Micellar rate effects in the alkaline fading of crystal violet in the presence of various surfactants

Article abstract of DOI:10.1016/j.molliq.2014.11.013

The influence of surfactants of various types (nonionic, cationic, anionic, and zwitterionic) on the rate of reaction of the crystal violet, cationic triphenylmethane dye, with the hydroxide ion in unbuffered aqueous solutions was studied using the spectrophotometric method. It was found that the values of the rate constants increase in the presence of the cationic, nonionic and zwitterionic surfactants, and decrease on the addition of the anionic surfactant at concentrations both below and above the CMC. The variation of the rate constants along with the change in the concentration of the surfactants may be explained on the basis of conception about distribution of the reagents between aqueous and micellar phases and the changing of their properties depending on local microenvironment. The character of the salt effects upon the rate constants was explained. It was revealed, that the Piszkiewicz's model of micellar catalysis is inapplicable even semi-quantitatively to the most of the examined systems.

Full text of DOI:10.1016/j.molliq.2014.11.013

MOLLIQ-04528; No of Pages 6
Journal of Molecular Liquids xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
1
2
Micellar rate effects in the alkaline fading of crystal violet in the presence
of various surfactants
3
Q6 Kateryna V. Roshchyna, Sergey V. Eltsov ⁎, Anna N. Laguta, Nikolay O. Mchedlov-Petrossyan
4
Department of Physical Chemistry, Kharkov V. N. Karazin National University, Svoboda Square 4, Kharkov 61022, Ukraine
5
a r t i c l e i n f o
a b s t r a c t
OOF
6
7
8
9
Article history:
The influence of surfactants of various types (nonionic, cationic, anionic, and zwitterionic) on the rate of reaction 16
of the crystal violet, cationic triphenylmethane dye, with the hydroxide ion in unbuffered aqueous solutions was 17
studied using the spectrophotometric method. It was found that the values of the rate constants increase in the 18
presence of the cationic, nonionic and zwitterionic surfactants, and decrease on the addition of the anionic sur- 19
factant at concentrations both below and above the CMC. The variation of the rate constants along with the 20
change in the concentration of the surfactants may be explained on the basis of conception about distribution 21
of the reagents between aqueous and micellar phases and the changing of their properties depending on local 22
microenvironment. The character of the salt effects upon the rate constants was explained. It was revealed, 23
that the Piszkiewicz's model of micellar catalysis is inapplicable even semi-quantitatively to the most of the ex- 24
Received 6 October 2014
Accepted 6 November 2014
Available online xxxx
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Keywords:
Crystal violet
Alkaline dye fading
Surfactant
Micellar rate effects
Molecular spectroscopy
amined systems.
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© 2014 Published by Elsevier B.V.
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1. Introduction
of these reactions is analyzed in terms of the micellar pseudophase 53
model, which assumes that reactants are partitioned between the aque- 54
ous and micellar pseudophases [10]. An alternative model of micellar 55
catalysis has been proposed by Bruice [23] and developed by 56
3 Q2 7
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
The present paper is devoted to the analysis of the effect of
surfactants on the rate of the alkaline fading of the cationic dye crystal
violet.
Piszkiewicz [24–26].
57
Numerous attempts have been made to investigate the change of the
rate constants of different reactions by micelles of surfactants. Colloidal
surfactants are amphiphilic compounds, which form micelles in solu-
tions. Micelles effect on the rate of reactions owing to: (i) concentration
of reagents within their volume; (ii) stabilization or destabilization of
substrates, intermediates, and products; (iii) specific orientation of re-
agents that changes their physico-chemical properties [1]. Micelles pro-
vide microreaction medium that is similar to that for many biochemical
processes. Kinetic rules of reaction proceeding in micelles have general
analogy to enzymatic catalysis [2,3]. Micellar kinetic effects depend on
the nature of the surfactants hydrophobic and hydrophilic groups, and
on the counter-ion [4,5].
The effect of the micelles on the rate of alkaline fading of CV is similar 58
to that of amphiphilic macrocycles like calixarenes and cyclodextrins. In 59
such systems, the change in observed rate constants is caused by inter- 60
action between CV and macrocycle, which proceeds as host–guest com- 61
plex formation reaction [10,27,28].
62
Recently, several papers concerning the fading of some triphenyl- 63
methane dyes in micellar media have been published [15,16,18–20,29, 64
30]. The Piszkiewicz's model was successfully used for describing the 6Q 58
variation of the rate constants along with the surfactant concentration 66
[16,19,20,29,30]. Similar model has been used in other papers [15,18]. 67
However, in our previous work [22], we have studied the fading reac- 68
tions of these dyes in micellar media and a weak correlation of experi- 69
Among the most widely investigated reactions in micelles is the al-
mental data with the Piszkiewicz's model was found.
70
UNCORRECTED PR
kaline fading of triphenylmethane dyes: crystal violet (CV) [6–13], mal-
achite green [6,14–19], and brilliant green [6,20–22]. This is due to their
high solubility both in aqueous and micellar phases. The important
property of these reactions is that its rate is very convenient for simple
kinetic measurements. The effect of the micellar systems on the kinetics
This model is often used for the processing of such kind of exper- 71
imental data. Therefore, we decided to examine the applicability of 72
the Piszkiewicz's model for the proceeding of the reaction in the 73
presence of the cationic surfactants cetyltrimethylammonium bro- 74
mide (CTAB) and dimeric bis-quaternary ammonium bromide sur- 75
factant (16–4–16, gemini surfactant), the anionic surfactant 76
sodium dodecyl sulfate (SDS), the zwitterionic surfactant 3- 77
(
dimetyldodecylammonio)-propanesulfonate (DMDAPS), and non- 78
⁎
Corresponding author.
E-mail address: eltsov@karazin.ua (S.V. Eltsov).
ionic surfactants Triton-X-100 (TX-100) and Brij-35 in both pre- 79
http://dx.doi.org/10.1016/j.molliq.2014.11.013
167-7322/© 2014 Published by Elsevier B.V.
0
Please cite this article as: K.V. Roshchyna, et al., Micellar rate effects in the alkaline fading of crystal violet in the presence of various surfactants, J.
Mol. Liq. (2014), http://dx.doi.org/10.1016/j.molliq.2014.11.013

2
K.V. Roshchyna et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
8
8
0
1
micellar and micellar concentration range in non-buffered systems
with the fixed NaOH concentration.
8
8
2
3
2. Experimental
2.1. Materials
8
8
8
8
8
8
9
9
9
9
9
9
9
9
4
5
The samples of CTAB, SDS, TX-100 and Brij-35 were purchased from
Merck (99% of the main substance). 3-(dimetyldodecylammonio)-
propanesulfonate was from Fluka. Gemini surfactant was synthesized
and purified by S. V. Shekhovtsov (Department of Physical Chemistry,
Kharkov V. N. Karazin National University) [31]. All other chemicals
were of analytical reagent grade. All reagents were used without further
purification. Aqueous solution of NaOH was prepared from saturated
6
7
8
Scheme 1.
Q2
9
0
without surfactant equals 0.157 M−1 s−1. This value is in good agree- 133
ment with a number of previously reported values given in Table 1.
.2. Influence of the media on the rate constants of the reaction
1
stock solution using CO
atmosphere.
2
-free water and kept protected from the
134
135
2
Q3 9
4
Critical micelle concentrations (CMCs) of surfactants in pure
3
−
4
1
water at 25 °C are as follows. Gemini 16–4–16: 1 × 10
M
−
4
−3
5
[31], CTAB: 9.8 × 10
M [32], SDS: 8.2 × 10
M [32], TX-100:
M [34], and DMDAPS:
The effect of surfactants on the rate constant of the reaction is shown 136
in Figs. 1–3. The analysis of kinetic data obtained can be made on the 137
basis of the following considerations. 138
−
4
−5
6
2.4 × 10
M [33], Brij-35: 6.8 × 10
M [35].
−
3
7
2.7 × 10
9
8
2.2. Kinetic measurements
1) The successive addition of the surfactant to the solution may lead to 139
the formation of the pre-micellar and then the micellar aggregates. 140
The presence of the aggregates causes the distribution of the dye be- 141
tween aqueous phase and the aggregates. The most probable loca- 142
tion of the dye in the micellar pseudophase is the Stern layer of the 143
micelles of ionic surfactants and the interfacial layer for the aggre- 144
gates of nonionic ones. The change of the reaction rate at the addi- 145
tion of the surfactant indicates that the dye interacts with the 146
surfactant molecules or ions and aggregates. There are two parallel 147
9
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
9
In water solution, crystal violet exists in the intensively colored cat-
+
+
00 ionic form (CV ). The reaction of CV with the hydroxide ion results in
01 the fading of the color due to formation of the colorless carbinol. The re-
02 action is given in Scheme 1.
03
The k
1
and k
2
on Scheme 1 are rate constants for direct and reverse
−
04 reactions of the OH ion addition to the dye cation. At the pH value of
05 solution used in our experiments, the reaction proceeds to the complete
06 fading of the dye and the formation of colorless solution at the end of the
07 reaction. This means that under these conditions, k
08
−
reactions of the crystal violet cation with the OH ions in the pres- 148
1
≫ k
2
.
ence of surfactant, namely: in the aqueous phase and in the dye–sur- 149
factant aggregates [1]. Under these conditions, the experimentally 150
determined rate constant is an effective, or observed, value that is 151
dependent on the rate constants of the processes in each phase 1Q 5 12 0
and on the local concentrations of the reacting ions. The increase in 153
the total surfactant concentration in solution causes the increase of 154
The kinetic measurements were made spectrophotometrically,
09 with cuvettes having an internal thickness of 1.00 cm, at 25 °C
10 employing the SF-46 and Hitachi U-2000 UV–visible spectrophotom-
11 eters equipped with a cuvette holders through which a thermostated
12 water was continuously circulated. The reaction of CV alkaline fad-
13 ing was followed by observing the decrease in absorbance at 590 nm
+
the degree of dye binding by the aggregates [36].
155
+
14 as a function of time. In all kinetic runs, the initial CV concentration
2) As the crystal violet fading is a reaction between ions, the effect of 156
the solvent on its rate constant is partly described by Scatchard 157
15 of 1.00 × 10− M was used, whereas that of sodium hydroxide con-
5
−
3
+
−
16 centration was 4.6 × 10 M. The last-named corresponds the initial
17 pH value of solution in the concentration scale ≈ 11.7. Under the
18 above conditions, [CV ] ≪ [OH ] and the reaction proceeds as the
19 pseudo-first-order one. The observed pseudo-first-order rate con-
20 stants, k′, were obtained from the slopes of the dependence of ln A
21 versus time using the equation:
equation [37]. The CV and OH ions are oppositely charged. As it 158
follows from Scatchard equation, in this case the decrease in the rel- 159
ative permittivity of the local surrounding of the dye cation leads to 160
the increase in the rate constant. This corresponds to the increase in 161
the reaction rate when the dye cation moves from aqueous phase to 162
the location site in the Stern layer of the micelle, because the effec- 163
tive relative permittivity of the interfacial micellar layer, where the 164
+
−
0
lnA ¼ lnA −k t;
ð1Þ
dye is localized, is lower than that of the bulk water [1].
165
0
3
) The hydroxide ions are distributed between bulk phase and the mi- 166
cellar pseudophase where the dye cations are localized on the sur- 167
face. In general, at equilibrium the distribution of the charged 168
particles between two phases is described by the equation following 169
1
0
23 where A and A are the absorbance of the solution at time t and the
initial absorbance, respectively. Eq. (1) describes the experimental
24 dependences of ln A on t as a straight line; the good linearity was ob-
25 served for all experimental data with correlation coefficient more
26 than 0.96 and the standard error for k′ always less than 5%. The
27 second-order rate constants of the reaction were calculated as k =
1
1
1
1
1
equation [36].
170
171
ꢀ
ꢁ
−
½iꢀm
f i;w
zi FΨ
28 k′/[OH ].
¼ f ꢁ γ ꢁ exp −
ð2Þ
i
½
iꢀ
RT
w
i;m
1
1
29 3. Results and discussion
m w
where [i] and [i] are concentrations of the component i in micellar 173
30 3.1. Rate constant of the reaction in water
pseudophase and water, respectively; γ is the transfer activity coef-
i
ficient of the species from water to micellar phase; fi,m and fi,w are the 174
1
1
31
The rate constant of interaction between crystal violet and hydrox-
concentration activity coefficients in the micellar and the aqueous 175
32 ide ion determined in the present work at 25 °C in aqueous solution
phases, respectively; z is the charge of the ion; Ψ is the difference 176
i
between the electrical potentials of the phases; T is the absolute tem- 177
1
Hereafter, 1 M = 1 mol dm−3
.
perature; F is the Faraday constant; and R is the gas constant.
178
Please cite this article as: K.V. Roshchyna, et al., Micellar rate effects in the alkaline fading of crystal violet in the presence of various surfactants, J.
Mol. Liq. (2014), http://dx.doi.org/10.1016/j.molliq.2014.11.013

K.V. Roshchyna et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
3
t1:1
t1:2
Table 1
The rate constant of interaction between crystal violet and hydroxide ion at 25 °C reported in the literature.
1
t1:3
Rate constant, M− s−1
Comment
The pseudo-first-order rate constant is given, as 1.7 × 10− s−1 at pH = 12
Reference
3
t1:4
t1:5
t1:6
t1:7
t1:8
t1:9
0.17
0.205
0.16
0.16
0.22
0.157
[6]
[7,8]
[9]
[10]
[11]
–
−
The value is determined as a slope of the dependence of observed pseudo-first-order rate constants on OH concentration
The value is given as 0.47 min− at 0.05 M sodium hydroxide solution
Determined at NaOH concentration of 0.1 M
1
Given as 0.040 min− at 30 °C and NaOH concentration of 0.003 M
1
Present work
1
1
1
1
79
Sacrificing the accuracy, it may be assumed that the ratio of the con-
80 centration activity coefficients and the transfer activity coefficient of the
81 ions do not depend on the type of the surfactant. This gives the interre-
82 lation:
3.3. Effect of nonionic, cationic and mixed cationic–nonionic surfactants on 210
the rate of fading 211
The data shown in Figs. 1 and 2 testify that the presence of CTAB, TX- 212
00, and Brij-35 increases the reaction rate, the effect being more 213
1
ꢀ
ꢁ
−
½
OH ꢀ
FΨ
RT
m
w
expressed for cationic surfactant CTAB. The difference between the ef- 214
fect of cationic and nonionic surfactants can be explained by the as- 215
sumption that owing to the positive charge of the surface of CTAB 216
¼
A ꢁ exp
;
ð3Þ
−
½
OH ꢀ
w
OH
m
OH
−
ꢁ γ
−
=f
−
¼ const:
−
1
1
84 where A ¼ f
OH
micelles the effective concentration of OH ions in the locus of the 217
Eq. (3) indicates that depending on the type of surfactant, three dif-
85 ferent cases are possible:
dye is higher than in the case of TX-100 and Brij-35 nonionic aggregates. 218
The accelerating of the process at the increase in surfactant concentra- 219
OOF
−
−
tion is caused by the rise of the degree of the binding of the dye. Also, 220
1
1
1
86
87
88
(i) for nonionic surfactants, Ψ = 0 and [OH ]
m
/[OH ]
w
= A;
−
−
even before the CMC some slight (ca. 20%) growth of the k value takes 221
place. The reason is obviously the formation of mixed dye + surfactant 222
(ii) for anionic surfactants, Ψ b 0 and [OH ]
m
/[OH ]
w
b A;
N A.
−
−
(iii) for cationic surfactants, Ψ N 0 and [OH ]
m
/[OH ]
w
aggregates.
223
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
89
90 the [OH−]
Thus, at constant total concentration of hydroxide ions in solution,
value on the surface of cationic micelles will be higher,
The effects of the CTAB–TX-100 mixtures, in which the molar ratios 224
of the surfactants are equal to 1:1 and even 1:4, are closer to that of pure 225
CTAB. The magnitude of the effect is intermediate between these ob- 226
m
91 whereas on the surface of anionic micelles lower, as compared with
92 the nonionic micelles.
served for pure CTAB and TX-100 (Fig. 1a).
227
93
The degree of the dye binding by micelles is determined mainly by
The addition of the dimeric bis-quaternary ammonium surfactant 228
6–4–16 to the aqueous crystal violet solution leads to the increase in 229
+
94 two factors: hydrophobicity of the CV ions and the sign of the charge
95 of a micellar surface. The binding of the dye cations is promoted for
96 the negatively charged surface of colloidal particles in the case of SDS
97 micelles. At pre-micellar concentrations of SDS, the ionic pairs and
98 other dye–surfactant associates are formed. At the concentrations
99 where the formation of micelles begins, the ionic pairs are destructed
00 due to simultaneous interaction of the cation with several surfactant an-
01 ions [38–40]. The increase in the reaction rate in the presence of the cat-
02 ionic surfactant indicates that despite the same charge of the dye and
03 surfactant ions, the binding of the dye nevertheless occurs due to hydro-
04 phobicity of the dye. The binding of the dye to the micelles of nonionic
05 surfactants TX-100 and Brij-35 occurs due to hydrophobic interactions
06 only [40]. Irrespective of the type of the surfactant used, the aromatic
07 rings of the dye will probably be immersed into micellar core to mini-
08 mize their contact with water, but one or two of the dimethylamino
09 groups will be located on the surface being hydrated by water.
1
the reaction rate (Fig. 2b). This effect is similar to that observed for an- 230
other cationic surfactant, CTAB, though it is less expressed. 231
3.4. Effect of anionic and mixed anionic–nonionic surfactants on the rate of 232
fading 233
The addition of anionic surfactant SDS leads to sharp decrease in re- 234
action rate (Fig. 3). In fact, the rate of the reaction considerably de- 235
creases in the pre-micellar region of concentrations and further 236
additions of SDS give but small change in the rate constant. The ob- 237
served decrease in the rate constants is probably caused by the interac- 238
tion of the dye cations with anions of surfactant, which leads to the 239
redistribution of the charge in cation and the lowering of positive charge 240
on the dye carbocation. Also, the carbinol formation may be decelerated 241
5
4
3
0,4
0
,3
,2
0
2
UNCORRECTED PR
0,1
1
0
,0
0
0
0
,00
0,05
0,10
0,15
0,20
,000
0,005
0,010
0,015
0,020
3
[
Brij-35]x10 , M
[
Surfactant], M
(a)
(b)
Fig. 1. The effect of cationic, nonionic surfactants and their mixture on the observed second-order rate constants of the alkaline fading of crystal violet at 25 °C, 4.6 × 10−3 M NaOH. (a) • —
TX-100, ▼ — CTAB, ° — mixture of TX-100 and CTAB (molar ratio 1:1), Δ — mixture of TX-100 and CTAB (molar ratio 4:1); (b) • — Brij-35. Solid line drawn using calculated values according
Q1 to Eq. (7) of Piszkiewicz's model.
Please cite this article as: K.V. Roshchyna, et al., Micellar rate effects in the alkaline fading of crystal violet in the presence of various surfactants, J.
Mol. Liq. (2014), http://dx.doi.org/10.1016/j.molliq.2014.11.013

4
K.V. Roshchyna et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
1
1
0
0
0
0
0
,2
,0
,8
,6
,4
,2
,0
4
3
2
1
0
0
0,0000
0,0005
0,0010
0,0015
0,0020
,000
0,002
0,004
0,006
0,008
[
Gemini surfactant], М
[
СTAB], M
(a)
(b)
Fig. 2. (a) The effect of cationic surfactant CTAB in the presence of sodium bromide and (b) the effect of the dimeric bis-quaternary ammonium cationic surfactant 16–4–16 on the observed
second-order rate constants of the alkaline fading of crystal violet at 25 °C. (a) • — СТАB without salt, ° — СТАB +0.01 М NaBr, ▼ — СТАB +0.10 M NaBr, ∇ — СТАB +0.2 M NaBr, — СТАB
+
0.40 M NaBr; (b) • — gemini surfactant without salt, ° — gemini surfactant +0.01 М NaBr, ▼ — gemini surfactant +0.04 M NaBr.
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
42 because the dye cations are hidden in the clusters of ionic pairs,
this case is enhanced by interaction with positively charged portion of 264
zwitterionic molecule, but the effect of the concentrating of OH ions 265
+
−
−
43 CV DS
44
.
Absorption spectra of CV+ in SDS solutions at varying concentrations
is probably absent.
266
45 of the surfactant have been measured (Fig. 4). The absorption band of
46 the dye at low SDS concentrations is widened due to the multiplicity
47 of the states of the dye ions and interaction of their chromophoric sys-
48 tems with each other. At the reaching of CMC value in micellar solutions,
49 the observed absorption band of the dye shows solvatochromic shift in
50 comparison that in water. In the range of SDS concentrations below
3.6. Micellar effects in the presence of sodium bromide
267
The addition of salts to solution containing surfactants leads to de- 268
crease in CMC according to the well-known equation [32]
269
+
−
51 CMC, the ionic pairs CV DS , their aggregates, and other associates of
logCMC ¼ −a logc þ b;
ð4Þ
i
+
52 CV with dodecyl sulfate ions are formed. These ionic pairs have been
53 shown to be surface-active; they are readily adsorbed on the water/air
+
+
−
where a and b are some constants depending on a kind of surfactant and 271
temperature, and c — concentration of counter-ions in a bulk phase of
the solution.
x y z
54 interface [40]. Near the CMC, the mixed micelles (CV ) (Na ) (DS )
i
55 with the prevalence of surface-active anions. In such systems, the dye
56 cation may be considered as a kind of a counter-ion competing with
272
+
So, the higher the salt concentration the lower the CMC of the surfac- 273
tant is. For example, at 25 °C the CMC of SDS in the solution containing 274
57 Na [36]. Finally, homomicelles of SDS with the ions of dyes isolated
58 from each other adsorbed on them are formed.
−
3
−4
0
.10, 0.20, and 0.40 M of sodium chloride equal 1.62 × 10 , 8.3 × 10
275
−
4
and 5.2 × 10
M, correspondingly [32]. The last value is almost 16 276
2
59 3.5. Effect of zwitterionic surfactant on the rate of fading
times lower, than in the salt-free system. It is similarly possible to as- 277
sume that in the solutions containing different concentrations of NaBr, 278
the formation of the surfactant aggregates begins at much smaller con- 279
centrations, than in 4.6 × 10 M NaOH solution. Therefore in the pres- 280
ence of the salt the influence of the surfactant on the reaction rate will 281
2
2
2
2
60
The effect of the addition of zwitterionic surfactant DMDAPS on the
−
3
61 rate of alkaline fading of crystal violet is shown in Fig. 3 (inset). The in-
62 fluence of this surfactant on the rate is similar to that observed in the
63 presence of nonionic surfactants. The binding of the dye by micelles in
appear at lower surfactant concentrations.
282
The effect of addition of NaBr on the reaction rate is shown on Figs. 3 283
and 4. Gradual increasing of NaBr concentration in system with CTAB 284
0
0
0
0
0
0
0
0
0
0
,18
,16
,14
,12
,10
,08
,06
,04
,02
,00
0
0
,30
,25
A 1,0
8
7
0,20
1
0
0
0
0
0
,8
,6
,4
,2
,0
2
0
0
,15
3
6
,10
0
4
,000
0,005
0,010
0,015
0,020
[
DMDAPS], M
5
0
,000 0,002 0,004 0,006 0,008 0,010 0,012
[
SDS], M
4
50
500
550
600
650
700
Wavelength, nm
Fig. 3. The influence of SDS on the observed second-order rate constants of the alkaline
fading of crystal violet at 25 °C: • — SDS, ° — SDS +0.40 M NaBr. Inset: the effect of zwit-
terionic surfactant DMDAPS on the observed second-order rate constants of the alkaline
fading of crystal violet at 25 °C.
4
Fig. 4. Absorption spectra of crystal violet at different concentrations of SDS (c∙10 , M): 1 —
0.0, 2 — 0.4, 3 — 0.8, 4 — 1, 5 — 2, 6 — 10, 7 — 60, and 8 — 100.
Please cite this article as: K.V. Roshchyna, et al., Micellar rate effects in the alkaline fading of crystal violet in the presence of various surfactants, J.
Mol. Liq. (2014), http://dx.doi.org/10.1016/j.molliq.2014.11.013

K.V. Roshchyna et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
5
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
85 leads to progressive decreasing in the rate constants. In the case of the
86 gemini 16–4–16, the salt effect is of the same sign; however, high
87 NaBr concentrations precipitate the surfactant. Comparison of the rate
88 constants determined in the absence and presence of salt in system
89 with SDS (Fig. 3) shows that the addition of NaBr leads to decrease in
90 the rate constants in the pre-micellar region of SDS concentrations
91 and to their increase at concentrations near and above CMC.
92
When the surfactant concentration exceeds the CMC value, the pres-
93 ence of electrolyte in solution will lead to the shielding of surface poten-
94 tial of a micelle and the compression of a double electric layer of the
95 micelle of ionic surfactants. This leads to a decrease in absolute value
96 of the surface potential in the locus of dye and to some decrease in the
Scheme 2.
Q3
n
+
+
[
D] /[D
n
− CV ]. Eq. (2.3) corresponds to the interaction of the CV 331
−
−
bound by catalytic micelle and OH with the formation of the same 332
product; km is the second-order rate constant for this process. It follows 333
from Scheme 2, that for the observed rate constant as a function of the 334
97 concentration of OH of ions in Stern layer according to Eq. (3). In
98 terms of the ion-exchange model, the decrease in the [OH−
99 may be explained taking into account of the excess of the Br ions in
00 the system [36]. It is natural that the last effect is imposed on results
01 in all area of existence of micelles in the presence of salt.
m
] value
−
surfactant concentration [24–26]:
335
n
k ½Dꢀ þ k K
02
It is clearly seen that in the presence of the proper salt background,
m
w
n
D
k ¼
:
ð5Þ
K
þ ½Dꢀ
03 the dependences of the observed k values on the surfactant concentra-
04 tion practically reach the plateau. The corresponding quantities are
05 compiled in Table 2. The changes of the rate constants are in line with
06 the above considered electrostatic approach.
D
337
We have applied the Piszkiewicz's model in the form of Eq. (5) for
OOF
describing of variation of the rate constants along with the concentra- 338
tion of surfactant in solution below and above the CMC values. The non- 339
07
On the other hand, in the cationic and non-ionic surfactant solutions
08 with low ionic strength the k values are increasing monotonously,
09 whereas in the SDS-containing systems, especially at low surfactant
10 concentrations, they are decreasing sharply. For such cases, we under-
11 took the attempt to use the afore-mentioned Piszkiewicz's model.
linear method of the least squares was used for calculations.
340
Unfortunately, this approach was absolutely inapplicable for several 341
systems (Brij 35, gemini 16–4–16), where the confidential intervals of 342
all the required parameters included zero. In the other systems, the 343
standard deviations were as a rule insufficiently high; the correspond- 344
3
12 3.7. Application of the Piszkiewicz's model
ing values of k
m
, n, and K
D
thus obtained are gathered in Table 3.
345
As one can see, correlation coefficients of the approximation (r) are 346
good, but standard errors for the parameters are usually close to the 347
values and sometimes exceed them. This fact clearly demonstrates 348
that in general Piszkiewicz's model does not correctly describe the var- 349
iation of rate constants in these systems. This conclusion was also point- 350
ed out for the kinetics of brilliant green fading in micellar solutions [20]. 351
Hence, the micellar effects should be accounted for without relying the 352
3
3
13
The Piszkiewicz's model [24–26] is based on the mechanism de-
14 scribed in Scheme 2.
kw
þ
−
CV þ OH → CV−OH;
ð2:1Þ
ð2:2Þ
3
3
16
17
Piszkiewicz's model.
353
þ
þ
CV þ nD ⇄ D −CV ;
n
Indeed, the pre-micellar dye–surfactant interactions are of stepwise 354
nature and thus it is difficult to expect a constant n value. On the other 355
hand, the manifold n values for cationic dyes of similar structure, such as 356
KD
3
3
19
20
1.07 [30] 3.5 [19], 3.98 [21], 4.03 [17], 4.45 [29] in CTAB and 0.8 [17], 1.3 357
km
þ
−
D −CV þ OH → CV−OH:
ð2:3Þ
n
[30], 1.46 [29], and 1.68–1.78 [20] in SDS solutions, probably reflect just 358
the formal fitting of the data. If the dye–micelle interaction may be ex- 359
3
22
According to Scheme 2, the kinetic mechanism of bimolecular reac-
pected only, then it should be formally accepted that n = 1.
360
+
3
3
3
3
3
3
3
3
23 tion of the alkaline fading of CV in aqueous solution in the presence of
24 surfactant, or detergent (D) includes three steps. Eq. (2.1) reflects the
For the systems containing cationic surfactants (CTAB, its mixtures 361
with TX-100, and the gemini surfactant), the n values are close to 362
unity (Table 2). In this case Eq. (7) may be transformed to the well- 3Q 6 13 1
+
−
25 interaction of the free CV and OH with the formation of the colorless
is the second-order rate constant for
26 product (CV − OH) in water; k
w
known Menger equation [5] in the following form
364
27 this process. Step (2.1) is the equilibrium of the binding of the dye cation
28 with several numbers of detergent molecules (n) with the formation of
kw þ kmKass½Mꢀ
k ¼
;
ð8Þ
+
29 catalytic micelles (D
n
− CV ); K
D
is the equilibrium constant of dissoci-
1 þ K ½Mꢀ
ass
+
D
30 ation of the last-named giving the free dye and detergent, K = [CV ]
where [M] is the concentration of the micelles in solution, and Kass 366
is the association constant of the reagent and the micelle; [M] =
t2:1
t2:2
t2:3
Table 2
UNCORRECTED PR
+
−
The observed k values for the CV + OH reaction at the plateau of the k vs. csurf.
dependence.
a
Table 3
t3:1
tQ3:52
k, M− s−1
1
k/k
t2:4
System
w
a
Kinetic parameters of Eq. (7).
t2:5
Surfactant-free water
CTAB + 0.10 M NaBr
CTAB + 0.20 M NaBr
CTAB + 0.40 M NaBr
0.157
0.49
0.33
0.17
1.00
3.10
2.08
1.05
2.1 × 10
1.5 × 10
3.45
, M−1 s−1
r2
Surfactant
CTAB
k
m
K
D
n
t3:3
t2:6
t2:7
16 ± 10
CTAB–TX-100 (1:1) 12 ± 10
CTAB–TX-100 (1:4) 5.3 ± 1.6
0.01 ± 0.01
0.018 ± 0.015
(8 ± 5) × 10
(3 ± 7) × 10
(6 ± 12) × 10
1.13 ± 0.12 0.996
1.08 ± 0.14 0.996
1.09 ± 0.07 0.999
0.995
0.983
1.29 ± 0.04 0.999
1.83 ± 0.19 0.987
t3:4
t3:5
t3:6
t3:7
t3:8
t3:9
t3:10
t2:8
b
−5
−4
−2
−3
t2Q:94
t2:10
t2:11
t2:12
SDS
3.3 × 10
2.4 × 10−
0.54
3
Triton X-100
DMDAPS
SDS
0.69 ± 0.08
0.298 ± 0.009
−6
−6
3.0 ± 0.4
2.3 ± 0.3
SDS + 0.40 M NaBr
a
TX-100
a
−4
−6
DMDAPS
0.29
1.84
(0 ± 4) × 10
(6.8 ± 2.7) × 10
(4.8 ± 2.4) × 10
(5 ± 3) × 10−
3
−7
SDS–TX–100 (1:4)
a
At 4.6 × 10−3 M NaOH.
At surfactant concentration of 0.02 M.
t2:13
t2:14
b
a
Standard errors of the parameters are given.
t3:11
Please cite this article as: K.V. Roshchyna, et al., Micellar rate effects in the alkaline fading of crystal violet in the presence of various surfactants, J.
Mol. Liq. (2014), http://dx.doi.org/10.1016/j.molliq.2014.11.013

6
K.V. Roshchyna et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
3
3
3
3
3
67 ([D] − CMC)/naggr, where naggr is the aggregation number of the
68 surfactant. At moderate concentrations of surfactant it may be as-
5. Uncited reference
4Q 1 13 3
69 sumed that [M] ≈ [D]/naggr
.
[41]
414
Q7 012 Indeed, dividing the numerator and denominator of Eq. (7) by K
D
71 and assuming that [D] ≈ [M]naggr, it gives
kw þ k ½Mꢀn =ðKDÞ
Acknowledgments
415
m
aggr
k ¼
:
ð9Þ
1
þ ½Mꢀn
aggr
=ðKDÞ
The authors express their gratitude to Sergey V. Shekhovtsov for the 416
synthesis of the gemini surfactant 16–4–16.
3
3
73
417
The ratio naggr/K
74 stant between CV and micelle
D
may be considered as binding or association con-
+
hD−CVþi ꢂh þi
ꢃ
hD−CVþi ꢂh þi
ꢃ
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CV ½Mꢀ
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87
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5
88 droxide ion the variation of the rate constants along with the change in
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Please cite this article as: K.V. Roshchyna, et al., Micellar rate effects in the alkaline fading of crystal violet in the presence of various surfactants, J.
Mol. Liq. (2014), http://dx.doi.org/10.1016/j.molliq.2014.11.013