Abstract
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.
Highlights
Hydrophobic and hydrophilic interactions are competing effects influencing the kinetics and dynamics of many processes in chemistry, biochemistry, and physics
With Tm < 100 °C, C2OOEt can be regarded as a surface-active ionic liquids (SAILs) composed from a quaternary ammonium-based cation and a relatively small chloride anion
According to their melting points, the alcohol, ether, and polyether-functionalized compounds are molten salts with a relatively low Tm. These results demonstrate that the appropriate functionalization with a polar residue on the quaternary ammonium group crucially changes the thermal properties of these systems, leading to the formation of molten salts or even SAILs
Summary
Hydrophobic and hydrophilic interactions are competing effects influencing the kinetics and dynamics of many processes in chemistry, biochemistry, and physics. This class of cationic surfactants is probably the most studied model for investigating the effects of alkyl chain length and counterions on micellar properties.[4−12] little is known about quaternary ammonium salts carrying a functional group in addition to the long alkyl chain on the quaternary ammonium Such substitution should significantly affect headgroup hydration and counterion binding and allow the tuning of micelle properties. Examples for such compounds were synthesized by Swain et al.[13] who prepared various alkyldimethylammonium bromides and iodides with the fourth group being an alkyl chain spacer terminated by a carboxylate, ester, amide, or nitrile group. These authors found that the melting points of almost all of these compounds were below 200 °C, making their cations interesting as potential candidates for the preparation of surface-active ionic liquids (SAILs).[14]
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