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Influence of aminosilane surface modification and dyes adsorption on zeta potential of spherical silica particles formed in emulsion system
Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 87 /94 www.elsevier.com/locate/colsurfa
Influence of aminosilane surface modification and dyes adsorption on zeta potential of spherical silica particles formed in emulsion system Teofil Jesionowski * Institute of Chemical Technology and Engineering, Poznan University of Technology, Pl. M. Sklodowskiej-Curie 2, 60-965 Poznan, Poland
Abstract Studies were performed on the synthesis of spherical silica in an emulsion system. The obtained silica was subjected to surface modification using N -2-(aminoethyl)-3-aminopropyltrimethoxysilane in order to increase adsorptive properties of the silica surface. On surfaces of the unmodified silica material and the aminosilane-functionalised silica C.I. Reactive Blue 19 and C.I. Acid Red 18 were adsorbed. The obtained silica and its derivatives were subjected to physicochemical evaluation: particle size and polydispersity were examined, specific surface area (BET) was determined, surface composition was defined using elemental analysis. The principal aim of the studies was to define electrokinetic (zeta) potential and determination of its alterations as a function of pH in all types of the tested silica colloids. For the purpose the technique of electrophoretic light scattering (ELS) was applied. Effect of ionic strength on stability of tested colloids was also examined. The tested aminosilane was found to exert critical influence on values of zeta potential and, thus, on values of isoelectric point (i.e.p.). The modified silicas reached peak stability within the pH range of 2 /4 manifesting zeta potential within the range of ( /20) to ( /45) mV. # 2003 Elsevier B.V. All rights reserved. Keywords: Zeta potential; Silica, surface modification; Amino-silane; Organic dye
1. Introduction Familiarity with electrochemical properties of colloids has proven very important for various branches of science and for multiple technological processes including in particular adsorptive processes.
* Tel.: /48-61-6653-720; fax: /48-61-6653-649. E-mail address: [email protected] Jesionowski).
(T.
The most important parameter which defines surface properties of solids in aqueous solutions is zeta potential (z). The electrokinetic (zeta) potential has been described in detail in multiple most valuable scientific monographs [1 /8]. Detailed knowledge of colloid characteristics, i.e. of its surface potential, structure of its electric double layer (e.d.l.) or of its isoelectric point (i.e.p.) as well as the data on particle size and shape, homogeneity and surface chemistry allow to understand colloid behaviour in several specific
0927-7757/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-7757(03)00237-1
88
T. Jesionowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 87 /94
complex systems, including polymer dispersions, inorganic-organic hybrids, etc. Determination of electrokinetic properties of particle suspensions in solutions takes advantage of methods and investigation techniques such as electrophoresis, electroacoustics, streaming and sedimentation potential determination, electroosmosis [1,2,5,6,9 /16]. Multiple different substances, both of natural and synthetic origin, are subjected to comprehensive testing. Several literature reports are available on determination of zeta potential or i.e.p. of minerals, mainly of montmorillonite [17] and kaolin [18,19] due to their specific structure and the potential for cation exchange, which exert significant effect of electrokinetic properties. Effect of silanes on stability of studied colloids was also examined [18]. The other tested substances included mica [20], pyrite and arsenopyrite [21], semectites [22] or quarz [23]. Alterations of i.e.p. due to electrolyte adsorption and evaluation of stability of synthetic titanium, aluminium, zirconium and other colloids were also thoroughly described in the literature [24 /30]. Among synthetic colloids the highest investigative and application significance is shown by synthetic silica. Due to the presence of very reactive surface silanol groups ( /Si /OH) its surface is capable of adsorption (or chemisorption) of both organic and inorganic compounds. For example, the following reactions may take place on the SiO2 surface in the medium of various type electrolytes:
SiOH
SiOHH 2 X
SiOH X SiOH H
(1) (2)
SiOH
SiOHH An 2 An X
SiOHCt X SiO Ct H
(3) (4)
The above reactions (1 /4) explain changes in surface charge and in zeta potential in the medium of electrolytes during change in pH. Colloidal silica demonstrates a variable structure (surface chemistry and morphology) as an effect of a preparation procedure. The so called fumed silica belongs to most widely used silicas. It originates from hydroxyl hydrolysis processes of organic silica compounds at a very high temperature and it consists of particles of a relatively small grain size and of irregular structure. Electrochemical
properties of the silica were widely tested and explained in various reports [31 /34]. Suhara and co-workers [35,36] explained in addition effect of quaternary ammonium compounds on reactive potential of amorphic silica and alterations induced by addition of various electrolytes. In other reports [37 /40] zeta potential and alterations of i.e.p. were determined for silica which manifested a spherical particle shape, very high monodisperse character and a slightly higher particle diameter (mostly 0.2 /2 mm) and which was obtained in the so called Sto¨ber’s technique or in silica/large molecular weight organic compound dispersive systems to establish, i.a., their stability [41 /44]. In studies performed till now, formation of monodisperse spherical silicas was investigated from sodium metasilicate solutions in emulsion systems, i.e. by an alternative technique to the Sto¨ber’s approach [45 /47]. Conditions of emulsion formation and of precipitation process were also established. Physicochemical parameters of such precipitated and alkoxysilane-modified silicas were also comprehensively examined [48,49]. The so prepared spherical silicas find broad application in production of polymer composites [50] or in composing new pigments by adsorption of organic dyes on their surface [51]. In present study, synthesis of monodisperse spherical silica was conducted, employing precipitation in an emulsion system. The silica was subjected to surface modification using aminosilane. Selected dyes were also adsorbed in order to alter surface chemistry of the obtained hybrid systems. The procedures permitted to alter surface character and provided a model for studies on alteration of zeta potential as a function of pH in the formed colloid systems.
2. Experimental 2.1. Silica support preparation The silica was synthesised by precipitation from aqueous solutions of sodium metasilicate and hydrochloric acid in an emulsion system [48,51]. The formation process of spherical colloidal silica is schematically presented in Fig. 1.
T. Jesionowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 87 /94
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The solutions of the appropriate dye (2 mg cm 3) were introduced to the silica-containing reactor. 2.2. Physicochemical evaluation
Fig. 1. Schematically procedure of spherical silica precipitation from emulsion systems.
Surface modification of the obtained silica was performed using N -2-(aminoethyl)-3-aminopropyltrimethoxysilane-U-15D-(three or five weight parts by mass of SiO2), pre-hydrolysed in a solution (methanol/water, 4:1, v/v) prepared directly before the modification in order to avoid ageing effects. The modification was performed in a specially designed reactor [52] in the course of 1 h and the solvent was distilled off. Adsorption of the organic dyes, purchased from Boruta-Kolor, of the formula presented below, was conducted on the silica surface modified with three weight parts of aminosilane and, for comparison, on the unmodified silica surface.
The unmodified and the functionalised silica samples were characterised by quantitive elemental analysis (C, H, N, S analysis), BET measurements, and by SEM or DLS techniques. Chemical constitutions and physical properties of the organically treated silica powders are listed in Table 1. Aqueous electrophoretic data were obtained using a ZetaPlus Brookhaven Instrument. The zeta potential (z) was calculated from the electrophoretic mobility. The electrokinetic potential of amorphous silicas was determined at the ionic strength of 0.001 and 0.01 mol dm 3 of NaCl. The pH of solutions was adjusted with NaOH or HCl. Mili-Q water was applied in each experiment.
3. Results and discussion Studies on zeta potential, effect of pH and of ionic strength allowed to obtain most valuable data on surface chemistry and, indirectly, on stability of colloidal silica dispersions and their derivatives. The relation between zeta potential and pH for the unmodified silica, silica modified with N -2(aminoethyl)-3-aminopropyltrimethoxysilane (three or five weight parts per 100 weight parts of SiO2), and obtained pigments is illustrated in Fig. 2. In this case, concentration of the applied electrolyte amounted to 0.001 mol dm 3.
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Results of electrokinetic studies reflected the acid-base alterations taking place due to surface modification of SiO2 using aminosilane. The i.e.p., which is a quantitative reflection of silica surface acidity or alkalinity, amounted to 1.70 for the unmodified silica. The relatively low value of i.e.p. for the silica resulted from the very high content of carbon in its structure (see Table 1). The content
H2 N(CH2 )2 NH(CH2 )3 Si(OCH3 )3
3H2 O
0
3CH3 OH
H2 N(CH2 )2 NH(CH2 )3 Si(OH)3 (5)
which is followed by condensation of the silane with SiO2 surface:
(6)
stemmed from the technology of the silica formation by precipitation in an emulsion system. As expected, the silica surface modification with U15D silane clearly shifted i.e.p. toward higher values of pH. Following modification with three weight parts of the silane, the i.e.p. was 6.10 and following modification with five weight parts of the silane it amounted to 6.75. According to Jacobasch [53], dissociation of /NH2 groups plays a significant role in alteration of solid surface charge, i.a., in surface charge of SiO2. Surface modification of precipitated silica follows the aminosilane hydrolysis reaction:
In cases of high H ion density, induction of NH3 takes place, with the resulting positive surface charge of the modified silica. Increasing H ion concentration restricts the dissociation and the surface charge decreases. Adsorption of organic pigment on the surface of modified or unmodified silica evidently shifted i.e.p. value to acidic pH. Definitely more extensive changes in the parameter were noted for the silanemodified silica. The mechanism of interaction between aminosilane-modified silica and C.I. Reactive Blue 19 pigment may acquire the following form (atomic bonds are formed between the modified silica surface and functional groups of the pigment):
(7)
0.00 0.00 0.00 0.15 0.00 0.28 0.00 0.35 0.55 0.54 0.10 0.49 2.04 2.90 2.96 2.47 2.16 2.52 SiO2-K7-HCl SiO2-K7-HCl SiO2-K7-HCl SiO2-K7-HCl SiO2-K7-HCl SiO2-K7-HCl
/ 3 w/w U-15D / 5 w/w U-15D / 3 w/w U-15D/C.I. Reactive Blue 19 / C.I. Reactive Blue 19 / 3 w/w U-15D/C.I. Acid Red 18
Spherical Spherical Spherical Spherical Spherical Spherical
0.059 0.005 0.005 0.005 0.023 0.005 792 507 442 395 489 385 182 119 208 96 174 113
10.21 14.75 15.76 15.81 10.41 15.09
S N H C Particle size (nm) Particle shape Polydispersity Specific surface area (m2 g 1) Silica powder
Table 1 The physicochemical characteristics of the silica materials applied in this work
Surface composition (%)
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The suggested mechanism (7) explained changes in the studied i.e.p. as a function of pH values. Alterations in zeta potential as a function of pH are also presented in Fig. 2, with particular attention given to the modified silica pigmented with C.I. Acid Red 18. Effect of the acidic pigment was also evident. In this case, i.e.p. was shifted toward lower pH values (as it happened following adsorption of the blue dye). Its value changed from 6.10 to 5.50. Zeta potentials for analogous samples at the ionic strength equal 0.01 mol dm 3 NaCl are presented in Fig. 3. Small alterations were observed in values of the i.e.p. It amounted to 1.30 for unmodified silica, 5.0 for silica modified with three weight parts of aminosilane and 7.1 for silica modified with five weight parts of the silane, respectively. Effect of ionic strength was definitely more pronounced in the case of silica modified with N -2-(aminoethyl)-3-aminopropyltrimethoxysilane than in the case of silica pigmented with C.I. Acid Red 18. This probably resulted from a strong electrostatic interaction in the studied dispersive system:
ð8Þ More pronounced alterations in i.e.p.s were typical for silica pigmented with C.I. Reactive Blue 19. Slightly lower differences were noted for the silica pigmented with the acidic agent, C.I. Acid Red 18. The differences probably resulted from chemical structure of the applied pigments and certainly from mechanism of interaction of the pigments with the modified silica surface. Covalent interactions (7) are much stronger than electrostatic ones (8). The almost identical course of the relation between zeta potential and pH was observed in the course of measurements performed at the ionic strength of 0.01 mol dm 3 NaCl for the silica activated with either C.I. Reactive Blue 19 or C.I. Acid Red 18 (Fig. 3).
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Fig. 2. Zeta potential of the unmodified silica, silica modified with three and five weight parts by mass of aminosilane, and obtained inorganic-organic hybrids as a function of pH (ionic strength equal 0.001 mol dm 3 of NaCl).
It should also be mentioned that aminosilanemodified silica and the silica with the adsorbed pigments reached peak stability within the acidic
range of pH (pH 2/4). In such cases, z potential acquired values within the range of ( /20) to ( /45) mV.
Fig. 3. Zeta potential of the unmodified silica, silica modified with three and five weight parts by mass of aminosilane, and obtained inorganic-organic hybrids as a function of pH (ionic strength equal 0.01 mol dm 3 of NaCl).
T. Jesionowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 87 /94
4. Summary 1) Modification of silica surface with N -2-(aminoethyl)-3-aminopropyltrimethoxysilane induced a clear shift in i.e.p. toward high pH values. The shift reflected the amount of aminosilane used for modification. 2) Slight differences in i.e.p. of modified silicas were observed when the parameter was measured at various ionic strengths. 3) Adsorption of organic pigments on modified (particularly aminosilane-modified) and unmodified silica surfaces clearly shifted i.e.p. toward acidic pH. 4) The type of interaction (atomic bond or electrostatic interaction) on the surface of the modified silica and following adsorption of pigments significantly influenced i.e.p. More evident alterations were observed when the interaction involved formation of a covalent bond (e.g. following adsorption of C.I. Reactive Blue 19 pigment).
Acknowledgements This work was supported by the Poznan University of Technology Research Grant No. BW 32/ 008/2002.
References [1] R.K. Iler, The Chemistry of Silica, Chichester, 1979. [2] R.J. Hunter, Zeta Potential in Colloid Science, Academic Press, London, 1981. [3] A.P. Legrand, The Surface Properties of Silicas, Wiley, Chichester, 1998. [4] R.A. Williams, Colloid and Surface Engineering: Application in the Process Industries, Butterworth Heinemann, Oxford, 1992. [5] R.J. Hunter, Introduction to Modern Colloid Science, Oxford University Press, London, 1993. [6] R.J. Hunter, Fundations of Colloid Science, second ed, Oxford University Press, New York, 2001. [7] E. Papirer, Adsorption on Silica Surface, Marcel Dekker, New York, 2000. [8] M. Kosmulski, Chemical Properties of Material Surfaces, Marcel Dekker, New York, 2001. [9] R.J. Hunter, Colloids Surf. A 141 (1998) 37.
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[10] S.G. Malghan, Electroacoustics for Characterization of Particulates and Suspensions, NIST Special Publication No 856, National Institute of Standards and Technology, Gaithersburg, MD, 1992. [11] B. Chu, Laser Light Scattering: Basic principles and practice, Academic Press, New York, 1991. [12] N. Buske, H. Gedan, W. Katz, H. Lichtenfeld, H. Sonntag, Colloid Polym. Sci. 258 (1980) 1303. [13] A.W. Adamson, A.P. Gast, Physical Chemistry of Surface, sixth ed, Wiley, New York, 1997. [14] G.H. Bolt, Soil Chemistry, Elsevier, Amsterdam, 1979. [15] J. Lyklema, Fundamentals of Interface and Colloid Science, vol. 1, Academic Press, London, 1991. [16] J. Lyklema, Fundamentals of Interface and Colloid Science, vol. 2, Academic Press, London, 1995. [17] K. Ma, A.C. Pierre, Colloids Surf. A 155 (1999) 359. [18] B. Braggs, D. Fornasiero, J. Ralston, R.S. Amart, Clays Clay Miner. 42 (1994) 123. [19] S.B. Johnson, D.R. Dixon, P.J. Scales, Colloids Surf. A 146 (1999) 281. [20] P.G. Hartley, I. Larson, P.J. Scales, Langmuir 13 (1997) 2207. [21] A.A. Sirkeci, Miner. Eng. 13 (2000) 1037. [22] F. Thomas, L.J. Michot, D. Vantelon, E. Montarges, B. Prelot, M. Cruchaudet, J.F. Delon, Colloids Surf. A 159 (1999) 351. [23] M. Kosmulski, J. Colloid Interf. Sci. 250 (2002) 99. [24] D. Nei Furlong, Surface chemistry of silica coatings titania, in: H. Bergna (Ed.), The Colloid Chemistry of Silica, Advances in Chemistry Series No 234, ACS, 1994. [25] E. Chibowski, L. Holysz, Colloids Surf. A 105 (1995) 211. [26] M. Rasmusson, S. Wall, Colloids Surf. A 122 (1997) 169. [27] C.-W. Won, B. Siffert, Colloids Surf. A 131 (1998) 161. [28] W. Janusz, E. Skwarek, A. Galgan, Polish J. Chem. 76 (2002) 745. [29] L. Machbaur, P.-C. Nkinamubanzi, A. Nonat, J.-C. Mutin, J. Colloid Interf. Sci. 202 (1998) 261. [30] M. Kosmulski, J.B. Rosenholm, J. Colloid Interf. Sci. 248 (2002) 30. [31] V.M. Gun’ko, V.I. Zarko, R. Leboda, E. Chibowski, Adv. Colloid Interf. Sci. 91 (2001) 1. [32] S. Spange, A. Reuter, S. Prause, C. Bellmann, J. Adhesion Sci. Technol. 14 (2000) 399. [33] M. Kosmulski, P. Eriksson, J. Gustafsson, J.B. Rosenholm, Radiochim. Acta 88 (2000) 701. [34] M. Kosmulski, J. Hartikainen, E. Maczka, W. Janusz, J.B. Rosenholm, Anal. Chem. 74 (2002) 253. [35] T. Suhara, T. Kanemaru, H. Fukui, M. Yamaguchi, Colloids Surf. A 95 (1995) 1. [36] T. Suhara, H. Fukui, M. Yamaguchi, Colloids Surf. A 101 (1995) 29. [37] J. .Sˇkvarla, Colloids Surf. A 110 (1996) 135. [38] M. Kosmulski, P. Eriksson, C. Brancewicz, J.B. Rosenholm, Colloids Surf. A 162 (2000) 37. [39] P. Pendleton, J. Colloid Interf. Sci. 227 (2000) 227. [40] C. Walldal, Colloids Surf. A 194 (2001) 111.
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[41] M.J. Percy, C. Barthet, J.C. Lobb, M.A. Khan, S.F. Lascelles, M. Vamvakaki, S.P. Armes, Langmuir 16 (2000) 6913. [42] T. Okubo, M. Suda, J. Colloid Interf. Sci. 213 (1999) 565. [43] D. Bauer, H. Buchhammer, A. Fuchs, W. Jaeger, E. Killmann, K. Lunkowitz, R. Rehmet, S. Schwarz, Colloids Surf. A 156 (1999) 291. [44] S. Schwarz, K. Lunkowitz, B. Keßler, U. Spiegler, E. Killmann, W. Jaeger, Colloids Surf. A 163 (2000) 17. [45] T. Jesionowski, J. Dispersion Sci. Technol. 22 (2001) 363. [46] T. Jesionowski, Colloids Surf. A 190 (2001) 153.
[47] T. Jesionowski, Powder Technol. 127 (2002) 56. [48] T. Jesionowski, J. Mater. Sci. 37 (2002) 5275. [49] T. Jesionowski, Physicochem. Problems Miner. Proc. 36 (2002) 243. [50] T. Jesionowski, K. Bula, J. Janiszewski, J. Jurga, Composite Interf. 10 (2003) 1 /18. [51] T. Jesionowski, Dyes Pigments 55 (2002) 133. [52] L. Domka, A. Krysztafkiewicz, W. Krysztafkiewicz, Polish Patent 115 671, 1982. [53] H.J. Jacobasch, Prog. Org. Coat. 17 (1989) 115.
Influence of aminosilane surface modification and dyes adsorption on zeta potential of spherical silica particles formed in emulsion system Teofil Jesionowski * Institute of Chemical Technology and Engineering, Poznan University of Technology, Pl. M. Sklodowskiej-Curie 2, 60-965 Poznan, Poland
Abstract Studies were performed on the synthesis of spherical silica in an emulsion system. The obtained silica was subjected to surface modification using N -2-(aminoethyl)-3-aminopropyltrimethoxysilane in order to increase adsorptive properties of the silica surface. On surfaces of the unmodified silica material and the aminosilane-functionalised silica C.I. Reactive Blue 19 and C.I. Acid Red 18 were adsorbed. The obtained silica and its derivatives were subjected to physicochemical evaluation: particle size and polydispersity were examined, specific surface area (BET) was determined, surface composition was defined using elemental analysis. The principal aim of the studies was to define electrokinetic (zeta) potential and determination of its alterations as a function of pH in all types of the tested silica colloids. For the purpose the technique of electrophoretic light scattering (ELS) was applied. Effect of ionic strength on stability of tested colloids was also examined. The tested aminosilane was found to exert critical influence on values of zeta potential and, thus, on values of isoelectric point (i.e.p.). The modified silicas reached peak stability within the pH range of 2 /4 manifesting zeta potential within the range of ( /20) to ( /45) mV. # 2003 Elsevier B.V. All rights reserved. Keywords: Zeta potential; Silica, surface modification; Amino-silane; Organic dye
1. Introduction Familiarity with electrochemical properties of colloids has proven very important for various branches of science and for multiple technological processes including in particular adsorptive processes.
* Tel.: /48-61-6653-720; fax: /48-61-6653-649. E-mail address: [email protected] Jesionowski).
(T.
The most important parameter which defines surface properties of solids in aqueous solutions is zeta potential (z). The electrokinetic (zeta) potential has been described in detail in multiple most valuable scientific monographs [1 /8]. Detailed knowledge of colloid characteristics, i.e. of its surface potential, structure of its electric double layer (e.d.l.) or of its isoelectric point (i.e.p.) as well as the data on particle size and shape, homogeneity and surface chemistry allow to understand colloid behaviour in several specific
0927-7757/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-7757(03)00237-1
88
T. Jesionowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 87 /94
complex systems, including polymer dispersions, inorganic-organic hybrids, etc. Determination of electrokinetic properties of particle suspensions in solutions takes advantage of methods and investigation techniques such as electrophoresis, electroacoustics, streaming and sedimentation potential determination, electroosmosis [1,2,5,6,9 /16]. Multiple different substances, both of natural and synthetic origin, are subjected to comprehensive testing. Several literature reports are available on determination of zeta potential or i.e.p. of minerals, mainly of montmorillonite [17] and kaolin [18,19] due to their specific structure and the potential for cation exchange, which exert significant effect of electrokinetic properties. Effect of silanes on stability of studied colloids was also examined [18]. The other tested substances included mica [20], pyrite and arsenopyrite [21], semectites [22] or quarz [23]. Alterations of i.e.p. due to electrolyte adsorption and evaluation of stability of synthetic titanium, aluminium, zirconium and other colloids were also thoroughly described in the literature [24 /30]. Among synthetic colloids the highest investigative and application significance is shown by synthetic silica. Due to the presence of very reactive surface silanol groups ( /Si /OH) its surface is capable of adsorption (or chemisorption) of both organic and inorganic compounds. For example, the following reactions may take place on the SiO2 surface in the medium of various type electrolytes:
SiOH
SiOHH 2 X
SiOH X SiOH H
(1) (2)
SiOH
SiOHH An 2 An X
SiOHCt X SiO Ct H
(3) (4)
The above reactions (1 /4) explain changes in surface charge and in zeta potential in the medium of electrolytes during change in pH. Colloidal silica demonstrates a variable structure (surface chemistry and morphology) as an effect of a preparation procedure. The so called fumed silica belongs to most widely used silicas. It originates from hydroxyl hydrolysis processes of organic silica compounds at a very high temperature and it consists of particles of a relatively small grain size and of irregular structure. Electrochemical
properties of the silica were widely tested and explained in various reports [31 /34]. Suhara and co-workers [35,36] explained in addition effect of quaternary ammonium compounds on reactive potential of amorphic silica and alterations induced by addition of various electrolytes. In other reports [37 /40] zeta potential and alterations of i.e.p. were determined for silica which manifested a spherical particle shape, very high monodisperse character and a slightly higher particle diameter (mostly 0.2 /2 mm) and which was obtained in the so called Sto¨ber’s technique or in silica/large molecular weight organic compound dispersive systems to establish, i.a., their stability [41 /44]. In studies performed till now, formation of monodisperse spherical silicas was investigated from sodium metasilicate solutions in emulsion systems, i.e. by an alternative technique to the Sto¨ber’s approach [45 /47]. Conditions of emulsion formation and of precipitation process were also established. Physicochemical parameters of such precipitated and alkoxysilane-modified silicas were also comprehensively examined [48,49]. The so prepared spherical silicas find broad application in production of polymer composites [50] or in composing new pigments by adsorption of organic dyes on their surface [51]. In present study, synthesis of monodisperse spherical silica was conducted, employing precipitation in an emulsion system. The silica was subjected to surface modification using aminosilane. Selected dyes were also adsorbed in order to alter surface chemistry of the obtained hybrid systems. The procedures permitted to alter surface character and provided a model for studies on alteration of zeta potential as a function of pH in the formed colloid systems.
2. Experimental 2.1. Silica support preparation The silica was synthesised by precipitation from aqueous solutions of sodium metasilicate and hydrochloric acid in an emulsion system [48,51]. The formation process of spherical colloidal silica is schematically presented in Fig. 1.
T. Jesionowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 87 /94
89
The solutions of the appropriate dye (2 mg cm 3) were introduced to the silica-containing reactor. 2.2. Physicochemical evaluation
Fig. 1. Schematically procedure of spherical silica precipitation from emulsion systems.
Surface modification of the obtained silica was performed using N -2-(aminoethyl)-3-aminopropyltrimethoxysilane-U-15D-(three or five weight parts by mass of SiO2), pre-hydrolysed in a solution (methanol/water, 4:1, v/v) prepared directly before the modification in order to avoid ageing effects. The modification was performed in a specially designed reactor [52] in the course of 1 h and the solvent was distilled off. Adsorption of the organic dyes, purchased from Boruta-Kolor, of the formula presented below, was conducted on the silica surface modified with three weight parts of aminosilane and, for comparison, on the unmodified silica surface.
The unmodified and the functionalised silica samples were characterised by quantitive elemental analysis (C, H, N, S analysis), BET measurements, and by SEM or DLS techniques. Chemical constitutions and physical properties of the organically treated silica powders are listed in Table 1. Aqueous electrophoretic data were obtained using a ZetaPlus Brookhaven Instrument. The zeta potential (z) was calculated from the electrophoretic mobility. The electrokinetic potential of amorphous silicas was determined at the ionic strength of 0.001 and 0.01 mol dm 3 of NaCl. The pH of solutions was adjusted with NaOH or HCl. Mili-Q water was applied in each experiment.
3. Results and discussion Studies on zeta potential, effect of pH and of ionic strength allowed to obtain most valuable data on surface chemistry and, indirectly, on stability of colloidal silica dispersions and their derivatives. The relation between zeta potential and pH for the unmodified silica, silica modified with N -2(aminoethyl)-3-aminopropyltrimethoxysilane (three or five weight parts per 100 weight parts of SiO2), and obtained pigments is illustrated in Fig. 2. In this case, concentration of the applied electrolyte amounted to 0.001 mol dm 3.
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T. Jesionowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 87 /94
Results of electrokinetic studies reflected the acid-base alterations taking place due to surface modification of SiO2 using aminosilane. The i.e.p., which is a quantitative reflection of silica surface acidity or alkalinity, amounted to 1.70 for the unmodified silica. The relatively low value of i.e.p. for the silica resulted from the very high content of carbon in its structure (see Table 1). The content
H2 N(CH2 )2 NH(CH2 )3 Si(OCH3 )3
3H2 O
0
3CH3 OH
H2 N(CH2 )2 NH(CH2 )3 Si(OH)3 (5)
which is followed by condensation of the silane with SiO2 surface:
(6)
stemmed from the technology of the silica formation by precipitation in an emulsion system. As expected, the silica surface modification with U15D silane clearly shifted i.e.p. toward higher values of pH. Following modification with three weight parts of the silane, the i.e.p. was 6.10 and following modification with five weight parts of the silane it amounted to 6.75. According to Jacobasch [53], dissociation of /NH2 groups plays a significant role in alteration of solid surface charge, i.a., in surface charge of SiO2. Surface modification of precipitated silica follows the aminosilane hydrolysis reaction:
In cases of high H ion density, induction of NH3 takes place, with the resulting positive surface charge of the modified silica. Increasing H ion concentration restricts the dissociation and the surface charge decreases. Adsorption of organic pigment on the surface of modified or unmodified silica evidently shifted i.e.p. value to acidic pH. Definitely more extensive changes in the parameter were noted for the silanemodified silica. The mechanism of interaction between aminosilane-modified silica and C.I. Reactive Blue 19 pigment may acquire the following form (atomic bonds are formed between the modified silica surface and functional groups of the pigment):
(7)
0.00 0.00 0.00 0.15 0.00 0.28 0.00 0.35 0.55 0.54 0.10 0.49 2.04 2.90 2.96 2.47 2.16 2.52 SiO2-K7-HCl SiO2-K7-HCl SiO2-K7-HCl SiO2-K7-HCl SiO2-K7-HCl SiO2-K7-HCl
/ 3 w/w U-15D / 5 w/w U-15D / 3 w/w U-15D/C.I. Reactive Blue 19 / C.I. Reactive Blue 19 / 3 w/w U-15D/C.I. Acid Red 18
Spherical Spherical Spherical Spherical Spherical Spherical
0.059 0.005 0.005 0.005 0.023 0.005 792 507 442 395 489 385 182 119 208 96 174 113
10.21 14.75 15.76 15.81 10.41 15.09
S N H C Particle size (nm) Particle shape Polydispersity Specific surface area (m2 g 1) Silica powder
Table 1 The physicochemical characteristics of the silica materials applied in this work
Surface composition (%)
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The suggested mechanism (7) explained changes in the studied i.e.p. as a function of pH values. Alterations in zeta potential as a function of pH are also presented in Fig. 2, with particular attention given to the modified silica pigmented with C.I. Acid Red 18. Effect of the acidic pigment was also evident. In this case, i.e.p. was shifted toward lower pH values (as it happened following adsorption of the blue dye). Its value changed from 6.10 to 5.50. Zeta potentials for analogous samples at the ionic strength equal 0.01 mol dm 3 NaCl are presented in Fig. 3. Small alterations were observed in values of the i.e.p. It amounted to 1.30 for unmodified silica, 5.0 for silica modified with three weight parts of aminosilane and 7.1 for silica modified with five weight parts of the silane, respectively. Effect of ionic strength was definitely more pronounced in the case of silica modified with N -2-(aminoethyl)-3-aminopropyltrimethoxysilane than in the case of silica pigmented with C.I. Acid Red 18. This probably resulted from a strong electrostatic interaction in the studied dispersive system:
ð8Þ More pronounced alterations in i.e.p.s were typical for silica pigmented with C.I. Reactive Blue 19. Slightly lower differences were noted for the silica pigmented with the acidic agent, C.I. Acid Red 18. The differences probably resulted from chemical structure of the applied pigments and certainly from mechanism of interaction of the pigments with the modified silica surface. Covalent interactions (7) are much stronger than electrostatic ones (8). The almost identical course of the relation between zeta potential and pH was observed in the course of measurements performed at the ionic strength of 0.01 mol dm 3 NaCl for the silica activated with either C.I. Reactive Blue 19 or C.I. Acid Red 18 (Fig. 3).
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Fig. 2. Zeta potential of the unmodified silica, silica modified with three and five weight parts by mass of aminosilane, and obtained inorganic-organic hybrids as a function of pH (ionic strength equal 0.001 mol dm 3 of NaCl).
It should also be mentioned that aminosilanemodified silica and the silica with the adsorbed pigments reached peak stability within the acidic
range of pH (pH 2/4). In such cases, z potential acquired values within the range of ( /20) to ( /45) mV.
Fig. 3. Zeta potential of the unmodified silica, silica modified with three and five weight parts by mass of aminosilane, and obtained inorganic-organic hybrids as a function of pH (ionic strength equal 0.01 mol dm 3 of NaCl).
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4. Summary 1) Modification of silica surface with N -2-(aminoethyl)-3-aminopropyltrimethoxysilane induced a clear shift in i.e.p. toward high pH values. The shift reflected the amount of aminosilane used for modification. 2) Slight differences in i.e.p. of modified silicas were observed when the parameter was measured at various ionic strengths. 3) Adsorption of organic pigments on modified (particularly aminosilane-modified) and unmodified silica surfaces clearly shifted i.e.p. toward acidic pH. 4) The type of interaction (atomic bond or electrostatic interaction) on the surface of the modified silica and following adsorption of pigments significantly influenced i.e.p. More evident alterations were observed when the interaction involved formation of a covalent bond (e.g. following adsorption of C.I. Reactive Blue 19 pigment).
Acknowledgements This work was supported by the Poznan University of Technology Research Grant No. BW 32/ 008/2002.
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