Introduction
The hydrophobization of clay minerals, in particular
the montmorillonites, the clay minerals in the bento-
nites, with cationic surfactants is used on an industrial
scale. The organic derivatives find application as rhe-
ological control agents in aqueous and nonaqueous
dispersions, like drilling fluids, paints, oils, lubricants,
greases, resins, emails, and inks, and are used as pol-
ishing materials, chromatographic supports, and in
cosmetic formulations [1, 2]. Modification of the clay
mineral surface by organic cations provides a simple
method to tailor adsorbents for organic compounds,
especially pollutants [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
14, 15, 16, 17, 18, 19, 20]. The interlayer spaces of
surface-modified clay minerals provide extremely small
reaction spaces which allow the preparation of colloidal
metals and transition-metal and semiconductor oxides
and sulfides on a nanometer scale [21, 22, 23, 24]. The
current efforts to construct clay mineral–polymer
nanocomposites require hydrophobized clay minerals
which must be dispersed as highly as possible within the
polymer matrix [25].
A condition for an optimal application of hydro-
phobized bentonites is not only the choice of the modi-
fying organic cations but also the degree of cation
exchange (partial, saturated, or oversaturated) [2]. Many
of the uses just mentioned also require a careful choice of
the colloidal properties of the hydrophobized bentonites
when they have to be dispersed in water, organic solvents,
ORIGINAL CONTRIBUTION
Colloid Polym Sci (2003) 281: 293–301
DOI 10.1007/s00396-002-0759-z
M. Janek
G. Lagaly
Interaction of a cationic surfactant
with bentonite: a colloid chemistry study
Received: 27 May 2002
Accepted: 27 June 2002
Published online: 26 September 2002
Ó Springer-Verlag 2002
M. Janek (&)
Institute of Inorganic Chemistry,
Slovak Academy of Sciences,
842 36 Bratislava, Slovak Republic
E-mail: Marian.Janek@itc-wgt.fzk.de
Tel.: ++49-7247-824102
Fax: ++49-7247-826639
G. Lagaly
Institute of Inorganic Chemistry,
University of Kiel, 24098 Kiel, Germany
M. Janek
Research Center Karlsruhe GmbH,
Institute of Technical Chemistry,
Water Technology and Geotechnology,
P.O. Box 3640, 76021 Karlsruhe, Germany
Abstract Cationic surfactants are
strongly coagulating agents for clay
mineral dispersions. The critical
coagulation concentration of cetylt-
rimethylammonium chloride
(CTMAC) is 0.09 mEql
–1
for a
0.025% sodium montmorillonite
dispersion and increases to
0.35 mEql
–1
for a 0.5% dispersion.
The very low concentrations are
caused by the strong adsorption of
the organic cations on the clay
mineral surface. The particles
aggregate in different ways. Phase
diagrams in relation to the mass
content of CTMAC and bentonite
reveal four domains of different
states of the dispersions. As long as
the amount of CTMAC added is
below the cation exchange capacity,
n, the dispersion consists of fine flocs
and separates into a two-phase sys-
tem of large volume and a clear
supernatant (domain I). At
CTMAC amounts near n, the fine
flocs rearrange into voluminous flocs
(domain II) and at CTMAC amounts
of about 2n into compact fine flocs
(domain II) as a consequence of
recharging of the particles. Unlike
type-I dispersions, the type-III
dispersions exhibit Newtonian flow,
even at relatively high bentonite con-
centrations (2.5% w/w and above),
and the flocs settle to sediments of
distinctly smaller volumes.
Keywords Bentonite Æ Cetyl-
trimethylammonium chloride Æ
Coagulation Æ Hexadecyltri-
methylammonium chloride Æ
Montmorillonite