Apparent molar volumes of benzyltrimethylammonium bromide and its homologs in aqueous solution at 15, 25, and 35°C

Article abstract of DOI:10.1023/A:1014402102186

Apparent molar volumes at infinite dilution of benzyltrimethylammonium bromide and its butyl and hexyl homologs at 15, 25, and 35°C and of dibenzyldimethylammonium bromide at 25°C in aqueous solution were estimated from density measurements. The additivity rule for the contribution of the methylene groups to the apparent molar volumes was found to be obeyed within a broad range of homologs, which covers the parent salt and the dodecyldimethylbenzylammonium bromide. The volumetric contribution of the phenylene (-C₆H₄-) group was estimated to be 61 cm³-mol^-1 at 25°C. A value of - 16.9 ± 0.3 cm³-mol^-1 was suggested for the volumetric contribution of the N⁺ fragment to the apparent molar volume of alkylbenzyldimethylammonium salts.

Full text of DOI:10.1023/A:1014402102186

Journal of Solution Chemistry, Vol. 30, No. 12, December 2001 ( C 2002)
Apparent Molar Volumes
of Benzyltrimethylammonium Bromide and its
Homologs in Aqueous Solution at 15, 25, and 35 C
B. Tutaj,1 A. Gonza´lez-Pe´rez,2 J. Czapkiewicz,1
J. L. Del Castillo,2 and J. R. Rodriguez2,*
Received July 1, 2001
Apparent molar volumes at infinite dilution of benzyltrimethylammonium bromide and
its butyl and hexyl homologs at 15, 25, and 35 C and of dibenzyldimethylammonium
bromide at 25 C in aqueous solution were estimated from density measurements. The
additivity rule for the contribution of the methylene groups to the apparent molar vol-
umes was found to be obeyed within a broad range of homologs, which covers the parent
salt and the dodecyldimethylbenzylammonium bromide. The volumetric contribution
1
of the phenylene (–C6H4–) group was estimated to be 61 cm3-mol at 25 C. A value
1
of 16.9 0.3 cm3-mol was suggested for the volumetric contribution of the N+
fragment to the apparent molar volume of alkylbenzyldimethylammonium salts.
KEY WORDS: Apparent molar volume; benzyltrimethylammonium bromide;
homologs; dibenzyldimethylammonium bromide; group contributions; additivity.
1. INTRODUCTION
Two recent papers of Gianni and Lepori(1,2) on the additivity scheme for
group contributions to the partial molar volume of ionic organic solutes in aqueous
solution have prompted us to extend our studies on aqueous solutions of long-chain
alkyldimethylbenzylammonium halides to the short-chain homologs.
In previous studies, we have reported, among others, the apparent molar
6)
volumes V ofthechlorides(alkyl=decyl, dodecyl, tetradecyl, andhexadecyl)(3
and bromides (alkyl = decyl and dodecyl),(7) in the pre- and postmicellar region
at various temperatures. For obvious reasons, the range of molalities available for
1Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Krakow, Poland.
2Universidad de Santiago de Compostela, Facultad de Fisica, Departamento de Fisica de la Materia
Condensada, E-15706 Santiago de Compostela, Spain; email: fmjulio@usc.es.
1101
C
0095-9782/02/1200-1101/0 2002 Plenum Publishing Corporation

1102
Tutaj, Gonza´lez-Pe´rez, Czapkiewicz, Del Castillo, and Rodriguez
density studies in the premicellar region was narrow and, hence, the precision of the
apparentmolarvolumesatinfinitedilution V o ofthesurfactants, wasratherlow. Itis
worthnoting, however, thatthesevaluesappearedself-consistentwithrespecttothe
generally accepted additivity rule for the volumetric contribution of the methylene
group in the alkyl chain(1,2) and to the difference in the apparent molal volumes
suggested for the chloride and bromide ions.(8) Furthermore, the V o value for the
parent benzyltrimethylammonium cation estimated by extrapolation from our data
for the long-chainhomologs wasfoundtoagree, withinthe experimental error, with
the values derived from the apparent molar volumes of benzyltrimethylammonium
chloride reported by Yatome et al.,(9) Boyd,(10) and Owens et al.(11)
We decided to check whether the additivity rule for the volumetric contribu-
tion of methylene groups is valid in this broad range of homologs and to estimate
the contribution of the N+ ionic constituent to the limiting apparent molal volume
of the cation, which bears a benzyl group and two methyl groups linked directly to
the nitrogen atom. We present here our results on density measurements of aque-
ous solutions of benzyltrimethylammonium bromide (C1BBr) and its short-chain
alkyl homologs, namely the butyl (C4BBr) and hexyl (C6BBr) salts at 15, 25, and
35 C. We report also densitometric data for dibenzyldimethylammonium bromide
(DiBBr) at 25 C. This salt was chosen to investigate whether the additivity rule
holds also for the volumetric contribution of the phenylene groups in the case of
bulky cations.
2. EXPERIMENTAL
C1BBr, C4BBr, and C6BBr were prepared by quaternization of benzyl-
dimethylamine with an appropriate alkyl bromide in acetone. The reaction for
C1BBr was carried out at around 0 C, whereas synthesis of the two higher ho-
mologs was carried out at room temperature. The products were recrystallized
from acetone and acetone/ethanol mixtures. The detailed description of the syn-
thetic and purification procedures will be given elsewhere. The air-dried salts were
anhydrous by Karl–Fischer analysis. Results of potentiometric titration of the bro-
mide ion with a standarized solution of silver nitrate indicated high purity of the
salts.
DiBBr was synthesized as follows: the reagents and the solvent were cooled
to about 10 C and a mixture of 58.45 g of benzyl bromide (Aldrich) and 46.21 g
of N,N-dimethylbenzylamine (Aldrich) in 100 g of acetone was then prepared
and placed, stoppered, overnight at 10 C, and then kept for 3 days at room
temperature. The copious product was filtered, washed with cold acetone, and
recrystallized several times from acetone. The purity of the salt was checked as
above.
Deionized and distilled water was used for preparation of the solutions.
Densities of the solutions were measured using a vibrating-tube density meter

Volumes of Benzyltrimethylammonium Bromide
1103
(Anton Paar DMA 60/602). Temperatures were kept constant within 0.001 C.
3
The experimental error in the density was within 0.001 kg-m
.
3. RESULTS AND DISCUSSION
In the concentration and temperature ranges investigated, densities of aqueous
solutions of the alkyldimethylbenzylammonium bromides increase with molality.
The observed relations are, to a satisfactory approximation, linear. The slopes of
density vs. molality plots, 1 /1m, decrease with growth of the number, n, of
carbon atoms in the alkyl chain, as illustrated in Fig. 1, where the data for decyl-
(C10BBr) and dodecyldimethylbenzylammonium bromides (C12BBr)(7) at 25 C
are included. The 1 /1m against n dependence can be expressed by the relation:
1 /1m = 0.05325 0.00148n
(1)
with a correlation coefficient r = 0.9998.
The apparent molal volumes, V at 15, 25, and 35 C for benzyltrimethy-
lammonium bromide and its homologs and at 25 C for DiBBr, listed in Table I,
together with the molalities m and densities were calculated from the equation:
M
103(
)
o
V =
(2)
m
o
Fig. 1. Plot of 1 /1m against n, the number of carbon atoms in the alkyl chain of CnBBr salts at
25 C.

1104
Tutaj, Gonza´lez-Pe´rez, Czapkiewicz, Del Castillo, and Rodriguez
Table I. Molalities, Densities, and Apparent Molar Volumes of C1BBr, C4BBr, and C6BBr at 15,
25, and 35 C and of DiBBr at 25 C
15 C
25 C
35 C
m
V
V
V
1
3
1
3
1
3
1
(mol-kg
)
(g-cm
)
(cm3-mol
)
(g-cm
)
(cm3-mol
)
(g-cm
)
(cm3-mol
)
C1BBr
0.02021
0.02971
0.04200
0.06282
0.08357
0.10490
0.12560
0.15870
1.000215
1.000732
1.001400
1.002484
1.003600
1.004734
1.005819
1.007609
174.8
174.9
175.0
175.8
175.6
175.5
175.6
175.1
0.998158
0.998669
0.999318
1.000406
1.001484
1.002605
1.003660
1.005331
175.1
175.5
176.0
176.4
176.6
176.5
176.7
176.8
0.995100
0.995592
0.996226
0.997295
0.998360
0.999449
1.000502
1.002141
177.7
178.0
178.2
178.3
178.3
178.3
178.2
178.4
C4BBr
0.021668
0.031604
0.042218
0.063367
0.085766
0.104831
0.125527
0.156825
0.209300
1.000170
1.000673
1.001199
1.002251
1.003348
1.004273
1.005281
1.006793
1.009299
222.7
222.2
222.1
221.9
221.9
221.9
221.8
221.6
221.4
0.998092
0.998572
—
1.000114
1.001168
1.002091
1.003055
1.004520
1.006944
224.1
224.0
—
223.6
223.8
223.5
223.5
223.4
223.2
0.995075
0.995546
0.996032
0.997029
0.998084
0.998943
0.999886
1.001304
1.003659
224.0
224.1
224.6
224.4
224.3
224.5
224.5
224.4
224.2
C6BBr
0.02285
0.03224
0.04190
0.06268
0.08668
0.10452
0.15665
0.20889
0.31480
0.02285
1.000189
1.000639
1.001095
1.002075
1.003189
1.004003
1.006404
1.008758
1.013399
1.000189
252.4
252.3
252.3
252.2
252.2
252.3
252.0
251.8
251.4
252.4
0.998082
0.998512
0.998917
0.999855
1.000947
1.001741
1.004004
1.006264
1.010618
0.998082
255.2
255.0
255.7
255.3
254.9
254.8
254.7
254.4
254.3
255.2
0.995041
0.995452
0.995872
0.996773
0.997802
0.998568
1.000754
1.002916
1.007132
0.995041
257.0
257.1
257.1
257.1
257.1
257.0
256.9
256.7
256.6
257.0
DiBBr
0.020975
0.031410
0.041990
0.064102
0.084860
0.104440
0.167080
0.250650
0.344800
0.998480
0.999203
0.999925
1.001416
1.002804
1.004103
1.008197
1.013574
1.019227
237.9
237.5
237.4
237.5
237.5
237.5
237.3
236.9
237.2

Volumes of Benzyltrimethylammonium Bromide
1105
where
is the density of pure water and M is the molar mass of the solute.
o
Theapparentmolarvolumesofthesaltsatinfinitedilution V o, werecalculated
by applying the relation:
V = V o + Avm1/2 + Bvm
(3)
where Av is the Debye–Hu¨ckel limiting law coefficient and Bv is an adjustable
parameter that measures the deviations from the limiting law due to ion–ion inter-
actions, which may vary with size of the cation and temperature. The parameter Av
is 1.696, 1.868, and 2.046 cm3-kg1/2-mol 3/2 at 15, 25, and 35 C, respectively.(8)
As Millero pointed out,(8) when concentrations are expressed as molarities C
the densities of aqueous solutions of the electrolytes can be determined from the
relation:
M
oV o
103
Av
103
Bv
o
103
o
=
+
C
C3/2
C2
(4)
o
A more lengthy relation can be derived when concentrations are expressed as
molalities. Since the 1 /1C against n can be expressed by a relation similar to
(1), 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 Eq. (4)
are negligibly small or cancel out. Growth of density with molarity indicates thus
that the M
oV o term in the first parameter in the Eq. (4) 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 comparatively high, as found in the case
of chlorides(3) and hydroxides.(12,13)
The plots of V
1.868 m1/2 vs molality for C1BBr, C4BBr, and C6BBr at
25 C are shown in Fig. 2.
The values of V o and of the Bv parameters for C1BBr and its two homologs at
15, 25, and 35 C are shown in Table II, together with the standard deviations. The
mean value for the methylene group contribution to the apparent molar volumes
1
in the C1BBr to C6BBr for the homologs amounts to 16.0 0.2 cm3-mol
.
As can be seen in Fig. 2, V for C1Br decrease upon dilution, whereas those
found for its two homologs increase, which is consistent with expectations that
stronger ion–counterion interactions should occur in the case of the parent salt. In-
spection of the Bv values presented in Table II confirms this observation, since the
parameters for C1BBr are systematically higher compared with those estimated
for the two homologs at 15, 25, and 35 C. The drop in the value of Bv estimated for
C1BBr at 35 C is, however, unexpected unless it can be assumed that in the tem-
perature range of 25–35 C the properties of C1BBr switch from structure breaking
to structure making. The high thermal expansivity of C1BBr in this temperature
range, which can be seen upon inspection of the data in Table II, is here worth men-
tioning. Spectroscopic absorption data in the UV range for aqueous solutions of the
benzyltrimethylammonium chloride(14) seem to provide evidence for specificity of

1106
Tutaj, Gonza´lez-Pe´rez, Czapkiewicz, Del Castillo, and Rodriguez
Fig. 2. Plots of V
1.868 m1/2 vs. molality of C1BBr, C4BBr, and C6BBr at 25 C.
this salt at room temperature, since its molar absorptivity at the absorption max-
1
imum of 262.2 nm amounted to 427 cm2-mmol and was substantially higher
than the molar absorptivities at the corresponding maxima of the remaining benzyl-
trialkylammonium chlorides studied. For the benzyltriethylammonium chloride,
1
the absorptivity decreased dramatically to 372 cm2-mmol (at 262.4 nm). Upon
passing to higher homologs, the effect almost leveled off.
We will now discuss the V o values for C1BBr and its homologs at 25 C in
view of the general additivity scheme for group contributions to V suggested by
Gianni and Leppori.(1,2) According to these authors, ionic partial molar volumes
can be estimated through the simple equation:
X
V2 = A +
nBi + C
(5)
where V2 is the partial molar volume of the solute, A is a constant covolume term
(equal to 9.20) Bi is the contribution of the ith group, which appears ni times in
the solute molecule, and C is a correction term for polyfunctional compounds and
peculiar structures. This term will not be taken into account here. The following Bi
values were taken from Ref.(2) and applied for the estimation of the V o values of the
organic cations: 21.44 cm3-mol 1 for CH3, 15.93 for CH2, 7.99 for Car, and 12.91
for CHar Two Bi values have been suggested for N+, namely, 14.94 cm3-mol
1
in case of tetraalkylammonium cations (with R = Et to Pe) and 11.34 for cations
of the RNMe3+ and R2NMe2+ type. None of these two values appear to conform to
the situation when the nitrogen atom is linked directly to a benzyl group. The best

Volumes of Benzyltrimethylammonium Bromide
Table II. Apparent Molar Volumes at Infinite Dilu-
1107
tion and the Bv Coefficient for Benzyltrimethylammo-
niumBromide(C1BBr), Butyldimethylbenzylammonium
Bromide (C4BBr), and Hexyldimethylbenzylammonium
Bromide (C6BBr) at 15, 25, and 35 C
V o
(cm3-mol
Bv
1
3
1
Salt
)
(10 g2-cm 3-mol
)
15 C
C1BBr
C4BBr
C6BBr
174.8 0.2
222.2 0.1
252.2 0.04
0.27 2.5
8.19 1.0
5.52 0.3
25 C
C1BBr
C4BBr
C6BBr
175.1 0.2
223.8 0.1
255.0 0.2
7.68 2.1
7.46 0.9
6.27 1.0
35 C
C1BBr
C4BBr
C6BBr
177.7 0.1
224.0 0.1
256.8 0.02
0.88 2.1
2.64 1.0
4.64 0.2
fit of the experimental values of V to those predicted by Eq. (5) occurs when a
value of 16.9 0.3 cm3-mol 1 is assumed for N+ in the case of the C1B+ cation
and its C4 to C12 homologs. This value was calculated on basis of our data and
the data for benzyltrimethylammonium chloride taken from Boyd(10) and Owens
et al.(11) The experimental and calculated values of V o for the C1B+ cation and
its homologs were estimated by assuming that the apparent molal volume of the
bromide and chloride anions amounts to 30,1 and 23.2 cm3-mol 1, respectively.(2)
The relevant data are collected in Table III where, for comparison, the V o
calc
values calculated for N+ = 14.94 cm3-mol are included. It can be seen that
1
the value of N+ equal to 16.9 cm3-mol yields V o values, which agree
1
calc
satisfactorily with the experimental data within a broad range of homologous
alkyldimethylbenzylammonium halides.
The set of data for benzyltrimethylammonium bromide and its homologs
enables us to estimate the contribution of the phenylene, C6H4 , group to the
apparent molal volumes of the CnB+ cations by taking into account relevant vol-
umetric data for tetramethylammonium salts and their alkyltrimethylammonium
homologs. Forthispurpose, wewilluse V o = 107.3cm3-mol 1, reportedbyVerrall
and Conway(15) for tetramethylammonium chloride and V o = 256.9 cm3-mol
1
cited by Wettig and Verall(16) for decyltrimethylammonium bromide. The differ-
ence in apparent volumes of C1B+ and the tetramethylammonium cation amounts
to 60.9 cm3-mol 1. The corresponding value estimated from the data for the decyl
salts amounts to 61.0 cm3-mol 1. Thus, the data for this broad range of compounds

1108
Tutaj, Gonza´lez-Pe´rez, Czapkiewicz, Del Castillo, and Rodriguez
Table III. Apparent Molar Volumes at Infinite Dilution for Benzyltrimethylammonium Cation
(C1B+) and its Monoalkyl Homologs (CnB+) Estimated form Experimental Data, V oexp, and
from the Additivity Scheme, V o in cm3-mol
1
calc
Ion
V o
V o (N+
=
16.9 0.3)
V ocalc(N+
=
14.94 1.4)
exp
calc
C1B+
145.0a 145.9b
145.7c 147.8d
193.7a
145.1
147.1
C4B+
192.9
224.7
288.5
320.3
194.8
226.7
290.4
322.3
C6B+
224.9a
C10B+
C12B+
287.2e
319.6e
aPresent results.
bRef. (10).
cRef. (11).
d Ref. (9).
eRef. (7).
appearssatisfactorilyself-consistent. Fromthevolumetricpointofview, thepheny-
lene group appears to be equivalent to almost four methylene groups. A similar
correspondence was suggested for hydrophobicity of the two groups, which can
be evaluated by comparison of surface activity or critical micelle concentration
Fig. 3. Plots of V
1.868 m1/2 vs. molality of DiBBr at 25 C.

Volumes of Benzyltrimethylammonium Bromide
1109
of appropriate pairs of amphiphiles. Such results indicate that the hydrophobic
character of a phenylene group corresponds to that of an aliphatic hydrocarbon
chain composed of 4 0.5 methylene groups.(17,18)
The influence of a second benzyl group at the nitrogen atom on the apparent
molal volume of the ammonium cation can be seen upon inspection of the plot of
V
1.868 m1/2 vs. molality of the solutions of DiBBr at 25 C, presented in Fig. 3.
The extrapolated value of V o and the value of Bv amount to 237.2 0.1 cm3-mol
and 3.94 0.1, respectively. The values of V for this salt show a concentration
1
dependence similar to that found for alkyldimethylbenzylammonium bromides.
The difference in V o found for DiBBr and C1BBr amounts to 62.1 cm3-mol
.
1
This value is only slightly higher compared to the volumetric contribution of the
phenylene group estimated for the set of alkyldimethylbenzylammonium salts and
indicates that introduction of a second phenylene group at the nitrogen atom of
the organic cation does not lead to substantial changes in the group contribution
scheme.
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