Structure effects of amphiphilic and non-amphiphilic quaternary ammonium salts on photodegradation of Alizarin Red-S catalyzed by titanium dioxide

Article abstract of DOI:10.1039/c6ra25421g

The role of surfactants such as single- and double-tailed tetraalkylammonium bromide and various non-amphiphilic tetraalkylammonium salts was investigated on the TiO₂ photocatalyzed degradation of 3,4-dihydroxy-9,10-dioxo-2-anthracenesulfonic acid sodium salt (Alizarin Red-S, ARS) in an air-equilibrated alkaline medium under UV light irradiation. Absorption spectral analysis showed that the photodegradation efficiency of the dye was significantly enhanced by the addition of cationic surfactants. Two interesting findings emerged from this study: ARS was almost completely degraded at surfactant concentrations close to 1 mM (values well above the cmc in the experimental conditions); moreover, on increasing the surfactant concentration, the photocatalytic reaction became less and less efficient and significantly dependent on the surfactant headgroup size. The presence of a maximum of efficiency depending on the surfactant concentration was due to the combination of catalytic and inhibiting processes. The first one likely depended on the ability of the surfactant to improve the ARS approach to the semiconductor through the formation of cationic bilayers on the TiO₂ particles; this effect made the ARS more easily oxidized by TiO₂. This catalytic action of the surfactant was opposed by the increase of the micellar aggregate number in the aqueous bulk, which competes with the TiO₂ sites in associating to the dye. This was supported by the results obtained with the non-amphiphilic alkylammonium bromide. In this case a higher amount of salt must be added to reach the same maximum efficiency of ARS photooxidation. This is due to the lower capability of neutralization of these salts for both the ARS and the TiO₂ surface; the inhibiting effect was not evidenced anymore.

Full text of DOI:10.1039/c6ra25421g

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PAPER
Structure effects of amphiphilic and non-
amphiphilic quaternary ammonium salts on
photodegradation of Alizarin Red-S catalyzed by
titanium dioxide†
Cite this: RSC Adv., 2017, 7, 361
a
ab
ab
Nicoletta Spretic
*
*
Paola Anastasio, Tiziana Del Giacco,
Raimondo Germani,
and Matteo Tieccoa
The role of surfactants such as single- and double-tailed tetraalkylammonium bromide and various non-
amphiphilic tetraalkylammonium salts was investigated on the TiO2 photocatalyzed degradation of 3,4-
dihydroxy-9,10-dioxo-2-anthracenesulfonic acid sodium salt (Alizarin Red-S, ARS) in an air-equilibrated
alkaline medium under UV light irradiation. Absorption spectral analysis showed that the
photodegradation efficiency of the dye was significantly enhanced by the addition of cationic
surfactants. Two interesting findings emerged from this study: ARS was almost completely degraded at
surfactant concentrations close to 1 mM (values well above the cmc in the experimental conditions);
moreover, on increasing the surfactant concentration, the photocatalytic reaction became less and less
efficient and significantly dependent on the surfactant headgroup size. The presence of a maximum of
efficiency depending on the surfactant concentration was due to the combination of catalytic and
inhibiting processes. The first one likely depended on the ability of the surfactant to improve the ARS
approach to the semiconductor through the formation of cationic bilayers on the TiO2 particles; this
effect made the ARS more easily oxidized by TiO2. This catalytic action of the surfactant was opposed by
the increase of the micellar aggregate number in the aqueous bulk, which competes with the TiO2 sites
in associating to the dye. This was supported by the results obtained with the non-amphiphilic
alkylammonium bromide. In this case a higher amount of salt must be added to reach the same
maximum efficiency of ARS photooxidation. This is due to the lower capability of neutralization of these
salts for both the ARS and the TiO2 surface; the inhibiting effect was not evidenced anymore.
Received 18th October 2016
Accepted 12th December 2016
DOI: 10.1039/c6ra25421g
www.rsc.org/advances
compounds dissolved or dispersed in aqueous media.2–6 Among
the oxidative photodegradation methods, the use of titanium
Introduction
dioxide has found wide use, because of its photostability, non
toxicity, cheapness and strong oxidizing power. Heterogeneous
photocatalysis has proved to be an efficient destructive tech-
nology, leading to the complete mineralization of most organic
pollutants.7–9 During the past decade successful improvements
in TiO2 photoactivity have been made on the oxidation process:
nanostructurated composite materials, as noble metal nano-
particles (Ag, Au and Cu NPs), increase the process efficiency
when immobilized on semiconductor surface; doped-TiO2 with
metallic and not metallic species also showed interesting
results in this eld.10 Surfactants play a key-role in this topic,11
generally favoring the adsorption of the dye onto the semi-
conductor surface due to their unique chemical properties.
Previous works reported that TiO2 is a good photocatalyst for
ARS (a water-soluble widely used textile dye) and its removal
from the aqueous phase.7 The efficiency of the dye photo-
degradation is reported to be inuenced by pH, one of the most
important factor controlling both the adsorption of dye on the
The release of dyes and their degradation products in waste
waters by textile industries is one of the main causes which
determine environmental damage in the water ows and
therefore may cause disorders to human health.1 To reduce the
risk of environmental pollution from these species, in recent
years great efforts have been made by developing new chemical
methods to improve the oxidative degradation of organic
a
`
Dipartimento di Chimica, Biologia e Biotecnologie, Universita di Perugia, Via Elce di
Sotto 8, 06123 Perugia, Italy. E-mail: tiziana.delgiacco@unipg.it
b
`
Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Universita di
Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy
c
`
Dipartimento di Scienze Fisiche e Chimiche, Universita dell'Aquila, Via Vetoio, 67100
L'Aquila, Italy
† Electronic supplementary information (ESI) available: Tables of the numerical
values of the gures. Photodegradation of ARS with TiO2 as a function of
ObisDTABr and DTABr concentration. LCMS-ESI analysis of the ARS
photodegradation with TiO2 in the presence of CTABr. FTIR analysis of the
CTABr photostability with TiO2. See DOI: 10.1039/c6ra25421g
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semiconductor and the reactivity of the process. In most cases,
the photodegradation was performed with UV-illuminated
titanium dioxide, but in some works visible radiation by
photosensitization was used, certainly nalized to the use of the
free and inexhaustible sunlight, of which 40% is visible light
and less than ca. 5% is UV. Even though there is a basic
difference in the primary steps of photodegradation depending
on the excitation wavelength (visible or UV irradiation), the
photoassisted degradation leads to the dye radical cation
formation as an intermediate, which is the main responsible of
the degradation of the dye itself.12–15
Our long-standing interest in the mechanistic investigation
of TiO2 sensitized photooxidation of various organic compound
has recently led us to focus our attention on the effect of the
surfactant structure;16–18 the photodegradation efficiency of ARS
catalyzed by TiO2 was studied in air-equilibrated basic aqueous
dispersion under UV light irradiation using surfactants
Scheme 1 Cationic surfactants used in this work.
differing in their counterions.19 A series of surfactants such as
2ꢀ
cetyltrimethylammonium salts (CTAX, with X ¼ Brꢀ, Clꢀ, SO4
,
CH3SO3ꢀ, 4-CH3C6H4SO3ꢀ) were used. From these studies
emerged that the photocatalytic efficiency was signicantly
dependent both on the surfactant concentration and on the
nature of the counterion Xꢀ.
Changes in the molecular structures of surfactants lead to
signicant changes in their properties and in the properties of
their micellar aggregates. For these reasons the use of synthetic
novel amphiphiles represents a step ahead in the chemistry of
these molecules, and it can lead to important advantages.20,21 The
topics where these molecules nd an effective role are in fact
wide, that is because small chemical modications of the struc-
tures can provoke dramatic changes on their effect.22,23 In this
work we extended the analysis of the structural effects of tetraal-
kylammonium bromide surfactants on the TiO2-mediated pho-
todegradation of ARS, considering other surfactants structures,
varied in their hydrophobic chains portions and in their head-
group size. Thus we have investigated single-chain surfactants,
such as alkylammonium bromide, with different headgroup size
(cetyltrimethylammonium, CTA, cetyltriethylammonium, CTEA,
cetyltripropylammonium, CTPA, and cetyltributylammonium,
CTBA) and variable length tail (dodecyltrimethyl, DTA), and
gemini surfactants, with varying tail and two quaternary ammo-
nium headgroups connected by a diethylene oxide spacer (2-[2-
Scheme 2 Quaternary ammonium salts used in this work.
butylpyridinium bromide (BPyBr). This comparison has shown
that the hydrophobic degree of the salt strongly inuences the
reactivity of the oxidation process of the dye.
Experimental
Materials
(dimethylalkylazanium)ethoxy]ethyl-dimethylalkylazanium, Obis- Alizarin Red S (Carlo Erba) and TiO2 (P25, Degussa, 99.5%,
alkyl-TA). The comparison between single-chain and gemini dried in vacuo) were analytical grade reagents. Milli-Q water (18
systems has given also information on the role played by positive MU, pH 5.9) was used for all photodegradation experiments.
charge number of the surfactant headgroups. All the surfactant CTABr and DTABr surfactants were purchased from Sigma-
structures are shown in Scheme 1. The use of non-amphiphilic Aldrich and recrystallized from acetone and methanol before
ammonium salts in TiO2-catalyzed degradation processes has use. CTEABr, CTPABr, CTBABr, ObisCTABr and ObisDTABr
not received the same attention of amphiphilic salts so far;24–26 surfactants were synthesized by quaternization reaction of the
however this could help the understanding of the role of the correspondent tertiary amines with the appropriate alkyl halide
headgroup portions of the surfactants in this process. Therefore and puried by subsequent crystallizations from acetone or
in this work we have also compared the ability of the various ethyl acetate with different amounts of methanol.27 Quaternary
surfactants in assisting the ARS degradation with that of not ammonium salts TMABr, TEABr, TPABr and TBABr (Sigma-
amphiphilic alkylammonium salts (structures in Scheme 2), such Aldrich and Carlo Erba, purity > 98%) were dried in vacuo over
tetraalkylammonium bromide (TRABr), 6-azoniaspiro[5.5]unde- P2O5 before use. Spiro cyclic quaternary ammonium salts ASBr
cane bromide (ASBr), 6,9-diazoniaspiro[5.2.3.2.]hexadecane and DASBr were synthesized as reported in literature.28 BQBr
dibromide (DASBr), butylquinuclidinium bromide (BQBr) and and BPyBr were synthesized by quaternization reaction of the
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correspondent tertiary amines with butyl bromide and puried procedure. The chromatogram highlighted a main peak at
by subsequent crystallizations from acetone/methanol mixture. 0.484 min acquisition time, which was assigned to phthalic acid
by MS spectrum (negative ion acquisition): calcd for C8H5O4 [M
ꢀ H]ꢀ: 165.1266, found: 165.0199 (data in ESI†).
Photodegradation experiments
The photostability of the surfactant was checked as follows.
An identical procedure was followed both with surfactants and
An aqueous dispersion of TiO2 (20 mg in 50 ml) was irradiated
with non-amphiphilic salts. The degradation experiments were
in the presence of 50 mM CTABr, the surfactant taken as test,
performed by an Applied Photophysics multilamp apparatus
for 30 min. Subsequently, the semiconductor particles were
with 12 phosphor-coated uorescent lamps (15 W each) emit-
removed by centrifugation (see above) aer adding of HCl and
ting at 355 nm (Dl1/2 ¼ 20 nm), radiation adsorbed mainly by
the supernatant was analyzed directly. The analysis of the irra-
the semiconductor, as the dye absorbs more weakly at these
diated samples was always compared with that of the corre-
wavelengths.29
sponding not-irradiated samples, prepared following the same
TiO2 suspensions were prepared by adding P25 TiO2 powder
experimental procedure. The photostability of the surfactant
(0.4 g lꢀ1) to 50 ml of an aqueous solution containing ARS (0.20
was monitored by IR spectroscopic methods, using a Perkin
mM) and various concentrations of ammonium salt. The
Elmer Spectrum RX I FT-IR System instrument. The FTIR
ꢁ
degradations were carried out at 20 C. The starting pH of the
spectra were calculated from 16 scans at 1 cmꢀ1 resolution. The
irradiated suspensions was adjusted to 12 using NaOH. Before
intensities of the bands at 2924 and 2854 cmꢀ1 (C–H stretching
irradiation, the suspensions were magnetically stirred in the
modes) and at 1470 cmꢀ1 (C–H bending modes), all mainly due
dark for 30 min to reach equilibrated adsorption between TiO2,
to the methylene tails, were evaluated before and aer irradia-
dye and atmospheric oxygen. The irradiated dispersion was
tion (data in ESI†).30
kept under constant air-equilibrated and stirred condition.
Micellar parameter determination by surface tension
Analytical procedures
measurements
The degradation process was followed by UV-Vis spectropho-
The critical micellar concentration (cmc) values of the various
cationic surfactants were measured at pH 12 in the absence of
tometric analysis. Mixture samples (2 ml) were collected at
different irradiation time. To recover quantitatively the dye
¨
TiO2 according to the Du Nouy ring method using a Fisher
adsorbed on TiO2, an equal volume of ethanol was added to the
sample, together with 0.2 ml of HCl 1.2 M; in this condition ARS
was all protonated. The suspension was then shaken by vortex
mixer and centrifuged at 10 ꢁC for 15 min at 12 000 rpm to
remove TiO2. The supernatant (2 ml) was carefully transferred
from the centrifugation tube into a 4 ml quartz cuvette for UV/
VIS analysis. Absorption spectra were recorded on a Shimadzu
UV-2401 PC spectrophotometer connected to a CPS-controller
Surface Tensiometer (manual model). The measurements were
performed on solutions (from 1.0 to 0.0050 mM) prepared by
adding rates of a surfactant stock solution. The cmc values were
obtained by the intersection of the two linear correlations ob-
tained in the graph of the surface tensions in function of ꢀlog
[surfactant], whereas the ionization degree (a) was determined
by the ratio of the slopes of the two correlations. The average
ꢁ
value of three measurements at 25 C was taken as the result.
ꢁ
thermostat kept at 25.0 ꢂ 0.1 C.
The average error was estimated to be ca. 5%.
The kinetic experiments were performed using 6–12
phosphor-coated uorescent lamps for irradiating the samples.
Rate constants for ARS degradation were obtained by moni-
toring the change of absorbance at the maximum wavelength
Results and discussion
TiO2-photocatalyzed degradation of ARS assisted by
ꢁ
(423 nm) as a function of the irradiation time at 25 C. The k
surfactants
values were the average of at least three determinations. The
average error estimated on the rate constants was ꢂ10%.
In a previous paper, we reported that the TiO2-photodegradation
Product analysis was carried out as follows. An aqueous efficiency of ARS assisted by cetyltrimethylammonium salts
dispersion of TiO2 (40 mg in 100 ml) was irradiated in the improved by increasing the pH value up to 12.19 The pH affected
presence of 1.0 mM CTABr for 30 min. Subsequently, the both the amount of hydroxyl radicals produced by oxidation of
semiconductor particles were removed by centrifugation aer OHꢀ ions available on the semiconductor surface7 and also the
adding of ethanol and HCl (see above). The supernatant was stabilization, due to the electrostatic attraction between the
carefully transferred from the centrifugation tubes into a sepa- photocatalyst surface (negatively charged at this pH) and the
rating funnel and water was added to the mixture. The aqueous cation radical intermediates. On the basis of these results, all the
layer was extracted three times with chloroform. The residue, experiments presented in this work were carried out at pH 12.
obtained aer removal of the solvent under reduced pressures
Single-chain surfactants. It is known that excited Alizarin red
from the combined organic extracts, was diluted with methanol. adsorbed on the TiO2 surface was subjected to oxidation
A LCMS-ESI analysis was performed on the diluted samples by following a photo-assisted mechanism.31 ARS photodegradation
an Agilent 6540 UHD accurate mass Q-TOF LC/MSMS instru- produced, in the absence of additives, species as hydrogen
ment. The analysis of the irradiated samples was always peroxide and phthalic acid as principal products, in addition to
compared with that of the corresponding not-irradiated carbon dioxide and smaller carbonyl compounds. Phthalic acid
samples, prepared following the same experimental was also detected by HPLC-HRMS analysis (see Experimental) in
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our experimental condition, where the degradation of Alizarin weakly on the surfactant structure up to a concentration of
red was performed under UV light irradiation of the semi- about 1 mM, where the maximum efficiency of ARS degradation
conductor in the presence of CTABr. In this case a photo- is reached (about 90%). By comparing the minimum value of
catalytic degradation mechanism should be operative,19 the ratio C15/C0 (ca. 0.1) with that determined in the absence of
according to which conduction band electrons and valence CTRAX (ca. 0.9), it is possible to establish a signicant catalytic
band holes are formed by TiO2 irradiation in the primary step.8,9 efficiency by the surfactant of almost 10 times. This concen-
Evidently, the same crucial intermediates, such as ARS radical tration is well above the cmc values of all CTRAX examined;
cations and oxygen reactive species, are involved in the photo- indeed, the cmc values ranged from 0.23 to 0.056 mM in alka-
degradation process in both the irradiation conditions.
line medium (Table 1) increasing the chain length of the alkyl
Blank experiments, performed on irradiated ARS solutions residues of the headgroup. As expected, the cmc values at pH 12
but not containing TiO2, or on dark TiO2/ASR dispersions, are lower than in neutral water (Table 1), according to the effect
showed that no dye degradation occurred along a time shorter due to the addition of ionic species (as Na+OHꢀ). The second
than 120 min. It is also well known that the surfactant mole- interesting evidence is that, for surfactant concentration above
cules themselves are photocatalytic degraded over TiO2 1.0 mM, the photocatalytic degradation of the dye is strongly
powder,32,33 but we checked by FTIR analysis (data in ESI†), that inuenced on the headgroup structure. In particular, the extent
the photooxidation of cetyltrialkylammonium bromide, the of photodegradation does not change with propyl- and butyl-in
surfactant taken as test, by irradiated semiconductor particles is the headgroup, while with methyl and ethyl it decreases rapidly
rather slow during the time in which ARS degrades. Indeed, the with the monomer concentration until reaching a plateau value
CTABr disappearance percentage, measured as transmittance (at ca. 3 mM surfactant), corresponding to 40% and 60% of
change of the surfactant characteristic peaks (see Experimental) degraded Alizarin, respectively.
before and aer the irradiation, is less than 2%.
These results are conrmed by kinetic analysis of the
The effect of single-tailed surfactants has been examined in degradation obtained in the presence of CTABr or CTBABr, the
terms of variation of the headgroup size in the CTRABr series two surfactants at the extremes of the degradation efficiency.
(Scheme 1). For this purpose, the experiments were performed Indeed, the degradation data (C/C0 vs. time), determined on
in the presence of surfactants at concentrations ranging 1.0 mM CTRAX solutions, were well tted by a pseudo-rst order
between 0.20 and 5.0 mM, keeping constant the ARS and TiO2 kinetic law (inset a in Fig. 1) with slightly different observed rate
concentrations.
constants (0.050 and 0.058 minꢀ1 for CTBABr and CTABr,
The shape of the absorption spectra recorded aer 15 min respectively). In the presence of 5.0 mM surfactant the observed
irradiation in the presence of surfactant were very similar to rst order rate constant with CTBABr (0.061 minꢀ1) was found
those recorded without surfactant (as an example, in Fig. S1 and similar to that at 1.0 mM and greater than that determined with
S2† the spectra in the presence of CTABr and CTBABr are CTABr (0.016 minꢀ1) by a factor of ca. 4 (inset b in Fig. 1), which
shown); this indicates that the surfactant role is mainly to affect is almost the same reactivity ratio determined aer 15 minutes
the process kinetics. The degradation proles, shown in Fig. 1, of irradiation.
are reported as the changes of concentration of ARS as a func-
The graphics reported in Fig. 1 are very similar to that
tion of [CTRAX]; the changes of ARS concentration are already reported for the same system, ARS/TiO2/pH 12, but in
expressed as the ratio of residual ARS concentration aer the presence of cetyltrimethylammonium salt with different
15 min irradiation (C15) on its initial concentration (C0) (both counterions (CTAX),19 at least as regards the trend of the
values determined by absorbance measurements, see Experi- degradation efficiency up to 1.0 mM surfactant concentration,
mental). The rst observation emerging from the analysis of which corresponds to the ARS degradation maximum. Also in
these proles is that the photocatalytic efficiency depends this case the formation of positively charged close-packed
Table 1 Values of cmc and a in water and cmc in NaOH 0.01 M at
25.0 ꢁ
C
NaOHa (pH 12)
cmc (mM)
H2Ob
Surfactant
cmc (mM)
a
CTABr
CTEABr
CTPABr
CTBABr
DTABr
Obis(CTABr)
Obis(DTABr)
0.23
0.24
0.15
0.056
1.5
0.92
0.79
0.56
0.33
1.6
0.25
0.36
0.46
0.51
0.28
0.49
0.26
Fig. 1 Photodegradation of ARS (0.20 mM) with TiO2 (0.4 g lꢀ1) at pH
12 after 15 min of irradiation as a function of CTABr (C), CTEABr ( ),
CTPABr ( ) and CTBABr ( ) concentration. Insets: photodegradation
kinetics in the presence of CTABr (C) and CTBABr ( ) at (a) 1.0 mM and
(b) 5.0 mM concentration. The solid lines represent the corresponding
first order kinetic fittings.
0.0049
0.033
1.0
a
b
Obtained by surface tension measurements.
conductivity.34
Determined by
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bilayers on the TiO2 surface is supposed,11,30 which has the role
of approaching the dye to the semiconductor, otherwise
impossible being both negatively charged. Interestingly, in this
range of surfactant concentrations, the ARS degradation effi-
ciency is very little affected both by the steric hindrance of the
headgroup alkyl chains and by the cmc values (Table 1); this
probably can be due to a combination of hydrophobic and
electrostatic interactions.
At monomer concentrations higher than 1.0 mM, the dye
competed between two association sites, the TiO2 surface and
the micellar aggregates of CTABr and CTEABr, while with larger
headgroup size (CTPABr and CTBABr) the degradation effi-
ciency did not undergo any variation. In particular, the dye
oxidation efficiency decreased in the order of alkyl chain length
methyl < ethyl < propyl y butyl, i.e. by decreasing the micellar
dissociation degree a (Table 1), a parameter correlated with the
positive charge density on the micellar surface.34 In the case of
CTAX surfactants, an opposite behavior was instead observed:
micelles more ionized (with greater a value) were able to bind
more efficiently the negatively charged dye, subtracting it from
the photodegradation process. The association degree of ARS to
the micellar aggregates evidently does not depends only on the
charge density, but also on the hydrophobic nature of the
micelle surface. Previous studies showed that an increase in the
headgroup size not only lowered the cmc value and increased
the a parameter, but also reduced the water content and the
hydrophobicity of the interface.27 In particular, the alkyl chains
of CTPABr and CTBABr did not extend into the water, but were
oriented along the micellar surface to reduce water-alkyl group
contact. Therefore, the reduced inhibition of the photo-
degradation with increasing of the headgroup size can be
interpreted in terms of increased difficulty of ARS to intercalate
into the micelle because of steric effect due to the alkyl chains.
We observed this effect also with other aromatic molecules
(sodium trans-orthomethoxycinnamate) not intercalating in
CTBABr micelles due to steric hindrance of the headgroups in
other previous studies.34
Double-tailed surfactants. Two dimeric quaternary ammo-
nium surfactants with C16 and C12 tail length and same dieth-
ylene oxide spacer (ObisCTABr and ObisDTABr) were compared
with the corresponding monomeric surfactants (CTABr and
DTABr). Based on previous studies on aggregate structures at
solid–liquid interface as a function of surfactant geometry,35,36 it
is plausible to assume that ObisCTABr and ObisDTABr, being
symmetric geminis and with large spacer, show spherical
micelle geometry in neat water and hemimicellar/admicelle
structures adsorbed on the negatively charged TiO2 surface,
similarly to what is observed with the correspondent single-
tailed surfactants.
Fig. 2 Photodegradation of ARS (0.20 mM) with TiO2 (0.4 g lꢀ1) at pH
12 after 15 min of irradiation as a function of ObisCTABr (B) and CTABr
(C) concentration.
cmc values (0.049 and 2.3 mM at pH 12, Table 1). The other
relevant aspect concerns the shape of the proles, namely the
curve appeared less broaden going from CTABr to ObisCTABr,
reaching a plateau value from ca. 60% to 82% of residual ARS.
The highest inhibition effect on the dye degradation process
observed with ObisCTABr is in line with its much lower cmc
value and higher a parameter (cmc: 0.049 and 0.23 mM, a: 0.36
and 0.25 for ObisCTABr and CTABr, respectively, Table 1).
Indeed, the lower cmc value implied that, in the presence of the
same amount of surfactant, the gemini system provided
a greater number of micellar aggregates able to associate ARS
and thus to subtract it from the TiO2 catalytic action. Further-
more, the ObisCTABr micelle aggregates, being more ionized
because less associated with the counterions, interacted more
effectively with the negatively charged ARS.
The comparison of the proles C15/C0 against the surfactant
concentration obtained with ObisDTABr and DTABr (Fig. S3 in
ESI†) pointed out smaller differences between the two systems.
The maximum efficiency of degradation with ObisDTABr (ca.
85%) was obtained at 0.5 mM, a concentration that is half of the
value found for the corresponding single-tailed DTABr
(maximum efficiency ca. 80%); the C15/C0 values at the plateau
in this case were more similar (0.7 and 0.63 for ObisDTABr and
DTABr, respectively). These results were in line with the similar
cmc and a values for the two systems, as observed from the data
collected in Table 1 (cmc: 1.0 and 1.6, a: 0.26 and 0.28 for
ObisDTABr and DTABr, respectively). In this case the favorable
adsorption by electrostatic and hydrophobic interaction of
ObisRTABr surfactants on the TiO2, due to the double charge
and double chain in its structure, seemed to play a role only in
anticipating, in terms of concentration, the maximum photo-
degradation point of ARS.
TiO2-photocatalyzed degradation of ARS assisted by tetraalkyl
quaternary ammonium salts
In Fig. 2 the degradation proles of the concentration
change of ARS, expressed as C15/C0 ratio as a function of
surfactant concentration, for ObisCTABr and CTABr are In order to investigate the role of the amphiphilic character of
compared. Two most signicant aspects emerged from the the quaternary ammonium salts in assisting the TiO2 photo-
analysis of these proles. The rst one was that the maximum catalyzed degradation of Alizarin, we examined numerous
photocatalytic efficiency for ObisCTABr, ca. 65% of degraded bromide salts, with variously structured ammonium groups,
ARS, was at a concentration (0.2 mM) 5 times lower than that for not able to form micelles in the same experimental conditions
CTABr (1 mM). Both values are higher than the corresponding described above.
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known to assist the ARS degradation in micellar media,19 was
achieved at lower surfactant concentrations because of the
importance of non-electrostatic hydrocarbon tail–tail
interactions.
Effect of the conformational freedom. To deepen the inter-
vention of structural change of the salt on the ARS oxidative
degradation, the inuence of the steric hindrance of the groups
on the charged ammonium was examined. For this purpose, the
derivatives of aza-spiro bromide (ASBr) quinuclidinium
bromide (BQBr) and pyridinium bromide (BPyBr), together with
the acyclic salt TPABr were investigated in the same experi-
mental condition already reported. The corresponding struc-
tures are depicted in Scheme 2.
From the C15/C0 ratio as a function of salt concentration
proles shown in Fig. 4, it is observable that the catalytic effi-
ciency decreased going from the structure with a higher degree
of conformational freedom, such that of TPABr, to the more
rigid molecular structures of BQBr and ASBr. To explain this
trend, the hydrophobicity of alkyl residues can not be invoked,
since the number of the carbon atoms is similar. Indeed, in the
case of BQBr and ASBr, the ammonium positive charge should
have been more available for the interaction both with the dye
and with the negatively charged catalyst surface, compared with
the higher steric hindrance of TPABr.
The analogous behavior of BQBr and ASBr suggests that the
ammonium charge availability is similar with the two salts.
Evidently, in the BQBr structure, the conformational freedom of
the butyl residue is balanced by the rigidity of quinuclidinium
portion to the same extent of the two rigid chair conformation
of ASBr.
Effect on the chain length of the alkyl groups. In Fig. 3 C15/C0
ratios as a function of salt concentration proles of tetramethyl-
, tetraethyl-, tetrapropyl- and tetrabutylammonium bromide
(TMABr, TEABr, TPABr and TBABr, respectively, Scheme 2), are
collected. With all the ammonium salts, the efficiency degra-
dation of ARS increased with increasing both the salt concen-
tration and, at the same salt concentration, the length of the
alkyl chains. At concentrations $50 mM the catalytic effect
reached a constant maximum value for all the investigated
systems.
Comparing these results with those collected with the cor-
responding surfactants (CTABr, CTEABr, CTPABr and CTBABr),
the main evidences were: (i) the maximum catalytic effect was
still very high, of the same entity (90%) but reached at much
higher concentration of salt; (ii) the inhibition effect due to the
competition with the micelles was obviously missing; (iii) the
chain length effect of the alkyl residue on the efficiency of the
degradation process was roughly kept, i.e., butyl > propyl > ethyl
> methyl. Indeed tetrabutylammonium, being the soest
cation, neutralized better the negative charge of the dye, which
is also so since the charge is delocalized on the aromatic rings.
This neutralization could allow a more effective approach of the
dye to the semiconductor, a necessary condition to facilitate
kinetically the electron transfer from the dye to the positive hole
photogenerated on the TiO2 surface. At the same time, the tet-
rabutylammonium group could create a coating on the catalyst
surface not only by electrostatic interactions, but also by
hydrophobic interactions between the butyl chains, reducing in
this way their steric hindrance. The trimethylammonium, being
the hardest of the four salts examined, could prefer to interact
electrostatically by the positive charged nitrogen with the
surface hard TiOꢀ groups, rather than with the dye.7 So, the less
Alizarin molecules were in proximity of the semiconductor, the
less efficient was their photodegradation. TEABr and TPABr
clearly followed the trend due to the hydrophobicity of the
system.
The weaker effect of BPyBr on the degradation reaction may
seem unexpected. Also in this case the hydrophobicity can not
be inuent, since its carbon atom number similar to that of the
others (9, 12, 11 and 10 for BPyBr, TPABr, BQBr and ASBr,
respectively). The rigidity of the BPyBr should have determined
a better accessibility of the ammonium, but two main factors
could hinder the neutralization: the rst is electronic cloud of
the aromatic ring that repulses the negatively charged dye and
then the approach of the dye to the semiconductor; the second
factor could be the different hybridization of the nitrogen atom
(sp2 in pyridinium, sp3 in the other cases) that determines
a lower positive charge density on nitrogen.
The concentration of 50 mM, in correspondence of which the
photodegradation reached its maximum value, corresponds to
the situation of maximum adsorbed amount of Alizarin on the
semiconductor in the presence of different salts. It is plausible
that the formation of positive charged close-packed bilayers of
surfactant molecules adsorbed on the TiO2 surface, which are
Fig. 3 Photodegradation of ARS (0.20 mM) with TiO2 (0.4 g lꢀ1) at pH Fig. 4 Photodegradation of ARS (0.20 mM) with TiO2 (0.4 g lꢀ1) at pH
12 after 15 min of irradiation as a function of TMABr (C), TEABr ( ), 12 after 15 min of irradiation as a function of TPABr ( ), BPyBr ( ), ASBr
TPABr ( ) and TBABr ( ) concentration.
( ), BQBr ( ).
366 | RSC Adv., 2017, 7, 361–368
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accomplished by its hydrophobic character, the availability of
the positive charge on the ammonium group and the number of
charged nitrogen atoms.
The advantages of using non-amphiphilic salts were that the
dye photodegradation process did not undergo inhibition,
although the lack of hydrophobic tails required greater salt
concentrations to achieve values of degradation efficiency
comparable to those obtained with the surfactants. This effect
was however offset by lower environmental impact of non-
amphiphilic alkylammonium salts with respect to the corre-
sponding surfactants.
Fig. 5 Photodegradation of ARS (0.20 mM) with TiO2 (0.4 g lꢀ1) at pH
12 after 15 min of irradiation as a function of ASBr ( ), DASBr ( )
concentrations. Insets: photodegradation kinetics in the presence of
1.0 mM ASBr (a) and DASBr (b) and corresponding kinetic fittings.
Acknowledgements
Thanks are due to MIUR (Ministero dell'Istruzione,
`
dell'Universita e della Ricerca Scientica), Italy (PRIN 2010–
Effect of the number of positive charges. The effect of the
charge number of the salt on the photodegradation efficiency
was evaluated considering the two aza spiro systems ASBr and
DASBr (Scheme 2). As observed in Fig. 5, where the C15/C0 vs.
concentration proles are shown, with both the salts an excel-
lent percentage degradation (ca. 90%) was obtained at high
concentrations. The interesting difference, instead, was that
with ASBr the degradation process was slower; indeed the dye
degradation maximum was reached at 10 mM concentration,
a doubled value than that observed with DASBr (5 mM). This
was conrmed by kinetic analysis performed with the two salts
(both 1.0 mM). The observed rst order rate constant of the
degradation process assisted by ASBr (0.034 minꢀ1) was found
smaller than that determined with DASBr (0.071 minꢀ1) by
a factor of ca. 0.5. These results indicated that the inuence of
the two salts could be ascribed to a pure charge concentration
effect of charge concentration on the oxidative process kinetic.
2011, grant number 2010FM738P) and to the University of
Perugia (Fondo Ricerca di Base 2015) for nancial support.
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