 23
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
The Journal of Physical Chemistry
1
Structures and Acidity Constants of Silver-Sulfide
2
Complexes in Hydrothermal Fluids: A First
3
Principles Molecular Dynamics Study
4
Mengjia He, Xiandong Liu*, Xiancai Lu, Chi Zhang, Rucheng Wang
5
State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and
6
Engineering, Nanjing University, Nanjing 210046, P. R. China
7
*
8
+86 25 89680700
Corresponding author. E-mail:
[email protected]; Fax: +86 25 83686016; Tel:
ACS Paragon Plus Environment
The Journal of Physical Chemistry
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
9
ABSTRACT
10
In order to quantify the speciation and structures of silver-sulfide complexes in
11
aqueous solutions, we have carried out systematic first principles molecular dynamics
12
(FPMD) simulations at three temperatures (25°C, 200°C, and 300°C). It is found that
13
mono-sulfide (i.e., Ag(HS)) and di-sulfide species (i.e., Ag(HS)2-) are the major
14
silver-sulfide species over a wide T-P range, while Ag(HS)32- can hold stably only at
15
ambient temperatures and Ag(HS)43- does not exist even at the ambient conditions.
16
Ag(H2S)+ has a tetrahedral structure up to 300°C (i.e., Ag(H2S)(H2O)3+). Ag(H2S)2+
17
remains 4-coordinated to 200°C (i.e., Ag(H2S)2(H2O)2+) but it transforms to
18
3-coordinate at 300°C (i.e., Ag(H2S)2(H2O)+). All of the other mono- and di-sulfide
19
species (Ag(HS)(H2O)0, Ag(HS)(OH)-, Ag(HS)(H2S)0, Ag(HS)2- and AgS(HS)2-) have
20
two-fold linear structures. For their solvation structures, the H2S ligands donate weak
21
H-bonds to water O; the HS- ligands accept weak H-bonds from water H; the dangling
22
S2- form strong H-bonds with H of water molecules and the OH- ligands can form
23
strong H-bonds as donors and weak H-bonds as acceptors. We further calculated the
24
acidity constants (i.e. pKas) of Ag(H2S)+ and Ag(H2S)2+ complexes using FPMD
25
based vertical energy gap method. Based on the calculated pKas, the mono- and
26
di-sulfide species distributions versus pH have been derived. We found that for
27
mono-sulfide species, Ag(HS)(H2O)0 is the major species in near neutral pH, while
28
Ag(H2S)(H2O)3+ and Ag(HS)(OH)- exist in the acid and alkaline pH range at
29
T300°C
59
absorption spectroscopy (XAS) measurements and FPMD, Pokrovski, et al.
60
proposed that the major species included AgCl2- and AgCl32- in aqueous solutions of
61
200-300°C (at least 6m Cl).
62
10
. Most of these data
suggested that the major species of silver chloride complexes
14
, Sverjensky, et al.
15
suggested that AgCl43- existed at high chloride
11
. Using in situ X-ray 12
It is suggested that silver sulfide complexes are more significant than silver 5, 16
63
chloride complexes in natural ore forming fluids
64
been paid to sulfide complexes. Treadwell and Hepenstrick 17 and Schwarzenbach and
. Surprisingly, less attention has
ACS Paragon Plus Environment
The Journal of Physical Chemistry
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
18
Page 4 of 23
suggested that, at the ambient conditions, AgHS(aq), Ag(HS)2-, and
65
Widmer
66
Ag2S(HS)22- species are dominative in acidic, neutral, and alkaline aqueous sulfide
67
solutions, respectively. Stefánsson and Seward 5 derived the same conclusions at higher
68
temperatures by calculating the solubility constant. In contrast, Ag(HS)2- was proposed
69
to be the dominant species at near-neutral to alkaline pH conditions by measuring the
70
solubility of Ag2S in 25~300°C with total sulfide between 0.2 and 1.4 m 19.
71
The molecular-level information of silver sulfide complexes in geological fluids,
72
especially at elevated T-P conditions has been poorly documented. To our knowledge,
73
these species have not been investigated by using the third-generation synchrotron
74
sources (e.g., extended x-ray absorption fine structure (EXAFS) spectroscopy).
75
Acidity constants (pKa’s) are the key thermodynamic parameters determining Ag
76
speciation because sulfide ligands (i.e., H2S, HS-, and S2-) can easily gain or lose
77
proton. However, the acidity constants of silver sulfide complexes have not been
78
reported up to now.
79
Combining electronic structures calculation with molecular dynamics sampling,
80
FPMD simulation can serve as an effective approach to investigate metal speciation in
81
aqueous solutions
82
ore-forming metal elements. The examples include Au+-HS
83
Cu+-HS--Cl-
84
Zr4+/Hf4+ monomers 27.
85
20
. FPMD has been successfully applied to many important
, Au+-HS-/OH-/S32-
22
23
, Cu+/Au+-Cl-
24-25
21
, Ag+-Cl-
complexes, Ti4+
26
11-12
,
and
Acidity constant can be accurately predicted by the FPMD based vertical energy 28-29
86
gap method developed by the Sprik group
87
molecular acids spanning over 20 pKa units, indicating an accuracy of 2 pKa units 28-31.
88
This technique has successful applications on many organic and inorganic systems
89
containing main group and transition metal elements
90
molybdic acid, arsenite, and thioarsenite species have been computed from ambient
. This method has been tested on
32-37
ACS Paragon Plus Environment
. Recently, the pKa’s of
Page 5 of 23
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
The Journal of Physical Chemistry
91
up to 300°C and an accuracy of 2 pKa units has been achieved, which demonstrates
92
that the method is valid at elevated temperatures 32, 38.
93
In this study, FPMD simulations are carried out to investigate the structures and
94
acidity constants of possible Ag+-sulfide species with S:Ag ratios ranging from 1 to 4
95
at the temperatures of 25°C, 200°C, and 300°C. The results of Ag+-HS- complexes
96
have been compared with Ag+/Au+/Cu+-Cl- and Au+/Cu+-HS- complexes. The pKa’s of
97
Ag(H2S)+ and Ag(H2S)2+ complexes are calculated using the FPMD based vertical
98
energy gap method and the distributions of mono- and di-sulfide species with respect
99
to pH have been derived.
100
2. Methodology
101
2.1 Systems
102
The simulation box is a periodically repeated cubic box with a side length of
103
12.43 Å. The numbers of water molecules are derived from the equation of state of
104
water 39, and the details of the simulated systems are listed in Table 1. The pressures
105
at 200°C and 300°C are the saturated vapor pressures.
106
Ag+-sulfide complexes investigated include possible species with S:Ag ratios
107
ranging from 1 to 4. The number of water under the three conditions was estimated
108
with N-n in AgSn systems (here N represents the number of water in Table 1, and n
109
stands for the number of S atom in Ag+-sulfide complexes).
110
Table 1. T-P conditions, water densities, and number of water molecules Density of liquid water /g·cm-3 Number of water (N)
T-P conditions
111 112
Ambient
1.0
64
200°C-1.55MPa
0.86
55
300°C-8.59MPa
0.71
45
2.2 FPMD Set-up The CP2K/QUICKSTEP package (http://www.cp2k.org)
40
was applied to carry
113
out the calculations. Density functional theory (DFT) is implemented with a mixed
114
Gaussian
and
plane
waves
approach
41
.
Goedecker-Teter-Hutter
ACS Paragon Plus Environment
(GTH)
The Journal of Physical Chemistry
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
42
Page 6 of 23
115
pseudopotentials
116
functional 43-44 was used. DZVP basis sets were used for O, H, S and Ag. A cutoff of
117
280 Ry was set for the electron density. Born-Oppenheimer molecular dynamics
118
(BOMD) simulations were performed with a Nóse–Hoover thermostat
119
step of 0.5 fs. For each system, the production simulation was run for 8.0~20.0 ps
120
following an equilibration for at least 4.0 ps.
121
2.3 pKa calculations
122
were used to avoid the core state calculation. The BLYP density
45
and a time
To calculate the acidity constant, the half-reaction scheme of the vertical energy 28-30
. With this method, a proton H+ of the acid AH is
123
gap method was applied
124
gradually transformed to a "dummy" atom (i.e., a classical particle having no charge).
125
The free energy (denoted as ∆dp °) of the transformation is expressed with the
126
thermodynamic integration relation,
127
∆dp ° = η〈∆dp 〉
(1)
128
The vertical energy gap ∆dp is defined as the difference of potential energies of the
129
reactant state and the product state, which is calculated by analyzing the MD
130
trajectories. is a coupling parameter, increasing from 0 to 1 (i.e. from the
131
protonated state to the deprotonated state). The subscript is a reminder meaning
132
that the averages are evaluated over the restraints, as explained in the following.
133 134 135
The auxiliary restraining potential is used to keep the dummy in a location which resembles that of the acid proton of the reactant state:
= ∑bonds ( − ) + ∑angles ( − )
(2)
136
where d0 and α0 are the equilibrium values for the bonding and angle bending of the
137
proton in the protonated state, respectively. These values are determined from the free
138
FPMD simulations according to the prescriptions of previous studies
139
the are tabulated in Table 2.
28, 30
. Details of
140 141
Table 2. Acids computed and values of the parameters restraining the dummy for the
142
harmonic potentials (eqn (2)) are tabulated. is the number of restrained bonds,
ACS Paragon Plus Environment
Page 7 of 23
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
The Journal of Physical Chemistry
143
is the equilibrium bond distance, is the coupling constant for bond-restraint.
144
is the number of restrained angles, is the equilibrium angle, is the coupling
145
constant for angle-restraint. Hd means the dummy proton. Equilibrium bond distances
146
are in Bohr, angles are in radians, and coupling constants are in a.u. Acid
nd
d0
Ag(HHdS)(H2S)-
1
2.60
kd
nα
0.1
2
α0
kα
1.65 (Ag-S-Hd) 0.1 1.63 (H-S-Hd)
Ag(HHdS)(HS)0
1.68 (Ag-S-Hd) 1
2.59
0.1
2
0.1 1.62 (H-S-Hd)
Ag(HdS)(HS)-
1
2.57
0.1
1
Ag(HHdS)(H2O) +
1
2.55
0.1
2
1.68(Ag-S-Hd)
0.1
1.68 (Ag-S-Hd) 0.1 1.62 (H-S-Hd)
Ag(HHdO)(HS)0
2.02 (Ag-O-Hd) 1
1.89
0.1
0.1
2 1.92 (H-O-Hd)
Ag(HdS)(H2O)0
1
2.40
0.1
1
1.70(Ag-S-Hd)
0.1
147 148 149 150 151 152
The same procedure is applied to transform one proton of a hydronium into a
dummy. The free energy of this process is derived as d〈∆dp H3 O+ 〉 . In practice, we apply the three-point (TP) Simpson rule to calculate the integral (Eqn (1)), which needs simulations at =0, 0.5, and 1, separately:
∆dp = $ (〈∆ 〉 + 〈∆ 〉 ) + % 〈∆ 〉.'
(3)
153
The deprotonation free energy values for hydronium were taken from our previous
154
work
155
respectively.
32
, which are 15.35 eV, 14.76 eV, and 14.11 eV at 25°C, 200°C, and 300°C,
156
The final formula of pKa is given by:
157
2.30* T pK a = η〈∆dp 〉 − d〈∆-. /0 12 〉 + * Tln5c°Λ% 2 8
(4)
158
where c°=1 mol/L is the unit molar concentration, ΛH+ is the thermal wavelength of
159
proton 29. The third term takes the translational entropy from the acid dissociation into
ACS Paragon Plus Environment
The Journal of Physical Chemistry
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
160
account. It is approximated by the free energy of a free proton at standard
161
concentration, equivalently -3.2 pKa units 29.
162
3. Results and discussion
163
3.1 Structures
164
3.1.1 mono-sulfide species
165
Representative snapshots of Ag(H2S)+ species at the three conditions are
166
displayed in Fig. 1. It is observed that the RDF peaks and CN curves of Ag-O denote
167
3 water molecules around Ag+ (Fig. 2a) and the complex has a tetrahedral structure
168
(i.e., Ag(H2S)(H2O)3+) (Fig. 1abc). All of the RDF curves show no peaks in Fig. 2b at
169
high and low temperatures, which implies that S atom of H2S does not form H-bonds
170
with solvating water, in line with Fig. 1abc. At 25°C, the peaks of RDF and the
171
plateaus of CN for HH2S-Owater (Fig. 2c) show that the H atom of H2S ligand forms one
172
H-bond with water, but the H-bond can not hold as temperature increases (Fig. 1abc).
173
Upon proton dissociation, Ag(H2S)+ becomes linear (i.e., Ag(HS)(H2O)0) as
174
shown in Fig. 1def. The RDF and CN curves for Ag-OH2O suggest that Ag+ bonds with
175
one water molecules at all temperatures (Fig. 2d and Fig. 1def). The S atom of HS-
176
ligand forms three H-bonds with water H at the ambient conditions, and the average
177
number of the H-bonds decreases to two at higher temperatures (Fig. 2e and Fig.
178
1def). The H atom of HS- ligand forms no obvious H-bonds at the three temperatures
179
(Fig. 2f and Fig. 1def). The linear structure of Ag(HS)(H2O)0 is similar with Au(HS)0
180
21
181
and CuCl(H2O)0).
, Cu(HS)0
22
, AuCl0 and CuCl0
11
(i.e., Au(HS)(H2O)0, Cu(HS)(H2O)0, AuCl(H2O)0
182
There are two possible acid dissociation products for Ag(HS)(H2O)0 (i.e.,
183
Ag(HS)(OH)- and AgS(H2O)-). The latter is ruled out by the pKa results below (in
184
section 3.2). Akin to Ag(HS)(H2O)0, Ag(HS)(OH)- has a linear structure (Fig. 1ghi),
185
where Ag+ has no direct interaction with water molecules as shown on the RDF
186
curves (Fig. 2g). The O atom of OH- ligand forms 2 H-bonds with water H at all
187
temperatures (Fig. 2h and Fig. 1ghi). Similarly with Fig. 2c, the RDF and CN curves
ACS Paragon Plus Environment
Page 8 of 23
Page 9 of 23
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
The Journal of Physical Chemistry
188
show that the H atom of OH- ligand attracts an O atom from solvating water forming
189
an H-bond at 25°C, and the H-bond breaks down at high temperatures (Fig. 2i and Fig.
190
1ghi).
191 192
Fig. 1 Snapshots of Ag(H2S)+ , Ag(HS)(H2O)0, and Ag(HS)(OH)-. Some water
193
molecules have been removed for clarity.
ACS Paragon Plus Environment
The Journal of Physical Chemistry
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
194 195
Fig. 2 RDFs and CNs derived from the trajectories of Ag(H2S)+ , Ag(HS)(H2O)0, and
196
Ag(HS)(OH)-.
197
3.1.2 di-sulfide species
198
The simulations show that Ag(H2S)2+, Ag(HS)(H2S)0, and Ag(HS)2- are stable at
199
the three conditions (see snapshots in Fig. 3). The RDF peaks of Ag-O (around 2.5 Å)
200
and the CN plateaus for Ag(H2S)2+ (Fig. 4a) indicate that Ag+ has 2 H2O ligands up to
201
200°C and 1 H2O at 300°C. Thus Ag(H2S)2+ has a tetrahedral structure (i.e.,
202
Ag(H2S)2(H2O)2+) at T
