Effect of cationic structure of surface active ionic liquids on their micellization: A thermodynamic study

Simple ApproximaTion for Aggregation Number Determination by Isothermal Titration Calorimetry: STAND-ITC

A new proposal to obtain aggregation numbers from isothermal titration calorimetry dilution experiments is described and tested using dodecyl trimethyl ammonium bromide, dodecyl methylimidazolium chloride, dodecyl methylimidazolium sulfonate, and didecyl methylimidazolium chloride aqueous solutions at different temperatures. The results were compared to those obtained from fluorescence measurements and also with data from the literature. In addition to the aggregation number, the molar free energy to transfer a solute molecule from the aggregate to the bulk solution, the enthalpy corresponding to the formation of the self-assembled suprastructures, the molar heat corresponding to the dilution of monomers and aggregates, and an offset parameter to account for unpredictable external contributions are simultaneously obtained using the same method. The new equations are compared to those obtained from previous proposals, and they are also analyzed in detail to assess the impact of each fitting parameter in the profile of the calorimetric isotherm. This new approach has been implemented in a computational code that automatically determines the fitting parameters as well as the corresponding statistical uncertainties for the large variety of calorimetric profiles that have been tested. Given the high sensitivity of the dilution experiments to the aggregation number for relatively small assemblies, our approach is proposed also to quantify the oligomerization state of biomolecules such as proteins and peptides.

Applications of isothermal titration calorimetry in pure and applied research from 2016 to 2020

The last 5 years have seen a series of advances in the application of isothermal titration microcalorimetry (ITC) and interpretation of ITC data. ITC has played an invaluable role in understanding multiprotein complex formation including proteolysis-targeting chimeras (PROTACS), and mitochondrial autophagy receptor Nix interaction with LC3 and GABARAP. It has also helped elucidate complex allosteric communication in protein complexes like trp RNA-binding attenuation protein (TRAP) complex. Advances in kinetics analysis have enabled the calculation of kinetic rate constants from pre-existing ITC data sets. Diverse strategies have also been developed to study enzyme kinetics and enzyme-inhibitor interactions. ITC has also been applied to study small molecule solvent and solute interactions involved in extraction, separation, and purification applications including liquid-liquid separation and extractive distillation. Diverse applications of ITC have been developed from the analysis of protein instability at different temperatures, determination of enzyme kinetics in suspensions of living cells to the adsorption of uremic toxins from aqueous streams.

Interplay between aggregation number, micelle charge and hydration of catanionic surfactants

Catanionic mixtures are commonly used in applications due to synergetic properties of both cationic and anionic surfactants. To better understand the mechanism of the micellization process of salt-free catanionic surfactants, alkyltrimethylammonium alkanecarboxylates, [CxMe3N]+[Cy]-, with medium to long alkyl chains on both cation and anion (x,y = 6-10), were investigated in aqueous solution by density and zeta potential measurements, isothermal titration calorimetry (ITC), and dielectric relaxation spectroscopy (DRS). The obtained ITC data was analysed with the help of a two-step model equation, yielding the thermodynamic parameters, micelle charge and aggregation numbers. Comparison with the "parent" decyltrimethylammonium chloride and sodium decanoate reveals that combined dehydration of both alkyl chains increases entropy upon micellization. In the first step neutral smaller micelles with partly dehydrated alkyl chains are formed, while in the second step larger charged micelles with fully dehydrated alkyl chains are equally favourable. At low temperature both formations are thermodynamically equivalent, while with increasing temperature neutral micelles become more entropically favourable and charged micelles more enthalpically favourable. The resulting average micelle charge and average aggregation number are decreasing with temperature. From the DRS spectra, effective hydration numbers of the free monomers and micelles were deduced and are comparable to the "parent" cationic surfactant micelles.

Insight into the Hydration of Cationic Surfactants: A Thermodynamic and Dielectric Study of Functionalized Quaternary Ammonium Chlorides

Hydrophobic interactions are one of the main thermodynamic driving forces in self-assembly, folding, and association processes. To understand the dehydration-driven solvent exposure of hydrophobic surfaces, the micellization of functionalized decyldimethylammonium chlorides, XC₁₀Me₂N⁺Cl^-, with a polar functional group, X = C₂OH, C₂OMe, C₂OC₂OMe, C₂OOEt, together with the "reference" compound decyltrimethylammonium chloride, C₁₀Me₃N⁺Cl^-, was investigated in aqueous solution by density measurements, isothermal titration calorimetry (ITC), and dielectric relaxation spectroscopy (DRS). From the density data, the apparent molar volumes of monomers and micelles were estimated, whereas the ITC data were analyzed with the help of a model equation, yielding the thermodynamic parameters and aggregation number. From the DRS spectra, effective hydration numbers of the free monomers and micelles were deduced. The comprehensive analysis of the obtained results shows that the thermodynamics of micellization are strongly affected by the nature of the functional group. Surprisingly, the hydration of micelles formed by surfactant cations with a single alkyl chain on quaternary ammonium is approximately the same, regardless of the alkyl chain length or functionalization of the headgroup. However, notable differences were found for the free monomers where increasing polarity lowers the effective hydration number.

Micellization, surface activities and thermodynamics study of pyridinium-based ionic liquid surfactants in aqueous solution

The micellization and surface activity properties of long-chain pyridinium ionic liquids n-alkyl-3-methylpyridinium bromide ([C_nmpy][Br], n: the carbon numbers of hydrophobic tails, n = 12, 14, 16) in aqueous solution were systematically investigated through electronic conductivity measurement, surface tension, and ultraviolet-absorption spectra. The surface chemical parameters and thermodynamics parameters were obtained. The [C_nmpy][Br] ionic liquids exhibit higher surface activities than conventional surfactants with corresponding alkyl chain lengths. The effects of inorganic salts (LiBr, NaBr, MgBr₂), organic alcohols (C₂H₅OH, C₃H₇OH, C₄H₉OH, C₅H₁₁OH) and temperature on the critical micelle concentration (CMC) values of [C_nmpy][Br] aqueous solutions were also investigated. The CMC values remarkably decreased with the addition of inorganic salts. The CMC values increased slightly in the presence of ethanol, but decreased gradually as the chain length of the alcohol increased. The CMC values assumed a trend of decreasing and then increasing with the increase of temperature. The calculation results of thermodynamic parameters show that both adsorption and micellization processes of [C_nmpy][Br] are spontaneous; the enthalpy of [C₁₂mpy][Br] is negative at 293.15 K and becomes negative with temperature increasing. For [C₁₄mpy][Br] and [C₁₆mpy][Br] this transition occurs at 288.15 K and the micellization process is entropy-driven in the investigated temperature range.

This paper contains details on the micellization, surface activity properties, thermodynamics and effects of additives of [C_nmpy][Br].

Total Description of Intrinsic Amphiphile Aggregation: Calorimetry Study and Molecular Probing

Isothermal titration calorimetry (ITC) is an apt tool for a total thermodynamic description of self-assembly of atypical amphiphiles such as anionic boron cluster compounds (COSAN) in water. Global fitting of ITC enthalpograms reveals remarkable features that differentiate COSAN from classical amphiphiles: (i) strong enthalpy and weak entropy contribution to the free energy of aggregation, (ii) low degree of counterion binding, and (iii) very low aggregation number, leading to deviations from the ideal closed association model. The counterion condensation obtained from the thermodynamic model was compared with the results of 7Li DOSY NMR of Li[COSAN] micelles, which allows direct tracking of Li cations. The basic thermodynamic study of COSAN alkaline salt aggregation was complemented by NMR and ITC experiments in dilute Li/NaCl and acetonitrile aqueous solutions of COSAN. The strong affinity of acetonitrile molecules to COSAN clusters was microscopically investigated by all-atomic molecular dynamics simulations. The impact of ionic strength on COSAN self-assembling was comparable to the behavior of classical amphiphiles, whereas even a small amount of acetonitrile cosolvent has a pronounced nonclassical character of COSAN aggregation. It demonstrates that large self-assembling changes are triggered by traces of organic solvents.