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.
A number of studies of micellar aggregation in aqueous solutions of ethylene oxide-propylene oxide block copolymers — using high sensitivity differential scanning calorimetry (HSDSC) — are reviewed. The review attempts to show how the calorimetric output can be analysed, using a model fitting procedure, to obtain estimates for various thermodynamic parameters, which characterise the micellization event, as observed by HSDSC. These important parameters include:
T1/2 the temperature at which half the surfactant has been incorporated into micelles;
Hcal — the calorimetric enthalpy for the process which is measured by integration of the calorimetric output;
HvH — the van't Hoff enthalpy — which characterises the functional dependence of the equilibrium composition of the system upon temperature and which is derived from the application of the van't Hoff isochore to the data analysis procedure;
Cp — the heat capacity change between the initial and final states;and n the aggregation number.Using this data it is possible to examine how extent of aggregation functionally varies with temperature. Subsequent interpolation of these thermal aggregation plots permits an examination of how the extent of aggregation is affected by changes in solution composition under isothermal conditions. A large body of data is presented which shows how co-solvents, co-solutes and pH affect the aggregation process in aqueous solution.
The thermal denaturation of β-lactoglobulin in the presence of urea and alkylurea solutions were measured. In the presence
of a high concentration of urea this protein shows not only heat but also cold denaturation. For studying the effect of temperature
two methods were used, differential scanning calorimetry (DSC) and UV-spectroscopy. DSC provides direct model-independent
determination of the transition enthalpy in comparison with UV-spectroscopy, which gives only apparent or van't Hoff enthalpy
of transition. The UV-melting curves were analyzed on the basis of a two-state approximation. The apparent standard enthalpies
of thermal denaturation, ΔHapp.o
, were compared with calorimetric ones.
Authors:Michele Iafisco, Ismaela Foltran, Michele Di Foggia, Sergio Bonora, and Norberto Roveri
the van'tHoffenthalpy (Δ H VH ) for a two-state process by using the Eq. 1 [ 11 ]:
where T d is the maximum peak temperature, and R is the gas constant. Therefore, the comparison of the values obtained for Δ H VH with the calorimetric
Authors:Alberto Albis, José Manuel Lozano, Javier Sancho, and Carmen M. Romero
be modeled as a two-state process without the presence of intermediate states [ 8 ]. The calorimetric and van'tHoffenthalpies for both forms of the protein are very close indicating a high cooperativity of the denaturation process and confirming the