Determination of an ion exchange constant by the use of a kinetic probe: A new semiempirical kinetic approach involving the effects of 3-F- and 4-F-substituted benzoates on the rate of piperidinolysis of anionic phenyl salicylate in aqueous cationic micelles

Article abstract of DOI:10.1021/la100863q

Pseudo-first-order rate constants (k_obs) for the nucleophilic substitution reaction of piperidine (Pip) with ionized phenyl salicylate (PS^-), obtained at a constant [Pip]_T (= 0.1 M), [PS ^-]_T (= 2×10^-4 M), [CTABr]_T (cetyltrimethylammonium bromide), ≤0.06 M NaOH, and a varying concentration of MX (= 3-FC₆H₄CO₂Na, 3-FBzNa and 4-FC ₆H₄CO₂Na, 4-FBzNa), follow the kinetic relationship k_obs = (k₀ + θK^X/S[MX])/(1 + K^X/S[MX]) which is derived by the use of the pseudophase micellar (PM) model coupled with an empirical equation. The empirical equation explains the effects of [MX] on CTABr micellar binding constant (K_S) of PS^- that occur through X^-/PS^- ion exchange. Empirical constants θ and K^X/S give the parameters F _X/S and K_X/S, respectively. The magnitude of F _X/S gives the measure of the fraction of micellized PS^- transferred to the aqueous phase by the limiting concentration of X^- through X^-/PS^- ion exchange. The values of F_X/S and K_X/S have been used to determine the usual thermodynamic ion exchange constant (K_X^Y) for ion exchange process X ^-/Y^- on the CTABr micellar surface. The values of K _X^Br (where Br = Y) have been calculated for X = 3-FBzNa and 4-FBzNa. The mean values of K_X^Br are 12.8 ± 0.9 and 13.4 ± 0.6 for X^- = 3-FBz^- and 4-FBz ^-, respectively. Nearly 3-fold-larger values of K_X ^Br for X = 3-FBz^- and 4-FBz^- than those for X = Bz^-, 2-ClBz^-, 2-CH₃Bz^-, and the 2,6-dichlorobenzoate ion (2,6-Cl₂Bz^-) are attributed to the presence of wormlike micelles in the presence of >50 mM 3-FBz^- and 4-FBz^- in the [CTABr]_T range of 5-15 mM. Rheological properties such as shear thinning behavior of plots of shear viscosity versus the shear rate at a constant [3-FBz^-] or [4-FBz^-] as well as shear viscosity (at a constant shear rate) maxima as a function of the concentrations of 3-FBz^- and 4-FBz^- support the conclusion, derived from the values of K_X^Br, for the probable presence of wormlike/viscoelastic micellar solutions under the conditions of the present study.

Full text of DOI:10.1021/la100863q

pubs.acs.org/Langmuir
© 2010 American Chemical Society
Determination of an Ion Exchange Constant by the Use of a Kinetic Probe: A
New Semiempirical Kinetic Approach Involving the Effects of 3-F- and 4-F-
Substituted Benzoates on the Rate of Piperidinolysis of Anionic Phenyl
Salicylate in Aqueous Cationic Micelles
Nor Saadah M. Yusof and M. Niyaz Khan*
Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
Received March 1, 2010. Revised Manuscript Received May 20, 2010
Pseudo-first-order rate constants (kobs) for the nucleophilic substitution reaction of piperidine (Pip) with ionized phenyl
salicylate (PS-), obtained at a constant [Pip]T (=0.1 M), [PS-]T (=2 ꢀ 10-4 M), [CTABr]T (cetyltrimethylammonium
bromide), e0.06 M NaOH, and a varying concentration of MX (= 3-FC6H4CO2Na, 3-FBzNa and 4-FC6H4CO2Na,
4-FBzNa), follow the kinetic relationship kobs = (k0 þ θKX/S[MX])/(1 þ KX/S[MX]) which is derived by the use of the
pseudophase micellar (PM) model coupled with an empirical equation. The empirical equation explains the effects of [MX] on
CTABr micellar binding constant (KS) of PS- that occur through X-/PS- ion exchange. Empirical constants θ and KX/S give
the parameters FX/S and KX/S, respectively. The magnitude of FX/S gives the measure of the fraction of micellized PS-
transferred to the aqueous phase by the limiting concentration of X- through X-/PS- ion exchange. The values of FX/S and
K
X/S have been used to determine the usual thermodynamic ion exchange constant (KXY) for ion exchange process X-/Y- on
the CTABr micellar surface. The values of KXBr (where Br = Y) have been calculated for X = 3-FBzNa and 4-FBzNa. The
mean values of KXBr are 12.8 ( 0.9 and 13.4 ( 0.6 for X- = 3-FBz- and 4-FBz-, respectively. Nearly 3-fold-larger values of
KXBr for X = 3-FBz- and 4-FBz- than those for X = Bz-, 2-ClBz-, 2-CH3Bz-, and the 2,6-dichlorobenzoate ion (2,6-
Cl2Bz-) are attributed to the presence of wormlike micelles in the presence of >50 mM 3-FBz- and 4-FBz- in the [CTABr]T
range of 5-15 mM. Rheological properties such as shear thinning behavior of plots of shear viscosity versus the shear rate at a
constant [3-FBz-] or [4-FBz-] as well as shear viscosity (at a constant shear rate) maxima as a function of the concentrations of
3-FBz- and 4-FBz- support the conclusion, derived from the values of KXBr, for the probable presence of wormlike/
viscoelastic micellar solutions under the conditions of the present study.
Br
accurately either KX or KX because the magnitudes of KX and
I. Introduction
Br
KX are very technique-dependent.6,7 For instance, the reported
Quaternary ammonium surfactants with certain structurally
typical counterions have been shown to form viscoelastic surfactant
solutions1 that have various potential commercial uses2 including
effective drag reducers in turbulent flow.3 Moderately hydro-
phobic organic anions binding to cationic micelles is believed to
be important in stabilizing viscoelastic micellar structures.4 Simi-
larly, a recent study concludes that rodlike micelle formation occurs
because of an increase in headgroup-counterion association and
dehydration on the micellar surface.5 These studies predict that (a)
the cationic micellar binding constant (KX) of viscoelastic counter-
ion X should be larger than that (KY) of nonviscoelastic counterion
Y with comparable molecular structural features and (b) the ion
exchange constant for X, KXBr (= KX/KBr where KBr is the cationic
micellar binding constant of counterion Br-), should be larger than
that for Y, KYBr (= KY/KBr). Practically, it is not easy to determine
values of KXCl for X = 2,6-Cl2Bz- on the CTACl micellar surface
are 1.3 ( 0.1, 16.2 ( 5.5, and 16.8 ( 1.0 as determined by the use of
respective ion-selective electrode measurements, 1H NMR measure-
OH
ments, and a flotation technique.7 Similarly, the values of KBr
,
determined by the use of variety of physicochemical techniques,
vary over the range of 3-34.8
A reliable correlation between the viscoelastic behavior of
surfactant solutions and the values of KXBr for various counter-
ions (X) is expected to be achieved if the values of KXBr have been
Br
determined by the use of the same technique. The values of KX
have been determined for different X counterions by the use of a
new technique9 involving an empirical kinetic approach that
consists of the classical pseudophase micellar (PM) model
coupled with an empirical equation (eq 1).8b,10
o
KS
KS
¼
ð1Þ
*Corresponding author. E-mail: niyaz@um.edu.my.
1 þ KX=S½MXꢁ
(1) (a) Ahmed, T.; Aramaki, K. J. Colloid Interface Sci. 2008, 327, 180–185. (b)
Sharma, S. C.; Shrestha, R. G.; Shrestha, L. K.; Aramaki, K. J. Phys. Chem. B 2008,
112, 10520–10527.
In eq 1, KS and KSo represent cationic micellar binding constants
of reactive counterion S- in the presence and absence of inert
(2) Attarzadeh, H. J. Res. Med. Sci. 2006, 11, 111–112.
(3) (a) Zakin, L.; Zhang, Y.; Ge, W. In Giant Micelles: Properties and Applica-
tions; Zana, R., Kaler, E. W., Eds.; Surfactant Science Series; CRC Press: Boca Raton,
FL, 2007; Vol. 140, Chapter 16, pp 473-492.(b) White, A. Nature 1967, 214, 585–586.
(c) Roze, G. D.; Foster, K. L. J. Non-Newtonian Fluid Mech. 1989, 31, 59–85.
(d) Elson, T. P.; Garside, J. J. Non-Newtonian Fluid Mech. 1983, 12, 121–133.
(e) Ohlendorf, D.; Interthal, W.; Hoffmann, H. Rheol. Acta 1986, 25, 468–486.
(4) Rehage, H.; Hoffmann, H. Rheol. Acta 1982, 21, 561–563.
(7) Magid, L. J.; Han, Z.; Warr, G. G.; Cassidy, M. A.; Butler, P. D.; Hamilton,
W. A. J. Phys. Chem. B 1997, 101, 7919–7927.
(8) (a) Romsted, L. S. In Surfactant in Solutions; Mittal, K. L., Lindmann, B., Eds.;
Plenum: New York, 1984; Vol. 2, pp 1015-1068.(b) Khan, M. N.; Ariffin, Z. J. Chem.
Soc., Perkin Trans 2 2000, 2503–2510.
(5) Geng, Y.; Romsted, L. S.; Menger, F. J. Am. Chem. Soc. 2006, 128, 492–501.
(6) (a) Khan, M. N.; Naoliya, J.; Dahiru, M. J. Chem. Res. 1988, 116-117, (S) .
Khan, M. N.; Naoliya, J.; Dahiru, M. J. Chem. Res. 1988, 1168-1176, (M) .(b) Khan,
M. N. J. Mol. Catal. A: Chem. 1995, 102, 93–101. (c) Khan, M. N. J. Phys. Org.
Chem. 1996, 9, 295–300.
(9) Khan, M. N. Micellar Catalysis; Surfactant Science Series; CRC/Taylor &
Francis: Boca Raton, FL, 2007; Vol. 133, Chapter 3.
(10) (a) Khan, M. N. J. Org. Chem. 1997, 62, 3190–3193. (b) Khan, M. N.; Ariffin,
Z.; Lasidek, M. N.; Hanifiah, M. A. M.; Alex, G. Langmuir 1997, 13, 3959–3964.
(c) Khan, M. N.; Ismail, E. J. Mol. Liq. 2003, 107, 277–287.
Langmuir 2010, 26(13), 10627–10635
Published on Web 06/07/2010
DOI: 10.1021/la100863q 10627

Article
Yusof and Khan
counterion X-, respectively. The symbol KX/S represents an em-
pirical constant whose magnitude is the measure of the ability of X-
to expel S- from the cationic micellar pseudophase into the aqueous
phase through the occurrence of ion exchange X-/S- on the
cationic micellar surface. It has been concluded elsewhere8b,11 that
the values of KX/S and KBr/S (for ion exchange Br-/S-) are related to
the usual thermodynamic ion exchange constant, KXBr (= KX/KBr).
The values of KXBr, obtained from this new approach/technique, for
a few inorganic and organic anions (X) are listed elsewhere.8b,11 The
value of KXBr with MX=3-ClC6H4CO2Na (an inert salt with an
anion acting as a viscoelastic counterion of a CTABr micelle12) was
found to be more than 12-fold larger than that with MX =
2-ClC6H4CO2Na (an inert salt with an anion acting as a non-
viscoelastic counterion of CTACl12).13 Thus, the values of KXBr for
X = viscoelastic as well as nonviscoelastic counterions would have
predictive power for a counterion X to be either viscoelastic or
nonviscoelastic. Furthermore, the kinetic approach of the present
study is rather new and provides a quantitative interpretation of the
effects of the inert salt concentration on the rate constants for
cationic micellar-mediated organic reactions. The present study was
Br
aimed at using this new technique to determine the values of KX
for MX = 3-FC6H4CO2Na (3-FBzNa) and 4-FC6H4CO2Na (4-
FBzNa) at different total concentrations of CTABr. The results and
their probable explanation(s) are described in this article.
II. Experimental Section
Reagent-grade chemicals such as phenyl salicylate (PSL),
cetyltrimethylammonium bromide (CTABr), piperidine (Pip),
and 3-fluorobenzoic acid (3-FBzH) as well as 4-fluorobenzoic
acid (4-FBzH) were commercial products of the highest available
purity. However, these acids were recrystallized before use. Stock
solutions (0.01 M) of PSL were prepared in acetonitrile.
Figure 1. Plots showing the dependence of kobs upon [MX] (MX =
3-FBzNa) for the piperidinolysis of PS- at 2.0 ꢀ 10-4 M PS-, 0.1 M
Pip, e0.06 to >0.03 M NaOH, 35 ꢀC, and[CTABr]T/mM = 5 (b),6
(2), 7(9), 10(O), and15(Δ). Thedashedlinesaredrawnthroughthe
calculated data points from eq 3 with kinetic parameters (k0, θ, and
KX/S) listed in Table 2 at [MX]0op = 0.0. The solid lines are drawn
through the calculated data points from eq 3 with kinetic parameters
(k0, θ, and KX/S) listed in Table 2 at [MX]0op/mM = 5.9 (b), 7.3 (2),
9.4 (9), 13.1 (O), and 18.8 (Δ).
Kinetic Measurements. The rate of the nucleophilic reaction
of Pip with ionized phenyl salicylate (PS-) was measured spectro-
photometrically at 35 ꢀC by monitoring the disappearance of
reactant (PS-) at 350 nm as a function of reaction time (t) in the
presence of 2 ꢀ 10-4 M PS-, 0.1 M Pip, and >0.03 M NaOH and
desired concentrations of CTABr and 3-FBzNa or 4-FBzNa. The
details of the kinetic procedure and product characterization
study were the same as described elsewhere.14
piperidine, cetyltrimethylammonium bromide, phenyl salicylate,
and [MX] (= 3-FBzNa or 4-FBzNa). The concentrations of
3-FBzNa and 4-FBzNa were varied within the range of 0.03-
0.20 M. The sample solutions were left in a water bath at a con-
stant temperature of 35 ꢀC for at least 10 min. The rheological
measurements were carried out using the Brookfield R/Sþ rhe-
ometer, and the temperature was controlled by an external
temperature controller. However, because of the air-conditioned
room, the temperature varied slightly from 35 ꢀC, which is
expectedto cause a negligible error. A coaxial double-gapcylinder
(CC-DG) was used with the required volume of sample for every
run as 16 mL. For every repeat measurement, a new sample was
used. By fixing the shear rate (γ) range at 1-15 s-1 in 2400 s and
1-100 s-1 in 800 s for all runs and 1-1000 s-1 in 8000 s for certain
runs, the dependent shear stress (τ) and shear viscosity (η)
detected by the machine were recorded after each 80 s.
All ofthekinetic runswere carried out under pseudo-first-order
kinetic conditions ([Pip]T/[PS-]T = 500 where [ ]T represents the
total concentration). The observed kinetic data (Aob vs t where
Aob represents the absorbance value at any reaction time t) were
found to fit to eq 2 for a reaction period equivalent to ∼6 to ∼8
half-lives of the reaction.
Aob ¼ ½R0ꢁδap expð - kobstÞ þ A¥
ð2Þ
In eq 2, [R0] is the initial concentration of PSL, δap is the apparent
molar absorptivity of the reaction mixture, kobs is the pseudo-first-
order rate constant, and A¥=Aob at t=¥. The nonlinear least-squares
technique was used to calculate kobs, δap, and A¥ from eq 2, and the
observed data fit to eq 2 was found to be satisfactory in terms of the
percent residual errors (RE=100 ꢀ (Aob i-Acald i)/Aob i where Aob i
and Acald i represent observed and least-squares calculated values of
absorbance at the ith reaction time (ti)) as well as standard deviations
associated with calculated parameters kobs, δap, and A¥.
III. Results
Effects of [3-FC6H4CO2Na] ([3-FBzNa]) and [4- FC6H4-
CO2Na] ([4-FBzNa]) on kobs for the Reaction of Pip with PS-
at a Constant [CTABr]T and 35 ꢀC. More than 10 kinetic runs
were carried out within the [MX] (where MX = 3-FBzNa or
4-FBzNa) range of 0.0 toe0.30 M at constant values of [CTA-
Br]T, 0.1 M Pip, and 2 ꢀ 10-4 M PSL. The values of [NaOH] were
varied from 0.030 to 0.060 M. Similar observations were obtained
at different [CTABr]T within the range of 0.005-0.015 M. The
values of kobs at different [CTABr]T are shown in Figures 1 and 2
Rheological Measurements. Samples with total volume of
25 mL for each sample in the rheological measurements were
prepared by mixing constant amounts of sodium hydroxide,
(11) Khan, M. N.; Ismail, E. J. Mol. Liq. 2007, 136, 54–63.
(12) Lu., B.; Li, X.; Scriven, L. E.; Davis, H. T.; Talmon, Y.; Zakin, J. L.
Langmuir 1998, 14, 8–16.
(13) Khan, M. N.; Ismail, E. J. Phys. Chem. A 2009, 113, 6484-6488 and
references cited therein
(14) Khan, M. N. J. Chem. Soc., Perkin Trans 2 1989, 199–208.
10628 DOI: 10.1021/la100863q
Langmuir 2010, 26(13), 10627–10635

Yusof and Khan
Article
Figures 3 and 4 for MX = 3-FBzNa and 4-FBzNa, respectively.
Figures 3 and 4 represent the shear thinning behavior that is a
typical characteristic of wormlike micelles.15-19 The values of η at
γ=1.5 s-1 (i.e., η1.5γ) and constant [MX] were obtained by the
interpolation of the plots of η versus γ (Figures 3 and 4). The
values of η1.5γ were calculated from the empirical relationship η=
ηoe-Rγ (where ηo and R are considered to be empirical constants)
by the use of observed data points (i.e., η vs γ) at the lower γ
values (<10 s-1). The calculated values of η1.5γ at different [MX]
are shown graphically in Figure 5 for MX=3-FBzNa and MX=
4-FBzNa. Although the derivation of the value of η at a specific
shear rate by the interpolation process using a small segment of
the plot of η versus γ is considered to be reliable, the values of η at
a shear rate of 100 s-1 (i.e., η100γ) were directly obtained from the
observed data of η versus γ at different [MX]. The values of η100γ
at different [MX] are shown graphically in Figure 6 for MX=
3-FBzNa and MX=4-FBzNa. Figures 5 and 6 reveal that the
nature of the plots remained essentially unchanged with the
change in the shear rate from 1.5 to 100 s-1.Viscosity maxima
as a function of salt and surfactant concentration are ubiquitous
in the literature for wormlike micellar systems.16,19,20
IV. Discussion
The details of the following observations have been described
elsewhere.13,21 (a) The products of the reaction of Pip with PSL are
N-piperidinylsalicylamide (P1) and phenol (P2). (b) The PSL exists
in nearly 100% ionized form (PS-) under the experimental condi-
tions of entire kinetic runs of the present study (i.e., [PSL]T
=
[PS-]T). (c) The rates of the uncatalyzed and hydroxide ion-
catalyzed hydrolysis of PS- are negligible compared with the rate
of piperidinolysis of PS- under the present experimental condi-
tions. (d) The concentration of protonated piperidine ([PipHþ]) is
negligible compared with [Pip] in the presence of 0.03-0.060 M
NaOH and 0.0-0.015 M CTABr, and thus [Pip]T ≈ [Pip]. This fact
is also evident from the kobs values listed in Table 1. (e) The values
of kobs at [MX] = 0 (Figures 1 and 2 as well as Tables I-IV, SI) are
more than 10-fold smaller than kobs for the reaction of Pip with PS-
at 0.1 M Pip and [CTABr]T =0 (Table 1).22,23 The mechanistic
details of these observations have been described elsewhere.10b,22 In
view of these observations, a brief reaction scheme for the cleavage
of PS-, under the present experimental conditions, may be repre-
sented by Scheme 1, where kn represents the nucleophilic second-
order rate constant for the reaction of Pip with PS- at [Pip] ≈
[Pip]T and consequently kobs = kn[Pip]T.
Figure 2. Plots showing the dependence of kobs upon [MX] (MX =
4-FBzNa) for the piperidinolysis of PS- at 2.0 ꢀ 10-4 M PS-, 0.1 M
Pip, e0.06 to>0.03 M NaOH, 35 ꢀC, and [CTABr]T/mM = 5 (b),6
(2), 7(9), 10(O), and15(Δ). Thedashedlinesaredrawnthroughthe
calculated data points from eq 3 with kinetic parameters (k0, θ, and
KX/S) listed in Table 2 at [MX]0op = 0.0. The solid lines are drawn
through the calculated data points from eq 3 with kinetic parameters
(k0, θ, and KX/S) listed in Table 2 at [MX]0op/mM = 4.5 (b), 4.7 (2),
5.2 (9), 7.3 (O), and 9.8 (Δ).
as well as in Tables I-IV in the Supporting Information (SI). The
values of δap turned out to be almost independent of [MX], and
the mean values of δap (=δapav), at different [CTABr]T, are shown
in Tables I-IV (SI). The calculated values of A¥ revealed a
definite but mild absorption due to 3-FC6H4CO2- and 4-FC6H4-
-
CO2 at 350 nm (δ = ∼10-14 M-1 cm-1 with δ representing
the molar absorptivity of 3-FC6H4CO2- or 4-FC6H4CO2-). The
values of δ remained unchanged within the [CTABr]T range of
0.005-0.015 M.
A critical look at the observed data, represented by Figure 1 as
well as Tables I and II, reveals that the values of kobs are almost
independent of [3-FBzNa] within the ranges of 0.0-0.005 M at
Effects of [3-FBzNa] and [4-FBzNa] on kobs for the
Reaction of Pip with PS- in the Absence of CTABr at 35
ꢀC. To determine the probable effect of inert salts 3-FBzNa and
4-FBzNa on kobs in the absence of micelles, a few kinetic runs
were carried out at 0.1 M Pip, 2 ꢀ 10-4 M PSL, and within the
[3-FBzNa] or [4-FBzNa] range of 0.0 to e0.3 M. The concentra-
tion of NaOH was varied within the range of 0.030 to e0.060 M.
The calculated values of kinetic parameters kobs, δap, and A¥ at
different [3-FBzNa] or [4-FBzNa] are summarized in Table 1.
Rheological Properties. The rheological behavior of CTABr/
3-FBzNa and CTABr/4-FBzNa were examined at 34.3 ( 0.5 and
34.2 ( 0.3 ꢀC under steady shear. The micellar solutions con-
tained constant concentrations of all of the additives (i.e., 2 ꢀ10-4
M PS-, 0.1 M Pip, 15 mM CTABr, constant [NaOH] (g0.03
to e0.05 M), and [MX] (g20 to 200 mM for MX = 3-FBzNa as
well as MX = 4-FBzNa)). The log-log plots of shear viscosity (η)
versus shear rate (γ) at different values of [MX] are shown in
(15) Lu, T.; Huang, J.; Li, Z.; Jia, S.; Fu, H. J. Phys. Chem. B 2008, 112, 2909–
2914.
(16) Raghavan, S. R.; Kaler, E. W. Langmuir 2001, 17, 300-306 and references
therein.
(17) Lin, Y.; Han, X.; Huang, J.; Fu, H.; Yu, C. J. Colloid Interface Sci. 2009,
330, 449–455.
(18) Shrestha, R. G.; Aramaki, K. J. Nepal Chem. Soc. 2008/2009, 23, 65–73.
(19) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.;
Miller, W. G. J. Phys. Chem. 1992, 96, 474–484.
(20) (a) Shubert, B. A.; Wagner, N. J.; Kaler, E. W. Langmuir 2004, 20, 3564–
3573. (b) Ali, A. A.; Makhloufi, R. Colloid Polym. Sci. 1999, 277, 270–275. (c)
Hoffmann, H. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prudhomme,
R., Eds.; ACS Symposium Series 578; American Chemical Society: Washington, DC,
1994; pp 2-31.(d) Magid, L. J. J. Phys. Chem. 1998, 102, 4064–4074. (e) Aswal, V. K.;
Goyal, P. S.; Thiyagarajan, P. J. Phys. Chem. B 1998, 102, 2469–2473. (f) Ali, A. A.;
Makhloufi, R. Phys. Rev. E. 1997, 56, 4474–4478.
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cited therein.
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Langmuir 2010, 26(13), 10627–10635
DOI: 10.1021/la100863q 10629

Article
Yusof and Khan
Table 1. Kinetic Parameters kobs, δob, and A¥ Calculated from Equation 2 for the Reaction of Pip with PS- in the Presence of MX and the Absence
of CTABra
[MX] =
3-FC6H4CO2Na
4-FC6H4CO2Na
-1
-1
102 [MX] M
104kobs s-1
10-1δ
M
cm-1
102A¥
104kobs s-1
10-1δ
M
ob
cm-1
102A¥
ob
0.0
0.2
0.5
0.8
1.0
1.5
2.0
2.5
3.0
4.0
5.0
7.0
10
340 ( 1.0b
338 ( 1.8
342 ( 1.7
324 ( 1.7
345 ( 1.6
332 ( 1.5
333 ( 1.8
317 ( 1.2
329 ( 1.3
322 ( 1.2
321 ( 1.3
305 ( 1.1
304 ( 1.6
278 ( 1.2
340 ( 1.0
338 ( 1.8
671 ( 1
682 ( 2
671 ( 2
678 ( 5
679 ( 2
675 ( 2
671 ( 2
666 ( 1
669 ( 1
671 ( 1
675 ( 1
675 ( 1
679 ( 2
687 ( 1
671 ( 1
682 ( 2
1.8 ( 0
340 ( 0.4b
329 ( 0.3
305 ( 0.9
334 ( 0.8
332 ( 0.7
310 ( 1.0
313 ( 0.8
318 ( 0.3
340 ( 0.9
327 ( 1.0
333 ( 0.1
323 ( 0.8
314 ( 0.5
313 ( 1.4
294 ( 0.4
285 ( 0.9
705 ( 1
695 ( 2
707 ( 3
708 ( 2
717 ( 7
697 ( 3
701 ( 2
711 ( 2
699 ( 7
682 ( 9
712 ( 1
702 ( 1
702 ( 1
711 ( 1
704 ( 1
706 ( 2
1.9 ( 0.1
1.9 ( 0.3
2.2 ( 0.2
2.1 ( 0.1
2.5 ( 0.3
4.5 ( 0.8
5.2 ( 0.3
5.9 ( 0.8
6.7 ( 0.9
8.9 ( 0.3
11 ( 0.1
16 ( 0.3
23 ( 0.1
27 ( 0.1
35 ( 0.1
43 ( 0.2
1.6 ( 0.1
1.6 ( 0.1
1.1 ( 0.3
1.9 ( 0.1
1.6 ( 0.1
1.7 ( 0.1
2.1 ( 0.1
1.9 ( 0.1
1.7 ( 0.1
2.0 ( 0.1
1.9 ( 0.1
2.1 ( 0.1
2.6 ( 0.1
1.8 ( 0.0
1.6 ( 0.1
15
20
30
a [Phenyl salicylate]0 = 2 ꢀ 10-4 M, 35 ꢀC, λ = 350 nm and [piperidine]T = 0.1 M, where [ ]T represents the total concentration and the aqueous
reaction mixture for each kinetic run contained 2% v/v CH3CN. The required amounts of 3- or 4-FC6H4CO2Na ([MX]) were generated in the reaction
mixture by using the stock solution (wM) of 3- or 4-FC6H4CO2H prepared in (w þ 0.05) M aqueous NaOH. The stock solution of NaOH was used to
produce a fixed known concentration (0.03 M) of NaOH in the reaction mixture for each kinetic run. Thus, the maximum concentration of added NaOH
was 0.06 M. b Error limits are standard deviations.
Figure 4. Plot showing the dependence of shear viscosity (η) upon
shear rate (γ) for samples where [PSL] = 2.0 ꢀ 10-4 M, [NaOH] =
0.03 M, [Pip] = 0.1 M, [CTABr] = 0.015 M, and [4-FBzNa] = 0.0
(ꢀ), 0.02 (0), 0.04 (9), 0.06 (Δ), 0.07 ()), 0.08 (•), 0.10 (O), 0.15 (2),
and 0.20 M (().
Figure 3. Plot showing the dependence of shear viscosity (η) upon
shear rate (γ) for samples where [PSL]=2.0 ꢀ 10-4 M, [NaOH]=
0.03 M, [Pip] = 0.1 M, [CTABr] = 0.015 M, and [3-FBzNa] = 0.0
()), 0.20 (ꢀ), 0.03 ((), 0.05 (9), 0.06 (Δ), 0.07 (•), 0.08 (O), 0.10 (2),
and 0.15 M (0).
0.015 M CTABr has caused a nonlinear increase in kobs with the
increase in [3-FBzNa]. Similarly, Figures 2 as well as Tables III
and IV reveal that the values of kobs are independent of [4-FBzNa]
within its ranges of 0.0-0.002 M at 0.005 and 0.006 M CTABr,
0.005, 0.006, and 0.007 M CTABr; 0.0-0.01 at 0.010 M CTABr;
and 0.0-0.015 at 0.015 M CTABr. A further increase in [3-
FBzNa] at 0.010 M 3-FBzNa and 0.005-0.007 M CTABr, 0.015
M 3-FBzNa and 0.010 M CTABr, and 0.020 M 3-FBzNa and
10630 DOI: 10.1021/la100863q
Langmuir 2010, 26(13), 10627–10635

Yusof and Khan
Article
Figure 5. Plotofshear viscosity (η) atγ=1.5 s-1 vs[MX] ([MX] =
[3-FBzNa] (b) and [4-FBzNa] (2)) at 0.015 M CTABr, 2.0 ꢀ 10-4
PS-, 0.03 M NaOH, 0.1 M Pip, and ∼35 ꢀC.
Figure 6. Plot of shear viscosity (η) at γ = 100 s-1 vs [MX]
([MX] = [3-FBzNa] (b) and [4-FBzNa] (2)) at 0.015 M CTABr,
2.0 ꢀ 10-4 PS-, 0.03 M NaOH, 0.1 M Pip, and ∼35 ꢀC.
0.0-0.005 M at 0.007 and 0.010 M CTABr, and 0.0-0.008 at
0.015 M CTABr. As the values of [4-FBzNa] increase from 0.005
M at 0.005 and 0.006 M CTABr, from 0.008 M at 0.007 and 0.010
M CTABr, and from 0.010 M at 0.015 M CTABr, the values of
kobs also increase nonlinearly. These observations cannot be
attributed to merely an inert salt effect because the values of kobs
are almost independent of [3-FBzNa] and [4-FBzNa] within its
range of 0.0-0.3 M (Table I). Similar observations have been ob-
tained for 3-ClC6H4CO2Na (a viscoelastic salt additive for CTABr
micelles12).13 As described earlier in the text, the rate of piperidino-
lysis of PS- is >10-fold larger in the aqueous phase than in the
CTABr micellar pseudophase. Thus, the most obvious source of the
nonlinear increase in kobs with the increase in [MX] (Figures 1 and 2
as well as Tables I-IV) is the transfer of micellized PSL (PS-M) to
the aqueous phase through the X-/PS- ion exchange. All possible
ion exchange processes, in the present reaction system, are X-/PS-,
X-/Br-, X-/HO-, HO-/Br-, HO-/PS-, and Br-/PS-. But in
view of the explanations described elsewhere,9,11 the most effective
ion exchange that could significantly affect kobs, is X-/PS- under
the experimental conditions of this study.
Scheme 1
and the aminolysis of phthalimide, phenyl salicylate, phenyl
benzoate, and N-benzylphthalimide obtained at a constant con-
centration of cationic micelles, amine nucleophiles (such as
piperidine, Pip), and NaOH.9,11,24 The nonlinear least-squares
technique was used to calculate unknown parameters θ and KX/S
P
as well as least-squares di2, (where di = kobsi - kcald1i, with kobsi
and kcald1i representing respective experimentally determined and
least-squares-calculated rate constants at the ith value of [MX])
from eq 3 by the use of an experimentally determined value of k0
at [MX] = 0. These results are summarized in Table 2. The extent
of reliability of the observed data fit to eq 3 is evident from the
standard deviations associated with least-squares-calculated
parameters θ and KX/S, as shown in Table 2 and in Figures 1
We attempted to fit the observed data (kobs vs [MX]), shown
graphically in Figures 1 and 2 as well as in Tables I-IV, to an
empirical equation (eq 3)
k0 þ θKX=S½MXꢁ
kobs
¼
ð3Þ
1 þ KX=S½MXꢁ
where θ and KX/S are empirical constants, k0 = kobs at [MX] = 0,
and [micelles] ¼ 0. Equation 3 has been found to explain satis-
factorily the effects of various inert inorganic and organic salts on
pseudo-first-order rate constants (kobs) for alkaline hydrolysis
(24) Khan, M. N.; Cheong, M. Y.; Ariffin, A. AIP Conf. Proc. 2009, 1136, 380–
384.
Langmuir 2010, 26(13), 10627–10635
DOI: 10.1021/la100863q 10631

Article
Yusof and Khan
Table 2. Values of Empirical Constants θ and KX/S Calculated from Equation 3 (Where [MX] = [MX]Sef with Zero and Nonzero [MX]0op Values)
for Different MX in CTABr Micellesa
P
[CTABr]T mMb
104ko s-1c
[MX]0op mM
104 θ s-1
KX/S M-1
KX/S M-1
FX/S
KX/Sn M-1
KX
108 di2
Br
MX = 3-FC6H4CO2Na
5.0
5.0
6.0
6.0
7.0
7.0
10.0
10.0
15.0
15.0
29.5
29.5
28.8
28.8
27.9
27.9
27.4
27.4
26.0
26.0
5.9
0
7.3
0
9.4
0
13.1
0
254 ( 8d
275 ( 14
229 ( 6
256 ( 16
241 ( 7
283 ( 24
219 ( 6
284 ( 37
209 ( 23
897 ( 1280
12.8 ( 1.1d
9.37 ( 1.15
10.1 ( 0.6
6.92 ( 0.95
8.18 ( 0.53
5.09 ( 0.87
6.76 ( 0.38
3.47 ( 0.80
4.59 ( 0.91
0.54 ( 0.86
461e
337
434
298
409
255
480
246
487
57.2
0.78f
0.84
0.70
0.78
0.74
0.87
0.67
0.87
0.64
2.74
358g
284
304
233
301
220
321
214
311
157
14.3h
11.3
12.2
9.32
12.1
8.81
12.9
8.56
12.4
6.28
219.5
579.3
74.68
385.9
63.09
460.4
24.98
411.9
40.91
255.2
18.8
0
MX = 4-FC6H4CO2Na
5.0
5.0
6.0
6.0
7.0
7.0
10.0
10.0
15.0
15.0
29.9
29.9
29.1
29.1
28.5
28.5
26.6
26.6
25.2
25.2
4.5
0
4.7
0
5.2
0
7.3
0
269 ( 6
289 ( 12
266 ( 3
289 ( 11
263 ( 9
288 ( 17
253 ( 10
294 ( 24
254 ( 27
472 ( 191
11.9 ( 0.7
9.15 ( 0.92
9.76 ( 0.26
7.44 ( 0.64
8.28 ( 0.63
6.25 ( 0.76
6.23 ( 0.53
4.23 ( 0.64
3.76 ( 0.65
1.39 ( 0.71
428
329
420
320
414
313
442
300
399
147
0.82
0.88
0.81
0.88
0.80
0.88
0.77
0.90
0.78
1.44
352
291
341
283
333
275
342
270
310
213
14.1
11.6
13.7
11.3
13.3
11.0
13.7
10.8
12.4
8.51
154.6
507.6
26.80
285.2
155.6
433.4
109.2
362.8
60.57
9.8
0
224.8
a [MX] = 3-FBzNa and 4-FBzNa. b Total concentration of CTABr. c k0=kobs at [MX]=0. d Error limits are standard deviations. e KX/S=KX/S (1þKS
o
[CTABr]T) where KSo = 7000 M-1. f FX/S=θ/kW where kW=kobs at [CTABr]T = 0, [Pip]T=0.1 M, and the value of kW under such conditions is 32.7 ꢀ
10-3 s-1 at 35 ꢀC. g KX/Sn = FX/SKX/S. h KXBr = KX/Sn/KBr/Sn, where KBr/Sn = 25 M-1
.
and 2 where dashed lines are drawn through the least-squares-
calculated rate constants obtained from eq 3, with the calculated
parameters listed in Table 2.
Scheme 2
Significant negative deviations of the observed data points from
the corresponding calculated data points with the decrease in [MX]
at its extreme lower values as well as higher values of [CTABr]T may
be explained as follows. The addition of X- to the reaction mixture
containing CTABr micelles causes the most hydrophilic counterions
HO- to move first from the cationic micellar pseudophase to the
bulk aqueous phase. Once such decreasing effects of [MX] upon
op
op
[HO-M] and [Br-M] at the respective [MX]OH and [MX]Br
values of θand KX/S (atthese[MX]0op values) at different [CTABr]T
for MX = 3-FBz and 4-FBz are shown in Table 2. It is evident from
the plots (solid lines) of Figures 1 and 2 as well as Tables I-IV that
op
op
(where [MX]OH and [MX]Br represent the optimum/critical
values of [MX] at which a further increase in [MX] has no effect
on ion exchange X-/HO- and X-/Br-, respectively) are attained,
P
the values of di2 and RE2 (=100 ꢀ (kobs -kcald2)/kobs, where
kcald2 represents the least-squares-calculated rate constant obtained
from eq 3 with [MX] = [MX]Sef and parameters θ and KX/S listed in
ef
ef
a further increase in [MX] at [MX] = [MX]S (where [MX]S is
the effective value of [MX] affecting the X-/S- ion exchange) causes
the expulsion of S- ions from cationic micelles into the aqueous
phase. Thus,
Table 2) are more reliable compared to the corresponding values of
P
di2, kcald1, and RE1 especially at lower and higher values of [MX]
and [CTABr]T, respectively. Thus, the calculated values of θ and
KX/S are more reliable when the data treatment with eq 3 was
ef
op
op
½MXꢁS ¼ ½MXꢁ - ð½MXꢁOH þ ½MXꢁBr
Þ
ð4Þ
carried out by considering [MX]=[MX]Sef at [MX]0 ¼ 0.
op
ef
and eq 4 predicts that [MX]S ≈ [MX] when [MX] . [MX]0op (=
[MX]OH þ [MBr]Brop). The increasing negative deviations of the
op
The effects of CTABr micelles on the rate of reaction of Pip
with PS- in the absence and presence of an inert salt have been
explained in terms of the pseudophase micellar (PM) model of
micelles (Scheme 2).10b,25,26
observed data points from dashed lines in Figures 1 and 2 with
decreasing values of [MX] at the lower end of the [MX] range
op
show that the magnitudes of [MX]o are no longer negligible
ef
In Scheme 2, Dn represents a CTABr micelle and KS and KN are
the CTABr micellar binding constants of PS- and Pip, respec-
tively. Symbols knW and knM represent nucleophilic second-order
rate constants for the reactions of Pip with PS- in the respective
aqueous phase and micellar pseudophase. Subscripts W and M
stand for water and the micellar pseudophase, respectively. The
observed rate law (rate=kobs[S]T where [S]T = [PS-W] þ [PS-M])
compared to [MX] at [MX] e [MX]S
.
The values of [MX]0op were estimated as follows. The nonlinear
least-squares technique was used to calculate unknown parameters
P
θ and KX/S as well as the least-squares di2 from eq 3 (with the
replacement of [MX] by [MX]Sef =[MX] - [MX]0op) at a given
P
(presumed) value of [MX]0op. The values of di2 were determined
at different given (presumed) values of [MX]0op, and the typical
P
[MX]0op value, at which the value of di2 turned out to be mini-
mum or nearly a minimum, was considered to be the most reliable
kinetic value of [MX]0op. Such [MX]0op values as well as calculated
(25) Khan, M. N.; Yusoff, M. R. J. Phys. Org. Chem. 2001, 14, 74–80.
(26) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698–4703.
10632 DOI: 10.1021/la100863q
Langmuir 2010, 26(13), 10627–10635

Yusof and Khan
Article
at nearly zero concentration of nonionized phenyl salicylate and
Scheme 2 can lead to eq 5
physical reality, this relationship is true only if the limiting
concentration of X- can cause 100% transfer of S- micellized
counterions (i.e., PS-M) from the micellar pseudophase to the
aqueous phase. The limiting concentration of a salt MX is defined
as the optimum/critical value of [MX] at which the values of kobs
become independent of [MX] (i.e., at the optimum value of [MX],
ðknW þ kmrMKNKS½DnꢁÞ½Pipꢁ
ð1 þ KS½DnꢁÞð1 þ KN½DnꢁÞ T
kobs
¼
ð5Þ
where kmrM=knM/VM (VM is the micellar molar volume9), [Dn]=
[CTABr]T - cmc (cmc is the critical micelle concentration), and
[Pip]T = [PipW] þ [PipM] under the typical reaction conditions
where the total concentration of protonated piperidine([PipHþ]T)
is approximately zero. The reported value of the cmc is <1 ꢀ 10-4
M in the presence of 2 ꢀ 10-4 M PS- and in the absence of any
inert salt.10b,27 The presence of an inert salt is expected to decrease
the cmc, and thus under the present experimental conditions, the
value of the cmc must be significantly lower than 1 ꢀ 10-4 M. It is
therefore apparent that [Dn] ≈ [CTABr]T under the experimental
conditions of the present study.
K
X/S[MX] . 1 and θKX/S[MX] . k0 in eq 3). However, a
moderately hydrophilic counterion (such as HO- or Br-) cannot
be expected to expel a moderately hydrophobic counterion (such
as PS-) completely from the micellar pseudophase to the aqueous
phase even if the values of [HO-]T/[PS-]T and [Br-]T/[PS-]T are
very large. Thus, the value of empirical constant FX/S in eq 7 must
be within the range of e1.0 to >0. The value of FX/S should
measure the fraction of the micellized counterions PS- trans-
M
ferred to the aqueous phase by the optimum concentration of MX
through ion exchange X-/PS- provided that 1 . KN[Dn]. In
another words, the value of FX/S, obtained under the condition
1 . KN[Dn], may be considered to be the measure of the micellar
penetration of X- relative to that of PS- where the hydropho-
bicity of PS- is larger than that of X-.
It is evident from eq 8 that the values of θ should be inde-
pendent of [Dn] or [CTABr]T because 1 . KN[Dn] under the
experimental conditions of the present study. The calculated
values of θ (Table 2) are almost independent of [CTABr]T for
MX (= 3-FBzNa and 4-FBzNa).
It has been almost certain by direct/indirect experiments of a
diverse nature that the micellar environment of micellized solubi-
lizates of different hydrophobicity and steric requirements is not
homogeneous in terms of water concentration, polarity, relative
permittivity, and perhaps molecular fluidity. The respective hydro-
phobicity and hydrophilicity of the micellar environment increases
and decrease continuously from the exterior regions of the head-
groups to the interior of the core of a micelle. In view of the energy
requirements, a solubilizate molecule is expected to acquire a
micellar location where various hydrophilic, electrostatic (in the
case of an ionic micelle only), hydrophobic and steric interactions
between the solubilizate molecule and the micellar environment give
the most stable state. Thus, the value of FX/S may be considered to
be a measure of the cationic micellar penetration of X- relative to
that of S- (i.e., PS-), where the hydrophobicity of S- is larger than
that of X- (= 3-FBz- and 4-FBz-).
The reported value of KN (for N = Pip) for a tetradecyltri-
methylammonium bromide (TTABr) micelle is ∼0.3 M
.
-1 28,29 The
value of KN for a CTABr micelle should not be significantly
different from the KN for TTABr.30 The value of KN would not
be expected to change appreciably in the presence of varying
concentrations of MX because piperidine is a nonionic, highly
hydrophilic molecule. The value of the cetyltrimethylammonium
chloride (CTACl) micellar binding constant of nonionic benzimida-
zole, a relatively much more hydrophobic molecule compared with
Pip, increased from 43 to 68 M-1 with the increase in [NaCl] from
0.0 to 1.0 at 0.01 M CTACl.31 Thus, it is apparent that 1 . KN[Dn]
at [CTABr]T e 0.02 M, and under such conditions, eq 5 can be
reduced to eq 6, where [Dn] ≈ [CTABr]T.
ðknW þ kmrMKNKS½DnꢁÞ½PipꢁT
kobs
¼
ð6Þ
1 þ KS½Dnꢁ
As concluded earlier in the text, the occurrence of the X-/PS- ion
exchange (where X- = 3-FBz- and 4-FBz-) is the most effective
cause of the nonlinear increase in kobs with the increase of [MX] at
constant [CTABr]T and [PS-]T. The effect of [MX] on the distribu-
tion of PS- between the aqueous phase and the micellar pseudophase
(due to X-/PS- ion exchange) is expected to be represented by eq 1.
If this conclusion is correct, then eqs 1 and 6 can lead to eq 3 with
ðknW þ kmrMKNKS ½DnꢁÞ½Pipꢁ
o
T
A satisfactory fit of the observed data to eq 3 with [MX] replaced
ko
¼
ð7Þ
o
by ([MX] - [MX]oop) reveals that θ is kinetically independent of
W
1 þ KS ½Dnꢁ
[MX] and hence kn MX=knW under such conditions. The values of
where knW[Pip]T = kobs at[Dn] = [MX] = 0 and KN isassumed to
FX/S were calculated from eq 8 where 1 . KN[Dn], knW=32.7 ꢀ
be independent of [MX]
10-2 M-1 s-1 13
, and [Pip]T=0.1 M, and these results, at different
MX
FX=Skn
½PipꢁT
1 þWKN½Dnꢁ
[CTABr]T, are summarized in Table 2. The average values of FX/S
,
θ ¼
ð8Þ
as shown in Table 3, reveal that CTABr micellar penetration of
3-FBz- (FX/S =0.70) is only slightly (∼12%) lower than that of
4-FBz- (FX/S = 0.80). These observations are supported by the
study on investigations of monofluoro-substituted benzoates at the
TTAþ (tetradecyltrimethylammonium ion) micellar interface where
1H NMR measurements suggest both 3-FBz- and 4-FBz- localize
in the Stern layer and are intercalated among the micellar(CH3)3Nþ
headgroups.32 The values of FX/S for X=3-FBz- and 4-FBz- are
larger than those for X=3-CH3Bz- and 4-CH3Bz- by respective
0.2 and 0.3 units.33 Considerably low depths of CTABr micellar
penetration of 3-CH3Bz- and 4-CH3Bz- compared with those
of 3-FBz- and 4-FBz- may be attributed to the hydrophobicity
and steric requirements of micellized 3-CH3Bz-/4-CH3Bz- and
where kn MX[Pip]T=kobs at a typical value of [MX] and [Dn]=
W
0 and FX/S (=θ/(kn MX[Pip]T) under the condition 1 . KN[Dn])
W
are empirical constants whose magnitude should vary from e1.0
to > and
KX=S
KX=S
¼
ð9Þ
o
1 þ KS ½Dnꢁ
Perhaps it is noteworthy that in a strict mathematical sense θ =
MX
kn
[Pip]T provided that 1 . KN[Dn], but in view of the
W
(27) Khan, M. N.; Arifin, Z. J. Colloid Interface Sci. 1996, 180, 9–14.
(28) Iglesias, E.; Leis, J. R.; Pefia, M. E. Langmuir 1994, 10, 662–669.
(29) Fernandez, A.; Iglesias, E.; Garcia-Rio, L.; Leis, J. R. Langmuir 1995, 11,
1917–1924.
(30) Bravo, C.; Herves, P.; Leis, J. R.; Pena, M. E. J. Colloid Interface Sci. 1992,
153, 529–536.
(31) Romsted, L. S. J. Phys. Chem. 1985, 89, 5113–5118.
(32) Vermathen, M.; Stiles, P.; Bachofer, S. J.; Simonis, U. Langmuir 2002, 18,
1030–1042.
(33) Khan, M. N.; Kun, S. Y. J. Chem. Soc., Perkin Trans. 2 2001, 1325–1330.
Langmuir 2010, 26(13), 10627–10635
DOI: 10.1021/la100863q 10633

Article
Yusof and Khan
Table 3. Average Values of FX/S and KXBr for Different MX in the Presence of CTABr Micellesa
102FX/S
KX
cmc2
op
Br
X
[MX]o
3-FC6H4CO2- (3-FBz-)
4-FC6H4CO2- (4-FBz-)
nonzero
zero
nonzero
zero
zero
71 ( 6b
122 ( 85
80 ( 2
100 ( 25
70
12.8 ( 0.9b
8.85 ( 1.80
13.4 ( 0.6
10.6 ( 0.4
5.8d
∼12.9 mMc
∼13.8 mMc
C6H5CO2- (Bz-)
2-ClBz-
200 mM CTACl þ g 50 mM BzKe
4.0 f
3-ClBz-
nonzero
72
50g
2,6-Cl2Bz-
2-CH3Bz-
3-CH3Bz-
4-CH3Bz-
5.0h
nonzero
nonzero
nonzero
43
50
48
4.9i
17.7i
16.7i
a Unless otherwise noted, cationic micelles are CTABr. b Error limits are standard deviations. c Reference 32; cmc2 values are for TTAX micelles.
d Reference 34. e Reference 20b. f Reference 25. g Reference 13. h Reference 7. i The values of KXBr were recalculated from the observed data published
as ref 11.
n
n
3-FBz-/4-FBz- where the hydrophobicity and molecular size of
substituent CH3 is significantly larger than those of F. The value
of FX/S (= 0.70) for X = 3-FBz- is comparable with those for
X = 3-ClBz- (FX/S = 0.72)13 and X = H (FX/S = 0.70).34 This
similarity may be explained qualitatively in terms of polarity, size
and polarizability of these counterions (3-FBz- and 3-ClBz-)
where the effects of polarity/steric requirements and size/polariz-
ability of counterions on their ionic micellar penetration are
expected to oppose each other.
should be normalized, i.e. KX/S = FX/SKX/S and KY/S
=
n
MY
FY/SKY/S where FX/S = θ/kn MX [Pip]T and FY/S = θ/kn
with 1 . KN [Dn] (where N = Pip). Thus, under such conditions,
the ratio KX/S/KY/S should be replaced by the ratio KX/Sn/KY/Sn in
eq 10.
Because the valuesofKX/S, listedinTable 2, werederived bythe
use of eq 3, these KX/S values have been normalized (i.e., KX/S
FX/SKX/S) so that KX/S values could be used to calculate ion
exchange constants (KXY) from eq 10. The calculated values of
KX/Sn (= FX/SKX/S, Table 2) and the reported value of 25 M-1 for
KBr/Sn with S = PS- give the values of KXBr (by the use of eq 10
with Y = Br) for different X species. These calculated values of
KXBr for X = 3-FBz- and 4-FBz- at different [CTABr]T are also
W
W
n
=
n
The values of KX/S were calculated from eq 9 with KS0 = 7.0 ꢀ
103 M-19,35 and [Dn] ≈ [CTABr]T (as concluded earlier in the
text). These calculated values of KX/S at different [CTABr]T for
3-FBz- and 4-FBz- are summarized in Table 2. The values
of KX/S are almost independent of [CTABr]T within its range
5-15 mM (Table 2).
The empirical definition of KX/S implies that the value of KX/S is
proportional to thevalue of KX (ionic micellar bindingconstant of
counterion X) and inversely proportional to the value of KS (ionic
micellar binding constant of another counterions S). Thus,
KX/S = ΩSKX/KS where ΩS represents proportionality constant.
The magnitude of ΩS is assumed to depend only upon the mole-
cular characteristics of counterion S (the counterion which is
expelled by another counterion X from the ionic micellar pseudo-
phase to the aqueous phase at a constant [S]T and [ionic micelle]
andit isindependentofthemolecularcharacteristics of counterion
X (the counterion which expels the counterion S from the ionic
micellar pseudophase to the aqueous phase at a constant [S]T,
[ionic micelle] and varying [X]T). Similarly, for another counterion
Y (i.e., for another ion exchange Y-/S-), KY/S = ΩSKY/KS. The
relationships: KX/S = ΩSKX/KS and KY/S = ΩSKY/KS lead to
eq 10
Br
shown in Table 2. The average values of KX (obtained at
different [CTABr]T for a typical MX) for X = 3-FBz- and
4-FBz- are summarized in Table 3.
The cationic micellar growth has been found to cause an
increase in the degree of counterion binding, β (= 1 - R).36
However, the reported values of R are 0.16, 0.15, and 0.16 for
respective counterions 2-FBz-, 3-FBz-, and 4-FBz- of the TTAþ
micellar surface. Similar observations were also obtained with
TTAþ/2-CH3Bz-, TTAþ/3-CH3Bz-, and TTAþ/4-CH3Bz- mi-
cellar systems.32 Although these values are nearly the same,
various physical techniques such as conductometric and 1H
NMR measurements revealed the first cmc (cmc1) and second
cmc (cmc2) for TTAþ/3-FBz, TTAþ/3-CH3Bz-, TTAþ/4-FBz-,
and TTAþ/4-CH3Bz- micelles whereas cmc2 was not detected for
either TTAþ/2-FBz- or TTAþ/2-CH3Bz-.32 These observations
show that, perhaps, the values of R and β are not the true
indicators of micellar structural transitions where the counterions
are moderately hydrophobic.
The values of KXBr for X = Bz-, 2-ClBz-, 3-CH3Bz-, and the
2,6-dichlorobenzoate ion (2,6-ClBz-) are not appreciably differ-
ent from each other (Table 3), and these counterions of CTAþ
micelles are known to produce only spherical micelles at e20 mM
KX=S
KY=S
KX
KY
¼
ð10Þ
Br
CTABr.7,12,37,38 However, the values of KX for X = 3-ClBz-
where KX/KY (ꢂ KXY) represents usual ion exchange constant for
ion exchange process X-/Y- occurring at cationic micellar sur-
face, i.e. KXY = ([XM] [YW])/([XW] [YM]).
It is, perhaps, noteworthy that the use of eq 10 is correct only if
KX/S and KY/S have been determined experimentally from eq 1
where optimum concentration of counterions X or Y has caused
>90% expulsion of counterions S from micellar pseudophase to
aqueous phase. If the values of KX/S and KY/S have been
determined experimentally by the use of eq 3, then these values
and 4-ClBz- are nearly 10-fold larger than those for X = 2-ClBz-
and 2,6-dichlorobenzoate. Counterions 3-ClBz-, 4-ClBz-, and
3,5-dichlorobenzoate result in viscoelastic surfactant solutions
when mixed with CTABr at e20 mM CTABr and e0.3 M inert
Br
salt for these counterions.7,12,37,38 The value of KX for X =
2-FBz- could not be determined because of experimental com-
plications caused by the moderate reactivity of 2-FBz- toward
(36) (a) Abdel-Rahem, R. Adv. Colloid Interface Sci. 2008, 141, 24-36 and
refrerences cited therein.(b) Rodriguez, A.; Graciani, M. M.; Bitterman, K.; Carmona,
A. J.; Moya, M. L. J. Colloid Interface Sci. 2007, 313, 542–550.
(37) Gravsholt, S. J. Colloid Interface Sci. 1976, 57, 575–577.
(38) Rao, U. R. K.; Manohar, C.; Valaulikar, B. S.; Iyer, R. M. J. Phys. Chem.
1987, 91, 3286–3291.
(34) Khan, M. N.; Ismail, E. J. Dispersion Sci. Technol., in press.
(35) (a) Khan, M. N.; Ismail, E. Int. J. Chem. Kinet. 2001, 33, 288–294. (b) Khan,
M. N.; Arifin, Z.; Wahab, I .A.; Ali, S. F. M.; Ismail, E. Colloids Surf., A 2000, 163,
271–281.
10634 DOI: 10.1021/la100863q
Langmuir 2010, 26(13), 10627–10635

Yusof and Khan
Article
PS- and other unresolved degradation paths of 2-FBz- under the
mM [4-FBzNa]. These images show the presence of only spherical
micelles at 3 mM 4-FBzNa but only mixed spherical and short
threadlike micelles (TLM) at 20 mM 4-FBzNa. The shear
viscosity of the aqueous solutions containing 5 mM CTACl and
3-20 mM 4-FBzNa remained 1 cP at shear rates of 10 and
1000 s-1. In the present study, the shear viscosity of the aqueous
solutions containing 15 mM CTABr and 20 mM 4-FBzNa
changes from 4.2 to 3.0 cP with the change in shear rate from
21 to 1000 s-1. Similarly, shear viscosity changes from 5.2 to 3.2
cP with the change in the shear rate from 21 to 1000 s-1 at 15 mM
CTABr and 40 mM 4-FBzNa.
Br
reaction kinetic condition. However, the value of KX for X=
Br
2-FBz- may not be significantly different from KX for X =
Br
2-ClBz- and 2-CH3Bz-. Thus, nearly 3-fold larger values of KX
for X=3-FBz- and 4-FBz- than those for X=Bz-, 2-ClBz-, and
2-CH3Bz- are in favor of the respective presence and absence of
cmc2 for TTAþ/3-FBz-, TTAþ/4-FBz-, and TTAþ/2-FBz-
micelles provided that TTAþ and CTAþ do not differ signifi-
cantly in their effects on micellar growth. The value of KXBr (=
50) for X=3-ClBz- is nearly 3-fold larger than those for 3-FBz-
and 4-FBz-, which may be attributed to the relative polarity/
hydrophobicity of Cl and F.
Viscosity maxima as a function of the concentration of the
counterionic salt are known as the characteristic features of the
wormlike micellar solutions.16,19,20 The maxima in Figures 3 and 4
occur at [MX]/[CTABr]T ≈ 5, where the value of [CTABr]T was
kept constant at 15 mM. Similarly, a viscosity maximum was
found to occur at [BzK]/[CTACl]T=1 with constant [CTACl]T=
200 mM.20b The cationic surfactant solutions containing 200 mM
CTACl as well as <∼50 mM and >∼50 mM BzK behave like
nonviscoelastic and viscoelastic solutions, respectively,20b but the
aqueous micellar solutions containing 10 mM CTABr, e0.08 M
NaOH, 0.1 M Pip, 2 ꢀ 10-4 M PS-, and BzNa within the concen-
tration range of 0.0-500 mM remained essentially spherical at
35 ꢀC.34 The reasons for the maxima, such as those in Figures 3 and
4, are not yet clearly understood, but the possibilities include (a) a
transition from linear to branched micelles or a maximum in
micellar contour length16 and (b) the viscosity increase before a
maximum can be attributed to the 1D micellar growth and increased
density in the network structure whereas the viscosity decrease after
the maximum is due to a decrease in the rigidity of the network
structure (i.e., micellar breaking).18
V. Conclusions
Nearly 3-fold larger values of KXBr for X=3-FBz- and 4-FBz-
than for X = 2-ClBz- may be attributed to the presence of a
second cmc (i.e., cmc2) at ∼13 mM TTAþ3-FBz- and ∼14 mM
Br
TTAþ4-FBz- as reported elsewhere.32 The values of KX for
X=3-FBz- and 4-FBz- give quantitative evidence of the percep-
tion that the strong micellar binding of certain counterions is the
cause of the micellar structural transition from spherical to
cylindrical to wormlike entangled micelles. The observed shear
viscosity maxima as a function of the concentration of 3-FBzNa
and 4-FBzNa (Figures 5 and 6) may be attributed to the possi-
ble presence of the viscoelastic behavior of these surfactant
solutions.16,19,20
Acknowledgment. We thank the University of Malaya (UM)
for financial assistance through research grant RG022/09AFR
and the Centre of Ionic Liquids, UM for permission to carry out
the rheological study.
Supporting Information Available: Pseudo-first-order rate
constants (kobs) for the reaction of piperidine with anionic
phenyl salicylate (PS-) at a constant [CTABr]T and different
[3-FC6H4CO2Na] and [4-FC6H4CO2Na]. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
Recently, Ge and co-workers39 have reported the cryo-TEM
images of the CTACl/4-FBzNa system at 5 mM CTACl and 3-20
(39) Ge, W.; Kesselman, E.; Talmon, Y.; Hart, D. J.; Zakin, J. L. Non-
Newtonian Fluid Mech. 2008, 154, 1–12.
Langmuir 2010, 26(13), 10627–10635
DOI: 10.1021/la100863q 10635