Mechanochemical Behavior of Surface Radicals in Ground Quartz

ARTICLE pubs.acs.org/JPCC

Mechanochemical Behavior of Surface Radicals in Ground Quartz Francesco Delogu* Dipartimento di Ingegneria Chimica e Materiali, Universit
a degli Studi di Cagliari, via Marengo 2, I-09123 Cagliari, Italy

bS Supporting Information ABSTRACT: This work focuses on the chemical reactivity of quartz powders undergoing mechanical activation. The powders were submitted to mechanical loads in the presence of a free radical scavenger dissolved in ethanol. A gradual consumption of the scavenger was observed, ascribable to the chemical reactivity of the active sites generated at the surface of quartz powders by fracture and attrition. The apparent surface density of the active sites was indirectly measured by relating the observed reaction kinetics to the specific surface area of quartz powders. It is shown that not only the apparent surface density of the active sites generated by fracture is higher than the one of the active sites formed by attrition but also that it is even higher than the maximum possible surface density of dangling bonds. This points out the high reactivity of the surface sites generated by fracture and identifies the nature of mechanochemical effects in the present case.

1. INTRODUCTION Mechanochemistry is the specific branch of chemistry that relates to the effects of nonhydrostatic stresses and plastic strain on the chemical processes responsible for the change of the energy and entropy as well as of the structure and composition of molecules, crystals, and other aggregates of matter.1
6 Recently, the directional character of mechanical stresses has been exploited to selectively break and reform covalent bonds in individual molecules.6
10 These studies have clearly demonstrated that purely mechanical forces can activate covalent systems and have a definite chemical effect.6
10 Such novel achievements are only the last exciting experimental evidence brought to light in the long history of Mechanochemistry,11 which for centuries has kept its main focus on solid phases.12 Starting from the early years of 19th century, the field has grown slowly for decades until an explosion of interest took place during the 1980s, connected with the potential of mechanochemical methods in the preparation of nanostructured phases and amorphous alloys.13 At present, mechanochemistry is a well-established field in Materials Science and Solid-State Chemistry, with an impressive number of investigations carried out on the most different solid
solid, solid
liquid, and solid
gas systems.13
15 Yet, the conceptual framework regarding mechanochemical effects is still largely fragmentary and unsatisfactory. Except for a few valuable contributions,5,16 the thermodynamics and the kinetics of mechanochemical transformations are not well understood. Because of the far-from-equilibrium conditions imposed by the mechanical loading, heat and mass transfer regimes deviating from conventional ones must be expected.16
23 In particular, the observed transformations should be described within the conceptual framework of nonequilibrium thermodynamics.16
23 In spite of this, no r 2011 American Chemical Society

comprehensive approach to mechanochemical transformations has been developed yet along such line.15 At the same time, experimental evidence often lack of the necessary accuracy to address the theoretical approaches and point out atomistic effects. As a consequence, many fundamental questions regarding the mechanochemical behavior of solid phases wait for a definite response. One of these concerns the exact nature of mechanochemical effects in phase transformations or chemical reactions undergone by mechanically activated solids. The present study focuses precisely on such question by investigating the reactivity of the active sites generated at the surface of quartz powders during their mechanical grinding. The generation of radicals at the surface of ground quartz is known since the 1960s24 and has been intensely scrutinized in view of its importance for human health.25
28 Any fracture of the SiO2 crystalline lattice determines the formation of at least two types of surface sites.29 One is due to the homolytic cleavage of the tSi— O— chemical bond, which results in the generation of the silyl tSi• and siloxyl tSi
O• radicals. The other, originated by the heterolytic cleavage of the tSi
O
chemical bond, allows the formation of the tSi+ and tSi—O
charged species. Theoretical studies suggest that more complicate species can also form.30 All of the possible surface active sites can either recombine to form siloxane tSi—O—Sit units or react with their chemical environment.31 The reaction of quartz powders with suitable chemical compounds provides a useful tool to measure the reactivity of the active sites and quantify their surface density. The potential of this method has been clearly demonstrated in a recent investigation aimed at quantifying the kinetics of radical generation during Received: July 6, 2011 Revised: August 26, 2011 Published: September 26, 2011 21230

dx.doi.org/10.1021/jp206354p | J. Phys. Chem. C 2011, 115, 21230–21235

The Journal of Physical Chemistry C

Figure 1. The mechanochemical reactor at rest with the quartz powders at the bottom of the reactor chamber, the ball, the ethanol solution containing DPPH, and a small volume at the top occupied by Ar gas.

the grinding of quartz powders.32 Carried out in ethanol (C2H5OH) in the presence of the free radical 2,2-diphenyl-1picrylhydrazyl (DPPH), the mechanical activation of quartz was shown to govern the rate of DPPH consumption.32 DPPH was consumed by reaction with the hydrogen radicals H• generated by the interaction of the surface sites with C2H5OH.32 Two different contributions to the formation of surface sites were identified, connected respectively with the fracture of quartz particles and the activation of their surfaces by attrition.32 As usual, the rates of the different processes observed were referred to time. Although apparently obvious in the light of conventional chemical kinetics, the choice of time is not the most suitable one for mechanically activated processes. In this regard, it must be noted that two different time scales govern the rate of mechanical activation processes. One is the time scale underlying the intrinsic chemical reactivity governed by surface species lifetime, typically of a few nanoseconds for quartz surfaces embedded in a reactive environment.33
35 The other is the time scale of consecutive collisions, generally on the order of a few milliseconds or more.2
4,12,13,15 The difference between the mentioned time scales is such that the rate of mechanical activation is only controlled by the time scale of collisions. It follows that referring the mechanochemical reactivity to time can be misleading. In the light of the above-mentioned considerations, in the present study the mechanochemical reactivity is referred to the individual events responsible for the mechanical processing, i.e. the collisions between grinding tools. It is shown that this choice allows the identification of mechanochemical effects and their quantification.

ARTICLE

2. EXPERIMENTAL OUTLINE Pure quartz powders were dispersed in absolute ethanol and mechanically processed under Ar atmosphere in the presence of the free radical DPPH. The mechanochemical reactivity of the system was investigated by measuring the specific surface area of quartz powders and the concentration of DPPH in ethanol as a function of the number of collisions. 2. 1. Materials. α-Quartz powders with a nominal purity of 99.5% and average particle size in the range of 53 and 0.1 μm were purchased from Aldrich. The powders were sieved to narrow the particle size distribution and obtain a batch including particles with average size between 53 and 44 μm. According to the analysis carried out, the specific surface area of the sieved powders amounts to 0.041 ( 0.002 m2 g
1. Experiments were carried out on such powders. Absolute ethanol and DPPH were purchased from Aldrich and used as supplied. 2. 2. Mechanical Processing. The mechanical treatment of quartz powders was carried out by employing a suitably designed mechanochemical reactor. It substantially consists of a stainless steel cylindrical reactor, with a stainless steel ball inside, fixed on a mechanical arm susceptible of a vertical harmonic oscillation. In the present case, it was operated at amplitude and frequency values such to determine a single collision every 5 s. A ball of 20 g was used, and collisions occurred with an average energy of about 0.08 J. Experiments were carried out with a total mass mp of powder equal to 2 g. A schematic description of the reaction chamber is shown in Figure 1. The reaction chamber was almost completely filled with 30 mL of ethanol. The remaining volume was occupied by Ar gas. Because of the characteristic features of the reactor displacements, collisions always occur on the bottom of the reactor. Here, the ball traps part of the quartz powder and dissipates its energy, which is completely transferred to the powders. The top basis of the reactor is equipped with a gastight septum connector, which allows the sampling of the liquid phase. 2. 3. Specific Area of Quartz Powders. The mechanical processing was interrupted after selected numbers of collisions. Powders were separated by the ethanol solution by centrifugation for 15 min at 20000 rpm. Afterward, the powders were kept for 2 h at 393 K to dry, degassed at 300 K, and exposed to nitrogen at about 77 K in a Fisons Sorptomatic 1900 apparatus. The specific surface area was estimated by physical adsorption of nitrogen according to the so-called BET method. 2. 4. Scanning Electron Microscopy (SEM). SEM observation was used to monitor the size reduction of quartz powders and measure their size distribution. A Zeiss EVO LS15 microscope was employed. 2. 5. DPPH Concentration in Ethanol. Being a reactive scavenger, the free radical DPPH is normally used as an indicator of the radical character of a chemical reaction.36 Because of a strong absorption band approximately centered around 520 nm, it exhibits an intense violet color in solution, whereas it becomes pale yellow or even colorless when neutralized by H• radical species.36 The color change permits to monitor with accuracy the variation in DPPH concentration upon reaction with radicals by UV
vis spectrophotometry. In the present work, the DPPH concentration was estimated by using a Varian Cary 50 Scan apparatus. A sample of the ethanol solution separated by centrifugation from the quartz 21231

dx.doi.org/10.1021/jp206354p |J. Phys. Chem. C 2011, 115, 21230–21235

The Journal of Physical Chemistry C

ARTICLE

powders was introduced into a suitable PMMA cuvette with optical path 1 cm long. The absorption spectrum was recorded in the wavelength range between 300 and 900 nm. The values of DPPH concentration were worked out from the absorption curves by using an optical absorption coefficient of 11300 L mol
1 cm
1.37 The initial concentration of DPPH in ethanol was equal to 0.1 M.

3. RESULTS A few optical absorption curves are shown in Figure 2. Data regard samples of ethanol solution separated by quartz powders that have undergone an increasing number of collisions. The curves exhibit two absorption peaks at wavelengths λ equal to about 343 and 516 nm. Two isosbestic points are observed roughly at 370 and 470 nm. It can be seen that the relative intensity of the absorption peaks decreases as the number of collisions increases. This indicates a consumption of the free radical DPPH in the ethanol solution. Such consumption can be ascribed to the interaction of DPPH with the H• radicals formed in solution by reaction of the C2H5OH molecules with the active sites at the surface of quartz powders, and in particular with the tSi• and tSi—O• radicals. The reaction scheme can be summarized as indicated below:32 S
O
Si  f  S• þ  S
O•

ð1Þ

S• þ C2 H5 OH f  S
OC2 H5 þ H•

ð2Þ

S
O• þ C2 H5 OH f  S
OH þ • C 2 H4 OH

ð3Þ

DPPH þ H• f DPPH
H

ð4Þ

First, in 1, the mechanical activation determines the formation of surface radicals. Then, the silyl and siloxyl radicals react with ethanol molecules, eqs 2 and 3, to form hydrogen radicals. Finally, in eq 4, the hydrogen radicals combine with the DPPH molecules to form the neutralized DPPH
H species. The number NDPPH of DPPH moles in ethanol solution was measured by UV
vis spectrophotometry for all of the samples collected after a selected number n of collisions have occurred. The results are shown in Figure 3a. NDPPH undergoes a continuous decrease. The rate of decrease becomes increasingly lower as n increases until, after approximately 4000 collisions, it reaches a plateau value and NDPPH keeps decreasing according to a linear trend. Because of the connection between DPPH concentration and number of reactive species generated at the quartz surface, the NDPPH data in Figure 3a were compared with the values of the average specific surface area S of quartz powders. The data shown in Figure 3b indicate that S increases according to an asymptotic trend, reaching a plateau after about 4000 collisions. The changes in the number NDPPH of DPPH moles and in the specific surface area S of quartz powders seem to be intimately correlated. Indeed, both these quantities exhibit a two-regime variation, the boundary between the two regimes being approximately the same, i.e., 4000 collisions. Here, it is also worth noting that the mechanical processing of the ethanol solution 0.1 M in DPPH in the absence of quartz powders does not induce any significant decrease of the DPPH concentration. More specifically, the number NDPPH of DPPH moles decreases roughly by only 0.3 mmol after 8000 collisions have taken place. Such a decrease can be tentatively ascribed to

Figure 2. The intensity I of the optical absorption of ethanol solutions as a function of the UV
vis radiation wavelength λ. Arrows indicate the relative displacement of curves. Data refer to samples collected after the quartz powders have undergone 0 (black), 600 (red), 1400 (green), 2400 (blue), 5200 (cyan), and 10000 (magenta) collisions.

Figure 3. (a) The number NDPPH of DPPH moles in ethanol solution and (b) the average specific surface area S of quartz powders as a function of the number n of collisions occurred. Best-fitted curves are also shown.

the interaction of DPPH molecules with some sort of reactive center generated by collision at the stainless steel surfaces of reactor and ball. In any case, the very small decrease observed in the absence of quartz powders permits to exclude any significant spurious effect of the stainless steel reactivity toward the DPPH on the observed reactivity of quartz powders.

4. DISCUSSION In the light of the reaction kinetics pointed out in previous work,32 the NDPPH and S data are expected to correlate. In particular, it must be expected that the rate at which NDPPH decreases is somewhat proportional to the surface area S available to the reaction at any given collision. Unfortunately, the scatter affecting the experimental points prevents their direct use to investigate such correlation. For this reason, a suitable model must be developed. A line of approach to the question is suggested by a closer look at the plot of the average surface area S shown in Figure 3b. The experimental points can be satisfactorily interpolated by the mathematical expression S ¼ Sin expð
knÞ þ Sf in ½1
expð
knÞ

ð5Þ

where Sin and Sfin represent the initial and final S values, and k is the apparent rate constant for the surface area increase. Such an 21232

dx.doi.org/10.1021/jp206354p |J. Phys. Chem. C 2011, 115, 21230–21235

The Journal of Physical Chemistry C expression can be interpreted as the weighted average of the surface areas Sin and Sfin exhibited by two different fractions of quartz powders, respectively equal to exp(
kn) and 1
exp(
kn). The former measures the fraction of quartz powders retaining the initial specific surface area Sin, whereas the latter relates to the amount of quartz powders that have reached the final specific surface area Sfin consequent to collision events. The capability of eq 5 of satisfactorily interpolating the experimental points in Figure 3b suggests that, at any given collision, a certain amount of quartz powders changes discontinuously its average specific surface area from Sin to Sfin. Within this framework, the apparent rate constant k represents an average measure of the fraction of quartz powders susceptible of changing their specific surface area from Sin to Sfin during individual collision events. A support to the above-mentioned scenario comes from SEM observations. A statistical analysis of SEM pictures was performed to work out the statistical distribution of the particle size of quartz powders after different numbers of collisions have occurred. Distributions (see Supporting Information for details) exhibit evident bimodal features, with peaks roughly around 0.6 and 50 μm. The relative height of the two peaks changes with the number n of collisions. In particular, the height of the peak at 0.6 μm increases as n increases, whereas the height of the peak at 50 μm decreases. The bimodality of statistical distributions and the simultaneous change of their peaks heights suggest that the quartz powders undergo, on the average, a discontinuous fragmentation process. More specifically, the average particle size changes from about 50 to 0.6 μm without intermediate values. Although this is only a very rough description of the fragmentation process, it seems to catch one of its fundamental features. Regarding the specific surface area, it must be noted that particles of size around 0.6 and 50 μm exhibit a geometric specific surface area of about 3.81 and 0.045 m2 g
1, respectively. These estimates are remarkably close to the Sin and Sfin values obtained by best-fitting the experimental points in Figure 3b with eq 5, which are approximately equal to 0.035 ( 0.003 and 3.673 ( 0.004 m2 g
1, respectively. The best-fitting was carried out by performing a regression analysis aimed at minimizing the leastsquares. The linear regression coefficient measuring the agreement between the theroretical curve and the experimental points amounts to about 0.99. The observed agreement between the Sin and Sfin values obtained by the BET physisorption method and by SEM observations is relatively unusual. In general, the specific surface areas estimated by the BET method are significantly larger than the ones obtained by direct SEM observation. The reason is that, being based on the physisorption of an inert gas, the BET method includes in the measured area also the porosity not detectable by direct SEM observation. Therefore, the agreement between specific surface area values obtained by BET method and SEM observations indicates that the quartz powders do not exhibit open porosity susceptible of gas physisorption. In addition, it suggests a rationale for the capability of eq 5 of best-fitting the experimental S values plotted in Figure 3b. In accordance to the results of SEM observations, it can be inferred that, at any given collision, a fraction of the powder charge changes its specific surface area discontinuously from about 0.03 to 3.67 m2 g
1. The best-fitting of the experimental S points also provides a value for the rate constant k, approximately equal to 1.1  10
3 ( 0.1  10
3. Correspondingly, the mass of quartz powders effectively involved in the change of specific surface area at individual collisions, given by the product kmp, amounts to about

ARTICLE

2.2 mg. It follows that the maximum increase of surface area per collision, expressed by kmp(Sfin
Sin), is equal to about 8.0  10
3 m2. Following the indications given by the experimental evidence, it can be reasonably assumed that the total mass mp of quartz only includes two fractions exp(
kn) and 1
exp(
kn) of powders with average specific surface areas Sin and Sfin, respectively. Of course, this assumption is extremely rough. However, it allows developing a simplified kinetic model for the DPPH consumption that connects the DPPH consumption with the number of active sites at the surface of quartz powders. Two different contributions must be included in the total number of active sites. One is the number Nf of active sites generated by fracture of quartz particles at collisions, with consequent increase of the specific surface area S. The other is the number Na of active sites generated by attrition between quartz particles with no significant change of S. Such distinction is necessary in view of the fact that, in principle, the active sites formed by fracture or by attrition can have a different chemical activity. The DPPH consumption can be described by the equation
dNDPPH ¼ ðNf þ Na Þdn

ð6Þ

The quantities Nf and Na are proportional, respectively, to the surface area generated by fracture at individual collisions and to the total surface area of powder particles involved in individual collisions. The former quantity can be obtained by differentiating eq 1, which provides the increment of specific surface area at individual collisions as a function of the number n of collisions already occurred. Then, the surface area actually generated by fracture can be obtained by multiplying the increment of specific surface area by the mass mp of powder charge. Indicating with α and β the proportionality constants, Nf and Na can be expressed as follows Nf ¼ αkmp ðSf in
Sin Þexpð
knÞ

ð7Þ

Na ¼ βkmp S

ð8Þ

Taking into account eqs 7 and 8, the solution of eq 6 is NDPPH
NDPPH, in ¼
mp fβkSf in þ ðα
βÞðSf in
Sin Þ½1
expð
knÞg ð9Þ where NDPPH,in is the initial number of DPPH moles. Here, it is worth noting that no constraint is imposed to the proportionality constants α and β. It can be only inferred that their values must somewhat relate to the number γ of dangling bonds that can be formed by homolytic and heterolytic cleavage when surfaces are generated by fracture. Such number expresses the maximum possible number of active sites per surface unit and can be roughly determined by considering the density of Si
O units at the surfaces exhibiting the lower energy.34 An average between the (001), (011), (101), and (112) surfaces yields a maximum surface density γ of dangling bonds equal to about 2.1  10
5 mol m
2. The values of the proportionality constants α and β can be only determined by best-fitting eq 9 to the experimental NDPPH data shown in Figure 3a. A regression analysis to minimize the least-squares was carried out by using the Sin, Sfin, and k values previously determined by interpolating the experimental S points 21233

dx.doi.org/10.1021/jp206354p |J. Phys. Chem. C 2011, 115, 21230–21235

The Journal of Physical Chemistry C with eq 5. The best-fitted curve is shown in Figure 3a. It can be seen that the theoretical curve interpolates the experimental points fairly well, providing estimates for the proportionality constants α and β roughly amounting to 8.10  10
5 ( 0.08  10
5 and 4.7  10
6 ( 0.4  10
6 mol m
2, respectively. The linear regression coefficient measuring the goodness of the bestfit amounts to about 0.97. Within the framework of the simplified kinetic model developed, the constants α and β can be regarded as a measure of the apparent surface density of the active sites responsible for the DPPH consumption. Whereas α refers to the sites associated with the surfaces generated by fracture, β refers to the sites created at surfaces by attrition. The contribution of the α active sites to the DPPH consumption is related to the processes responsible for the increase of the specific surface area S and thus exhibits a transient character. Conversely, β active sites can be continuously generated by attrition at a rate dependent on the amount of powders processed at individual collisions and on the extension of surface area available to attrition. The α and β values must be compared with the maximum possible surface density γ of dangling bonds roughly equal to 2.1  10
5 mol m
2. It appears that only about the 20% of the maximum possible number of β active sites is involved in DPPH consumption as a consequence of attrition processes. In contrast, the number of α active sites on surfaces formed by fracture is about 4 times larger than γ. This apparently unreasonable result can be explained by hypothesizing a very high reactivity for surfaces formed by fracture. Although the number of active sites per surface unit cannot be larger than γ, it must be taken into account that the surfaces formed by fracture necessarily undergo significant relaxation and reconstruction processes.38
42 Precisely during such stages, quartz surfaces could transiently form unstable species able to repeatedly interact with the ethanol solution.38
42 This would induce the formation of an anomalously large number of H• radicals, which would explain the rate enhancement of the DPPH consumption process. The reliability of the model developed to describe the kinetics of DPPH consumption was tested by obtaining independent estimates of the surface density β of active sites generated by attrition. To such aim, quartz powders that already reached different asymptotic values Sfin of specific surface area were used (see Supporting Information for details). The powders exhibit a specific surface area Sfin between 3.69 and 24.67 m2 g
1. They are no longer subject to significant fracture process upon further mechanical processing, so that the Sfin values cannot change. Therefore, active sites can be generated only by attrition. The powders were processed in the mechanochemical reactor under the same previously described conditions. However, in this case an ethanol solution with higher concentration in DPPH, namely, 1 M, was used to facilitate quantitative analyses. As expected, in this case the DPPH consumption is linear and the β values obtained by best-fitting the experimental points range between 3.9  10
6 and 5.2  10
6 mol m
2, with an average error of about (0.3  10
6, in fairly good agreement with the previously determined value of 4.7  10
6 mol m
2. The results obtained suggest that, despite its roughness, the model is able to catch the fundamental features underlying the DPPH consumption process. Therefore, the apparent surface density α of the active sites generated by fracture should also be considered accurate enough to point out an anomalously high reactivity. From the point of view of mechanochemistry, precisely the greatly enhanced chemical reactivity of the active sites

ARTICLE

at surfaces generated by fracture can be interpreted as a definite mechanochemical effect. The experimental findings, and their subsequent modelingbased analysis, definitely point out a marked difference in the chemical reactivity of the reactive centers formed either by fracture of the powder particles, with consequent increase of the specific surface area, or by attrition between particle surfaces, with no significant change in the specific surface area of quartz. In particular, it appears that the active sites at the surfaces generated by fracture are about 20 times more reactive than the ones generated by attrition. The comparison of the proportionality constant α and β values with the mximum possible surface density γ of dangling bonds suggests that the enhanced reactivity of mechanically processed quartz powders must be ascribed not only to extensive factors, but also to intensive ones. In other words, it is not the number of active centers available to the reaction to determine the reactivity enhancement, but rather the capability of the active sites generated by fracture to exhibit unusually high degrees of chemical reactivity. This particular behavior can be tentatively rationalized by taking into account a least two factors. First, the attainment of very high local temperatures on very short time intervals at collision-induced fracture events.43
45 Apart from inducing an anomalously high mobility of atomic species, such local temperature rises can result in local electronic excitations, which in turn can potentially deeply affect the chemical reactivity of the atomic species involved.45 Second, the complex sequence of local processes enabling the relaxation and possibly reconstruction of the surfaces generated by fracture.38
42 Being relaxation and reconstruction times longer than the time intervals required for the chemical interaction between the surfaces and the liquid chemical surrounding, it must be expected that the repeated change of local chemical bonds at the surfaces could significantly affect the apparent chemical reactivity of the quartz surfaces. Finer effects can be also identified on a theoretical basis. For example, the chemical reaction of small silica clusters emitted during the fracture process.15,43
45 However, the roughness of the experimental data here discussed prevents any supported analysis of such hypothetical contributions.

5. CONCLUSIONS Quartz powders were submitted to mechanical activation in the presence of an ethanol solution of the free radical DPPH. The mechanical processing determines the formation of active sites at the surface of quartz particles. These sites interact with ethanol molecules and generate hydrogen radicals, which in turn neutralize the free radical DPPH. The kinetics of DPPH consumption exhibits two regimes, which indicates an intimate relationship between DPPH neutralization and total area of quartz powders available to reaction. More specifically, experimental evidence suggest that the two regimes of DPPH consumption are connected with the different contributions coming from active sites formed by fracture of quartz particles or by attrition between them with no significant increase of specific surface area. A simplified kinetic model, able to satisfactorily describe the reaction kinetics, was developed. By assumption of the existence of two types of active sites, the model equations are able to interpolate fairly well the experimental points. In turn, this allows to estimate the apparent surface density of active sites generated by fracture or by attrition. The results obtained indicate that the 21234

dx.doi.org/10.1021/jp206354p |J. Phys. Chem. C 2011, 115, 21230–21235

The Journal of Physical Chemistry C active sites generated by fracture exhibit a significantly higher reactivity than the ones generated by attrition. The surface density of the former sites is also higher than the maximum possible number of dangling bonds per surface unit. This unexpected finding suggests that a role in the reaction kinetics is played by relaxation processes operating at the surfaces generated by fracture. Such processes could promote the repeated interaction of surface sites with the ethanol solution, which could explain the high apparent surface density of the sites generated by fracture. This latter feature represents a truly mechanochemical effect.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures depicting the statistical distribution p(d) of the size d of quartz powders and the number NDPPH of DPPH moles in ethanol solution as a function of the number n of collisions occurred. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Dr. Paola Meloni, Dipartimento di Ingegneria Chimica e Materiali, University of Cagliari, and Ing. Gianfranco Carcangiu, Istituto di Geologia Ambientale e Geoingegneria, CNR, are gratefully acknowledged for the measurements carried out at the Laboratorio per lo studio dei Materiali “Colle di Bonaria”. Dr. Ing. Annalisa Vacca and Dr. Ing. Anna Da Pozzo, Dipartimento di Ingegneria Chimica e Materiali, University of Cagliari, are also gratefully acknowledged for assistance in UV
vis spectrophotometry. Financial support has been given by the University of Cagliari. ’ REFERENCES (1) Butyagin, P. Yu. Russ. Chem. Rev. 1971, 40, 901. (2) Boldyrev, V. V. Bull. Div. Chem. Sci., Akad. Nauk SSSR 1990, 39, 2029. (3) Gilman, J. J. Science 1996, 274, 65. (4) Gutman, E. M. Mechanochemistry of Materials; Cambridge International Science Publishing: Cambridge, 1998. (5) Levitas, V. I. In High Pressure Surface Science and Engineering; Gogotsi, Y., Domnich, V., Eds.; Institute of Physics: Bristol, 2004; p 159, Chapter 3. (6) Beyer, M. K.; Clausen-Schaumann, H. Chem. Rev. 2005, 105, 2921. (7) Sheiko, S. S.; Sun, F. C.; Randall, A.; Shirvanyants, D.; Rubinstein, M.; Lee, H.; Matjyaszewski, K. Nature 2006, 440, 191. (8) Davis, A. D.; Hamilton, A.; Yang, J.; Cremar, L. D.; Van Gough, D.; Potisek, L. S.; Ong, M. T.; Braun, P. V.; Martinez, T. J.; White, S. R.; Moore, J. S.; Sottos, N. R. Nature 2009, 459, 68. (9) Kon^opka, M.; Turansky , R.; Reichert, J.; Fuchs, H.; Marx, D.; Stich, I. Phys. Rev. Lett. 2008, 100, 115503. (10) Caruso, M. M.; Davis, D. A.; Shen, Q.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Chem. Rev. 2009, 109, 5755. (11) Takacs, L. J. Met. JOM 2000, January issue, 12. (12) Heinicke, G. Tribochemistry; Akademie Verlag: Berlin, 1984. (13) Suryanarayana, C. Prog. Mater. Sci. 2001, 46, 1. (14) Rodriguez, B.; Bruckmann, A.; Rantanen, T.; Bolm, C. Adv. Synth. Catal. 2007, 349, 2213.

ARTICLE

(15) Experimental and Theoretical Studies in Modern Mechanochemistry, Eds. Delogu, F., Mulas, G.; Transworld Research Network: Kerala, 2010. (16) Klika, V.; Marsík, F. J. Phys. Chem. B 2009, 113, 14689. (17) Sieniutycz, S.; Berry, R. S. Phys. Rev. A 1992, 46, 6359. (18) Jou, D.; Casas-Vazquez, J.; Lebon, G. Extended Irreversible thermodynamics; Springer: Berlin, 1993. (19) Compte, A.; Jou, D. J. Phys. A: Math. Gen. 1996, 29, 4321. (20) Jou, D.; Casas-Vazquez, J.; Lebon, G. Rep. Prog. Phys. 1999, 62, 1035. (21) Galenko, P.; Jou, D. Phys. Rev. E 2005, 71, 046125. (22) Galenko, P.; Lebedev, V. Int. J. Thermodyn. 2008, 11, 21. (23) Fox-Rabinovich, G. S.; Gershman, I. S.; Yamamoto, K.; Biksa, A.; Veldhuis, S. C.; Beake, B. D.; Kovalev, A. I. Entropy 2010, 12, 275. (24) Arends, J.; Dekker, A. J.; Perdok, W. G. Phys. Status Solidi 1963, 3, 2275. (25) Fubini, B.; Giamello, E.; Pugliese, L.; Volante, M. Solid State Ionics 1989, 32
33, 334. (26) Costa, D.; Fubini, B.; Giamello, E.; Volante, M. Can. J. Chem. 1991, 69, 1427. (27) Fubini, B.; Bolis, V.; Cavenago, A.; Volante, M. Scand. J. Work Environ. Health 1995, 21, 9. (28) Hasegawa, M.; Ogata, T.; Sato, M. Powder Tech. 1995, 85, 269. (29) Hochstrasser, G.; Antonini, J. F. Surf. Sci. 1972, 32, 644. (30) Narayanasamy, J.; Kubicki, J. D. J. Phys. Chem. B 2005, 109, 21796. (31) Saruwatari, K.; Kameda, J.; Tanaka, H. Phys. Chem. Minerals 2004, 31, 176. (32) Damm, C.; Peukert, W. Langmuir 2009, 25, 2264. (33) Fubini, B.; Arean, C. O. Chem. Soc. Rev. 1999, 28, 373. (34) Murashov, V. V.; Demchuck, E. J. Phys. Chem. B 2005, 109, 10835. (35) Lopes, P. E. M.; Demchuck, E.; Mackerell, A. D., Jr. Int. J. Quantum Chem. 2009, 109, 50. (36) Hristea, E. N.; Caproiu, M. T.; Pencu, G.; Hillebrand, M.; Constantinescu, T.; Balaban, A. T. Int. J. Mol. Sci. 2006, 7, 130. (37) Schultz, R.; Henglein, A. Z. Naturforsch. 1953, 8b, 160. (38) Rignanese, G.-M.; De Vita, A.; Charlier, J.-C.; Gonze, X.; Car, R. Phys. Rev. B 2000, 61, 13250. (39) Rountree, C. L.; Kalia, R. K.; Lidorikis, E.; Nakano, A.; Van Brutzel, L.; Vashishta, P. Annu. Rev. Mater. Res. 2002, 32, 377. (40) Muralidharan, K.; Simmons, J. H.; Deymier, P. A.; Runge, K. J. Non-Cryst. Solids 2005, 351, 1532. (41) Musso, F.; Ugliengo, P.; Solans-Monfort, X.; Sodupe, M. J. Phys. Chem. C 2010, 114, 16430. (42) Giordano, S.; Mattoni, A.; Colombo, L. In Reviews in Computational Chemistry; Lipkowitz, K. B., Eds.; John Wiley and Sons: New York, 2011; Chapter 1, p 1. (43) Delogu, F. Phys. Rev. B 2006, 74, 035406. (44) Delogu, F. Phys. Rev. B 2009, 80, 014115. (45) Delogu, F. Phys. Rev. B 2010, 82, 205415.

21235

dx.doi.org/10.1021/jp206354p |J. Phys. Chem. C 2011, 115, 21230–21235