Determination of Contact Angles, Silane Coverage, and

Article pubs.acs.org/Langmuir

Determination of Contact Angles, Silane Coverage, and Hydrophobicity Heterogeneity of Methylated Quartz Surfaces Using ToF-SIMS Susana Brito e Abreu* and William Skinner Ian Wark Research Institute, ARC Special Research Centre for Particle and Material Interfaces, University of South Australia, Mawson Lakes, South Australia 5095, Australia ABSTRACT: Methylated quartz surfaces are extensively used in colloid science for wettability studies and the control and impact of hydrophobicity in key physicochemical processes. In this study, time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been used to correlate the surface chemistry of trimethylchlorosilane-methylated quartz surfaces with the contact angle. Models have been developed for the calculation of both advancing and receding contact angles based on measurements of the ToF-SIMS signals for SiC3H9+ (TMCS) and Si+ (quartz). These models enable the contact angle across surfaces and, more importantly, that of individual particles to be determined on a micrometer scale. Distributions of contact angles in large ensembles of particles, therefore, can now be determined. In addition, from the ToF-SIMS analysis, the surface coverage of the methylated species can be quantitatively determined, in line with the Cassie equation. Moreover, advancing and receding contact angle maps can be calculated from ToF-SIMS images, and hence the variation in microscopic hydrophobicity (e.g., at the particle level) can be extracted directly from the images. TMCS solution.9 However, it can be reasonably expected that, when carried out properly (i.e., fully eliminating water vapor, minimizing solution mixing issues, and ensuring that the silane is unpolymerized) using TMCS, vapor and solution methylation methods should yield the same resulting average surface hydrophobicity, as measured by the contact angle, on a scale greater than a few micrometers, regardless of whether they are applied to plates or particles of the same material. The mechanism of silane adsorption is well understood and involves the selective attachment of organosilyl groups (SiMe3) to the free silanol groups on the surface, accompanied by the production of HCl.1 Because of the steric hindrance promoted by the SiMe3 groups, only about 60% of the free silanol groups will react. Experimental and theoretical studies (ref and references therein, ref 2) have shown that for a fully hydroxylated surface there is an average of 4.6 SiOH/nm2 and that a fully methylated surface covers about 2.5 SiOH/nm2, leaving about 2.1 SiOH/nm2. The maximum surface coverage of a methylated silica surface corresponds to an area fraction of about 0.7.10 Small molecules having an electron donor atom such as N or O that is able to form a hydrogen bond with the free silanol groups will tend to adsorb to the surface.1 For instance, unreacted silanol groups will readily react with water, physically or chemically adsorbing to the surface, and with

1. INTRODUCTION The wettability of surfaces is an important property of materials; it is inherent to industrial processes involving solid−liquid separation such as froth flotation, flocculation, adhesion, and water purification.1 Silica is commonly used as a solid model in wettability studies for its versatility in modifying original surface properties. In addition, silica can be used in the form of flat surfaces (low surface area), particles (high surface area), and smooth or rough surface morphologies. Quartz particles,2−5 glass spheres,6 and quartz plates7 have been used as model systems in studies involving mineral flotation. The silica surface, which is naturally hydrophilic (because of hydroxylation), can be rendered hydrophobic easily by the use of temperature and/or chemical treatments. One of the most studied mechanisms of surface coatings of silica surfaces is the formation of thin layers of chemisorbed organosilyl molecules. Compounds of the type SiR3X (where R is an organic chain and X is a halogen) are often used, with trimethylchlorosilane (TMCS) being a popular example because of the ease of adsorption. Adsorption, partial or complete, can be achieved by either exposure of the silica surface to TMCS vapor or immersion in TMCS solutions (at different concentrations) for given reaction times.1,2 Vapor methylation has been found to produce uniform monolayers of reduced lateral heterogeneity (i.e., chemical hysteresis). The solution methylation method, however, may result in a patchlike morphology, rendering the surface chemically rough on the nanometer scale.8,9 This effect has been attributed to possible water contamination of the © 2012 American Chemical Society

Received: January 24, 2012 Revised: April 24, 2012 Published: April 25, 2012 7360

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air plasma treatment. Here, we have relied on residual water vapor in the low-vacuum environment to contribute to surface hydroxylation. This method was chosen to avoid contamination and etching effects that might result from caustic treatments. The power of the plasma was set to low (∼7 W applied to the rf coil). After the treatment, the samples were immediately transferred to the ToF-SIMS instrument for surface analysis and were carried in a desiccator during the transport. Quartz particles were treated with acid and alkali solutions to remove any organic contamination and restore the surfaces' full hydroxylation. The quartz particles were rinsed with deionized water, dried in an oven at 110 °C overnight, and stored in sealed glass containers.2 2.2.2. Methylation. Quartz plates were methylated with trimethylchlorosilane (TMCS) by the vapor deposition method. The methylation was performed in a desiccator under vacuum inside a glovebag under argon flow. The TMCS vapor (pure or a toluene solution) was allowed to come into contact with the freshly hydroxylated quartz plates for given reaction times. Partial methylation was achieved by varying the TMCS concentration (diluted solutions from the stock solution of 11% by volume) and the time of vapor exposure (up to 30 min). Quartz particles were methylated in a glovebox under nitrogen by a TMCS solution in cyclohexane for given concentrations and reaction times in order to obtain particulate samples with different degrees of hydrophobicity.2 2.3. Contact Angle Measurements. The contact angles of the quartz plates were measured immediately after the ToF-SIMS analysis using a custom-designed sessile drop instrument following a traditional procedure.22 A water droplet (Millipore water, 18 MΩ cm resistivity) was formed at the end of a syringe and placed on the surface. The static advancing and receding angles were measured by increasing or decreasing the volume of the water droplet, respectively, until observing that the movement of the three-phase contact line and the droplet had come to a rest. At a given three-phase contact point, the droplet profile was recorded by digital photography and the angle was extracted using the DropSnake (drop analysis) plug-in of the ImageJ software.23 “Equilibrium” contact angles were measured by allowing a water droplet to rest on the surface (detached from the syringe), and the droplet profile was taken. The contact angles were measured on at least six different areas of the plate surface. Each advancing and receding contact angle determined is the average of the contact angles measured at three different contact points within these areas. 2.4. ToF-SIMS Analysis. 2.4.1. Quartz Plates. The ToF-SIMS instrument used in this study was the PHI TRIFT V nanoTOF model from Physical Electronics Inc. using a pulsed liquid metal 79Au+ primary ion beam operating at 30 kV energy. The high spectral resolution mode was used by selecting the “bunched” Au1 instrumental settings. The time for data acquisition was 1 min/frame with an ion dose of 4 × 1010 ions/cm2, which is well below the static limit. Positive secondary ion mass spectra were collected from 6−10 different areas of 500 × 500 μm2 across the plate surface. The analysis was conducted in a vacuum on the order of 10−6 Pa or better. Charge compensation was provided by an electron gun and Ar+ ions. 2.4.2. Quartz Particles. The quartz particles were mounted on an indium foil on a silicon wafer and introduced into the fore-vacuum chamber of the ToF-SIMS instrument. For the analysis of the particles, the high spatial resolution mode was used by selecting the “unbunched” Au1 instrumental settings. “Raw” data files of 256 pixel × 256 pixel images were collected for positive secondary ions, and the spectra from specific regions of interest (ROI) (i.e., particle-specific data) were extracted after analysis using offline software (WinCadenceN). Areas of raster size 125−175 μm were analyzed for 2 min, with an ion dose of less than 1012 ions/cm2, which is below the static limit. The analysis was conducted in a vacuum on the order 10−6 Pa or better. The electron flood gun was used for charge neutralization. The statistical analysis of ToF-SIMS data was performed with the STATISTICA software package.24

organic contamination from the air. To restore the silica surface to its original state and obtain an hydroxylated surface free of organic contaminants, plasma treatments have proven effective.11 This is particularly important prior to the modification of the surface by the chemical adsorption of organic molecules. The number of residual silanol groups available on the modified silica surface constitutes a significant parameter in controlling the surface properties of the newly formed layer. The relative numbers of silanol and adsorbate groups on the surface as well as their distribution (heterogeneity) will determine the overall hydrophobicity of the modified silica surface. A powerful technique capable of providing the distribution of chemical species on the surface with great sensitivity is time-of-flight secondary ion mass spectrometry (ToF-SIMS). This technique associates imaging with mass spectral information of secondary ions that are emitted when the surface is bombarded by an energetic ion beam (e.g., Ga+, Au+n clusters, or C60).12 The imaging feature of this technique allows for the selection of individual regions of interest down to a few micrometers in lateral dimension, individual particles, for example, and collecting the corresponding mass spectra from them. Several studies have used this technique to determine the effects of physical/chemical treatments on the surface chemistry of the silica, looking at characteristic ToF-SIMS signals.11,13−16 For instance, the ratio of characteristic fragments such as SiOH+/Si+ and OH−/O− has been used as an indication of hydroxylation levels, and the C3H9Si+ signal, as a relative measure of the surface coverage. Theoretical and experimental evidence of linear trends observed between the intensity of ToF-SIMS signals and surface coverage is found in the literature.17−19 In our previous studies, we have shown that ToF-SIMS can be used quantitatively to determine the advancing contact angle of mineral surfaces.20,21 We were able to directly correlate ToFSIMS secondary ions with the water advancing contact angle of the reactive mineral, chalcopyrite (CuFeS2), in the presence of a dithiophosphate surfactant (collector). The derived ToFSIMS equation determines the chemical contribution to the contact angle based on the intensities of secondary ions characteristic of the mineral matrix, collector, and oxidation of selected regions of interest on the micrometer scale, whether particulate or planar surfaces. In this study, we extend this approach to quartz samples, plates, and particles subjected to TMCS methylation treatment. We develop ToF-SIMS models to infer the contact angle statistically, advancing and receding, directly from ToF-SIMS measurements of the characteristic secondary ions of the TMCS−quartz system.

2. EXPERIMENTAL SECTION 2.1. Materials. UV-grade quartz wafers of 1 mm thickness, twoside polished (Groiss Herbert A & Co, Victoria, Australia), were cut into smaller plates with dimensions of approximately 1 cm2 for subsequent treatment and surface analysis. Ground quartz particles in the size range of 10−50 μm were used in this study. Trimethylchlorosilane (99%) was purchased from Aldrich. Solutions of TMCS in analytical-grade anhydrous toluene were used for partial methylation. 2.2. Surface Preparation. 2.2.1. Hydroxylation. Prior to the methylation treatments, the quartz plates were hydroxylated in a plasma reactor. The quartz plates were cleaned with Millipore water and introduced into the plasma reactor chamber (Harrick PlasmaFlo PDC-002). Vacuum was applied for about 2 min prior to the 15 min 7361

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3. RESULTS AND DISCUSSION 3.1. Correlation of the ToF-SIMS Signals with the Contact Angle. Trimethylchlorosilane-methylated quartz is a system where the characteristic ToF-SIMS signals of the surface are well established. In previous studies, Si+, SiO+, and SiOH+ have been used to represent the quartz substrate.11,14 Representative of the adsorbed trimethylchlorosilane molecule (TMCS) is the C3H9Si+ signal, the trimethylsilyl fragment (TMS) that is anchored to the surface by a condensation reaction between the TMCS and SiOH sites. In this work, plate samples were treated with TMCS such that surfaces with different degrees of hydrophobicity were produced. These surfaces were analyzed by ToF-SIMS, and the corresponding contact angles, advancing and receding, were measured. The correlation between the ToF-SIMS secondary ion intensities and the contact angle was examined. It was observed that the normalized C3H9Si+ signal (i.e., normalized to the total ion yield) increases with the contact angle. This trend, however, is not observed for the Si+ or SiOH+ signals. In a separate examination involving the methylation of quartz samples with different levels of thermally controlled hydroxylation, the intensity of the normalized Si+ signal did not change significantly with subsequent TMCS methylation treatment (Chart 1). This result indicates that the Si+ signal does not

Chart 2. Relationship between the Advancing and Receding Contact Angles of Methylated Quartz Plates and the ToFSIMS Ratio SiC3H9+/Si+, Showing a Linear Dependencea

Chart 1. ToF-SIMS-Normalized Intensity of the Si+ Signal before and after Methylation with Trimethylchlorosilane for Heat-Treated Quartz Platesa

a

The error bars represent 95% confidence intervals.

suggesting a linear dependence in both cases. To analyze this correlation further, linear regression analysis (LRA) was applied to the ToF-SIMS and contact angle data. By applying LRA, calibration curves may be generated, allowing the direct calculation of the advancing and receding contact angles from the ToF-SIMS measurement of the SiC3H9+ and Si+ ion signals. The resulting regression lines are displayed in Chart 3, and the models for these curves are listed in Table 1 along with the Chart 3. Correlation between the Contact Angle, Advancing and Receding, and the ToF-SIMS Ratio SiC3H9+/Si+a

a

Solid symbols: heat treatment only. Open symbols: after methylation. The error bars represent 95% confidence intervals.

depend on the methylation treatment, in agreement with earlier observations. For example, the Si+ signal was almost absent on the spectrum of a TMCS-methylated Ag substrate.16 Other studies have used the normalization of the SiOH+ signal to the Si+ signal as an indication of the hydroxylation levels on the surface.11,14 Normalizing the C3H9Si+ signal to the Si+ signal (i.e., C3H9Si+/Si+) therefore allows the elimination of the contribution of surface-contaminating hydrocarbons that may have been adsorbed from the air as well as fluctuations in the intensity of the primary ion beam. Hence, we have used the SiC3H9+/Si+ ratio in all subsequent data analysis for this work. The prepared quartz samples were analyzed by ToF-SIMS, and the contact angles were measured. The relationship between the ToF-SIMS ratio SiC3H9+/Si+ and the corresponding contact angles, advancing and receding, is shown in Chart 2,

a

The bands around the regression line represent 95% confidence intervals (---) and prediction intervals (···) for the correlation.

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approaches saturation. Hysteresis of a similar magnitude has been observed for similarly prepared types of surfaces.28 In this study, the maximum advancing contact angle obtained was approximately 80°. Several studies have observed maximum advancing contact angles of about 88° for a fully methylated quartz surface.10,28 However, lower contact angle values have been determined for methylated quartz surfaces using the same methylation route as in this study.9 A possible factor that may have contributed to this result is not having achieved the complete hydroxylation of the quartz surface prior to methylation treatments. In our case, the plasma pretreatment conditions may not have produced the highest achievable silanol coverage. 3.2. Correlation between the TMCS Surface Coverage and the ToF-SIMS Ratio. The TMCS−quartz system has been experimentally confirmed to follow the Cassie equation (eq 3) in previous studies.2,10 Cassie’s equation,29 applied to heterogeneous smooth surfaces, determines the average equilibrium contact angle of the surface as the contribution of the contact angle (θ) of each surface chemical composition (i) and its corresponding fractional surface area (f). By applying this equation to TMCS-treated quartz surfaces, we obtain eq 4, where f SiOH and f TMS represent the area fractions of silanol (substrate) and trimethylsilyl groups (TMS), respectively, and θSiOH and θTMS represent the corresponding contact angles, 0 and 110°,30 respectively. Rewriting eq 4, we obtain eq 5, which directly relates the average contact angle to the fractional area of TMS groups on the surface (f TMS).

Table 1. Models of the ToF-SIMS−Contact Angle Correlations and Goodness of Fit Results R2a

model (eq 1) (eq 2)

+

θadv = 98.8

SiC3H 9 + 36.1 Si+

0.985

SiC3H 9+ + 7.9 Si+

0.959

θrec = 119.0

a

Proportion of variance about the mean contact angle as explained by the ToF-SIMS ratio.

statistical results of the regression analysis. The correlation based on the SiC3H9+/Si+ ratio from ToF-SIMS accounts for 98.5% of the advancing and 95.9% of the receding contact angle variation. The coefficient of the ToF-SIMS ratio in eqs 1 and 2 includes information about the contact angle sensitivity and ion yield sensitivity of the ratio. Coverage information is contained within the ratio itself. The derived models (Table 1) can be used to predict the advancing and receding contact angles of unknown samples. In this case, the error in prediction is given by the prediction interval, which will be larger than the confidence interval because it accommodates the error associated with future observations in addition to the error from the fitted model (Chart 3). The error in prediction is affected by two components: (1) the variance obtained from the fitted regression model and (2) the deviation between the predictor value (SiC3H9+/Si+) and the mean of the observed values.25 As the SiC3H9+/Si+ value moves away from the mean value, the error in prediction increases. This effect is also observed for the confidence intervals; the confidence bands get larger as they approach the extremes of the models (Chart 3). The values of the calculated advancing and receding contact angles depend on the value of SiC3H9+/Si+ (the mean of at least six areas on the surface) and on the errors arising from the regression models. Three sources of error can be identified in this study: (1) measurement error, (2) sample variation, and (3) lack of fit. The measurement error is the error associated with the experimental measurements. For instance, the error in the receding contact angle measurements was found to be larger than the advancing contact angles. This result may be explained by the difficulty in determining the receding contact angles precisely.26 Reorientation of the surface groups upon contact with the water droplets may occur, resulting in the alteration of the surface tension.27 The sample variation arises from differences between samples with the same values of the ToF-SIMS ratio SiC3H9+/Si+. The lack of fit refers to imperfections in the relationship between the contact angle and the ToF-SIMS ratio SiC3H9+/Si+. In this study, the variation in the data explained by the correlations is close to 100%, meaning that the total errors arising from this study are low. Hysteresis, the difference between the advancing and receding contact angles, is present to some extent in most practical studies because it depends on many factors departing from ideality (e.g., surface roughness, chemical heterogeneity, and rigidity). In practical terms, the observed hysteresis is an average of many events.27 In this study, the hysteresis was found to be on average 23°, with chemical heterogeneity most certainly the major contributing factor. The observed hysteresis decreases slightly for higher contact angle values, which reflects lesser chemical heterogeneity as the surface coverage

n

cos θ =

∑ fi cos θi i=1

(3)

cos θ = fSiOH cos(θSiOH) + fTMS cos(θTMS)

(4)

cos θ = 1 + fTMS (cos(θTMS) − 1)

(5)

The “equilibrium” contact angles were measured in this study (Chart 4). It was observed that the values lie between the advancing and receding curves in accordance with practical approximations that calculate the equilibrium contact angle as the average of the advancing and receding angles when the surface roughness is small. Using eq 5, we can calculate the area fraction of TMS from the measured contact angles, directly correlating the TMS uptake with the ToF-SIMS ratio SiC3H9+/ Si+ (Chart 5). The regression model (eq 6) and goodness of fit results for this correlation are presented in Table 2. The area fraction of TMS groups adsorbed on the quartz surface is 1.22 times the ToF-SIMS ratio SiC3H9+/Si+. The regression model explains 97.3% of the variability in the data. Using this model (eq 6), one can determine the surface coverage directly from the ToF-SIMS measurement of SiC3H9+/Si+. This is an important result and enables adsorption studies to be performed for TMCS-methylated surfaces using ToF-SIMS without needing further treatments. The ToF-SIMS ratio SiC3H9+/Si+ can be used to calculate the area fraction of TMS on the quartz surface, which follows the Cassie equation. A modified Cassie equation (eq 7) therefore, can be obtained, allowing direct calculation of the equilibrium average contact angle from the ToF-SIMS ratio SiC3H9+/Si+ of planar or, more importantly, particulate methylated surfaces. Additionally, the lateral variation in adsorbed coverage (and hence the contact angle) may be 7363

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monitored at dimensions determined by the primary ion beam (practically, ∼1 μm).

Chart 4. Equilibrium Contact Angles of Methylated Quartz Plates versus the ToF-SIMS Ratio SiC3H9+/Si+a

cos θ = 1 + 1.22

SiC3H 9+ Si+

(cos(110°) − 1)

(7)

3.3. Contact Angle Heterogeneity. The ToF-SIMS− contact angle models (eqs 1 and 2) allow one to determine the contact angle of selected regions of interest (ROI) such as different areas of a plate or individual particles. Using this approach, the contact angle variation across surfaces, plates, or particles and the distribution of contact angles within particle ensembles can be determined. In this study, we compare the contact angle variation of quartz surfaces that have been methylated (using TMCS) by different methods: plates by vapor-phase adsorption and particles by solution adsorption. We use eq 1 to determine the variation of the advancing contact angle across a plate sample and the distribution of contact angles observed within a particulate sample, with similar average contact angles. Approximately 40 regions of interest (ROI) were analyzed in each sample. For the plate samples, an ROI of approximately 170 μm raster size was analyzed across the surface, and for the particulate samples, individual particles in the size range of 10−50 μm were analyzed. Two ranges of hydrophobicity (average contact angle) are examined. Chart 6 compares the distribution of the a

The measured values lie between the advancing and receding contact angles (top and bottom lines, respectively). The error bars represent 95% confidence intervals.

Chart 6. Histogram of the Distribution of Contact Angles of Two Samples, Plate and Particles, Methylated by Different Methods with Average Contact Angles of 40 and 39°, Respectivelya

Chart 5. Correlation between the Area Fraction of Trimethylsilyl Groups (TMS) and the ToF-SIMS Ratio SiC3H9+/Si+a

a

calculated advancing contact angles obtained for a plate and particle ensemble with average contact angles of 40 and 39°, respectively. The plate and particles exhibit similar contact angle distributions, which are quite narrow about the mean (with a standard deviation of 1° in both cases). For higher average contact angles, however, both the plate and particles show a broader contact angle distribution around the mean values, 58 and 56°, respectively (Chart 7). Statistically, the distributions are of similar width although in the particulate sample there are several outliers exhibiting high-contact-angle surfaces. These examples suggest that the methylation of quartz by different methods is likely to produce surfaces with chemical heterogeneity of similar magnitudes for the same degree of hydrophobicity. Plates and particles treated similarly with

a The bands around the regression line represent 95% confidence intervals (---) and prediction intervals (···) for the correlation.

Table 2. ToF-SIMS−TMS Calibration Curve and Goodness of Fit Results R2*

model (eq 6)

fTMS = 1.22

SiC3H 9+ 0.973 Si+

The curves represent normal distributions.

0.973

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particularly important for studies using methylated quartz particles because it is often assumed that for a particle sample with a given average contact angle all particles will exhibit similar hydrophobicity when in fact there can be a broad contact angle distribution within the particle ensemble, which can have an impact on the expected behavior. Studies of this type include the use of methylated quartz particles for modeling flotation and, for example, to determine the critical contact angle of fine particles for flotation.3,4,6 Indeed, chemical heterogeneity within particulate samples has been reported in several studies.7,31−33 Fine particles are more difficult to study than larger particles because they often pose problems with contact angle measurements (e.g., the formation of particle aggregates). The use of ToF-SIMS to determine individual particle contact angles provides valuable and unique information that can help greatly in exploring studies of this type. In addition, using this approach, no pretreatments of the samples are required for the contact angle measurements. Wetting during the contact angle measurements is a requirement for the traditional techniques, whether the samples be planar or particulate. For the latter, separation into size fractions is often required for the contact angle measurements using the traditional methods (e.g., Washburn,34 equilibrium capillary pressure35), which is not needed in this case because the ToF-SIMS analysis can be performed on different particle sizes down to a few micrometers within a complex mixture of particle sizes and particle compositions. Another important result arising from this study is the use of the ToF-SIMS models to determine the contact angle of

Chart 7. Histogram of the Distribution of Contact Angles of Two Samples, Plate and Particles, Methylated by Different Methods, with Average Contact Angles of 58 and 56°, Respectivelya

a

The curves represent normal distributions.

TMCS have been found to exhibit comparable hydrophobicity, regardless of surface roughness differences.10 The above results, together with the variability in surface chemistry observed along the contact angle curves (Chart 2) suggest that at high and low coverages of TMCS less chemical heterogeneity is observed compared with that at intermediate coverage. This is

Chart 8. Contact Angle Line Scans of Two Methylated Quartz Particles, Images A and B, in a ToF-SIMS Image with an Average Advancing Contact Angle of 51° and a Receding Contact Angle of 26°a

The line scans (from top to bottom) show how the microscopic hydrophobicity varies along the particle (16 μm section). The error bars represent 90% prediction intervals. a

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this method does not require the surfaces to be exposed to any liquids, and in the case of particulate samples, particle separation into size fractions is not needed as part of the contact angle measurements, in contrast with the traditional techniques. Additionally, in line with the Cassie equation, we were able to determine the coverage of the methylation species on micrometer scales. This allows, for example, adsorption studies to be performed using ToF-SIMS. Moreover, advancing and receding contact angle maps can be performed on ToFSIMS images using the derived linear models, and the variation of hydrophobicity within individual particles (i.e., microscopic hydrophobicity) can be extracted. We have shown how this variation can be considerably large within a single particle, which in part explains the complexity of the mechanism of flotation. Our previous studies20,21 used a Ga+ primary ion beam in the ToF-SIMS measurements. It is important to note that the use of a different primary ion, in this case Au+, does not alter or detract from the methodology. However, we expect a greater sensitivity (higher secondary ion yield) using Au+. In future work, it may be possible to compare data acquired using different ion beams in order to extract the ion yield contributions to the coefficient in the advancing and receding contact angle models (eqs 1 and 2).

methylated plates and particles, suggesting that the derived models may be applied to TMCS-methylated surfaces with different morphologies or methylation routes. 3.4. Contact Angle Variability on the Micrometer Scale. It is now possible to predict advancing and receding contact angles from discrete regions of a surface using ToFSIMS measurements. By extension, it is therefore possible to observe contact angle variation over short length scales. In this study and in previous studies,20,21 the distribution of contact angles of individual particles within a large ensemble has been examined. Clearly, a single particle contact angle is an average across its surface. Therefore, what does the variation of the contact angle across an individual particle look like? With the ToF-SIMS instrument, pixel by pixel mass spectra are collected over the field of view. We can therefore apply the equations for advancing and receding contact angles to either the secondary ion image or line-scan data to extract changes on the micrometer scale. Chart 8 presents 16-μm-long calculated contact angle line scans for two quartz particles from a ToF-SIMS image with average advancing and receding contact angles of 51 and 26°, respectively. The variability in the contact angle along the line scans is considerable. For example, in the case of line scan A, the advancing contact angle varies between 37 and 86°. The receding contact angle varies between 12° and a maximum of 67°. In froth flotation mineral processing, the receding contact angle is considered to be central to the bubble−particle attachment mechanism, and this result highlights the complex motion of the three-phase (air−water−surface) contact line across such a surface. This observation helps to explain the stochastic nature of predicting the recovery of individual particles into flotation concentrates (i.e., the probability of a bubble contacting a particle in a sufficiently hydrophobic surface region during the flotation and remaining attached in an inherently turbulent system. Of course, we are considering only the surface chemical contribution to hydrophobicity. Clearly, surface roughness will increase the complexity of the system.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +610883026457. E-mail: susana.britoeabreu@postgrads. unisa.edu.au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Australian Minerals Industry Research Association (AMIRA) International for financial support. We are also grateful to Dr. Daniel Chipfunhu for providing TMCS−methylated quartz particles and Dr. Catherine Whitby for assistance with the plasma reactor.

4. CONCLUSIONS Contact angle measurements of extended surfaces are quite trivial to perform; however, examining the hydrophobicity of discrete regions on the micrometre scale and individual particle surfaces is highly problematic. We have now extended the use of ToF-SIMS beyond that in previous studies20 to study the important TMCS−quartz system and have shown that this technique can address these issues. In particular, determining the contact angle, advancing and receding, across a surface and between particles in a large ensemble is now possible. The derived ToF-SIMS models have been applied to methylated surfaces, plates and particles, using different methylation methods, and the results suggest that the models may be applied to any type of methylated surface (e.g., different morphologies) and may be used to detect methylation routeinduced artifacts or deliberate patterning. Moreover, these surfaces also appear to show very similar hydrophobicity heterogeneity. This method of determining the contact angle introduces a new capability: the determination of the receding contact angle of individual particles and hence the distribution of receding contact angles in particulate samples. This will be a valuable addition to studies such as flotation, where receding contact angles have considerable importance. Studies involving fine particles and the complex chemical patterning of extended surfaces will also benefit from using this approach. Moreover,

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REFERENCES

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dx.doi.org/10.1021/la300352f | Langmuir 2012, 28, 7360−7367