Clay Minerals (1983) 18, 447-458.
REACTION
PRODUCTS
OF ORGANIC
MOLECULES
WITH ACID-TREATED
MONTMORILLONITE
R.
FAHN
DYE
AND K . F E N D E R L
Siid-Chemie A.G., Postfach 20 22 40, 8000 Mh'nchen 2, FRG
(Received 18 April 1983)
A B S T R A C T : During the reaction of solutions of the leuco dyes crystal violet lactone
and N-benzoyl leuco methylene blue with acid-treated montmarillonite (as occurs with
carbonless copying papers), most of the offered dye is intercalated within the interlayers of the
montmorillonite structure remaining after acid attack. This phenomenon was demonstrated by
XRD and also by high-resolution transmission electron microscopy. During acid treatment,
octahedral cations are dissolved from exposed edges of the montmorillonite lamellae and
amorphous silica builds up in these areas. As a result, the BET(N2) specific surface area
increases from ~60 mZ/g to ~300 mZ/g and, consequently, the theoretical reactivity towards
leuco dyes. Nevertheless, only small quantities of the dye molecules are adsorbed onto the
amorphous silica. The excellent colour intensity and, in particular, the resistance to fading in
the light of the montmorillonite dye complexes may be explained by the protected position of
the dye molecules within the interlayers of the montmorillonite.
Montmorillonite is capable o f intercalating organic molecules between its unit layers
(Fahn, 1963) and such c o m p o u n d s with long-chain quaternary alkyl a m m o n i u m ions
have attained much importance in the paint industry as thixotropic and thickening agents
( F a h n & Buckl, 1968). Reactions o f dye molecules with acid-activated montmorillonite are
also becoming important in their capacity as colour developers in the acceptor sheet of
carbonless copying papers (von Raven & F e n d e d , 1980; Fenderl & v o n Raven, 1982).
The present paper describes the results o f an investigation into the reactions o f dye
molecules with acid-activated montmorillonite to form light-resistant stable complexes.
Particular aspects examined were the a m o u n t o f dye uptake as a function o f time, and
the physical and chemical changes occurring in the acid-activated montmorillonite as a
result of dye adsorption.
MATERIALS
Montmorillonite
Tests were m a d e with acid-activated Bavarian bentonite, which is marketed under the
brand n a m e C O P I S I L | During acid-activation the exchangeable cations are replaced by
hydrogen ions, and aluminium, magnesium and iron are dissolved from the octahedral
sheet. The extent o f cation removal depends on acid concentration, temperature and time.
9 1983 The Mineralogical Society
R. Fahn and K. Fenderl
448
Amorphous silica builds up at the boundaries of the silicate lamellae and, as a result, the
specific surface increases considerably from ~60 m2/g to ~300 mZ/g, with a consequent
increase in theoretical sorptive capacity (Fahn, 1979).
Some chemical and physical properties of COPISIL are presented in Table 1.
Dyes
The leuco compounds, crystal violet lactone (CVL) and N-benzoyl leuco methylene blue
(N-BLMB), usually used in the manufacture of carbonless copying papers, served as dye
precursors. On contact with the acid-activated montmorillonite the leuco dyes react
according to the following equations, developing the coloured compounds (Sliwka, 1975):
H3C\
,CH3
N.
.<-.
H~C/ Y
"~
..-~.
( ~
~
-HaC
N/
\CH3
,CH3-]G
H3C
\CH 3
C
C=O
COPISIL~-
N..
H3C
CH3
H3C
crystal violet lactone
H3C~
N
crystal violet
/CH3
S
CH 3
H3C\
N
/CH3
N
S
N
o
~N /
~
C~
~
coH2IOIL)
~N / ~
I
\r,/
+
9 89
H3C~N~S~N\cH31
N-benzoyl leuco
methylene blue
--
methylene blue
CVL reacts with the Lewis acid COPISIL, opening the lactone ring and producing a
cationic dye. Another cationic dye (methylene blue) is formed during the reaction with
N-BLMB.
Operation of carbonless copying papers
For colour development, the front of a base paper sheet (CF) is covered with a coating
colour containing COPISIL as the main component, together with water and binders such
Reaction of leuco dyes with acid-montmorillonite
449
TABLE 1. Chemical and physical properties of
COPISIL.
Representative chemical composition (%)
SiOz
72.0
AI203
13.6
Fe203
3.3
CaO
0.6
MgO
2.0
Na20
0.1
K20
0.8
Loss on ignition
7.3
pH value (10% slurry)
3.5
Brightness (Elrepho at 457 nm)
77%
Specific surface (BET/N 0
290 m2/g
Mean particle size (by sedimentation)
2.5 gm
Water content
5.2%
as polystyrene butadiene latex, C M C , or starch (Rohmann & Schoepke, 1982). The
coating colour is usually adjusted to a pH value of 9 - 1 0 and applied to the paper as a thin,
uniform layer with a coating weight of 6 - 8 g / m 2.
The back of a second paper sheet (CB) is coated with micro-gelatine or polyurethane
capsules ( 1 - 1 0 gm) which contain a solvent with the leuco dyes dissolved therein
(Brunner, 1982; Baxter, 1974). Under pressure, for instance that resulting from the stroke
of a typewriter, the micro capsules at this exact point break, the liquid is released and is
adsorbed by the C F side, and the colour develops on contact with C O P I S I L .
EXPERIMENTAL
Adjustment of pH value
In order to adjust the pH-value of the acid-activated bentonite C O P I S I L (usually ~ 3.5),
500 g of the clay were first suspended in distilled water to give a 15 wt % suspension.
Caustic soda solution was then added until the pH of the suspension reached steady values
CB
FIG. 1. Operation ofcarbonless copying papers. CB and CF described in text.
450
R. Fahn and K. Fenderl
of either 6.8 or 9.5. The suspension was filtered and the filter cake washed with a little
distilled water. The reaction product was dried to constant weight at l l 0 ~
and
subsequently ground.
Dye adsorption
0.02 to 2 g portions of C V L and N-BLMB were dissolved in 100 and 200 g of toluene,
respectively. 20 g of acid-activated C O P I S I L (on an oven-dried basis) were added with
stirring. After contact times ranging from 10 min to 10 days, the coloured adsorbate was
separated by means of a glass filter funnel. The filter cake was repeatedly suspended in
fresh toluene and filtered until the filtrate was nearly free of dye. The collected filtrate was
used for the quantitative determination of non-adsorbed C V L or N-BLMB. The filter cake
was dried at 60~ in a vacuum drying oven for 18 h and subsequently dried in high
vacuum for 16 h to remove the adherent toluene.
Quantitative determination of CVL
6 ml of saturated anhydrous zinc chloride solution in pure acetone were added to 2 ml
of toluenic C V L solution, the concentration of which was in the range of 0 . 0 1 - 0 . 1 0 % by
weight. After 2 min, the resulting deep-blue coloured solution was measured with a UV
spectrophotometer in a 1 cm cell at 384 nm against pure acetone and a calibration curve
constructed.
Quantitative determination of N-BLMB
Amounts of N-BLMB ranging from 0.07 to 1.0 g were placed in a 250 ml beaker,
100 ml of distilled water and 20 ml of concentrated hydrochloric acid added, and the beaker
covered with a watch glass and brought to the boil. After 5 rain boiling, the solution was
transferred into a 1 1 flask and filled to volume with distilled water. 2.0 ml of this solution
were pipetted into another dry flask and 2.0 ml of 1 Y sodium nitrite solution were added.
After shaking, the solution was allowed to stand for 10 min. Following dilution with 50 ml
of ethanol, the solution was transferred to a 500 ml flask and filled to volume with ethanol.
The blue solution was then measured with a UV spectrophotometer in a 1 cm cell at
655 nm against pure ethanol and a calibration curve constructed.
RESULTS
AND
DISCUSSION
Dependence of dye adsorption on time
For the original acid-activated bentonite C O P I S I L (pH value 3.8) and the dye precursor
CVL, the adsorption time was varied in the range 10 min to 10 days. The C and N
contents of the toluene-free deep-blue reaction products ('adsorbates') were analysed. The
results obtained, and the molecular weight of C V L (415.5 g), served as basis for
calculating the amount of adsorbed C V L (Table 2).
The data in Table 2 show that the adsorption time exercises a significant influence on
the amount of adsorbed CVL. Note that the amount of adsorbed C V L calculated from the
difference between C V L offered and C V L not adsorbed is of the order of 6% at an
adsorption time of 10 min and that it rises to nearly 9% after contact times of several days.
Reaction of leuco dyes with acid-montmorillonite
451
TABLE2. Dependence of amount of CVL adsorbed by COPISIL on adsorption time.
CVL adsorbed
estimated from
CVL per 100 g of COPISIL
Adsorption
time
Offered
g
Filtrate
g
Adsorbed
g
CVL
adsorbed
%
C content
of adsorbate
%
N content
of adsorbate
%
10 min
lOmin
10 min
1 day
3 days
10 days
1.6
3.2
6.4
10.0
10.0
10.0
0
0
0.2
0-6
0.3
0.2
1.6
3.2
6.2
9.4
9.7
9-8
1.6
3.1
5.8
8.6
8.8
8.9
1.6
3.1
6.2
8.6
8.5
--
1.8
3.1
5.6
----
The amounts of adsorbed C V L determined in this manner correspond well with those
calculated from the C and N contents.
Adsorption tests carried out with the dye precursor N - B L M B showed a similar
dependence on the time of adsorption.
Dependence of dye adsorption on pH value of the acid-activated bentonite
Adsorption tests were made on acid-activated bentonites adjusted to different p H values
with both dye precursors, the offered amounts o f C V L and N - B L M B varying in the range
0 . 2 - 9 g per 100 g adsorbent. In all cases, the adsorption time was only 10 min in order
to approach the very short contact times pertaining to conditions of use of carbonless
copying papers.
Similar adsorption curves were given by C V L and N - B L M B (Figs 2 and 3). Both the
dye precursors are fully adsorbed (up to ~ 3 % ) at low concentrations offered, and this is
g CVLadsorbed/
7.
100g of COPISIL
PH 3,8
pH 6,5
6"
~H
>///
5-
9,5
~.
3"
"
2.
1.
0
0
~
~
~
~
s
6
C V L o f f e r e d / 1 0 0 g of COPISIL
_-_
~
g
FIG. 2. Effect ofpH on CVL adsorption by COPISIL.
452
R. Fahn and K. FenderI
N- BLMB adsorbed /
100g of COPISIL
g
7
pH 3,8
6-
pH 6,5
54-
pH 9,5
3210i
0
N-BLMB offered/100g of COPISIL
nl
i
Fro. 3. EffectofpH on N BLMB adsorption by COPISIL.
independent of the pH value of the acid-activated bentonite. At higher dye concentrations
CVL and N-BLMB are not fully adsorbed. In this context it is worth noting that the initial
acid-bentonite (pH - 3 . 8 ) adsorbed higher amounts than when adjusted to pH values
of 6.5 and 9.5.
Table 3 shows that in this series of experiments also, the amounts of adsorbed dye
precursors determined by the difference between dye offered and dye not adsorbed
correspond well with those quantities calculated from the C and N contents of the
adsorbates.
The specific surface area is reduced by the pH adjustment as well as by the dye
adsorption. As can be seen from Table 3, the initial specific surface of the acid-activated
bentonite is 290 m2/g but is lowered to 260 m2/g after adjustment to pH 6.5 and to 155
m2/g after adjustment to pH 9.5. Furthermore, Table 3 reveals that adsorption of CVL
and N-BLMB reduces the specific surface area of the three adsorbents, generally in
proportion to the extent of leuco dye adsorption. On the whole, reduction of the specific
surface area resulting from pH adjustment of COPISIL is more important than that
caused by dye adsorption.
Dependence of colour development and resistance to fading in light of paper coated with
COPISIL on pH value
Preparation of CF papers. Coating colours were prepared according to the formula
100 parts of COPISIL to 11 parts of latex at pH values of 3.8, 6.5 and 9.5; pH adjustment was by means of 30% caustic soda solution. Using a rod, the colour was coated on
the base paper at a coating weight of 6.5 g/m 2. These CF papers were cut into strips of
100 cm z for the dipping test.
Reaction of leuco dyes with acid-montmorillonite
453
TABLE 3. Dependence of amount of CVL and N-BLMB adsorbed by COPISIL on pH.
Amount of CVL adsorbed after 10 min
pH of
acid
activated
bentonite
3.8
6.5
9.5
Dye
o~red
g
Amount of N-BLMB adsorbed after 10 min
From From Specific
C eont, N cont. sur~ce
%
%
m~
g
%
0
1.6
3.2
6.4
7.0
9-0
0
1-6
3.2
6.2
6.3
0
1.6
3,1
5.8
5.9
0
1.6
3.1
6.2
0
1.8
3-1
5.6
290
278
269
258
0
0
0
0
290
3.2
6.0
3.1
5.7
3.1
5.9
3.2
5.5
275
262
7.2
6.7
0
0-4
1.6
3.2
6.4
7.0
9.0
0
0-4
1.6
3-2
5.7
5.9
0
0.4
1.6
3-1
5-4
5-6
0
0
0
0.4
1-6
3.1
5.3
0
260
1.7
3-2
0
0.4
1-6
3.2
5.6
0
1-6
3.1
260
260
243
238
3.2
6.0
3.2
5.1
239
225
5.7
5.4
0
0.4
1.6
3.2
6.4
0
0.4
1.6
3.0
4-1
0
0.4
1.6
2.9
3.1
0
0.4
1.6
2-8
3-0
0
0
155
2.9
3.4
3.0
3.2
127
127
0
0.4
1.6
2.9
3.9
0
0
1.7
3-0
1.7
3.0
155
145
134
g
From From Specific
C cont. N cont. sur~ee
%
%
m2/g
%
Dipping test. S t r i p s w e r e d i p p e d for 15 s i n t o 100 g o f 0 . 0 0 5 % t o l u e n i c s o l u t i o n s o f
C V L a n d N - B L M B . I m m e d i a t e c o l o u r d e v e l o p m e n t t o o k place, b l u e in t h e c a s e o f C V L
a n d light t u r q u o i s e for N - B L M B . I n t h e tests, h i g h l y diluted l e u c o d y e s o l u t i o n s w e r e u s e d
in o r d e r t o o b t a i n realistic c o l o u r intensities. A f t e r t h e e x c e s s liquid h a d d r i p p e d off, t h e
m o i s t p a p e r w a s d r i e d w i t h a h a i r - d r y e r for 3 0 s a n d t h e n k e p t in t h e d a r k for 15 m i n . T h e
initial c o l o u r d e v e l o p m e n t w a s t h e n m e a s u r e d u s i n g a n E l r e p h o s p e c t r o p h o t o m e t e r fitted
w i t h a R Y filter. R e s i s t a n c e t o f a d i n g in t h e light w a s d e t e r m i n e d b y e x p o s i n g t h e C F
s t r i p s to i r r a d i a t i o n in a X e n o n test u n i t ( S u n T e s t e r ) f o r 2 h a n d t h e n m e a s u r i n g t h e R Y
value. R e s u l t s are g i v e n in T a b l e 4. N o t e t h a t t h e l o w e r t h e v a l u e o f R Y , t h e m o r e i n t e n s e is
the image.
TABLE 4. Dipping test with COPISIL CF papers of coating colours of different pH values
Dipping test
pH values of coating colours
Using
Concentration
%
Colour development
3.8
6- 5
9- 5
CVL
0.005
RY after 15 min in the dark (%)
RY after 2 h in Sun-Tester (%)
61
68
47
64
38
61
N-BLMB
0.005
(RY after 15 min in the dark) (%)
RY after 1/2 h in Sun-Tester (%)
RY after 2 h in Sun Tester (%)
(84)
63
59
(83)
63
59
(81)
61
59
CVL/N-BLMB
(3: 1)
0.005
RY after 15 min in the dark (%)
RY after 2 h in Sun-Tester (%)
64
66
51
62
45
63
R. Fahn and K. Fenderl
454
There is a distinct pH dependence, not only of the initial colour development but also of
the resistance of CVL to fading. In the acid and neutral range, colour development and
intensity are inferior to that at pH 9.5. This may well be the reason for adjusting the pH
value of the coating colours to between 9 to 10 during the production of carbonless
copying papers.
For the dye N-BLMB, the pH dependence is not so marked because this so-called
secondary dye precursor produces complete colour development only several hours later
on exposure to light. When using a mixture of CVL and N - B L M B - - a s is often done in
practice for obtaining blue copies--the initial colour development again is clearly
dependent on the pH-value. The colour fading resistance is only little affected, however,
because of the N-BLMB component.
Dependence of dye bonding on pH
Bonding of CVL and N-BLMB to COPISIL at a dye level of 3.2% (similar to that
present in carbonless copying paper) occurs within the interlayer region of the remaining
silicate layers of the acid-treated bentonite and not to the amorphous silica. Tests have
shown that when treating dye-loaded samples (pH values of 3.8, 6-5 and 9.5) with 2% hot
soda lye the amorphous silica is dissolved and thus removed. By measuring the carbon
and nitrogen contents, the dye amounts bound to the remaining blue coloured residue
were determined. Table 5 shows the results for CVL.
From the C content, ~5% CVL is bound to the residue after removal of the amorphous
silica (this amounted to more than 40%). Intercalation of the dye molecules between the
silicate layers would favour resistance to fading in the light because of the protected
position of the dye molecules. When the dyes are adsorbed only superficially, e.g. by
precipitated silica, rapid fading of the developed dye occurs.
Proof of dye intercalation between the silicate layers of COPISIL by X-ray diffraction and
electron microscopy
The chemical investigations described previously indicated that only a small amount of
dye was adsorbed by the amorphous silica and that the major part was associated with the
residual montmorillonite structure.
An X R D trace of COPISIL shows no distinct 001 spacing (Fig. 4a), but with 3.2%
N-BLMB addition a reflection occurs at 14-5 A (Fig. 4b). Following removal of the
TABLE5. Dependenceof dye bond to COPISILon pH.
Residue without
amorphous silica
Calculateddye adsorption
per 100 g of COPISIL
Dye offered Dye adsorbed Amorphous
CVL adsorbed = residue + amorphous
g CVL/100
CVL
silica
C cont. fromC content,
silica
COPlSIL g COPISIL
%
%
%
%
%
pH 3.8
pH 6.5
pH 9.5
3.2
3.2
3.2
3.1
3.1
2.9
42.0
41.5
43-4
3.6
3.7
3.8
4.8
4.9
5.0
2.8
2.9
2.8
Reaction of leueo dyes with aeid-montmorillonite
|
~<
455
_.~
e~
! t"
'~ .A/i'~L - -
,I
v.
~
~11
10--9--8
|
i
7
r
"
6--5--4--3
0
FIG. 4. XRD traces of: (a) COPISIL; (b) COPISIL with 3.2% N-BLMB; (c) COPISIL with
3.2% N-BLMB after removalof amorphous silica. Cu-Ka radiation.
amorphous silica, the intensity of this reflection increases and it is also seen to consist of
two components (Fig. 4 c ) - - a peak at ~14-3 ~ corresponding to interlayers filled with
dye molecules and a peak at ~12-9 /k corresponding to interlayers containing water
molecules. Intercalation of the dye in the silicate interlayers of COPISIL appears
therefore to have increased their degree of orientation normal to the e axis as indicated in
Fig. 5.
The contrast between the disordered silicate layers in COPISIL and the high degree of
alignment in the c direction after dye intercalation can also be demonstrated by electron
microscopy (Fig. 6). In Fig. 6a, which is of COPISIL before dye addition, crystal edges
have been wedged open due to octahedral cation removal, and large amounts of
amorphous silica are visible adjacent to these areas. Groups of silicate layers are also
disordered with respect to each other. In Fig. 6b, which is of COPISIL containing
N-BLMB, the silicate layers show much better alignment and it is possible to recognize
differences in interlayer distances corresponding to interlayers filled with dye and those
filled with exchangeable cations and water molecules.
456
R. Fahn and K. Fenderl
(c)
(b)
(a)
FIG. 5. Schematic representation of: (a) silicate layers of montmorillonite; (b) disorientation of
acid-treated montmorillonite layers with amorphous silica in edge regions of crystallites;
(c) alignment affect of intercalated dye molecules.
CONCLUSIONS
The dye adsorption of the colour developer COPISIL is dependent both on time and pH
value. At offered dye levels of up to 3% of CVL and N-BLMB adsorption is complete
within a contact time of 10 min. A level of 3% dye is similar to that used in most of the
blue-writing carbonless copying papers, so that the results are relevant to common
practice. At 3% dye addition the pH-dependence is small. At higher dye levels the
adsorption is lowest at pH 9.5 for both CVL and N-BLMB. It is possible that this is
related to the observed decrease of specific surface area at increasing pH values.
With CVL, the immediate colour development is very sensitive to pH, being best in the
alkaline range. This is taken advantage of in practical applications. The resistance to
fading in the light is less dependent on pH, but is worst at pH 9.5. For N-BLMB the
immediate colour development is poor but improves with increasing incident light
radiation.
It has been shown that the dye is bound substantially in the montmorillonite structure
remaining after the acid treatment. Indeed, even though the specific surface area is
increased by the amorphous silica left after partial dissolution of the octahedral sheet at
exposed crystal edges, dye adsorption takes place only to a relatively low extent. The
bonding of the cationic dyes CVL and N-BLMB between the strongly disoriented montmorillonite layers is similar to that of exchangeable cations. Thus the silicate layers are
re-arranged in parallel order and the interlayer distance of ~10 /k in the dry state is
expanded to ~14.5 .A. Good resistance to fading in the light probably results from the
protected position of the dye molecules between the layers.
Reaction of leuco dyes with aeid-montmorillonite
FIG. 6. High-resolution (magnification •
transmission electron micrographs of
(a) COPISIL with disoriented silicate layers (b) COPISIL with intercalated N-BLMB and
well-aligned silicate layers. (a) is of a suspension on the microscope grid, (b) is of a resinembedded specimen prepared by microtomy.
457
R. Fahn and K. Fenderl
458
ACKNOWLEDGEMENTS
We thank Prof. H. K6ster, Lehrstuhl fiir Mineralogie der Technischen Universit~it Mfinchen, for performing
the electron microscopy, and his colleague Mr. Vali, Diplom-Mineraloge, for preparing the samples and the
images.
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editor). Plenum Press.
BRUNNER F. (1982)Pulp and Paper International May, 54.
FAHN R. (1963)Kolloid-Zeitschrift 187, 120.
FAHN R. (1979) Acid-activated clays and their adsorption properties. SME-AIME Fall Meeting, Tucson,
Arizona.
FAnN R. & BUCKLH. (1968) Keramische Zeitschrift 5, 1.
FENDERL K. & VON RAVEN A. (1982) Proc. TAPPI Coating Conf., 37.
ROrIMANN M.E. & SCHOEPKE H. (1982) Wochenblattfiir Papierfabrikation 110, 767.
SUWKA W. (1975)Angew. Chemie 87, 556.
VON RAVEN A. & FENDERL K. (1980) Wochenblattfffr Papierfabrikation 10g, 607.