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Abstract  

The dilution enthalpies of D-mannitol and D-sorbitol in aqueous sodium chloride solution at various concentrations have been determined by isothermal microcalorimetry at 298.15 K. The homogeneous enthalpic interaction coefficients over a quite large range of concentration of aqueous sodium chloride solutions have been calculated according to the excess enthalpy concept. The results show that enthalpic pairwise interaction coefficients (h 2) of D-mannitol and D-sorbitol are positive in aqueous sodium chloride solution and become more positive with increase of the concentration of sodium chloride. The results are interpreted in terms of the different conformations of the two polyols, solute-solute and solute-solvent interactions involved by solvent effects.

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the molar dilution enthalpies of the salt solutions (where m i and m f are the values of initial and final concentrations of the solvent, respectively): (5) Molar dilution enthalpy of the electrolyte ( ) can be presented as ( 6 ): (6

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Journal of Thermal Analysis and Calorimetry
Authors: V. Arcoleo, M. Goffredi, G. La Manna, V. Liveri, F. Aliotta, and M. Fontanella

Abstract  

The enthalpies of dilution of water/lecithin/cyclohexane microemulsion-gels at variousR values (R=[water]/[lecithin]) and molar enthalpies of solution of water in lecithin/cyclohexane at a fixed lecithin concentration were determined calorimetrically at 25°C. Through a description of the process of dilution of water/lecithin/cyclohexane microemulsion-gels as one involving mainly the scission of lecithin reversed micelles in to smaller ones, the concentration dependence of the enthalpy was rationalized. Surprisingly, in order to account for the dilution enthalpies, it was not necessary to hypothesize a thermal effect arising from the breakage of the micellar network present in the micremulsion-gels. This result indicates that the building-up of the micellar network mainly arises from topological entanglements and steric hindrances among the lecithin reversed micelles. Attribution of the molar enthalpy of solution of water essentially to the thermal effect arising from the intermicellar aggregation process induced by the water addition allowed a rough estimation of the scission energy (the energy necessary to break an Avogadro's number of micelles) and of the micellar aggregation number.

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Abstract  

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 (∆H dil) was calculated and graphed versus concentration in order to determine the micellization enthalpy (∆H mic) and CMC. In addition to the CMC and ∆H mic, 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 (∆G mic), entropy of the micellization (∆S mic), and specific heat capacity of the micellization (∆C P,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 ∆G mic was found to be negative, implying, as expected, that micellization occurs spontaneously once the CMC has been reached. The values of ∆G mic were found to become more negative with increasing temperature and the ∆S mic was found to decrease with increasing temperature in both models.

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Journal of Thermal Analysis and Calorimetry
Authors: Xing Xiaoling, Xue Liang, Zhao Fengqi, Yi Jianhua, Gao Hongxu, Xu Siyu, Pei Qing, Hao Haixia, and Hu Rongzu

diss H partial ) calculated by formula above are (2) and (3) respectively.The empirical formula of dilution enthalpy (Δ dil H 1,2 ) for CL-20 is

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the dissolution enthalpies of two of the products in 4 mol L −1 HCl at 298.15 K, respectively; is the dilution enthalpy of water in 4 mol L −1 HCl at 298.15 K; is the enthalpy value for the designed reaction. Results

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]. The dilution enthalpy Δ dil H (J) is determined by measuring thermal power P (μW) and flow rates of solution and solvent ( f A and f B, mg s −1 ): 1 in which m x,i is the initial molality of the solution before dilution and M x is the molar

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The value of c.m.c., was regarded as the concentration where the second derivate of the Δ H dil versus C s curve displayed zero—Eq. 8 . 8 The dilution enthalpy curve for ionic surfactants loses its sigmoidal shape. The determination of the

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heat of dilution and mixing via the y component, i.e., the active principle. Consequently, Δ H mix [( m x i )( m y i ) → m x f , m y f ] should coincide with Δ H dil ( m x i → m x f ) + Δ H *, i.e., the dilution enthalpy of the probe plus am

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