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Preparation of colloidal silica from sodiummetasilicate solution and sulphuric acid inemulsionmedium


Colloids and Surfaces A: Physicochemical and Engineering Aspects 190 (2001) 153– 165 www.elsevier.com/locate/colsurfa

Preparation of colloidal silica from sodium metasilicate solution and sulphuric acid in emulsion medium
Teo?l Jesionowski *
Institute of Chemical Technology and Engineering, Poznan Uni6ersity of Technology, Pl. M. Sk?odowskiej-Curie 2, 60 -695 Poznan, Poland ?

Abstract The studies were directed toward production of silicas from emulsion systems. Precipitation of colloidal silica from sodium metasilicate solution was performed using sulphuric acid. Optimum emulsion composition and precipitation parameters were worked out. As emulsi?ers non-ionic surfactants were used (Rokafenol N-5, N-6 and N-9). Moreover, basic physicochemical parameters of obtained silicas were evaluated. Surface character (zeta potential) was examined by direct measurement of electrophoretic mobility (ELS). Particle morphology and morphology of the dispersion forms were examined using scanning electron microscopy. Polydisperse character and particle sizes of agglomerates were established using dynamic light scattering. Moreover, speci?c area (BET) and pore volume of studied colloidal silicas was estimated. The studies showed that the basic aim of this study was reached, i.e., highly dispersed silicas were obtained by precipitation from emulsion systems. ? 2001 Elsevier Science B.V. All rights reserved.
Keywords: Precipitated silica; Emulsion; Agglomerate; Aggregate structures

1. Introduction The developing specialization branch of colloid chemistry and, in particular, industrial mastering of silica gel production and of production of highly dispersed silicas have permitted the introduction to the technology of several processes, based on the use of such silicas which demonstrate speci?c properties. Studies on physicochemical properties of silica adsorbents and of highly
* Tel.: +48-61-6653626; fax: +48-61-6653649. E-mail address: teo?l.jesionowski@put.poznan.pl (T. Jesionowski).

dispersed silicas as well as detailed recognition of their properties permitted the use of substances to resolve engineering and equipment problems. Conditions were created to implement new methods, in technology of gas and liquid drying, selective adsorption, some contact processes in industry and in ?lling of plasto- and elastomers [1,2]. Application of highly dispersed silicas in the industry of paints and varnishes represents a very interesting problem. On the basis of the compounds inorganic pigments are produced, particularly required and valued in production of paints and varnishes. In the paints they, moreover, play

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the role of carriers, opalescing agents, densifying agents as well as tixotropic factors, which prevent sedimentation of pigmented paint components [3]. Highly dispersed silicas ?nd application in toothpastes as whitening and cleaning agents, which do not damage tooth enamel. Moreover, they are applied to control rheologic properties of the toothpaste [4]. What is more signi?cant, vast amounts of silicas are used in the food and pharmaceutical industries as anti-agglomerating agents, agents which improve liquidity of powders and decrease their higroscopic character. Highly dispersed silicas found application also in the cosmetic industry, as densifying agents [5]. Nanometric colloidal silicas may be obtained in several chemical processes. The most important include such methods as hydrolysis of alkoxysilane esters [6], combustion of silica halides [7,8] and precipitation from the aqueous solution of sodium metasilicate [9,10]. In parallel to the common application of silicas obtained by precipitation, studies are conducted on formation of nanometric silicas in emulsion systems [11–17]. In this study, we present the way of obtaining silica from aqueous solution of sodium metasilicate and sulphuric acid by precipitation technique

in emulsion systems. We have decided to employ emulsion in the system of cyclohexane – water using emulsi?ers non-ionic surfactants, formed by oxyethylenation of nonylphenol.

2. Experimental

2.1. Materials
In order to obtain silica, sodium metasilicate solution (water glass) was used. Sulphuric acid was employed as a precipiting agent. An organic medium for emulsion formation was provided by cyclohexane. In the studies, non-ionic surfactants like Rokafenol N-5, N-6 and N-9 (nonylphenyl polyoxyethylene glycol ethers) were applied. Their general formula has the following form:

where mean n= 5, 6 or 9. They were produced by Chemical Works Rokita S.A., Brzeg Dolny (Poland).

Table 1 Physicochemical parameters of silicas obtained in emulsion medium by adding the emulsion of sodium metasilicate to emulsion of sulphuric acid Sample no. Emulsi?ers (mass ratio) Bulk density (g dm?3) Paraf?n oil absorbing Dibutyl phthalate capacity (cm3 100 g?1) absorbing capacity (cm3 100 g?1) Water absorbing capacity (cm3 100 g?1)

Rapid stirring motor 1 2 3 4 5 6 Ultrasonic bath 7 8 9 10

N-5/N-6 N-5/N-6 N-5/N-6 N-5/N-6 N-5/N-6 N-5/N-6

(1:1.5) (1:1.5) (1:1.5) (1:1.5) (1:1.5) (1:1.5)

209.6 196.7 185.2 271.3 – – 219.3 211.1 168.8 –

350 470 450 180 – – 300 200 280 –

200 250 250 150 – – 180 100 200 –

200 200 180 170 – – 300 250 250 –

N-5/N-6 (1.5:1) N-5/N-6 (1:1.5) N-5 (2.5) N-6 (2.5)

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Table 2 Physicochemical parameters of silicas obtained in emulsion medium by adding the emulsion of sodium metasilicate to emulsion of sulphuric acid in presence of coagulant Na2SO4 Sample no. Emulsi?ers (mass ratio) Bulk density (g dm?3) Paraf?n oil absorbing Dibutyl phthalate capacity (cm3 100 g?1) absorbing capacity (cm3 100 g?1) Water absorbing capacity (cm3 100 g?1)

Rapid stirring motor 11 12 13 14 15 16 17 Ultrasonic bath 18 19 20 21 22 23 24 25 26 Homogenizer 27 27a 28 28a 29 29a 30 30a

N-5/N-6 (1:1.5) N-5/N-6 (1.5:1) N-9 (2.5) N-5 (2.5) N-6 (2.5) N-9/N-6 (1.5:1) N-9/N-5 (1.5:1)

130.0 163.8 141.7 119.0 170.6 138.0 216.0

450 400 400 650 450 950 400 600 500 800 900 850 500 850 800 700 800 900 950 850 650 700 650 800

400 250 150 300 250 400 200 450 300 500 550 500 260 500 500 500 450 400 700 600 450 400 450 600

250 400 150 300 250 300 150 300 300 350 500 450 400 450 450 450 250 250 500 400 350 200 250 300

N-5/N-6 (1:1.5) 99.2 N-5/N-6 (1:1.5) 76.9 N-5/N-6 (1.5:1) 87.4 N-9 (2.5) 86.7 N-9 (1.5) 92.1 N-5 (2.5) 119.9 N-6 (2.5) 86.6 N-5/N-6 (1.5:1) 97.7 N-9/N-5 (1.5:1) 107.0 N-5/N-6 (1:1.5) N-5/N-6 (1:1.5) N-9 (2.5) N-9 (2.5) N-5 (2.5) N-5 (2.5) N-6 (2.5) N-6 (2.5) 126.0 108.0 91.0 97.6 151.4 150.3 163.5 97.6

2.2. Procedures and methods

2.2.1. Practical preparation of emulsion Two emulsions were prepared, of which one (E1) contained in its constant composition: 100 cm3 5% sodium metasilicate (in which the coagulant, sodium sulphate (VI), was dissolved) and 110 cm3 of cyclohexane. The third component of E1 emulsion was an emulsi?er. The second emulsion (E2) contained in its constant composition 33 cm3 5% H2SO4 and 35 cm3 cyclohexane. The third component of E2 emulsion was the emulsi?er of the same structure as in E1 but in appropriately lower amounts.

Surfactants (at various mass ratios) were dissolved in cyclohexane and, then, the aqueous phase was added in few portions (for E1 5% water solution of sodium metasilicate and for E2 5% water solution of sulphuric acid) and the mixture was agitated in a mechanical shaker for 15 min. So prepared emulsions were ready for precipitation process.

2.2.2. Process of preparing highly dispersed silica in emulsion system At a laboratory scale, the process of precipitation of silica was conducted in order to establish parameters needed to obtain products of best properties.

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2.2.2.1. Dispersion (precipitation) using a rapid stirring motor. The precipitation was conducted in the reactive vessel of 0.5 dm3 capacity. In the

stand a rapid stirring motor EUROSTAR type mixer (digital IKA LABORTECHNIK) was ?xed, which secured intense mixing (2000 1

Fig. 1. Relation between silica bulk density and the type of applied emulsi?er in the process of precipitation. Table 3 Zeta potential, electrophoretic mobility and polydispersity of precipitated silicas. Sample no. Rapid stirring motor 2 11 13 14 15 Ultrasonic bath 19 21 23 24 Homogenizer 27 28 29 30 Zeta potential (mV) Electrophoretic mobility ((m s?1)/(V cm?1)) Polydispersity

?32.43 ?28.88 ?24.88 ?16.09 ?29.81 ?13.30 ?17.94 12.46 ?34.27 ?11.41 ?29.38 ?26.98 ?18.69

?2.53 ?2.26 ?1.94 ?1.26 ?2.33 ?1.04 ?1.40 ?0.97 ?2.68 ?0.89 ?2.30 ?2.11 ?1.46

0.005 0.005 0.005 0.005 0.005 0.209 0.149 0.063 0.190 0.005 0.136 0.005 0.005

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Fig. 2. Particle size distribution of silica using rapid stirring motor (sample 2).

min ? 1) of the system. E2 emulsion was placed in the reactive vessel and subjected to intense mixing. E1 emulsion was dosed to E2 emulsion at a constant rate, using a peristaltic pump of PP2B-15 type. Due to the reaction, which took place in the reactive vessel, the silica containing emulsion was obtained. The emulsion was heated to 80°C to destabilise it. Then, cyclohexane was separated from the emulsion by distillation. The subsequent stage involved ?ltration of the remaining mixture under lowered pressure. In this way, the obtained sample was washed with warm water and, then, with acetone to wash out the remaining surfactants. Acetone was separated by distillation. Subsequently, the sample was subjected to drying for 48 h in a stationary drier, at 105°C.

capacity. The mixing took advantage of the homogenizer of ULTRA TURRAX T25 basic type (IKA LABORTECHNIK, Germany) at 11000 l min ? 1 or 19000 l min ? 1. The remaining functions corresponded to those employed using ultrasonic ?eld or rapid stirring motor.

2.2.2.2. Precipitation using ultrasonic bath. The reactor vessel of 0.5 dm3 capacity was placed in an ultrasonic bath of INTERSONIG-102 type, (30 kHz) of 100 W power, in which emulsi?cation took place due to the action of ultrasonic waves. The remaining functions were performed analogously to those employed in the course of rapid mixing. 2.2.2.3. Dispersion using a homogenizer. Precipitation was performed in a reactive vessel of 0.5 dm3
Fig. 3. SEM photograph of silica using a rapid stirring motor (sample 2).

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Fig. 4. Particle size distribution for silica using rapid stirring motor (sample 11).

2.2.3. Physicochemical e6aluation of the formed silicas Following the precipitation, the silicas were subjected to physicochemical tests, their bulk densities were estimated as well as water, dibutyl phthalate and paraf?n oil absorption capacities. Studies on morphology and microstructure of the granules were performed in order to obtain data on dispersion, particle shape and morphology of the granules, structure of individual particles and on silica aggregation and agglomeration type following precipitation. The studies were conducted using scanning electron microscopy (SEM). The observations were performed in the Philips SEM 515 microscope. Laser Doppler electrophoretic light scattering determinations were performed with a ZetaPlus instrument (Brookhaven Instruments Inc., USA), in the reference beam mode at the wavelength of laser light source of 635 nm, sampling time 256 ms, modular frequency 250 Hz and the scattering angle 15°. The standard error of the zeta potentials, converted from the experimentally determined electrophoretic mobilities according to the Smoluchowski limit of the Henry equation, was typically B1.0%. The zeta potentials were obtained by averring 5– 10 runs.

Size distribution of silica particle agglomerates and aggregates were also estimated using a ZetaPlus instrument by dynamic light scattering method. In the instrument, the mobility rate is measured of loaded colloid particles, suspended in water.

Fig. 5. SEM photograph of silica using a rapid stirring motor (sample 11).

T. Jesionowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 190 (2001) 153–165 Table 4 Particles’ effective diameter and mean size of aggregates and agglomerates of precipitated silicas Sample no. Rapid stirring motor 2 11 13 14 15 Ultrasonic bath 19 21 23 24 Homogenizer 27 28 29 30
a

159

Effective diameter (nm)

Mean size of aggregatesa (nm)

Mean size of agglomeratesa (nm)

821.5 1559.9 985.3 1288.2 1281.5 619.8 808.5 808.4 720.8 1061.6 869.0 1100.5 1038.2

645.6 1251.2 839.7 1099.5 without 403.6/98 611.1 688.1 551.3 953.1 455.4/28 971.9 550.7/15

2000.0/25 without without without 1333.5/100 1217.5/100 1673.1/71 2262.6/27 2122.5/42 without 1155.7/100 without 1140.9/100

Maximum intensity of aggregates ?100, maximum intensity of agglomerates is presented in table.

Fig. 6. Particle size distribution of silica using an ultrasonic bath (sample 19).

Speci?c surface areas of silica powders were determined by N2 adsorption (BET method) using ASAP 2010 apparatus (Micrometrics Instrument Corporation). Moreover, the volume and size of pores of precipitated materials were examined. The samples were heated at 120 and 300°C for 2 h prior to the measurements.

3. Results and discussion Basic physicochemical parameters of silicas obtained by precipitation from sodium metasilicate emulsions and emulsions of suphuric acid are presented in Table 1. The obtained products showed relatively high

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Fig. 7. SEM photograph of silica using an ultrasonic bath (sample 19).

Fig. 9. SEM photograph of silica using an ultrasonic bath (sample 21).

bulk densities of around 170– 220 g dm ? 3 and low capacities to absorb paraf?n oil, ranging between 180 and 470 cm3 100 g ? 1. Capacity to absorb water varied between 170 and 300 cm3 100 g ? 1. Application of a coagulant (sodium sulphate (VI)), the substance which corrected morphology and structure of particle surface as well as surface character of the particles, resulted in signi?cant

changes in physicochemical parameters of the synthetized colloidal silicas. When a rapid stirring motor was used as a dispersing agent, the silicas obtained showed relatively slightly lower values of bulk density than in an analogous case but with no coagulant used (Table 2). The values ?tted the range of 130–216 g dm ? 3. Increase in the capacity to absorb the paraf?n oil and dibutyl phtha-

Fig. 8. Particle size distribution of silica using an ultrasonic bath (sample 21).

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161

Fig. 10. Particle size distribution of silica using a homogenizer (sample 27).

late was also observed and the values amounted to, respectively, 400– 950 cm3 100 g ? 1 and 150– 400 cm3 100 g ? 1. When the system was dispersed using ultrasound bath, a marked decrease in the bulk density took place. The values of bulk densities ranged from 76.9 to 119.9 g dm ? 3. The so low values of this typical parameter for powder substances might point to their extensive dispersion and permit to signi?cantly broaden applications of the material. The capacity to absorb paraf?n oil and dibutyl phthalate also signi?cantly increased to, respectively, 500–900 cm3 100 g ? 1 and 260– 550 cm3 100 g ? 1. The silicas exhibited high capacity to absorb water (300– 500 cm3 100 g ? 1). Basic physicochemical parameters of the silicas obtained in the homogenizer are presented in the Table 2, for two rates of homogenization, i.e., 11000 and 19000 l min ? 1. Application of a higher homogenization rate resulted in a decrease of bulk densities and an increase in the capacity to absorb paraf?n oil (e.g., samples 27a, 29a and 30a). The lowest values of bulk densities and the highest capacity to absorb paraf?n oil were obtained in the silicas prepared using ultrasonic bath. Positive results were also obtained using the homogenizer. The least favourable basic physicochemical parameters were shown by silicas precipitated using the rapid stirring motor. Changes in

bulk densities during application of various dispersing methods (rapid stirring motor, ultrasonic bath and homogenizer) and respective emulsi?er combinations are presented in Fig. 1. The use of a coagulant resulted in the marked decrease of bulk densities of the silicas as compared to the silica precipitated without its use (sample 2). In all emulgation systems the lowest

Fig. 11. SEM photograph of silica using a homogenizer (sample 27).

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Fig. 12. Particle size distribution of silica using a homogenizer (sample 29).

values of bulk density were obtained using ultrasonic bath. Somewhat higher values were observed using the homogenizer. Application of appropriate dispersing system had profound effect on values of bulk densities of the formed silicas. Values of zeta potential, electrophoretic mobility and of polydisperse character are presented in Table 3. Sample 2 obtained with no coagulant (Na2SO4) addition was characterized by the highest value of zeta potential (? 32.43 mV). In the case of applying the coagulant in all cases a signi?cant decrease in the potential was observed (when absolute values of the potential were taken into account). Most probably, this was linked to equilibration of charges on the surfaces of the double diffusion layer of silica and sodium sulphate (VI) (SiO? + Na+). The exception involved the sample 24. A lowered electrophoretic mobility was also observed, analogously to the change in zeta potential. Polydisperse character represents another parameter typical for dispersion systems. In the case of employing ultrasonic bath, an evident increase in polydisperse character took place. Increased value of this parameter pointed to growing stability of the dispersion and a markedly restricted tendency to form agglomerate and aggregate structures of the formed particles. Particle size distribution and electron microscopy photo-

graphs (SEM) of silicas precipitated in emulsion system (Rokafenol N-5 and Rokafenol N-6) using a rapid stirring motor and 5% H2SO4 solution (samples 2 and 12) are presented in Figs. 2–5. Effective particle diameters as well as mean values of aggregate and agglomerate sizes for silicas precipitated in the three dispersion systems are presented in Table 4.

Fig. 13. SEM photograph of silica using a homogenizer (sample 29).

T. Jesionowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 190 (2001) 153–165 Table 5 Speci?c surface area, volume and average size of pores of obtained silicas (evaporated at 120°C) Sample no. Speci?c surface area BET (m2 g?1) Total pores volume (cm3 g?1) , Pores volume 8.5–1500 A (cm3 g?1) Adsorption curve 2 11 19 21 27
a

163

Average size of , poresa (A)

Desorption curve 0.4617 0.7564 0.6600 0.6536 0.7130 46.883 52.7663 80.8047 61.9963 50.5473

189.17 276.88 121.82 162.10 266.23 Calculated from BET equation [4V/A].

0.4429 0.7305 0.4922 0.5025 0.6728

0.4631 0.7513 0.6609 0.6532 0.7193

Table 6 Speci?c surface area, volume and average size of pores of obtained silicas (evaporated at 300°C) Sample no. Speci?c surface area BET (m2 g?1) Total pores volume (cm3 g?1) , Pores volume 8.5–1500 A (cm3 g?1) Adsorption curve 2 11 19 21 27
a

Average size of , poresa (A)

Desorption curve 0.5110 0.8313 0.7444 0.7158 0.7727 39.6828 43.7713 73.5596 45.6625 44.6782

245.17 363.61 149.23 249.79 325.48 Calculated from BET equation [4V/A].

0.4864 0.7958 0.5488 0.5703 0.7271

0.4907 0.8330 0.7436 0.7174 0.7737

Both samples were precipitated in identical conditions with a single difference: the sample 11 was obtained with the addition of sodium sulphate (VI) as a coagulant. The obtained silicas showed presence of spherical particles of a variable diameter. The sample 2 demonstrated presence of particles of lower diameters (effective agglomerate diameter amounted to 821.5 nm) but it was less uniform than that of sample 11. Sample 11 exhibited extremely uniform character but contained agglomerates of large diameters (effective diameter amounted to 1559.9 nm). As shown in Fig. 2, the precipitated silica consisted of primary agglomerates of high intensity and secondary agglomerates, present in a markedly lower quantity. The band of primary agglomerates (aggregates) was contained within 580–715 nm (maximum intensity of 100 corresponded to agglomerate diameter of 645.6 nm).

On the other hand, the band of secondary agglomerates of low intensity occupied the range of 1800–2250 nm (maximum intensity of 25 corresponded to the secondary agglomerate diameter of 2000 nm). In the case of the silica (sample 11) precipitated in presence of Na2SO4, a very uniform product was obtained (Figs. 4 and 5). In this case a very narrow band (typical for very uniform products) was obtained, within the range of 1230–1265 nm (maximum intensity of 100 corresponded to the primary agglomerate diameter of 1251.2 nm). Particle size distributions and electron microscopic photographs of silicas precipitated using an ultrasonic bath and various emulsi?ers, used to prepare emulsion from which the silicas were precipitated, are shown in Figs. 6–9. Independently of the emulsion system used, the application of an ultrasonic bath promoted decrease in agglomerate

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formation. The obtained silica showed relatively low effective agglomerate diameters following precipitation of silica from the system of emulsion containing two Rokafenols N-5 and N-6. The obtained silica showed effective diameter of agglomerates of 619.8 nm and nonhomogenous particles of a variable shape. As shown in Fig. 6, the precipitated silica (sample 19) showed presence of two bands of primary agglomerates, of which the less intense corresponded to low agglomerate diameters of 340– 450 nm (maximum intensity of 98 corresponded to the agglomerate diameter of 403.6 nm) and the more intense one corresponded to the diameters of 1030– 1360 nm (maximum intensity of 100 corresponded to the agglomerate diameter of 1217.5 nm). Silicas precipitated from the emulsion containing Rokafenol N-9 using ultrasonic bath (Figs. 7 and 9), similarly to silicas precipitated using a rapid stirring motor, contained no spheric particles in their structure. Particle size distribution for the silica precipitated in the presence of Rokafenol N-9 (sample 21) is presented in Fig. 8. Silica precipitation from similar systems of emulsion using a homogenizer resulted in similar relations as those noted during precipitation in other mixing systems. The best silicas were obtained during precipitation from the emulsion system, which contained Rokafenol N-5 and Rokafenol N-6 (sample 27), and in the system containing exclusively Rokafenol N-5 emulsi?er (sample 29). In both cases, the obtained silicas contained spherical particles and showed high uniformity. The silicas precipitated from the emulsion system (Rokafenol N-5 and N-6) exhibited high effective diameter of agglomerates (1061.6 nm) but were extremely uniform (Figs. 10 and 11). They were characterized by the presence of a band of a narrow range of agglomerate diameters, ranging between 940– 960 nm (maximum intensity of 100 corresponded to the agglomerate diameter of 953.1 nm). Also the silica precipitated from Rokafenol N-5 (sample 29) containing emulsion, using as a coagulant sodium sulphate (VI), exhibited highly uniform character and effective diameter of agglomerates of 1100.5 nm. Silica particles showed mostly a spherical shape. The silica was characterized by presence of an intense narrow

band within the range of 955–975 (maximum intensity of 100 corresponded to the agglomerate diameter of 971.9 nm) (see Figs. 12 and 13). Speci?c areas and pore characteristics for silicas precipitated from emulsions, following their degassing at 120 and 300°C, are presented in Tables 5 and 6. As shown in the tables, the temperature of preliminary processing in the silica roasting process exerted a signi?cant effect on its speci?c area, which was linked to changes in development of its outer surface. The silicas roasted (degassed) at lower temperature exhibited lower size of speci?c surface area and this probably re?ected presence on silica surfaces of adsorbed components (surfactants) of emulsion from which the silica was precipitated. The adsorbed organic substances blocked active centres on the silica surface and, therefore, lower N2 adsorption took place during estimation of BET isotherm. The silicas degassed at higher temperature (300°C) (Table 6) showed de?nitely higher speci?c areas, which was caused by thermic desorption of emulsion components and uncovering of the surface-active centres. The highest speci?c areas were exhibited by silicas precipitated from emulsions containing a mixture of N-5 and N-6 emulsi?ers (samples 11 and 27). Both silicas showed similar values of bulk densities. Total pores volume as well as an average size of pores showed similar values and at that stage of studies it was dif?cult to establish effect of precipitation parameters on those values.

4. Conclusions

?

?

Application of sodium sulphate (VI) as a coagulant in the process of dispersion formation resulted in signi?cant favourable alterations in physicochemical parameters of synthetized colloidal silicas in the emulsion system The most marked decrease in value of bulk density (76.9–163.8 g dm ? 3) and increase in capacity to absorb paraf?n oil (500– 950 dm3 100 g ? 1) took place when ultrasonic bath or homogenizer were used as an equipment to form dispersion in the process of precipitation.

T. Jesionowski / Colloids and Surfaces A: Physicochem. Eng. Aspects 190 (2001) 153–165 ?

165

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Silica (sample 2) obtained with no coagulant (Na2SO4) addition exhibited the highest absolute values of zeta potential (?32,43 mV). The use of ultrasonic bath resulted in an evident increase in polydispersity; somewhat lower values of polydispersity were observed when a homogenizer was used. Application of Rokafenol N-5 and N-6 mixture in the precipitation process from the emulsion system promoted precipitation of clearly spherical particles. On the other hand, application of Rokafenol N-9 resulted in formation of particles of an irregular shape. Silica precipitated from the emulsion containing Rokafenol N-6 was composed of spherical particles of low diameters and of particles of a variable shape, which showed strong tendency to form agglomerates. The effective diameter of such agglomerates was 1038.2 nm. Silicas degassed at a lower temperature showed lower values of speci?c area than silicas degassed at the temperature of 300°C. This re?ected probably presence of the adsorbed on the surface non-ionic surfactants, originating from the emulsion used to form colloidal silicas.

the studies. This work was supported by the research grant no. DS 32/008/2000.

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Acknowledgements The author is indebted to ‘‘ROKITA’’ S.A., Brzeg Dolny for the gift of the emulsi?ers used in

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