Apparent and partial molar volumes of long-chain alkyldimethylbenzylammonium chlorides and bromides in aqueous solutions at T = 15 °C and T = 25 °C

Article abstract of DOI:10.1016/S0021-9614(03)00155-1

Density measurements of dodecyl- (C₁₂DBACl), tetradecyl- (C ₁₄DBACl), hexadecyldimethylbenzylammonium chloride (C ₁₆DBACl) and of decyl- (C₁₀DBABr) and dodecyldimethylbenzylammonium bromide (C₁₂DBABr)

Full text of DOI:10.1016/S0021-9614(03)00155-1

J. Chem. Thermodynamics 35 (2003) 1983–1992
www.elsevier.com/locate/jct
Apparent and partial molar volumes of
long-chain alkyldimethylbenzylammonium
chlorides and bromides in aqueous solutions
at T ¼ 15 °C and T ¼ 25 °C
a,
*
a
b
a
A. Gonz ꢀa lez-P ꢀe rez , J.M. Ruso , J. Nimo , J.R. Rodr ꢀı guez
a
Grupo de Ciencia y Tecnolog ꢀı a de Coloides, Departamento de F ꢀı sica de la Materia Condensada,
Facultad de F ꢀı sica, Universidad de Santiago de Compostela, Campus Sur s/n 15782,
Santiago de Compostela, Spain
b
Grupo de Biof ꢀı sica e Interfases, Departamento de F ꢀı sica Aplicada, Facultad de F ꢀı sica,
Universidad de Santiago de Compostela, Campus Sur s/n 15782, Santiago de Compostela, Spain
Received 9 April 2003; accepted 23 July 2003
Abstract
Density measurements of dodecyl- (C12DBACl), tetradecyl- (C14DBACl), hexadecyldim-
ethylbenzylammonium chloride (C16DBACl) and of decyl- (C10DBABr) and dodecyldimeth-
ylbenzylammonium bromide (C12DBABr) in aqueous solutions at T ¼ 15 °C and T ¼ 25 °C
have been carried out. From these results, apparent and partial molar volumes were calcu-
lated. Positive deviations from the Debye–H u€ ckel limiting law provide evidence for limited as-
sociation at concentrations below the critical micelle concentration. The change of the
apparent molar volume upon micellization was calculated. The relevant parameters have been
presented in function of the alkyl chain length. Apparent molar volumes of the present com-
m
/
m
/
pounds in the micellar phase, V , and the change upon micellization, DV , have been dis-
cussed in terms of temperature and type of counterion.
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Alkyldimethylbenzylammonium chlorides; Alkyldimethylbenzylammonium bromides; Micel-
lization; Density; Apparent molar volumes
*
Corresponding author. Tel.: +34-981-563100; Fax: +34-981-520676.
E-mail address: fmjulio@usc.es (J.R. Rodr ꢀı guez).
0
021-9614/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0021-9614(03)00155-1

1984
A. Gonz ꢀa lez-P ꢀe rez et al. / J. Chem. Thermodynamics 35 (2003) 1983–1992
1
. Introduction
The dualistic character of amphiphilic compounds having both hydrophilic and
hydrophobic moieties is the basis of their relation to both external and internal in-
terfaces in aqueous systems. Self-association of the amphiphilic compounds is a pos-
sible way of eliminating the energetically unfavourable contact between the nonpolar
part and water while simultaneously retaining the polar part in an aqueous environ-
ment. The physical phenomenon responsible for such behaviour is referred to as the
hydrophobic effect and arises from a subtle balance between intermolecular energies
and entropies [1]. An equilibrium solution of amphiphiles in water corresponds to a
system of aggregates, co-existing with a nearly constant concentration of monomers.
Unlike in ordinary solutions, the micelles in amphiphile solutions can respond to
variations on thermodynamic parameters (such as total concentration, or tempera-
ture, or ionic strength) by changing their size and shape distributions. This behaviour
resembles that of a system governed by multiple chemical equilibrium [2]. The pres-
ence of changes in some physical properties of amphiphiles in aqueous solutions is
assumed to be indicative of an aggregation state and the discontinuity was identified
with the critical micelle concentration (cmc) of surfactant systems [3].
Alkyldimethylbenzylammonium chlorides and bromides are used in cosmetic
products including hair conditioners and hair colouring preparations. Besides, being
surfactant antistatic agents, the alkyldimethylbenzylammonium compounds work as
biocides in various cosmetic and detergent products. Among others, the biocidal
properties are used, when alkyldimethylbenzylammonium is added to all-purpose
or specialized cleaning agents. The earliest studies on micellar properties of alkyldim-
ethylbenzylammonium salts were reported by Walton et al. [4] and Cushman et al.
[
5]. A large number of scientific application reports arises from their well known ger-
micidal activity as antiseptics and disinfectants agents [6]. More recently, a survey of
literature on the fundamental micellar properties of this group of surfactants was re-
ported by Del Castillo et al. [7]. Extensive studies on the thermodynamics of micellar
solubilization of co-surfactants in mixed binary surfactant systems containing alkyl-
dimethylbenzylammmonium salts have been carried out by Treiner et al. [8–10].
Their conductance results revealed the existence of a second cmc for C DBACl
14
and C16DBACl [10,11].
In previous papers, we discussed changes in the micellar properties of alkyldim-
ethylbenzylammonium chlorides as a function of alkyl chain length, type of counter-
ion, and temperature [12–14]. In this paper, we report our results on apparent and
partial molar volume measurements at T ¼ 15 °C and T ¼ 25 °C. Apparent molar
volumes at infinite dilution and apparent molar volumes of the micelles are calcu-
lated and discussed in function of the alkyl chain length and the type of counterion.
2
. Materials and methods
C12DBACl was synthesized by quaternization of N; N-benzyldimethylamine with
dodecyl chloride by applying a procedure similar to that used for the synthesis of the

A. Gonz ꢀa lez-P ꢀe rez et al. / J. Chem. Thermodynamics 35 (2003) 1983–1992
1985
TABLE 1
Densities, q, apparent molar volumes, V
T ¼ 15 °C and T ¼ 25 °C
/
, and partial molar volumes, V
2
, for C
n
DBACl and C
n
DBABr at
m
q
V
/
V
2
(
mol ꢀ kgꢁ
1
)
(g ꢀ cmꢁ3
)
(cm ꢀ mol
12DBACl
T ¼ 15 °C
3
ꢁ1
)
(cm ꢀ mol
3
ꢁ1
)
C
ꢂ
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.999099
0.999106
0.999108
0.999110
0.999110
0.999113
0.999113
0.999115
0.999113
0.999085
0.998951
0.998771
0.998765
0.998578
0.998449
0.997917
0.997398
.0019
.0029
.0038
.0046
.0056
.0060
.0073
.0084
.0133
.0308
.0541
.0580
.0792
.0994
.1847
.2726
335.9
336.9
337.6
338.2
338.8
339.5
339.8
340.7
341.4
344.2
349.0
348.3
347.6
347.8
347.7
347.9
347.9
336.7
337.0
337.6
337.5
337.7
337.9
338.5
341.3
345.1
346.5
346.2
346.8
347.1
347.1
347.1
T ¼ 25 °C
ꢂ
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.997043
0.997041
0.997039
0.997037
0.997034
0.997030
0.997028
0.997026
0.997020
0.996975
0.996801
0.996591
0.996562
0.996360
0.996176
0.995453
0.994727
.0019
.0029
.0038
.0046
.0056
.0060
.0073
.0084
.0133
.0308
.0541
.0580
.0792
.0994
.1847
.2726
342.0
342.4
342.6
342.9
343.3
343.5
343.3
343.7
346.1
348.9
349.5
349.4
349.9
350.1
350.2
350.3
342.6
343.2
343.6
344.1
344.6
344.8
345.5
346.0
348.1
350.7
351.4
351.0
350.5
350.7
350.5
350.7
C
14DBACl
T ¼ 15 °C
369.9
0
0
0
0
0
0
0
.0024
.0029
.0038
.0046
.0071
.0083
.0097
0.999094
0.999089
0.999079
0.999070
0.999040
0.999026
0.999009
383.1
384.7
382.3
380.0
380.2
379.3
379.2
371.4
373.3
374.4
376.6
377.1
377.6

1986
A. Gonz ꢀa lez-P ꢀe rez et al. / J. Chem. Thermodynamics 35 (2003) 1983–1992
TABLE 1 (continued )
m
q
V
/
V
2
mol ꢀ kgꢁ
1
)
(g ꢀ cmꢁ3
)
(cm ꢀ mol
3
ꢁ1
)
(cm ꢀ mol
3
ꢁ1
)
(
0
0
0
0
0
0
0
0
0
0
.0113
.0140
.0150
.0171
.0191
.0218
.0235
.0255
.0266
.0312
0.998990
0.998960
0.998949
0.998928
0.998907
0.998879
0.998862
0.998841
0.998830
0.998785
377.9
378.3
378.4
378.3
378.4
378.5
378.5
378.6
378.5
378.5
379.1
379.2
379.1
379.1
379.0
379.1
379.0
379.0
379.0
379.1
T ¼ 25 °C
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.0014
.0019
.0024
.0029
.0046
.0071
.0083
.0097
.0113
.0140
.0150
.0171
.0191
.0218
.0235
.0248
0.997042
0.997038
0.997032
0.997026
0.997004
0.996971
0.996955
0.996935
0.996912
0.996875
0.996861
0.996833
0.996807
0.996772
0.996750
0.996732
369.8
371.8
373.8
374.9
377.6
379.4
379.9
380.4
380.8
381.3
381.4
381.6
381.6
381.8
381.8
381.9
377.1
380.2
381.1
381.1
382.7
383.3
382.8
382.9
382.9
383.1
382.9
383.0
383.0
383.1
383.0
383.1
C
16DBACl
T ¼ 15 °C
410.9
0
0
0
0
0
0
0
0
0
.0060
.0094
.0100
.0149
.0185
.0187
.0235
.0296
.0303
0.999012
0.998961
0.998951
0.998883
0.99883
411.1
411.2
411.1
411.1
411.1
411.1
411.1
411.1
411.1
411.1
411.2
411.0
411.1
411.0
411.1
411.1
411.2
0.998828
0.998758
0.998669
0.998659
T ¼ 25 °C
0
0
0
0
0
0
0
0
.0060
.0094
.0100
.0149
.0188
.0235
.0296
.0303
0.996947
0.996882
0.996870
0.996783
0.996716
0.996633
0.996530
0.996514
413.5
414.6
414.7
414.9
414.9
415.0
414.9
415.1
414.8
415.7
415.3
415.5
415.5
415.5
415.5
415.5

A. Gonz ꢀa lez-P ꢀe rez et al. / J. Chem. Thermodynamics 35 (2003) 1983–1992
TABLE 1 (continued )
1987
m
q
V
/
V
2
(
mol ꢀ kgꢁ
1
)
(g ꢀ cmꢁ3
)
(cm ꢀ mol
10DBABr
T ¼ 15 °C
313.1
3
ꢁ1
)
(cm ꢀ mol
3
ꢁ1
)
C
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.0065
.0070
.0105
.0148
.0168
.0206
.0224
.0277
.0355
.0447
.0538
.0563
.0710
.1593
.2596
.2824
.3866
.5127
0.999380
0.999399
0.999550
0.999728
0.999812
0.999970
1.000040
1.000250
1.000540
1.000850
1.001170
1.001254
1.001760
1.004770
1.007880
1.008590
1.011690
1.015170
313.6
313.7
314.2
314.7
315.0
315.5
315.8
319.2
320.6
320.7
320.7
320.3
320.2
321.0
320.7
320.5
320.5
320.6
313.2
313.4
313.8
313.9
314.0
314.3
314.6
315.5
316.9
317.4
317.6
318.3
319.2
320.0
320.0
320.0
320.2
T ¼ 25 °C
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.0070
.0104
.0148
.0151
.0205
.0206
.0224
.0277
.0304
.0355
.0405
.0447
.0520
.0538
.0563
.0648
.0710
.0955
.2066
.5953
0.997318
0.997454
0.997626
0.997643
0.997850
0.997855
0.997927
0.998131
0.998200
0.998370
0.998538
0.998680
0.998926
0.998986
0.999070
0.999340
0.999530
1.000320
1.003730
1.014050
317.5
317.4
317.6
317.4
317.6
317.5
317.5
317.5
318.7
319.4
319.8
320.0
320.4
320.5
320.6
321.1
321.4
321.9
322.7
323.2
317.6
317.6
317.6
317.6
317.6
317.6
317.6
323.0
323.4
323.6
323.5
323.4
323.6
323.4
323.3
323.5
323.2
323.3
323.6
323.9
C
12DBABr
T ¼ 15 °C
344.6
0
0
0
0
0
.0017
.0021
.0028
.0035
.0045
0.999165
0.999180
0.999208
0.999237
0.999276
344.9
345.0
345.1
345.2
345.4
345.1
344.9
345.2
344.7

1988
A. Gonz ꢀa lez-P ꢀe rez et al. / J. Chem. Thermodynamics 35 (2003) 1983–1992
TABLE 1 (continued )
m
q
V
/
V
2
mol ꢀ kgꢁ
1
)
(g ꢀ cmꢁ3
)
(cm ꢀ mol
3
ꢁ1
)
(cm ꢀ mol
3
ꢁ1
)
(
0
0
0
0
0
0
0
0
0
0
.0056
.0068
.0078
.0098
.0107
.0124
.0133
.0151
.0172
.0202
0.999318
0.999360
0.999386
0.999450
0.999479
0.999534
0.999564
0.999628
0.999689
0.999776
345.2
346.2
347.6
348.5
349.1
349.3
349.4
349.4
350.2
350.9
345.6
352.9
353.4
353.4
352.6
352.7
352.3
352.3
352.2
352.5
T ¼ 25 °C
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.0017
.0021
.0028
.0035
.0045
.0056
.0068
.0078
.0098
.0107
.0124
.0133
.0151
.0172
.0202
0.997103
0.997117
0.997142
0.997168
0.997202
0.997242
0.997274
0.997299
0.997352
0.997382
0.997430
0.997450
0.997499
0.997558
0.997639
349.8
349.7
349.6
349.9
349.7
349.7
351.5
352.4
353.6
353.7
354.0
354.6
355.0
355.3
355.7
349.7
349.7
349.7
349.7
349.7
357.0
358.8
358.0
358.4
357.9
358.1
357.8
357.8
357.8
358.1
*
Reference [24].
decyl homologue [7]. The surfactant was purified by several recrystallizations from
acetone. C14DBACl and C16DBACl with an assay of >99% and >97%, respectively,
were from Fluka. Both compounds were used without further purification.
C
10DBABr with a purity of 99% was synthesized by quaternization of equimolar
amounts of N; N-benzyldimethylamine and decyl bromide in a double volume of
ethyl alcohol. The reaction was carried out at 80 °C for 72 h. C DBABr with a pu-
12
rity of >99% was from Fluka. The five studied compounds were found to be in the
form of monohydrates as determined by the Karl–Fischer method. Bidistilled water
and solutions were properly degassed using an ultrasound unit.
The measurement of densities were carried out by using an Anton Paar DMA 60/
02 vibrating tube densimeter operating under the static option. Solutions were pre-
6
pared by mass using a Mettler H51 precision balance. The molalities of the solutions
ꢁ
1
were prepared within an accuracy of ꢃ0.0001 mol ꢀ kg . The temperature inside the
vibrating-tube cell was measured using Anton Paar DT 100-20 and DT 100-30 digital
thermometers and was regulated better than ꢃ0.01 K using a HETO DBT thermo-
stat. The apparatus was calibrated at the beginning of every set of experiments using
water and air (at atmospheric pressure) as references (see table 1). Taking into

A. Gonz ꢀa lez-P ꢀe rez et al. / J. Chem. Thermodynamics 35 (2003) 1983–1992
1989
account that the relative uncertainty in the difference between the densities of the
sample and pure water is mainly due to the difference in the square of the corre-
sponding oscillating periods, and that the relative uncertainty of a single period
ꢁ
6
founded in our case was about 10 , then the uncertainty in the density is assumed
to be less than 5 ꢀ 10ꢁ g ꢀ cmꢁ3.
6
3
. Results and discussion
The values of densities, q, of aqueous solutions of C DBACl (n ¼ 12, 14, and 16)
n
and C DBABr (n ¼ 10, 12) at T ¼ 15 °C and T ¼ 25 °C are listed in table 1. In an
n
ionic surfactant system, below the cmc, the dependence of the density on molar con-
centration c can be expressed as
ðq ꢁ q Þ ¼ ½ðM
c
þ M
f
Þ ꢁ ðV
c
þ V
f
Þq ꢄc;
ð1Þ
0
0
where M
f
, M
c
, V
f
c
, and V are the molar masses and the apparent molar volumes of the
free surfactants ions (f), and counterions (c), respectively, and q is the density of
0
pure water. If we assume that the apparent molar volumes are negligibly concen-
tration-dependent, then equation (1) predicts a linear relation between density and
concentration. However, this was found not to be true in the case of C12DBACl at
T ¼ 15 °C, where a change in the slope of the density vs molality plot from positive to
negative can be detected (see table 1). Such behaviour is not observed for the same
surfactant at T ¼ 25 °C and for all the other surfactants. This change in slope, in-
dicates, according to equation (1), that the V
c
þ V
f
terms increase with temperature.
This effect is related to the relaxation of structured water engaged in the solvation of
the hydrocarbon chain, the headgroup, and the counterion upon increase in tem-
perature.
The slopes of plots of density vs molality, for the bromides, are positive over the
entire molality and temperature ranges studied; however, slopes for chlorides are
negative over the whole set of data.
From the density values, apparent molar volumes of surfactants in water were cal-
culated according to the equation [15]
V
/
¼ M=q ꢁ ðq ꢁ q Þ=ðmqq Þ;
ð2Þ
where M is the molar mass of the solute in and m its molality. The partial molar
volumes V were calculated from V using the equation
¼ V þ mðoV =omÞT;P
0
0
2
/
V
2
/
/
:
ð3Þ
2
The values of partial molar volumes, V , are also listed in table 1 together with the
values of the apparent molar volumes.
In the premicellar region, the values of V can be expressed by the relation
/
0
/
1=2
V
/
¼ V þ A
V
m
þ B
V
m;
ð4Þ
is the Debye–
0
where V is the value of apparent molar volume at infinite dilution, A
V
/
H u€ ckel limiting law coefficient which amounts to 1.696 cm ꢀ kg ꢀ molꢁ3=2 and 1.865
3
1=2

1990
A. Gonz ꢀa lez-P ꢀe rez et al. / J. Chem. Thermodynamics 35 (2003) 1983–1992
TABLE 2
Values obtained for the adjustable parameter B
V
of equation (4)
3
ꢁ2
B
V
/(cm ꢀ kg ꢀ mol
)
T ¼ 15 °C
T ¼ 25 °C
C
C
C
12DBACl
10DBABr
12DBABr
338 ꢃ 20
340 ꢃ 16
59 ꢃ 6
)2 ꢃ 3
52 ꢃ 5
)7 ꢃ 8
3
1=2
ꢁ3=2
cm ꢀ kg ꢀ mol
at T ¼ 15 °C and T ¼ 25 °C, respectively, and B is an adjustable
V
parameter which measures individual deviations from the limiting law. The fit of the
0
V values to this equation yields the limiting value V . The parameters B obtained
/
/
for the studied systems are listed in table 2. It is instructive to observe those values.
V
While for the chlorides B remains constant with the temperature, for the bromides
V
the slope changes from positive to almost zero (the zero value is inside the error
bars). If we observe the dependence with hydrophobic moiety of the molecule, there
is no significant change for the bromides, with n ¼ 10 and 12. However, for both
dodecyl a great change can be observed. All these facts indicate that the counterions
play the main role in the deviations from the limiting law. Finally, positive deviations
from the Debye–H u€ ckel limiting law provide evidence for limited association at
concentrations below the cmc.
As Millero pointed out [15], the densities of aqueous solutions of the electrolytes
can be determined from the relation
ꢀ
ꢁ
0
/
ꢁ3
3=2
ꢁ3
2
q ¼ q þ M ꢁ q V c ꢁ 10 A q c ꢁ 10 B q c :
ð5Þ
0
0
V
0
V
0
A more lengthy relation can be derived when concentration are expressed as mol-
alities. Since oq=oc is linear with n, it can be immediately seen that the linearity of
density vs molarity plots in the low concentration range indicates that the second
and third terms in equation (5) are negligibly small or cancel out. Increase of density
0
/
with molarity indicates thus that the M ꢁ q V term in the first parameter in
0
equation (5) is positive. This term appears to be negative when the molecular weight
of the counterion in surfactants is low and its apparent molar volume is compara-
tively high, as found in the case of chlorides.
0
The values of V/ for C
1
DBACl from Boyd [16] and Owens et al. [17], for
C DBACl from Gonz ꢀa lez-P ꢀe rez et al. [12], and for C DBACl from present results,
10
12
together with the values for alkyltrimethylammonium chlorides (C TACl) for
n
C TACl from Verrall and Conway [18], for C TACl and C TACl from Milioto
1
8
12
0
/
et al. [19] at T ¼ 25 °C are listed in table 2. The values of V for C10DBABr are
3
ꢁ1
3
ꢁ1
3
C
°
12.6 cm ꢀ mol and 317.4 cm ꢀ mol at T ¼ 15 °C and T ¼ 25 °C and for
3
ꢁ1
3
ꢁ1
12DBABr are 344.6 cm ꢀ mol and 349.5 cm ꢀ mol at T ¼ 15 °C and T ¼ 25
C, respectively. The values for the bromides confirm those reported earlier [20].
0
Linear fitting of the values of V at T ¼ 25 °C (collected in table 3) in function of
/
the number of carbon atoms in the alkyl chain (C ) yields slopes of 15.7 ꢃ 0.1
n
3
ꢁ1
3
ꢁ1
cm ꢀ mol and 15.8 ꢃ 0.1 cm ꢀ mol for the C DBACl and C TACl salts, respec-
n
n
tively. These values are in satisfactory agreement with the result of 16.0 ꢃ 0.2

A. Gonz ꢀa lez-P ꢀe rez et al. / J. Chem. Thermodynamics 35 (2003) 1983–1992
1991
TABLE 3
0
/
Apparent molar volumes at infinite dilution, V , for C
n
DBACl (T ¼ 15 °C and T ¼ 25 °C) and C
n
TACl
(
T ¼ 25 °C)
n
0
/
3
ꢁ1
)
V /(cm ꢀ mol
C
n
DBACl
n
C TACl
T ¼ 15 °C
T ¼ 25 °C
168.9
T ¼ 25 °C
b;c
107.3d
217.2e
1
8
1
1
0
2
307.2a
335.6
310.7a
341.6
280.9e
a
b
c
d
e
Reference [7].
Reference [16].
Reference [17].
Reference [18].
Reference [19].
3
ꢁ1
cm ꢀ mol reported by Tutaj et al. [21] for short-chain alkyldimethylbenzylammo-
nium bromides C DBABr (n ¼ 1, 4, and 6).
n
In the post-micellar region, the values of V have been fitted to the asymptotic
/
function
h
i
cmc
m
/
V
/
¼ V/ þ DV ðm ꢁ McmcÞ=fB þ ðm ꢁ McmcÞg ;
cmc
ð6Þ
m
/
m
where V/ is the value of the apparent molar volume at the cmc, DV is the dif-
cmc
ference between the limiting value at high concentration of surfactant, V , and V
.
/
/
m
The DV/ can be identified with the change in the apparent molar volume upon
micellization. In the case of C DBACl and C DBACl the reported values of V are
restricted to the post-micellar region only because of the low cmc values. For all the
14
16
/
m
m
studied compounds the values of V/ and DV/ are listed in table 4. De Lisi et al. [22]
TABLE 4
m
m
Limiting apparent molar volumes of micelles of C
at T ¼ 15 °C and T ¼ 25 °C
n
DBACl, V/ , and the change upon micellization, DV/
,
m
3
ꢁ1
m
3
ꢁ1
)
n
V/ /(cm ꢀ mol
T ¼ 15 °C
)
DV/ /(cm ꢀ mol
T ¼ 15 °C
T ¼ 25 °C
T ¼ 25 °C
C
n
DBACl
10
12
14
16
314.2a
347.75
379
317a
7.0a
9.35
15.0
18.7
5.7a
6.6
350.6
383.1
414.85
10.6
11.2
411.0
C
n
DBABr
1
1
0
2
313.7
344.9
317.1
349.8
6.8
7.7
6.3
8.1
a
Reference [7].

1992
A. Gonz ꢀa lez-P ꢀe rez et al. / J. Chem. Thermodynamics 35 (2003) 1983–1992
m
found negative slopes of DV/ of sodium octyl, decyl, and dodecyl sulphates in water
against temperature in the temperature range from 25 °C to 130 °C at different
pressures. This negative slope value is attributed to the expansibility of the surfactant
in the micellar state being smaller than that in the aqueous phase. Atwood et al. [23]
found this decrease with temperature for tricyclic drugs, and the effect has been
attributed to the dehydration of the ionic head group.
Acknowledgements
This work received financial support from Xunta de Galicia (PGI-
DIT03PXIB20601PR).
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JCT 03/030