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J. Phys. Chem. C 2007, 111, 8708-8715
ARTICLES Adsorption at a Carbon Black Microparticle Surface in Aqueous Colloids Probed by Optical Second-Harmonic Generation† Hong-fei Wang,‡ Thomas Troxler,§ An-gong Yeh,| and Hai-lung Dai*,§ Institute of Chemistry, Chinese Academy of Sciences, No. 2, 1st North Street, ZhongGuanCun, Beijing, China 100080, Department of Chemistry & Laboratory for Research of Structure of Matters, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, and DuPont AutomotiVe Products, Marshall Laboratory, 3401 Grays Ferry AVenue, Philadelphia, PennsylVania 19146 ReceiVed: October 19, 2006; In Final Form: December 26, 2006
The surface properties of carbon black microparticles in aqueous solution are characterized through adsorption of a hydrophobic ion (malachite green) monitored by optical second-harmonic generation (SHG). Carbon black particle surfaces with 14 different O/C ratios (from 0.028 to 0.270) have been examined. The adsorption isotherm of MG on the surface of carbon black particles weighing as little as 0.010 g can be measured using SHG measured during continuous titration of the MG molecule into the solution. The SHG data allows determination of the adsorption free energy and reveals coherent interference between the SH fields from the surface-adsorbed MG molecules and the carbon black particle. The change in the coherent interference implies a change of adsorption orientation of MG on surfaces with different O/C ratios. There is strong correlation found between the observed adsorption characteristics and the O/C ratio of the surfaces. Furthermore, MG adsorption isotherms of surfaces with higher O/C ratios appear similar to the ones of surfaces preadsorbed with surfactants that can increase surface hydrophilicity and dispersion in aqueous solution. All observations indicate that particle surfaces with higher O/C ratios appear more hydrophilic. This study demonstrates once again the many advantages of SHG as an effective surface analytical technique of colloidal particles.
I. Introduction Microparticles, the dispersant, and the solvent are the basic components that form dispersed colloidal systems that have many real-life and industrial applications.1,2 Being able to characterize and even control the surface structure of the microparticles is important to the applications as modification of the particle surface is often one approach taken to improve the dispersibility and rheological properties of a colloidal system. Tremendous efforts have been employed, for many cases quite successfully, in the design and synthesis of a variety of polymeric dispersant (or surfactant) molecules with both hydrophilic and hydrophobic functional groups for modifying particle surface properties through adsorption of these molecules onto the particle surface. Many attempts have also been made to directly modify the chemical composition and functionality of particle surfaces in order to achieve preferred interactions with the solvent molecules or affect adsorption of surfactants. Adsorption at the solid (particle)-liquid (solvent) interface is a complex phenomenon. In order to understand the effect of surface modification, through either adsorption or direct chemi†
Part of the special issue “Kenneth B. Eisenthal Festschrift”. * To whom correspondence should be addressed. Phone: 215-204-2897. Fax: 215-204-1255. E-mail:
[email protected]. ‡ Chinese Academy of Sciences. § University of Pennsylvania. | DuPont Automotive Products.
cal reactions, on macroscopic dispersibility and rheological properties, it requires experimental methods being able to characterize surface structure at the microscopic level and the adsorption kinetics. Preferably, these characterizations can be performed in situ to reflect the correlation between the microscopic conditions and the macroscopic observations.3 Conventional methods for examining colloidal properties, however, are generally neither in situ nor with interface specificity. Many methods for monitoring adsorption, i.e., establishing the adsorption isotherm, have involved spectroscopic analyses of the aliquot solutions before and after establishing the adsorption equilibrium following addition of a known quantity of surfactant. Such indirect methods can be time consuming, tedious, and prone to errors induced by contamination, as known by researchers in the field.4 Development of nonlinear optical techniques such as secondharmonic generation (SHG) and sum-frequency generation (SFG) for colloidal and nanoparticle surfaces since the late 1990s has provided effective tools for direct, in situ measurement of the microparticle surface buried in the colloid.5-26 In SHG a coherent optical signal is detected at twice the frequency of the incident laser, while in SFG a coherent optical signal is detected at the sum of the two simultaneously incident laser frequencies. The surface sensitivity of SHG and SFG arises from the lack of inversion symmetry at the surface of a centrosymmetric medium. In materials with inversion symmetry, dipole contribution to SHG is symmetry forbidden.27-30 The origin of the SH
10.1021/jp066873i CCC: $37.00 © 2007 American Chemical Society Published on Web 03/03/2007
Adsorption at a Carbon Black Microparticle Surface signal from micrometer or submicrometer centrosymmetric particle surfaces was first demonstrated by Eisenthal and coworkers in 1996.5,6 A recent review by Eisenthal surveyed their applications to the studies of various microparticle surfaces.26 Among the many applications, direct measurements of the surface adsorption, surface electric potential, and transmembrane kinetics of microemulsion and liposome vesicle surfaces in aqueous solutions demonstrate the versatility of the SHG technique.11-16 Theoretical descriptions of SHG and SFG from centrosymmetric particle surfaces have also been established.17-21 In order to make continuous in situ measurements of the particle surface adsorption processes, Dai and co-workers employed a liquid jet sample circulation system and made it possible to record the complete adsorption isotherm in a relatively short time.23-25 Dai and co-workers also developed a displacement method so that SHG from a dye molecule can be used to probe the adsorption of a molecule such as polymeric surfactants that cannot produce SHG.23 With these developments it is possible to make in situ characterization of adsorption on industrially important microparticles and powders, such as the carbon black particles studied in this work. Carbon black, with many uses in industrial applications, has been extensively characterized.31 Carbon black powder in various sizes is widely used as a filler to modify the mechanical, electrical, and optical properties of the medium in which it is dispersed.31 These applications include elastomers, plastics, paints, and inks. As printing pigment, carbon black accounts for about 70% of the total pigment volume used.32 It has been recognized that the primary carbon black parameters influencing its applications are particle size (or specific surface area), structure, and surface activity.33 In addition, because of its spherical morphology, some researchers have recognized that carbon black can be used as a material for examining the effects of surface treatments on surface structure and properties.34 For better control and improvement of the surface-related properties, especially hydrophilicity and rheological properties, treatment of carbon black particle surfaces with oxygen and other plasma sources has been reported.33-35 Analytical methods such as FTIR, XPS/ESCA, STM, etc., have been used to obtain carbon black surface structural information. Inverse gas chromatography has been used to measure filler surface energy.31 It was found that the plasma can peel off the outer layers of the particle, and the surface oxygen to carbon ratio (O/C ratio) can be significantly increased up to 0.42, which can be measured with XPS/ESCA.34,35 Through BET isotherm measurement it was also found that such treatments do not significantly change the carbon black particle size and porosity.34 Though such qualitative characterizations of the surface structure are feasible, quantitative correlations between the O/C ratio and surface parameters such as adsorption free energy would be desirable for microscopic understanding of the effect of the surface treatment. Understanding adsorption on the carbon black particle surface has additional significance. Reactions on the surface of atmospheric carbon particles and environmental black carbon originated from natural and anthropogenic sources affect the environment and climate on earth in complex ways.36,37 Information useful for estimating the capacity of adsorption of organic compounds on carbon black surfaces in atmosphere and aqueous environment directly impact the assessment of pollution and climate. Toward this purpose, carbon materials as adsorbents in aqueous solutions have been intensively studied.38 In this report, we present a study of the adsorption energetics and hydrophilicity of surfaces of submicrometer carbon black
J. Phys. Chem. C, Vol. 111, No. 25, 2007 8709 particles in aqueous solution probed by adsorption of a hydrophobic ion through second-harmonic generation. Carbon black particles with different surface oxygen/carbon ratios are examined so that the adsorption properties can be compared with the surface chemical composition. Adsorption of the organic dye cation malachite green (MG) is used as a probe of the hydrophilicity of the surfaces. Adsorption of the MG ion can be detected by the SHG method due to the intrinsic surface sensitivity of the nonlinear optical phenomenon and resonant enhancement of SHG of the molecule. We show that SHG data reveal the adsorption free energy as well as the change of adsorption structure of MG on carbon black surface with different surface O/C ratios. This study demonstrates a fast in situ analytical tool for quantitative characterization of the adsorption kinetics onto the carbon black microparticle surfaces in solution and suggests that surface treatments in the form of both chemical modification and surfactant adsorption can result in surfaces with desired hydrophilicity for adsorption. II. Experimental Section The experimental setup is the same as reported elsewhere.23 A Ti-Sapphire femtosecond laser (Coherent Mira Seed pumped by a Coherent Innova 300 Ar+ ion laser) generates nominally 50 fs laser pulses at a 76 MHz repetition rate and tunable from 760 to 870 nm. In this experiment the wavelength is set at 846 nm, and the power is 0.5 W. The laser beam, ∼1 mm in diameter, is focused with a 2 in. fl lens into the sample, a liquid jet described below. A filter (Schott RG695) is placed before the lens to cut off any possible SHG from elements in the optical path until this point. The fundamental light and the secondharmonic signal after the sample jet are collected by a 2 in. f1 lens and sent through a telescope for a 5 times beam size reduction before being refocused into a monochromator (JarrelAsh 1/4 meter). Combination of the monochromator, which is set at 423 nm with a 2 nm spectral resolution, and filters (Schott BG39) placed in front of the entrance effectively eliminates fluorescence and scattering signals at other wavelengths. The second-harmonic signal is then detected by a photomultiplier tube (Hamamatsu R585). The output is preamplified by a Stanford Research Systems SR440 amplifier and processed by a photon counter (SR400). The high repetition rate of the laser pulses allows phase-sensitive suppression of the noise and correlated photon counting to improve the signal/noise. The sample is prepared in a flow/titration system.23 The sample solution is circulated from a reservoir with a total volume of 500 mL. Pumped with a liquid pump at a pumping speed of 200 mL/min, a liquid jet is formed through a nozzle made out of pressed copper tubing with 1/16 in. inner diameter. The laser intercepts the center of the jet perpendicularly and passes through the jet with a path length about 2 mm. Using the jet, instead of a steady cell, avoids nonlinear optical signals generated from the windows of the cell as well as accumulation and coagulation of particles/dye/surfactant on the window walls. The liquid is then collected and returned to the reservoir, forming a complete circulation loop with a circulation period of about 20 s. A magnetic stirrer constantly stirs the solution in the reservoir, and an ultrasound dispenser is used to prevent aggregation of the particles. Because the total liquid volume in the circulation tubing is about 20 mL or less, such small volume does not cause significant error of the concentration reading as the reservoir of 500 mL is under constant stirring. The dye molecules, dissolved at a known high concentration in the same solvent as the colloid, are added into the reservoir by a digital
8710 J. Phys. Chem. C, Vol. 111, No. 25, 2007
Wang et al.
TABLE 1: Carbon Black Sample O/C Ratio and Relevant Parameters Determined from Nonlinear Least-Squares Fitting of MG Adsorption Isotherms sample no. CB 1 CB 2 CB 3 CB 4 CB 5 CB 6 CB 7 CB 8 CB 9 CB10 CB11 CB12 CB13 CB14
O/C ratio
K (×106)
MG strength (arb. unit)
phase angle (deg)
0.028 0.038 0.063 0.086 0.137 0.138 0.149 0.176 0.176 0.238 0.253 0.253 0.266 0.270
21.1 ( 0.7 16.7 ( 0.9 11.0 ( 1.1 13.0 ( 0.8 6.5 ( 0.6 7.4 ( 0.7 8.0 ( 0.5 7.3 ( 0.6 6.0 ( 0.5 6.1 ( 0.6 7.3 ( 0.5 8.5 ( 0.8 5.3 ( 1.0 4.9 ( 1.2
1.20 ( 0.21 2.14 ( 0.17 2.44 ( 0.22 2.25 ( 0.17 1.96 ( 0.11 1.68 ( 0.15 1.54 ( 0.18 1.76 ( 0.14 1.98 ( 0.21 2.11 ( 0.17 1.92 ( 0.15 1.86 ( 0.22 2.38 ( 0.24 2.07 ( 0.23
65.8 ( 2.0 121.3 ( 1.5 107.8 ( 0.8 123.4 ( 1.7 83.1 ( 1.8 97.8 ( 1.2 94.0 ( 2.3 93.9 ( 2.1 92.3 ( 1.8 89.4 ( 1.7 91.4 ( 2.4 91.6 ( 1.6 90.5 ( 0.9 78.2 ( 5.2
titration burette (CAT Contiburret). The titration amount is kept below 20 mL, and the effect of the volume change is included in the analysis. The Marshall Laboratory of DuPont Automotive Products provided the carbon black particle samples. About a dozen carbon black samples were surface treated from the same lot of carbon black particles. The average diameter of these particles is 160 ( 40 nm. Though the surface treatment process is not specified here, the effect of the treatment in surface modification is represented by the O/C ratio. In our experiments, Sample 1 (CB1) is the untreated control, Sample 2 (CB2) has been treated with a nonreactive process so that the particle surface was cleaned of weakly adsorbed species with little chemical modification, and the rest of the carbon black samples (CB3CB14) had their surfaces chemically modified with different time duration in the same surface-reactive treatment process. The larger the sample number, the longer the treat time and more surface modification. Their corresponding O/C ratios measured with XPS/ESCA are listed in Table 1. For each sample, 0.01 g of carbon black is dispersed into 500 mL of deionized water where an ultrasonic disperser is used to prevent coagulation. Since carbon black is in such a low concentration, coagulation has not been a problem during the experiment. The pH of the solution was adjusted with 0.01 N HCl solutions to 4.0 ( 0.2. The dye molecule used here is malachite green (MG) from an aqueous solution of malachite green chloride (Aldrich) of 100.0 µM. The structure of the MG cation is shown in Scheme 1. The DuPont Marshall Laboratory provided the surfactants with the trade name of VGP15401 and G1322. They are both methacrylate-based, water-soluble polymer mixture with a distribution of hydrocarbon chain length. VGP15401 differs from G1322 in the functional groups on the polymer chain. VGP15401 has mostly -COOH, -OH, and -SH groups, while G1322 has the -NH2 group. At pH 4, VGP15401 is negatively charged while G1322 is positively charged. The numberaveraged molecular weight of VGP15401 is Mn ) 3300, while the weight-averaged molecular weight is Mw ) 8500. The polymer content by weight is 30.04%. The VGP15401 stock solution concentration is 5.00 g/L with pH ) 4.0 ( 0.2, while G1322 has Mw ≈ 2000 and a polymer content 40.0%. The G1322 stock solution is 4.98 g/L with pH ) 4.0 ( 0.2. The solution pH is controlled around 4.0 because MG is in its singlecharged (green) form between pH 2.0 and 6.5.8
SCHEME 1: Malachite Green Molecular Structure
III. Theoretical Model Analysis Before we present the experimental observations, we first describe the theoretical model that is developed for analyzing the SH intensity measured from the colloidal system as a function of the concentration of the MG dye or the surfactant. As we learned from previous studies,6,23,39 the total SHG signal obtained from a colloidal system can be generally represented as
ISH ) B + |b + aθD exp(iΦ)|2
(1)
In eq 1 b represents the bulk and clean surface contribution to the second harmonic from the particles which may coherently interfere with the surface contribution aθD exp(iΦ). Here a corresponds to the SH polarization at full surface dye coverage, θD is the surface dye coverage (as a fraction of the full coverage), and exp(iΦ) is the phase factor between the particle and the surface dye SHG contributions. All other possible incoherent contributions to the signal detected in the experiments are summarized in the background term B. As we shall discuss in detail later, for the systems examined here the background term B is mainly from the incoherent hyper Rayleigh scattering and negligible in comparison with the other terms. Thus, we can further simplify eq 1 by normalizing the total SH signal with the term b2, which is the SH intensity from the particle without any dye adsorption. If we define the ratio A ) a/b, which describes the relative strength of the relative contributions from the particle and the adsorbed molecules to the SH field, then eq 1 becomes
) [1 + AθD exp(iΦ)]2 INORM SH
(2a)
) [1 + AθD cos Φ]2 + [AθD sin Φ]2 (2b) Assuming that the adsorption follows Langmuir kinetics, the coverage θD can be related to the total concentration CD of the dye molecules, including both the molecules in the liquid and the ones adsorbed on surface, the maximum number density of the adsorbed molecules NDmax and the equilibrium constant KD, 1 defined as KD ) k-1 D /kD in the adsorption equation 1 kD
} filled surface site MG in liquid + empty surface site {\ k-1 D (3) as
θD ) (CD + NDmax + 55.5/KD) -
x
(CD + NDmax + 55.5/KD)2 4CDNDmax
2NDmax (4)
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J. Phys. Chem. C, Vol. 111, No. 25, 2007 8711
Figure 1. Adsorption isotherm of MG on carbon black surfaces. Only 6 of the total 14 curves are shown here. The dots are experimental data, and the solid lines are fitting curves. The curves falls into three categories: Type I, CB1; Type II, CB2 and CB4; Type III, CB8, CB10, and CB14.
where 55.5 is the molar concentration of water, CD is the dye concentration, and NDmax is total surface number density when a full adsorption monolayer is reached. As it has been shown,6,23 eqs 2 and 4 together can be used to relate the SHG signal as a function of CD and the experimentally measured SH intensity as a function of CD data can be fitted to give parameters NDmax and KD. It is important to recognize that eq 4 is a modification of the Langmuir model in the case of substantial depletion of molecules from the solution due to adsorption, such as the case of MG adsorption of polystyrene particles.6 If depletion of molecules from the solution due to adsorption is negligible, i.e., CD . NDmax, the standard Langmuir model should be used40,41
θD )
1 55.5 1+ KDCD
(5)
In this case, MG adsorption onto carbon black surfaces, depletion of MG from solution due to adsorption is negligible; NDmax in eq 4 is no longer a comparable number to CD and 55.5/ KD. NDmax therefore becomes a redundant parameter if eq 4 is used in a nonlinear least-squares fitting of the data and cannot be determined with statistical certainty. In our analysis of the SH intensity as a function of MG concentration, we used eqs 2 and 5 but not eq 4, so that all parameters determined have statistical certainty. IV. Results and Discussion A. Adsorption of MG on Carbon Black Surfaces. Adsorption isotherms, expressed in the form of SH intensity as a function of MG concentration, of 14 carbon black sample (0.010 g/500 mL) surfaces in water at pH ) 4.0 ( 0.2 were measured. In order to avoid unnecessary congestion on the graph, we present 6 of the total 14 measured isotherm curves (Figure 1). Each isotherm curve takes about 20 min titration to complete. Each curve was normalized to the SH signal at zero MG concentration. For all the samples, this signal level is about the
same at 2500 counts per 3 s, comparing with the water background signal at about 20 counts per 3 s. The curves are shockingly different from the other SHG isotherm curves previously reported in that some of the curves show a decrease in SH intensity as MG is added into the solution. The fact that all samples with different surface conditions (see below) produce the same amount of SH signal indicates that the origin of this signal comes from the bulk phase of the carbon black particle. This is because the samples all have very different surface O/C ratios, and the surface contributions should therefore vary. We assign this signal as the SHG signal, which is coherent, instead of hyper Rayleigh scattering, which is incoherent, from the carbon black particles. This is because in Figure 1 we see not only in some curves that the SH intensity increases with MG concentration in a few curves it even decreases with MG concentration. The decrease can only arise from destructive interference between the SHG from surfaceadsorbed MG and that from the particle. The presence of interference, a coherent phenomenon, suggests that at zero MG concentration the SH intensity from carbon black particles can be used as the internal standard against which the SHG for the surface can be measured quantitatively. In Figure 1 none of the isotherms reached the plateau that corresponds to saturation of adsorption. We did not go to higher MG concentration because in order to reach saturation the total MG concentration in the solution would be so high that twophoton fluorescence (TPF) from the MG molecules in solution would become strong. At the level of MG concentrations we used, such TPF contribution is still negligible. The initial portion of the isotherms is sufficient for accurate model analysis for determination of the fitting parameters as discussed below. At the initial part of the isotherm the surface coverage is relatively low and the interaction between adsorbate molecules is generally weak. Therefore, there is no reason to expect that the orientation of the MG adsorbed at the surface would change and cause adverse SH responses like those observed in some of the curves. This expectation supports the analysis of those adverse behaviors as from interference. The adsorption isotherms, including the ones shown in Figure 1, can be clearly classified into three distinctive types. The first is the isotherm of the untreated control sample (CB 1), which stands alone on top of the graph with a monotonous increase of SH signal with the increase of MG concentration. The second includes those for CB2, CB3 (not shown), and CB4. In each of these curves the SH signal decreases as the MG concentration increases initially. However, as the MG concentration continues to increase, the SH signal turns over and increases with the MG concentration. The third type includes all of the other 10 samples, from CB5 to CB14 (only CB8, CB10, and CB14 are shown; those not shown follow these three curves very closely). Here the SH signal increases monotonously with the MG concentration but at a slow rate. The difference between types I and III is obvious: the type I curve is convex, while type III curves are distinctively concave. The different behaviors of the isotherm curves can be described by eq 2 as interference between the particle contribution and the surface-adsorbed molecule contribution to the SH intensity. If the contributions to the total SH signal from the bulk term and the surface term (AθD exp(iΦ)) in eq 2a are comparable, the interference between these two terms with different relative magnitudes generates different shape curves. This kind of interference has been previously observed, for example, the one observed by Wang et al.39 where the SH intensity was measured as the DEPNA concentration at the air/
8712 J. Phys. Chem. C, Vol. 111, No. 25, 2007 water interface. Basically, when the relative phase angle Φ in eq 2 is >90°, i.e., cos Φ < 0, the first term [1 + AθD cos Φ]2 in eq 2b is smaller than 1 and would continuously get smaller as the surface coverage θD becomes bigger. This is the case of destructive interference. At further increases of θD, the decrease of total SH signal caused by the first term in eq 2b can be compensated by the second term [AθD sin Φ]2. In this case, we should see a turn over when the second term is big enough. This is the case for the type II curves in Figure 1. On the other hand, when the relative phase angle Φ in eq 2 is 0, all terms should add up constructively. In this case, the total SH signal should increase monotonously with the surface coverage θDsthe behavior of the type I curve in Figure 1. Intuitively, we could expect that the type III curves, the ones with a slow initial increase, may fall in between the type I and type II cases, i.e., the relative phase angle lies close to 90°. This is because when Φ ) 90°, cos Φ is close to zero and there should be a slow rise of the total signal as the surface coverage increases. These expectations are all confirmed by quantitative analysis of the curves below. Nonlinear least-squares analysis of the curves in Figure 1 using the model described in section III gives adsorption free energies of MG cation on the various carbon black particle surfaces as about -9 to -10 kcal/mol. In comparison, the adsorption free energy for MG cation on PSS particle surfaces is -12.4 kcal/mol.6,23 This indicates significantly weaker adsorption onto the carbon black surfaces than that of PSS surfaces. With the amount of carbon black particles in solution, depletion of the MG cation from the solution is negligible. It is estimated that in order to observe depletion, the density of the carbon black particles has to be increased by at least 1 or 2 orders of magnitude. Consequently, eq 5, instead of eq 4, is used together with eq 2 for the fitting. In our experiments a continuous flow jet is used for interacting with the laser. This allows the condition of the solution during titration to be monitored continuously, which means hundreds of data points rather than a limited number of points when a static cell is used can be readily measured in a reasonably short time. Fitting these data, even when only the initial portion of the isotherm is recorded, gives precise determination of the adsorption equilibrium constant KD and the maximum adsorption density NDmax. All fitting results are presented in Table 1. The phase angle of the CB1 adsorption isotherm curve was determined as 65.8° ( 2.0°, an angle indeed much smaller than 90° for displaying the type I curve behavior. The phase angles of CB2, CB3, and CB4 are 121.3° ( 1.5°, 107.8° ( 0.8°, and 123.4° ( 1.7°, respectively. These three phase angles characterize the type II curves as discussed above. Phase angles of all other samples are nearly 90°, also as described above. In Table 1 the oxygen to carbon ratios (O/C ratio) of carbon black particles, measured with XPS/ESCA,35 are also listed. The O/C ratio provides a measure for the chemical composition change at the particle surface due to surface treatment. A higher O/C ratio means a higher concentration of oxygen species on carbon black particle surfaces after longer treatment time. The oxygen content usually is attributed to surface -C-OH, -Cd O, or -COOH functionalities.33,35 A comparison of this piece of structural information with the parameters characterizing MG ion adsorption on the same surface should provide us a better understanding of the effect of the surface treatment. There are several observations one can make in this comparison. (1) There is only a small difference in the O/C ratio between CB1 (O/C ) 0.028), the untreated control sample, and CB2
Wang et al.
Figure 2. Equilibrium constant K (solid dot) and adsorption free energy ∆G° (open circle) versus O/C ratio. Both K and ∆G° reach a plateau at an O/C ratio ≈.14.
(0.038), the slightly treated sample. However, the SHG adsorption isotherm of CB2 is dramatically different from that of CB1, as illustrated in Figure 1. This indicates that even with the slightest treatment of the carbon black surface the surface structure and adsorption properties can be significantly modified. A reasonable explanation for this is that the initial step for the surface treatment is to chemically remove the species originally adsorbed on the carbon black surfacesa cleaning process. The XPS/ESCA measurement cannot distinguish such cleaning processes. This may be attributed to the fact that these species are not strongly bound to the surface and, therefore, not detected by XPS/ESCA. The comparison between the measured O/C ratios and the SHG behaviors demonstrates that the O/C ratio alone cannot tell us the actually surface property of the carbon black particles. (2) For the surfaces cleaned from species weakly adsorbed on the particle surface there is a strong correlation between the surface adsorption free energy and the surface O/C ratio. From the fitted equilibrium constant in Table 1, the adsorption free energy can be calculated according to the relationship ∆G° ) -RT ln K. The results of KD (black dot) and ∆G° (open circle) values versus the O/C ratio are plotted in Figure 2. The general trend observed is that KD decreases and ∆G° becomes less negative with the surface O/C ratio. As the surface O/C ratio increases, it appears that the likeliness for MG to adsorb onto the carbon black surface decreases. Thus, a higher O/C ratio indicates higher hydrophilicity of the surface. Previously adsorption of hydrophobic dye molecules such as rose bengal has been used as a probe of the hydrophobicity of a surface.3 Here, the hydrophobic MG cation serves also as a useful probe of hydrophobicity. In Figure 2, as the O/C ratio reaches a level of about 0.14, KD and ∆G° both reach a plateau. This indicates that once the surface oxidation reaches a certain level, further increasing the O/C ratio would not significantly improve the surface hydrophilicity. This observation suggests that long-time surface treatment may not be helpful in improving the adsorption properties of the carbon black particle surface. (3) It can be shown that the MG molecular orientation on the carbon black particle surfaces changes with the surface O/C ratio. Figure 3 shows the relative phase angle and MG SH field strength (parameter A) versus the O/C ratio. The relative phase
Adsorption at a Carbon Black Microparticle Surface
J. Phys. Chem. C, Vol. 111, No. 25, 2007 8713
Figure 3. Relative phase angle (solid dot) and MG field strength (parameter A, open circle) versus O/C ratio.
angle is related to the adsorption structure within the surface layer. As we discussed above, the contribution of the SH signal is from the bulk phase of the particle and the adsorbed MG molecules. When the surface of carbon black is modified, the bulk-phase contribution does not change significantly, as the particle bulk phase is still the same. However, since the surface is modified, the interaction between the adsorbed MG molecule and the carbon black surface, which directly determines the adsorption energetics and structure, changes. The change in the adsorption energetics can be directly measured as the change of adsorption free energy ∆G° with the O/C ratio. On the other hand, the structural change is reflected in the change of the relative phase angle. It is well known that the phase of the SH field is related to the molecular orientation to the surface normal.42,43 Generally, when a molecule changes its orientation by 180°, the SH field generated changes its sign, i.e., cos 180° ) -1. However, for intermediate values of the phase angle, the quantitative contribution of the phase angle change is also affected by the relative magnitude (relative strength) between the two contributing terms. One cannot model and calculate the phase relationship between the SHG contributed from the adsorbed MG cations and that from the particle bulk without knowing the molecular orientation and polarizability, which are generally not available except for a recent determination of adsorption of MG on differently charged PS particle surfaces.25 For planar surfaces, the general description of the orientational functional, which contains all the orientational and phase information, can be derived.44-46 However, the interference and phase between different origins remain a difficult problem in theoretical modeling. The MG strength factor in Figure 3 appears to track the change in the phase angle. This strength factor also reflects the orientation of MG at the surface, and its change suggests the change of adsorption orientation with varying O/C ratios of the surface. The significant change of both the phase angle and the SH strength factor suggests significant change of the MG orientation when the O/C ratio changes. Scheme 1 shows the molecular structure of MG. The two amino groups are hydrophilic, while the phenyl groups are hydrophobic. Depending on the nature of the surface, different parts of the MG molecule should show
Figure 4. Adsorption isotherms of MG on CB2 (O/C ratio ) 0.038, upper panel) and CB12 (O/C ratio ) 0.253, lower panel) samples measured under three different conditions: no surfactant, VGP15401covered, and G1322-covered carbon black surfaces.
preferential interaction with the surface. On the basis of the observed trend of hydrophobicity with the O/C ratio, we speculate that when the carbon black surface has a low O/C ratio, MG may have its aromatic rings closer to the surface, while when the carbon black surface has a high O/C ratio, the two amino groups may be closer to the surface. In the analysis of the SHG data we focused on MG cation adsorption on the carbon black surface. Adsorption of the counterion, the chloride anion, is judged much less likely because the oxygen-containing surface is electronegative and the aromatic rings of MG provide additional attraction to carbon black surface. Any chloride anions at the surface region may affect MG adsorption but will not contribute to the SH signal. B. Surface Modification of Carbon Black with Surfactant Adsorption. In industrial applications surfactant adsorption on carbon black surfaces has been used to improve the particle hydrophilicity and rheological properties for better dispersion in water. Surfactant adsorption can also be used here to modulate carbon black surface hydrophobicity and examine the following
8714 J. Phys. Chem. C, Vol. 111, No. 25, 2007 MG adsorption behavior to see if it is consistent with the observed adsorption pattern of carbon black with different surface O/C ratios. From the MG adsorption behavior on particles with different O/C ratios, monitored by SHG, we deduced that the untreated carbon black particles, with type I adsorption isotherm, have the least hydrophilicity and are the hardest to be dispersed in water. The low O/C ratio surfaces with type II isotherms have relatively low surface hydrophilicity. The high O/C ratio surfaces with type III isotherms, on the other hand, have relatively high hydrophilicity. On the basis of these deductions, we speculate that by improving the surface hydrophilicity of type II carbon black surfaces with surfactant adsorption, the MG adsorption isotherm on such modified surfaces should also be changed into the high hydrophilicity type, i.e., the type III isotherm. Figure 4 shows two sets of such MG adsorption isotherms measured through SHG. The upper diagram includes three isotherms for carbon black sample CB2 (0.020 g/L in water, pH ) 4.0 ( 0.2), which has O/C ratio ) 0.038. The adsorption isotherms of MG on bare CB2 surfaces, the ones not modified by surfactant adsorption, are unmistakably type II. However, when the colloid containing the same amount of CB2 is mixed with either 0.020 g/L VGP15401 or G1322 surfactant, the MG adsorption isotherm in both cases turned into type III. The lower diagram shows three isotherms for sample CB12, which has O/C ratio ) 0.253. The three adsorption isotherms for CB12 without adsorption, CB12 covered with VGP15401, and CB12 covered with G1322 are all type III and nearly nondistinguishable from each other. It is revealing that the VGP15401- and G1322-covered carbon black surfaces show similar MG adsorption isotherms as those of CB12 with high O/C ratio. Qualitatively, both surfactants have been known to have the ability to improve surface hydrophilicity. However, because VGP15401 has mostly -COOH, -OH, and -SH groups while G1322 has mostly a -NH2 group, at pH ≈ 4 one surface is nearly negatively charged while the other is positively changed. The similarity of the adsorption isotherms on these two surfaces indicates that the interaction between the adsorbed MG molecule and the surfactant-covered surfaces is not dominated by the charge-charge interaction. Hydrophobic interactions between MG molecules and between MG and the surface also play important roles. This is consistent with the findings from the adsorption of MG on differently charged PS particle surfaces.25 The adsorption behavior of the MG ion demonstrates that the surface hydrophilicity of carbon black can be modified using different approaches. Both use of surfactants/dispersants as well as direct alteration of surface chemical composition can achieve similar surface hydrophilicity as probed by the MG adsorption. V. Concluding Remarks Adsorption of a hydrophobic ion, MG, has been used as a probe of the hydrophilicity of the surface of carbon black microparticles in aqueous solution. Carbon black particle surfaces with various O/C ratios, which have been determined by the ESCA/XPS technique, have been studied. Adsorption was characterized with adsorption isotherms measured by the SHG method. Three types of adsorption isotherms have been observed and analyzed quantitatively using the Langmuir adsorption model. Analysis allows determination of the adsorption free energy and reveals optical interference between the SH fields from the adsorbed MG molecule and the carbon black particle itself. The different optical interference implies that the orientation of the MG ion adsorbed on surfaces with varying
Wang et al. O/C ratios is different. Both the adsorption free energy and the implied orientation change serve as indicators for the change of hydrophilicity of the surface. A clear correlation can be identified between the carbon black surface O/C ratios and the adsorption free energies/orientation change. The correlation indicates that the higher O/C ratio surfaces are more hydrophilic. This deduction is consistent with the observation that adsorption of MG ion on carbon black particles whose surfaces are adsorbed with surfactants that increase the hydrophilicity and dispersion in aqueous solutions is similar to that on the high O/C ratio particle surfaces. This study shows once again that SHG is an effective and versatile technique for characterizing colloidal particle surface properties through sensitive, in situ measurement of adsorption on the particle surface. Acknowledgment. H.L.D. and A.G.Y. acknowledge the National Science Foundation GOALI Grant and the DuPont Marshall Laboratory for support of the collaborative research. H.L.D. acknowledges equipment support from the NSF MRSEC Program (grant no. DMR05-20020). H.F.W. is thankful for support by the Natural Science Foundation of China (NSFC, Nos. 20425309 and 20533070) References and Notes (1) In Principles of Powder Technology; Rhodes, M. J., Ed.; John Wiley & Sons, Inc.: New York, 1990. (2) In The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet, 2nd ed.; Evans, D. F., Wennerstro¨m, H., Eds.; WileyVCH: New York, 1999. (3) In Particle and Surface Characterization Methods; Mu¨ller, R. H., Mehnert, W., Eds.; Medpharm Scientific Publishers: Stuttgart, 1997. (4) In Adsorption from Solution at the Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: London, 1983; Chapter 1, p 13. (5) Wang, H. F.; Yan, E. C. Y.; Borguet, E.; Eisenthal, K. B. Chem. Phys. Lett. 1996, 259, 15-20. (6) Wang, H. F.; Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. J. Phys. Chem. B 1998, 102, 4446-4450. (7) Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. J. Phys. Chem. B 1998, 102, 6331-6336. (8) Yan, E. C. Y.; Eisenthal, K. B. J. Phys. Chem. B 1999, 103, 60566060. (9) Yan, E. C. Y.; Eisenthal, K. B. J. Phys. Chem. B 2000, 204, 66866689. (10) Liu, Y.; Dadap, J. I.; Zimdars, D.; Eisenthal, K. B. J. Phys. Chem. B 1999, 103, 2480-2486. (11) Srivastava, A.; Eisenthal, K. B. Chem. Phys. Lett. 1998, 292, 345351. (12) Liu, Y.; Yan, E. C. Y.; Eisenthal, K. B. Biophys. J. 2001, 80, 10041012. (13) Liu, Y.; Yan, E. C. Y.; Zhao, X. L.; Eisenthal, K. B. Langmuir 2001, 17, 2063-2066. (14) Yan, E. C. Y.; Eisenthal, K. B. Biophys. J. 2000, 79, 898-903. (15) Shang, X. M.; Liu, Y.; Yan, E. C. Y.; Eisenthal, K. B. J. Phys. Chem. B 2001, 105, 12816-12822. (16) Liu, J.; Shang, X. M.; Pompano, R.; Eisenthal, K. B. Faraday Discuss. 2005, 129, 291-299. (17) Roke, S.; Roeterdink, W. G.; Wijnhoven, J. E. G. J.; Petukhov, A. V.; Kleyn, A. W.; Bonn, M. Phys. ReV. Lett. 2003, 91, 258302. (18) Dadap, J. I.; Shan, J.; Eisenthal, K. B.; Heinz, T. F. Phys. ReV. Lett. 1999, 83, 4045-4048. (19) Yang, N. P.; Angerer, W. E.; Yodh, A. G. Phys. ReV. Lett. 2001, 87, 103902. (20) Roke, S.; Bonn. M.; Petukhov, A. V. Phys. ReV. B 2004, 70, 115106. (21) Dadap, J. I.; Shan, J.; Heinz, T. F. J. Opt. Soc. Am. B 2004, 21, 1328-1347. (22) Shan, J.; Dadap, J. I.; Stiopkin, I.; Reider, G. A.; Heinz, T. F. Phys. ReV. A 2006, 73, 023819. (23) Wang, H. F.; Troxler, T.; Yeh, A. G.; Dai, H. L. Langmuir 2000, 16, 2475-2481. (24) Eckenrode, H. M.; Dai, H. L. Langmuir 2004, 20, 9202-9209. (25) Eckenrode, H. M.; Jen, S. H.; Han, J.; Yeh, A. G.; Dai, H. L. J. Phys. Chem. B 2005, 109, 4646-4653. (26) Eisenthal, K. B. Chem. ReV. 2006, 106, 1462-1477. (27) Chen, C. K.; Heinz, T. F.; Ricard, D.; Shen, Y. R. Phys. ReV. B 1983, 27, 1965-1979.
Adsorption at a Carbon Black Microparticle Surface (28) Shen, Y. R. Nature 1989, 337, 519-525. (29) Eisenthal, K. B. Chem. ReV. 1996, 96, 1343-1360. (30) Heinz, T. F. ‘Second-order nonlinear optical effects at surfaces and interfaces. In Nonlinear Surface Electromagnetic Phenomena; Ponath, H., Stegeman, G., Eds.; Elsevier: Amsterdam, 1991; pp 353-416. (31) In Carbon Black Science and Technology; Donnet, J.-B., Bansal, R. C., Wang, M.-J., Eds.; Marcel Dekker, Inc.: New York, 1993. (32) Thayer, A. M. Chem. Eng. News 2002, 80, 23-30. (33) Boehm, H. P. Carbon 1994, 32, 759-769. (34) Cascarini De Torre, L. E.; Bottana, E. J.; Martı´nez-Alonso, A.; Cuesta, A.; Garcı´a A. B.; Tasco´n, J. M. D. Carbon 1998, 36, 277-282. (35) Takada, T.; Nakahara, M.; Kumagai, H.; Sanada, Y. Carbon 1996, 34, 1087-1091. (36) Nienow, A. M.; Roberts, J. T. Annu. ReV. Phys. Chem. 2006, 57, 105-28. (37) Koelmans, A. A.; Jonker, M. T. O.; Cornelissen, G.; Bucheli, T. D.; Van Noort, P. C. M.; Gustafsson, O ¨ . Chemosphere 2006, 63, 365377.
J. Phys. Chem. C, Vol. 111, No. 25, 2007 8715 (38) Radovic, L. R.; Moreno-Castilla, C.; Rivera-Utrilla, J. Chem. Phys. Carbon 2001, 27, 227-405. (39) Wang, H. F.; Borguet, E.; Eisenthal, K. B. J. Phys. Chem. A 1997, 101, 713-718. (40) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361-1403. (41) Adamson, A. W. Physical Chemistry of Surfaces, 7th ed.; John Wiley & Sons, Inc.: New York, 1997. (42) Heinz, T. F. In Nonlinear Optical Effects at Surface and Interfaces, in Nonlinear Surface Electromagnetic Phenomena; Ponath, H., Stegman, G., Eds.; Elsevier: New York, 1991; p 353. (43) Reider, G. A.; Heinz, T. F. Second-order Nonlinear Optical Effects at Surfaces and Interfaces: Recent Advances. In Photonics Probes of Surfaces; Helevi, P., Ed.; Elsevier: New York, 1995; pp 413-78. (44) Rao, Y.; Tao, Y. S.; Wang, H. F. J. Chem. Phys. 2003, 119, 52265236. (45) Wang, H. F. Chin. J. Chem. Phys. 2004, 17, 362-368. (46) Zhang, W. K.; Wang, H. F.; Zheng, D. S. Phys. Chem. Chem. Phys. 2006, 8, 4041-4052.
