Aggregation behavior of sodium 3-(octyloxy)-4-nitrobenzoate in aqueous solution

Article abstract of DOI:10.1039/c8nj03440k

Ester 3-(octanoyloxy)-4-nitrobenzoic acid is a standard for the assay of the activity of phospholipase enzymes (the main toxins involved in tissue-damaging after snake bites). Because of its amphiphilic nature, it probably behaves as a surfactant but the instability of the ester bond prevents its characterization. Its ether analogue, 3-(octyloxy)-4-nitrobenzoic acid, is also an interesting compound as it is an inhibitor of the phospholipase activity and can be accepted as a model of the ester derivative. Its sodium salt, stable at basic pH, was studied by surface tension measurements, isothermal titration calorimetry, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The aggregation number, fraction of bound counterions, critical micelle concentration and thermodynamic parameters involved in demicellization were obtained. The positive value for the change in the heat capacity for demicellization indicates that a larger hydrophobic surface area of each monomer is exposed to a hydrophilic environment after dissociation. Semiempirical calculations are in agreement with DLS and TEM measurements. For several carboxylate surfactants, the plot of enthalpy vs. entropy is linear. Although the slope has been named the compensation temperature, T_c, it merely is an experimental temperature without any extra-thermodynamic meaning.

Full text of DOI:10.1039/c8nj03440k

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Aggregation behavior of sodium 3-(octyloxy)-
4-nitrobenzoate in aqueous solution
Cite this: New J. Chem., 2018,
42, 19407
a
b
c
Victor H. Soto, M. Pilar Vazquez-Tato, Francisco Meijide, c*
´
a
b
Marıa Jose Alvarado, Julio A. Seijas,
Bruno Lomonte
Santiago de Frutos,
´
´
d
c
and Jose Vazquez Tato
´ ´
Ester 3-(octanoyloxy)-4-nitrobenzoic acid is a standard for the assay of the activity of phospholipase
enzymes (the main toxins involved in tissue-damaging after snake bites). Because of its amphiphilic nature, it
probably behaves as a surfactant but the instability of the ester bond prevents its characterization. Its ether
analogue, 3-(octyloxy)-4-nitrobenzoic acid, is also an interesting compound as it is an inhibitor of the
phospholipase activity and can be accepted as a model of the ester derivative. Its sodium salt, stable at basic
pH, was studied by surface tension measurements, isothermal titration calorimetry, dynamic light scattering
(DLS), and transmission electron microscopy (TEM). The aggregation number, fraction of bound counterions,
critical micelle concentration and thermodynamic parameters involved in demicellization were obtained. The
positive value for the change in the heat capacity for demicellization indicates that a larger hydrophobic
surface area of each monomer is exposed to a hydrophilic environment after dissociation. Semiempirical
calculations are in agreement with DLS and TEM measurements. For several carboxylate surfactants, the
plot of enthalpy vs. entropy is linear. Although the slope has been named the compensation temperature,
Received 10th July 2018,
Accepted 25th October 2018
DOI: 10.1039/c8nj03440k
rsc.li/njc
c
T , it merely is an experimental temperature without any extra-thermodynamic meaning.
aqueous media, particularly at pH values far from neutrality,
the surfactant activity and the self-aggregation behavior of
The main toxins involved in tissue-damaging after snake bites these compounds are unknown. Both factors are important in
have been identified as the members of the phospholipase A the enzyme catalysis as the catalytic efficiency is strongly
1
. Introduction
2
1
3
(
PLA ) and metalloproteinase families. Their rapid activities dependent on the length of the alkyl chain.
2
2
often limit the ability of antivenom to prevent tissue damage.
These phospholipases are a class of calcium dependent nitrobenzoic acid, is also an interesting compound as it is an
enzymes that catalyze the hydrolysis of the fatty acid ester in the inhibitor of the PLA activity (the unpublished results will not
-position of 3-sn-phospholipids. To understand the chemical be described in this paper). Its sodium salt (NaOctylBz, Fig. 1)
mechanism of enzymatic catalysis 3-(acyloxy)-4-nitrobenzoic acids can be accepted as a model of the ester derivative, and its
The ether analogue of the octanoyl derivative, 3-(octyloxy)-4-
2
2
(
with acyl groups ranging from butyryl to dodecanoyl) were
3
designed for mimicking the essential structural features of a
phosphatidylcholine. The octanoyl derivative has become a stan-
4
dard for the spectrophotometric assay of PLA s because of the
2
easy measurement of UV absorption of 3-hydroxy-4-nitrobenzoate.
Due to their amphiphilic nature, they probably behave as
surfactants but because of the instability of the ester bond in
a
Escuela de Qu ´ı mica, Centro de Investigaci o´ n en Electroqu ´ı mica y Energ ´ı a Qu ´ı mica
(
CELEQ), Universidad de Costa Rica, San Jos ´e , Costa Rica
b
c
Departamento de Qu ´ı mica Org a´ nica, Facultad de Ciencias, Universidad de
Santiago de Compostela, Avda. Alfonso X El Sabio s/n, 27002 Lugo, Spain
Departamento de Qu ´ı mica F ´ı sica, Facultad de Ciencias, Universidad de Santiago
de Compostela, Avda. Alfonso X El Sabio s/n, 27002 Lugo, Spain.
E-mail: francisco.meijide@usc.es
d
Instituto Clodomiro Picado, Facultad de Microbiolog ´ı a,
Fig. 1 Synthesis of 3-(octyloxy)-4-nitrobenzoic acid, and numbering of
Universidad de Costa Rica, San Jos ´e 11501, Costa Rica
the NaOctylBz molecule.
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6
stability in aqueous solution allows an easy characterization as minimized using the semiempirical program MOPAC2016 with
a surfactant. The system was studied in NaCl aqueous solution PM7 Hamiltonian, using localized molecular orbitals in solving
by surface tension measurements, isothermal titration calori- the SCF equations (MOZYME) and water medium by the COSMO
metry (ITC), dynamic light scattering (DLS), and transmission implicit solvation model.
electron microscopy (TEM). The obtained results, particularly
the surface tension behavior and the thermodynamics of its
Synthesis
self-aggregation process, could be used as a reference for 3-(Octyloxy)-4-nitrobenzoic acid was synthesized by reaction
3-(acyloxy)-4-nitrobenzoic acids.
of methyl 3-hydroxy-4-nitrobenzoate 1 with octyl iodide 2
following the steps indicated in Fig. 1.
Synthesis of 1. A mixture of 3-hydroxy-4-nitrobenzoic acid
(
2
0.45 g, 2.63 mmol) and p-toluenesulphonic acid (0.45 g,
.63 mmol) in 50 ml of methanol was heated at 65 1C for
2. Materials and methods
Enthalpies associated with the demicellization of NaOctylBz 60 h under an inert atmosphere. After removing the solvent
were determined in a MicroCal ITC200 titration calorimeter at in vacuo, the reaction mixture was redissolved in ethyl acetate
constant temperature. The heat exchange for each injection was and washed with 3 ꢀ 50 ml of water. The organic phase was
2 4
calculated using the Origin program supplied by MicroCal dried over Na SO and concentrated in vacuo. The product was
which accepts fast mixing in the measuring cell during each purified by column chromatography (stationary phase: silica
injection from the syringe. In a typical experiment, 2 ml of a gel, mesh: 70 : 230; mobile phase: ethyl acetate : hexane 2 : 1,
B20 mM solution of NaOctylBz in NaCl and NaOH (B20 mM) Rf = 0.87). Yield: 90.9%.
were injected into an aqueous solution with identical NaCl and
NaOH concentrations. Experimental titration curves were analyzed 31.5 mmol), HI (3.99 g, 31.2 mmol, 7 ml, 57%) and 85%
as indicated below. PO (5.95 g, 60.7 mmol, 7 ml) was stirred and heated at
Synthesis of 2. A mixture of octanol (4.10 g, 4.97 ml,
H
3
4
Surface tension measurements were carried out using a 130 1C for 8 h. The mixture was extracted with 50 ml of hexane
Drop volume Tensiometer TVT2 from Lauda. The temperature and this organic phase was washed with a saturated solution of
was kept constant by recirculating water from a PolyScience sodium thiosulfate until the disappearance of the brown color.
2 4
9100 thermostat. The surface tension was determined as the The organic phase was dried over Na SO and concentrated in
average value of four sets of 4–6 drops each. Sets of solutions vacuo. The product was purified by column chromatography
were prepared according to the step-by-step dilution-extraction (stationary phase: silica gel, mesh: 70 : 230; mobile phase:
5
method.
hexane, R = 0.85). Yield: 83.5%.
f
A Brookhaven instrument constituted by a BI-2030AT digital
Synthesis of methyl 3-(octyloxy)-4-nitrobenzoate 3. A mixture
correlator with 136 channels and a BI-200SM goniometer was of 1 (2.55 g, 14.2 mmol), 2 (1.4 equivalents, 4.77 g, 19.9 mmol),
used for DLS measurements. The light source was a Melles Grio and K CO (4 equivalents, 7.85 g, 56.8 mmol) in DMF (250 ml)
2
3
He–Ne laser operating at 632 nm. In the DLS experiments the was warmed to 50 1C and allowed to reflux overnight under an
intensity–intensity autocorrelation function was measured at inert atmosphere.
a particular q value, and related to the normalized electric
The reaction mixture was then poured into ethyl acetate
field autocorrelation function g1 by the Siegert relation. The (250 ml), and the organic phase was washed with 2 ꢀ 50 ml
autocorrelation function was analyzed using a NLLS/CONTIN portions of brine and 2 ꢀ 50 ml of water, dried over Na SO ,
2
4
algorithm (from Brookhaven software). From the so called and concentrated in vacuo. The product was purified by column
apparent diffusion coefficient, Dapp, an apparent hydrodynamic chromatography (stationary phase: silica gel, mesh: 70 : 230;
f
radius Rapp can be calculated using the Stokes–Einstein mobile phase : ethyl acetate : hexane 1 : 1.1. R = 0.43) yield: 83.7%.
equation, Rapp = k
B
T/6pZDapp, where k
B
is the Boltzmann constant,
Synthesis of 3-(octyloxy)-4-nitrobenzoic acid. 3 (3.52 g,
T is the absolute temperature and Z is the solvent viscosity.
11.38 mmol) was refluxed in 1 M KOH (50 ml) in dry methanol
TEM images were obtained using a JEOL JEM-1011 operating for 1 h. The solvent was removed in vacuo and the reaction
at 80 kV and equipped with a Mega View III camera. Samples mixture was poured into water (50 ml) and neutralized with
were equilibrated at the desired temperature, and then a drop concentrated HCl until the precipitation of the product. Then it
of the solution was deposited onto carbon-coated copper grids was filtered and dried in a vacuum oven. Yield: 41.2%.
1
and dried.
H NMR (600 MHz, CDCl
, d, J2-6 = 1.38 Hz); 7.77 (H
6-2 = 1.40 Hz); 4.20 (H , t, J8-9 = 6.52 Hz); 1.88 (H
6.52 Hz); 1.51 (H10, m, J10-9, J10-11 = 6.50 Hz); 1.40–1.28
= 7.03 Hz).
3
), d (ppm): 7.85 (H
, dd, J6-5 = 8.42 Hz,
, m, J9-8, J9-10
5
, d, J5-6 =
NMR spectra were recorded on a 600 MHz Bruker Ascend 8.42 Hz); 7.81 (H
2
6
equipment. Molecular mass was measured using a MS/MS:ULPC/MS
SYNAPT equipment.
J
8
9
=
Semiempirical calculations were carried out to construct (H –H , m); 0.91 (H , t, J
15-14
1
1
14
15
1
3
model micelles with different numbers of monomers packed
C NMR (151 MHz, CDCl ), d (ppm): 169.57 (C ), 151.98 (C ),
3 7 3
in a spherical arrangement and a fraction of bounded counterions 143.26 (C ), 133.56 (C ), 125.22 (C ), 121.79 (C ), 115.92 (C ),
4
1
5
6
2
according to the experimental results. The elaborate design and 70.06 (C
standard protocols are described at http://www.biomolecular- (C10), 22.65 (C13), 14.10 (C15).
modeling.com/Abalone/manual-Scripts.html. The clusters were Molecular weight: 296.149 (M + 1); theoretical: 296.150.
8
), 31.77 (C14), 29.19, 29.16 (C11, C12), 28.80 (C
9
), 25.79
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Solutions of NaOctylBz were prepared by neutralizing the conditions with 0.5 M NaCl were chosen to carry out ITC
appropriate quantities of 3-(octyloxy)-4-nitrobenzoic acid with experiments. Fig. 3(a) shows an example of an ITC measure-
aqueous sodium hydroxide.
ment for determining the thermodynamic parameters corres-
ponding to the demicellization process of NaOctylBz micelles.
In this experiment, 2 ml of a 19.08 mM solution of NaOctylBz in
0.5 M NaCl and 20.79 mM NaOH were injected into an aqueous
solution with identical NaCl and NaOH concentrations.
Experiments were carried out at different temperatures in
the range of 15–45 1C, all enthalpy demicellization values being
positive, i.e., the micellization process is exothermic.
3. Results and discussion
Surface tension measurements have been performed in NaCl
0.1–0.5 M) and NaOH (0.020 M) aqueous solutions. This
(
basicity assures complete ionization of 3-(octyloxy)-4-nitrobenzoic
acid for which a pKa value of 3.40 has been estimated
The calorimetric titration curves were analyzed following the
(MarvinSketch 6.1.0).
17
methodology published by Olesen et al. The method assumes
Fig. 2 shows the results of the surface tension measurements,
11
the mass-action model as well as the monodispersity of
g. From the breaking point of the dependence g with
ln[surfactant], values for the critical micelle concentration,
cmc, and surface tension at cmc, gcmc, were obtained. Table 1
shows the measured values for cmc and some values published
for other octyl surfactants having positive, negative or neutral
heads. In comparison to these surfactants, the cmc values for
NaOctylBz suggest that it is an excellent surfactant, mainly if
the alkyl tail length is taken into consideration. The observed
value for NaOctylBz is also lower than the published values
micelles. For an ionic surfactant, the equilibrium constant,
K
n
mic, for the formation of an aggregate M , constituted by the
association of n surfactant monomers, S, and bn counterions B
(where b is the degree of counterion binding) is
n
nb
nS + nbB 2 M
n
K
mic = [M
n
]/([S] [B] )
However, if counterions are in a large excess compared to
nb
free surfactants (as those mentioned above), [B] can be
1
7,18
7
accepted as being constant (pseudo-first order approximation)
for other longer alkyl chain surfactants such as SDS. Only
nb
n
and Kmic may be approximated by Kmic = K/[B] where K = [M ]/[S] .
the monoester b-D-fructofuranosyl-6-O-octyl-a-D-glucopyranoside
n
8
This constant would correspond to the equilibrium formation of an
aggregate from a pseudo-non-ionic surfactant,
has a similar cmc value.
The addition of NaCl significantly diminishes the surface
tension and cmc (Fig. 2). From the dependence of cmc with the
concentration of added NaCl, the fraction of bound counter-
ions (b = number of counterions/aggregation number) may be
nS 2 Mn.
17
After several transformations the following equation is
9
–11
estimated
as 0.53. High values for the fraction of bound
derived
counterions have also been measured for other sodium alkanoates
n
ꢂ1ꢂ1o
1
2
13
12
such as octanoate (=0.664; 0.58 ) and dodecanoate (=0.458;
0
d ln ðd½Sꢁ=dS Þ
2
tot
n ꢂ 1 ðn ꢂ 1Þ dS
13
14
¼
þ
(1)
.55; 0.46 ), and other n-octyl surfactants (Table 1).
dln Stot
n
n
dStot
As cmc is higher, the more concentrated solution of the surfac-
1
5
tant in the syringe must be used to perform ITC experiments.
The amount dS/dStot is obtained from calorimetric data as
By using a relatively high concentration of added salt, it is
commonly possible to overcome the solute–solute interactions
which increase with solute concentration. For these reasons,
dS
dDH
DHdemic
¼
(2)
dStot
1
6
and taking into account previous cmc values, experimental where DH is the enthalpy change after adding a small amount
of surfactant from the syringe into the measuring cell and
DHdemic is the enthalpy of demicellization. The analysis of the
experimental data according to eqn (1) affords the value of n, the
enthalpy of demicellization and the involved enthalpy of dilution
17
as optimization parameters. The optimization process is facili-
tated by considering that the intercept must be in the range of
0
.66–1 since 3 r n r N, and that the slope and slope/intercept
are equal to n provided that the aggregation number is large
enough. Furthermore, as n can be calculated from the intercept
and the slope, both values have to be self-consistent.
Once n is known, the equilibrium constant is obtained from
ꢀ
ꢁ1ꢂn
2
n ꢂ 2
2n ꢂ n
K ¼ 2
cmc
(3)
3
2
2
n ꢂ n
2n ꢂ 2
1
8
This equation is identical to the one published by Phillips
once a neutral micelle is considered. It must be noticed that
cmc is expressed as the mole fraction of the surfactant, Xcmc
Fig. 2 Surface tension of NaOctylBz vs. ln([surfactant]/mM) in [NaOH] =
0
0
.020
M and NaCl (concentrations/M: 0.5 squares; 0.3 triangles;
.1 circles). Temperature 25 1C.
.
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Table 1 cmc values, aggregation number (n) and fraction of bound counterions (b) for micellization of n-octyl surfactants. Temperature 25 1C
exceptions are indicated)
(
3
ꢂ3
Surfactant
10 cmc/(mol dm
)
n
b
Ref.
NaOctylBz
n-Octyl sulphate
2.01, 0.5 M NaCl; 2.40, 0.3 M NaCl; 4.29, 0.1 M NaCl
64.7 glycine buffer, pH 3.2
Fig. 4
0.53
This work
19
1
1
10–117
30
20
21
22
23
n-Octyl-sulfonate
148; 0.2 M NaCl
55
9
1
25 (23 1C)
Sodium octanoate
n-Octyl trimethylammonium bromide
314
9
0.664
0.71
0.35
12
24
25
260, pH 3.2; 251, pH 7.0; 215, pH 10.0
340
20
1
-Methyl-3-octyl-1H-imidazolium chloride
100–220; 220–234; 220 ꢃ 1.5
26
27
28
29
2
2
2
00; 263
27
0.35
0.45
0.45
20
n-Octyltetraoxyethylene, C
8
8
E
4
6
7.3
23
30
31
30
8.56
n-Octylhexaoxyethylene, C
E
10.1
n-Octyl-dimethylphosphine oxide
N,N-Dimethyloctylamine-N-oxide
42–46.31
206.9
41
15
31 and 32
33
31
34
35
36
1
50
Micelles highly hydrated (10 water molecules/monomers)
86 (27 1C)
24.1, 0 M NaCl; 13.3; 1 M NaCl
14
15
1
Octyl glucoside
2
2
2
2
2
1
2
2.60, 300 K
2.60, 300 K
0.5–20.9
7.8
5; 30 1C
7; 0.47 M NaCl
4
37
45
37 and 38
39
40
41
42
43
8
Monoester b-D-fructofuranosyl-6-O-octyl-
a-D-glucopyranoside
2.34–3.26
Fig. 3 (a) Example of a calorimetric titration curve from addition of [NaOctylBz] = 19.08 mM (0.5 M NaCl; 20.79 mM NaOH) in water solution with
identical ion concentrations. Temperature 35 1C. (b) Determination of the aggregation number according to eqn (1). Concentration of the injected
1
9.08 mM NaOctylBz; 0.5 M NaCl, 20.79 mM NaOH. Temperature 15 1C.
The calculation of the free energy change associated with
demicellization, DGdemic, follows directly as
Experimental ITC enthalpograms are particularly sensitive
to several calorimetric effects which may introduce different
4
4
kinds of systematic errors. When studying demicellization
processes the dilution enthalpy of monomers and micelles can
ꢀ
ꢁ
RT
n
DGdemic ¼
ln Kmic
(4)
1
7
be particularly important. Olesen et al. have discussed the
effects of the dilution enthalpy, eDHdil, as well as errors in total
+
bn
Obviously the contribution of the term [Na ] to DGdemic heat, on the numerical analysis outlined above, and how they
+
depends on Na concentration, the absolute value being can be manually tuned to prevent biases derived from systematic
ꢂ1
+
ꢂ1
1
.7 kJ mol at 25 1C and [Na ] = 0.5 M.
errors. For NaOctylBz an average value of eDHdil = 0.6 ꢃ 0.5 kJ mol
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was used to fit the experimental results. This value is comparable to
the one used by Olesen et al. for SDS and those values published by
38
Paula et al. for sodium deoxycholate.
Fig. 3(b) shows an example of the calculation of the aggrega-
tion number. In this case, intercept = 0.9513 and slope = 19.56
2
(r
= 0.9945), from which a value of 21.7 was deduced for n.
Fig. 4 shows the obtained values for n at different temperatures.
1
7
Regarding the aggregation number, Olesen et al. have
already commented on the general limitations of the method.
The obtained values for NaOctylBz are very close to those
2
3
determined by Tartar and Lelong for sodium octylsulphonate
4
5
46
(
=25), by De Lisi et al. for sodium nonanoate (=18 ꢃ 2), and
30
by Anderson and Olofsson for C E (=23). It is far from the
one determined by Kameyama and Takagi for octylglucoside
=87) and higher than the one (=9) published by Park et al. for
8
4
4
7
1
2
(
sodium octanoate.
Semiempirical calculations indicate that the interior of a
NaOctylBz micelle with an aggregation number n = 23 is an
open structure, the inside being easily accessible to solvents
(Fig. 5, top). In these calculations, the solvent is modeled as a
continuum.
The number of sodium ions was fitted to the half aggrega-
+
tion number (i.e., one Na for each two carboxylate groups),
according to the fraction of bound counterions (see above).
It may be noticed that the cations are located in the periphery
of the aggregate and that, in average, it may be considered that
counterions shield the electrostatic interactions between the
4
8
carboxylate groups, as proposed for bile salts. This agrees
4
9
with recent results of Long et al. who have shown significant
penetration of water into micelle interiors, well beyond the first
few carbon atoms adjacent to the head group both in anionic
+
Fig. 5 Calculated structure of NaOctylBz micelles with n = 23 and 12 Na
counterions (top) and with n = 100 (bottom).
(alkanoates) and cationic micelles. According to these authors
the hydrophobic core of a micelle is surrounded by a highly
corrugated surface containing hydrated non-polar cavities
This open structure is susceptible to further aggregation of
whose depth increases with increasing surfactant chain length. monomers at concentrations above cmc. In fact, aggregation
1
3
Medos and Bester-Rogac have noticed that the micelle core is numbers as high as one hundred may be packed into NaOctylBz
still in contact with water molecules for a series of C –C micelles without increasing their diameter (being approx.
8
12
3
4
alkanoates. From DLS experiments Courchene has found that 3.7 nm) very much (Fig. 5, bottom). This size is compatible
dimethyloctylamine oxide micelles are highly hydrated with with both DLS and TEM results. From CONTIN analysis of DLS
10 water molecules for each surfactant monomer.
experiments (in 10.23 NaOctylBz mM, 0.5 M NaCl and 22.6 mM
NaOH) two populations with sizes centered at 5.62 and 177 nm
were obtained, although the second one is negligible when a
plot of (number of particles) vs. diameter is taken into con-
sideration (Fig. 6). Similar results were obtained for 14.26 mM
NaOctylBz in 0.1 M NaCl and 20 mM NaOH. Under the last
conditions TEM images (Fig. 7, top) were obtained, and from
the analysis of the smaller particles (in a number of 107) a
diameter of 7.5 ꢃ 2.2 nm was obtained (Fig. 7, bottom).
Fig. 4 shows the plot of DHdemic vs. temperature, the slope
ꢂ1 ꢂ1
being 227 J mol
K
which corresponds to the change in the
heat capacity for demicellization, DC . This value is close to the
p
ꢂ1 ꢂ1
one (=246 J mol
K ) deduced from the results of Perspicace
4
0
et al. at around 30 1C for b-octyl glucoside.
3
0–33,37,40,50
The published values
for DC
p
for octyl surfactants
4
0
50
ꢂ1 ꢂ1
are in the range of 246 –460 (for C
8 4
E )
J mol
p
K . DC 4 0
Fig. 4 Aggregation numbers (ꢄ) and enthalpies of demicellization (J) of
5
0
NaOctylBz as a function of temperature; 0.5 M NaCl and 20 mM NaOH.
has been observed for other alkyl surfactants including
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5
2
atoms of the alkyl chain. Similarly, a linear dependence is
observed when the Gibbs energy of dissolution of liquid
alkanes into water is plotted against the number of hydrogen
atoms (or equivalently the number of carbon atoms) of the alkyl
5
3
chain. The large positive heat capacity difference between
gaseous and dissolved nonpolar molecules in water has been
correlated with the number of water molecules in the first
5
4
solvation shell around the nonpolar molecule. It has to be
noticed that the DC values for the enthalpies of liquid–liquid
p
5
5
and gas–liquid transfers are similar. These considerations
support the fact that the dissociation of the NaOctylBz aggre-
gates implies the transfer of monomers from a non-polar
environment into a polar one (water), i.e., a larger hydrophobic
surface area of monomers is exposed to a hydrophilic environ-
Fig. 6 CONTIN analysis of the autocorrelation function obtained in a DLS
experiment with 10.23 mM NaOctylBz. Temperature 25 1C, 0.5 M NaCl ment following the dissociation.
and 22.6 mM NaOH. Plot of intensity (top) and number of particles
The values for cmc from calorimetric titration curves have
been determined by different methods depending on the
(
bottom) vs. diameter.
3
0
definition of cmc. For instance, Andersson and Olofsson have
defined cmc as the concentration where 1% of surfactant is in
1
8
the micellar form. Phillips has defined cmc as the point
corresponding to the maximum change in gradient in an ideal
property–concentration relationship. Therefore he, as well as
Olesen et al., calculated it by making the third derivative of such
56
a relationship zero. According to Evans and Wennerstr o¨ m
cmc would correspond to the concentration at which an added
monomer is as likely to enter a micelle as to remain in solution.
57
This definition has been accepted by Olofsson and Loh who
recommended that the cmc corresponds to the inflexion point of
the calorimetric titration curve. In fact, this criterion has been
37,38,58–60
widely used.
The difference between the calculated values for cmc by the
17,57
two methods may be dependent on the aggregation number
to which the width of the sigmoidal curve is also related).
(
However since n 4 20 for NaOctylBz, the difference is rather
small (Table 2) and the use of any set of values for the calculation
of DGdemic (and DSdemic) does not introduce significant biases.
6
1
Medos and Bester-Rogac have studied the micellization of
a set of long-chain carboxylates in water by ITC in the temperature
range between 288.15 and 328.15 K, the center being
o
T = 308.15 K for all systems except for sodium dodecanoate,
NaC12 (=313.15 K). They analyzed the experimental data by treat-
ing the micellization process as a two-step process, and estimated
all the thermodynamic parameters of micellization for both steps.
In particular, the heat capacities of micellization for the first
Fig. 7 TEM image taken from a solution of 14.26 mM NaOctylBz, 0.1 M
NaCl and 20 mM NaOH. Scale bar 200 nm (top). Size distribution of
particles with diameters lower than 15 nm (bottom).
and second steps were temperature independent in the examined
n-octyltrimethyl- temperature range. They also noticed a balance between entropy
3
7,38
octyl derivatives such as octyl glucoside,
2
4
19
ammonium bromide, and n-octyl sulphate. Since DC
DHdemic will be negative at low temperatures and positive at of Gibbs free energy. In what follows we shall refer to the
high temperatures, and the comparison between DC published first process for which linear dependences of both enthalpy and
values for different surfactants is better than the comparison entropy with temperature are observed.
p
4 0, and enthalpy which results in a weak temperature dependence
p
between DHdemic ones.
For a set of systems, the plot of the enthalpy vs. entropy data
The transfer of alkyl surfactant monomers from a micelle to was traditionally used to determine the so-called compensation
6
2–64
the bulk water has many facts in common with the dissolution temperature,
5
1
of liquid alkanes into water. An example is the linear relation-
ship found when the Gibbs energy change of micellization
ꢀ
ꢁ
ꢀ
ꢁ ꢂꢀ
ꢁ
@
DHdemic
@DHdemic
@T
@DSdemic
@T
Tc ¼
¼
@
DSdemic
(or equivalently ln cmc) is plotted against the number of carbon
P
P
P
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Table 2 cmc values and thermodynamic parameters of demicellization of around 23, being highly hydrated and half neutralized by
NaOctylBz at different temperatures. 0.5 M NaCl and 20 mM NaOH
p
bound counterions. Since DC 4 0 for demicellization the
3
ꢂ3
dissociation of the aggregates implies the transfer of monomers
from a non-polar environment into a polar one (water). The
present results could be used as reference values for 3-(octa-
noyloxy)-4-nitrobenzoic acid.
For a set of long-chain carboxylates, the plot of the enthalpy
vs. entropy data of demicellization is linear, the slope being
1
3
0 cmc (mol dm )
rd
Inflexion
point
DGdemic
(kJ mol
DSdemic
(J mol
ꢂ1
ꢂ1 ꢂ1
T/1C
derivative
)
K )
1
2
2
3
3
4
4
5
0
5
0
5
0
5
0.96
1.02
1.12
1.34
1.33
1.67
1.72
1.05
1.11
1.21
1.28
1.61
1.72
1.80
24.4
24.9
25.2
25.3
25.5
25.8
26.1
ꢂ59.9
ꢂ63.7
ꢂ47.0
ꢂ51.2
ꢂ44.7
ꢂ43.5
ꢂ39.5
traditionally called the compensation temperature, T
c
. However
T
c
is a pseudo-compensation temperature without any extra-
thermodynamic physical meaning since it is merely the central
value of the experimental interval of temperatures at which the
dependencies of enthalpy and entropy changes are analysed.
Fig. 8 shows this type of plot for the long-chain carboxylates
6
1
of Medos and Bester-Rogac, the slope being T
exception of NaC12 is made) which perfectly agrees with T
The values are T = 313.15 K and T = 317.7 ꢃ 0.9 K for NaC
c
= 308.0 ꢃ 0.9 K
(
o
.
Conflicts of interest
o
c
12
There are no conflicts to declare.
and T = 303.15 and T = 302 ꢃ 6 K for NaOctylBz (the dark
o
c
6
5
point in Fig. 8). In a recent revision we have demonstrated
that it is possible to obtain as many T values as the number of
temperature intervals at which the dependencies of enthalpy
c
Acknowledgements
and entropy changes with temperature are analysed, the value The authors thank the Ministerio de Ciencia y Tecnolog ´ı a
of each T will be the central value T
of each temperature (Project MAT2013-45657-P) for financial support. V. H. S. and
interval. That is to say, T is simply such experimental T
M. J. A. thank AECI (Beca Mutis)/University of Costa Rica for a
c
o
c
o
.
Consequently, T is a pseudo-compensation temperature without research scholarship.
c
any extra-thermodynamic physical meaning. Obviously this is also
the case of previous carboxylate surfactants.
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