The micellization behavior of the long-chain carboxylatessodium and potassium octanoate (NaC8 and KC8), sodium decanoate (NaC10), potassium decanoate (KC10), cesium decanoate (CsC10), choline ...decanoate (ChC10), and sodium dodecanoate (NaC12)in aqueous solutions were studied using isothermal titration calorimetry (ITC) in the temperature range between 288.15 and 328.15 K. Experimental data were analyzed by help of an improved model treating the micellization process as a two-step process. Furthermore, consideration of the state of the stock and titrated solutions during the experiment allowed for the elimination of all usually used empirical parameters. The proposed approach represents thus an essential improvement of the thermodynamic analysis of the micellization process and turned out to be (only) effective for the description of the micellization at carboxylates with moderate alkyl chain length (C8 and C10). By fitting the model equation to the experimental data, all the thermodynamic parameters of micellization for both steps were estimated. It was found that the first step is endothermic and thus a solely entropy driven processes in the studied temperature range for all investigated systems. The same goes also for the second step, except for KC10, Cs10, and NaC12 where at temperatures above ∼320 K the micellization was detected as an exothermic process. The delicate balance between entropy and enthalpy results in weak temperature dependence of (negative) Gibbs free energy which turned out as almost counterion independent quantity. The carboxylic groups are namely able to form H-bonds with water molecules, and it is quite likely that they remain strongly hydrated even upon micellization. Thus, the interactions with counterions are less expressed in comparison to those observed by other ionic surfactants (alkyl sulfates and cationic surfactants), where the micellization process was found to be an exothermic process even below ∼300 K.
Molar conductivities, Λ, of dilute solutions of the ionic liquids (ILs) 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF4), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4), ...1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), 1-hexyl-3-methylimidazolium tetrafluoroborate (hmimBF4), and 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (hmimNTf2) in acetonitrile (AN) were determined as a function of temperature in the range 273.15–313.15 K. The data were analyzed with Barthel’s lcCM model to obtain limiting molar conductivities, Λ∞(T), and association constants, K A °(T) of these electrolytes. The temperature dependence of these parameters, as well as the extracted limiting cation conductivities, λ i ∞, were discussed. Additionally, dielectric spectra for hmimNTf2 + AN were analyzed in terms of ion association and ion solvation and compared with the inference from conductivity. It appears that in dilute solutions the imidazolium ring of the cations is solvated by ∼6 AN molecules that are slowed by a factor of ∼8–10 compared to the bulk-solvent dynamics. Ion association of imidazolium ILs to contact ion pairs is only moderate, similar to common 1:1 electrolytes in this solvent.
The relative position of the hydroxylic and carboxylic groups in the isomeric hydroxybenzoate (HB) anions is experimentally known to have a large impact on the thermodynamics of micellization of ...cationic surfactants, such as dodecyltrimethylammonium chloride (DTAC), and on the structure of the resulting micelles. To understand the effect of the different isomers on the molecular level, we employed atomistic molecular dynamics simulations to study systems containing infinitely long cylindrical DTAC micelles in aqueous solutions of the sodium salts of all three isomers of HB at a temperature and a pressure of 298.15 K and 1 atm. In all studied systems, the number of DTAC unimers is identical to the number of HB anions. At this concentration, the initially cylindrical micelles remain stable, irrespective of the nature of the isomer, whereas micelles rapidly disintegrated in the absence of HB anions. The HB isomers decrease the line density of unimers along the micellar axis and its concomitant thickness in the order o-HB > m-HB > p-HB. It is further observed that o-HB anions penetrate more deeply into the micellar core, induce a more ordered internal structure of the micelle, and are oriented more strongly than the other two isomers. In addition, the ortho isomer shows two different preferential orientations with respect to the radial direction of the cylindrical micelle; it can either be incorporated almost completely into the micelle or it can be attached through hydrogen bonding to one of those o-HB anions that are already incorporated into the micelle, and thus stick out of the micellar surface.
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.
Ionic liquids belong to the most investigated systems in the recent years and this field is still significantly growing with an increased focus on developing ionic liquids for specific applications, ...along with fundamental research. In the present featured paper the similarity and differences between common, "classical" electrolytes and common surfactants and ionic liquids along with the surface-active ionic liquids are discussed in order to stress their significance and point out on their weak points. A short survey of the literature data reveals namely that ionic liquids in solutions behave like "classical" electrolytes and can be described by existing models in the range of their validity. There is still a lack of models describing well the concentrated electrolyte solutions and here ionic liquids, less limited by solubility as common electrolytes, can serve as model systems. The micellization of surface-active ionic liquids in aqueous solutions can be described in the same way as for common surfactants, but surface-active ionic liquids offer more possibilities to study specific ion and isomer effects. They are also quite promising systems to study the aggregation processes in non-aqueous solutions.
The model of counterion-induced metallacarborane aggregation in water was verified on sodium bis(1,2-dicarbollide), one of the most studied examples of metallacarboranes. This case study introduces ...a novel approach in anionic boron cluster self-assembly that resembles the behavior of polyoxometalate nanoions. The key prerequisite of the counterion binding to the aggregates is the strongly uneven charge distribution of cobalt bis(dicarbollide) clusters. Simultaneously, the size of the counterion should fit into the void between the metallacarborane clusters within the aggregate. In consequence, compact pentamers with two bound sodium cations via Coulombic-driven assembly are formed. This behavior cannot be explained solely by the hydrophobic effectthe paradigm of metallacarborane aggregation in the current literature. The pentamers can merge into bigger nanostructures via the second aggregation mechanism that sheds light on the complex behavior of metallacarboranes in water. The proof-of-concept of counterion-induced metallacarborane aggregation and the proposed total description of the self-assembly via the two-process model are results of calorimetry modeling, quantum chemistry calculations, and molecular dynamics simulations.
A systematic investigation of the micellization process of a biocompatible zwitterionic surfactant 3-(3-cholamidopropyl)-dimethylammonium-1-propanesulfonate (CHAPS) has been carried out by isothermal ...titration calorimetry (ITC) at temperatures between 278.15 K and 328.15 K in water, aqueous NaCl (0.1, 0.5, and 1 M), and buffer solutions (pH = 3.0, 6.8, and 7.8). The effect of different cations and anions on the micellization of CHAPS surfactant has been also examined in LiCl, CsCl, NaBr, and NaI solutions at 308.15 K. It turned out that the critical micelle concentration, cmc, is only slightly shifted toward lower values in salt solutions, whereas in buffer media it remains similar to its value in water. From the results obtained, it could be assumed that CHAPS behaves as a weakly charged cationic surfactant in salt solutions and as a nonionic surfactant in water and buffer medium. Conventional surfactants alike, CHAPS micellization is endothermic at low and exothermic at high temperatures, but the estimated enthalpy of micellization, ΔH M 0, is considerably lower in comparison with that obtained for ionic surfactants in water and NaCl solutions. The standard Gibbs free energy, ΔG M 0, and entropy, ΔS M 0, of micellization were estimated by fitting the model equation based on the mass action model to the experimental data. The aggregation numbers of CHAPS surfactant around cmc, obtained by the fitting procedure also, are considerably low (n agg ≈ 5 ± 1). Furthermore, some predictions about the hydration of the micelle interior based on the correlation between heat capacity change, Δc p,M 0, and changes in solvent-accessible surface upon micelle formation were made. CHAPS molecules are believed to stay in contact with water upon aggregation, which is somehow similar to the micellization process of short alkyl chain cationic surfactants.
•Electrical conductivity was measured for five carboxylates in a broad temperature range.•CMC and β parameter were estimated.•By using pseudo-phase separation model and Gibbs–Helmholtz equation ...thermodynamic parameters of micellization were obtained.•Δmiccpo were determined from the temperature dependence of ΔmicHo.•Relation between Δmiccpo and the removal of water from the contact with surfactant molecules is discussed.
The micellization behaviour of the long-chain carboxylates: sodium octanoate (NaC8), sodium decanoate (NaC10), potassium decanoate (KC10), caesium decanoate (CsC10) and sodium dodecanoate (NaC12) in aqueous solutions were studied using electrical conductivity measurements between the temperatures (278.15 and 328.15)K. By using the pseudo-phase separation model and Gibbs–Helmholtz equation, the thermodynamic parameters for the micellization process were obtained from the temperature dependence of the critical micelle concentration, CMC, and the degree of micelle ionisation, β. The heat capacity of micellization, Δmiccpo, estimated from the temperature dependence of enthalpy of micellization, ΔmicHo, were correlated with the size of the non-polar accessible surface area of the solvent, which is removed from contact with water during micellization. In all systems investigated, results suggest that the micelle core is still in contact with water molecules.
Effect of NaCl on the enthalpic and self-association behavior of dodecyltrimethylammonium chloride at 298.15
K.
Thermodynamics of micelle formation of the cationic surfactant dodecyltrimethylammonium ...chloride (DTAC) in water and aqueous NaCl solutions were investigated. Isothermal titration calorimetry (ITC) has been used to study the effect of the added NaCl on the critical micelle concentration,
cmc, and enthalpy of micellization, Δ
micH
o
, between 278.15 and 318.15
K. Gibbs free energy, Δ
micG
o
, and entropy, Δ
micS
o
, were deduced by taking into account the counterion binding. From the temperature dependence of Δ
micH
o
the heat capacities of micellization,
Δ
mic
c
p
o
, were determined.
NaCl shifts
cmc strongly towards lower values, indicating the screening of the repulsions of the polar head groups by counterions; but it influences Δ
micH
o
at higher concentrations of salt only.
Δ
micG
o
is always negative and slightly temperature dependent. The temperature dependence of Δ
micS
o
indicates that the process of micellization is entropically driven. Δ
micH
o
decreases strongly with increasing temperature and passes through zero (endothermic to exothermic processes). The temperature dependence of the critical micelle concentration exhibits a minimum characterized by Δ
micH
o
=
0, where Δ
micG
o
is of purely entropic contribution in all solutions.
Δ
mic
c
p
o
are strongly negative in all solvents, relating directly to the removal of water accessible non-polar surface of DTAC in the presence of excess counterions also.
PDF
