- Email: [email protected]
Novel titration method for surface-functionalised silica
Applied Surface Science 257 (2011) 2576–2580
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Novel titration method for surface-functionalised silica Kai Hofen a , Siegfried Weber a , Chiu Ping Candace Chan b , Peter Majewski b,∗ a b
Department of Biotechnology, University of Applied Sciences, Mannheim, Germany School of Advanced Manufacturing and Mechanical Engineering, Mawson Institute, University of South Australia, Mawson Lakes Blvd, Mawson Lakes 5095, Australia
a r t i c l e
i n f o
Article history: Received 7 June 2010 Received in revised form 28 September 2010 Accepted 7 October 2010 Available online 17 October 2010 Keywords: Functionalised silica Titration
a b s t r a c t This paper describes three inexpensive and fast analytical methods to characterise grafted particle surfaces. The reaction of silica with (3-aminopropyl)triethoxysilane, (3-mercaptopropyl)trimethoxysilane and N-(phosphonomethyl)iminodiacetic acid hydrate, respectively, leads to NH2 -, SO3 H- or COOHfunctionalised silica, which were characterised by X-ray photoelectron spectrometry and titration in nonaqueous media as well as with two titration methods in a water-based environment. In the work presented, factors influencing the titrations are pointed out and solutions are presented to overcome these limiting factors are shown. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Although the sol–gel chemistry of silanes is well investigated in various publications, e.g. [1–3], there is still room for their applications in industrial processes. Examples for successfully introduced processes based on silanes are the production of tires with a low rolling resistance by crosslinking rubber and silica particles via silanes containing mercapto groups [4], fluorosilanes for self-cleaning surfaces, e.g. [5], and Scratch Resistant and Transparent UV-Protective Coatings using 3-glycidoxypropyl-trimethoxysilane, e.g. [6–8]. A promising technology for a contemporary large-scale production is the treatment of water with functionalised silica particles [9,10]. The application of sol–gel processes in industrial processes leads to a demand for cheap, fast and easy-to-use analytical methods. In numerous publications within the last decades methods like ToFSIMS, XPS, AFM or solid-state NMR are used. There is no doubt that these methods give excellent results but they are very expensive and not usable at production sites. Furthermore it is difficult to make quantitative statements about the surface modification of particles which is also important in research. The present work adopts a potentiometric titration method in a nonaqueous solvent of the oil industry [11] to characterise silica particle surfaces carrying ionic groups. It comprises the difficulties, like diffusion effects, that occur during titration of analytes which are bound to a surface. Subsequently the titration is adopted to
∗ Corresponding author. E-mail address: [email protected] (P. Majewski). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.10.025
work in aqueous media to reduce costs and time as well as making it more environmentally friendly. 2. Experimental procedures 2.1. Materials All organic solvents, acids, bases, sodium chloride and hydrogen peroxide were p.a. quality and purchased from Merck Chemicals. Amorphous silica powder (Ultrasil 7000) and (3aminopropyl)triethoxysilane (AMPTS) were industry-grade and kindly allocated by Evonik Industries, Germany. As provided by the manufacturer, Ultrasil 7000 has a concentration of 1.1 OH groups/nm2 and a BET surface area of 165 m2 /g. This results in a quantity of 0.3014 mmol OH/g. Assuming that each silane molecule binds to one surface hydroxy group without crosslinking of the silane molecules themselves the utilised amount of silane means an allocation of 100% plus a 10% excess. (3-Mercaptopropyl)trimethoxysilane (MPTMS, purity 97%), N(phosphonomethyl)-iminodiacetic acid hydrate (PMIDA, purity 99%), and Oxone were purchased from Sigma–Aldrich. All these chemicals were used as obtained without further purification. The applied water was produced with a Milli-Q water purification device from Millipore. 2.1.1. Synthesis of functionalised silica particles NH2 -functionalised silica particles were prepared by suspending 30 g of the silica particles in 200 ml of toluene. Subsequently, a solution of 10 mmol of silane in 100 ml toluene was dropped to the suspension under constant stirring within 15 min and the resulting suspension was stirred for another 105 min. Finally, the suspen-
K. Hofen et al. / Applied Surface Science 257 (2011) 2576–2580
sion was filtered and the particles were washed three times with 100 ml toluene and soaked dry. Eventually, the particles were dried at 60 ◦ C in a vacuum oven for 4 h. With this procedure the theoretical standardised amount of amino groups on the silica surface is 0.333 mmol/g. SO3 H-functionalised silica particles were prepared by suspending 5 g of the silica particles in 200 ml of toluene. Subsequently, 1.02, 2.04 and 3.06 mmol of MPTMS per gram silica, respectively, was dropped to the suspension under constant stirring within 15 min and the suspension was stirred for another 105 min. The suspension was filtered and the particles were washed three times with 100 ml toluene and soaked dry. Subsequently, the powder was stirred in 200 ml Oxone (KHSO5 , KHSO4 , K2 SO4 ) at 60 ◦ C for 24 h and eventually filtered. The particles were then dried at 60 ◦ C in a vacuum oven for 4 h. With this procedure the theoretical standardised amount of sulfonate groups on the silica surface is 1.02, 2.04 and 3.06 mmol/g, respectively. COOH-functionalised silica particles were prepared by stirring 5 g of the silica particles in 250 ml of Milli-Q water containing 0.26 and 1.5 mmol of PMIDA per gram silica, respectively. The suspension was stirred for 20 h and subsequently filtered. Eventually, the particles were dried at 60 ◦ C in a vacuum oven for 4 h. With this procedure the theoretical standardised amount of carboxy groups on the silica surface is 0.52 and 3 mmol/g, respectively, considering that PMIDA contains two carboxy groups per molecule.
2577
2.1.4. XPS The XPS instrument used to conduct the experiment was a Kratos Axis Ultra (Kratos Analytical Ltd., Wharfside, UK), which used a Mg K␣ source (1253 eV) operated at 300 W. Samples were mounted on a stainless steel stub for analysis. A fixed pass energy of 93.9 eV for wide (or survey) scans and of 29.35 eV for narrow (or multiplex) scans was used. All XPS measurements were acquired at a take-off angle of 45◦ to the surface and were referenced to the C 1s peak at 284.7 eV. The XPS spectra were curve-fitted using the CASA programme version 2.1.35. 3. Results and discussion 3.1. XPS analyses The XPS analyses of the coated materials clearly indicate the presence of the functional group on the surface of the silica particles. Fig. 1 shows the XPS spectrum of NH2 -functionalised silica exhibiting besides the Si 2p peak the N 1s and C 1s peaks which can be attributed to amino groups on the surface. In Fig. 2, the XPS spectrum of SO3 H-functionalised silica shows the S 2s and S 2p peaks as well as the C 1s peak besides the Si2p peak, which can be attributed to sulfonic groups on the surface. Fig. 3 presents the XPS spectrum of COOH-functionalised silica, clearly exhibiting the Si 2p, N 1s, C 1s and a weak P 2s peak, which are believed to belong to carboxy groups on the surface.
2.1.2. Preparation procedure for the nonaqueous titration The titration in nonaqueous media was performed in a mixture of 55 vol% toluene and 45 vol% 2-propanol. The required amount of grafted silica particles was transferred together with 80 ml solvent into a beaker. The vessel was covered and the particles were allowed to swell without stirring for 15–60 min. The titration was carried out with 0.1 mol/l perchloric acid.
18
x 10
survey [R1,1]
4
Si2p C1s 16
2.1.3. Preparation procedure for the aqueous titration 2.1.3.1. NH2 -functionalised silica particles. Purified water was used as solvent for the direct titration as well as for the back titration. 2 g of the grafted particles, 10 g of sodium chloride and 80 ml of water were used.
The titrations were performed with a Metrohm 716 DMS Titrino using a Metrohm pH glass electrode in the aqueous environment and a Metrohm Solvotrode in the nonaqueous media. It is essential to stir the sample until no agglomerates and gel clouds are visible before the titration is started. 2.1.3.2. SO3 H and COOH functionalised silica particles. Back titration: 1 g of the silica particles and 1 g of sodium chloride were weighed into a clean beaker. Subsequently, 10 ml of a 0.1 mol/l solution of sodium hydroxide and 70 ml of Milli-Q water were added, and the suspension was stirred for 18 h. Three drops of phenolphthalein were added to the beaker and the solution titrated with a 0.1 mol/l solution of hydrochloric acid. The end point was reached when the colour changed from pink to colourless. At least three measurements were conducted per powder batch.
12
N1s
CPS
(a) Direct titration: The mixture was stirred for 24 h on a magnetic stirrer. The amount of released hydroxy groups was determined by titration with 0.1 mol/l solution of hydrochloric acid. (b) Back titration: An exact amount of 10 ml of 0.1 mol/l solution of hydrochloric acid was added to the suspension. The mixture was stirred for 24 h on a magnetic stirrer to neutralise the hydroxy groups. Afterwards the excess of acid was determined by titration with 0.1 mol/l solution of sodium hydroxid.
14
10
8
6
4
400
350
300
250
Binding Energy (eV) Fig. 1. XPS spectrum of NH2 -functionalised silica.
200
150
2578
K. Hofen et al. / Applied Surface Science 257 (2011) 2576–2580
Si-SAM 30/4
x 103
survey [R1,1]
x 104
Si2p 80
Si2p
22
20
70
18
60
16
50
CPS
CPS
C1s 14
12
S2p 30
C1s
P2s N1s
40
S2s 10
20 8
10
6
4
300
280
260
240
220
200
180
160
Binding Energy (eV)
400
350
300
250
200
150
Binding Energy (eV) Fig. 2. XPS spectrum of SO3 H-functionalised silica. Fig. 3. XPS spectrum of COOH-functionalised silica.
3.2. Titration results for NH2 -functionalised silica (Table 1) 3.2.1. Titration in the nonaqueous media To maximise the protonation of the amino groups in the nonaqueous media perchloric acid was chosen as the acid. Since organic solvents are a problematic medium for titrations a monotone equilibrium titration was carried out, which consists of the following automated steps: 1. a defined constant volume of the standard solution is injected; 2. after a defined constant time interval to allow the potential to stabilise the potential is measured and determined whether the equilibrium is reached.
Table 1 Concentrations [mmol/g] of amino groups obtained with different titration methods using silica with a theoretical concentration of 0.333 mmol amino groups per g silica. Nonaqueous
Direct, aqueous
Back titration
1 2 3 4 5 6
0.2885 0.2912 0.2928 0.2932 0.2884 0.2893
0.2605 0.2604 0.2597 0.2603 0.2607 –
0.2742 0.2749 0.2736 0.2773 0.2774 0.2767
Average Standard deviation
0.2906 2.12 × 10−3
0.2603 0.35 × 10−3
0.2757 1.66 × 10−3
These steps are repeated until the titration is completed. There were three parameters expected to influence the titration process: 1. The sample weight: It should be large enough to provide a good precision. On the other hand the amount of modified silica for the analysis should be as low as possible. 2. The volume aliquot: It should be as small as possible to obtain a good precision. 3. Diffusion effects in the pores of the silica [12] or in the surface structure [13]. The result of each titration was the amount of amino groups in mmol/g grafted silica. The optimum sample weight was determined with a series of measurements. Fig. 4 shows the concentration of amino groups on the surface obtained as a function of the sample weight. The correlation indicates that the titration approaches asymptotically a limiting value of the surface group concentration and that a minimum sample weight of 2 g of grafted silica is needed for obtaining a reliable result. Thus this amount was chosen for further experiments. The volume aliquot was altered beginning with a value of 0.1 ml. The data obtained with this aliquot had a very low signal-tobackground ratio, which led to random results. With a volume aliquot of 0.2 ml the results were reproducible and the following experiments were carried out with this value.
0.35
0.35
0.30
0.30
amino groups (standardized) [mmol/g]
amino groups (standardized) [mmol/g]
K. Hofen et al. / Applied Surface Science 257 (2011) 2576–2580
0.25 0.20
interval 45s
0.15
interval 90s 0.10 0.05 0.00
0
2
4
0.25 0.20 0.15 0.10 0.05 0.00
6
2579
sample weight [g]
0
50
100
150
me intervall [s]
Fig. 4. Dependence of the titration result on the sample weight.
To study possible diffusion effects the time interval before the potential is determined was varied. The correlation between amino group concentration and stirring time (Fig. 5) also shows an asymptotical approach towards a limiting value of the surface group concentration, which is comparable to that in Fig. 4. Diffusion effects are most probably also responsible for the influence of the sample weight on the titration result. Since the titration takes longer with a higher sample mass diffusion affects the result less. For a reliable result a stirring time of at least 90 s was required. 3.2.2. Direct titration in aqueous media An excess of sodium chloride was added to the particle suspension to provide a large excess of chloride ions that replace quantitatively the hydroxide ions, the counter ions of the ammonium groups, at the particle surface (Scheme 1, bottom). Diffusion effects were eliminated by a long stirring time. According to Table 2 10% less amino groups were found than in organic solvents. This can be explained by deprotonation of the amino groups through the released hydroxide ions.
Fig. 5. Dependence of the titration result on the time interval.
3.2.3. Back titration in aqueous media To overcome this problem an alternative titration procedure was tested: an excess of hydrochloric acid was added during the preparation of the sample suspension. This ensures an acidic pH during the whole preparation and titration process. Therefore the protonation equilibrium is shifted towards the ammonium groups. The excess of sodium chloride shifts the reaction equilibrium in Scheme 1, top towards the salt. In contrast to the direct titration a blank value with unmodified silica of 0.0422 mmol/g was found. It is most possibly caused by impurities of the silica. These effects and the elimination of the diffusion influence result in a 5% lower value compared to that of the nonaqueous titration. The precision of the back titration is with a standard deviation of 1.66 × 10−3 mmol/g still good. As this method was performed as a dynamic titration the titration time was reduced by a factor of 20 compared to the titration in nonaqueous media and by factor of 10 compared to the direct titration.
Table 2 Concentrations of surface groups obtained through back titration of SO3 H- and COOH-functionalised silica with different theoretical concentrations of the surface groups (all concentrations in mmol/g). Sulfonate, 1.02
Sulfonate, 2.04
Sulfonate, 3.06
Carboxy, 0.52
Carboxy, 3.0
1 2 3
0.81 0.80 0.79
0.79 0.80 0.82
0.84 0.83 0.82
0.59 0.58 0.58
0.87 0.82 0.81
Average Standard deviation
0.81 14 × 10−3
0.81 17 × 10−3
0.83 10 × 10−3
0.58 6 × 10−3
0.83 34 × 10−3
Scheme 1. Protonation/deprotonation equilibrium of NH2 -functionalised silica particles in water (top) and hydroxide/chloride exchange reaction between protonated NH2 -functionalised silica particles and sodium chloride (bottom).
2580
K. Hofen et al. / Applied Surface Science 257 (2011) 2576–2580
3.3. Titration results for SO3 H- and COOH-functionalised silica The results of the titration experiments for the SO3 H- and COOH-functionalised silica are listed in Table 2. For the SO3 Hfunctionalised silica, an increase of the concentration of MPTMS in the synthesis does not result in the expected increase of the measured functional group concentration. This phenomenon might be explained by the fact that there is a maximum value for the concentration of sulfonic groups that can be reached on the surface of the particles and that the concentration of MPTMS used in the synthesis (0.8 mmol/g) is higher than this limit. For the COOH-functionalised silica, an increase of the functional group concentration with increasing PMIDA content in the synthesis is evident. It should be noted that the concentration of PMIDA in the synthesis for these titrations was only 0.26 and 1.5 mmol/g, respectively, as PMIDA has two carboxy groups per molecule. The fact that for a PMIDA concentration of 1.5 mmol/g significantly lower COOH concentrations than the theoretical value are obtained is believed to be also due to the fact that the concentration of functional groups on the surface of the silica is limited, in this case to about 0.8 mmol/g. The reason for the limitation of the concentration of SO3 H and COOH groups on the surface of the silica particles could not be studied in this work and will be dealt with in future publications.
The experiments performed in a nonaqueous environment reveal the influencing factors, of which diffusion is the most important one. If possible, titration in an aqueous medium should be preferred because of the costs and deposition problems associated with nonaqueous systems. In addition, a switch to a dynamic titration mode in the aqueous system reduces the titration time by a factor of 10. Acknowledgements SW and KH would like to thank the Karl-Völker-Stiftung an der Hochschule Mannheim for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusions [12]
Traditional titrations can be used for the characterisation of grafting processes with ionic coatings. They provide a fast and cheap way to obtain information about the quality of the layers.
[13]
S. Onclin, B.J. Ravoo, D.N. Reinhoud, Angew. Chem. Int. Ed. 44 (2005) 6282–6304. C.J. Brinkler, G.W. Scherer, Sol–Gel Science, Academic Press, Oxford, UK, 1990. E.P. Plueddemann, Silane Coupling Agents, Plenum Press, New York, 1991. A. Blume, M. Janik, J.-P. Gallas, F. Thibault-Starzyk, A. Vimont, Kaut. Gummi Kunstst. 61 (7–8) (2008) 359–362. M. Oles, E. Nun, Patent Publication Number WO/2004/101880, 2004. D.K. Wang, J.H. Moon, Y.G. Shui, K.T. Jung, D.H. Kim, D.W. Lee, J. Sol–Gel Sci. 1–3 (2003) 783–787. M. Groenewolt, Prog. Org. Coat. 61 (2008) 106–109. K.S. Finnie, D.J. Waller, F.L. Perret, A.M. Krause, H.Q. Lin, J.V. Hanna, C.J. Barbe, J. Sol–Gel Sci. Technol. 49 (2009) 12–18. P. Majewski, C.P. Chan, Int. J. Nanotech. 5 (2008) 291–298. P. Majewski, C.P. Chan, J. Nanosci. Nanotech. 8 (2008) 6519–6522. DIN ISO 3771 Mineral Oil Products; Total Base Number, Determination by Potentiometric Titration, Deutsches Institut für Normung, Berlin, Germany, 1985. J. Hagen, Industrial Catalysis: A Practical Approach, Wiley–VCH, Weinheim, Germany, 2006. F. Bauer, H.-J. Glasel, U. Decker, H. Ernst, A. Freyer, E. Hartmann, V. Sauerland, R. Mehnert, Prog. Org. Coat. 47 (2003) 147–153.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Novel titration method for surface-functionalised silica Kai Hofen a , Siegfried Weber a , Chiu Ping Candace Chan b , Peter Majewski b,∗ a b
Department of Biotechnology, University of Applied Sciences, Mannheim, Germany School of Advanced Manufacturing and Mechanical Engineering, Mawson Institute, University of South Australia, Mawson Lakes Blvd, Mawson Lakes 5095, Australia
a r t i c l e
i n f o
Article history: Received 7 June 2010 Received in revised form 28 September 2010 Accepted 7 October 2010 Available online 17 October 2010 Keywords: Functionalised silica Titration
a b s t r a c t This paper describes three inexpensive and fast analytical methods to characterise grafted particle surfaces. The reaction of silica with (3-aminopropyl)triethoxysilane, (3-mercaptopropyl)trimethoxysilane and N-(phosphonomethyl)iminodiacetic acid hydrate, respectively, leads to NH2 -, SO3 H- or COOHfunctionalised silica, which were characterised by X-ray photoelectron spectrometry and titration in nonaqueous media as well as with two titration methods in a water-based environment. In the work presented, factors influencing the titrations are pointed out and solutions are presented to overcome these limiting factors are shown. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Although the sol–gel chemistry of silanes is well investigated in various publications, e.g. [1–3], there is still room for their applications in industrial processes. Examples for successfully introduced processes based on silanes are the production of tires with a low rolling resistance by crosslinking rubber and silica particles via silanes containing mercapto groups [4], fluorosilanes for self-cleaning surfaces, e.g. [5], and Scratch Resistant and Transparent UV-Protective Coatings using 3-glycidoxypropyl-trimethoxysilane, e.g. [6–8]. A promising technology for a contemporary large-scale production is the treatment of water with functionalised silica particles [9,10]. The application of sol–gel processes in industrial processes leads to a demand for cheap, fast and easy-to-use analytical methods. In numerous publications within the last decades methods like ToFSIMS, XPS, AFM or solid-state NMR are used. There is no doubt that these methods give excellent results but they are very expensive and not usable at production sites. Furthermore it is difficult to make quantitative statements about the surface modification of particles which is also important in research. The present work adopts a potentiometric titration method in a nonaqueous solvent of the oil industry [11] to characterise silica particle surfaces carrying ionic groups. It comprises the difficulties, like diffusion effects, that occur during titration of analytes which are bound to a surface. Subsequently the titration is adopted to
∗ Corresponding author. E-mail address: [email protected] (P. Majewski). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.10.025
work in aqueous media to reduce costs and time as well as making it more environmentally friendly. 2. Experimental procedures 2.1. Materials All organic solvents, acids, bases, sodium chloride and hydrogen peroxide were p.a. quality and purchased from Merck Chemicals. Amorphous silica powder (Ultrasil 7000) and (3aminopropyl)triethoxysilane (AMPTS) were industry-grade and kindly allocated by Evonik Industries, Germany. As provided by the manufacturer, Ultrasil 7000 has a concentration of 1.1 OH groups/nm2 and a BET surface area of 165 m2 /g. This results in a quantity of 0.3014 mmol OH/g. Assuming that each silane molecule binds to one surface hydroxy group without crosslinking of the silane molecules themselves the utilised amount of silane means an allocation of 100% plus a 10% excess. (3-Mercaptopropyl)trimethoxysilane (MPTMS, purity 97%), N(phosphonomethyl)-iminodiacetic acid hydrate (PMIDA, purity 99%), and Oxone were purchased from Sigma–Aldrich. All these chemicals were used as obtained without further purification. The applied water was produced with a Milli-Q water purification device from Millipore. 2.1.1. Synthesis of functionalised silica particles NH2 -functionalised silica particles were prepared by suspending 30 g of the silica particles in 200 ml of toluene. Subsequently, a solution of 10 mmol of silane in 100 ml toluene was dropped to the suspension under constant stirring within 15 min and the resulting suspension was stirred for another 105 min. Finally, the suspen-
K. Hofen et al. / Applied Surface Science 257 (2011) 2576–2580
sion was filtered and the particles were washed three times with 100 ml toluene and soaked dry. Eventually, the particles were dried at 60 ◦ C in a vacuum oven for 4 h. With this procedure the theoretical standardised amount of amino groups on the silica surface is 0.333 mmol/g. SO3 H-functionalised silica particles were prepared by suspending 5 g of the silica particles in 200 ml of toluene. Subsequently, 1.02, 2.04 and 3.06 mmol of MPTMS per gram silica, respectively, was dropped to the suspension under constant stirring within 15 min and the suspension was stirred for another 105 min. The suspension was filtered and the particles were washed three times with 100 ml toluene and soaked dry. Subsequently, the powder was stirred in 200 ml Oxone (KHSO5 , KHSO4 , K2 SO4 ) at 60 ◦ C for 24 h and eventually filtered. The particles were then dried at 60 ◦ C in a vacuum oven for 4 h. With this procedure the theoretical standardised amount of sulfonate groups on the silica surface is 1.02, 2.04 and 3.06 mmol/g, respectively. COOH-functionalised silica particles were prepared by stirring 5 g of the silica particles in 250 ml of Milli-Q water containing 0.26 and 1.5 mmol of PMIDA per gram silica, respectively. The suspension was stirred for 20 h and subsequently filtered. Eventually, the particles were dried at 60 ◦ C in a vacuum oven for 4 h. With this procedure the theoretical standardised amount of carboxy groups on the silica surface is 0.52 and 3 mmol/g, respectively, considering that PMIDA contains two carboxy groups per molecule.
2577
2.1.4. XPS The XPS instrument used to conduct the experiment was a Kratos Axis Ultra (Kratos Analytical Ltd., Wharfside, UK), which used a Mg K␣ source (1253 eV) operated at 300 W. Samples were mounted on a stainless steel stub for analysis. A fixed pass energy of 93.9 eV for wide (or survey) scans and of 29.35 eV for narrow (or multiplex) scans was used. All XPS measurements were acquired at a take-off angle of 45◦ to the surface and were referenced to the C 1s peak at 284.7 eV. The XPS spectra were curve-fitted using the CASA programme version 2.1.35. 3. Results and discussion 3.1. XPS analyses The XPS analyses of the coated materials clearly indicate the presence of the functional group on the surface of the silica particles. Fig. 1 shows the XPS spectrum of NH2 -functionalised silica exhibiting besides the Si 2p peak the N 1s and C 1s peaks which can be attributed to amino groups on the surface. In Fig. 2, the XPS spectrum of SO3 H-functionalised silica shows the S 2s and S 2p peaks as well as the C 1s peak besides the Si2p peak, which can be attributed to sulfonic groups on the surface. Fig. 3 presents the XPS spectrum of COOH-functionalised silica, clearly exhibiting the Si 2p, N 1s, C 1s and a weak P 2s peak, which are believed to belong to carboxy groups on the surface.
2.1.2. Preparation procedure for the nonaqueous titration The titration in nonaqueous media was performed in a mixture of 55 vol% toluene and 45 vol% 2-propanol. The required amount of grafted silica particles was transferred together with 80 ml solvent into a beaker. The vessel was covered and the particles were allowed to swell without stirring for 15–60 min. The titration was carried out with 0.1 mol/l perchloric acid.
18
x 10
survey [R1,1]
4
Si2p C1s 16
2.1.3. Preparation procedure for the aqueous titration 2.1.3.1. NH2 -functionalised silica particles. Purified water was used as solvent for the direct titration as well as for the back titration. 2 g of the grafted particles, 10 g of sodium chloride and 80 ml of water were used.
The titrations were performed with a Metrohm 716 DMS Titrino using a Metrohm pH glass electrode in the aqueous environment and a Metrohm Solvotrode in the nonaqueous media. It is essential to stir the sample until no agglomerates and gel clouds are visible before the titration is started. 2.1.3.2. SO3 H and COOH functionalised silica particles. Back titration: 1 g of the silica particles and 1 g of sodium chloride were weighed into a clean beaker. Subsequently, 10 ml of a 0.1 mol/l solution of sodium hydroxide and 70 ml of Milli-Q water were added, and the suspension was stirred for 18 h. Three drops of phenolphthalein were added to the beaker and the solution titrated with a 0.1 mol/l solution of hydrochloric acid. The end point was reached when the colour changed from pink to colourless. At least three measurements were conducted per powder batch.
12
N1s
CPS
(a) Direct titration: The mixture was stirred for 24 h on a magnetic stirrer. The amount of released hydroxy groups was determined by titration with 0.1 mol/l solution of hydrochloric acid. (b) Back titration: An exact amount of 10 ml of 0.1 mol/l solution of hydrochloric acid was added to the suspension. The mixture was stirred for 24 h on a magnetic stirrer to neutralise the hydroxy groups. Afterwards the excess of acid was determined by titration with 0.1 mol/l solution of sodium hydroxid.
14
10
8
6
4
400
350
300
250
Binding Energy (eV) Fig. 1. XPS spectrum of NH2 -functionalised silica.
200
150
2578
K. Hofen et al. / Applied Surface Science 257 (2011) 2576–2580
Si-SAM 30/4
x 103
survey [R1,1]
x 104
Si2p 80
Si2p
22
20
70
18
60
16
50
CPS
CPS
C1s 14
12
S2p 30
C1s
P2s N1s
40
S2s 10
20 8
10
6
4
300
280
260
240
220
200
180
160
Binding Energy (eV)
400
350
300
250
200
150
Binding Energy (eV) Fig. 2. XPS spectrum of SO3 H-functionalised silica. Fig. 3. XPS spectrum of COOH-functionalised silica.
3.2. Titration results for NH2 -functionalised silica (Table 1) 3.2.1. Titration in the nonaqueous media To maximise the protonation of the amino groups in the nonaqueous media perchloric acid was chosen as the acid. Since organic solvents are a problematic medium for titrations a monotone equilibrium titration was carried out, which consists of the following automated steps: 1. a defined constant volume of the standard solution is injected; 2. after a defined constant time interval to allow the potential to stabilise the potential is measured and determined whether the equilibrium is reached.
Table 1 Concentrations [mmol/g] of amino groups obtained with different titration methods using silica with a theoretical concentration of 0.333 mmol amino groups per g silica. Nonaqueous
Direct, aqueous
Back titration
1 2 3 4 5 6
0.2885 0.2912 0.2928 0.2932 0.2884 0.2893
0.2605 0.2604 0.2597 0.2603 0.2607 –
0.2742 0.2749 0.2736 0.2773 0.2774 0.2767
Average Standard deviation
0.2906 2.12 × 10−3
0.2603 0.35 × 10−3
0.2757 1.66 × 10−3
These steps are repeated until the titration is completed. There were three parameters expected to influence the titration process: 1. The sample weight: It should be large enough to provide a good precision. On the other hand the amount of modified silica for the analysis should be as low as possible. 2. The volume aliquot: It should be as small as possible to obtain a good precision. 3. Diffusion effects in the pores of the silica [12] or in the surface structure [13]. The result of each titration was the amount of amino groups in mmol/g grafted silica. The optimum sample weight was determined with a series of measurements. Fig. 4 shows the concentration of amino groups on the surface obtained as a function of the sample weight. The correlation indicates that the titration approaches asymptotically a limiting value of the surface group concentration and that a minimum sample weight of 2 g of grafted silica is needed for obtaining a reliable result. Thus this amount was chosen for further experiments. The volume aliquot was altered beginning with a value of 0.1 ml. The data obtained with this aliquot had a very low signal-tobackground ratio, which led to random results. With a volume aliquot of 0.2 ml the results were reproducible and the following experiments were carried out with this value.
0.35
0.35
0.30
0.30
amino groups (standardized) [mmol/g]
amino groups (standardized) [mmol/g]
K. Hofen et al. / Applied Surface Science 257 (2011) 2576–2580
0.25 0.20
interval 45s
0.15
interval 90s 0.10 0.05 0.00
0
2
4
0.25 0.20 0.15 0.10 0.05 0.00
6
2579
sample weight [g]
0
50
100
150
me intervall [s]
Fig. 4. Dependence of the titration result on the sample weight.
To study possible diffusion effects the time interval before the potential is determined was varied. The correlation between amino group concentration and stirring time (Fig. 5) also shows an asymptotical approach towards a limiting value of the surface group concentration, which is comparable to that in Fig. 4. Diffusion effects are most probably also responsible for the influence of the sample weight on the titration result. Since the titration takes longer with a higher sample mass diffusion affects the result less. For a reliable result a stirring time of at least 90 s was required. 3.2.2. Direct titration in aqueous media An excess of sodium chloride was added to the particle suspension to provide a large excess of chloride ions that replace quantitatively the hydroxide ions, the counter ions of the ammonium groups, at the particle surface (Scheme 1, bottom). Diffusion effects were eliminated by a long stirring time. According to Table 2 10% less amino groups were found than in organic solvents. This can be explained by deprotonation of the amino groups through the released hydroxide ions.
Fig. 5. Dependence of the titration result on the time interval.
3.2.3. Back titration in aqueous media To overcome this problem an alternative titration procedure was tested: an excess of hydrochloric acid was added during the preparation of the sample suspension. This ensures an acidic pH during the whole preparation and titration process. Therefore the protonation equilibrium is shifted towards the ammonium groups. The excess of sodium chloride shifts the reaction equilibrium in Scheme 1, top towards the salt. In contrast to the direct titration a blank value with unmodified silica of 0.0422 mmol/g was found. It is most possibly caused by impurities of the silica. These effects and the elimination of the diffusion influence result in a 5% lower value compared to that of the nonaqueous titration. The precision of the back titration is with a standard deviation of 1.66 × 10−3 mmol/g still good. As this method was performed as a dynamic titration the titration time was reduced by a factor of 20 compared to the titration in nonaqueous media and by factor of 10 compared to the direct titration.
Table 2 Concentrations of surface groups obtained through back titration of SO3 H- and COOH-functionalised silica with different theoretical concentrations of the surface groups (all concentrations in mmol/g). Sulfonate, 1.02
Sulfonate, 2.04
Sulfonate, 3.06
Carboxy, 0.52
Carboxy, 3.0
1 2 3
0.81 0.80 0.79
0.79 0.80 0.82
0.84 0.83 0.82
0.59 0.58 0.58
0.87 0.82 0.81
Average Standard deviation
0.81 14 × 10−3
0.81 17 × 10−3
0.83 10 × 10−3
0.58 6 × 10−3
0.83 34 × 10−3
Scheme 1. Protonation/deprotonation equilibrium of NH2 -functionalised silica particles in water (top) and hydroxide/chloride exchange reaction between protonated NH2 -functionalised silica particles and sodium chloride (bottom).
2580
K. Hofen et al. / Applied Surface Science 257 (2011) 2576–2580
3.3. Titration results for SO3 H- and COOH-functionalised silica The results of the titration experiments for the SO3 H- and COOH-functionalised silica are listed in Table 2. For the SO3 Hfunctionalised silica, an increase of the concentration of MPTMS in the synthesis does not result in the expected increase of the measured functional group concentration. This phenomenon might be explained by the fact that there is a maximum value for the concentration of sulfonic groups that can be reached on the surface of the particles and that the concentration of MPTMS used in the synthesis (0.8 mmol/g) is higher than this limit. For the COOH-functionalised silica, an increase of the functional group concentration with increasing PMIDA content in the synthesis is evident. It should be noted that the concentration of PMIDA in the synthesis for these titrations was only 0.26 and 1.5 mmol/g, respectively, as PMIDA has two carboxy groups per molecule. The fact that for a PMIDA concentration of 1.5 mmol/g significantly lower COOH concentrations than the theoretical value are obtained is believed to be also due to the fact that the concentration of functional groups on the surface of the silica is limited, in this case to about 0.8 mmol/g. The reason for the limitation of the concentration of SO3 H and COOH groups on the surface of the silica particles could not be studied in this work and will be dealt with in future publications.
The experiments performed in a nonaqueous environment reveal the influencing factors, of which diffusion is the most important one. If possible, titration in an aqueous medium should be preferred because of the costs and deposition problems associated with nonaqueous systems. In addition, a switch to a dynamic titration mode in the aqueous system reduces the titration time by a factor of 10. Acknowledgements SW and KH would like to thank the Karl-Völker-Stiftung an der Hochschule Mannheim for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusions [12]
Traditional titrations can be used for the characterisation of grafting processes with ionic coatings. They provide a fast and cheap way to obtain information about the quality of the layers.
[13]
S. Onclin, B.J. Ravoo, D.N. Reinhoud, Angew. Chem. Int. Ed. 44 (2005) 6282–6304. C.J. Brinkler, G.W. Scherer, Sol–Gel Science, Academic Press, Oxford, UK, 1990. E.P. Plueddemann, Silane Coupling Agents, Plenum Press, New York, 1991. A. Blume, M. Janik, J.-P. Gallas, F. Thibault-Starzyk, A. Vimont, Kaut. Gummi Kunstst. 61 (7–8) (2008) 359–362. M. Oles, E. Nun, Patent Publication Number WO/2004/101880, 2004. D.K. Wang, J.H. Moon, Y.G. Shui, K.T. Jung, D.H. Kim, D.W. Lee, J. Sol–Gel Sci. 1–3 (2003) 783–787. M. Groenewolt, Prog. Org. Coat. 61 (2008) 106–109. K.S. Finnie, D.J. Waller, F.L. Perret, A.M. Krause, H.Q. Lin, J.V. Hanna, C.J. Barbe, J. Sol–Gel Sci. Technol. 49 (2009) 12–18. P. Majewski, C.P. Chan, Int. J. Nanotech. 5 (2008) 291–298. P. Majewski, C.P. Chan, J. Nanosci. Nanotech. 8 (2008) 6519–6522. DIN ISO 3771 Mineral Oil Products; Total Base Number, Determination by Potentiometric Titration, Deutsches Institut für Normung, Berlin, Germany, 1985. J. Hagen, Industrial Catalysis: A Practical Approach, Wiley–VCH, Weinheim, Germany, 2006. F. Bauer, H.-J. Glasel, U. Decker, H. Ernst, A. Freyer, E. Hartmann, V. Sauerland, R. Mehnert, Prog. Org. Coat. 47 (2003) 147–153.