Comparison of the DSC Curves Obtained for Aqueous Solutions of Nonionic and Ionic Surfactants

Thermodynamics of micellization from heat-capacity measurements

Differential scanning calorimetry (DSC), the most important technique for studying the thermodynamics of structural transitions of biological macromolecules, is seldom used in quantitative thermodynamic studies of surfactant micellization/demicellization. The reason for this could be ascribed to an insufficient understanding of the temperature dependence of the heat capacity of surfactant solutions (DSC data) in terms of thermodynamics, which leads to problems with the design of experiments and interpretation of the output signals. We address these issues by careful design of DSC experiments performed with solutions of ionic and nonionic surfactants at various surfactant concentrations, and individual and global mass-action model analysis of the obtained DSC data. Our approach leads to reliable thermodynamic parameters of micellization for all types of surfactants, comparable with those obtained by using isothermal titration calorimetry (ITC). In summary, we demonstrate that DSC can be successfully used as an independent method to obtain temperature-dependent thermodynamic parameters for micellization.

Effect of hydrophobic interactions on volume and thermal expansivity as derived from micelle formation

Volumetric parameters have long been used to elucidate the phenomena governing the stability of protein structures, ligand binding, or transitions in macromolecular or colloidal systems. In spite of much success, many problems remain controversial. For example, hydrophobic groups have been discussed to condense adjacent water to a volume lower than that of bulk water, causing a negative contribution to the volume change of unfolding. However, expansivity data were interpreted in terms of a structure-making effect that expands the water interacting with the solute. We have studied volume and expansivity effects of transfer of alkyl chains into micelles by pressure perturbation calorimetry and isothermal titration calorimetry. For a series of alkyl maltosides and glucosides, the methylene group contribution to expansivity was obtained as 5 uL/(mol K) in a micelle (mimicking bulk hydrocarbon) but 27 uL/(mol K) in water (20 °C). The latter value is virtually independent of temperature and similar to that obtained from hydrophobic amino acids. Methylene contributions of micellization are about -60 J/(mol K) to heat capacity and 2.7 mL/mol to volume. Our data oppose the widely accepted assumption that water-exposed hydrophobic groups yield a negative contribution to expansivity at low temperature that would imply a structure-making, water-expanding effect.

Volume and expansivity changes of micelle formation measured by pressure perturbation calorimetry

We present the application of pressure perturbation calorimetry (PPC) as a new method for the volumetric characterization of the micelle formation of surfactants. The evaluation is realized by a global fit of PPC curves at different surfactant concentration ranging, if possible, from below to far above the CMC. It is based on the knowledge of the temperature dependence of the CMC, which can for example be characterized by isothermal titration calorimetry. We demonstrate the new approach for decyl-β-maltopyranoside (DM). It shows a strong volume increase upon micelle formation of 16 ± 2.5 mL/mol (+4%) at 25 °C, and changes with temperature by -0.1 mL/(mol K). The apparent molar expansivity (E(S)) decreases upon micelle formation from 0.44 to 0.31 mL/(mol K) at 25 °C. Surprisingly, the temperature dependence of the expansivity of DM in solution (as compared with that of maltose) does not agree with the principal behavior described for polar (E(S)(T) decreasing) and hydrophobic (E(S)(T) increasing) solutes or moieties before. The results are discussed in terms of changes in hydration of the molecules and internal packing of the micelles and compared with the volumetric effects of transitions of proteins, DNA, lipids, and polymers.

Thermodynamics of micelle formation of alkyltrimethylammonium chlorides from high performance electric conductivity measurements

Understanding micelle formation requires its complete thermodynamic characterization. In this work micellization of the model ionic surfactants decyltrimethylammonium chloride (DETAC), dodecyltrimethylammonium chloride (DTAC) and tetradecyltrimethylammonium chloride (TTAC) was investigated by high performance conductivity measurements, using instrumentation developed in our laboratory. The temperature dependence of the critical micelle concentration exhibits a minimum characterized by Delta(mic)H(0)=0 (endothermic to exothermic process) and the degree of micelle ionization increases slightly with increasing temperature. The temperature change of Delta(mic)S(0) indicates that the process of micellization is entropically driven. Delta(mic)G(0) is always negative and slightly temperature dependent. The temperature dependence of the thermodynamic parameters is discussed in terms of the alkyl chain length and nature of the counterion. The micellization process is more favourable for surfactants with longer alkyl chain length and larger (less hydrated) counterions.

Denaturation of Bovine β-Lactoglobulin in the Presence of n-Octyl-, Decyl-, and Dodecyldimethylphosphine Oxides

Denaturation of bovine beta-lactoglobulin (beta-L) in pH 2.9, 0.2 M glycine buffer was investigated by DSC in the presence of three nonionic surfactants, n-octyldimethylphosphine oxide (APO8), n-decyldimethylphosphine oxide (APO10), and n-dodecyldimethylphosphine oxide (APO12), and by ITC at temperatures from 25 to 61 degrees C. The thermal transition was eliminated when the molar ratio of surfactant to beta-L was between 88 and 133, 21-25, and 11-22 depending upon the protein concentration for APO8, APO10, and APO12, respectively. A protocol was developed that may be used for future studies that involve ligands with large temperature-dependent heats of dilution. Approximately 30 mol of APO10 and APO12 per mole of beta-L were bound at 45 degrees C and 37 degrees C, respectively, with an average affinity of (2.5 +/- 0.7) x 10(3) M(-1). This amount of surfactant would cover about 50% of the protein surface and may correspond to a new class of nonspecific neutral ligand binding sites that facilitate the digestion of lipids by neonatal calves. Titration of beta-L into 2% solutions of APO8, APO10, and APO12 at various temperatures between 25 and 61 degrees C yielded enthalpy changes with the same temperature dependence as for the thermal denaturation of beta-L without surfactant at much higher temperatures.

The temperature dependence of the heat capacity change for micellization of nonionic surfactants

The thermodynamic parameters that govern micelle formation by four different nonionic surfactants were investigated by ITC and DSC. These included n-dodecyldimethylphosphine oxide (APO12), Triton X-100 (TX-100), n-octyltetraoxyethylene (C8E4), and N,N-dimethyloctylamine-N-oxide (DAO8). All of these surfactants had been previously investigated by solution calorimetry over smaller temperature ranges with conflicting conclusions as to the temperature dependence of the heat capacity change, DeltaCp, for the process. The temperature coefficient of the heat capacity change, B (cal/mol K2), was derived from the enthalpy data that were obtained at small intervals over a broad temperature range. The values obtained for each of the surfactants at 298.2 K for DeltaCp and B were -155+/-2 and 0.50+/-0.36 (APO12), -97+/-3 and -0.24+/-0.18 (TX-100), -105+/-2 and 1.0+/-0.3 (C8E4), and -82+/-1 and 0.36+/-0.04 (DAO8), cal/mol K and cal/mol K2, respectively. The resulting B-values did not correlate with the cmc, aggregation number, or structure of the monomer in an obvious way, but they were found to reflect the relative changes in hydration of the polar and nonpolar portions of the surfactant molecule as the micelles are formed. An analysis of the data obtained from DSC scans was used to describe the temperature dependence of the critical micelle concentration, cmc. An abrupt increase in heat capacity was observed for TX-100 and C8E4 solutions of 36.5+/-0.5 and 21+/-5 cal/mol K, respectively, as the temperature of the scan passed through the cloud point. This change in heat capacity may reflect the increased monomer concentration of the solutions that accompanies phase separation, although other interpretations of this jump are possible.

A comparative study of the physicochemical properties of perfluorinated and hydrogenated amphiphiles

In this work we studied and compared the physicochemical properties of perfluorinated (sodium perfluoroheptanoate, C7FONa, and perfluorooctanoate, C8FONa) and hydrogenated (sodium octanoate, C8HONa, decanoate, C10HONa, and dodecanoate, C12HONa) amphiphiles. First, we determined their Krafft points to study the solubility and appropriate temperature range of micellization of these compounds. The critical micelle concentration (cmc) and ionization degree of micellization (beta) as a function of temperature (T) were estimated from conductivity data. Plots of cmc vs T appear to follow the typical U-shaped curve with a minimum T(min). The results show that the surfactants with CF2/CH2 ratio of 1.5 between alkyl chains (C12HONa-C8FONa and C10HONa-C7FONa) have nearly the same minimum value for cmc against temperature. The comparison between the cmc of hydrogenated amphiphiles and the corresponding perfluorinated amphiphiles must be done at this point. Thermodynamic functions of micellization were obtained by applying different theoretical models and choosing the one that best fit our experimental data. Although perfluorinated and hydrogenated amphiphiles present similar thermodynamic behavior, we have found a variation of 1.3 to 1.7 in the CF2/CH2 ratio, which did not remain constant with temperature. In the second part of this study the apparent molar volumes and adiabatic compressibilities were determined from density and ultrasound velocity measurements. Apparent molar volumes at infinite dilution presented the ratio 1.5 between alkyl chains again. However, apparent molar volumes upon micellization for sodium perfluoroheptanoate indicated a different aggregation pattern.

Observation of complex thermal transitions for mixed micelle solutions containing alkyldimethylphosphine oxides and phospholipids and the accompanying cloud points

The thermal properties of various mixtures of two nonionic surfactants, decyldimethylphosphine oxide (APO10) and dodecyldimethylphosphine oxide (APO12) and two phospholipids, dimyristoylphosphatidyl choline (DMPC) and dipalmitoylphosphatidyl choline (DPPC), were examined by differential scanning calorimetry at various mole fractions. The addition of APO12 to DMPC multilamellar vesicles lowered the temperature of the main transition, produced considerable broadening, and eliminated the pre-transition. Phase separation, as evidenced by the existence of a cloud point, T(cp), occurred when the mole fraction of APO12, with respect to DMPC was 0.58 and above. A small abrupt increase in heat capacity was observed at, or slightly above, the cloud point of APO12 and all mixed micelle solutions. It appeared that mixed micelles coexisted with mixed bilayers when the mole fraction was between 0.58 and 0.75 and perhaps as low as a mole ratio of 0.32. All of the mixtures, except APO12/DMPC, exhibited a clear endotherm below the temperature corresponding to the cloud point, which likely reflects the growth in micellar size. Overlapping chain length dependent endothermic peaks, perhaps resulting from reorganization and/or continued association of the micelles, were observed above the cloud point for all of the mixtures except for APO10/DMPC solutions. However, solutions of mixed micelles consisting of APO10/DMPC with mole fractions of surfactant between 0.81 and 0.93 portrayed a broad unidentified exotherm of about 2+/-1 kcal/mol, which was centered nearly 10-20 degrees C above the cloud point.

Triton promotes domain formation in lipid raft mixtures

Biological membranes are supposed to contain functional domains (lipid rafts) made up in particular of sphingomyelin and cholesterol, glycolipids, and certain proteins. It is often assumed that the application of the detergent Triton at 4 degrees C allows the isolation of these rafts as a detergent-resistant membrane fraction. The current study aims to clarify whether and how Triton changes the domain properties. To this end, temperature-dependent transitions in vesicles of an equimolar mixture of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, egg sphingomyelin, and cholesterol were monitored at different Triton concentrations by differential scanning calorimetry and pressure perturbation calorimetry. Transitions initiated by the addition of Triton to the lipid mixture were studied by isothermal titration calorimetry, and the structure was investigated by (31)P-NMR. The results are discussed in terms of liquid-disordered (ld) and -ordered (lo) bilayer and micellar (mic) phases, and the typical sequence encountered with increasing Triton content or decreasing temperature is ld, ld + lo, ld + lo + mic, and lo + mic. That means that addition of Triton may create ordered domains in a homogeneous fluid membrane, which are, in turn, Triton resistant upon subsequent membrane solubilization. Hence, detergent-resistant membranes should not be assumed to resemble biological rafts in size, structure, composition, or even existence. Functional rafts may not be steady phenomena; they might form, grow, cluster or break up, shrink, and vanish according to functional requirements, regulated by rather subtle changes in the activity of membrane disordering or ordering compounds.

Thermodynamics of Micellization of Alkyldimethylbenzylammonium Chlorides in Aqueous Solutions

Specific conductivities of alkyldimethylbenzylammonium chlorides (alkyl=decyl-, dodecyl-, tetradecyl-, and hexadecyl-) in aqueous solutions were measured as a function of molality and temperature. Critical micelle molalities (cmc) and degrees of ionization of the micelles, beta, were estimated from the dependence of the specific conductivity on molality. It was found that temperature dependence of cmc is U-shaped with a minimum shifting toward higher temperatures with a decrease in the chain length of the alkyl group. The temperature dependence of ln xcmc (where xcmc is the cmc in mole fraction units) was fitted to the equation of Muller, which we modified by taking into account the temperature dependencies both of beta and of change in heat capacity upon micellization. From the fitting parameters, Gibbs free energies, enthalpies, and entropies of micellization as a function of temperature were estimated.