The 'hydrophobic effect' of the dissolution process of non-polar substances in water has been analysed under the light of
a statistical thermodynamic molecular model. The model, based on the distinction between non-reacting and reacting systems explains the changes of the thermodynamic functions with temperature in aqueous systems. In the dissolution of non-polar
substances in water, it follows from the model that the enthalpy change can be expressed as a linear function of the temperature
(ΔHapp =ΔHø +nwCp,wT ). Experimental solubility and calorimetric data of a large number of non-polar substances nicely agree with the expected
function. The specific contribution of nw solvent molecules depends on the size of solute and is related to destructuring (nw >0) of water molecules around the solute. Then the study of 'hydrophobic effect' has been extended to the protein denaturation
and micelle formation. Denaturation enthalpy either obtained by van't Hoff equation or by calorimetric determinations again
depends linearly upon denaturation temperature, with denaturation enthalpy, ΔHden , increasing with T . A portion of reaction enthalpy is absorbed by a number nw of water molecules (nw >0) relaxed in space around the denatured moieties. In micellization, an opposite process takes place with negative number
of restructured water molecules (nw <0) because the hydrophobic moieties of the molecules joined by hydrophobic affinity occupy a smaller cavity.
Authors:S. Milioto, S. Causi, R. Crisantino, and R. De Lisi
Densities, heat capacities, enthalpies of dilution at 298 K and osmotic coefficients at 310 K of octyltrimethylammonium chloride were measured as functions of concentration. From the experimental data, the partial molar volumes, heat capacities, relative enthalpies, nonideal free energies and entropies at 298 K were derived as functions of concentration. A comparison between the above data and those of dodecyltrimethylammonium chloride reported in the literature shows that the increase of the alkyl chain length shifts the apparent molar volumevs. concentration curves towards greater values and the heat capacity, relative enthalpy and free energyvs. concentration curves towards smaller values. By assuming the pseudo-phase transition model the properties of micellization (ΔYm) were graphically evaluated. TheΔYm values of OTAC compared with those of DTAC are consistent with the increase of the hydrophobicity by increasing the alkyl chain length.
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.
Authors:Guangyue Bai, Vasco Castro, Marieta Nichifor, and Margarida Bastos
Dextran modified with deoxycholic acid (Dex-DCA) was synthesized by grafting DCA along the polymer backbone, with degrees
of substitution (DS)—2% and 3%. The thermodynamics of the association processes of the mixed systems is followed by isothermal
titration calorimetry for sodium deoxycholate/sodium dodecyl sulfate (NaDCA/NaDS), Dex-DCA with different surfactants—Dex-DCA/NaDS,
Dex-DCA/NaDCA, and Dex-DCA/DTAB (dodecyltrimethylammonium bromide). Calorimetric measurements for the micellization processes
of the pure surfactants in aqueous solution were also performed for comparison with the results obtained for the mixed systems.
We have obtained and herein present the enthalpies of micelle formation and critical micelle concentrations for the referred
pure surfactants, as well as the interaction and aggregation enthalpies for the mixed systems-surfactant/polymer. The dependence
of the observed aggregation behavior on the surfactant and temperature is discussed in detail. Finally, we should stress that
calorimetry allowed us to ascertain a very important fact in polymer/surfactant interaction. From the comparison between NaDCA/NaDS
and Dex-DCA/NaDS calorimetric titration curves, we could clearly see that the interaction between Dex-DCA and NaDS is driven
by the interaction between the bile acid moiety and the surfactant.
Three environmentally and agrochemicaly important mercury species: methylmercury, phenylmercury and mercury(II) are separated
within 4 minutes as bromocomplexes by micellar liquid chromatography using very short reversed-phase (RP) C18 columns (up
to 30 mm). The micellar mobile phase containig 0.05M cetyltrimethylammonium bromide (CTMA+Br−), 1% (v/v) 2-propanol, 0.001M cyclohexylenediaminetetraacetic acid (DCTA) and sulfuric acid (pH 2) showed good selectivity
in mixed reversed-phase and anion-exchange mode. The above mentioned separation order in which organomercurials are cluted
far behind the void volume of the column, but before the mercury(II) peak is advantageous in all instances where mercury(II)
is present in real samaples in great excess. Environmental and agrochemical samples contain humic material which does not
interfere in this particular system. The low cost photometric detection at 500 nm after post-column derivatization by CTMA+Br− micellized dithizone is almost free from interferences and enables detection limits at the 1–3 ng level (e.g., 0.1 ppm Hg)
for 20 μl samples.
Authors:Y. Li, G. Fei, Z. Honglin, L. Zhen, Z. Liqiang, and L. Ganzuo
The power–time curves of micellar formation of two anionic surfactants, sodium laurate (SLA) and sodium dodecyl sulfate (SDS),
in N,N-dimethyl acetamide (DMA) in the presence of various long-chain alcohols (1-heptanol, 1-octanol, 1-nonanol and 1-decanol)
were measured by titration microcalorimetry at 298 K. The critical micelle concentrations (CMCs) of SLA and SDS under various
conditions at 298 K were obtained based on the power–time curves. Thermodynamic parameters (
) for micellar systems at 298 K were evaluated according to the power–time curves and the mass action model. The influences
of the number of carbon-atom and the concentration of alcohol were investigated. Moreover, combined the thermodynamic parameters
at 303, 308 and 313 K in our previous work and those of 298 K in the present work for SLA and SDS in DMA in the presence of
long-chain alcohols, an enthalpy–entropy compensation effect was observed. The values of the enthalpy of micellization calculated
by direct and indirect methods were made a comparison.
At a given surfactant-surfactant ratio, the enthalpies of transfer ΔH (W→W+S) of pentanol 0.03m from water to sodium dodecylsulfate (NaDS)-dodecyldimethylamine oxide-water mixtures as functions of the surfactants mixture
concentration (mt) were determined. ForXNaDS=0.9, ΔH (W→W+S) increases monotonically withmt such as observed for pure surfactants. ForXNaDS=0.12 and 0.3, ΔH (W→W+S) increases withmt up to 0.12m beyond which it decreases withmt. AtXNaDS=0.6, two monotonic curves can be distinguished in the ΔH (W→W+S)vs. mt trend. Experimental data were fitted through an equation previously reported for additives in pure surfactants derived by
assuming the pseudo-phase transition model for the micellization and a mass action model for the distribution of the additive
between the aqueous and the micellar phases. This method did permit to simultaneously obtain the distribution constant of
the alcohol between the aqueous and the micellar phase (and, then, the standard free energy of transfer) and its enthalpy
of transfer from the aqueous to the micellar phases. By combining these properties the standard entropies of transfer were
calculated. From these results, the excess properties of pentanol in the mixed micelles were calculated as a function of the
mixture composition. The excess enthalpies and entropies are positive and compensate with each other leading to null values
for the excess free energies in the whole range of the mixed micelles composition.
Authors:Henryk Piekarski, Katarzyna Łudzik, and Michał Wasiak
Zana , R . Alkanediyl-α,ω- bis (dimethylalkylammonium bromide) surfactants: 10. Behavior in aqueous solution at concentrations below the critical micellization concentration: an electrical conductivity study . J