Synthesis and characterization of HDTMA-organoclays: Insights into their structural properties

Abstract

This study aims to synthesize and characterize organoclays developed from an Argentinian montmorillonite (Bent) using hexadecyltrimethylammonium bromide (HDTMA-Br) as the intercalation agent. Subsequently, an adsorption mechanism is proposed. The obtained organoclays were more hydrophobic than the starting clay. Surfactant molecules were adsorbed initially through cation exchange in sites placed in the interlayer space of the clay. Adsorption in such sites continued until the interlayer space was saturated. Depending on the surfactant loading introduced during the intercalation process, different organizations of surfactant in the interlayer were obtained. Further adsorption of surfactant occurred in the mesopores generated by tactoids in the “house of cards” organization. This process kept surfactant molecules relatively free and out of the interlayer space.

Full text

Quim. Nova, Vol. XY, No. 00, 1-6, 200_
Artigo
doi number
*e-mail: pnaranjo@unsa.edu.ar
SYNTHESIS AND CHARACTERIZATION OF HDTMA-ORGANOCLAYS: INSIGHTS INTO THEIR
STRUCTURAL PROPERTIES
Pablo M. Naranjoa,*, José Molinab, Edgardo Ling Sham a,c and Elsa M. Farfán Torresa,b
aInstituto de Investigaciones para la Industria Química, Consejo Nacional de Investigaciones Científicas y Técnicas, Av. Bolivia
5150 (A4408FVY) Salta, Argentina
bFacultad de Ciencias Exactas, Universidad Nacional de Salta, A4408FVY Salta, Argentina
cFacultad de Ingeniería, Universidad Nacional de Salta, A4408FVY Salta, Argentina
Recebido em 13/03/2014; aceito em 16/09/2014; publicado na web em 13/11/2014
This study aims to synthesize and characterize organoclays developed from an Argentinian montmorillonite (Bent) using
hexadecyltrimethylammonium bromide (HDTMA-Br) as the intercalation agent. Subsequently, an adsorption mechanism is proposed.
The obtained organoclays were more hydrophobic than the starting clay. Surfactant molecules were adsorbed initially through cation
exchange in sites placed in the interlayer space of the clay. Adsorption in such sites continued until the interlayer space was saturated.
Depending on the surfactant loading introduced during the intercalation process, different organizations of surfactant in the interlayer
were obtained. Further adsorption of surfactant occurred in the mesopores generated by tactoids in the “house of cards” organization.
This process kept surfactant molecules relatively free and out of the interlayer space.
Keywords: nanoclay; surfactant; adsorption; cation exchange.
INTRODUCTION
Organic Pillared clays, also known as organoclays or nanoclays,
are obtained by cation exchange of interlayer cations, component
of 2:1 type clay minerals, with quaternary ammonium cations. This
process leads to the formation of materials with hydrophobic charac-
teristics that can be applied in a variety of processes, such as catalytic
processes, rheological control agents in paintings and lubricants;
polymer and plastics matrix reinforcing; adsorbents for effluent
treatments, oil spilling, releasing active matrix, etc.1-6
The application of these materials depends strongly on the ob-
tained structure, which, at the same time, conditions its properties.
Therefore, understanding the relation between both the method and
synthesis parameters used, and the developed structure, is of the
utmost importance.
Among variables that affect the synthesis process, the properties
of the employed clay must be considered: chemical composition,
cation exchange capacity (CEC), charge density, specific surface
area, etc.; and also those related to the synthesis process itself: kind
and concentration of surfactant, time of contact, temperature, etc.
Some authors have demonstrated that the structure and properties
developed by these materials are correlated directly with the organiza-
tion of organic cations in the interlayer space,3 showing the importance
of determining the type of arrangement developed in organoclays.
The purpose of this work was to synthesize and characterize
organoclays from an Argentinian montmorillonite (Bent) with he-
xadecyltrimethylammonium bromide (HDTMA-Br) as intercalation
agent. The influence of the amount of intercalated hexadecyltri-
methylammonium cation (HDTMA+) characterizing the obtained
products was evaluated. Their structures were correlated with models
of distribution and ordering of HDTMA+ molecules intercalated in
the intra and extra layer space.
EXPERIMENTAL
Materials
The used clay was an Argentinian bentonite sample (Bent) from
Rio Negro Province, with mineralogy: 84% Na-montmorillonite,
4% quartz and 12% feldspars, determined by Rietveld method;7 the
CEC was 0.837 mol kg-1, determined by the Cu(EDA)22+ method.8
HDTMA-Br was supplied by Merck, (99%), used without further
purification. All solutions and dispersions were prepared using
deionized water obtained with SQC 3 Reverse Osmosis, from Water
Factory System® equipment.
Samples preparation
Organoclays were obtained by putting in contact a 5% w/v sus-
pension of starting clay with a determined volume of HDTMA-Br
5 × 10-3 mol L-1 solution for 2 h with orbital shaking at room
temperature.“Surfactant loading” (SL) is defined as the x fraction of
the CEC that is replaced by HDTMA+:
SL = (mol HDTMA+ / mol exchange cations) CEC = x CEC
To obtain different SL, different volumes of HDTMA-Br
5 × 10-3 mol L-1 solution were used. Solid products were isolated by
centrifugation and repeatedly washed with distilled water until they
were free of bromide ions (determined by AgNO3 test). Furthermore,
the solids were dried at 60 ºC over night, milled in agate mortar, kept
under conditions of controlled humidity, and denoted as Bent followed
by the SL (Bent-0.4; Bent-0.8, etc.).
Desorption of the surfactant was analyzed through leaching
tests. A suspension of each organoclay was prepared in distilled
water and then shaken for 0.5 h on an orbital shaker at room tem-
perature, after which it was separated by centrifugation. The leach-
ing process was repeated “n” times. Solids obtained in this way are
indicated with the name of the initial organoclay followed by “Ln”
(Bent-0.4-L3, Bent-0.8-L7, etc).

Naranjo et al.
2Quim. Nova
Materials characterization
Differential thermal analysis (DTA) was performed with a
Rigaku TAS 1100 from room temperature to 1200 °C, using about
20 mg samples, with a heat temperature rate of 20 °C min-1 in static
air atmosphere.
X-ray diffraction (XRD) studies were obtained on oriented sam-
ples by spreading the sample suspension on glass slides and further
drying (48 h) at room temperature with a relative humidity of 0.47.
Analyses were performed using a Philips PW 1710 diffractometer
using CuKα radiation (0.154 nm). Measuring conditions were: a
power supply of 40 kV and 30 mA; 1° (2θ) divergence and detector
slits, 0.02° (2θ) step size, counting time of 10 s step-1 and patterns
collected from 3° to 70° (2θ). In cases where overlapping peaks
were detected, mathematical deconvolutions were performed with
OriginPro 8 software. The positions and peak areas were calculated
using the Gaussian functions, applying a fitting algorithm for non-
linear least squares.
KBr pressed discs of dried montmorillonite and organoclays
with a sample to KBr relationship of 1:100 approximately, ground
in agate mortar and pressed at a pressure of 3 ton, were analyzed by
FTIR in a Spectrum GX Perkin Elmer Infrared spectrometer, between
4000 and 400 cm-1.
N2 adsorption isotherms were obtained on a Micromeritics sorp-
tometer ASAP 2020 V3.03 E, at -196 °C, previously outgassing solids
at 100 ºC for 10 h (or until a pressure lower than 8 × 10-6 mmHg was
reached). BET surface was calculated with at least 5 points with a
minimum linear correlation coefficient of 0.999. Micropore Volume
(VμP) was calculated with the t-plot method using Harkins-Jura-Boer
equation; Total Pore Volume (VTP) by Gurvitch method, interpolat-
ing at p/p0 = 0.98; and the Mesopore Volume (VmP) by the difference
between VTP and VμP. Pore size distribution was conducted using
Halsey equation over desorption branch.
Using Mopac 2009 software, a computational simulation of the
surfactant molecule in all-trans conformation was performed. A semi-
empirical method with a PM6 base was used for its optimization. In
this way, molecular dimensions and surface area were calculated.
Thickness values for the multilayer structures were also calculated
through techniques of computational calculation.
RESULTS AND DISCUSSION
TGA
Thermal analysis of Bent samples showed the two typical re-
gions associated with water-loss in smectites (Figure. 1).9 The water
adsorbed by external surfaces and hydration water of the interlayer
cations were removed between 80 and 150 °C, while structural water
was released at temperatures higher than 500 °C. Thermal treatment
of organoclays samples produced further mass loss at temperatures
of 180 to 780 °C,10,11 corresponding to organic matter oxidation and
charcoal formation.12 Temperature ranges were directly related with
the SL of each sample.
To determine the mass loss scheme in each sample, ∆mi,α was
defined as the mass loss produced in the α temperature range for the
organoclays with an i surfactant load.
Range 1: Room Temperature – 180 ºC. Within this range of
temperatures, physisorbed and interlayer water and/or of water
of hydration of exchangeable cations (non structural water) were
eliminated,13 both in the starting clay and in the organoclays. This
mass loss was associated with the presence of endothermic peaks
in the DTA curve.
Range 2: 180 °C – 800 °C. In this range, a mass loss of 3% was
observed in Bent clay that corresponds to dehydroxylation of clay
layers.10,14,15
For organoclays, three mass losses in this range of temperatures
were observed, associated with several (from 3 to 5) exothermic
events (Figure 1). Initial and final temperatures of each one of these
mass losses varied with the surfactant loading, but fell among the
following subdivisions of range 2 (Table 1).
Range 2-a: Mass loss was associated with a very important
exothermic event (Figure 1-B); its position depends on the SL. The
amount of mass eliminated in this stage increased with the SL up to
2.0 CEC (∆m0.4,2-a = 2.5%, ∆m0.8,2-a = 4.0% and ∆m2.0,2-a = 22.5%)
and then kept constant (∆m3.0,2-a = 22.0%).
Range 2-b: The second stage of mass loss was associated with
various exothermic events, whose positions depended on the SL
(Figure 1). The amount of mass eliminated in this stage increased
with surfactant loading up to 0.8 CEC (∆m0.4,2-b = 4.5%, ∆m0.8,2-b =
8.2%,) and kept constant up to 2.0 CEC, whereas for higher SL mass,
loss decreased slightly (∆m2.0,2-b = 8.5% and ∆m3.0,2-b = 7.5%). This
slight decrease could be attributed to the fact that as SL increased, a
higher amount of heat was released by combustion of organic groups
in the 2-a range, which provided enough energy for the combustion
of molecules that should be eliminated in range 2-b.
Range 2-c: A mass loss associated with a weak exothermic peak
was observed (Figure 1). In this stage, mass loss due to dehydro-
xylation of layers and the third stage of mass loss associated with
surfactant combustion were overlapped.
To calculate the mass of organic compounds eliminated in this
stage, the fact that mass corresponding to dehydroxylation of layers
represents a 3% of Bent clay mass was taken into account.
The amount of organic compounds eliminated in this stage follows
the same trend that the one observed in Range 2-b. The proportion of
mass loss at this range of temperatures corresponding only to com-
bustion of organic compounds (after subtracting the contribution of
the dehydroxylation of layers) was 3.6%, 6.6%, 6.9% and 6.8% for
Bent-0.4, Bent-0.8, Bent-2.0 and Bent-3.0 respectively.
Range 3: 800 ºC – 1200 ºC. For Bent sample, an endothermic
maximum at 867 °C and two exothermic maximums at 906 and
1145 °C were found, all of them without any associated mass loss
(Figure 1). These peaks were attributed to structural rearrangements
and/or formation of new phases such as spinel, cristobalite or
mullite.10
To perform a deeper analysis of the processes of mass
loss observed at the different temperature ranges, the relation
Ri(α/β)=(∆mi,α)/( ∆mi,β) was defined, where i and ∆mi,α have the
meaning indicated in section 3-1, and β is a range of temperatures
different from α.
Figure 1. TGA (A) and DTA (B) of indicated samples. Temperature range limits
are indicated with vertical lines. Note different scales in y axis of figure 1-B

Synthesis and characterization of HDTMA-organoclays 3
Vol. XY, No. 00
Ri(2-a/2-b) and Ri(2-a/2-c), increased with the surfactant loading,
whereas Ri(2-b/2-c) kept practically constant (Figure 2). The slight
decrease of Ri(2-b/2-c) with SL was due to the strong release of heat
that took place in stage 2-a, which was discussed before.
Thermograms of leached organoclays were also obtained. In
general, they showed a similar behavior to starting organoclays. For
surfactant loadings up to 0.8 CEC, values of ∆m2-a, ∆m2-b and ∆m2-c
were not modified with leaching treatments. For superior SL, the
∆m2-a decreased with the lecheates, whereas ∆m2-b and ∆m2-c kept
constant (Figure 3).
Taking into account Figures 2 and 3, ∆m2-b and ∆m2-c showed the
same behavior. To facilitate the discussion of the results, stages 2-b
and 2-c will be analyzed together, and be named as stage 2-(b+c).
The surfactant mass that was eliminated in stage 2-a decreased
with lecheates and led to the conclusion that the ∆m2-a corresponds
to molecules of surfactant weakly attached to the clay. On the other
hand, in stage 2-(b+c), the loss of surfactant molecules more strongly
attached to the surface was produced, as ∆m2-(b+c) did not decrease
with lecheates.
XRD studies
X-ray diffraction patterns (Figure 4) of different organoclays
showed values of the X-Ray diffraction peak corresponding to d(001)
distance that increased from 12.1 Å for the starting clay up to 19.2 Å
for the organoclay with higher SL (3.0 CEC). This increase (Table 2)
indicated that at least a fraction of the cationic surfactant has replaced
the hydrated interlayer cations.7
In Bent-0.8 the X-Ray diffraction peak d(001) was actually com-
posed by the overlapping of more than one component. In Bent-2.0
and in Bent-3.0 the presence of other X-Ray diffraction peaks of
lower intensity was also observed. This behavior is an indicator of
interstratification. The area of those peaks is related to the amount
of layers with a determined value of d(001).
In table 2, the positions of d(001) X-Ray diffraction peaks obtained
by deconvolution of XRD pattern are shown. From these distances,
the basal spacing was calculated, taking into account that the thick-
ness of the layer was 9.5 Å.16 In the cases in which there are more
than one d(001) line, the percentages of the area of each one of them
in respect to the total area were calculated.
Table 1. Temperatures of the different steps observed in TG, and position of the centre of the ATD peaks related to the steps
Sample Limits (°C) Maximum T (°C)
Range 2-a Range 2-b Range 2-c Range 2-a Range 2-b Range 2-c
Bent - - 448 – 690 - - 662*
Bent 0.4 220 - 343 343 - 573 573 – 784 291 322 655
Bent 0.8 212 - 321 321 - 560 560 - 779,5 311 329, 401, 534 650
Bent 2.0 184 - 334 334 - 554 554 – 758 300 346, 417, 516 668
Bent 3.0 185 - 313 313 - 544 544 – 782 282 342, 548 648
*Endothermic event that corresponds to dehydroxylation of layers.
Figure 2. Ri(α/β) (equation 4) in function of surfactant loading. (): Ri(2-
-a/2-b), (): Ri(2-a/2-c), (): Ri(2-b/2-c)
Figure 3. Washing effect from a Bent-2.0 sample. Squares indicate mass loss
in range 2-a, circles in range 2-b and triangles in range 2-c
Figure 4. X-ray diffraction pattern of indicated samples: Bent-0.4 (a), Bent-
0.8 (b), Bent-2.0 (c) and Bent-3.0 (d)

Naranjo et al.
4Quim. Nova
To determine the type of structure developed by the surfactant
intercalated within clay layers, the molecule of HDTMA+ was mod-
elled in an all-trans conformation.
Considering both the computational models and the interlaminar
distances experimentally obtained, different structures that the surfac-
tant could present in the interlamelar space were proposed (Table 2).
N2 adsorption
For expandable clays, N2 adsorption studies give a sub-evaluation
of the specific surface area value because they only provide informa-
tion about the external surface.7,17 They also provide information about
the meso and macropores that could be generated by the ordering of
tactoids in the structures of the “house of cards” type18 or, in the case
of mixture of clays with other materials of bigger grain size, by the
spatial ordering of particles.19
The N2 adsorption isotherm of Bent sample presented a hysteresis
loop type B, indicating mesopores presence (Figure 5).20 This hys-
teresis loop decreased with the SL until it disappeared for Bent-2.0
and Bent-3.0 samples. Pore size distribution (Figure 6-A) displayed
a maximum of pore size of 20.6 Å for Bent sample, increasing up to
25.1 Å for Bent-0.4 and Bent-0.8 samples. A maximum in the curve
of pore size distribution for Bent-2.0 and Bent-3.0 samples was not
observed. The value of BET surface area decreased from Bent (56 m2
g-1) to almost 10 m2 g-1 for clays with higher SL (Table 3).
Total pore volume (VTP) increased with the SL from Bent to
Bent-0.8, and then decreased for organoclays with higher SL (Table
3). Studies of scanning electron microscopy (not shown) have shown
that the starting clay presents a rather compact structure with a great
fraction of the layers presenting a face to face ordering. On the other
hand, organoclays present a more opened structure called “corn
flakes”. This change in the structure generated the biggest porosity
detected in organoclays. When the SL exceeded a certain value, the
excess of surfactant occupied mesopores or blocked their entries,
and, in this way, porosity decreased.
XRD results indicated that the interlayer space was never higher
than 10 Å, so that observed mesopores (within 20.6 and 25.1 Å)
corresponded to the pores generated by the “house of cards” order-
ing type of tactoids. This indicate that, apart from the amount of
Table 2. Peak position, basal spacing and structure proposed for the surfactant
in the interlayer space. Values obtained by XRD analyses
Sample (2θ)d(001)
(Å)
l
(Å)
a
(%) Stucture
Bent 7.3 12.1 2.6 100 -
Bent-0.4 6.22 14.2 4.7 100 Monolayer
Bent-0.8 6.08 14.5 5.0 33 Monolayer
5.27 16.8 7.3 67 Ps. Bilayer/Bilayer
Bent-2.0
9.28 9.5 0.0 4 Collapsed
6.54 13.5 4.0 4 Monolayer
4.78 18.5 9.0 92 Bilayer/Ps. Trilayer
Bent-3.0
9.06 9.8 0.3 6 Collapsed
6.30 14.0 4.5 2 Monolayer
4.60 19.2 9.7 92 Bilayer/Ps. Trilayer
l = Basal spacing; a = % of X-Ray diffraction peak area.
Figure 5. N2 adsorption-desorption isotherms. Symbols indicate: () Bent;
() Bent-0.4; () Bent-0.8; () Bent-2.0 and () Bent-3.0 samples
Table 3. BET surface area, C constant, micropore (VμP), mesopore (VmP)
and total pore (VTP) volume and fractal dimension of indicated samples.
Values obtained by N2 adsorption/desorption isotherms
Surfactant Loading
Bent Bent-0.4 Bent-0.8 Bent-2.0 Bent-3.0
SBET (m2/g) 56 25 27 11 12
C (BET) 348 72 100 29 29
VμP (cm3/g) 0.008 0.000 0.000 0.000 0.000
VmP (cm3/g) 0.077 0.098 0.116 0.052 0.059
VTP (cm3/g) 0.085 0.098 0.116 0.052 0.059
D 2.71 2.51 2.50 2.46 2.45
Pore size (Å) 20.6 25.1 25.0 - -
Figure 6. Pore size distribution (A) and Fractal dimension (B). Symbols
indicate: () Bent; () Bent-0.4; () Bent-0.8; () Bent-2.0 and ()
Bent-3.0 samples

Synthesis and characterization of HDTMA-organoclays 5
Vol. XY, No. 00
HDTMA+ adsorbed in the interlayer space, there was another frac-
tion of surfactant adsorbed in the external surface, occupying and/or
blocking the mesopores.
To examine if an effect of surface coverage was present, frac-
tal dimensions (D) were calculated using equations developed by
Frenkel, Halsey and Hill (FHH Theory).21 D can be considered as an
operational measure of surface roughness. Generally, D is placed be-
tween 2 (soft and regular surface) and 3 (surface extremely irregular).
In this work, as the starting solid (Bent) presents hysteresis, the
equation used for calculating the fractal dimension was D = 3 – 1/m,
where (1/m) was the absolute value of the slope obtained in the
Ln(Ln(P0/P)) vs Ln(Vads/VM) graph (Figure 6-B).
Starting clay presented a high value for the fractal dimension
(DBent = 2.71), similar to the value obtained by Wang for Saz-1 clay
(DWang = 2.74),21 indicating a very irregular surface. The D value of
organoclays decreased as surfactant loading increased. This decrease
was produced in two stages: the first stage for Bent-0.4 and Bent-0.8
(DBent-0.4 = 2.51, DBent-0.8 = 2.50), that depends on the elimination of
micropores as well as on the “softness” of the surface by the surfactant
and the second stage for Bent-2.0 and Bent-3.0 (DBent-2.0 = 2.46, DBent-3.0
= 2.45) due to the increase in the amount of molecules of surfactant
in the organoclay and the blocking of mesopores.
As the fractal dimension decreased and Total Pore Volume in-
creased together with the SL for Bent-4.0 and Bent-0.8 solids, the
un-intercalated surfactant covered the external surface as well as the
internal surfaces of the walls of pores formed by tactoids grouping
in “house of cards” type structures.
Consequently, from nitrogen adsorption isotherms it could be
concluded that cationic surfactant, besides intercalating within clay
layers, was adsorbed over the external surface and over the internal
walls of mesopores, coating and decreasing surface roughness. It
also fills or blocks mesopores, decreasing hysteresis and making it
disappear from organoclays with higher SL.
FTIR
IR spectra of samples (Figure 7) showed that the absorption bands
corresponding to the starting clay were also seen in organoclays, ap-
proximately in the same positions.
Symmetric bending bands, rocking bands, and symmetric and
asymmetric stretching of methylene (νs(CH2) and νas(CH2) respec-
tively) were observed in all organoclays. ν(N-H) peak starts to be seen
from a SL of 2.0 CEC, as corresponds to free HDMTA+ molecules.22
This result agrees with the results obtained in N2 adsorption studies,
indicating that for Bent-2.0 and Bent-3.0 organoclays, part of the
surfactant is adsorbed out of the interlayer space, filling or blocking
mesopores.
The more intense bands in the spectrum of surfactant correspond
to symmetric (νs (CH2)) and asymmetric (νas (CH2)) stretching of
methylene groups of the carbon chain of surfactant. As SL increases,
the intensity of the bands increases and the frequency decreases, ac-
cording to literature.21,23-27
Modelling the developed structures
Molecular modelling calculation showed that the wider surface
that a HDTMA+ molecule can cover is of approximately 100 Å2.
Thermogravimetry indicates the amount of molecules that were
absorbed for each SL, and from DRX results the structure of the
interlayer space of each organoclay was obtained. Using these data,
the surface that can cover those surfactant molecules (SHDTMA+) could
be calculated in the following way:
SHDTMA+ (m2/g) = f*nHDTMA+ (mol)*NA*sHDTMA+ (Å2)*10-20 (m2Å-2) /
Mclay (g) (1)
where SHDTMA+ is the specified surface (in m2 g-1) that n moles of
adsorbed surfactant (nHDTMA+) can cover, taking into account the
surfactant structure in the interlayer space (monolayer, bilayer, etc);
NA is the Avogadro number; sHDTMA+ is the surfactant molecule area
(Å2); 10-20 is the conversion factor between m2 and A2; Mclay is the
clay mass (g), and f is a factor that takes into account the amount
of surfactant layers present in the interlayer space of the clay. The f
factor is obtained knowing that, for example, if a bilayer is formed,
the surface covered by the surfactant will be equivalent to the clay
surface, as the surface of the inferior layer and the superior layer will
both be considered, and therefore fbilayer = 1. In the same way, if a
monolayer is formed the factor must be fmonolayer = 2, and for a trilayer
ftrilayer = 2/3. For the intermediate structures, factors are calculated as
the average of the structures that make them up. So, fps-bilayer = (2+1)/2
= 1.5, and fps-trilayer = (1+2/3)/2 = 5/6.
In the case of Bent-0.4 organoclays, the structure that was formed
is a monolayer and the factor that was applied is fmonolayer = 2. For the
other cases, the structures were interstratified, so that the factor that
was applied is a weighted average, taking into account the percentages
of the different structures present in each organoclay. For Bent-2.0
and Bent-3.0 cases the percentage of collapsed layers were also taken
into account (Table 2).
Equation (1) can be used with the total mass of absorbed organic
compounds or with the mass of any of the fractions determined by
thermogravimetry. In each case, the surface that the surfactant mol-
ecules of the employed range can cover will be obtained. The results
obtained employing the total amount of the adsorbed organic com-
pounds are shown as SHDTMA+, Total, and the amount absorbed in range
2-(b+c) as SHDTMA+, 2-(b+c) (Table 4). The percentage that this surface
represents in respect to the total surface of starting clay (621 m2 g-1)10
is also indicated.
For SL higher than 0.8, SHDTMA+, Total exceeded 100% of covered
surface, which corroborated the fact that a fraction of surfactant is
adsorbed out of the interlayer surface in the mesopores generated
by tactoids ordering, causing the decrease of the pore volume and
the elimination of the hysteresis loop that was observed in the N2
adsorption isotherm. On the other hand, SHDTMA+, 2-(b+c) never exceeded
100%, achieving 99% of surface covering for a SL of 0.8. The result
confirmed that the surfactant corresponding to stage 2-(b+c) was
Figure 7. IR spectra of indicated samples. Bent (a), Bent-0.4 (b), Bent-0.8
(c), Bent-2.0 (d), Bent-3.0 (e) and HDTMA-Br (f)

Naranjo et al.
6Quim. Nova
Table 4. Covered surface (%) obtained applying equation 1
Sample Factor Range 2 (Total Organic) Range 2-(b+c)
sHDTMA+, Total (m2/g) % covered surface sHDTMA+, 2-(b+c) (m2/g) % covered surface
Bent-0.4 2.00 475 77 383 65
Bent-0.8 1.50 701 113 579 99
Bent-2.0 0.96 1081 174 506 86
Bent-3.0 0.94 983 158 447 76
adsorbed between layers. SHDTMA+, 2-(b+c) decreased for the organoclays
with higher SL (2.0 and 3.0) which could be due to the excessive
release of heat that was generated by the combustion of surfactant
in range 2-a, as was explained before.
CONCLUSIONS
Obtained organoclays resulted to be more hydrophobic than the
starting clay.
Surfactant, both in molecular and cationic form, was adsorbed
in the Bent clay in at least two kinds of different sites, indicated by
the different thermal stabilities.
From characterization results and molecular modelling calcula-
tion, a mechanism of adsorption is proposed. The surfactant was
adsorbed initially, in his cationic form, in sites placed in the interlayer
space of the clay through cation exchange. Adsorption in these kind
of sites continued until the interlayer space was saturated. Depending
on the quantity introduced in the intercalation process, different
organizations of surfactant in the interlayer, both in molecular and
cationic form, were obtained, varying from monolayer in Bent-0.4
to Bilayer/Pseudo-trilayer in Bent-3.0.
Further adsorption of surfactant, principally in molecular form,
occurred in the mesopores generated by tactoids ordered in the “house
of cards” type structure. This process left surfactant molecules relati-
vely free, out of the interlayer space. This fact could be favourable or
unfavourable, and must be analyzed for each potential application in
particular (catalytic processes, rheological control agents in paintings
and lubricants; polymer and plastics matrix reinforcing; adsorbents
for effluent treatments, oil spilling, releasing active matrix, etc.).
ACKNOWLEDGEMENTS
The funding for this work was granted by “Consejo de
Investigación de la UNSa” (Project Nº 1632) and SECyT FONCyT-
ANCyP (Project 1360). Authors acknowledge Lic. L. Davies, Ing.
S. Locatelli, Dra. D. Acosta and Ing. J. Villarroel Rocha for their
technical assistance and clarifying discussions. Pablo Naranjo thanks
CONICET and Chubut Province for their fellowships.
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