Clay Minerals (1998)33,467~174
Application of phenyl salicylate-sepiolite
systems as ultraviolet radiation filters
C. DEL H O Y O , M. A. V I C E N T E *
AND V. R I V E S 1
Departamento de Qulmica lnorgdnica, Universidad de Salamanca, Salamanca, Spain, and *Instituto de Recursos
Naturales y Agrobiologia, CSIC, Cordel de Merinas, s/n, Salamanca, Spain
A B S T R ACT: The interaction between phenyl salicylate and sepiolite has been studied using drug-
clay systems obtained by melting and grinding. The samples have been characterized by powder Xray diffraction, differential thermal and thermogravimetric analyses, and Vis-UV and FT-IR
spectroscopies. 'Free' water molecules are steadily substituted by the drug molecules, without any
chemical change as shown by FT-IR. The systems prepared improved the protecting ability of the
pure sepiolite or the pure drug against ultraviolet radiation, especially in the so-called 'C' range
(290-190 nm).
The interaction between drugs and clay minerals is
one of the most widely studied fields within clay
science (Su & Carstensen, 1972; MacGinity &
Lach, 1976; Porubcan et al., 1978) due to the many
different applications of these systems. In previous
papers (Vicente et al., 1989; del Hoyo et al., 1993)
we have described the use of drug-clay systems
prepared by conventional impregnation methods as
radiation protectors. The increasing demand in
products to be used as radiation protectors against
the s o - c a l l e d ' C ' u l t r a v i o l e t r a d i a t i o n
(190-290 nm) to avoid skin cancer, has led to
study of the interactions between organic molecules, already known as radiation absorbers, with
clay minerals. Phenyl salicylate, also known as
salol, is a common component of radiation
protectors. However, this product, synthesized
through reaction of phosphorus oxychloride with a
mixture of phenol and salycilic acid, has a very low
solubility in water, 1 g/6670 mL (Merck Index,
1989), and so the impregnation method cannot be
used to prepare drug-clay systems. Alternative
methods have been used previously in the literature,
such as melting the organic molecule onto the clay,
and grinding mixtures of both (Ogawa et al., 1991,
1992; del Hoyo et al., 1995) to obtain intercalates
or organic-inorganic compounds by solid-state
reaction. We have previously reported on the
phenyl salicylate/montmorillonite interaction (dei
Hoyo et al., 1996), and we have reported a
comparative study, regarding the interaction of
this drug with sepiolite, to ascertain the differences
when using a fibrous instead of a layered clay.
Phenyl salicylate (Fig. 1) is a white powder which
melts at 41~ and boils at 173~ it is highly soluble
in acetone and chloroform (Merck Index, 1989).
EXPERIMENTAL
The sepiolite used was from Vallecas (Madrid,
Spain), commercially known as PANGEL S-9, and
was kindly supplied by TOLSA, SA (Madrid,
Spain). Its specific surface area was 328 m z g-1
COO"0
i Corresponding author.
OH
FI~. 1. Molecular structure of phenyl salicylate.
9 1998 The Mineralogical Society
468
C del Hoyo et al.
and its cation exchange capacity 5.2 mEq/100 g.
Phenyl salicylate was purchased from Fluka
(Germany).
The clay was characterized by elemental
chemical analysis, exchange capacity, powder Xray diffraction, FT-IR spectroscopy, nitrogen
adsorption at -196~ for specific surface area
and porosity assessment, and by differential thermal
and thermogravimetric analyses. Its ability to
absorb radiation was checked by Diffuse
Reflectance Vis-UV spectroscopy (del Hoyo, 1995).
Samples prepared by grinding were obtained by
intimately mixing 1, 2, 3, 5, 10, 25, 50, 75 or 90 g
of the drug with I00 g of clay, and grinding for 10
min in a ball mill; the optimum grinding time had
been determined previously (del Hoyo, 1995).
Samples for melting were prepared by mixing the
drug and the clay in the same weight proportions,
and heating the mixture at 43~ for 24 h. In both
series of samples, light absorption was measured in
the 500-190 nm range in a Shimadzu Vis-UV
spectrometer provided with an integrating sphere to
record the spectra by the diffuse reflectance
technique (Vis-UV/DR); the spectra were plotted
in a Shimadzu PR-1 plotter connected to the
spectrometer. The reference material used was
MgO, and the slit selected was 5 nm.
The DTA curves were recorded in a PerkinElmer DTA-1700 apparatus, with a vertical furnace,
chromel-alumel thermocouples, at a heating rate of
5~ min-1. The TG curves were recorded in a
Perkin-Elmer TGS-2 thermobalance. Both instruments were coupled to a Perkin-Elmer 3600 Data
Station and all thermal analyses were carried out in
air.
The FT-IR spectra were recorded in a PerkinElmer FT-1730 instrument, connected to a PerkinElmer 3700 data station using KBr pellets; a
nominal resolution of 4 cm -1 was used, and 100
scans were averaged to improve the signal-to-noise
ratio.
In order to assess the suitability of the drug/clay
systems prepared as radiation protectors in creams,
we have also checked the removal of the drug from
the clay surface, as its removal would probably
decrease the protection ability. The technique was
as follows: 100 mg of the drug-clay system
containing 50 mg drug/100 mg clay were suspended
in 50 ml of an aqueous solution containing 2.92 g
NaCI/I and 0.745 g KCI/I at pH = 5.5, in order to
reproduce the composition of human sweat (Vicente
et al., 1989). The suspension was immersed in a
water bath at 40~ and was continuously stirred;
after 15 min it was centrifuged and half of the
supematant liquid was removed, adding an identical
volume of solvent. The process was repeated six
times, each one after a 15 min stirring period. As
the drug is not soluble in water, the amount of drug
still remaining on the clay surface was directly
measured from the Vis-UV/DR spectrum of the
solid at a given wavelength.
For the sake of brevity, only data corresponding
to the most concentrated (and in some cases, least
concentrated) samples are presented, for both the
series prepared by melting and by grinding.
RESULTS
AND DISCUSSION
The Vis-UV/DR spectra of parent clay and the pure
drug are included in Fig. 2, together with the
spectra corresponding to the drug-clay systems,
prepared by melting and by grinding, with the
largest and the smallest drug/clay ratios. The VisUV/DR spectra of the samples prepared by melting
show three maxima at 212, 255 and 312nm;
however, the systems prepared by grinding show a
single band, with two overlapped maxima at 220
and 284 nm; these spectra are almost coincident
with those recorded for the drug/montmorillonite
system (del Hoyo et al., 1995), and the bands are
recorded in the spectral range expected for the
chromophores existing in this molecule. As shown
in Fig. 2, the spectra of the drug/clay samples are
not the mere superposition of the spectra of the
drug and the clay. The reflectance, for a given
series of samples, increases with increasing drug
content. The spectra of the samples prepared by
melting resemble that of the pure drug, extending in
a slightly wider wavenumbers range, while the
spectra of the samples prepared by grinding are
dominated by a single band, extending in a
narrower wavenumbers range.
Therefore, we conclude that both methods,
melting and grinding, are valid for preparation of
drug/clay systems to improve the ultraviolet
radiation absorption ability, especially in the
290-190 nm range.
Results from the drug removal studies were as
follows: removal corresponded to 19.2% for
samples prepared by grinding, but only to 3.8%
for those prepared by melting. Nevertheless, both
values are rather low, thus indicating that the
systems are fairly stable under the experimental
conditions (close to human physiology ones). In
Ultraviolet radiation filters
469
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FxG. 2. Vis-UV spectra (Diffuse reflectance) of: (a) sepiolite; (b) phenyl salicylate; (c) and (d) samples prepared
by melting; (e) and(f) samples prepared by grinding.
other words, the protecting ability is preserved after
placinging the samples in contact with water.
Due to the fibrous nature of the clay, the powder
XRD diagrams of the samples contain the same
diffraction lines as the diagrams corresponding to the
original pure support. Only after 10 rain grinding of
the samples with the highest drag content are some
diffraction peaks due to the drug detected. As
expected, the profile for sepiolite ground for
10 rain is identical to that of the original sample,
as this material only undergoes changes after at least
15 min grinding (Cornejo & Hermosin, I986).
The DTA curve for natural sepiolite shows an
endotherrnic effect at 70~ with a minimum at
120~ due to removal of 'free' water molecules. A
broad endothermic effect is recorded between 240
and 360~ with a weak minimum at 325~ due to
removal of 'bonded' water molecules. The weak
endothermic effect at 815~ followed by a sharp
exothermic effect at 830~ are due to dehydroxylation of the sepiolite structure, and formation of
clinoenstatite, respectively (Mackenzie, 1970).
The DTA curve for the clay ground for 10 min is
shown in Fig. 3e. The intense endothermic effect at
120~ is due to removal of 'free' water molecules.
The high temperature (>800~
effects are coincident with those recorded for pure sepiolite. It can
be concluded that no important changes in the DTA
curve develop for the ground clay.
The DTA profile for phenyl salicylate is shown
in Fig. 4a. A first, sharp, endothermic effect is
recorded at 59c'C, followed by a broader
470
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F[o. 3. (a) DTA curve of sepiolite; (b) TG curve of sepiolite; (c) DTA curve of the sample obtained by melting;
(d) TG curve of the sample obtained by melting; (e) DTA curve of ground sepiolite; (f) TG curve of ground
sepiolite; (g) DTA curve of the sample obtained by grinding; (h) TG curve of the sample obtained by grinding.
endothermic effect at 285~
and several
exothermic effects at 415 and 524~
The
corresponding TG diagram, Fig. 4b, shows that
weight loss starts above 150~ thus suggesting that
the sharp endothermic effect at 59~ is due to
melting (an endothermic, non-weight-loss process).
The endothermic effect at 285~ probably corresponds to the weight loss up to 296~
Decomposition and boiling probably take place
simultaneously, thus corresponding to the
exothermic effects at 415 and 524~ to combustion
of the residue, accounting for the weight loss above
296~
The DTA curve for the sample obtained by
melting is shown in Fig. 3c. Three effects are
recorded: the first, at 53~ is due to melting of the
drug. The effect recorded for pure sepiolite
(Fig. 3a) due to removal of 'free' water molecules,
is not recorded, probably because these water
molecules are substituted by drug molecules. Also,
the endothermic effect due to removal of 'bonded'
water molecules, recorded at 320~ for pure
sepiolite, is absent. Combustion of the organic
molecules accounts for the exothermic effect with a
maximum at 422~
Figure 3g shows the DTA curve for the sample
prepared by grinding. The first endothermic effect,
at 53~ should undoubtedly be ascribed to melting
of the drug. The second endothermic effect, at
97~ probably due to removal of water molecules,
is very weak, thus suggesting that most of the water
molecules have been substituted by drug molecules.
The endothermic effect at 241~ is mainly due to
partial decomposition of phenyl saticylate, while
burning of the organic molecules gives rise to the
main exothermic effect at 435~
Data from the TG analysis are in full agreement
with the DTA data discussed above. The TG curve
for parent sepiolite is shown in Fig. 3b. A sharp
weight loss is recorded up to 128~ (amounting to
7.1% of the initial sample weight), followed by a
smaller weight loss up to 236~ (0.9% of the initial
471
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TEMPERATURE('C}
Flo. 4. (a) DTA curve of phenyl salicylate; (b) TG curve of phenyl salicylate.
sample weight). A change in the slope of the TG
curve is recorded at 236~
and between this
temperature and 340~ 2.3% weight is lost. From
this temperature upwards the slope again decreases,
and a steady, small weight loss is recorded between
500 and 650~
Total weight loss up to 800~
corresponds to 15.5% of the initial sample weight.
The first weight loss corresponds to the intense
endothermic peak on the DTA curve, while the
medium temperature weight loss should be associated with the ill-defined endothermic DTA peak
at 326~ the weight loss between 340 and 800~ is
so small, and extended in a so wide temperature
range, that no defined DTA effect is recorded.
The TG curve of the ground (10 min) sepiolite is
shown in Fig. 3f. Up to four weight losses can be
discerned on this curve. The first one is much more
pronounced than the others, and is recorded
between 41 and 125~ (8.3% initial sample
weight), coinciding with the endothermic DTA
effect at 119~ for the ground sepiolite (Fig. 3e)
due to removal of free water molecules in the
channels of the crystal network. The second weight
loss, between 213 and 318~
is much weaker,
amounting to only 2.5% of the initial sample
weight, due to removal of bonded water molecules.
The third weight loss (428-672~ 3.1% weight) is
due to removal of sepiolite hydroxyl groups.
472
C. del Hoyo et al.
Finally, a fourth weight loss is recorded between
803 and 876~ corresponding to 1.0% of the initial
sample weight.
The TG curve shown in Fig. 3d corresponds to
the drug-clay sample prepared by the melting
method. The profile is rather similar to that
included in Fig. 4b for the pure drug. Weight loss
starts at a higher temperature than for the original
sepiolite, indicating that the free water has been
completely substituted by drug molecules. The
weight loss starts at 177~ The bonded water is
lost between 280 and 340~ corresponding to 3%
of the initial sample weight.
The TG curve corresponding to the sample
prepared by grinding is shown in Fig. 3h. Again
no weight loss is recorded below 100-110~
indicating that the free water existing in the original
support has been completely substituted by drug
molecules. The first weight loss extends from 115
to 199~ amounting to 40.4%, and is caused by
almost complete removal of the drug. Between 205
and 740~
a residual weight loss is recorded
(11.3%), due to removal of structural water
molecules and burning off of the drug residues.
The FT-IR spectroscopy has been used to assess
the chemical state of the drug adsorbed on the
sepiolite surface, The spectrum for the original
sepiolite is shown in Fig. 5a. The different
components of the broad band recorded around
3500 cm - l (hydroxyl stretching mode) are due to
the different types of hydroxyl groups (structural
and belonging to water molecules) in the sepiolite
structure. So, the band corresponding to type I or
free water molecules, is recorded at 3565 cm -I.
The signal due to bonded water (type II) is recorded
at 3419 cm -1, and the bending mode gives rise to
the medium intensity band at 1667 cm -I. For the
structural (type III) water molecules, the stretching
band and the deformation band are recorded at 3689
and 1618 cm -1, respectively. The lattice vibration
modes give rise to bands in the 1100-450 cm -j
range (Hayashi, Otsuka & Imai, 1969).
The spectrum shows slight changes when the
sepiolite has been ground for 10 rain (Fig. 5b). The
bands due to the deformation mode of free and
structural water are slightly shifted, and no change
was observed for the lattice vibrations.
The spectrum of pure phenyl salicylate is shown
in Fig. 5c. The C=O stretching mode gives rise to
the intense absorption at 1684 cm -1. The C - O
bond is responsible for the bands at 1214 and
1125 cm -1 (del Hoyo, 1995). The phenolic OH
group stretching mode gives rise to the band at
3434 cm -1. The C - H stretching modes give rise to
a series of weak absorptions slightly above
3000 cm -1, centred m a i n l y at 3060 and
3024 cm-k The skeletal modes are responsible for
the bands at 1616, 1584, 1482, and 1460 cm-k
The spectra corresponding to the drug-clay
systems studied are shown in Figs. 5d and e.
These spectra can be analysed taking into account
those of the sepiolite and of the drug. For the
system prepared by melting (Fig. 5d) the stretching
mode of OH groups corresponding to type III water
accounts for the band at 3680 cm-1; the corresponding deformation band is recorded at
1616 cm -1. The C=O stretching mode gives rise
to the band at 1685 cm -1. The bands at 1191 and
1304 cm - l are due to coupling between the C - O
stretching mode and the out-of-plane deformation of
the hydroxyl group. The very weak band at
3075 cm -1 is due to C - H stretching of the
aromatic moieties. The shift between the positions
of these bands and those recorded for bulk drug are
within experimental error, indicating that no sort of
decomposition takes place when the drug is
supported by melting on the sepiolite surface.
The spectrum corresponding to the drug/clay
system prepared by grinding is shown in Fig. 5e. In
this case, the C=O stretching mode is recorded as a
very intense, sharp, band at 1690 cm-1; the skeletal
modes give rise to the bands at 1619, 1579, 1482
and 1453 cm -l. As for the sample prepared by
melting, no shift with respect to the positions of the
bands for the pure drug was observed, thus
indicating that the organic molecules do not
decompose when supported on the sepiolite surface.
However, from an overall point of view, the
interaction between phenyl salicylate and the
sepiolite surface seems to be rather weak, as the
positions of the bands do not shift very much from
the positions for the pure drug and the pure
sepiolite. The main drug bands (1700-1500 cm -1
range) shift towards lower wavenumbers. This is
related to the lesser strength of the chemical bonds
responsible for these bands, and such a shift is more
pronounced for the sample prepared by melting
(Fig. 5d). Also, for this sample, the absorption in
the OH- stretching region has increased to a wider
wavenumber range. This indicates that the interaction (despite being weak) between the drug and the
clay is stronger in the system prepared by melting.
Interaction of this drug with smectite was also
rather weak, with only minor changes in the FT-IR
473
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Fro. 5. FT-IR spectra of: (a) sepiolite; (b) ground sepiolite; (c) phenyl salicylate; (d) sample obtained by melting;
(e) sample obtained by grinding.
spectrum (del Hoyo et al., 1996). The difference,
however, exists in the XRD patterns, which showed
swelling of the layered structure in the case of the
drug-smectite system, a situation not attainable for
the fibrous sepiolite.
CONCLUSIONS
The systems formed by phenyl salicylate supported
on sepiolite, obtained by melting or by joint
grinding of both, exhibit an important increase in
their ability to absorb ultraviolet radiation, especially in the so-called 'C' range (290-190 nm).
Desorption of the drug from the clay, under
experimental conditions close to those of human
sweat, is very small. The studies by thermal
analysis and FT-IR spectroscopy indicate that the
drug molecules substitute the type I water
molecules in the sepiolite network, and that this
process takes place without any strong chemical
474
C del Hoyo et al.
modification of the drug molecule. With regard to
the differences between both sets of samples
prepared, the drug/clay interaction seems to be
slightly stronger for samples prepared by melting.
In this case, the dispersion of the drug molecules on
the clay surface is probably larger than when
simply ground. Also, diffusion (of both drug and
water molecules) is favoured in the samples
prepared by melting, because of the temperature
increase.
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