On the Interaction between Silica Surfaces and Surfactants. A 2D

J. Phys. Chem. C 2009, 113, 13309–13316

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On the Interaction between Silica Surfaces and Surfactants. A 2D Periodic B3LYP Investigation Geyser Ferna´ndez-Cata´,†,‡,§ Aurora Pe´rez-Gramatges,† Luis Javier Alvarez,§ Hansel Comas-Rojas,† and Claudio M. Zicovich-Wilson*,‡ Departamento de Radioquı´mica, Instituto Superior de Tecnologı´as y Ciencias Aplicadas, AV. SalVador Allende y Luaces, A. P. 6163, La Habana, Cuba, Facultad de Ciencias, UniVersidad Auto´noma del Estado de Morelos, AV UniVersidad 1001, Col. Chamilpa, 62209, CuernaVaca, Morelos, Mexico, and Laboratorio de Simulacio´n, Instituto de Matema´ticas, Unidad CuernaVaca, UniVersidad Nacional Auto´noma de Me´xico, A. P. 273-3 Admon. 3, 62251, CuernaVaca, Morelos, Mexico ReceiVed: February 12, 2009; ReVised Manuscript ReceiVed: May 9, 2009

The structural and electronic features of the interaction between silica surfaces and surfactants have been studied with high level computational techniques of simulation. Calculations have been performed at the B3LYP level of theory as implemented in the CRYSTAL06 code. Each of the periodic models adopted consists of a conveniently functionalized (001) surface of edingtonite and a layer of tetrametilammonium and other species like water and Cl- that mimic the interface between the silica and the surfactant solution moieties, in both acidic and basic media. Different degrees of coverage and hydration have also been considered. Our calculations indicate that besides the electrostatic interactions, polarization and charge-transfer processes play a nonnegligible role in the formation and stability of the silica surface. Introduction Organized mesoporous silica exhibits interesting properties suitable for applications in the fields of catalysis, separation processes, and host-guest chemistry, among others.1,2 The synthesis of these materials is attained in the presence of surfactants. The knowledge of the detailed mechanisms of this process is crucial for the design and development of new materials of this family. It is generally assumed that the interaction between the silica surfaces and the surfactant micelles is determinant of the structural properties of the formed inorganic phase. In the last 15 years a remarkable effort has been devoted to understand such interaction during and after the templating process.3-8 However, the effects that work at the microscopic level and determine the proper conditions to generate a stable and structured mesoporous phase are still a matter of discussion. This interaction has been studied through experimental techniques such as nuclear magnetic resonance,7-10 neutron scattering,11 fluorescence probing,12 small-angle X-ray scattering,7,13-15 conductivity measurements,7 and infrared spectroscopy.9 A large number of recent reviews devoted to this topic1,16-21 have addressed micrometer scale powders obtained through hydrothermal basic conditions and thin films growing spontaneously at the solution/air or solution/substrate interfaces in acidic media. These studies show that the formation of disordered silicabased solids with surfactants as templates proceeds through a cooperative self-assembly process. The mechanism in basic medium proposed by Frasch et al.12 is based on the occurrence of an oppositely charged polyelectrolyte/surfactant system, which involves the formation of siliceous prepolymers as the * To whom correspondence should be addressed. E-mail: claudio@ buzon.uaem.mx. † Instituto Superior de Tecnologı´as y Ciencias Aplicadas. ‡ Universidad Auto´noma del Estado de Morelos. § Universidad Nacional Auto´noma de Me´xico.

overall rate-controlling step. As the prepolymer grows, it interacts with a larger number of surfactant molecules in a cooperative manner, forming the initial silica-surfactant aggregates. Further polymerization and organization of the hybrid complexes take place during the precipitation-aging step to yield the mesoporous silica. The formation of silica-quaternary ammonium surfactant complexes seems to be driven by the electrostatic attraction between the negatively charged silica matrix and the positively charged surfactant head together with the hydrophobic interaction among surfactant chains.7 Acidic media synthesis provides the conditions for the formation of mesoporous silica with different morphologies, such as single crystals, thin films, fibers, spheres, etc.1,22 In such conditions, the prepolymeric silica species in solution are linear oligomers less condensed than in the basic medium process. The material thus appears to have weaker surfactant-silica interaction and more silanol groups at the surface than that synthesized in basic medium.23 In this case the interaction between silica precursors and surfactant micelles is mediated by negatively charged counterions (for example, chloride from hydrochloric acid, used to adjust pH).12,17 A few theoretical studies have been focused on the characterization of the interactions between the structure-directing agents, the solution species, and the silica surface at the electronic level,24-26 probably because of the difficulty in finding a suitable and representative model. According to the experimental evidence of the occurrence of two steps (prepolymerization and condensation) in the formation of the silica phase, the (001) seven-layer slab of edingtonite proposed a few years ago by Civalleri et al.27,28 is a good candidate to be used as a suitable model for the silica-surfactant interface. This model was established through an exhaustive study on the convergence of several properties as functions of the number of layers and basis set level. The structure consists of natrolite units interconnected by Si-O-Si bridges between the equatorial Si atoms, while the

10.1021/jp901320q CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

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Figure 1. Model for the surface. Left and right panels: top and lateral views, respectively.

axial ones may be terminated by either OH or basic O- anions, giving rise to surface silanol or silanolate groups, respectively (see Figure 1). An interesting feature of this model is that it can be seen as the condensation of secondary building units (the natrolite ones) mimicking the product expected from the above-described two-step polymerization mechanism of the silica phase. On the other hand, the number of degrees of freedom of the model makes it suitable for the computational treatment at a rather high level of theory. Obviously, this choice disregards the nonnegligible semidisordered character of the real system because of the periodic approach. The aim of this work is to provide a detailed description of the interactions occurring between silica surfaces and surfactant molecules in both acidic and basic media, at the electronic level by adopting reliable quantum chemistry methods. The level of theory considered is the B3LYP29 one as implemented in the CRYSTAL06 code,30 which has been tested in several cases involving silicates with reasonably good performance.31-34 Computational Details Periodic calculations were performed with a developing version of the CRYSTAL06 code,30 at the B3LYP level of theory.29 A Gaussian-type basis set has been adopted that consists of standard 6-31G* sets for H, C, N, and Cl, and modified 6-21G(d) and 6-31G(d) sets for Si and O atoms. In the latter, the exponents of the most external sp and d shells are 0.13 and 0.5 bohr-2 for Si, and 0.2742 and 0.6 bohr-2 for O, respectively. This basis set level has been adopted in previous studies of related systems (zeolites, layered silicates, etc) with good performances.32,34-36 The tolerances for the truncation of the infinite Coulomb and exchange sums together with the SCF convergence criterion are the defaults of the code,30 while the grid adopted for the numerical integration of the electronic density is a pruned one with 75 radial and a maximum of 434 angular points, generated through the Gauss-Legendre and Lebedev schemes, respectively (see LGRID keyword in the code manual). Shrinking factors 4 and 2 have been considered for the Pack-Monkhorst sampling in the Brillouin zone in the 1:1 and 1:2 coverages, respectively. For charged periodic models (such as the clean surface in the basic case), the Jellium approach has been employed to warrant the overall electroneutrality of the system.30 While in principle this approach can be also suitable to estimate the energy of charged fragments, it is not in general ensured that the accuracy level of the calculations performed for charged and neutral species is completely equivalent. For this reason we do not compute adsorption energies but a set of energetic indexes in which the contributions of the charged fragments cancel and therefore all contributing quantities are ensured to feature comparable accuracy.

Ferna´ndez-Cata´ et al. Geometry optimizations were performed using analytical gradient techniques for both atomic positions and lattice parameters.37 All structures have been fully optimized. The models considered in this work consist of a functionalized seven-layer slab of the (001) surface of edingtonite to represent the silica surface in accordance with the previous discussion, tetramethylammonium (TMA) modeling the surfactant head, and, in acidic media, chloride as counterion. The addition of one or two water molecules per TMA to the interface has been considered to simulate different hydration degrees. Two coverages have also been considered by adopting the supercell approach. The model adopted for the silica surface is depicted in Figure 1. The dangling bonds resulting upon the cut of the slab from the whole crystal may or not be saturated depending on the acid-base conditions and coverages to be simulated. In accordance with the elemental analysis of the material synthesized in basic conditions,12 which shows that the incorporation of residual solvated anions to the interface is very low, the model chosen for the inorganic part under these conditions features silanolate groups and tetramethylammonium ions in equal parts, ensuring electroneutrality. Dangling bonds of the surface O atoms are all saturated with H atoms, in models that simulate acid medium, giving rise to superficial silanol groups. Therefore, the charged species at the interface are TMA+ and chloride. This is in agreement with experimental evidence that chloride anions coming from HCl, added to provide acidic medium, are incorporated into the interface in a much larger extent than bromide ones which are the original surfactant counterions.12 The opposite surface of the slab, not active in the interactions we are interested in, is terminated with silanol groups in all models. Two coverages have been investigated for both acidic and basic conditions, namely, 1:1 and 1:2. The latter is simulated by considering a 2 × 2 supercell of the original slab. Only half of the superficial O atoms remain in the silanolate form (Si-O-), while the remainder are saturated by hydrogen in models representing basic conditions. They were chosen in such a way that they are not contiguous along the main lattice directions. The same number of TMA cations and silanolates in the system warrants the electroneutrality. In models with acid interfaces, all surface O atoms remain saturated with H atoms, while the adsorbates are in a 1:2 ratio with respect to the Si-OH groups at the active side of the slab. The energetic comparison between both coverages is performed according to the following expression

∆E ) E1:2 - (E1:1 + Esurf) where E1:2 and E1:1 are the energies of the system at low and high coverage, respectively, while Esurf corresponds to the clean surface. ∆E estimates the energy of the following hypothetical reaction

SiO2(coverage 1:1) + SiO2(clean) ) 2SiO2(coverage 1:2) that stoichiometrically reads

TMA+(Si5O11H)- + Si5O11H2 ) TMA+(Si10O22H3)TMA+Cl-(Si5O11H2) + Si5O11H2 ) TMA+Cl-(Si10O22H4)

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TABLE 1: Selected Geometrical Parameters of the Optimized Models in Acidic (A) and Basic (B) Conditionsa dist/Å

clean surface

1:1

1:2

O-H Si-O Cl-H TMA-O TMA-Cl z Cl z N (TMA) H2O[1]-Cl H2O[1]-TMA z O (H2O[1]) H2O-H2O H2O2-Cl H2O2-TMA z O (H2O[2]) surf. area (Å2)

0.96 1.63

1.00 1.62 2.03 2.18 2.61 5.58 5.63

1.00 1.62 2.03 2.26 3.43 4.83 5.11

1:1 H2O

1:2 H2O

1:1 2H2O

1:2 2H2O

0.99 1.60 2.03 2.27 2.73 5.64 5.54 2.23 2.20 3.42

1.00 1.61 1.98 2.81 2.56 5.91 5.44 2.19 2.15 4.29

0.99 1.62 2.04 2.41 2.91 5.55 5.68 2.37 2.29 3.22 3.05 2.31 2.19 3.38 45.1

0.98 1.62 2.15 2.23 3.01 4.72 5.46 2.29 2.23 2.22 3.58 2.32 2.37 3.50 43.5

1.58 2.27 4.76 1.61 2.43 3.93 1.68 2.20 3.29 44.9

1.57 2.09 4.51 1.55 2.30 4.59 1.69 2.46 3.83 42.5

(A)

45.2

44.4

43.2

44.7

40.8

(B) Si-O TMA-O z N(TMA) H2O[1]-O H2O[1]-TMA z O (H2O[1]) H2O-H2O H2O2-TMA z O (H2O[2]) surf. area (Å2)

1.55

42.0

1.57 1.91 4.13

44.2

1.56 2.03 4.03

42.4

1.58 2.08 4.41 1.69 2.20 3.96

44.3

1.57 2.11 4.47 1.72 2.11 4.39

42.1

Coverage is indicated as 1:x (x ) 1, 2), while for the hydration degree the number of water molecules per TMA is given. Details are given in the text. a

for the basic and acidic media, respectively. ∆E provides a comparative index of the energetic preference of the molecules to adsorb at different coverages. For each coverage and media, there are two hydration states considered by including one or two water molecules per TMA+. The starting points for the optimizations of the monohydrated models were built considering the geometry of the corresponding anhydrous system together with a water molecule located in the largest void region of the interface. The starting dihydrated geometries were obtained from the monohydrated ones in the same way. This procedure obviously does not exhaust all the possible arrangements of molecules along the interface but provides reasonable representations of sets of slightly energetically differing configurations, whose characterization provides very useful information on the actually disordered system. Hydration energies were estimated as the energies required to incorporate one water molecule into the systems. The values have been corrected for BSSE adopting the counterpoise scheme.38 Results The most relevant geometrical features of the models considered are documented in Table 1. Interatomic distances (R-X), positions along the z axis (z R), and surface areas per unit cell are shown. When R is a molecule, then the position of the central atom (N for TMA and O for H2O) is always considered to quantify the z position, while distances are estimated considering the H atom of R that is closest to X. As concerns the origin for the z axis, it has been taken to be on the layer of Si atoms at the equatorial part of the natriolite unit containing the active site and located furthest from the active surface. The original unit cell is also used to compute the area in the case of 1:2 coverage; hence, it is actually the supercell

Figure 2. Optimized structures of anhydrous systems in basic medium. Top and bottom, 1:1 and 1:2 coverages; right and left, top and lateral views, respectively. In both left-hand side panels, the 2D unit cell borders of both coverage models are depicted.

area divided by 4. The optimized structures are also depicted in Figures 2-7. Table 2 provides the Mulliken population analyses of the electronic structures of the optimized models. Atomic populations are given for the surface O and Si atoms together with the active H and adsorbed Cl- in the acidic case. Regarding the remaining Si and O atoms, their averaged populations, denoted as 〈X〉, are documented. For H2O and TMA+, group

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Figure 3. Optimized structures of monohydrated systems in basic medium. Top and bottom, 1:1 and 1:2 coverages; right and left, top and lateral views, respectively.

Figure 4. Optimized structures of dihydrated systems in basic medium. Top and bottom, 1:1 and 1:2 coverages; right and left, top and lateral views, respectively.

populations are provided, computed as the sum of all atomic contributions in each species. In order to provide an additional picture of the electronic structure of the models, the electron density and electrostatic potential maps have been considered. In Figure 8 they are shown together with the difference maps between them and the densities and potentials generated by the ideal superposition of noninteracting (fixed geometry) surface-interface systems for the acidic case. In the case of the basic interface, only the densities and their difference maps are shown in Figure 9, because the electrostatic potential of the separated systems are not to be computed since both moieties are infinitely charged species and the reference electrostatic energy level provided by the Jellium approach is arbitrary. This makes nonsense of the potential energy difference maps. Some energetic data are given in Tables 3 and 4. In the former, the first and second hydration energies of both acidic

Ferna´ndez-Cata´ et al.

Figure 5. Optimized structures of anhydrous systems in acidic medium. Top and bottom, 1:1 and 1:2 coverages; right and left, top and lateral views, respectively.

Figure 6. Optimized structures of monohydrated systems in acidic medium. Top and bottom, 1:1 and 1:2 coverages; right and left, top and lateral views, respectively.

and basic interfaces are listed, while the latter contains the energy differences between the two coverages considered at different hydration degrees. These data allows for comparison of stabilities between different interface organizations and compositions. Discussion Interface Organization. As it is shown in Figures 2-7, in all models considered the surfactant heads and the species in solution form interfaces located very close over the silica surface, in such a way that the pockets that appear between the natrolite cages are more or less filled by the organic moiety. Obviously, the particular organization of the species in each interface strongly depends on the conditions: acidity, coverage, and hydration degree.

Interaction between Silica Surfaces and Surfactants

Figure 7. Optimized structures of dihydrated systems in acidic medium. Top and bottom, 1:1 and 1:2 coverages; right and left, top and lateral views, respectively.

In both acidic and basic media, the main interaction between the surface and the interface is the electrostatic attraction between the TMA cation and the O atoms at the silica surface. In addition, the interfaces formed in acidic conditions envisage hydrogen bonds mainly involving the counterion, Cl-, and the surface silanol groups. As expected, the electrostatic interaction is stronger in the basic medium, as there is a net charge separation between the surface and the interface, with negative and positive charges owing to the presence of the silanolate groups and the surfactant heads, respectively. Such strong forces are reflected in the TMA-O distances and the position of the surfactant heads over the surface pockets, namely, z N(TMA), documented in Table 1. Both quantities are smaller in the basic media compared to the acidic conditions, the former being about 0.2-0.3 Å shorter, while the latter features a reduction of more than 1.0 Å. This indicates a much more effective penetration of the surfactant head into the surface pockets in basic conditions. The corresponding spatial arrangements are apparent from the pictures provided in Figures 2 and 5. The hydrogen bond between the Cl- and the silanol H atoms in the acid interface is quite weak, a fact reflected in the rather large Cl-H distances (Table 1) that are about 2.0 Å or even larger in a few cases. At the same time, the silanol O-H distance slightly increases with respect to the clean surface, revealing a moderate increase of the bond polarity. A remarkable point is that at 1:1 coverage each Cl- is bonded to a single H atom, while when decreasing to 1:2 it appears that the anion relocates over the surface so as to be shared by two silanol groups (see Figure 5). The energetic results documented in Table 4 agree with this picture, as the 1:2 coverage appears to be favored by about 11 kJ mol-1 with respect to the highest one, this fact being attributable to the additional stability provided by the hydrogen bonds between the Cl- and the free silanol groups when no water molecules are present. In the anhydrous basic case, it turns out that the highest coverage is energetically favored by about 21 kJ mol-1. The stronger electrostatic interaction between the

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13313 silanolate and the TMA+ in 1:1 with respect to 1:2 may be responsible for this energetic difference. This is in agreement with the shorter TMA-silanolate distance (Table 1) and the larger negative charge of the silanolate O atom (Table 2) in the system with higher coverage. However, special caution must be taken in interpreting the fragment charges obtained through the Mulliken analysis. It turns out that the charges on TMA+ are largest for the lowest coverage in contradiction with the previous interpretation. Interactions between the interface species strongly depend on the hydration degree, acid-base conditions, and surface coverage. In the anhydrous acid interface, both charged species, Cl- and TMA+, interact with each other through attractive electrostatic forces. In basic conditions, however, the lateral interactions are mainly repulsive between the TMA+ cations, albeit this repulsion is energetically compensated by the strong interaction with the silanol groups as it is discussed later. When adding some water content to the medium, the picture significantly differs. Water molecules are accommodated in the interface forming a grid of H-bond connections between them, the chloride anions in the acidic medium, the silanol groups, and the TMA+ cations. This gives rise to a certain degree of reorganization of the interfaces compared to the corresponding anhydrous cases (see Figures 3, 4, 6, and 7). Water molecules are in general disposed along a layer that lies closer to the silica surface than the other species in the interface, regardless of the acid-base medium considered. It arises from the data documented in Table 1 that the layer location is within the range of 2.2-4.3 and 3.3-4.6 Å above the z reference level, in the acidic and basic cases, respectively. The way the water molecules accommodate themselves in the interface, however, differs between the acidic and basic cases. In the former the molecules are oriented so as to establish an H-bond with the chloride (see H2O-Cl distances in Table 1) and a weaker bond of electrostatic character between the H atom at the other side of the molecule and the bridging O atoms located in the pockets. This occurs in general irrespective of the coverage and for both water molecules in the dihydrated models. This particular disposition of the molecules in the acid interface may be observed in Figures 6 and 7. The water molecule that is adsorbed first in the basic interfaces is strongly H-bonded to the silanolate group. This can be seen from the H2O[1]-O distances documented in Table 1 that range from 1.55 to 1.72 Å. The water molecule adsorbed next is always oriented toward the O atom of the previous one, also displaying a H-bond, with H-O distances of about 1.68 Å (see H2O-H2O entry in Table 1), strongly suggesting that a cooperative effect is acting that eventually could favor the formation of larger chains dangling from the silanolate group if more water molecules were added. By virtue of the amphoteric nature of the water molecules, they envisage H-bonds to both the anionic species and TMA+ through the H atoms of the methyl groups. Consequently, water plays a role of structural stabilizer/organizer of all the species in the interface. The distances between the water molecules and the closest methyl H atom are between 2.1 and 2.5 Å (Table 1) irrespective of the acidity, but displaying a trend to lower values as the coverage decreases. A similar trend takes place for the H2O-Cl distances in the acid interfaces. This effect may indicate that in models with high coverage, the repulsive forces brought forth by the closeness of the ions partially compensate the attractive electrostatic forces between the anions and cations contained in the interface.

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Ferna´ndez-Cata´ et al.

TABLE 2: Selected Mulliken Atomic Charges in the Optimized Models in Acidic (A) and Basic (B) Conditionsa q/e

clean surface

1:1

1:2

O Si 〈O〉 〈Si〉 H TMA Cl H2O[1] H 2 O2

-0.658 1.504 -0.737 1.496 0.319

-0.681 1.526 -0.745 1.505 0.324 0.769 -0.743

-0.691 (-0.688) 1.533 (1.531) -0.748 1.512 0.334 (0.331) 0.807 -0.755

O Si 〈O〉 〈Si〉 TMA H2O[1] H 2 O2

-0.860 1.367 -0.770 1.446

-0.817 1.518 -0.771 1.501 0.662

-0.801 (-0.677) 1.470 (1.518) -0.758 1.502 0.700

1:1 H2O

1:2 H2O

1:1 2H2O

1:2 2H2O

(A) -0.670 1.542 -0.746 1.511 0.311 0.725 -0.705 0.004

-0.685 (-0.681) 1.533 (1.555) -0.754 1.527 0.323 (0.334) 0.752 -0.718 0.001

-0.696 1.553 -0.746 1.515 0.316 0.738 -0.707 0.011 -0.015

-0.676 (-0.685) 1.529 (1.531) -0.749 1.513 0.326 0.765 -0.710 -0.011 0.021

(B) -0.814 1.529 -0.768 1.503 0.633 -0.029

-0.801 (-0.659) 1.481 (1.509) -0.757 1.506 0.691 -0.042

-0.833 1.537 -0.770 1.505 0.633 -0.002 -0.001

-0.809 (-0.707) 1.488 (1.531) -0.760 1.507 0.687 0.003 -0.001

a

Labels are the same as adopted in Table 1. Charges between brackets refer to the atoms around the free silanol in the 1:2 covering. See details in the text.

Figure 8. Charge density and electrostatic potential total and difference maps for the anhydrous system in acid medium: (a) charge density of the interacting system, (b) electrostatic potential of the interacting system, (c) charge density difference between interacting systems and the sum of noninteracting systems, (d) electrostatic potential difference between interacting systems and the sum of noninteracting systems, and (e) view of the atoms that lie close to the plane chosen for the maps. Isolevels depicted from -0.01 to 0.01 |e| at each 0.005 |e|.

The Inorganic Surface. The organization of the interface has some influence on the way the silica surface structurally relaxes upon adsorption. An indication of such behavior arises from the consideration of the unit cell surface areas of the different models documented in Table 1. It turns out that the interaction of the clean surfaces with the interfaces may afford a contraction or expansion of the unit cell area with dependence on whether the medium is acidic or basic, respectively. It also turns out that the models with 1:2 coverage have always the smallest areas of each series. The surface contraction observed in going from 1:1 to 1:2 coverage is a consequence of the overall action of attractive forces between the interface species and between them and the silica moiety. In this aspect, the water content of the interface plays a crucial role since the water molecules are able to establish attractive interactions with all species in solution and in the surface centers. The smallest surface area is reached in the monohydrated 1:2 coverage in

both acidic and basic media. The contraction is most pronounced in the former, but the incorporation of a second water molecule brings about a significant reexpansion. To a lesser extent, a similar behavior is observed for the basic case. Despite the fact that the silica surface is rather flexible, surface contractions seem to occur at the expense of the stability of the system. This would explain the hydration energies behavior of the acid 1:2 model (see Table 3). Energetic results indicate that, while the first hydration does not provide any stabilization of the system (barely 4.6 kJ mol-1), the second one is more favorable by 51 kJ mol-1, which can be in part attributed to the structural relaxation reflected in the surface expansion. The results for the systems at high coverage show a rather different situation. The incorporation of water molecules to the interface slightly changes the surface areas, while the second hydration energy is in all cases smaller than the first one. This is explained by the fact that the relatively high concentration

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Figure 9. Charge density difference maps for the anhydrous system in basic medium. Top and bottom: view from (1 -1 0) and (-1 -1-0) planes, respectively. Isolevels depicted from -0.02 to 0.02 |e| at each 0.002 |e|.

TABLE 3: First (1st) and Second (2nd) Hydration Energies (kJ mol-1) for All Conditions and Coverages basic

acid

hydration

1:1

1:2

1:1

1:2

1st 2nd

-57.3 -22.2

-58.6 -69.0

-30.5 -15.1

-4.6 -50.6

TABLE 4: Energy Differences (kJ mol-1) between 1:2 and 1:1 Coverages (See Expression in the Text) at Different Hydration States N H2O/c

basic

acid

0 1 2

20.5 24.7 -23.4

-10.9 18.0 -10.9

of species in the interface provides not too much room for the incorporation of new water molecules making the second hydration less favorable than in the low coverage case. This is in full agreement with the trends observed in Table 4 that indicate the dihydrated interface is more stable at low coverage regardless of the acidity. Electronic Features. The interaction between the silica surface and the surfactant heads is mainly driven by electrostatic forces. However, the present study gives evidence that the presence of the interface induces remarkable changes in the electronic structure of the silica moiety involving phenomena other than pure Coulombic interactions. A clear polarization of the O-H silanol group occurs in the acidic medium, attributable to the influence of the neighboring Cl-, as can be seen from the electrostatic potential and electronic density difference maps given in Figure 8 for the anhydrous case. Such a polarization is accompanied by a slight increase of the covalent character of the hydrogen bond between the silanol group and the chloride anion embodied by the small density excess along the Cl-H line. In a similar way, the electronic density difference map for the anhydrous basic interface (see Figure 9) shows that the influence of the closeness between the surfactant heads and the surface leads to an increase of the overall ionicity of the latter in a quite surprising delocalized manner. The increase of the ionicity of the surface upon interaction with the interfaces is also suggested by the Mulliken population

analysis given in Table 2. Considering the average charges of the atoms not involved in the silanol (silanolate) groups, namely, 〈Si〉 and 〈O〉, it arises that in the acidic case, both atom types experiment a slight charge increase with respect to the clean surface for all hydration levels and coverages. In the basic case, the charges of the O atoms practically do not change (it is less than 0.01 |e|) with the interaction of the interface, while there is a net increase of charge on the Si atoms in all interface compositions considered. This overall trend to increase the ionicity of the surface upon interaction with the interface is in agreement with a previous study on the synthesis of zeolite ITQ-12 through the F-route with trimethylimidazolium as template.32 In that case, it has been shown that the presence of the occluded F- anions and nitrogenated cations appears associated with an increase of the ionicity of the silica part of the material with respect to the pure silica material. The present study suggests that the enhancement of the ionic character of the as-synthesized silica may be a general feature when surfactants are present during the formation of the materials. It is worth noting that the polarization of the Si-O bonds lying close to the surface does favor the affinity between the surface and the interface by increasing the hydrophilicity of the former, which gives rise to an overall stabilization of the material synthesized in polar solvent medium. This effect, while looking reasonable for basic synthesis where there is a net charge separation between the solid and the liquid phases, is particularly surprising for the synthesis in acidic medium. Conclusions The interaction between the silica surface and a phase containing the surfactant heads and their environment in aqueous acidic and basic media has been studied by means of hybrid Kohn-Sham methods as implemented in the CRYSTAL06 code. The level of theory adopted allows for a proper description of both electrostatic and electronic effects. This provides a quite realistic picture of the phenomena that drive the stability of the interfaces formed upon the synthesis of mesoporous materials in acidic and basic conditions. The effects of coverage and hydration degree have been also considered in the models. Although electrostatic forces dominate the interaction between the silica surface and the surfactant heads, the present results provide strong evidence that other effects are playing nonnegligible roles in the stability of the systems. The presence of the interface species induces a clear increase of the ionicity of the Si-O bonds on the basic surface (particularly, those belonging to the silanolate groups) that enhances their hydrophilicity favoring, at the same time, the affinity with the polar phase. In the acidic case, a much smaller charge increase mainly affecting the silanol groups suggests a similar but less pronounced effect. Perhaps connected to this, the most energetically favored situations concern those with not too high coverage and quite large water content, at least in those models considered in the present work. Water molecules play the role of organizing and stabilizing the interface thanks to their amphoteric nature that allows them to form H-bonds with all species in solution and surface centers. Interestingly, water molecules that are located closer to the surface than the other species in solution do not display similar organizations in acidic and basic media. In the former they interact quite weakly through their H atoms with one chloride anion and one O-bridging of the surface. This situation allows the anion to participate in several interactions with the H atoms of the silanol groups and other water molecules. In the basic

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medium, some water molecules undergo quite strong H-bonds with the silanolate groups in a stoichiometric 1:1 relationship. The exceeding water molecules are not connected to the same anion, like in the acidic case, but form chains by linking themselves to the water molecules already adsorbed on the silanolate groups. The chains are rather stable thanks to the occurrence of sequences of H-bonds that are likely favored by cooperative effects. The network of forces between the species present in the interface generate moderate geometrical distortions in the surface compared to the clean situation. These distortions appear to be slightly more relevant in acidic than in basic media. This is to be connected to the previously discussed increase of the ionicity of the Si-O bonds with respect to the clean surface and agrees with previous results32 that suggest the synthesis of purely silicic phases in acidic medium with surfactants is favored by the increase of flexibility driven by the enhanced ionicity of the system. The present is a preliminary study that will be completed in further works by considering not only the surfactant heads but also larger hydrocarbon chains so as to include the hydrophobic interactions between surfactant molecules and a larger set of coverages and hydration levels. Acknowledgment. We thank the “FOMES2000 Co´mputo Cientı´fico” SEP project for generous allocation of CPU time on the IBM-p690 supercomputer at UAEM. Support from SEPCONACYT (project 46983) is also gratefully acknowledged. G.F.-C., H.C.-R, and A.P.-G thank Dr. Karen Edler for useful ideas and discussion. References and Notes (1) Wan, Y.; Zhao, D. Chem. ReV. 2007, 107, 2821. (2) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (3) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenkert, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (4) Ogawa, M.; Kikuchi, T. AdV. Mater. 1998, 10, 1077. (5) Zhang, J.; Luz, Z.; Zimmermamn, H.; Goldfarb, D. J. Phys. Chem. B 2000, 104, 279. (6) Vautier-Giongo, C.; Pastore, H. O. J. Colloid Interface Sci. 2006, 299, 874. (7) Tjandra, W.; Yao, J.; Tam, K. C. Langmuir 2006, 22, 1493. (8) Baccile, N.; Laurent, G.; Bonhomme, C.; Innocenzi, P.; Babonneau, F. Chem. Mater. 2007, 19, 1343.

Ferna´ndez-Cata´ et al. (9) Innocenzi, P.; Falcaro, P.; Grosso, D.; Babonneau, F. J. Phys. Chem. B 2003, 107, 4711. (10) Wang, L.; Exarhos, G. J. Phys. Chem. B 2003, 107, 443. (11) Brennan, T.; Roser, S. J.; Mann, S.; Edler, K. Chem. Mater. 2002, 14, 4292. (12) Frasch, J.; Lebeau, B.; Soulard, M.; Patarin, J.; Zana, R. Langmuir 2000, 16, 9049. (13) Gibaud, A.; Grosso, D.; Smarsly, B.; Baptiste, A.; Bardeau, J. F.; Babonneau, F.; Doshi, D. A.; Chen, Z.; Brinker, C. J.; Sanchez, C. J. Phys. Chem. B 2003, 107, 6114. (14) Brennan, T.; Roser, S. J.; Mann, S.; Edler, K. Langmuir 2003, 19, 2639. (15) Edler, K. J.; Goldar, A.; Hughes, A. V.; Roser, S. J.; Mann, S. Microporous Mesoporous Mater. 2005, 44-45, 661. (16) Patarin, J.; Lebeau, B.; Zana, R. Curr. Opin. Colloid Interface Sci. 2002, 7, 107. (17) Soller Illia, G. J. A. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093. (18) Davison, A. Curr. Opin. Colloid Interface Sci. 2002, 7, 92. (19) Ciesla, U.; Schuth, F. Microporous Mesoporous Mater. 1999, 27, 131. (20) Schuth, F.; Schmidt, W. AdV. Mater. 2002, 14, 629. (21) Berggren, A.; Palmqvist, A. E. C.; Holmberg, K. Soft Mater. 2005, 1, 219. (22) Mesa, M.; Sierra, L.; Guth, J.-L. Microporous Mesoporous Mater. 2007, 102, 70. (23) Lin, H. P.; Mou, C. Y. Acc. Chem. Res. 2002, 35, 927. (24) Fois, E.; Gamba, A.; Tabacchi, G.; Coluccia, S.; Martra, G. J. Phys. Chem. B 2003, 107, 10767. (25) Jorge, M.; Gomes, J. R. B.; Cordeiro, M. N. D. S.; Seaton, N. A. J. Am. Chem. Soc. 2007, 129, 15414. (26) Shah, K.; Chiu, P.; Jain, M.; Fortes, J.; Moudgil, B.; Sinnott, S. Langmuir 2005, 21, 5337. (27) Civalleri, B.; Cassasa, S.; Garrone, E.; Pisani, C.; Ugliengo, P. J. Phys. Chem. B 1999, 103, 2165. (28) Ugliengo, P.; Civalleri, B.; Dovesi, R.; Zicovich-Wilson, C. M. Phys. Chem. Chem. Phys. 1999, 1, 545. (29) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (30) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, Ph.; Llunell, M. CRYSTAL 2006 User’s Manual. (31) Cora`, F.; Alfredsson, M.; Mallia, G.; Middlemiss, D. S.; Mackrodt, W. C.; Dovesi, R.; Orlando, R. Struct. Bonding 2007, 113, 171. (32) Zicovich-Wilson, C. M.; San-Roma´n, M. L.; Camblor, M. A.; Pascal, F.; Durand-Niconoff, S. J. Am. Chem. Soc. 2007, 129, 11512. (33) Solans-Monfort, X.; Sodupe, M.; Branchadell, V.; Sauer, J.; Orlando, R.; Ugliengo, P. J. Phys. Chem. 2005, 109, 3539. (34) Torres, F. J.; Civalleri, B.; Pisani, C.; Ugliengo, P. J. Phys. Chem. B. 2006, 110, 10467. (35) Civalleri, B.; Zicovich-Wilson, C. M.; Ugliengo, P.; Saunders, V. R.; Dovesi, R. Chem. Phys. Lett. 1998, 292, 394. (36) Civalleri, B.; Ugliengo, P. J. Phys. Chem. B 2000, 104, 9491. (37) Doll, K.; Saunders, V. R.; Harrison, N. M. Int. J. Quantum Chem. 2001, 82, 1. (38) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553.

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