Langmuir 1996, 12, 3503-3510
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Water Adsorption at Pyrogenic Silica Surfaces Modified by Phosphorus Compounds V. V. Turov, V. M. Gun’ko,* V. I. Zarko, V. M. Bogatyr’ov, V. V. Dudnik, and A. A. Chuiko Institute of Surface Chemistry, 31 Prospect Nauki, Kiev 252022, Ukraine Received July 17, 1995. In Final Form: March 21, 1996X
Water adsorbed on pyrogenic silica modified by phosphorus chlorides, which were then hydrolyzed, was studied by 1H NMR, dielectric, and IR spectroscopies and quantum chemical ab initio and semiempirical AM1 and MNDO/H methods. Hydrophosphinic groups (HPA) bound to pyrogenic silica surfaces are stable at the adsorbed water content Cw e 12 wt %, which corresponds to water adsorption at standard conditions. Increase of the adsorbed water content leads to hydrolysis of the tSisOsP bonds via nucleophilic substitution SNi(Si) mechanism. Upon heating, the adsorbed phosphinic acid can cleave the tSisOsSit bridges and form new sites responsible for changes of structure of the adsorbed water clusters and chemical shift values for the 1H NMR spectra. Great clusters of the adsorbed water molecules, which form at Cw > 12 wt %, are localized near ionized bound hydrophosphinic groups or adsorbed acid molecules. For air/H2O/HPA/ SiO2, the adsorption energy of water decreases if the water content increases. The dielectric and 1H NMR spectra of H2O/HPA/SiO2 have significant distinctions in comparison with those for the parent pyrogenic silica, and these differences depend not only on the adsorbed water content but also on reaction temperature and other treatment parameters.
Introduction One method for the modification of dispersed silica is its interaction with phosphorus chlorides and oxychlorides.1-3 Surface compounds of phosphorus chlorides are hydrolyzable, and in the presence of water, they readily form chemisorbed derivatives of phosphorus oxyacids, which can be chemically bound to the surface via reaction with tSiOH groups or SisOsSi bridges. Then, those oxyacids bound to the surface can transform via hydrolysis and elimination of free acid molecules in a surface layer.4 Since phosphorus oxyacids can form donor-acceptor complexes with Si from tSiOH groups or cleave the siloxane bonds under some conditions, a few reactions may occur in the adsorption layers of the silica particles.5,6 The structure of the phosphorus compounds adsorbed on the silica surface depends on the temperature and adsorbed water content. A study of the adsorption layers on hydrated, dispersed particles by experimental and theoretical methods is of interest for understanding these processes. The NMR method is widely used for study of structures of the active surface sites and adsorption complexes on silica.7-11 The separated signals of the tSiOH and dSi(OH)2 groups at the silica surface were observed by the * Author to whom correspondence should be addressed. Fax, 380 044 264 0446; tel, 380 044 265 6731; e-mail, lena%
[email protected]. X Abstract published in Advance ACS Abstracts, June 15, 1996. (1) Koltsov, S. I.; Volkov, A. K.; Aleskovskiy, V. B. Zh. Fiz. Khim. 1970, 44, 2246. (2) Negievich, L. A.; Vinogradova, A. S.; Kachan, A. A. Ukr. Khim. Zh. 1976, 42, 1109. (3) Pavlov, V. V.; Tertykh, V. A.; Chuiko, A. A.; Bogatyrev, V. M. Dokl. Akad. Nauk Ukr. SSR, Ser. B 1979, N8, 639. (4) Bernstein, T.; Fink, P.; Mastikhin, V. M.; Subin, A. A. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1879. (5) Laskorin, B. N.; Strelko, V. V.; Strazesko, D. N.; Denisov, V. I. Sorbents on the Basis of Silica Gel; Atomizdat: Moscow, 1977. (6) Mitsyuk, B. M. Zh. Neorg. Khim. 1972, 42, 903. (7) Lippmaa, E. T.; Samoson, A. V.; Brei, V. V.; Gorlov, Yu. I. Dokl. Akad. Nauk USSR 1981, 259, 403. (8) Morrow, B. A.; Gay, I. D. J. Phys. Chem. 1988, 92, 5569. (9) Cruz, M. I.; Stone, W. E. E.; Fripiat, I. J. J. Phys. Chem. 1972, 76, 3078. (10) Piculell, L. J. Chem. Soc., Faraday Trans. 1 1986, 82, 387.
S0743-7463(95)00592-0 CCC: $12.00
29 Si NMR high-resolution method.7,8 The translational and rotational mobility of the adsorbed molecules of different natures was studied by measurement of the longitudinal and cross relaxation of the adsorbed compounds.9,10 Adsorbed phosphorus compound structure may also be studied by the 31P NMR method.4,11 In addition, the 1H NMR method allows us to investigate the adsorbed water layer characteristics and their influence on the surface groups and other adsorbed compounds.
Experimental Section The NMR spectra were recorded by a WP-100 SY (Bruker). Chemical shifts (δ) were determined relative to tetramethylsilane with a mean error of (0.2 ppm. The 1H NMR spectra of SiO2/ HPA/H2O (HPA ) hydrophosphinic acid) were obtained when the bandwidth of the NMR spectrometer was under 50 kHz for the separation of the signals of the adsorbed molecules, since superposition of the 1H signals of solids, which have greater width than the bandwidth of the spectrometer, does not influence the intensity and shape of the signals of these molecules. Dielectric Spectroscopy (DS) Method.12 Water adsorption on the patterns was performed at 300 K with mean errors near (0.01 g of water/g of oxide. The dielectric characteristics were measured by a Q-meter VM-560 (“Tesla”) at 0.15, 0.25, 0.45, 0.8, 1.3, 3.0, 8.0, and 9.0 MHz at T ) 100-300 K. The measurements of dielectric permittivity ′ and dielectric loss ′′ were taken by a thermochamber with programmed temperature changes. The heating rate β was equal to 0.05 K/s with relative mean errors ∆β ) (5%. The IR spectra (UR-20 spectrophotometer, Germany) of parent silica and SiO2/HPA show that the νSiO-H stretch band at 3750 cm-1 is absent for the modified oxide, independent of water content (Figure 1). The band of P-H at 2490 cm-1 is observed against the background of absorption of water molecules. The excessive adsorbed PCl3 content gives practical substitution of all surface OH groups, but as a result, dimer or longer compounds of hydrophosphinic acid can form at the silica surface. The SiO2 samples with bound hydrophosphinic groups (SiO2/ HPA) were produced from fine pyrogenic silica (Aerosil A-300 (11) Bogatyrev, V. M.; Brey, V. V.; Chuiko, A. A. Teor. Eksp. Khim. 1988, 24, 629. (12) von Hippel, A. R. Dielectrics and Waves; John Wiley & Sons, Inc.: New York, 1954.
© 1996 American Chemical Society
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Figure 1. The IR spectra of parent silica (1, 2) and SiO2/HPA (3, 4) for water adsorption at 300 K (2, 4) and after treatment at 430 K (1, 3).
Figure 2. Dependence of the 1H NMR spectra on temperature for water and hydrophosphinic acid at silica surfaces at Cw ) 12 wt % at T ) 270 K (1), 260 K (2), 250 K (3), 240 K (4), 230 K (5). with specific surface area of 300 m2 g-1, 99.5% purity, “Chlorovinyl”, Kalush, Ukraine) via reaction with PCl3 and H2O at T ) 400 K13-15 as follows: SiOH + PCl3
–HCl
SiOPCl2
+2H2O –2HCl
SiOP(OH)2 O SiOPOH
(1)
H
The HCl was removed by degassing at a reaction temperature. SiO2/HPA is favored over the parent SiO2 for an increase of the probability of protolytic surface reactions, which can occur with participation of HPA and water. This is of interest to change the adsorption characteristics of the oxide particles.
Results and Discussion The 1H NMR signal broadening, decrease of its intensity I, and shift of a peak to low magnetic field are observed (13) Morrow, B. A.; Lang, S. J. Langmuir 1994, 10, 756. (14) Koltsov, S. I.; Volkova, A. N.; Aleskovsky, V. B. Izv. Vuzov USSR Kim. Khim. Techn. 1969, 11, 1633. (15) Pavlov, V. V.; Tertykh, V. A.; Chuiko, A. A.; Bogatyr’ov, V. M. Dokl. Akad. Nauk USSR, Ser. B 1979, N8, 639.
Figure 3. Dependencies of the 1H NMR signal intensity on temperature for water adsorbed on silica/HPA for the Cw values 43.5 wt % (2), 26 wt % (3), 17 wt % (4) and 12 wt % (5) and for silica/HPA in water (1).
Figure 4. Dependence of the 1H NMR signal intensity on temperature for silica/HPA in CDCl3 for Cw 26 wt % (3), 17 wt % (4), and 12 wt % (5).
for temperature lowering (Figure 2). This broadening is caused by reduction of the adsorbed molecule mobility,16 and intensity lowering is connected with freezing of the water molecules in the adsorption layer at T < 273 K. The proton signal of the adsorbed molecules is detected as the protons of the tSiOH and HPA groups bound to surfaces do not contribute to the NMR spectra due to the short time (≈10-6 s) of cross-relaxation of protons in solids. The degree of surface hydration has an influence on the I(T) function (Figure 3). In the case of an inert solvent (Figure 4) and high hydration of the oxide particles, the intensity reduction is seen only for T < 230 K. In the same time for Cw ) 12 wt %, the intensity decreases for the total interval of measurement that is a result from partial freezing of adsorbed water. The I(T) dependence shown in Figures 3 and 4 was obtained upon heating the samples, previously cooled to 185 K, to ensure against an effect of overcooling of water at T < 273 K. The obtained dependence I(T) can be transformed into a function of the changes in the Gibbs free energy (∆G) (16) Mank, V. V.; Lebovka, N. I. NMR Spectroscopy of Water in Heterogeneous Systems; Naukova Dumka: Kiev, 1988.
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Figure 7. Dependence of the chemical shift on temperature for water adsorbed on silica/HPA for Cw 43.5 wt % (1), 26 wt % (2), 17 wt % (3), and 12 wt % (4).
Figure 5. Dependence of the Gibbs free energy increment on the nonfrozen water layer thickness at the ice/water/HPA/silica interfaces (1) and air/ice/HPA/silica (2-5) at Cw 43.5 wt % (2), 26 wt % (3), 17 wt % (4), and 12 wt % (5).
Figure 8. Dependence of the chemical shift on the water content at the temperatures 270 K (1), 260 K (2), 250 K (3), 240 K (4), and 230 K (5). Table 1. Dependence of ∆G on the Water Content for SiO2/HPA/H2O/Air Cw, wt % ∆Gmd, kJ mol-1 ∆Gsp, kJ g-1
Figure 6. Dependence of the Gibbs free energy increment on the nonfrozen water layer thickness for CDCl3/ice/water/HPA/ silica at Cw 26 wt % (1), 17 wt % (2), and 12 wt % (3).
reduced on the nonfrozen adsorbed water content.17-19 We assumed that at water molecule adsorption ∆G is equal to ∆Gice ) Gice(T)273K) - Gice(T). The function ∆G(Cw) depends on changes in the adsorption potential of the surface in the ice/water/solid system. The area limited by the curve of ∆G ) f(Cw) (∆Gsp) gives integral lowering of the free energy in the adsorbed water layer that equals the sum of the free surface energy of the adsorbent and the free energy of the interface of the adsorbed water/ air(chloroform). The free energy of the adsorbed water layer can also change upon the hydrolysis of the tSisOsP bonds and the appearance of the HPA molecules which are not bound at the surface layer. Notice that the water molecules, which are strongly bound to the surface and do not turn into ice at the lowest studied temperature, do (17) Thermodynamic Properties of Individual Materials (Handbook); Nauka: Moscow, 1978. (18) Turov, V. V. React. Kinet. Catal. Lett. 1993, 50, 243. (19) Turov, V. V.; Zarko, V. I.; Chuiko, A. A. Ukr. Khim. Zh. 1990, 56, 1262.
12 3.5 29.0
17 2.4 13.8
26 2.1 8.1
43 1.4 3.2
60 1.0 1.7
Table 2. Dependence of ∆G on the Water Content for SiO2/HPA/H2O/CDCl3 Cw, wt % ∆Gmd, kJ mol-1 ∆Gsp, kJ g-1
12 1.4 11.7
17 1.9 11.2
26 2.1 8.1
not contribute to ∆G. The mobility of these molecules is the lowest, and determination of the integral intensity for the corresponding wide 1H NMR signal is difficult. Besides, the influence of the adsorption potential in the interface region on the freezing temperature for adsorbed molecules is a controlling factor only for nonporous adsorbents. For porous adsorbents, ∆G of the adsorbed molecules depends on pore size. A maximum value of ∆G (∆Gmax) can be found via extrapolation of the obtained dependencies to Cw ) 0 (Figure 5) to obtain middle ∆Gmd ) 0.5∆Gmax and ∆Gsp ) ∆Gmd/Cw (Table 1). The ∆Gmax value is close to the average magnitude of the free energy of the first monolayer of adsorbed water, as the samples studied had sufficiently thick films of adsorbed water. The ∆Gmd value rises steeply when the hydration layer thickness decreases for the same content of nonfrozen adsorbed water (Table 1, Figure 5). This may result from solvate complex structure changes depending on the initial
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Table 3. Parameters of Molecules and Clusters with Trivalent and Penta-valent Phosphorus (ab initio) structurea
-EHF (au)
567.096494 P(OH)3 OdP(H)(OH)2 567.117434 R3SiOP(OH)2 1081.98574 R3SiOPO(H)OH 1082.00483 a
-EHOMO ELUMO (eV) (eV) µ (D) 10.46 13.46 10.52 12.95
4.87 5.18 4.68 4.70
3.22 1.38 2.71 1.36
qP
-qO
1.173 1.377 0.721 1.193 1.429 0.763
-qO(H)
qH
rPsO (nm)
0.853 0.786 0.850 0.780
0.461 0.493 0.471 0.494
0.16349 0.15882 0.16091 0.15852
rPdO (nm)
rPsH (nm)
rOH (nm)
basis set
0.09509 0.1453 0.1373 0.09514 0.09497 0.1460 0.1372 0.09515
6-31G* 6-31G* 6-31G* 6-31G*
R ) OH.
Table 4. Parameters of Adsorption Complexes of Water Molecules on the Cluster (O*3SiO)2Si(OR)sOsSi(OH)(OSiO*3)2 R
∆Et (kJ/mol)
OH HO...HOH OHH+ Si(OH)rOH2 P(HO)-OH
ELUMO (eV)
µ (D)
-qO
qH
0.52 0.44 -3.54 0.46 -0.43 -0.48 -0.93 -1.54 -0.55 -0.69 -1.28 -0.74 -0.49 -0.67 -0.42 -1.11 -0.66 -1.52 -0.64 -0.95
2.48 4.32
-36 -38 -37 158 159 -64 94 -64 86 -173 -8c -98
10.53 10.71 13.22 10.41 10.17 10.32 10.59 10.78 10.45 9.62 10.53 9.69 9.66 10.07 9.44 10.32 9.48 10.78 9.61 10.68
2.68 1.88 4.13 3.25 3.13 5.62 1.79 4.89 9.40 5.55 4.18 6.79 6.12 12.7 4.44 8.51 1.37
0.684 0.691 0.418 0.717 0.475 0.457 0.460 0.464 0.459 0.445 0.473 0.732 0.761 0.502 0.787 0.482 0.764 0.504 0.811 0.470
0.239 0.242 0.355 0.278 0.221 0.231 0.245 0.228 0.254 0.233 0.264 0.347 0.343 0.265 0.342 0.272 0.344 0.292 0.354 0.281
-56
10.43
-1.01
2.02
0.460
0.275
-28 -719 -55
P(O)(OH)2 P(HO)OP(HO)-OH P(HO)-OH...OH2 O(P(HO)H)...HOH PHOOPHOOH...OH2 PHOOPHOO-... H3+O PHOO-...H3+O PHOOH...OH2...OH2 PHOO-...H+OH2‚OH2 PHOOPHOOH‚OH2‚OH2 PHOOPHOO-‚H3+O‚H2O PHOOPOH‚5H2O PHOOPHOO-‚6H2O‚H+ P(H)OH • • • O O•••H
-EHOMO (eV)
P(H)OH
-41b
∆Et* a (kJ/mol)
-36 -38 -37 158 159 -32 47 -32 43 -29 -1
rOH (nm)
method
0.09 486 0.09 628 0.09 807 0.09 731 0.09 415 0.09 576 0.09 612 0.09 424 0.09 635 0.0958 0.09 652 0.144 439 0.1456 0.09 664 0.14 725 0.09 674 0.14 712 0.09 848 0.16 036 0.15 876
AM1 AM1 AM1 AM1 MNDO/H AM1 AM1 MNDO/H AM1 AM1 MNDO/H MNDO/H MNDO/H MNDO/H MNDO/H MNDO/H MNDO/H MNDO/H MNDO/H MNDO/H
0.15 954
MNDO/H
O
P(HO)OH...OdPH(OH)2 complex
-∆Et
-EHOMO
method
HOH...OH2 H3O+ H5O2+
-16 -551 -679
11.51 23.07 19.70
MNOD/H MNDO/H MNDO/H
complex
-∆Et
-EHOMO
method
H9O4+ H9O4+
-744 -703
15.78 17.46
MNDO/H AM1
a ∆E * is the change in total energy per one water molecule. b ∆E is the change in total energy for the reaction tSiOP(HO)OH + H O t t 2 f tSiOP(O)(OH)2 + H2. c H+ has the longest distance from surface.
surface hydration degree and occurrence of chemical reactions at the interfaces with water and HPA participation. When the boundary with air is replaced by the boundary with CDCl3 (Table 2, Figure 6), the ∆Gmd changes at Cw ) 12 wt % (6.67 mmol g-1), but for high Cw the ∆Gmd values are little different. However, the ∆Gsp values have more differences for the same Cw magnitudes. A significant decrease in the nonfrozen adsorbed water layer at temperature lowering is observed only for small Cw (Figure 7). The δ value dependencies on temperature and solvate layer thickness have an intricate shape (Figure 8). The δ value increases for temperature lowering independent of Cw, and this effect grows with a decrease in the solvate layer thickness. The δ(Cw) function has maximum for T > 240 K, but at T ) 230 K it increases monotonically at Cw diminution. The main reason for the shift of the 1H NMR signal to the low magnetic field is fast proton exchange between the OH groups of the adsorbed water and phosphinic acid molecules. Inasmuch as the δ value for phosphinic acid (δHPA ≈ 15 ppm) is higher than that for adsorbed water (δw ) 5 ppm),19 the average δ value depends on the water/ HPA ratio for each adsorption complex. Therefore, only a qualitative analysis can be performed for surface phenomena studied on the basis of the δ(Cw,T) functions. Since the surface OH groups of the oxide do not contribute to the chemical shift value for adsorbate, the high δ values (Figure 8) may be explained via the hydrolysis of the bound phosphinic groups followed by a growth of the HPA
(adsorbed acid) content upon formation of acidic H-bonded adsorption complexes. At phosphorus concentration CP ) 5.6 wt % on the average two HPA molecules fall on one tSiOH group. Inasmuch as these molecules have the proton-acceptor and electron-donor centers, they can form self-associates in the adsorption layer. If water is absent in the surface complexes, the cyclic dimers of HPA (e.g., between one chemically bound HPA group and another adsorbed HPA molecule) are most stable (see Table 4). If water is adsorbed in excess, the SisOsP bonds are hydrolyzed as follows
tSsOsPsOsX(H)(O) + H2O f tSiOH + HOPOX(H)(O) (2) where X ) H, POX′(H)(O). The minimum-studied content of adsorbed water is Cw ) 12 wt %, corresponding to five H2O for each residue bound acid (i.e., the equilibrium in eq 2 under such a condition may be shifted to the right). It is possible that the generation of complexes in which the HPA molecules do not have direct intermolecular bonds with the surface (i.e., they are linked to the surfaces via the water molecules) results in an increase in the δ value. The observed changes in the chemical shifts (Figures 6 and 7) may be explained from eq 2. Hydrolysis of the SisOsP bridge followed by cleavage of the SisOsSi bonds by free HPA molecules decreases the δ value at increasing temperature as the HPA molecules transfer from the adsorption state to the chemically bound state. The δ value diminution for the high adsorbed water amount is
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Figure 9. Dependence of log fmax on T-1 for silica/HPA at Cw 20 and 29 wt %.
due to a contribution of the water molecules as δw < δHPA. A δ growth for Cw ) 12 wt % at low temperatures is a result of water freezing and increasing the HPA contribution to the δ magnitude. The CDCl3 media has an influence on the hydration structures at the SiO2/HPA interfaces (Tables 1 and 2) that is conditioned by changes in size of the adsorbed water clusters and adsorbed water molecule distribution between different active surface sites. Using the DS method12,20 and theoretical simulation for the H2O/HPA/SiO2 system, we can obtain further information about the water cluster structures and the influence of the adsorbed and chemically bound HPA molecules on the silica surface properties. The SiO2/HPA samples with a phosphorus content of CP ) 5.6 wt % were studied by the DS method in the 0.1-10 MHz range at 100-300 K. Water adsorption on SiO2/HPA has been performed at p/ps ) 0.95. The ′ and ′′ values for SiO2/ HPA/H2O that have strong dependence on temperature and relaxation maxima at water adsorption are observed in the 180-210 K range (Figures 9 and 10). The conductivity of the hydrated SiO2/HPA samples linearly depends on the phosphorus content, and already at CP ) 2 wt % it is 2 orders of magnitude higher than that for water adsorbed on parent silica. This effect is attributable to weakly bound protons, whose content increases upon interaction of adsorbed water molecules with bound HPA groups leading to dissociation of the PO-H bonds. This effect also has an influence on the DS data. At standard conditions, the SiO2/HPA samples absorb nearly 12 wt % of water, but at Cw < 12 wt %, the relaxation maxima are not observed in the dielectric spectra of SiO2/HPA/H2O. We suggest that the dielectric loss for hydrated oxides is caused by relaxation of the polar adsorbed molecules; therefore, this process can be described according to the Debye formula12
′ ) R + (0 - R)/(1 + ω22) 2 2
′′ ) (0 - R)ωτ/(1 + ω τ )
(3) (4)
where ω is a cyclic frequency, τ is the relaxation time, 0 and R are low- and high-frequency magnitudes of permittivity, respectively. At dipole reorientation, a molecule overcomes the potential barrier, a value corresponding to (20) Poplavko, Yu. M. Physics of Dielectrics; Vyscha Shkola: Kiev, 1980.
Figure 10. The Cole-Cole diagrams for silica/HPA at 203 K for Cw 12 wt % (a), 20 wt % (b), and 29 wt % (c).
the activation energy (Ea) of the polarization. This process runs in a time
τ ) (2ν)-1 exp(Ea/kT)
(5)
where ν is a frequency of the particle vibration and k is the Boltzmann constant. Such system conductivity is a result from displacement current
I ) n0P/(12kTτ) ) (0 - R)/τ
(6)
where n0 is the amount of dipoles and P is their polarizability. From eqs 3-6, we can obtain temperaturefrequency characteristics
′ ) R + (n0P/12k)/{T[1 + ω2/4ν exp(2Ea/kT)]}
(7)
′′) (n0P/12k) exp(Ea/kT)/{2νT[1 + ω2/4ν2 exp(2Ea/kT)]} (8) Conditions for the ′(T) and ′′(T) maxima give the transcendental equations and their solutions in the first approach and at Ea . kT can be written for ′ as
(Ea/kT) ) ln(2ν/ω) - 2/(2ν/ω)
(9)
and for ′′ as
(Ea/kT) ) ln(2ν/ω)
(10)
at ωτ ) 1 (i.e., the ′(T) maximum has a shift to high temperature relative to maximum of ′′(T)). According to the equation ln ωmax ) (A/T + B) for ′′ (Figure 9), we can calculate the Ea value, which is equal to 31 kJ/mol for
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SiO2/HPA/H2O (for SiO2/H2O Ea ) 13-17 kJ/mol at Cw ) 4-6 wt %). Using eq 10, we obtain
ν ) (ω/2) exp(Ea/kT) ) 5.6 × 1013 s-1
(11)
According to the work,21 this value corresponds to a frequency of synchronous H+ transfers in water, which leads to the dipole reorientation. The time approaching τ ∼ ν-1 is consistent with rotation motion of a water molecule on an angle, which is sufficient for the dipole reorientation along external field.22 Thus, the polarization mechanism for adsorbed water (and HPA molecules) can be conditioned by both the molecule rotations and synchronous H+ transfers. For adsorbed HPA molecules in the presence of water, the second mechanism is preferential as acid molecules have more weakly bound protons and a higher inertia moment than for water molecules (i.e., for rotational frequencies fHPA < fw). The activation energy of the polarization of the SiO2/ HPA/H2O system is practically independent of Cw in the 12-29 wt % range, but for the parent silica, the Ea value depends on the water content at Cw > 5 wt % (Ea increases up to 21 kJ/mol for parent pyrogenic silica at Cw ) 45 wt % at 180-220 K). Consequently, the structures of the adsorbed water clusters on parent silica22 and SiO2/HPA have some distinctions that are due to the HPA availability, especially the PO-H bond hydrolysis. For all Cw magnitudes, the centers of the Cole-Cole diagrams drop below the X axis (Figure 10). According to Davidson and Cole,23 this is observed for compounds with hydrogen bonds. Besides, such displacement shows that a few relaxation times can be found for the adsorbed clusters.20 Therefore, we can use a new parameter R for characterization of the relaxation time distribution.20,24 For SiO2/ HPA/H2O the R value increases for Cw enhancement, and at Cw ) 12, 20, and 29 wt % R ) 0.407, 0.505, and 0.626, correspondingly (Figure 10). This result shows a heterogeneity of the surfaces or a rising of the association degree of adsorbed molecules in the clusters. The surface heterogeneity leads to the formation of adsorbed clusters on different active sites, and they can have varied size and other properties (′, ′′, etc.). Besides, at increasing Cw, it is possible for hydrolysis of the Si-O-P bonds to occur with formation of new active sites, which can give new hydrogen bonds with water molecules that have influence on the dielectric characteristics of the system. A growth of the static dielectric permittivity 0′ at increasing of Cw from 37 to 45 wt % (Figure 10) shows that the size of the adsorbed clusters increases. According to the results obtained by molecular dynamic method, the size of the water clusters has an influence on the static dielectric permittivity.25-27 Therefore, for the water clusters including n < 256 water molecules, the ′ value changes in the 25-60 range.26
Figure 11. Cluster of SiO2/HPA used in semiempirical calculations.
MNDO/H29,30 methods in a cluster approach. The silica surface fragments were modeled by the (O*3SiO)2Si(OR)sOsSi(OH)(OSiO*3)2 cluster (Figure 11), where O* is oxygen pseudoatom and R ) P(H)(O)OH or P(H)(O)OP(H)(O)OH. In reaction 1, the tSiOP(OH)2 groups form on the second stage upon hydrolysis of the PsCl bonds. However, such groups transform to tSiOP(H)(O)OH, as has been found from the IR spectra (Figure 1). We have studied two forms of molecules P(OH)3 and OdP(H)(OH)2 and bound groups by ab initio method using the Gaussian 92 program package with the 6-31G(d) basis set.31 According to these calculations (Table 3), the stability of OdP(H)(OH)2 is higher than that for P(OH)3, for both free molecules and bound groups. Besides, HPA has more acidic POsH groups (see qH, rOH). Upon water adsorption on silica surfaces with bound hydrophosphinic groups, it is possible to form hydrogenbonded complexes having different structures. The first H2O molecule forms the hydrogen bond with an acidic POH group H SiO
P
(21) Zatsepina, G. N. Physical Properties and Structure of Water; MGU: Moscow, 1987. (22) Tishchenko, V. A.; Gun’ko, V. M. Colloids Surf. A 1995, 101, 287. (23) Davidson, P. W.; Cole, R. H. J. Chem. Phys. 1952, 19, 1484. (24) Cole, K. S.; Cole, R. H. J. Chem. Phys. 1941, 9, 341. (25) Neumann, M. J. Chem. Phys. 1986, 85, 1567. (26) Anderson, J.; Ullo, J. J.; Yip, S. J. Chem. Phys. 1987, 87, 1726. (27) Alper, H. E.; Levy, R. M. J. Chem. Phys. 1989, 91, 1242. (28) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902.
(12)
H...OH2
O
Energy for such a bond (Table 4, ∆Et) is higher than that for tSiO(H)...HOH, HOH...OH2, or the second hydrogen bond in the complex H
Quantum Chemical Simulation The theoretical study of water interaction with bound tSiOP(H)(O)sOH or tSiOP(H)(O)OP(H)(O)sOH groups at the silica surface was performed with the AM128 and
O
SiO
P
O
H...O
H H...OH2
(13)
O
but it is near to ∆Et for the H bond between water and HPA molecules (Table 5). Thus, single H2O molecules at SiO2/HPA surfaces have to form hydrogen bonds with the (29) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899. (30) Burshtein, K. Ya.; Isaev, A. N. Zh. Strukt. Khim. 1984, 25, 25. (31) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92, Revision G.3; Gaussian, Inc.: Pittsburgh, PA, 1992.
Water Adsorption at Pyrogenic Silica Surfaces
Langmuir, Vol. 12, No. 14, 1996 3509
Table 5. Parameters for Acid Molecules and H-Bonded Complexes (OH)2OPX (X ) H, OH) (MNDO/H) compound
∆Et (kJ/mol)
-EHOMO (eV)
-ELUMO (eV)
µ (D)
qP
-qO(dP)
-qO(H)
-34 -35
11.10 11.50 11.10 10.71
-1.06 -0.71 -1.34 -0.61
1.59 2.45 2.86 2.17
1.130 1.332 1.144 1.133
0.667 0.649 0.693 0.686
0.461 0.478 0.453 0.483
(OH)2PHO OdP(OH)3 tPdO...HOH tPOH...OH2
-qO(w)
0.377 0.342
rO...H (nm)
qH
qH(w)
0.16 553 0.16 262
0.226 0.237 0.223 0.262
0.203 0.191
POH groups having stronger acidic properties than tSiOH and that causes the possibility of the H+ transfer
tSiOP(H)(O)sOH...OH2 f tSiOP(H)(O)sO-...+H3O (14) However, formation of the subsequent hydrogen bonds with other H2O molecules gives a significant decrease of total energy (Et) for such system (e.g., ∆Et for hydrogen bonds of the second H2O molecule with complex 14 is (-63 kJ/mol), twice as large as that (-27 kJ/mol) for complex 13 (Table 4)). Dipper stabilization of
tSiOP(H)(O)sO-...+H3O‚nH2O
(15)
Figure 12. The sections of PES for reaction 16: 1 is SN(Si) mechanism (the pathway is ACD in eq 16) and 2 is SN(P) mechanism (the pathway is ABD in eq 16); the reaction coordinate λ ) RrO(w)-X + βrO-H(w), X ) Si or P.
is observed with increasing the n value (∆Et/Nw < 0 already at n ) 5). Therefore, a great part of the adsorbed water molecules tends to diffuse to HPA bound groups and forms big water clusters (at high Cw) that leads to change of the chemical shift δ, dielectric, ′, ′′, and other properties of the system relative to parent silica. Besides, it is possible for hydrolysis of the SisOsP bonds to occur with subsequent dissociation of the new POsH bonds in product of eq 16D. Therefore, water molecule localization intensiOH2
Si O P C Si O P
+ H2O
SiOH + HOP
A
(16)
D Si O P B OH2
fies near the hydrophosphinic groups. The contacts between these adsorbed clusters create conditions for the proton conductivity found for such patterns. On the SiO2 surface, the stabilization of complex tSiOH2+ is observed even without water (Table 4), and its value corresponds to H+ stabilization by 4 H2O molecules (Table 4, H9+O4). It seems likely that a stabilization of ions at the POsH bond dissociation in the surface layers can occur not only for high coverage of surfaces by water but also at low Cw as a result of localization of the water molecules near HPA bound groups. The PO-...H+ structures create strong local electrostatic fields, and their interaction with water molecule dipoles leads to the localization of water molecules near separated ion pairs (i.e., the tSiOPO-...H+2n+1On complexes form). The reaction mechanism for eq 16 was studied by calculation of the sections of the potential energy surface (PES) by the AM1 method (Figure 12). The reaction coordinate λ was RrO(w)-X + βrO-H(w), where X ) Si or P; for high rO(w)-X values β ≈ 0 and for rO(w)-X < 0.18 nm β . R (O(w) and H(w) are atoms from a water molecule). As seen from the PES sections, the reaction which occurs via formation of the electron-donor bond SirOH2 in
Figure 13. The silica cluster with adsorbed water molecules.
intermediate C of eq 16 (mechanism is SNi(Si)) is preferential over that which occurs via complex B of eq 16 (Figure 12) (i.e., for the pathway ACD in eq 16, the activation energy value is lower than that for ABD). Consequently, the tSiOH groups will restore upon water interaction with bound HPA groups at the SiO2/HPA surface, but in the IR spectra the tSiOsH band at 3750 cm-1 (Figure 1) is not intensive because the tSiOH groups interact with water and phosphorus compounds localized near these groups. The free OP(H)(OH)2 molecules form strong hydrogen bonds at the SiO2/HPA surface (e.g., a cyclic complex with two H bonds (Table 5) with an ∆Et nearly twice as high as that for the complex with one H bond HOP(H)O...HOP(HO)OH that has influence on the δ(T,Cw) function). The polarization and dipolar relaxation of the water molecules adsorbed on the tSiOP(HO)sOH groups are conditioned by weakly bound protons and rotations of H2O dipoles. The activation energy of rotations (Er) of the H2O
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Langmuir, Vol. 12, No. 14, 1996
Turov et al.
molecules in H-bonded complexes in eqs 12 or 13 or Figure 13
••• •••
HOH•••OH2
SiOP(H)(O) O H•••OH2•••OH2
(17)
HOH•••OH2
changes in the 10-40 kJ/mol range. Modeling of these processes was performed by AM1 and dynamic reaction coordinate methods using the forced rotations of one H2O molecule of chosen angle from the equilibrium state and calculation of reorientation of this molecule and neighboring molecules.22 The lowest Er value corresponds to slow reorientation of the H2O dipole with nearly synchronous relaxation of the surroundings. In the case of fast rotations of the H2O molecule, when neighboring molecules do not have time for synchronous reorientation, the Er value increases to 40 kJ/mol. In the case of rigid surroundings for a H2O molecule having four hydrogen bonds, Er grows to 75-80 kJ/mol. For a molecule having only one hydrogen bond, the Er magnitude is equal to 3-10 kJ/mol in relation to the response of the surroundings. These values are close to those for the parent silica.22
Conclusion Fine pyrogenic silica particles containing bound hydrophosphinic groups have a stable state at Cw e 12 wt % that corresponds to water adsorption under standard conditions. Increasing the adsorbed water content leads to hydrolysis of the Si-O-P bonds, but upon raising the temperature, the adsorbed hydrophosphinic acid can cleave the Si-O-Si bonds resulting in the δ value decrease. Using the dependence of the 1H NMR signal intensity on temperature, the mean values of the free energy changes for water molecules adsorbed at the SiO2/ HPA interfaces can be found. For air/H2O/HPA/SiO2, the adsorption energy for water decreases when the adsorbed water content increases. Hydrolysis of the Si-O-P bonds adds to the content of weakly bound protons and the localization of the water molecules near these ionized bonds and has influence on the 1H NMR signal and ′ and ′′ values for the adsorption layers. Thus, modification of pyrogenic silica surfaces by hydrophosphinic acid changes the adsorption properties of the oxide that can be used to increase the intermolecular interaction between the fine silica particles and different polar adsorbates. Acknowledgment. We are grateful to the SherwinWilliams Co. (U.S.A.) for financial support of this work. LA950592C
