Authors:Marta Fernandez-Tarrio, Carmen Alvarez-Lorenzo, and A. Concheiro
comprises X-shaped copolymers formed by four poly(propylene oxide) (PPO) and
poly(ethylene oxide) (PEO) block chains bonded to an ethylene diamine central
group. Micellization behaviour of three representative Tetronics (T304, T904
and T1307) was characterized to gain an insight into the interactions between
the copolymer unimers and the state of water in their solutions. The enthalpy
of demicellization, recorded at 37°C in an isoperibol microcalorimeter,
indicated that the process was in all cases exothermic and the enthalpy ranked
in the order T1307≥T904>>T304. Micellization is entropy-driven owing to
hydrophobic interactions between the PPO chains.
showed that the crystallization and melting peaks of the free water remaining
in T304 and T904 solutions were progressively shifted toward lower temperatures
as the surfactant proportion increased, owing to a colligative effect. Bound
water corresponded to 3 water molecules per EO repeating unit. In the case
of T1307, which has longer PEO chains, a splitting of the melting peak was
observed, one peak appearing around 0°C due to free water and another
at –15°C due to interfacial water. As T1307 proportion raised, the
enthalpy of the former decreased, whilst the enthalpy of the latter increased.
In 40% T1307 solutions, interfacial water overcame the proportion of free
water; there being 1 interfacial and 3 bound water molecules per EO repeating
unit. Gaussian deconvolution of FTIR spectra also enabled to characterize
the evolution of free water as a function of Tetronic proportion. The dependence
of micellization and water interaction behaviour on Tetronics structure should
be taken into account to use these copolymers as drug solubilizers and micellar
Authors:Asghar Taheri-Kafrani and Abdol-Khalegh Bordbar
The micellization characteristics of sodium n-dodecyl sulfate (SDS) have been investigated by microcalorimetric technique at conditions close to the physiological ones.
The thermodynamics of micellization were studied at 20, 25, 30, 35 and 40 °C in 50 mM HEPES buffer, pH 7.4 and 160 mM NaCl
using isothermal titration calorimetric (ITC) technique. The calorimeter can operate in a stepwise addition mode, providing
an excellent method of determination of critical micelle concentration (CMC) and enthalpy of demicellization (and hence micellization).
It can as well distinguish between aggregating and non-aggregating amphiphiles (solutes) in solution. The dilution enthalpy
(∆Hdil) was calculated and graphed versus concentration in order to determine the micellization enthalpy (∆Hmic) and CMC. In addition to the CMC and ∆Hmic, the effective micellar charge fraction (α) of the ionic surfactant micellization process can also be determined from ITC
curves. The Gibbs free energy of the micellization (∆Gmic), entropy of the micellization (∆Smic), and specific heat capacity of the micellization (∆CP,mic) process have been evaluated by the direct calorimetric method (mass-action model) as well as by the indirect method of van’t
Hoff by processing the CMC and α results of microcalorimetry at different temperatures. The differences of the results obtained
by these two procedures have been discussed. The presence of NaCl (160 mM) in the solutions decreased the CMC of SDS. The
enthalpy changes associated with micelle dissociation were temperature-dependent, indicating the importance of hydrophobic
interactions. The ∆Gmic was found to be negative, implying, as expected, that micellization occurs spontaneously once the CMC has been reached. The
values of ∆Gmic were found to become more negative with increasing temperature and the ∆Smic was found to decrease with increasing temperature in both models.
.m.c has a shallow minimum near this temperature ( Figs. 5 , 6 ). As it can be seen from the presented data at low temperatures, the micellizationenthalpy at temperatures below 313.15 K for 8-8-8 and below 288.15 for 12-8-12 is endothermic and then