First evaluation of the thermodynamic properties for spheres to elongated micelles transition of some propanediyl-α,ω-bis(dimethylalkylammonium bromide) surfactants in aqueous solution

Article abstract of DOI:10.1021/jp0458912

The apparent and partial molar enthalpies, apparent molar volumes, and adiabatic compressibilities at 298 K of the aqueous solutions of the cationic gemini surfactants propanediyl-α,ω-bis(octyldimethylammonium bromide) (8-3-8) and propanediyl-α,ω-bis(dodecyldimethylammonium bromide) (12-3-12) have been measured as a function of concentration. The trends of the partial molar enthalpies versus concentration are the first well documented thermodynamic evidence of sphere to rod transition in the micellar phase, involving a detectable quantity of heat, and allow the determination of the change in enthalpy associated with this transition. The changes in enthalpies upon micellization and for the sphere to elongated micelles transition, ΔH_s-r have been obtained from the experimental data by using a pseudo-phase transition approach: -1.5 kJ mol^-1 for 8-3-8 and -3.9 kJ mol^-1 for 12-3-12. No evidence of the above transition is found in the trends of volumetric properties versus m. The apparent adiabatic molar compressibilities for the compounds under investigation are also reported here for the first time: a negative group contribution for the methylene group is evaluated, when the surfactants are present in solution as a single molecule, reflecting its solvation structure. In the micellar phase, the -CH₂- group contribution becomes positive. A value of 1.17 × 10^-3 cm³ bar^-1 mol^-1 for the change in adiabatic molar compressibility upon micellization is obtained. The lower values of the methylene group contributions to the volumetric properties for the monomers support the hypothesis of partial association of the chains before the cmc.

Full text of DOI:10.1021/jp0458912

1744
J. Phys. Chem. B 2005, 109, 1744-1749
First Evaluation of the Thermodynamic Properties for Spheres to Elongated Micelles
Transition of Some Propanediyl-r,ω-bis(dimethylalkylammonium bromide) Surfactants in
Aqueous Solution
Emilia Fisicaro,*,† Carlotta Compari,† Elenia Duce,† Cristina Contestabili,†
Guido Viscardi,‡ and Pierluigi Quagliotto‡
Dipartimento di Scienze Farmacologiche, Biologiche e Chimiche Applicate, UniVersita` di Parma,
Parco Area delle Scienze, 27A, 43100 Parma, Italy, and Dipartimento di Chimica Generale ed Organica
Applicata Applicata e Centro di Eccellenza NIS - Superfici ed Interfacce Nanostrutturate, UniVersita` di Torino,
Corso Massimo D’Azeglio, 48, 10125 Torino, Italy
ReceiVed: September 11, 2004; In Final Form: NoVember 5, 2004
The apparent and partial molar enthalpies, apparent molar volumes, and adiabatic compressibilities at 298 K
of the aqueous solutions of the cationic gemini surfactants propanediyl-R,ω-bis(octyldimethylammonium
bromide) (8-3-8) and propanediyl-R,ω-bis(dodecyldimethylammonium bromide) (12-3-12) have been measured
as a function of concentration. The trends of the partial molar enthalpies versus concentration are the first
well documented thermodynamic evidence of sphere to rod transition in the micellar phase, involving a
detectable quantity of heat, and allow the determination of the change in enthalpy associated with this transition.
The changes in enthalpies upon micellization and for the sphere to elongated micelles transition, ∆Hsfr, have
been obtained from the experimental data by using a pseudo-phase transition approach: -1.5 kJ mol-1 for
8-3-8 and -3.9 kJ mol-1 for 12-3-12. No evidence of the above transition is found in the trends of volumetric
properties versus m. The apparent adiabatic molar compressibilities for the compounds under investigation
are also reported here for the first time: a negative group contribution for the methylene group is evaluated,
when the surfactants are present in solution as a single molecule, reflecting its solvation structure. In the
micellar phase, the -CH2- group contribution becomes positive. A value of 1.17 × 10-3 cm3 bar-1 mol-1
for the change in adiabatic molar compressibility upon micellization is obtained. The lower values of the
methylene group contributions to the volumetric properties for the monomers support the hypothesis of partial
association of the chains before the cmc.
Introduction
become a preferred means of gene delivery into eukaryotic cells.
Gemini surfactants seem to be the most promising.5-8
In the field of cationic surfactants, the structures bearing an
ammonium headgroup were widely studied, due to their fast
and simple synthetic pathway.3,9-17 However, different and more
tailored structures were prepared in recent years, showing the
great interest connected with these kinds of molecules.18 With
the aim to collect chemicophysical data, useful to correlate the
structure of surfactants with the transfection ability, we under-
took the solution thermodynamic study of the simplest and best
known cationic gemini surfactants, the bisquaternary ammonium
bromides. These compounds have been extensively studied by
Zana et al.3,9-17 under the chemicophysical profile. Their results
are a very convenient starting point for the interpretation of our
thermodynamic data.
Research in the field of gemini surfactants, that is, surfactants
in which at least two identical moieties are bound together by
a spacer at the polar head level, has grown very quickly, due to
the their advantages over the monomeric surfactants with respect
to various applications. In fact, gemini surfactants show
increased surface activity, a lower critical micelle concentration
(cmc), and useful viscoelastic properties such as effective
thickening.1 Recently, many studies concerning the practical use
of gemini molecules have appeared in the literature, showing
that the first speculative interests were followed by practical
applications.1-3 Moreover, cationic bisquaternary ammonium
surfactants show stronger biological activity than the corre-
sponding monomers and are more active both on a molar and
on a weight scale as far as germicidal activity and protein
binding ability are concerned.2,4
Materials and Methods
Quite recently, the use of gemini surfactants as nonviral
vectors in gene therapy has been proposed.5-7 Although viral
vectors are generally very efficient in delivering genes into a
targeted cell, their use is not without the risk of adverse or
immunogenic reaction, or replication, depending on the virus
being used. As a result, nonviral vectors have in many cases
The surfactants propanediyl-R,ω-bis(octyldimethylammonium
bromide) (8-3-8) and propanediyl-R,ω-bis(dodecyldimethylam-
monium bromide) (12-3-12) were prepared by us, by a general
protocol similar to those found in the literature.19
Propanediyl-R,ω-bis(octyldimethylammonium bromide)
(8-3-8). The N,N,N-octyldimethylamine (45.77 g, 0.291 mol)
was dissolved in anhydrous ethanol and warmed at reflux. A
solution of 1,3-dibromopropane (29.28 g, 0.145 mol) in ethanol
was slowly added dropwise. The reaction was stopped after 20
* Corresponding author. E-mail: emilia.fisicaro@unipr.it.
† Universita` di Parma.
‡ Universita` di Torino.
10.1021/jp0458912 CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/15/2005

Thermodynamics of Sphere to Rod Transition
J. Phys. Chem. B, Vol. 109, No. 5, 2005 1745
h. The solution was evaporated, and the solid residue was
suspended twice in ethyl acetate, giving a white solid that was
crystallized from methanol/ethyl acetate. The crystals were
filtered on a buchner funnel under inert gas flux. The white
needle crystals were very hygroscopic. Yield: 47.28 g (63%).
1
Rf: 0.03 Al2O3 (ethyl acetate/methanol 75:25). H NMR (400
MHz, DMSO-d6, δ): 0.87 (t, 6H, 2 CH3), 1.29 (m, 20H, 10
CH2), 1.67 (m, 4H, N+CH2CH2), 2.17 (m, 2H, N+CH2CH2-
CH2N+), 3.06 (m, 12H, 4 N+CH3), 3.35 (m, 8H, 4 N+CH2).
Anal. Calcd for C23H52Br2N2: C, 53.49; H, 10.15; N, 5.42.
Found: C, 53.43; H, 10.19; N, 5.49.
Propanediyl-R,ω-bis(dodecyldimethylammonium bromide)
(12-3-12). The N,N,N-dodecyldimethylamine (10.45 g, 0.049
mol) was dissolved in anhydrous ethanol and warmed at reflux.
A solution of 1,3-dibromopropane (5.00 g, 0.025 mol) in ethanol
was slowly added dropwise. The reaction was stopped after 24
h. The solution was refrigerated at 0 °C, and a solid mass was
filtered. Further evaporation of the resulting ethanolic solution
gave another crop of solid product. The solid was crystallized
from ethanol/ethyl acetate. The crystals were filtered on a
buchner funnel under inert gas flux. The white needle crystals
were very hygroscopic. Yield: 11.60 g (75%). Rf: 0.03 Al2O3
Figure 1. Relative apparent molar enthalpies (full circles) and partial
molar relative enthalpies (empty circles) of the 8-3-8 gemini surfactant
as a function of surfactant molality.
For the sake of clarity, we recall that, with reference to the
state of infinite dilution, the molar enthalpy of dilution, ∆Hd,
is given by
∆Hd ) LΦ,f - LΦ,i
(1)
where LΦ is the relative apparent molar enthalpy and the indexes
f and i stand for the final (after dilution) and initial (before
dilution) concentrations, respectively.
For ionic surfactant in the premicellar region, the relative
apparent molar enthalpy can be expressed by means of a
polynomial in m1/2. Stopping the serial expansion at the third
term, we obtain
1
(ethyl acetate/methanol 75:25). H NMR (DMSO-d6, δ): 0.86
(t, 6H, 2 CH3), 1.24 (m, 20H, 10 CH2), 1.77 (m, 4H,
N+CH2CH2), 2.66 (m, 2H, N+CH2CH2CH2N+), 3.38 (m, 12H,
4 N+CH3), 3.53 (m, 4H, 4 N+CH2CH2-chain), 3.80 (m, 4H, 4
N+CH2CH2CH2N+). Anal. Calcd for C31H68Br2N2: C, 59.22;
H, 10.90; N, 4.46. Found: C, 59.18; H, 10.99; N, 4.40.
The solutions were prepared by weight using freshly boiled
doubly distilled water, stored under nitrogen. Solution concen-
trations are expressed by molality, m (mol kg-1).
LΦ ) ALm1/2 + BLm + CLm3/2
(2)
Methods. The enthalpies of dilution were measured by means
of the Thermometric TAM 2277 (flow-mixing cell) microcalo-
rimeter, equipped with a 221 Nano Amplifier instrument, at 298
K. The TAM thermostat (2277-011, from 5 to 90 °C) is stable
to within (0.1 mK. The freshly prepared surfactant solutions,
kept before injection at the experimental temperature by means
of a Heto cryothermostatic bath, were diluted into the “mixing”
measuring cell of the microcalorimeter in a 1:1 ratio by using
CO2-free water. The injection of the solutions and of water into
the calorimeter was done by means of a Gilson peristaltic pump,
Minipuls 2, and their flows were determined by weight.
Electrical calibrations were performed before and after each run.
The density, d, and sound velocity, u, of the solutions were
measured by a Paar DSA 5000 oscillating U-tube density
((0.000 001 g cm-3) and sound velocity ((0.1 m sec-1) meter
which measures to the highest accuracy in wide viscosity and
temperature ranges. Temperature is controlled within (1 × 10-3
K. On the basis of an additional measuring cell made of stainless
steel and on high resolution electronics, the sound velocity of
the sample can be determined accurately. Both measuring cells
are temperature controlled using a built-in solid state thermostat
and two integrated Pt(100) platinum thermometers. The instru-
ment was calibrated before each series of measurements by
degassed bidistilled water (d0 ) 0.997 043 g cm-3, u0 ) 1497
m sec-1 at 298 K) and dry air (d ) 0.001 169 g cm-3 at P )
1 bar).
where AL is the limiting Debye-Huckel slope for relative
enthalpies accounting for the long range electrostatic solute-
solute interactions. The parameters BL and CL are obtained from
the experimental points in the premicellar region by a least-
squares curve fitting.
In the micellar region, the apparent molar enthalpies are
evaluated by means of eq 1, and when a value of LΦ versus m
not experimentally measured is needed, by graphical interpola-
tion.
The partial molar enthalpies, L2, are determined by drawing
the best curve for the apparent molar enthalpies versus m and
then by calculating the partial molar quantities as ∆(mLΦ)/∆m
from points interpolated at regular intervals.
The heats of dilution and apparent and partial molar enthalpies
at 298 K, obtained by means of eqs 1 and 2, for the compounds
under study are available as Supporting Information. In Figures
1 and 2, the plots of the apparent and partial molar enthalpies
as a function of molality for the compounds under study are
shown.
The apparent molar volumes, VΦ, and adiabatic compress-
ibilities, Ks,Φ, were calculated by means of the following
equations26-29
103(d - d0)
M
d
VΦ )
-
(3)
(4)
mdd0
Mâs 103(âs,0d - âsd0)
Results
Ks,Φ
)
-
The experimental data have been expressed in terms of the
apparent and partial molar quantities of the solute, as usual in
solution thermodynamics, assuming infinite dilution as the
reference state. Apparent and partial molar quantities were
obtained from the experimental data using methods detailed
elsewhere.20-25
d
mdd0
where d is the density of the solution of molality m, M is the
molecular weight of the surfactants, d0 is the density of the
solvent, and âs,0 and âs (in units of bar-1) are the coefficient of
adiabatic compressibility of the solvent and of the solute,

1746 J. Phys. Chem. B, Vol. 109, No. 5, 2005
Fisicaro et al.
agree with the above observations. A cmc value of 55 mM and
a degree of ionization of R ) 0.27 for 8-3-812 and a cmc value
of 0.91 mM for 12-3-1210,11 are reported in the literature. In a
previous paper,9 a value of 20 mM has been also reported for
8-3-8. Conductivity data of 8-3-8 versus concentration, reported
by Frindi et al.,12 show a first break at 20 mM and a second
break at 55 mM. The value of concentration at which the second
break is occurring has been assumed to be the cmc, following
the spectrofluorometric and ultrasonic absorption data. From
our conductivity experiments, not reported here, a value of 56
mmol kg-1 for the cmc of 8-3-8 has been determined, in good
agreement with the above-reported literature values, and we have
used this value in the following calculations. The data obtained
from an electrical conductivity study of the above compounds
are interpreted in terms of ion pairing in the premicellar region
for the 8-3-8 compound.14 However, 12-3-12 behaves regularly
as a strong electrolyte.
Figure 2. Relative apparent molar enthalpies (full squares) and partial
molar relative enthalpies (empty squares) of the 12-3-12 gemini
surfactant as a function of surfactant molality.
respectively. The latter is calculated from sound velocity, u,
and density data as
Apparent and Partial Molar Enthalpies. In general for ionic
surfactants, as far as apparent and partial molar enthalpies as a
function of concentration are concerned, the curves, after
increasing in the premicellar region, tend to level off at
concentrations above the cmc, where they are almost parallel.
The lowering of the curves in the micellar region, proportional
to the number of carbon atoms in the alkyl chain, is attributed
to the electrostatic interactions in micellar solutions. The trends
for the compounds under study, reported in Figures 1 and 2,
show a very peculiar behavior. The curves of 12-3-12 at very
low concentrations are positive, as expected. On the contrary,
the trends of the apparent and partial molar enthalpy of 8-3-8
as a function of concentration in the premicellar region are
different: notwithstanding the greater value of the cmc, they
are always negative starting from the lowest concentrations
examined. The explanation of this different and unusual behavior
could be found in the predominant interactions in dilute solutions
depending on the length of the hydrophobic tails, as already
suggested from the interpretation of the conductivity data:8,14
for 8-3-8, ion pairing interactions involving exothermic pro-
cesses are prevalent, while 12-3-12 behaves regularly. The
partial molar enthalpies of the latter decrease sharply below the
cmc to a first plateau, due to the formation of spherical micelles;
then, a new smaller jump starting at a concentration greater than
0.020 m to a second plateau value at concentrations greater than
0.045 m is clearly seen. In the case of the dodecyl compound,
the dilution enthalpies have been measured up to a concentration
of 0.1 m: samples of greater concentration cannot be injected
in our flow-mixing calorimetric cell, due to the very high
viscosity, further evidence of a change in structure of the
solution. The above-reported well documented change in shape
of the micelles could offer a possible explanation of the observed
behavior, the first jump being associated with the formation of
spherical micelles and the second one with the transition from
spherical to elongated micelles.
âs ) 100/(u2d)
(5)
Experimental densities, sound velocities, and the data from eqs
3-5 are available as Supporting Information.
Discussion
Only a few data have been published until now about the
thermodynamics of gemini surfactant solutions,17,25,30 and a lot
of questions about their behavior are still open. For instance, it
is not completely clear if the group contribution approach is
valid also in the case of gemini surfactants and if it is possible
to use the group contribution obtained from the study of
monomers to predict the properties of gemini surfactant solu-
tions. Moreover, it is known13,15 that this class of surfactants
undergoes structural transitions in solution from spherical to
elongated micelles, but until now, in the literature, thermody-
namic parameters for such transitions have not been reported.
In general, as far as regular surfactants are concerned, phase
transitions in solutions involve too small of an amount of heat
to be detected by enthalpy measurements, but other second
derivative thermodynamic properties seem to be more sensitive
to structural transitions.21 The reasons why we have chosen to
perform measurements on two bisquaternary ammonium bro-
mide surfactants having the same spacer, constituted by three
methylene groups, and different chain lengths, namely, the
dioctyl (8-3-8) and didodecyl (12-3-12) compounds, are the
following: (i) the well documented sphere to rod transition for
the compound 12-3-1213,15 and for the compounds having a short
spacer and ii) conductivity data14 showing the different behavior
between 8-3-8 and 12-3-12 in the premicellar region. In fact,
some very interesting papers about aggregation number and
shapes of micelles of the didodecyl compound as a function of
the length of the spacer have appeared in the literature some
time ago,13,15 showing the structural changes of the 12-3-12
compounds as a function of concentration.
For 8-3-8, the first plateau ends at the cmc. Then, after a
jump, corresponding to the formation of spherical micelles, a
new plateau is reached. A less steep jump, centered around 0.15
m, is suggested to correspond to the formation of elongated
micelles, in accordance with literature data.13,15
Time-resolved fluorescence quenching (TRFQ) techniques for
the determination of the aggregation number and cryogenic
transmission electron microscopy (cryo-TEM) for the direct
visualization of the aggregates have been used. At concentrations
close to the cmc, micelles of 12-3-12 are spherical with an
aggregation number of 25; that is, about 50 dodecyl chains stick
together. This number increases significantly with concentration,
suggesting a change in shape toward elongated micelles,
confirmed by the cryo-TEM images and by the trends in solution
viscosity. Small angle neutron scattering (SANS) measurements
To obtain the enthalpy change upon micellization, ∆HM, we
have applied a pseudo-phase transition model, in which the
aggregation process is considered like a phase transition, taking
place at equilibrium. In this model, it is assumed that, at the
cmc, the partial molar properties present a discontinuity due to
the formation of the pseudo-phase. The micellization parameters
are obtained by extrapolating at the cmc the trends of partial

Thermodynamics of Sphere to Rod Transition
J. Phys. Chem. B, Vol. 109, No. 5, 2005 1747
Figure 4. Apparent molar volume (cm3 mol-1) of the 12-3-12 gemini
surfactant as a function of surfactant molality. Also included are results
from the fit of the experimental data by eq 6 (solid line).
Figure 3. Apparent molar volume (cm3 mol-1) of the 8-3-8 gemini
surfactant as a function of surfactant molality. Also included are results
from the fit of the experimental data by eq 6 (solid line).
molar properties before and after the cmc.20-25 Rather sharp
changes occur, in fact, around the cmc, but they are never
discontinuous, especially for surfactants with short hydrophobic
chains. Following this procedure, we have ∆HM ) -1.9 kJ
mol-1 for 8-3-8 and ∆HM ) -16.9 kJ mol-1 for 12-3-12 for
the formation of spherical micelles. The latter value is in good
agreement with that predicted in ref 17b for 12-3-12 from the
trends of micellization enthalpy of didodecyl compounds with
even s, from 2 to 12, measured by titration calorimetry.
In the case of 12-3-12, but never for 8-3-8, the enthalpies of
dilution have been also directly measured by means of titration
calorimetry by Bai et al.,30 in order to obtain the micellization
enthalpy. Their values, according to the observation of Gros-
maire et al.,17b are less exothermic than those found in the
present study and in ref 17b and seem to not be very reliable.
Moreover, the titration calorimetry data have never been
expressed in terms of apparent and partial molar enthalpies,
losing in this way, in our opinion, very interesting details on
the systems under investigation. In the sphere to rod transition,
in fact, a relevant quantity of heat seems to be involved. From
the plots in Figure 1, according to the interpretation suggested
above, the ∆Hsfr results are -1.5 kJ mol-1 for 8-3-8 and -3.9
kJ mol-1 for 12-3-12. Some time ago, we observed a comparable
trend for the partial molar enthalpies of an anionic surfactant,
the potassium 4-(hexylcarbonil-amino)-2-hydroxybenzoate
(KPAS-C7), in 0.02 m KOH solution.23,24 At a surfactant
concentration of ∼0.14 mol kg-1, a sudden change in slope
suggested a structural transition in solution, tentatively attributed
to a sphere to rod transition. At that time, we suggested, as a
determining factor, the possibility of an intramicellar rearrange-
ment of the hydrogen bonds between the -OH and the
carboxylic group of the salicylic moiety from intramolecular
to intermolecular hydrogen bonding. The results obtained by
the study of 8-3-8 and 12-3-12 gemini compounds seem to
support our hypothesis. In fact, when an intermolecular hydrogen
bond is formed, the system looks like a sort of “gemini”
surfactant with a very short spacer, similar to the 8-3-8
compound investigated here. This conformation could justify
the observed sphere to rod transition. Moreover, the value of
∆Hsfr for KPAS-C7, around -1.5 kJ mol-1, is nicely compa-
rable with that for 8-3-8.
Figure 5. Apparent molar isentropic compressibilities (cm3 mol-1
bar-1) of the 8-3-8 (circles) and of 12-3-12 (squares) gemini surfactants
as a function of surfactant molality. Also included are results from the
fit of the experimental data by eq 6 (solid line).
mental points has been determined in the same concentration
range, to more carefully look for evidence for sphere to rod
transition. Adiabatic compressibilities, on the contrary, are
reported here for the first time. Despite the great care we have
used in our experiments, the curves do not show peculiar
behavior around the concentrations at which the sphere to rod
transitions are expected. This means that the change in volume
involved in such transitions, if present, is of the same order of
magnitude as the experimental error. The change in volume upon
micellization is primarily due to the change in structure of the
water molecules involved in the hydrophobic effect31,32 and to
the change in electrostriction of the polar part of the molecule,
modulated by the value of R, the degree of ionization of the
micelle: volumetric properties, such as volumes and compress-
ibilities, are reflective, in fact, of the solute-solvent interactions.
It is obvious that a great change in these interactions occurs
when micelle formation begins. A further change in structure
of the micelles, in which solute-solute van der Waals interac-
tions are mainly involved, is expected to give rise to a smaller
change in apparent molar volume of surfactant, if any. The same
arguments are valid for compressibilities.
To extract micellization parameters, the volumetric data have
been analyzed assuming a pseudo-phase transition model.12,26
Following this model, the observed trends above the cmc can
be described by the equation
XΦ ) XΦ,Μ - (cmc∆XΦ,Μ)(1/m)
(6)
Volumes and Adiabatic Compressibilities. We show in
Figures 3 and 4 the trends of apparent molar volume for 8-3-8
and 12-3-12, respectively, and in Figure 5 the apparent molar
adiabatic compressibilities as a function of concentration. The
apparent molar volumes for m-s-m type gemini surfactants in
aqueous solution have already been investigated by Wettig and
Verral,26 but in the present work, a greater number of experi-
Here, X stands for the volume or for the adiabatic compress-
ibility. Knowing the values for the cmc, the values of XΦ,Μ
,
the property in the micellar phase, and ∆XΦ,Μ, the change in
property upon micellization, can be obtained by a least-squares
fit. In Table 1, the values so obtained, together with the values

1748 J. Phys. Chem. B, Vol. 109, No. 5, 2005
Fisicaro et al.
TABLE 1: Values of XΦ°, the Property at Infinite Dilution, XΦ,M, the Property in the Micellar Phase, ∆XΦ,M, the Change in
Property upon Micellization, XΦ,cmc, the Property at the cmc for the Compounds under Investigation
compound
8-3-8
X
Vb
Κs
Vb
Κs
XΦ°
XΦ,M
∆XΦ,M
9.5(2)
0.016 33(10)
11.7(2)
XΦ,cmc
cmca
437.3
446.5(1)
0.013 05(10)
579.6(2)
437.2(2)
-0.003 29(10)
567.8(2)
0.056
c
c
-0.003 842
571.2d
12-3-12
0.000 91
-0.005 36e
0.022 25(10)
0.025 72(10)
-0.003 48(10)
a In units of mol kg-1
.
b In units of cm3 mol-1
.
c In units of bar-1 cm3 mol-1 d From ref 26. e From the group contribution approach (ref 28).
.
of XΦ,cmc, the value at the cmc, obtained by
XΦ,cmc ) XΦ,Μ - ∆XΦ,Μ
1.5 × 10-3 cm3 bar-1 mol-1, reported in ref 27 for the change
in adiabatic molar compressibility upon micellization of alky-
ltrimethylammonium bromide monomers. The lower value
compared to the monomers could support the above-reported
hypothesis of partial association of the chains before the cmc.
(7)
are reported. The good agreement between the experimental and
computed data is shown in Figures 3-5, in which the solid line
represents the computed function. The values of apparent molar
volumes at the cmc and in the micellar phase agree very well
with those found by Wettig and Verral.26 Moreover, in the case
of 8-3-8, the experimental value of XΦ°, obtained by extrapola-
tion to infinite dilution of the data below the cmc (reported in
Table 1), is comparable to the values obtained by different group
contribution approaches.26,33,34 The group contribution of the
methylene group to the change in apparent molar volume upon
micellization, obtained from the comparison of 8-3-8 and 12-
3-12, is smaller compared to those obtained from the study of
normal surfactants.22 The reduction of this group contribution
in the case of gemini surfactants is confirmed by the data
reported in ref 26, if the compounds with spacer 3, excluding
8-3-8, are considered. A possible explanation of this behavior
is that the alkyl chains of the gemini surfactants are already
associated in solution, as we suggested some years ago.25
The isentropic compressibilies (see Figure 5) show trends as
a function of m, similar to those of volumes, with a sharp
increase at the cmc, ending in a plateau in the micellar region,
without showing peculiarities. Notwithstanding, it is generally
accepted that properties second or higher order as derivatives
of free energy are more sensitive to the transition in solution,
but this is not the case for the compressibilities we have
measured. Compressibility, as the other volumetric properties,
is strictly associated with the structure of the hydration sphere
of the molecule: the disruption of the rigid cavity hosting the
hydrophobic moiety of the surfactant molecule, together with
the reduction of the electrostriction due to the counterion binding
in micelles, allows a greater change in volume with pressure.
Once again, the analogy between the micellization process and
the folding of proteins or polypeptides is very strict: proteins
in the native state and folded polypeptides show positive
compressibility. On the contrary, negative compressibilities are
evaluated by the group contribution approach for the completely
unfolded state and negative compressibilities are shown by the
surfactants under investigation (see Table 1) below the cmc. It
is generally accepted that the group contribution of -CH2- to
the molar compressibility is negative (-1.9 × 10-4 cm3 bar-1
mol-1 in ref 28 and -1.6 × 10-4 cm3 bar-1 mol-1 in ref 29) at
infinite dilution. We have evaluated Ks,Φ° of 8-3-8 by extrapo-
lating at infinite dilution the experimental trend below the cmc,
and we have evaluated the value for 12-3-12 by applying the
first of the above group contributions (Table 1). Both values
are negative, as expected. In Table 1 are also reported the values
of Ks,Φ,cmc, the compressibility at the cmc, obtained by eq 7:
they are negative and the lengthening of the hydrophobic chain
lowers the values to a limited extent with respect to infinite
dilution. In the micellar phase, the -CH2- group contribution
becomes positive: from the data in Table 1, a value of 1.17 ×
10-3 cm3 bar-1 mol-1 can be obtained, lower than the value of
Conclusions
The evaluation of the apparent and partial molar enthalpies
at 298 K of aqueous solutions of the cationic gemini surfactants
propanediyl-R,ω-bis(octyldimethylammonium bromide) and
propanediyl-R,ω-bis(dodecyldimethylammonium bromide) as a
function of concentration allows the determination, besides the
enthalpy of micellization, of the change in enthalpy associated
with the sphere to rod transition in the micellar phase, which
has never been reported before. The changes in enthalpies upon
micellization and for sphere to elongated micelle transition have
been obtained from the experimental data by using a pseudo-
phase transition approach. The sphere to rod transition appears
to be an exothermic process, with the quantity of heat involved
increasing with increasing chain length. The ∆Hsfr results are
-1.5 kJ mol-1 for 8-3-8 and -3.9 kJ mol-1 for 12-3-12.
Moreover, the data reported here are a further contribution
supporting the interpretation of a similar trend in the partial
molar enthalpies of an anionic surfactant, the potassium
4-(hexylcarbonilamino)-2-hydroxybenzoate in 0.02 m KOH
solution, previously studied by us: due to the formation of
intermolecular hydrogen bonding, this surfactant can assume a
structure very similar to that of gemini compounds. The value
of ∆Hsfr is very similar to that for 8-3-8 and could support
this hypothesis.
On the other hand, no evidence of the above transition is
found in the trends of volumetric properties versus m, in the
limits of the accuracy of the experimental methods used.
The group contribution approach, using the values obtained
from the study of monomers, seems to not be suitable in the
case of gemini compounds.
The adiabatic molar compressibilities, reported here for the
first time, show a negative group contribution for the methylene
group, when the surfactants are present in solution as monomers,
reflecting its solvation structure. In the micellar phase, the
-CH2- group contribution becomes positive: a value of 1.17
× 10-3 cm3 bar-1 mol-1 for the change in adiabatic molar
compressibility upon micellization is obtained, lower than that
of the alkyltrimethylammonium bromide monomers. The lower
values of the methylene group contributions to the volumetric
properties with respect to the monomers support the hypothesis
of partial association of the chains below the cmc.
Acknowledgment. The authors are grateful to the Italian
Ministry for the Education, University and Research (MIUR),
COFIN 2002 project “Molecular Systems and Metallo-receptors
for the Activation and Transport of Dioxygen, Nitric Oxide,
and Biosubstrate”, COFINLAB 2001 “Superfici ed Interfasi
Nanostrutturate”, and to the ex 60% funds (University of Torino)
for financial support. The authors are particularly grateful to

Thermodynamics of Sphere to Rod Transition
J. Phys. Chem. B, Vol. 109, No. 5, 2005 1749
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Supporting Information Available: Tables collecting the
molalities, m, enthalpies of dilution, ∆Hd, apparent, LΦ, and
partial, L2, molar enthalpies of the 8-3-8 and 12-3-12 gemini
surfactants, in water at 298 K; tables reporting the densities, d,
apparent molar volumes, VΦ, sound velocities, u, coefficients
of adiabatic compressibility, âs, and isentropic compressibilities,
(18) Perez, L.; Torres, J. L.; Mansera, A.; Solans, C.; Infante, M. R.
Langmuir 1996, 12, 5296-5301.
Ks,Φ, as a function of molality, m, of the 8-3-8 gemini
(19) (a) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072-
1075. (b) Dam, T.; Engberts, J. B. F. N.; Kartha¨user, J.; Karaborni, S.; van
Os, N. M. Colloids Surf., A 1996, 118, 41-49.
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of InVestigation; Zana, R., Ed.; Marcel Dekker: New York, 1987; p 1 and
references therein.
surfactants, in water at 298 K. This material is available free of
charge via the Internet at http://pubs.acs.org.
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