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Examination of acid–base properties of alumina treated with silane coupling agents, by using inverse gas chromatography
Powder Technology 188 (2009) 229–233
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
Examination of acid–base properties of alumina treated with silane coupling agents, by using inverse gas chromatography Young-Cheol Yang a, Byoung-Gyu Kim a, Soo-Bok Jeong a,⁎, Pyoung-Ran Yoon b a b
Minerals and Materials Processing Division, Korea Institute of Geoscience & Mineral Resources, Deajeon 305-350, South Korea Department of Mineral Resources & Energy Engineering, Chonbuk National University, Jeonju 561-756, South Korea
A R T I C L E
I N F O
Article history: Received 11 February 2008 Received in revised form 21 April 2008 Accepted 25 April 2008 Available online 3 May 2008 Keywords: Inverse gas chromatography (IGC) Specific component of the free energy of adsorption (ΔGASP) Enthalpy of specific adsorption (ΔHSP A ) Surface acid–base property
A B S T R A C T This study examined the specific component of the free energy of adsorption, −ΔGASP, of the untreated and four types of silane coupling agent-treated alumina powders using inverse gas chromatography (IGC) by employing the adsorption of several polar and non-polar probes onto their surfaces at various temperatures. The acid–base properties of the untreated and surface-treated alumina powders were quantified using their KA and KD parameters, which reflect the ability of a surface to act as an electron acceptor and donor, respectively. The surface of the untreated alumina was found to be amphoteric and was able to function as both an electron acceptor and donor. The acid–base properties of the alumina surfaces treated with γ-glycidoxy propyl trimethoxy silane (GMS) and γ-amino propyl triethoxy silane (AES) were slightly basic, and those of the alumina surfaces treated with γ-methacryloxy propyl trimethoxy silane (MTMS) and γ-mercapto propyl trimethoxy silane (MCMS) were amphoteric. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In the first paper of this series, we described the surface properties of the untreated and silane coupling agent-treated alumina in terms of their dispersive force parameters, as determined by inverse gas chromatography (IGC). However, it is clear that the full description of the surface properties can only be achieved with the use of the acid–base interaction parameters. Acid–base interactions are important components of polar forces, and play a significant role in the adhesion of inorganic fillers to organic polymers. This study examined the acid–base properties of the same samples to complete the previously defined characterization of their surface properties. The following parameters were calculated in order to compare the acid–base properties of the untreated and silane coupling agenttreated alumina powders: the specific component of the free energy of adsorption (ΔGASP), the enthalpy of specific adsorption (ΔHSP A ), the acidic constant of a solid (KA), the basic constant of a solid (KD) and SC (KD/KA). 2. Experimental 2.1. Theory of inverse gas chromatography In IGC under infinite dilution conditions, the retention volume VN can be calculated using the following Eq. (1): VN ¼ ðtR t0 ÞjDC
⁎ Corresponding author. Tel.: +82 42 868 3576; fax: +82 42 861 9720. E-mail address: [email protected] (S.-B. Jeong). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.04.080
ð1Þ
where tR is the retention time of the probes, t0 is the zero retention time measured with a nonadsorbing probe such as methane, j is the compressibility factor depending on the pressure at the column inlet and outlet, and DC is the corrected flow rate. In practice, the retention time and retention volume, VN can be determined in a current chromatographic experiment: a larger VN will correspond to a higher affinity of a probe to the chromatographic support. In Eq. (1), j was calculated using the following Eq. (2) [1]: " # ðpi =p0 Þ2 1 j ¼ 1:5 ðpi =p0 Þ3 1
ð2Þ
where pi is the inlet pressure of the carrier gas, and p0 is the outlet pressure of the carrier gas, which is usually equal to atmospheric pressure. For a test substance, the free energy of adsorption, ΔGA, is the sum of the energies of adsorption due to the dispersive and specific interactions. The adsorption of non-polar probes such as n-alkanes, occur through dispersive interactions, whereas for polar probes, both London and acid–base interactions contribute to ΔGA. The model reported by Donnet et al., was used in this study because the injected probe is in the gaseous state [2]. In this model, ΔGA is given by the following equations: ½DGA ¼ DGDA þ DGSP A ¼ ½RTlnVN þ C h i ¼ K ðhrS Þ1=2 a0S ðhrL Þ1=2 a0L þ DGSP A
ð3Þ ð4Þ ð5Þ
230
Y.-C. Yang et al. / Powder Technology 188 (2009) 229–233
where ΔGAD and ΔGASP are the dispersive and specific components of the free energy of adsorption, respectively. In Eq. (4), R is the ideal gas constant, T is the absolute temperature and the value of the constant, C depends on an arbitrarily chosen reference state of the adsorbed molecule. In Eq. (5), K is a constant, hνS and hνL are the ionization potentials of the interacting materials, and α0 is the deformation polarizability of the molecules. Subscripts S and L refer to the solid and liquid phases, respectively. In the case of n-alkanes, ΔGA is equal to the free energy of adsorption corresponding to the dispersive interactions only, ΔGAD, i.e., [−ΔGASP] = 0 in Eq. (5). The term [K (hνS)1/2 α0S] is a characteristic of a given solid surface and is related to ΔGAD. Consequently, [RT lnVN + C] between an adsorbate and an adsorbent is a linear equation of the parameter [(hνL)1/2 α0L] with a slope of [K (hνS)1/2 α0S]. Polar testing probes [Lewis acids and bases, e.g. chloroform (CHCl3) and tetrahydrofuran (THF)] have their corresponding [−ΔGA] values above the reference line. The [−ΔGASP] value is determined by the vertical distance between the n-alkane plot and the data for the polar probe of interest. An examination of the temperature dependence of ΔGASP enables the enthalpy of specific adsorption, ΔHSP A , to be determined [3]: DHASP ¼
A DGSP A =T : Að1=T Þ
ð6Þ
The enthalpy of specific adsorption between the examined surface and the test solutes may be related to the acid–base properties of both species by using either Drago's equation or the following equation [3]: DHASP ¼ KD AN þ KA DN
ð7Þ
where AN and DN are the electron acceptor and donor numbers of the test solute, respectively, and denote the Gutmann numbers, AN, which indicates the ability of a surface to attract electrons (acidity), and DN, which quantifies its ability to release electrons (basicity), respectively [4]. The parameters KA and KD reflect the ability of a surface to act as an electron acceptor and donor, respectively. The ratio, KD/KA(SC), describes the characteristics of a surface (acidic or basic). KD and KA, determined according to the method using Gutmann's AN values in Eq. (7), are expressed in different units because Gutmann's AN and DN have in different units. Therefore, in order to obtain both sides of Eq. (7) in the same units, KD must be in kJ mol− 1 and KA needs to be dimensionless. Therefore, caution must be taken when making conclusions from their ratio because the KD/KA value is unclear. Further discussion will be based only on the KD, KA and SC values determined using the Riddle–Fowkes AN⁎ values [5] in Eq. (7). The procedure described above has been used to examine surface properties of silica, modified silica, oxides, various minerals and solid polymers [6–15].
Fig. 1. Variation of [RTlnVN] as a function of [(hνL)1/2 α0L] for different probes adsorbed on the untreated alumina, measured at 120 °C.
2.2. Materials The α-alumina (α-Al2O3) was obtained in powder form from the Bayer process. The particle size ranged from 0.73 to 180 μm. Four types of silane coupling agents were used in the adsorption experiments. Table 1 lists the chemical names and adsorption experimental conditions of the silane coupling agents. Methanol and distilled water were used as the solvent, and acetic acid was used as the catalyst for hydrolyzing the silane coupling agents. A homologous series of n-alkanes, heptane (C7H16), octane (C8H18), and nonane (C9H20), were was used as the non-polar probes, and THF (C4H8O), chloroform (CHCl3), acetonitrile (CH3CN), ethyl acetate (CH3 COOC2H5) and acetone (CH3COCH3) were used as the polar probes for the IGC experiment.
Table 1 Chemical names and experimental conditions for adsorption of silane coupling agents Denomination Chemical name in this paper MTMS
γ-methacryloxy propyl trimethoxy silane
GMS
γ-glycidoxy propyl trimethoxy silane γ-mercapto propyl trimethoxy silane
MCMS
AES
γ-amino propyl triethoxy silane
Solution
pH
Remarks
Alumina weight
Methanol (80 mL) Distilled water (20 mL) Distilled water Methanol (80 mL) Distilled water (20 mL) Distilled water
3.53–3.78
Acetic acid catalyzed
30 g
5.66– 6.88
Natural
4.64– 4.83
Acetic acid catalyzed
9.95–10.66 Natural
Fig. 2. Variation of [RTlnVN] as a function of [(hνL)1/2 α0L] for different probes adsorbed on the alumina surface-treated with γ-methacryloxy propyl trimethoxy silane, measured at 120 °C.
Y.-C. Yang et al. / Powder Technology 188 (2009) 229–233
231
Table 2 Characteristics of probes Probe
α0 1040
hν 1019
(hν)1/2 α0 1049
n-Heptane n-Octane n-Nonane Chloroform Acetonitrile Acetone Ethyl acetate THF
[Cm2 V− 1]
[CV]
[C3/2 m2 V− 1/2]
15.24 17.59 19.75 10.57 5.43 7.12 10.79 8.77
4.57 4.20 4.00 5.45 7.58 6.63 5.36 5.98
10.3 11.4 12.5 7.8 4.7 5.8 7.9 6.8
2.3. Adsorption experiment The experiment designed to modify the alumina surface was performed as follows. The solvent (100 mL), silane coupling agents (1.0 g) and alumina powder (30 g) were stirred with a magnetic bar stirrer for 1 h and then separated into solid and liquid components using a centrifugal separator. The pH is the most important factor in the hydrolysis of silane coupling agents. Therefore, the appropriate amount of acetic acid was added before stirring in accordance with the type of silane coupling agents in order to obtain a reasonable pH. Table 1 lists the conditions used in the adsorption experiment. The separated alumina was dried for 24 h at room temperature and then for 8 h at 105 °C. Various samples, i.e. the untreated alumina, and the
Fig. 3. Variations of [−ΔGASP/T] as a function of [1/T] (T in K) for some polar probes adsorbed on the untreated alumina.
MTMS, GMS, MCMS and AES-treated alumina, were prepared in this way for the IGC study at infinite dilution. Table 3 The specific component of the free energy of adsorption, −ΔGASP, of polar probes on the untreated and surface-treated alumina Probe
Temperature 110 °C
120 °C
130 °C
140 °C
6.84 20.84 20.00 16.78 17.11
6.60 20.04 18.97 15.84 16.24
6.33 19.08 17.88 14.95 14.82
130 °C
140 °C
150 °C
5.67 19.00 16.01 10.65 12.85
5.48 18.18 15.32 10.20 12.25
5.47 18.13 14.99 10.01 11.80
120 °C
130 °C
140 °C
6.59 16.92 14.62 11.07 12.86
6.02 15.76 13.48 10.23 12.04
5.37 14.41 12.41 9.29 10.65
−1 MCMS-treated alumina, −ΔGSP A (kJ mol ) Chloroform 5.47 Acetonitrile 21.24 Acetone 19.37 Ethyl acetate 15.78 THF 17.55
5.25 20.19 18.41 14.89 16.72
4.78 19.46 17.58 13.83 15.40
4.65 18.58 16.97 13.01 14.66
−1 AES-treated alumina, −ΔGSP A (kJ mol ) Chloroform 7.22 Acetonitrile 19.54 Acetone 16.62 Ethyl acetate 14.07 THF 16.33
6.90 18.45 15.47 13.14 15.45
6.34 17.12 14.38 12.46 14.25
5.74 15.98 13.36 11.13 13.14
−1 Untreated alumina, − ΔGSP A (kJ mol ) Chloroform 7.01 Acetonitrile 21.87 Acetone 20.95 Ethyl acetate 17.72 THF 18.28
Probe
Temperature 120 °C
−1 MTMS-treated alumina, −ΔGSP A (kJ mol ) Chloroform 5.80 Acetonitrile 19.77 Acetone 16.53 Ethyl acetate 11.08 THF 13.09
Probe
Since the particle size of the alumina powder was too small to be used as a chromatographic support, alumina disks were prepared by compressing the powders in an IR die at a pressure of 108 Pa. The disks were then hand-crushed and sieved to select the fraction of particles with diameters ranging from 250 to 425 μm. Particles of the correct size were introduced into a stainless steel column, which was 50 cm long and 3.17 mm in diameter. Approximately 1 g of each sample was used to fill the chromatographic column. Each column filled with the sample was conditioned at 200 °C for 12 h to remove any impurities. The IGC measurements were performed using a Hewlett Packard 6890
Temperature 110 °C
GMS-treated alumina, Chloroform Acetonitrile Acetone Ethyl acetate THF
2.4. IGC experimental conditions
−ΔGSP A
−1
(kJ mol ) 6.85 17.90 15.51 12.06 13.90
Fig. 4. Variations of [−ΔGASP/T] as a function of [1/T] (T in K) for some polar probes adsorbed on the alumina surface-treated with γ-methacryloxy propyl trimethoxy silane.
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Table 4 The enthalpy of specific adsorption, −ΔHSP A , of polar probes on the untreated and surface-treated alumina Probe
Samples Untreated alumina
−1 −ΔHSP A (kJ mol ) Chloroform 14.77 Acetonitrile 56.85 Acetone 60.05 Ethyl acetate 53.54 THF 62.32
MTMStreated alumina
GMStreated alumina
MCMStreated alumina
AES-treated alumina
12.01 36.49 36.66 27.58 34.02
30.60 62.37 56.83 48.17 56.41
13.68 55.08 53.54 53.01 58.72
29.71 64.84 58.16 52.60 56.17
Eq. (7) becomes
DHASP DN ¼ KA þ KD ANT ANT
ð8Þ
where the [−ΔHSP A /AN⁎] values of the polar probes varied linearly with [DN/AN⁎]. Fig. 5 shows the change in [−ΔHSP A /AN⁎] as a function of [DN/AN⁎] of the polar probes on the five types of sample. The slopes of each line correspond to the KA values, and the Y-axis intercept corresponds to the KD values. Table 6 shows the KA and KD values evaluated in accordance with the above method. KA indicates the surface acidity of the samples shown in Table 6, while KD indicates the basicity. The examination of the SC values shown in Table 6 indicates that the untreated alumina is amphoteric.
GC system that was equipped with a highly sensitive flame ionization detector (FID). The carrier gas was nitrogen (N2) and the flow rate was 10 mL/min. The temperature of the IGC measurement was varied from 110 to 150 °C. Very small amounts of the probes were injected using the following procedure. Ten to 20 μL of the probe was introduced through a septum into a 1 L flask, which was filled with N2. Subsequently, approximately 0.1 mL of the diluted probe was injected into the GC system. The physical and chemical properties of the probes were obtained from the CRC Handbook of chemistry and physics. 3. Results and discussion The −ΔGASP values for the various samples were evaluated using the well-established IGC method. Figs. 1 and 2 show the change in [RTlnVN] vs. [(hνL)1/2 α0L] for each probe used to evaluate the −ΔGASP values of the untreated alumina and MTMS-treated alumina from Eq. (5), respectively. Table 2 shows the calculated [(hνL)1/2 α0L] values of the probes. The [RTlnVN] values of the n-alkanes used as the non-polar probe vary linearly with [(hνL)1/2 α0L], and the [RTlnVN] values for the polar probes of interest were above the reference line, as shown in Figs. 1 and 2. The vertical distance between the n-alkane plot and the [RTlnVN] value for the polar probe corresponds to the −ΔGASP value. The −ΔGASP values of the alumina powders were calculated at various temperatures ranging from 110 to 150 °C. Table 3 shows the calculated −ΔGASP values. The −ΔGASP values decreased with increasing temperature for all samples. In addition, the range of IGC measuring temperatures differs according to the type of sample, i.e., the range of temperatures suitable for measuring their surface characteristics differs because of their different surface characteristics. However, the effect of the measuring temperature on the KA and KD values could be ignored because it did not affect the calculation of their acid–base surface characteristics using these parameters. This means that the KA and KD parameters are not related to the range of measuring temperature. Figs. 3 and 4 show the change in [−ΔGASP/T] as a function of the reciprocal absolute temperature. −ΔHSP A is the slope of the line obtained from Eq. (6). Table 4 shows the −ΔHSP A values calculated in accordance with the above method. The KA and KD parameters were calculated using these −ΔHSP A values. Table 5 lists Gutmann's AN and DN, and Riddle–Fowkes' AN⁎ numbers of the interested polar probes. Riddle–Fowkes' AN⁎ numbers in units of kcal mol− 1 were chosen instead of Gutmann's AN numbers.
Table 5 Values of electron acceptor and donor numbers of various polar probes Probe
AN (%)
AN⁎ (kcal mol− 1)
DN (kcal mol− 1)
DN/AN⁎
Chloroform Acetonitrile Acetone Ethyl acetate THF
23.1 21.3 12.5 9.3 8.0
5.4 4.7 2.5 1.5 0.5
0 14.1 17.0 17.1 20.0
0 3.0 6.8 11.4 40
Fig. 5. Evolution of [−ΔHSP A /AN⁎] as a function of [DN/AN⁎] of some polar probes adsorbed on the untreated and surface-treated alumina.
Y.-C. Yang et al. / Powder Technology 188 (2009) 229–233
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Table 6 Values of acidic and basic constants of surface of the untreated and surface-treated alumina
treated with MTMS and MCMS are suitable for both acidic and basic polymers.
Sample
KA
KD
SC = KD/KA
Acknowledgements
Untreated alumina MTMS-treated alumina GMS-treated alumina MCMS-treated alumina AES-treated alumina
0.73 0.39 0.64 0.69 0.64
0.65 0.53 1.35 0.61 1.31
0.89 1.36 2.11 0.88 2.05
This study was supported by the General Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Knowledge Economy of Korea. References
The SC values of the alumina treated with MTMS and MCMS were 1.36 and 0.88, respectively. Therefore, their surface properties were considered to be almost amphoteric. It was already reported that the fused silica treated with the same types of silane coupling agents were also amphoteric (the SC values were 1.36 and 0.77 for MTMS and MCMS, respectively) [16]. However, the SC values of the alumina treated with GMS and AES were 2.11 and 2.05, respectively. Therefore, their surface properties were considered to be slightly basic. The surface properties of the fused silica treated with the same types of silane coupling agents were also slightly basic (the SC values are 2.40 and 3.48 for GMS and AES, respectively) [16]. This means that the alumina treated with MTMS and MCMS are suitable for both acidic and basic polymers, while the alumina treated with GMS and AES are more suitable for basic polymers. 4. Conclusions In this study, a well-established IGC method was used to evaluate the acid–base properties of untreated and surface-treated alumina with MTMS, GMS, MCMS and AES. Using n-alkanes and polar solutes as probes, the −ΔGASP values were determined from the plots of [RTlnVN] as a function of [(hνL)1/2 α0L] of the solutes. The −ΔHSP A values were determined from the temperature dependence of −ΔGASP. These values correlated with Gutmann's electron donor numbers, DN, and the new electron acceptor numbers, AN⁎, which was introduced by Riddle– Fowkes in units of kcal mol− 1, i.e., the same units as the donor numbers. The acid–base properties of the above samples were characterized using two parameters describing their acidity (KA) and basicity (KD) in consistent units. The results suggested that the surface properties of the untreated alumina was amphoteric. The alumina treated with GMS and AES were slightly basic, while the alumina treated with MTMS and MCMS were amphoteric. Overall, the alumina treated with GMS and AES are more suitable for basic polymers, and the alumina
[1] A.T. James, A.J.P. Martin, Gas–liquid partition chromatography, Biochem. J. 50 (5) (1952) 679. [2] J.B. Donnet, S.J. Park, H. Balard, Evaluation of specific interactions of solid surfaces by inverse gas chromatography. A new approach based on polarizability of the probes, Chromatographia 31 (9/10) (1991) 434–440. [3] M. Nardin, H. Balard, E. Papirer, Surface characteristics of commercial carbon fibers determined by inverse gas chromatography, Carbon 28 (1) (1990) 43–48. [4] V. Gutmann, The Donor–Acceptor Approach to Molecular Interactions, Plenum Press, New York, 1978. [5] F.L. Riddle, F.M. Fowkes, Spectral shifts in acid–base chemistry. Van der walls contributions to acceptor numbers, J. Am. Chem. Soc. 112 (9) (1990) 3259. [6] H. Balard, M. Sidqi, E. Papirer, J.B. Donnet, A. Tuel, H. Hommel, A.P. Legrand, Study of modified silicas by inverse gas chromatography. Part III: influence of chain length on surface properties of silica grafted with α–ω diols, Chromatographia 25 (8) (1988) 712–716. [7] E. Papirer, H. Balard, Y. Rahmani, A.P. Legrand, L. Facchini, H. Hommel, Characterization by inverse gas chromatography of the surface properties of silicas modified by poly(ethylene glycols) and their models (oligomers, diols), Chromatographia 23 (9) (1987) 639–647. [8] E. Papirer, J. Perrin, B. Siffert, G. Philipponneau, Surfaces characteristics of aluminas in relation with polymer adsorption, J. Colloid Interface Sci. 144 (1) (1991) 263–279. [9] M. Sidqi, H. Balard, E. Papirer, Study of modified silicas by inverse gas chromatography. Influence of chain length of the conformation of n-alcohols grafted on a pyrogenic silica, Chromatographia 27 (7/8) (1989) 311–315. [10] M.M. Chehimi, E. Pigois-Landureau, Determination of acid–base properties of solid materials by inverse gas chromatography at infinite dilution, J. Mater. Chem. 4 (5) (1994) 741–745. [11] U. Panzer, H.P. Schreiber, On the evaluation of surface interactions by inverse gas chromatography, Macromolecules 25 (1992) 3633–3637. [12] P. Mukhopadhyay, H.P. Schreiber, Aspects of acid–base interactions and use of inverse gas chromatography, Colloids Surf., A Physicochem. Eng. Asp. 100 (1995) 47–71. [13] A. Voelkel, E. Andrzejewska, R. Maga, M. Andrzejewski, Characterization of dispersive and acid–base properties of crosslinked polymers by inverse gas chromatography, Polymer 34 (14) (1993) 3109–3111. [14] A. Voelkel, A. Krysztafkiewicz, Acid–base properties of silicas modified by organic compounds as determined by inverse gas chromatography, Powder Technol. 95 (2) (1998) 103–108. [15] Y.C. Yang, P.R. Yoon, Examination of the surface properties of kaolinites by inverse gas chromatography: dispersive properties, Korean J. Chem. Eng. 24 (1) (2007) 165. [16] Y.C. Yang, P.R. Yoon, Determination of acid–base properties of silicas by inverse gas chromatography: variation with surface treatment, Mater. Trans. 48 (2007) 1955–1960.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
Examination of acid–base properties of alumina treated with silane coupling agents, by using inverse gas chromatography Young-Cheol Yang a, Byoung-Gyu Kim a, Soo-Bok Jeong a,⁎, Pyoung-Ran Yoon b a b
Minerals and Materials Processing Division, Korea Institute of Geoscience & Mineral Resources, Deajeon 305-350, South Korea Department of Mineral Resources & Energy Engineering, Chonbuk National University, Jeonju 561-756, South Korea
A R T I C L E
I N F O
Article history: Received 11 February 2008 Received in revised form 21 April 2008 Accepted 25 April 2008 Available online 3 May 2008 Keywords: Inverse gas chromatography (IGC) Specific component of the free energy of adsorption (ΔGASP) Enthalpy of specific adsorption (ΔHSP A ) Surface acid–base property
A B S T R A C T This study examined the specific component of the free energy of adsorption, −ΔGASP, of the untreated and four types of silane coupling agent-treated alumina powders using inverse gas chromatography (IGC) by employing the adsorption of several polar and non-polar probes onto their surfaces at various temperatures. The acid–base properties of the untreated and surface-treated alumina powders were quantified using their KA and KD parameters, which reflect the ability of a surface to act as an electron acceptor and donor, respectively. The surface of the untreated alumina was found to be amphoteric and was able to function as both an electron acceptor and donor. The acid–base properties of the alumina surfaces treated with γ-glycidoxy propyl trimethoxy silane (GMS) and γ-amino propyl triethoxy silane (AES) were slightly basic, and those of the alumina surfaces treated with γ-methacryloxy propyl trimethoxy silane (MTMS) and γ-mercapto propyl trimethoxy silane (MCMS) were amphoteric. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In the first paper of this series, we described the surface properties of the untreated and silane coupling agent-treated alumina in terms of their dispersive force parameters, as determined by inverse gas chromatography (IGC). However, it is clear that the full description of the surface properties can only be achieved with the use of the acid–base interaction parameters. Acid–base interactions are important components of polar forces, and play a significant role in the adhesion of inorganic fillers to organic polymers. This study examined the acid–base properties of the same samples to complete the previously defined characterization of their surface properties. The following parameters were calculated in order to compare the acid–base properties of the untreated and silane coupling agenttreated alumina powders: the specific component of the free energy of adsorption (ΔGASP), the enthalpy of specific adsorption (ΔHSP A ), the acidic constant of a solid (KA), the basic constant of a solid (KD) and SC (KD/KA). 2. Experimental 2.1. Theory of inverse gas chromatography In IGC under infinite dilution conditions, the retention volume VN can be calculated using the following Eq. (1): VN ¼ ðtR t0 ÞjDC
⁎ Corresponding author. Tel.: +82 42 868 3576; fax: +82 42 861 9720. E-mail address: [email protected] (S.-B. Jeong). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.04.080
ð1Þ
where tR is the retention time of the probes, t0 is the zero retention time measured with a nonadsorbing probe such as methane, j is the compressibility factor depending on the pressure at the column inlet and outlet, and DC is the corrected flow rate. In practice, the retention time and retention volume, VN can be determined in a current chromatographic experiment: a larger VN will correspond to a higher affinity of a probe to the chromatographic support. In Eq. (1), j was calculated using the following Eq. (2) [1]: " # ðpi =p0 Þ2 1 j ¼ 1:5 ðpi =p0 Þ3 1
ð2Þ
where pi is the inlet pressure of the carrier gas, and p0 is the outlet pressure of the carrier gas, which is usually equal to atmospheric pressure. For a test substance, the free energy of adsorption, ΔGA, is the sum of the energies of adsorption due to the dispersive and specific interactions. The adsorption of non-polar probes such as n-alkanes, occur through dispersive interactions, whereas for polar probes, both London and acid–base interactions contribute to ΔGA. The model reported by Donnet et al., was used in this study because the injected probe is in the gaseous state [2]. In this model, ΔGA is given by the following equations: ½DGA ¼ DGDA þ DGSP A ¼ ½RTlnVN þ C h i ¼ K ðhrS Þ1=2 a0S ðhrL Þ1=2 a0L þ DGSP A
ð3Þ ð4Þ ð5Þ
230
Y.-C. Yang et al. / Powder Technology 188 (2009) 229–233
where ΔGAD and ΔGASP are the dispersive and specific components of the free energy of adsorption, respectively. In Eq. (4), R is the ideal gas constant, T is the absolute temperature and the value of the constant, C depends on an arbitrarily chosen reference state of the adsorbed molecule. In Eq. (5), K is a constant, hνS and hνL are the ionization potentials of the interacting materials, and α0 is the deformation polarizability of the molecules. Subscripts S and L refer to the solid and liquid phases, respectively. In the case of n-alkanes, ΔGA is equal to the free energy of adsorption corresponding to the dispersive interactions only, ΔGAD, i.e., [−ΔGASP] = 0 in Eq. (5). The term [K (hνS)1/2 α0S] is a characteristic of a given solid surface and is related to ΔGAD. Consequently, [RT lnVN + C] between an adsorbate and an adsorbent is a linear equation of the parameter [(hνL)1/2 α0L] with a slope of [K (hνS)1/2 α0S]. Polar testing probes [Lewis acids and bases, e.g. chloroform (CHCl3) and tetrahydrofuran (THF)] have their corresponding [−ΔGA] values above the reference line. The [−ΔGASP] value is determined by the vertical distance between the n-alkane plot and the data for the polar probe of interest. An examination of the temperature dependence of ΔGASP enables the enthalpy of specific adsorption, ΔHSP A , to be determined [3]: DHASP ¼
A DGSP A =T : Að1=T Þ
ð6Þ
The enthalpy of specific adsorption between the examined surface and the test solutes may be related to the acid–base properties of both species by using either Drago's equation or the following equation [3]: DHASP ¼ KD AN þ KA DN
ð7Þ
where AN and DN are the electron acceptor and donor numbers of the test solute, respectively, and denote the Gutmann numbers, AN, which indicates the ability of a surface to attract electrons (acidity), and DN, which quantifies its ability to release electrons (basicity), respectively [4]. The parameters KA and KD reflect the ability of a surface to act as an electron acceptor and donor, respectively. The ratio, KD/KA(SC), describes the characteristics of a surface (acidic or basic). KD and KA, determined according to the method using Gutmann's AN values in Eq. (7), are expressed in different units because Gutmann's AN and DN have in different units. Therefore, in order to obtain both sides of Eq. (7) in the same units, KD must be in kJ mol− 1 and KA needs to be dimensionless. Therefore, caution must be taken when making conclusions from their ratio because the KD/KA value is unclear. Further discussion will be based only on the KD, KA and SC values determined using the Riddle–Fowkes AN⁎ values [5] in Eq. (7). The procedure described above has been used to examine surface properties of silica, modified silica, oxides, various minerals and solid polymers [6–15].
Fig. 1. Variation of [RTlnVN] as a function of [(hνL)1/2 α0L] for different probes adsorbed on the untreated alumina, measured at 120 °C.
2.2. Materials The α-alumina (α-Al2O3) was obtained in powder form from the Bayer process. The particle size ranged from 0.73 to 180 μm. Four types of silane coupling agents were used in the adsorption experiments. Table 1 lists the chemical names and adsorption experimental conditions of the silane coupling agents. Methanol and distilled water were used as the solvent, and acetic acid was used as the catalyst for hydrolyzing the silane coupling agents. A homologous series of n-alkanes, heptane (C7H16), octane (C8H18), and nonane (C9H20), were was used as the non-polar probes, and THF (C4H8O), chloroform (CHCl3), acetonitrile (CH3CN), ethyl acetate (CH3 COOC2H5) and acetone (CH3COCH3) were used as the polar probes for the IGC experiment.
Table 1 Chemical names and experimental conditions for adsorption of silane coupling agents Denomination Chemical name in this paper MTMS
γ-methacryloxy propyl trimethoxy silane
GMS
γ-glycidoxy propyl trimethoxy silane γ-mercapto propyl trimethoxy silane
MCMS
AES
γ-amino propyl triethoxy silane
Solution
pH
Remarks
Alumina weight
Methanol (80 mL) Distilled water (20 mL) Distilled water Methanol (80 mL) Distilled water (20 mL) Distilled water
3.53–3.78
Acetic acid catalyzed
30 g
5.66– 6.88
Natural
4.64– 4.83
Acetic acid catalyzed
9.95–10.66 Natural
Fig. 2. Variation of [RTlnVN] as a function of [(hνL)1/2 α0L] for different probes adsorbed on the alumina surface-treated with γ-methacryloxy propyl trimethoxy silane, measured at 120 °C.
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Table 2 Characteristics of probes Probe
α0 1040
hν 1019
(hν)1/2 α0 1049
n-Heptane n-Octane n-Nonane Chloroform Acetonitrile Acetone Ethyl acetate THF
[Cm2 V− 1]
[CV]
[C3/2 m2 V− 1/2]
15.24 17.59 19.75 10.57 5.43 7.12 10.79 8.77
4.57 4.20 4.00 5.45 7.58 6.63 5.36 5.98
10.3 11.4 12.5 7.8 4.7 5.8 7.9 6.8
2.3. Adsorption experiment The experiment designed to modify the alumina surface was performed as follows. The solvent (100 mL), silane coupling agents (1.0 g) and alumina powder (30 g) were stirred with a magnetic bar stirrer for 1 h and then separated into solid and liquid components using a centrifugal separator. The pH is the most important factor in the hydrolysis of silane coupling agents. Therefore, the appropriate amount of acetic acid was added before stirring in accordance with the type of silane coupling agents in order to obtain a reasonable pH. Table 1 lists the conditions used in the adsorption experiment. The separated alumina was dried for 24 h at room temperature and then for 8 h at 105 °C. Various samples, i.e. the untreated alumina, and the
Fig. 3. Variations of [−ΔGASP/T] as a function of [1/T] (T in K) for some polar probes adsorbed on the untreated alumina.
MTMS, GMS, MCMS and AES-treated alumina, were prepared in this way for the IGC study at infinite dilution. Table 3 The specific component of the free energy of adsorption, −ΔGASP, of polar probes on the untreated and surface-treated alumina Probe
Temperature 110 °C
120 °C
130 °C
140 °C
6.84 20.84 20.00 16.78 17.11
6.60 20.04 18.97 15.84 16.24
6.33 19.08 17.88 14.95 14.82
130 °C
140 °C
150 °C
5.67 19.00 16.01 10.65 12.85
5.48 18.18 15.32 10.20 12.25
5.47 18.13 14.99 10.01 11.80
120 °C
130 °C
140 °C
6.59 16.92 14.62 11.07 12.86
6.02 15.76 13.48 10.23 12.04
5.37 14.41 12.41 9.29 10.65
−1 MCMS-treated alumina, −ΔGSP A (kJ mol ) Chloroform 5.47 Acetonitrile 21.24 Acetone 19.37 Ethyl acetate 15.78 THF 17.55
5.25 20.19 18.41 14.89 16.72
4.78 19.46 17.58 13.83 15.40
4.65 18.58 16.97 13.01 14.66
−1 AES-treated alumina, −ΔGSP A (kJ mol ) Chloroform 7.22 Acetonitrile 19.54 Acetone 16.62 Ethyl acetate 14.07 THF 16.33
6.90 18.45 15.47 13.14 15.45
6.34 17.12 14.38 12.46 14.25
5.74 15.98 13.36 11.13 13.14
−1 Untreated alumina, − ΔGSP A (kJ mol ) Chloroform 7.01 Acetonitrile 21.87 Acetone 20.95 Ethyl acetate 17.72 THF 18.28
Probe
Temperature 120 °C
−1 MTMS-treated alumina, −ΔGSP A (kJ mol ) Chloroform 5.80 Acetonitrile 19.77 Acetone 16.53 Ethyl acetate 11.08 THF 13.09
Probe
Since the particle size of the alumina powder was too small to be used as a chromatographic support, alumina disks were prepared by compressing the powders in an IR die at a pressure of 108 Pa. The disks were then hand-crushed and sieved to select the fraction of particles with diameters ranging from 250 to 425 μm. Particles of the correct size were introduced into a stainless steel column, which was 50 cm long and 3.17 mm in diameter. Approximately 1 g of each sample was used to fill the chromatographic column. Each column filled with the sample was conditioned at 200 °C for 12 h to remove any impurities. The IGC measurements were performed using a Hewlett Packard 6890
Temperature 110 °C
GMS-treated alumina, Chloroform Acetonitrile Acetone Ethyl acetate THF
2.4. IGC experimental conditions
−ΔGSP A
−1
(kJ mol ) 6.85 17.90 15.51 12.06 13.90
Fig. 4. Variations of [−ΔGASP/T] as a function of [1/T] (T in K) for some polar probes adsorbed on the alumina surface-treated with γ-methacryloxy propyl trimethoxy silane.
232
Y.-C. Yang et al. / Powder Technology 188 (2009) 229–233
Table 4 The enthalpy of specific adsorption, −ΔHSP A , of polar probes on the untreated and surface-treated alumina Probe
Samples Untreated alumina
−1 −ΔHSP A (kJ mol ) Chloroform 14.77 Acetonitrile 56.85 Acetone 60.05 Ethyl acetate 53.54 THF 62.32
MTMStreated alumina
GMStreated alumina
MCMStreated alumina
AES-treated alumina
12.01 36.49 36.66 27.58 34.02
30.60 62.37 56.83 48.17 56.41
13.68 55.08 53.54 53.01 58.72
29.71 64.84 58.16 52.60 56.17
Eq. (7) becomes
DHASP DN ¼ KA þ KD ANT ANT
ð8Þ
where the [−ΔHSP A /AN⁎] values of the polar probes varied linearly with [DN/AN⁎]. Fig. 5 shows the change in [−ΔHSP A /AN⁎] as a function of [DN/AN⁎] of the polar probes on the five types of sample. The slopes of each line correspond to the KA values, and the Y-axis intercept corresponds to the KD values. Table 6 shows the KA and KD values evaluated in accordance with the above method. KA indicates the surface acidity of the samples shown in Table 6, while KD indicates the basicity. The examination of the SC values shown in Table 6 indicates that the untreated alumina is amphoteric.
GC system that was equipped with a highly sensitive flame ionization detector (FID). The carrier gas was nitrogen (N2) and the flow rate was 10 mL/min. The temperature of the IGC measurement was varied from 110 to 150 °C. Very small amounts of the probes were injected using the following procedure. Ten to 20 μL of the probe was introduced through a septum into a 1 L flask, which was filled with N2. Subsequently, approximately 0.1 mL of the diluted probe was injected into the GC system. The physical and chemical properties of the probes were obtained from the CRC Handbook of chemistry and physics. 3. Results and discussion The −ΔGASP values for the various samples were evaluated using the well-established IGC method. Figs. 1 and 2 show the change in [RTlnVN] vs. [(hνL)1/2 α0L] for each probe used to evaluate the −ΔGASP values of the untreated alumina and MTMS-treated alumina from Eq. (5), respectively. Table 2 shows the calculated [(hνL)1/2 α0L] values of the probes. The [RTlnVN] values of the n-alkanes used as the non-polar probe vary linearly with [(hνL)1/2 α0L], and the [RTlnVN] values for the polar probes of interest were above the reference line, as shown in Figs. 1 and 2. The vertical distance between the n-alkane plot and the [RTlnVN] value for the polar probe corresponds to the −ΔGASP value. The −ΔGASP values of the alumina powders were calculated at various temperatures ranging from 110 to 150 °C. Table 3 shows the calculated −ΔGASP values. The −ΔGASP values decreased with increasing temperature for all samples. In addition, the range of IGC measuring temperatures differs according to the type of sample, i.e., the range of temperatures suitable for measuring their surface characteristics differs because of their different surface characteristics. However, the effect of the measuring temperature on the KA and KD values could be ignored because it did not affect the calculation of their acid–base surface characteristics using these parameters. This means that the KA and KD parameters are not related to the range of measuring temperature. Figs. 3 and 4 show the change in [−ΔGASP/T] as a function of the reciprocal absolute temperature. −ΔHSP A is the slope of the line obtained from Eq. (6). Table 4 shows the −ΔHSP A values calculated in accordance with the above method. The KA and KD parameters were calculated using these −ΔHSP A values. Table 5 lists Gutmann's AN and DN, and Riddle–Fowkes' AN⁎ numbers of the interested polar probes. Riddle–Fowkes' AN⁎ numbers in units of kcal mol− 1 were chosen instead of Gutmann's AN numbers.
Table 5 Values of electron acceptor and donor numbers of various polar probes Probe
AN (%)
AN⁎ (kcal mol− 1)
DN (kcal mol− 1)
DN/AN⁎
Chloroform Acetonitrile Acetone Ethyl acetate THF
23.1 21.3 12.5 9.3 8.0
5.4 4.7 2.5 1.5 0.5
0 14.1 17.0 17.1 20.0
0 3.0 6.8 11.4 40
Fig. 5. Evolution of [−ΔHSP A /AN⁎] as a function of [DN/AN⁎] of some polar probes adsorbed on the untreated and surface-treated alumina.
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Table 6 Values of acidic and basic constants of surface of the untreated and surface-treated alumina
treated with MTMS and MCMS are suitable for both acidic and basic polymers.
Sample
KA
KD
SC = KD/KA
Acknowledgements
Untreated alumina MTMS-treated alumina GMS-treated alumina MCMS-treated alumina AES-treated alumina
0.73 0.39 0.64 0.69 0.64
0.65 0.53 1.35 0.61 1.31
0.89 1.36 2.11 0.88 2.05
This study was supported by the General Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Knowledge Economy of Korea. References
The SC values of the alumina treated with MTMS and MCMS were 1.36 and 0.88, respectively. Therefore, their surface properties were considered to be almost amphoteric. It was already reported that the fused silica treated with the same types of silane coupling agents were also amphoteric (the SC values were 1.36 and 0.77 for MTMS and MCMS, respectively) [16]. However, the SC values of the alumina treated with GMS and AES were 2.11 and 2.05, respectively. Therefore, their surface properties were considered to be slightly basic. The surface properties of the fused silica treated with the same types of silane coupling agents were also slightly basic (the SC values are 2.40 and 3.48 for GMS and AES, respectively) [16]. This means that the alumina treated with MTMS and MCMS are suitable for both acidic and basic polymers, while the alumina treated with GMS and AES are more suitable for basic polymers. 4. Conclusions In this study, a well-established IGC method was used to evaluate the acid–base properties of untreated and surface-treated alumina with MTMS, GMS, MCMS and AES. Using n-alkanes and polar solutes as probes, the −ΔGASP values were determined from the plots of [RTlnVN] as a function of [(hνL)1/2 α0L] of the solutes. The −ΔHSP A values were determined from the temperature dependence of −ΔGASP. These values correlated with Gutmann's electron donor numbers, DN, and the new electron acceptor numbers, AN⁎, which was introduced by Riddle– Fowkes in units of kcal mol− 1, i.e., the same units as the donor numbers. The acid–base properties of the above samples were characterized using two parameters describing their acidity (KA) and basicity (KD) in consistent units. The results suggested that the surface properties of the untreated alumina was amphoteric. The alumina treated with GMS and AES were slightly basic, while the alumina treated with MTMS and MCMS were amphoteric. Overall, the alumina treated with GMS and AES are more suitable for basic polymers, and the alumina
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