Thermodynamics of Micelle Formation as a Function of Temperature: A High Sensitivity Titration Calorimetry Study

Abstract

Titration calorimetry was employed to measure the critical micelle concentration (cmc) and the heat of demicellization A/fdemic of the four surfactants octyl glucoside, sodium dodecyl sulfate (SDS), sodium cholate, and sodium deoxycholate at temperatures between 10 and 70-80 °C. From these data, the thermodynamic parameters AGdemic, Anemic, and ACp,demic associated with the demicellization process were calculated. Titration calorimetry has the advantage that the cmc and the thermodynamic parameter AFÍáemic can be directly measured, whereas with other methods /fdemic has to be calculated from the temperature dependence of the cmc, which requires high precision for the cmc data. Changes in temperature caused large variations of A//demic and ASdemic, whereas AGdemic remained virtually constant. Therefore, the changes in enthalpy and entropy almost completely compensate each other. At room temperature, the entropy was found to be the dominant factor responsible for micellization, whereas at elevated temperatures contributions from enthalpy dominate. These observations are in agreement with data of other processes where hydrophobic effects play a major role and were used to discuss the nature of the driving forces that rule micelle formation at various temperatures. Furthermore, predictions regarding the degree of hydration of the micelle interior were made. It is shown that titration calorimetry is an easy and fast method to determine the cmc and the demicellization enthalpy from a single experiment. For surfactants with low aggregation numbers the titration curves could be simulated using a mass action model.

Glycoprotein-surfactant interactions: A calorimetric and spectroscopic investigation of the phytase-SDS system

“…[23,39]. The CMC value of SDS in the protein free solution is best described by the inflexion point of this part of the curve [39]. Four separate trials in pure water at 24°C showed CMC = 8.1±0.5 mM SDS, which is in good accordance with recent data based on other experimental techniques [40].…”

“…This saturation is governed by the appearance of micelles in the aqueous bulk (see below) and thus essentially reflects an "apparent CMC" in the presence of the protein. Hence, in accordance with the suggestions of Paula et al [39] we use the inflexion point (D in Fig. 6) to identify saturation.…”

“…Thus, the concentration of SDS in the calorimetric cell is raised to CMC and additional micelles will not be dissolved; c.f. [23,39]. The CMC value of SDS in the protein free solution is best described by the inflexion point of this part of the curve [39].…”

Blood-Brain Barrier Permeation: Molecular Parameters Governing Passive Diffusion

“…At the CMC D the surface pressure, , becomes independent of the drug concentration indicating micelle formation. Micelles formed by amphiphilic drugs are generally less stable than micelles formed by detergents (Paula et al, 1995). The pK a values of drugs tend to shift (to lower values for bases and to higher values for acids) with increasing concentration (Fischer, 1998;Mayer et al, 1988) leading to further aggregation and finally precipitation.…”

Thermodynamics of micellisation: Sodium dodecyl sulfate/sodium deoxycholate with polyethylene glycol and model drugs

“…Not all studies have focused on the use of SDS based surfactants, for example, in the same year results were published for other surfactants as a function of temperature, determining values for the CMC, ∆H mic , ∆G mic and ∆S mic [5]. At 292 K the CMC for one surfactant was at a minimum of 7.8 mM, and the demicellisation enthalpy (i.e.…”