Influence of modification by N-2-(aminoethyl)-3-

aminopropyltrimethoxysilane on physicochemical

properties of bentonite

Michał Wieczorek*, Andrzej Krysztafkiewicz, Teofil Jesionowski

Institute of Chemical Technology and Engineering, Poznan University of Technology, pl. Marii Sklodowskiej-Curie 2, 60-965 Poznan, Poland

Accepted 25 September 2003

Abstract

The study was undertaken to establish the effect of bentonite modification with N-2-(aminoethyl)-3-aminopropyltrimethoxysilane on the

physicochemical properties of the product. The undesirable hydrophilic character of the bentonite surface was inhibited by introduction of

functional amino groups of defined chemical affinity to its surface. Elemental analysis proved the chemical nature of the modification.

Aminosilane modification of bentonite markedly increased the stability of its dispersion in water and altered its isoelectric point. It also

strongly decreased adsorptive properties of the material. The unmodified bentonite, Izol, exhibited an intense monodisperse character but

following the modification it contained a few secondary agglomerate structures.

q 2003 Elsevier Ltd. All rights reserved.

1. Introduction

Kaolins, talcs and bentonites belong to the family of white

silicate fillers. In contrast to the synthetic silicates, they are

moderately reinforcing fillers. Kaolin and in particular

bentonite exhibit high hydrophilicity, which is related to

the presence of silanol and aluminol groups on their surface

[1–8]. These groups may undergo protonation:

protonation of silanol group

protonation of aluminol group

The hydrophilicity of silica and silicate surfaces can be

reduced or inhibited by coupling, pro-adhesive compounds

and surfactants, including: silane, titanate, borate, zirconate

and hafnate coupling agents, metallocenes [9,10], and

quarternary ammonium salts [11–13]. These compounds

contain two types of functional groups: the alkoxy group,

exhibiting affinity to the filler, and amino, mercapto, etc.

groups with chemical affinity to the polymer or paints.

Silane molecules can interact with silicate surface silanol

groups in various ways [14,15], including the formation of

hydrogen bonds and proton transfer or condensation with

silanol groups (Fig. 1).

0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jpcs.2003.09.016

Journal of Physics and Chemistry of Solids 65 (2004) 447–452

www.elsevier.com/locate/jpcs

* Corresponding author. Tel.: þ48-61-6653626; fax: þ48-61-6653649.

E-mail address: michu@fct.put.poznan.pl (M. Wieczorek).

Following Conley and Althoff [16], we can ascribe the

Bronsted acidity of kaolins or bentonites to the presence of

silanol (xSi–OH) groups, arising from the cleavage of

the lattice. The Lewis acidity, how-

ever, is due to the presence of gibsite octahedrals, parti-

cularly aluminol (yAl–OH) groups. The presence of these

groups is indispensable for themodification of bentonitewith

compounds capable of condensing with them.

Modification of natural silicate surface with silane

coupling agents alters the hydrophilic surface character

and extends the application range of the natural fillers. They

may be used, first of all, in systems of non-polar polymers or

in paints with hydrophobic binders [17,18]. In some cases,

however, chemical affinity of a solid filler to a polymer

system of hydrophilic character can be reduced by

introduction of hydrophilic amino or mercapto groups,

present in silanes [19–21].

The study reported aimed at obtaining modified bentonite

with N-2-(aminoethyl)-3-aminopropyltrimethoxysilane and

at physicochemical, dispersive and structural evaluation of

the product. The results were also expected to corroborate to

the chemical character of the surface modification reaction.

2. Experimental

2.1. Materials

The bentonite Izol used was from Zebiec Company.

Selected physicochemical parameters of raw bentonite were

as follows: min. montmorillonite content 50%, min.

swelling 8–13 cm 2 g21, max. water content 10%, milling

80% under 0.056 mm. Physicochemical characteristics of

raw bentonite is given in Table 1.

N-2-(aminoethyl)-3-aminopropyltrimethoxysilane used

for modification of bentonite was purchased for UniSil

Company, Tarnow, Poland.

2.2. Methods

The modification was conducted in a 0.5 dm3 reactor,

to which 50 g bentonite and the modifying compound

solution were introduced. The content of the reactor was

thoroughly mixed for 1 h. Later the solvent was

evaporated and the dried product was ground for 30 min

to pass a 0.05 mm (270 mesh) sieve. The oversized

particles were removed. Methanol/water (1:1) mixture

served as a solvent.

The samples’ dispersion, particle size, grain morphology,

structure of individual particles and agglomeration were

studied by the scanning electron microscopy (SEM)

providing reliable images of the bentonite surface (using

Philips SEM 515 equipment).

Particle size and their size distribution, also representing

principal properties of bentonite, were measured by the

dynamic light scattering (DLS) technique, using ZetaPlus

apparatus (Brookhaven Instruments Co., USA).

The zeta potential (z) was calculated from the electro-

phoretic mobility. The electrokinetic potential of bentonites

was determined also in ZetaPlus instrument at the ionic

strength of 0.001 mol dm23 of NaCl. The pH of solutions

was adjusted with NaOH or HCl. Mili-Q water was applied

in each experiment.

Specific surface areas of bentonite powders were

determined by N2 adsorption (BET method) using ASAP

2010 instrument (Micrometrics Instrument Corporation).

Moreover, the volume and pore size of the samples were

Fig. 1. Mechanisms of reactions of silane coupling agent with silanol group

on bentonite surface (a) siloxane bond or (b) hydrogen bond.

Table 1

Principal physicochemical properties of unmodified and modified

bentonites

Parameter Izol Izol bentonite modified by

N-2-(aminoethyl)-3-amino

propyltrimethoxysilane

Bulk density (g dm23) 506 581

Absorptivity of water

(cm3 100 g21)

225 188

Absorptivity of dibutyl

phthalate (cm3 100 g21)

175 100

Absorptivity of paraffin

oil (cm3 100 g21)

100 150

Sedimentation in water (s) Few days Few hours

Sedimentation in petrol (s) 58 40

Sedimentation in xylene (s) 31 37

M. Wieczorek et al. / Journal of Physics and Chemistry of Solids 65 (2004) 447–452448

determined. The samples were heated at 120 8C for 2 h prior

to measurements.

The content of carbon, hydrogen and nitrogen present on

the modified bentonite surface was estimated using the

automatic type EA 1108 analyzer (Carlo-Erba, Italy).

The identification analysis was performed by the

WAXS technique. The results were analysed employing

the XRAYAN software. The diffraction patterns were

taken employing the horizontal diffractometer TUR-M62.

Nickel-filtered Cu Ka radiation ðl ¼ 1:5418 �AÞ was used

in the measurements. The conditions of measurements

were as follows: anode voltage—30 kV, anode current—

25 mA the measurement range of 2u : 3–608, measuring

step—0.048.

3. Results and discussion

As follows from Table 1, bentonite surface modification

with aminosilane promoted an increase in its bulk density

and capacity to absorb paraffin oil (from 100 cm3 100 g21

for the unmodified bentonite to 150 cm3 100 g21 for the

modified bentonite). For the modified bentonite the time of

sedimentation in three media (in water, gasoline and xylene)

has decreased. Importantly, aminosilane modification

induced a decrease in the bentonite surface capacity to

absorb water.

SEM micrographs of the bentonite unmodified and

modified with five weight parts of aminosilane, are

presented in Fig. 2. The unmodified bentonite particles are

flocculated in shape. The samples were relatively uniform.

The modification (Fig. 2b) destroyed the primary floccu-

lated structure and the particles became irregular in shape.

Also larger agglomerates of bentonite particles appeared

which confirmed the effect of silane modification on the

specific surface area and nitrogen adsorption on the surface

of the modified bentonite.

The modification-induced alterations in the morphologi-

cal structure of bentonite particles could be noted in the

results obtained using the DLS technique. The effect of the

modification on bentonite particle size distribution was

evident. The particle size distribution for the unmodified

Izol bentonite is presented in Fig. 3a. The curve shows two

Fig. 3. Multimodal particle size distribution of (a) unmodified Izol

bentonite and (b) Izol bentonite modified by aminosilane.

Fig. 2. SEM micrograph of (a) unmodified Izol bentonite and (b) Izol

bentonite modified by aminosilane.

M. Wieczorek et al. / Journal of Physics and Chemistry of Solids 65 (2004) 447–452 449

bands of different intensities. A narrow band was observed

in the range of 562–566 nm, which could be assigned to the

presence of bentonite primary particles. The maximum

intensity of 100 of the band corresponded to the particles of

564.5 nm in diameter. The other band of a very low intensity

reflected the presence of slightly larger particles and its

intensity was 14 for the particle diameter range from 572 to

574 nm. The bentonite manifested a very highly mono-

disperse character and its calculated polydispersity was as

low as 0.005. After the modification, the bentonite was less

uniform (Fig. 3b) and its polydispersity increased to 0.040.

The particle size distribution revealed two bands. One of

them was assigned to the presence of primary particles,

which had a significantly lower diameter than in the case of

the unmodified bentonite. The band appeared in the range

380–524 nm (maximum intensity of 100 corresponded to

the particles of 460.5 nm in diameter). Unfortunately,

secondary agglomerate structures were also present, as

indicated by a band in the range 1370–1885 nm (the

maximum intensity of 23 corresponded to the particles of

1771.2 nm in diameter). The results confirmed a decrease in

the activity of the surface groups.

Results of the study of the specific surface area and

porosity of the unmodified and the modified bentonite are

presented in Table 2 and Fig. 4. The unmodified Izol

bentonite manifested an average size of specific surface

area of 40.5 m2 g21. Thus, the bentonite could be

classified as a mesoporous adsorbent with an average

pore diameter of about 58.7 A. As indicated by the

isotherm of nitrogen adsorption–desorption (Fig. 4),

the amount of adsorbed N2 was relatively substantial,

exceeding 50 cm3 g21. After the bentonite surface

modification with aminosilane, the specific surface area

decreased to 12.4 m2 g21, which affected the course of

the curve of nitrogen adsorption–desorption on the

surface (Fig. 4). The amount of adsorbed nitrogen

decreased to about 27.5 cm3 g21. In the modified sample

the total pore volume radically decreased to

0.0270 cm3 g21 as compared to 0.0594 cm3 g21 for the

unmodified bentonite.

Electrokinetic study of the two samples of unmodified

and modified bentonite proved that the zeta potential

dependencies on pH were almost straight lines up to pH

of about 9 (Fig. 5). In the pH range 9–10 a rapid

decrease in zeta potential was observed in both curves

(particularly in the curve for the unmodified bentonite for

which the zeta potential decreased to about (250) mV).

Above pH 10 an increase in the zeta potential was

observed for both the unmodified and the modified

bentonite. The modification of bentonite surface with

aminosilane promoted an increase in the zeta potential

even to positive values at pH ,1.5.

The chemical composition, the unmodified and modified

was determined by elemental analysis. Results of the

elemental analysis of the bentonite samples studied are

listed in Table 3. The modified samples was characterised

Table 2

Structural and porosity characterisation of bentonites based on data of nitrogen adsorption/desorption on modified and unmodified bentonite surfaces

Type of bentonite BET surface

area (m2 g21)

Langmuir’s

surface area

(m2 g21)

Total surface of pores

of diameters between

17 and 3000 A (m2 g21)

(from adsorption curve)

Total surface of pores

of diameters between

17 and 3000 A (m2 g21)

(from desorption curve)

Total volume of

pores (below 700 A)

(cm3 g21)

Mean pore diameter

(from BET) (A)

Izol 40.5 55.5 24.2 33.3 0.05940 58.7

Izol modified by five

weight parts of N-2-

(aminoethyl)-3-amino-

propyltrimethoxysilane

12.4 17.2 9.7 14.9 0.02703 87.0

Fig. 4. Nitrogen adsorption/desorption isotherms of unmodified Izol and

modified Izol bentonite.

Fig. 5. Influence of pH on zeta potential of Izol bentonite and Izol bentonite

modified by aminosilane.

M. Wieczorek et al. / Journal of Physics and Chemistry of Solids 65 (2004) 447–452450

by a significant increase in the contents of carbon, hydrogen

and nitrogen.

The chemical character of the Izol bentonite surface

modification with aminosilane implied by our results is

excluded the possibility of intercalation of N-2-(ami-

noethyl)-3-aminopropyltrimethoxysilane into the inter-

packet montmorillonite structure. The X-ray analysis

proved that the modification of bentonite with five weight

parts of aminosilane failed to induce even a minimum shift

in the basic diffraction peaks describing the structure of the

silicate studied (Fig. 6). Thus, the applied technique of Izol

bentonite surface modification induced changes exclusively

in physicochemical and morphological properties of the

product.

4. Conclusions

Modification of Izol bentonite surface with N-2-(ami-

noethyl)-3-aminopropyltrimethoxysilane results in a

destruction of the primary flocculated structure, and

formation of particles of irregular shape, tending to join in

agglomerates.

Unmodified bentonite has highly uniform particles; its

polydispersity is as low as 0.005. The modification

markedly deteriorates the uniform character of bentonite

and results in a decrease in the mean particle diameter.

Bentonite modified with aminosilane exhibits a radically

decreased specific surface area and lower activity as the

active centres of the surface become blocked by the amine

groups of the silane.

Bentonite surface modification with aminosilane results

in an increase in the zeta potential.

The elemental analysis has proved the chemical

character of the bentonite surface modification with N-2-

(aminoethyl)-3-aminopropyltrimethoxysilane.

Acknowledgements

This work was supported by Poznan University of

Technology research grant DS. 32/115/2003.

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Content of carbon, hydrogen and nitrogen in samples of unmodified Izol

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xysilane

Bentonite Carbon

content (%)

Hydrogen

content (%)

Nitrogen

content (%)

Izol 0.46 0.65 –

Izol modified by five

weight parts N-2-(aminoethyl)-

3-aminopropyltrimethoxysilane

1.89 0.94 0.73

Fig. 6. X-ray analysis of the unmodified bentonite (a) and the bentonite

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