Download Critical Micelar Concentration and Thermodynamic Parameters of

Brazilian Chemical Society (SBQ). Division of Medicinal Chemistry. 4th Brazilian Symposium on Medicinal Chemistry
Critical Micelar Concentration and Thermodynamic Parameters of
Sodium Dodecil Sulfate and Triton X-100 micellization using Isothermal
Titration Calorimetry.
1
1, 2
Sciuti, V.F. *; Wiggers H.J.
1
2
1
; Montanari, C. A.
[email protected]
Universidade de São Paulo, Instituto de Química de São Carlos, NEQUIMED-PN http://www.iqsc.usp.br/nequimed
Universidade Federal de São Carlos, Departamento de Química, NEQUIMED-PN
Keywords: CMC, SDS, Triton X-100, ITC, thermodynamics parameters.
2
1
0
[Triton X-100] (mM)
0,6
0,4
(A)
0,2
0,2
0,4
0,6
4
6
8
10
12
14
16
18
-3
-4
-5
-7
TX-100
TX-100
SDS 31ºC SDS 25ºC SDS 31ºC
25ºC water 31ºC water
buffer
water
buffer
CMC(mM) - 8.9
4.9
4.9
0.3
0.3
Figure 2. Thermodynamic parameters of surfactants
micelization and CMC values.
As can be see from Figure 2, the Gibbs free
energy appears favorable in all micellization
processes that are entropy driven. The SDS shows a
favorable enthalpy, which can be attributed to ionic
contacts of the polar head-groups.
Conclusions
-0,5
-1,0
water 25 ºC
water 31 ºC
-1,5
-2,0
2
-2
0,8
(B)
0,0
0
-1
-1
CMC buffer
CMC water
0,8
-1
∆H (kcal mol )
1,0
0,0
0,0
∆H (kcal mol )
Phosphate buffer
10 mM 25 ºC
Phosphate buffer
10 mM 31 ºC
water 25 ºC
water 31 ºC
∆G
∆H
-T∆S
-6
Results and Discussion
The experiments were carried out with phosphate
buffer (10 mM) or pure water. The surfactants, with
concentration of 10 times above the CMC were
placed in the syringe and titrated over the same
solution into sample cell that contains no surfactants.
Experimental set: pH 7.1, 45 injections with intervals
of 250 s, temperature 25ºC and 31ºC.
The results of titrations are shown in Figure 1.
The
-1
The micellization of amphiphilic molecules like
tm
Sodium Dodecil Sulfate (SDS) and Triton X-100
starts when the surfactants concentrations reach the
Critical Micelar Concentration (CMC). The micelles
are good models to understand and study partition
1
coefficient .
In this work we used Isothermal Titration
2
Calorimetry (ITC) to determine the CMC and
thermodynamic parameters of micellization (∆G, ∆H,
tm
∆S) of SDS and Triton X-100 , aiming at to
establish appropriated conditions for partition
coefficient studies of small molecules.
the Boltzman equation (Triton X-100).
thermodynamic profiles are shown in Figure 2.
Energy (kcal mol )
Introduction
-2,5
[SDS] (mM)
Figure 1. Isotherms of the titrations of SDS (A) and
Triton X-100 (B).
The values of enthalpy variation before the CMC
(curve inflection) represent the sum of molar
enthalpies of micelles dilution, micelles rupture,
monomers solvation, Before the CMC just the
micelles dilution is registered.
The variation of molar enthalpy of demicellization
( ∆ desmic H = −∆ mic H ) is calculated by the difference of
the isotherms levels. The Gibbs free-energy for the
ionic
surfactants
is
obtained
by
the
equation ∆ mic G = RT (1 + f ) ln(cmc) ,
where
f
represents the ratio of counter-ions by the
3
aggregation number , for the Triton X-100, a non
ionic surfactant this value was taken as zero.
The CMC´s values can be obtained through the
first derivation of the isotherms (SDS) or adjusting on
The thermodynamic parameters and the CMCs of
the surfactants were determined using ITC.
The entropy’s contribution, in distinct solutions, for
the process of micellization is higher than that
demonstrated for enthalpy.
In addition, the
temperature variation causes major influences on
the enthalpy of micellization.
Acknowledgements
1
Sprunger, L.; Acre, W. E. Jr.; Michael H. J. Chem. Inf. Model. 2007,
47, 1808.
2
Ropers, M. H.; Czichocki ,G.; Brezesinski G. J. Phys. Chem. B. 2003,
107, 5281
3
Chatterjee, A.; Moulik, S. P.; Sanyal, S. K.; Mishra, B. K.; Puri P. M.
J. Phys. Chem. B. 2001, 105, 12823.
4th Brazilian Symposium on Medicinal Chemistry – BrazMedChem2008