- Email: [email protected]
Effects of surface-active substances on acid–base indicator reactivity in SiO2 gels
Journal of Non-Crystalline Solids 320 (2003) 168–176 www.elsevier.com/locate/jnoncrysol
Effects of surface-active substances on acid–base indicator reactivity in SiO2 gels Zhijian Wu b
a,b,*
, Kangtaek Lee b, Yanxin Lin a, Xinren Lan a, Liyao Huang
a
a College of Materials Science and Engineering, Huaqiao University, Quanzhou 362011, Fujian, PR China Department of Chemical Engineering, Yonsei University, 134 Shinchon-Dong, Sudaemun-Ku, Seoul 120-749, South Korea
Received 28 February 2002; received in revised form 4 October 2002
Abstract Methyl orange (MO) and phenolphthalein (P) doped SiO2 gels derived from tetraethyl orthosilicate (TEOS) and modified with cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and stearic acid (SA) are prepared. FTIR and TG analyses are used to characterize the gels. The color changes and leaching behavior of MO and P within the gels are investigated and explained on the basis of the gel structures and the reaction mechanism. The experimental results show that surface-active substances have obvious effects on the acid–base response and the leaching behavior of the acid–base indicators within the SiO2 gels. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Organically doped sol–gel materials, especially SiO2 sol–gel materials have attracted much attention due to their ability to reproduce and modify solution molecular activities within the sol–gel materials [1–6]. Generally the dopant properties are affected by the interfacial moieties of the sol–gel materials [7]. For SiO2 xerogels, silanols, siloxane bridges, residual water and alcohol molecules, unhydrolyzed alkoxy groups, and acid/base catalysts may affect the optical and chemical properties of the dopants. The co-entrapment of surface-active agents, especially surfactants, can further affect the dopant properties. Rottman et al.
*
Corresponding author. E-mail address: [email protected] (Z. Wu).
[1] have reported that the co-entrapment of cetyltrimethylammonium bromide (CTAB) with a series of pH indicators greatly modifies the indicating performance of the primary dopant within the tetramethoxysilane-derived sol–gel matrices. In this paper, the effects of three different surfaceactive substances (CTAB, SDS, and SA) are compared and the effects of drying temperature on the acid–base response and leaching behavior of the indicators are investigated.
2. Experimental procedure 2.1. Preparation of the gels Sols were prepared by mixing TEOS, ethanol, water, 0.1 M HCl solution, solutions of surfaceactive substances (0.054 M CTAB in ethanol;
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(03)00025-5
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
169
Table 1 Sol compositions and gelation time Sample 1 2 3 4 5 6 7 8 9 10 11 12
System
Molar ratio
Gelation timea (h)
TEOS
H2 O
E
HCl
SAS
TEOS–H2 O–E–HCl TEOS–H2 O–E–HCl– CTAB TEOS–H2 O–E–HCl– SDS TEOS–H2 O–E–HCl–SA
1 1
5 5
4.6 4.6
1.7 104 1.7 104
1.2 102
202 116
1
5
4.6
1.7 104
4.3 103
116
1
5
4.6
1.7 104
1.4 102
155
TEOS–H2 O–E–HCl– MO TEOS–H2 O–E–HCl– CTAB–MO TEOS–H2 O–E–HCl– SDS–MO TEOS–H2 O–E–HCl– SA–MO
1
5
4.6
1.7 104
1
5
4.6
1.7 104
1
5
4.6
1
5
TEOS–H2 O–E–HCl–P TEOS–H2 O–E–HCl– CTAB–P TEOS–H2 O–E–HCl– SDS–P TEOS–H2 O–E–HCl– SA–P
1 1
I
1.1 104
222
1.2 102
1.1 104
143
1.7 104
4.3 103
1.1 104
116
4.6
1.7 104
1.4 102
1.1 104
202
5 5
4.6 4.6
1.7 104 1.7 104
1.2 102
8.1 104 8.1 104
220 146
1
5
4.6
1.7 104
4.3 103
8.1 104
116
1
5
4.6
1.7 104
1.4 102
8.1 104
202
E – ethanol; SAS – surface-active substance; I – indicator. a This gelation time is the time needed for the sols to stop flowing after the culture plates are removed.
0.064 M SDS in water; 0.31 M SA in ethanol), and solutions of indicators (1.2 103 M MO in water; 0.018 M P in ethanol). The sol compositions are listed in Table 1. The total volume of each sol was 52 ml. The beakers containing the sols were covered with culture plates and kept for five days at room temperature. After that, the culture plates were removed to allow the solvents to evaporate and hence to convert the sols to gels. After gelation, the beakers were covered again for five days. Then the gels were air dried for five days, followed by drying at 50 °C for 24 h. Parts of the P-doped gels were further dried at 200 °C for 24 h to investigate the effect of drying temperature. All the gel products showed irregular shapes. 2.2. FTIR spectrum and TG analyses of the gels After grinding gels with a mortar, FTIR spectra of the gels were taken using KBr pellets on a
Spectrum 2000 FTIR spectrometer. Thermogravimetric analyses were conducted on a TA 5200 apparatus.
2.3. Determination of the color change pH value After the indicator-doped gels are put into water solutions at different pH values, the gel color will change due to the indicator acid–base response. The time needed for the gels to change their color is defined as response time. Initially, the prepared MO-doped gels were red, and their acid– base response time was relatively long (>30 min). The color change pH values of MO-doped gels were recorded with impregnation method. The MO-doped gels were impregnated in a series of solutions at different pH values to determine the pH values at which the original red gels turn yellow. For the determination of the pH values for
170
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
MO-doped gels to change color from yellow to red, some of the original MO-doped gels (red) were impregnated in a solution of pH 13.0 for 12 h, and then the yellow gels were impregnated in a series of solutions at different pH values. The P-doped gel products were initially colorless, and their acid–base response time was relatively short (<5 min). After the gels were put into water or solutions their color change pH values were determined by gradually increasing the pH values with NaOH solution. And the color change pH values of the gels from colorless to pink were recorded. Parts of the original gels were impregnated in NaOH solution of pH 13.0 for 5 min to allow their color to change into pink. Then HCl solution was gradually added to decrease the pH. The color change pH values from pink to colorless were recorded.
2.4. Determination of the indicator leaching from the gels The indicator-doped gels (0.5 g) were put into volumetric flasks with 50 ml solutions at pH 1.0, 7.0, and 13.0. The temperature of the flask was kept constant at 25 °C. Indicator leaching was recorded by determining the indicator concentrations in the solutions with UV–visible spectrophotometry. For the P-doped gels, the solution samples were adjusted to 13.0 with a dilute KOH solution, and the absorbance was recorded at 552 nm. For MO-doped gels, the absorbance was recorded at 510 nm for solutions at pH 1.0 and at 466 nm for solutions at pH 7.0 and 13.0. The indicator leaching behavior was compared according to the indicator leaching rate. The leaching rate is defined as the indicator leaching amount from the gels divided by the indicator total amount in the gels used for the leaching experiments. After soaking indicator concentration in the solutions was determined by UV–visible spectrophotometry. Then the indicator leaching amount could be calculated. The total amount of indicator in the gels was calculated from the gel weight and the initial concentration of indicator inside the gel products.
3. Results 3.1. FTIR spectra of the gels FTIR spectra of the gels are shown in Fig. 1. The assignments of the absorptions are listed in Table 2 [8]. All the FTIR spectra of the gels doped with surface-active substances exhibit absorptions related to the surface-active substances. On the other hand, all the FTIR spectra of the gels doped with acid–base indicators have no absorptions related to the acid–base indicators because of the low indicator contents in the gels. MO contents in Gels 5, 6, 7, and 8 are 0.060%, 0.055%, 0.059%, and 0.056% respectively. P contents in Gels 9, 10, 11, and 12 are 0.43%, 0.40%, 0.42%, and 0.40% respectively. Gels 1, 5 and 9 (without surface-active substances) and the gels with the same surface-active substances (Gels 2, 6 and 10 for CTAB, Gels 3, 7 and 11 for SDS, and Gels 4, 8 and 12 for SA) show the same IR absorptions, respectively.
Fig. 1. FTIR spectra of the gels dried at 50 °C: (a) Gels 1, 5 and 9 (without surface-active substances); (b) Gels 2, 6 and 10 (with CTAB), (c) Gels 3, 7 and 11 (with SDS) and (d) Gels 4, 8 and 12 (with SA).
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176 Table 2 IR absorption assignments of the gels Frequency (cm1 )
Assignments
2919–2927 2849–2855 1700 1630–1640 1470 1077–1084 944–955 792–797 553–561 450–462
Asymmetric stretch of methene Symmetric stretch of methene C@O stretch of carboxylic acid dimer H2 O bend Methene bend Asymmetric stretch of Si–O–Si Si–OH stretch Symmetric stretch of Si–O–Si Related to Si4 O12 ring structure Si–O–Si bend
3.2. TG analyses of the gels The TG curve shapes of Gels 1, 5 and 9 (without surface-active substances), Gels 2, 6 and 10 (with CTAB), Gels 3, 7 and 11 (with SDS), Gels 4, 8 and 12 (with SA) are all similar respectively. The TG curves of Gels 9, 10, 11 and 12 are shown in Fig. 2. The obvious weight loss below 130 °C can be assigned to the desorption of different solvents. The weight loss of Gels 9, 10, 11 and 12 is 14.3%, 15.9%, 17.0% and 16.1% respectively. The obvious weight loss between 250 and 400 °C (8.1%, calc. 6.8%), 450 and 600 °C (7.3%, calc. 6.2%), 650 and 700 °C (2.1%, calc. 2.0%) could be due to the thermal decomposition of CTAB (Gel 10), SA (Gel 12) and SDS (Gel 11) respectively. Because there exists an interference of the indicators and the hydroxyl groups, the determined weight loss is
Fig. 2. TG curves of the gels dried at 50 °C: (1) Gel 9 (without surface-active substances), (2) Gel 10 (with CTAB), (3) Gel 11 (with SDS) and (4) Gel 12 (with SA).
171
higher than the corresponding calculated weight loss. There is no obvious weight loss related to indicator decomposition because of the very low indicator contents in the gels. The TG curves in Fig. 2 do not show any obvious weight loss related to polymerization of silanols (loss of hydroxyl groups). According to Karmakar and Ganguli [8], TG analysis of the gels dried at 150 °C (these gels are similar to Gel 9 in our experiment, but without indicator doped) shows a weight loss of about 5% up to about 500 °C and virtually no further weight loss up to 1200 °C. Gel 9 shows a similar behavior. It shows a weight loss of 6.6% from 150 to 500 °C. 3.3. Acid–base response of indicators in the gels The response times of MO and P in the gels are listed in Table 3. The response time of MO is longer than that of P. Generally speaking, fast response time is easily achieved by reducing the particle dimensions of the gel products. Our experimental results show that the response time of the indicators within the gels is mainly determined by the nature of the indicator and the gel systems. Indicators in water solutions have definite indicating ranges. For example, the indicating ranges of MO and P in aqueous solutions are 2.9–4.6 (red–yellow) and 8.0–10.0 (colorless–pink), respectively. However, the indicators within the gels have no definite indicating ranges, so only the color change pH values can be determined. The indicating range of P in solution is generally considered to be 8.0–10.0, and it was determined to be 8.4–9.7 in our experiments. That is to say, with addition of NaOH solution to colorless P solution, the solution turns light pink when the solution pH value reaches 8.4. The solution color gradually deepens with further addition of NaOH solution until the solution pH value reaches 9.7. When dilute hydrochloric acid is gradually added to this solution, the solution color gradually becomes lighter up to a pH value of 8.4 where the solution becomes colorless. However, when Gel 9 (originally colorless) is put into distilled water, and NaOH solution is added, the color does not change into light pink until the solution pH value reaches 11.9. With the further addition of NaOH solution, the gel color becomes deep. If HCl
172
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
Table 3 Reaction parameters of MO and P in the gels Sample
Response time (min)
Color change pH value r!y
y!r
5
60 4
6
30 2
7
90 7
8
60 5
11.5 0.03– 12.5 0.04 2.0 0.04– 2.5 0.02 9.5 0.02– 10.5 0.05 10.5 0.05– 11.5 0.03
3.5 0.03– 2.5 0.05 1.0 0.03– 0.5 0.02 3.5 0.04– 2.5 0.01 3.5 0.03– 2.5 0.04
9 10 11 12
<5 <5 <5 <5
MO
Leaching rate after seven days (%) P c!p
p!c
11.9 0.07 12.3 0.08 12.5 0.07 12.5 0.06
8.9 0.04 8.5 0.05 a
11.4 0.04
pH 1.0
pH 7.0
pH 13.0
1.3 0.01
1.6 0.01
6.7 0.03
<0.05
<0.05
0.26 0.01
32.4 0.16
42.3 0.14
34.8 0.18
1.4 0.01
1.7 0.01
3.4 0.02
3.1 0.01 6.4 0.02 12.8 0.06 2.4 0.01
6.8 0.03 3.6 0.02 15.6 0.08 2.3 0.01
1.3 0.01 2.5 0.01 33.6 0.12 12.1 0.06
r – red, y – yellow, c – colorless, p – pink. a It is difficult to determine because of leaching.
solution is added to this gel-containing solution, the gel color does not turn colorless even when the solution pH reaches 11.9. The gel turns colorless when the solution pH reaches 8.9. It seems that this situation is not due to the response time, but that it is an intrinsic property of the indicator within the gels, since definite indicating ranges for MO and P doped gels could not be obtained even after the gels were soaked for seven days. The color change pH values of MO and P within the gels are listed in Table 3. For MO-doped gels only the color change pH range is given, because the color does not change sharply with pH.
time. With reference to the TG curves, the Pdoped gels dried at 50 and 200 °C were compared for their acid–base response and their leaching behaviors. The results are listed in Table 4. For Gels 9 and 10, the color change pH values increase with increasing drying temperature. For Gel 9 dried at 200 °C, its leaching rate in solution at pH 13.0 increases from 1.3% to 13.3%. For Gel 11 dried at 200 °C, its leaching rate increases from 12.8% to 27.9% at pH 1.0 and from 15.6% to 27.8% at pH 7.0.
4. Discussion 3.4. Leaching behavior of the indicators within the gels The leaching rates of the indicator after soaking the gels for seven days are listed in the last column of Table 3. Note that leaching rate of MO shows minimum with CTAB (Gel 6) and maximum with SDS (Gel 7). For P-doped gels, maximum leaching occurs with SDS (Gel 11). 3.5. Effects of drying temperature on the P reactivity within the gels P-doped gels were used for the drying temperature experiments because of their short response
The above experimental results demonstrate that classical solution chemistry is transferable to the xerogel environment, and the chemical performance of the dopant can be modified by surface-active substances. Surfactants can form various micellar structures in sol–gel starting reaction mixture, depending on the solvent concentration [10,11]. After gelation, these micellar structures can be retained in the gels [9]. For CTAB micelles, the micelle surfaces are positively charged, and the negatively charged polysilicate species will aggregate near the micelle surfaces (the situation of Figs. 3(a), (b) and 4(e), (f)). For SDS micelles, the micelle surfaces are negatively
13.3 0.05 3.2 0.01 27.6 0.11 9.7 0.03 9.2 0.01
a
11.4 0.04
a
pH 7.0
6.8 0.03 3.6 0.02 15.6 0.08 2.3 0.01 3.1 0.01 6.4 0.02 12.8 0.06 2.4 0.01
pH 1.0 p!c
9.5 0.04 9.7 0.02
12.8 0.03 13.0 0.05 12.7 0.02 12.3 0.06 9 10 11 12
c – colorless, p – pink. a It is difficult to determine because of leaching.
pH 7.0
6.9 0.02 6.7 0.02 27.8 0.12 1.0 0.01 0.8 0.01 5.0 0.02 27.9 0.14 1.7 0.01 8.9 0.04 8.5 0.05
11.9 0.07 12.3 0.08 12.5 0.07 12.5 0.06
pH 1.0 c!p p!c c!p
1.3 0.01 2.5 0.01 33.6 0.12 12.1 0.06
Sample dried at 50 °C Sample dried at 50 °C
pH 13.0
Leaching rate after seven-day soaking (%)
Sample dried at 200 °C Color change pH value Sample
Table 4 Effects of drying temperature on the P reactivity within the gels
Sample dried at 200 °C
pH 13.0
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
173
charged. Between the negatively charged micelles and the negatively charged polysilicate species there exists the counter-charged ions (Naþ , Hþ , etc.) (the situation of Figs. 3(c), (d) and 4(g), (h)). In aqueous solutions, there exist electrostatic and hydrophobic interactions between MO and the surfactants [12,13]. Gehlen and his coworkers demonstrated that MO interacts strongly with the cationic micelles cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB), but does not interact with the anionic micelle sodium dodecyl sulphate (SDS) through the studies of absorption and emission spectroscopy [14]. Bunton and his co-workers have shown that aromatic compounds with sulphonic acid groups are incorporated into the stern layers of cationic micelles in a sandwich arrangement and suggested that a van der Waals interaction between adjacent surfactant chains and the dye organic moiety (hydrophobic forces), causes alteration of the chromophore microenvironment [15,16]. For our gel products, there are electrostatic interactions, hydrogen bonds and van der Waals forces between the indicators and SiO2 gel matrix. Between the indicators and the surface-active substances there are electrostatic interactions, hydrogen bonds, van der Waals forces and hydrophobic interactions. The color change pH values of Gel 6 (with CTAB) from red to yellow or yellow to red are all lower than those of Gels 5, 7 and 8 (Table 3). This can be explained as follows. MO in Fig. 3(a) and (c) is in acidic form (red), and MO in Fig. 3(b) and (d) is in basic form (yellow). In Fig. 3(b), the basic form of MO is oriented fully within the surfactant micellar environment, because the negatively charged sulfonate groups of MO associate with the positively charged quaternary ammonium groups of the CTAB, whereas the non-charged phenylamine residue dissolves in the hydrophobic zone. Then, with decreasing pH the azophenylamine moiety becomes positively charged and hydrophilic, resulting in its expulsion from the hydrophobic region to the hydrophilic surroundings of the micellar phase. Because of the electrostatic attraction and hydrophobic interaction between the basic form of MO and the quaternary ammonium groups of the CTAB, a higher
174
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
Fig. 3. Schematic diagrams of the acid–base response of MO in the gels (a, b) Gel 6 (with CTAB) and (c, d) Gel 7 (with SDS).
concentration of protons is required to reach the azo functional group. The pH needed for basic form of MO to become acidic form is shifted to a lower value. The leaching rates of Gel 6 are very low. The leaching rates of Gel 7 (with SDS) are higher, especially in the solutions of pH 1.0 and 7.0. These results are probably caused by three factors: (1) For Gel 6 (with CTAB), cationic surfactant aggregates combine with the anionic silicate species to form a supramolecular structure [9]. The interaction between cationic surfactant aggregates and anionic silicate species are relatively strong so that MO within the gels is difficult to move. For Gel 7 (with SDS), both anionic surfactant aggregates and silicate species are negatively charged. They must be mediated by the positively charged counter ions, which must be present in stoichiometric amount. This suggests
that the combination between anionic surfactant aggregates and silicate species are relatively weak. Thus, MO within the SDS gels can easily move. (2) The solubility of SDS in water is much higher than that of CTAB. The leaching of MO in SDS gels is probably caused partly by the leaching of SDS from the gels. (3) The association between MO and SDS is weaker than that between MO and CTAB. The acid–base response of P within the gels is schematically represented in Fig. 4. P in Fig. 4(e) and (g) is in acidic form (colorless), and P in Fig. 4(f) and (h) is basic form (pink). The experimental results on the P-doped gels show that the leaching rate of Gel 11 (with SDS) is much higher than that of Gels 9, 10 and 12, especially in basic solution. There exists a strong electrostatic repulsion between the basic form of P and anionic surfactant aggregates (Fig. 4(h)). The hydrophobic interac-
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
175
Fig. 4. Schematic diagrams of the acid–base response of P in the gels. (e, f) Gel 10 (with CTAB), (g, h) Gel 11 (with SDS).
tion between the acidic form of P and SDS is very weak because of the short carbon chain length of SDS (Fig. 4(g)). Indicator reactivity in gels without surfaceactive substances shows some similarities with that in gels modified with stearic acid (SA). This may be partly because that SA does not form micelle effectively in the sols and the gels, which may be deduced from the FTIR data. The FTIR spectra of the gels modified with SA have an absorption assignable to C@O stretch of carboxylic acid dimer, which suggests that SA does not form micelle effectively in the sols and the gels.
5. Conclusions (1) Surface-active substances have obvious effects on the acid–base response and leach-
ing behavior of acid–base indicators within the SiO2 gels. This is probably caused by the modification of SiO2 gel structures by the surface-active substances, the electrostatic interactions and hydrophobic interactions between the indicators and the surface-active substances. (2) The response time of P-doped gels (<5 min) is shorter than that of MO-doped gels (>30 min). (3) There are no definite indicating ranges for MO and P within the gels. Only two color change pH values can be determined for either MO or P within the gels. It is demonstrated that this is not caused by response time. (4) The leaching rates of MO and P within the gels modified with SDS are obviously higher than that of other gels. The leaching rates of MO within the gels modified with CTAB are the lowest among those of all the gels.
176
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
Acknowledgements The authors thank the financial assistance from Research Foundation of Overseas Chinese Affairs Office, State Council, P.R. China, and Fujian Provincial Science and Technology Creation Foundation for Young Researcher (2001J023), P.R. China. Z.W. thanks the postdoctoral fellowship from Yonsei NT center at Yonsei University, Korea.
References [1] C. Rottman, G. Grader, Y.D. Hazan, S. Melchior, D. Avnir, J. Am. Chem. Soc. 121 (1999) 8533. [2] D.T. Kanti, I. Khan, D.L. Rousseau, J.M. Friedman, J. Am. Chem. Soc. 120 (1998) 10268. [3] T.K. Jain, I. Roy, T.K. De, A. Maitra, J. Am. Chem. Soc. 120 (1998) 11092.
[4] J. Blum, D. Avnir, H. Schumann, Chemtech 29 (1999) 32. [5] H. Schumann, M. Hasan, F. Gelman, D. Avnir, J. Blum, Inorg. Chim. Acta 280 (1998) 21. [6] R. Shacham, D. Avnir, D. Mandler, Adv. Mater. 11 (1999) 1544. [7] B. Dunn, J. Zink, Chem. Mater. 9 (1997) 2280. [8] B. Karmakar, G. De, D. Ganguli, J. Non-Cryst. Solids 272 (2000) 119. [9] A. Corma, Chem. Rev. 97 (1997) 2373. [10] M.T. Anderson, J.E. Martin, J.G. Odinek, P.P. Newcomer, Chem. Mater. 10 (1998) 1490. [11] K. Matsui, T. Nakazawa, H. Morisaki, J. Phys. Chem. 95 (1991) 976. [12] K.K. Karukstis, D.A. Savin, C.T. Loftus, N.D. DÕAngelo, J. Colloid Interface Sci. 203 (1998) 157. [13] R.K. Dutta, D.N. Bhat, Colloids Surf. A 106 (1996) 127. [14] M.H. Gehlen, M. Ferreira, M.G. Neumann, J. Photochem. Photobiol. A 87 (1995) 55. [15] C.A. Bunton, M.J. Minch, J. Hidalgo, L. Sepulveda, J. Am. Chem. Soc. 95 (1973) 3262. [16] C.A. Bunton, M.J. Minch, J. Phys. Chem. 87 (1974) 1490.
Effects of surface-active substances on acid–base indicator reactivity in SiO2 gels Zhijian Wu b
a,b,*
, Kangtaek Lee b, Yanxin Lin a, Xinren Lan a, Liyao Huang
a
a College of Materials Science and Engineering, Huaqiao University, Quanzhou 362011, Fujian, PR China Department of Chemical Engineering, Yonsei University, 134 Shinchon-Dong, Sudaemun-Ku, Seoul 120-749, South Korea
Received 28 February 2002; received in revised form 4 October 2002
Abstract Methyl orange (MO) and phenolphthalein (P) doped SiO2 gels derived from tetraethyl orthosilicate (TEOS) and modified with cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and stearic acid (SA) are prepared. FTIR and TG analyses are used to characterize the gels. The color changes and leaching behavior of MO and P within the gels are investigated and explained on the basis of the gel structures and the reaction mechanism. The experimental results show that surface-active substances have obvious effects on the acid–base response and the leaching behavior of the acid–base indicators within the SiO2 gels. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Organically doped sol–gel materials, especially SiO2 sol–gel materials have attracted much attention due to their ability to reproduce and modify solution molecular activities within the sol–gel materials [1–6]. Generally the dopant properties are affected by the interfacial moieties of the sol–gel materials [7]. For SiO2 xerogels, silanols, siloxane bridges, residual water and alcohol molecules, unhydrolyzed alkoxy groups, and acid/base catalysts may affect the optical and chemical properties of the dopants. The co-entrapment of surface-active agents, especially surfactants, can further affect the dopant properties. Rottman et al.
*
Corresponding author. E-mail address: [email protected] (Z. Wu).
[1] have reported that the co-entrapment of cetyltrimethylammonium bromide (CTAB) with a series of pH indicators greatly modifies the indicating performance of the primary dopant within the tetramethoxysilane-derived sol–gel matrices. In this paper, the effects of three different surfaceactive substances (CTAB, SDS, and SA) are compared and the effects of drying temperature on the acid–base response and leaching behavior of the indicators are investigated.
2. Experimental procedure 2.1. Preparation of the gels Sols were prepared by mixing TEOS, ethanol, water, 0.1 M HCl solution, solutions of surfaceactive substances (0.054 M CTAB in ethanol;
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(03)00025-5
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
169
Table 1 Sol compositions and gelation time Sample 1 2 3 4 5 6 7 8 9 10 11 12
System
Molar ratio
Gelation timea (h)
TEOS
H2 O
E
HCl
SAS
TEOS–H2 O–E–HCl TEOS–H2 O–E–HCl– CTAB TEOS–H2 O–E–HCl– SDS TEOS–H2 O–E–HCl–SA
1 1
5 5
4.6 4.6
1.7 104 1.7 104
1.2 102
202 116
1
5
4.6
1.7 104
4.3 103
116
1
5
4.6
1.7 104
1.4 102
155
TEOS–H2 O–E–HCl– MO TEOS–H2 O–E–HCl– CTAB–MO TEOS–H2 O–E–HCl– SDS–MO TEOS–H2 O–E–HCl– SA–MO
1
5
4.6
1.7 104
1
5
4.6
1.7 104
1
5
4.6
1
5
TEOS–H2 O–E–HCl–P TEOS–H2 O–E–HCl– CTAB–P TEOS–H2 O–E–HCl– SDS–P TEOS–H2 O–E–HCl– SA–P
1 1
I
1.1 104
222
1.2 102
1.1 104
143
1.7 104
4.3 103
1.1 104
116
4.6
1.7 104
1.4 102
1.1 104
202
5 5
4.6 4.6
1.7 104 1.7 104
1.2 102
8.1 104 8.1 104
220 146
1
5
4.6
1.7 104
4.3 103
8.1 104
116
1
5
4.6
1.7 104
1.4 102
8.1 104
202
E – ethanol; SAS – surface-active substance; I – indicator. a This gelation time is the time needed for the sols to stop flowing after the culture plates are removed.
0.064 M SDS in water; 0.31 M SA in ethanol), and solutions of indicators (1.2 103 M MO in water; 0.018 M P in ethanol). The sol compositions are listed in Table 1. The total volume of each sol was 52 ml. The beakers containing the sols were covered with culture plates and kept for five days at room temperature. After that, the culture plates were removed to allow the solvents to evaporate and hence to convert the sols to gels. After gelation, the beakers were covered again for five days. Then the gels were air dried for five days, followed by drying at 50 °C for 24 h. Parts of the P-doped gels were further dried at 200 °C for 24 h to investigate the effect of drying temperature. All the gel products showed irregular shapes. 2.2. FTIR spectrum and TG analyses of the gels After grinding gels with a mortar, FTIR spectra of the gels were taken using KBr pellets on a
Spectrum 2000 FTIR spectrometer. Thermogravimetric analyses were conducted on a TA 5200 apparatus.
2.3. Determination of the color change pH value After the indicator-doped gels are put into water solutions at different pH values, the gel color will change due to the indicator acid–base response. The time needed for the gels to change their color is defined as response time. Initially, the prepared MO-doped gels were red, and their acid– base response time was relatively long (>30 min). The color change pH values of MO-doped gels were recorded with impregnation method. The MO-doped gels were impregnated in a series of solutions at different pH values to determine the pH values at which the original red gels turn yellow. For the determination of the pH values for
170
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
MO-doped gels to change color from yellow to red, some of the original MO-doped gels (red) were impregnated in a solution of pH 13.0 for 12 h, and then the yellow gels were impregnated in a series of solutions at different pH values. The P-doped gel products were initially colorless, and their acid–base response time was relatively short (<5 min). After the gels were put into water or solutions their color change pH values were determined by gradually increasing the pH values with NaOH solution. And the color change pH values of the gels from colorless to pink were recorded. Parts of the original gels were impregnated in NaOH solution of pH 13.0 for 5 min to allow their color to change into pink. Then HCl solution was gradually added to decrease the pH. The color change pH values from pink to colorless were recorded.
2.4. Determination of the indicator leaching from the gels The indicator-doped gels (0.5 g) were put into volumetric flasks with 50 ml solutions at pH 1.0, 7.0, and 13.0. The temperature of the flask was kept constant at 25 °C. Indicator leaching was recorded by determining the indicator concentrations in the solutions with UV–visible spectrophotometry. For the P-doped gels, the solution samples were adjusted to 13.0 with a dilute KOH solution, and the absorbance was recorded at 552 nm. For MO-doped gels, the absorbance was recorded at 510 nm for solutions at pH 1.0 and at 466 nm for solutions at pH 7.0 and 13.0. The indicator leaching behavior was compared according to the indicator leaching rate. The leaching rate is defined as the indicator leaching amount from the gels divided by the indicator total amount in the gels used for the leaching experiments. After soaking indicator concentration in the solutions was determined by UV–visible spectrophotometry. Then the indicator leaching amount could be calculated. The total amount of indicator in the gels was calculated from the gel weight and the initial concentration of indicator inside the gel products.
3. Results 3.1. FTIR spectra of the gels FTIR spectra of the gels are shown in Fig. 1. The assignments of the absorptions are listed in Table 2 [8]. All the FTIR spectra of the gels doped with surface-active substances exhibit absorptions related to the surface-active substances. On the other hand, all the FTIR spectra of the gels doped with acid–base indicators have no absorptions related to the acid–base indicators because of the low indicator contents in the gels. MO contents in Gels 5, 6, 7, and 8 are 0.060%, 0.055%, 0.059%, and 0.056% respectively. P contents in Gels 9, 10, 11, and 12 are 0.43%, 0.40%, 0.42%, and 0.40% respectively. Gels 1, 5 and 9 (without surface-active substances) and the gels with the same surface-active substances (Gels 2, 6 and 10 for CTAB, Gels 3, 7 and 11 for SDS, and Gels 4, 8 and 12 for SA) show the same IR absorptions, respectively.
Fig. 1. FTIR spectra of the gels dried at 50 °C: (a) Gels 1, 5 and 9 (without surface-active substances); (b) Gels 2, 6 and 10 (with CTAB), (c) Gels 3, 7 and 11 (with SDS) and (d) Gels 4, 8 and 12 (with SA).
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176 Table 2 IR absorption assignments of the gels Frequency (cm1 )
Assignments
2919–2927 2849–2855 1700 1630–1640 1470 1077–1084 944–955 792–797 553–561 450–462
Asymmetric stretch of methene Symmetric stretch of methene C@O stretch of carboxylic acid dimer H2 O bend Methene bend Asymmetric stretch of Si–O–Si Si–OH stretch Symmetric stretch of Si–O–Si Related to Si4 O12 ring structure Si–O–Si bend
3.2. TG analyses of the gels The TG curve shapes of Gels 1, 5 and 9 (without surface-active substances), Gels 2, 6 and 10 (with CTAB), Gels 3, 7 and 11 (with SDS), Gels 4, 8 and 12 (with SA) are all similar respectively. The TG curves of Gels 9, 10, 11 and 12 are shown in Fig. 2. The obvious weight loss below 130 °C can be assigned to the desorption of different solvents. The weight loss of Gels 9, 10, 11 and 12 is 14.3%, 15.9%, 17.0% and 16.1% respectively. The obvious weight loss between 250 and 400 °C (8.1%, calc. 6.8%), 450 and 600 °C (7.3%, calc. 6.2%), 650 and 700 °C (2.1%, calc. 2.0%) could be due to the thermal decomposition of CTAB (Gel 10), SA (Gel 12) and SDS (Gel 11) respectively. Because there exists an interference of the indicators and the hydroxyl groups, the determined weight loss is
Fig. 2. TG curves of the gels dried at 50 °C: (1) Gel 9 (without surface-active substances), (2) Gel 10 (with CTAB), (3) Gel 11 (with SDS) and (4) Gel 12 (with SA).
171
higher than the corresponding calculated weight loss. There is no obvious weight loss related to indicator decomposition because of the very low indicator contents in the gels. The TG curves in Fig. 2 do not show any obvious weight loss related to polymerization of silanols (loss of hydroxyl groups). According to Karmakar and Ganguli [8], TG analysis of the gels dried at 150 °C (these gels are similar to Gel 9 in our experiment, but without indicator doped) shows a weight loss of about 5% up to about 500 °C and virtually no further weight loss up to 1200 °C. Gel 9 shows a similar behavior. It shows a weight loss of 6.6% from 150 to 500 °C. 3.3. Acid–base response of indicators in the gels The response times of MO and P in the gels are listed in Table 3. The response time of MO is longer than that of P. Generally speaking, fast response time is easily achieved by reducing the particle dimensions of the gel products. Our experimental results show that the response time of the indicators within the gels is mainly determined by the nature of the indicator and the gel systems. Indicators in water solutions have definite indicating ranges. For example, the indicating ranges of MO and P in aqueous solutions are 2.9–4.6 (red–yellow) and 8.0–10.0 (colorless–pink), respectively. However, the indicators within the gels have no definite indicating ranges, so only the color change pH values can be determined. The indicating range of P in solution is generally considered to be 8.0–10.0, and it was determined to be 8.4–9.7 in our experiments. That is to say, with addition of NaOH solution to colorless P solution, the solution turns light pink when the solution pH value reaches 8.4. The solution color gradually deepens with further addition of NaOH solution until the solution pH value reaches 9.7. When dilute hydrochloric acid is gradually added to this solution, the solution color gradually becomes lighter up to a pH value of 8.4 where the solution becomes colorless. However, when Gel 9 (originally colorless) is put into distilled water, and NaOH solution is added, the color does not change into light pink until the solution pH value reaches 11.9. With the further addition of NaOH solution, the gel color becomes deep. If HCl
172
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
Table 3 Reaction parameters of MO and P in the gels Sample
Response time (min)
Color change pH value r!y
y!r
5
60 4
6
30 2
7
90 7
8
60 5
11.5 0.03– 12.5 0.04 2.0 0.04– 2.5 0.02 9.5 0.02– 10.5 0.05 10.5 0.05– 11.5 0.03
3.5 0.03– 2.5 0.05 1.0 0.03– 0.5 0.02 3.5 0.04– 2.5 0.01 3.5 0.03– 2.5 0.04
9 10 11 12
<5 <5 <5 <5
MO
Leaching rate after seven days (%) P c!p
p!c
11.9 0.07 12.3 0.08 12.5 0.07 12.5 0.06
8.9 0.04 8.5 0.05 a
11.4 0.04
pH 1.0
pH 7.0
pH 13.0
1.3 0.01
1.6 0.01
6.7 0.03
<0.05
<0.05
0.26 0.01
32.4 0.16
42.3 0.14
34.8 0.18
1.4 0.01
1.7 0.01
3.4 0.02
3.1 0.01 6.4 0.02 12.8 0.06 2.4 0.01
6.8 0.03 3.6 0.02 15.6 0.08 2.3 0.01
1.3 0.01 2.5 0.01 33.6 0.12 12.1 0.06
r – red, y – yellow, c – colorless, p – pink. a It is difficult to determine because of leaching.
solution is added to this gel-containing solution, the gel color does not turn colorless even when the solution pH reaches 11.9. The gel turns colorless when the solution pH reaches 8.9. It seems that this situation is not due to the response time, but that it is an intrinsic property of the indicator within the gels, since definite indicating ranges for MO and P doped gels could not be obtained even after the gels were soaked for seven days. The color change pH values of MO and P within the gels are listed in Table 3. For MO-doped gels only the color change pH range is given, because the color does not change sharply with pH.
time. With reference to the TG curves, the Pdoped gels dried at 50 and 200 °C were compared for their acid–base response and their leaching behaviors. The results are listed in Table 4. For Gels 9 and 10, the color change pH values increase with increasing drying temperature. For Gel 9 dried at 200 °C, its leaching rate in solution at pH 13.0 increases from 1.3% to 13.3%. For Gel 11 dried at 200 °C, its leaching rate increases from 12.8% to 27.9% at pH 1.0 and from 15.6% to 27.8% at pH 7.0.
4. Discussion 3.4. Leaching behavior of the indicators within the gels The leaching rates of the indicator after soaking the gels for seven days are listed in the last column of Table 3. Note that leaching rate of MO shows minimum with CTAB (Gel 6) and maximum with SDS (Gel 7). For P-doped gels, maximum leaching occurs with SDS (Gel 11). 3.5. Effects of drying temperature on the P reactivity within the gels P-doped gels were used for the drying temperature experiments because of their short response
The above experimental results demonstrate that classical solution chemistry is transferable to the xerogel environment, and the chemical performance of the dopant can be modified by surface-active substances. Surfactants can form various micellar structures in sol–gel starting reaction mixture, depending on the solvent concentration [10,11]. After gelation, these micellar structures can be retained in the gels [9]. For CTAB micelles, the micelle surfaces are positively charged, and the negatively charged polysilicate species will aggregate near the micelle surfaces (the situation of Figs. 3(a), (b) and 4(e), (f)). For SDS micelles, the micelle surfaces are negatively
13.3 0.05 3.2 0.01 27.6 0.11 9.7 0.03 9.2 0.01
a
11.4 0.04
a
pH 7.0
6.8 0.03 3.6 0.02 15.6 0.08 2.3 0.01 3.1 0.01 6.4 0.02 12.8 0.06 2.4 0.01
pH 1.0 p!c
9.5 0.04 9.7 0.02
12.8 0.03 13.0 0.05 12.7 0.02 12.3 0.06 9 10 11 12
c – colorless, p – pink. a It is difficult to determine because of leaching.
pH 7.0
6.9 0.02 6.7 0.02 27.8 0.12 1.0 0.01 0.8 0.01 5.0 0.02 27.9 0.14 1.7 0.01 8.9 0.04 8.5 0.05
11.9 0.07 12.3 0.08 12.5 0.07 12.5 0.06
pH 1.0 c!p p!c c!p
1.3 0.01 2.5 0.01 33.6 0.12 12.1 0.06
Sample dried at 50 °C Sample dried at 50 °C
pH 13.0
Leaching rate after seven-day soaking (%)
Sample dried at 200 °C Color change pH value Sample
Table 4 Effects of drying temperature on the P reactivity within the gels
Sample dried at 200 °C
pH 13.0
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
173
charged. Between the negatively charged micelles and the negatively charged polysilicate species there exists the counter-charged ions (Naþ , Hþ , etc.) (the situation of Figs. 3(c), (d) and 4(g), (h)). In aqueous solutions, there exist electrostatic and hydrophobic interactions between MO and the surfactants [12,13]. Gehlen and his coworkers demonstrated that MO interacts strongly with the cationic micelles cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB), but does not interact with the anionic micelle sodium dodecyl sulphate (SDS) through the studies of absorption and emission spectroscopy [14]. Bunton and his co-workers have shown that aromatic compounds with sulphonic acid groups are incorporated into the stern layers of cationic micelles in a sandwich arrangement and suggested that a van der Waals interaction between adjacent surfactant chains and the dye organic moiety (hydrophobic forces), causes alteration of the chromophore microenvironment [15,16]. For our gel products, there are electrostatic interactions, hydrogen bonds and van der Waals forces between the indicators and SiO2 gel matrix. Between the indicators and the surface-active substances there are electrostatic interactions, hydrogen bonds, van der Waals forces and hydrophobic interactions. The color change pH values of Gel 6 (with CTAB) from red to yellow or yellow to red are all lower than those of Gels 5, 7 and 8 (Table 3). This can be explained as follows. MO in Fig. 3(a) and (c) is in acidic form (red), and MO in Fig. 3(b) and (d) is in basic form (yellow). In Fig. 3(b), the basic form of MO is oriented fully within the surfactant micellar environment, because the negatively charged sulfonate groups of MO associate with the positively charged quaternary ammonium groups of the CTAB, whereas the non-charged phenylamine residue dissolves in the hydrophobic zone. Then, with decreasing pH the azophenylamine moiety becomes positively charged and hydrophilic, resulting in its expulsion from the hydrophobic region to the hydrophilic surroundings of the micellar phase. Because of the electrostatic attraction and hydrophobic interaction between the basic form of MO and the quaternary ammonium groups of the CTAB, a higher
174
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
Fig. 3. Schematic diagrams of the acid–base response of MO in the gels (a, b) Gel 6 (with CTAB) and (c, d) Gel 7 (with SDS).
concentration of protons is required to reach the azo functional group. The pH needed for basic form of MO to become acidic form is shifted to a lower value. The leaching rates of Gel 6 are very low. The leaching rates of Gel 7 (with SDS) are higher, especially in the solutions of pH 1.0 and 7.0. These results are probably caused by three factors: (1) For Gel 6 (with CTAB), cationic surfactant aggregates combine with the anionic silicate species to form a supramolecular structure [9]. The interaction between cationic surfactant aggregates and anionic silicate species are relatively strong so that MO within the gels is difficult to move. For Gel 7 (with SDS), both anionic surfactant aggregates and silicate species are negatively charged. They must be mediated by the positively charged counter ions, which must be present in stoichiometric amount. This suggests
that the combination between anionic surfactant aggregates and silicate species are relatively weak. Thus, MO within the SDS gels can easily move. (2) The solubility of SDS in water is much higher than that of CTAB. The leaching of MO in SDS gels is probably caused partly by the leaching of SDS from the gels. (3) The association between MO and SDS is weaker than that between MO and CTAB. The acid–base response of P within the gels is schematically represented in Fig. 4. P in Fig. 4(e) and (g) is in acidic form (colorless), and P in Fig. 4(f) and (h) is basic form (pink). The experimental results on the P-doped gels show that the leaching rate of Gel 11 (with SDS) is much higher than that of Gels 9, 10 and 12, especially in basic solution. There exists a strong electrostatic repulsion between the basic form of P and anionic surfactant aggregates (Fig. 4(h)). The hydrophobic interac-
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
175
Fig. 4. Schematic diagrams of the acid–base response of P in the gels. (e, f) Gel 10 (with CTAB), (g, h) Gel 11 (with SDS).
tion between the acidic form of P and SDS is very weak because of the short carbon chain length of SDS (Fig. 4(g)). Indicator reactivity in gels without surfaceactive substances shows some similarities with that in gels modified with stearic acid (SA). This may be partly because that SA does not form micelle effectively in the sols and the gels, which may be deduced from the FTIR data. The FTIR spectra of the gels modified with SA have an absorption assignable to C@O stretch of carboxylic acid dimer, which suggests that SA does not form micelle effectively in the sols and the gels.
5. Conclusions (1) Surface-active substances have obvious effects on the acid–base response and leach-
ing behavior of acid–base indicators within the SiO2 gels. This is probably caused by the modification of SiO2 gel structures by the surface-active substances, the electrostatic interactions and hydrophobic interactions between the indicators and the surface-active substances. (2) The response time of P-doped gels (<5 min) is shorter than that of MO-doped gels (>30 min). (3) There are no definite indicating ranges for MO and P within the gels. Only two color change pH values can be determined for either MO or P within the gels. It is demonstrated that this is not caused by response time. (4) The leaching rates of MO and P within the gels modified with SDS are obviously higher than that of other gels. The leaching rates of MO within the gels modified with CTAB are the lowest among those of all the gels.
176
Z. Wu et al. / Journal of Non-Crystalline Solids 320 (2003) 168–176
Acknowledgements The authors thank the financial assistance from Research Foundation of Overseas Chinese Affairs Office, State Council, P.R. China, and Fujian Provincial Science and Technology Creation Foundation for Young Researcher (2001J023), P.R. China. Z.W. thanks the postdoctoral fellowship from Yonsei NT center at Yonsei University, Korea.
References [1] C. Rottman, G. Grader, Y.D. Hazan, S. Melchior, D. Avnir, J. Am. Chem. Soc. 121 (1999) 8533. [2] D.T. Kanti, I. Khan, D.L. Rousseau, J.M. Friedman, J. Am. Chem. Soc. 120 (1998) 10268. [3] T.K. Jain, I. Roy, T.K. De, A. Maitra, J. Am. Chem. Soc. 120 (1998) 11092.
[4] J. Blum, D. Avnir, H. Schumann, Chemtech 29 (1999) 32. [5] H. Schumann, M. Hasan, F. Gelman, D. Avnir, J. Blum, Inorg. Chim. Acta 280 (1998) 21. [6] R. Shacham, D. Avnir, D. Mandler, Adv. Mater. 11 (1999) 1544. [7] B. Dunn, J. Zink, Chem. Mater. 9 (1997) 2280. [8] B. Karmakar, G. De, D. Ganguli, J. Non-Cryst. Solids 272 (2000) 119. [9] A. Corma, Chem. Rev. 97 (1997) 2373. [10] M.T. Anderson, J.E. Martin, J.G. Odinek, P.P. Newcomer, Chem. Mater. 10 (1998) 1490. [11] K. Matsui, T. Nakazawa, H. Morisaki, J. Phys. Chem. 95 (1991) 976. [12] K.K. Karukstis, D.A. Savin, C.T. Loftus, N.D. DÕAngelo, J. Colloid Interface Sci. 203 (1998) 157. [13] R.K. Dutta, D.N. Bhat, Colloids Surf. A 106 (1996) 127. [14] M.H. Gehlen, M. Ferreira, M.G. Neumann, J. Photochem. Photobiol. A 87 (1995) 55. [15] C.A. Bunton, M.J. Minch, J. Hidalgo, L. Sepulveda, J. Am. Chem. Soc. 95 (1973) 3262. [16] C.A. Bunton, M.J. Minch, J. Phys. Chem. 87 (1974) 1490.