Heat Capacity Changes Accompanying Micelle Formation upon Burial of Hydrophobic Tail of Nonionic Surfactants

Micellization of Zwitterionic Surfactant with Opposite Dipoles is Differently Affected by Anions

We used isothermal titration calorimetry to investigate the influence of hydrotropic anions on the micellization of the zwitterionic surfactants N-dodecylphosphocholine (DPC) and N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DPS), as well as the effect of these anions on the micellar interface. Our results showed an increased enthalpic contribution to the Gibbs energy for micellization when the hydrotropic anions contained CF3 or C6H5 hydrophobic groups. We found that the critical micelle concentration of DPS was more sensitive to the effects of the anions than that of DPC, likely because the positive charge of DPS is located closer to the micellar core. Overall, the magnitudes of the anion effects on the micellar properties of DPC and DPS were relatively minor compared with those observed in cationic micelles. These findings offer valuable insights into the role of dipole orientation and charge distribution in the interactions between zwitterionic surfactants and anions as well as the effects of hydrotropic anions on these systems.

Exploring the role of polymer hydrophobicity in polymer-metal binding thermodynamics

Metal-chelating polymers play a key role in rare-earth element (REE) extraction and separation processes. Often, these processes occur in aqueous solution, but the interactions among water, polymer, and REE are largely under-investigated in these applications. To probe these interactions, we synthesized a series of poly(amino acid acrylamide)s with systematically varied hydrophobicity around a consistent chelating group (carboxylate). We then measured the ΔH of Eu³⁺ chelation as a function of temperature across the polymer series using isothermal titration calorimetry (ITC) to give the change in heat capacity (ΔC_P). We observed an order of magnitude variation in ΔC_P (39-471 J mol¹ K^-1) with changes in the hydrophobicity of the polymer. Atomistic simulations of the polymer-metal-water interactions revealed greater Eu³⁺ and polymer desolvation when binding to the more hydrophobic polymers. These combined experimental and computational results demonstrate that metal binding in aqueous solution can be modulated not only by directly modifying the chelating groups, but also by altering the molecular environment around the chelating site, thus suggesting a new design principle for developing increasingly effective metal-chelating materials.

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.

Two-Step Micellization Model: The Case of Long-Chain Carboxylates in Water

The micellization behavior of the long-chain carboxylates-sodium 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.

Thermodynamics of the multi-stage self-assembly of pH-sensitive gradient copolymers in aqueous solutions

The self-assembly thermodynamics of pH-sensitive di-block and tri-block gradient copolymers of acrylic acid and styrene was studied for the first time using isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) performed at varying pH. We were able to monitor each step of micellization as a function of decreasing pH. The growth of micelles is a multi-stage process that is pH dependent with several exothermic and endothermic components. The first step of protonation of the acrylic acid monomer units was accompanied mainly by conformational changes and the beginning of self-assembly. In the second stage of self-assembly, the micelles become larger and the number of micelles becomes smaller. While solution acidity increases, the isothermal calorimetry data show a broad deep minimum corresponding to an exothermic process attributed to an increase in the size of hydrophobic domains and an increase in the structure's hydrophobicity. The minor change in heat capacity (ΔCp) confirms the structural changes during this exothermic process. The exothermic process terminates deionization of acrylic acid. The pH-dependence of the ζ-potential of the block gradient copolymer micelles exhibits a plateau in the regime corresponding to the pH-controlled variation of the micellar dimensions. The onset of micelle formation and the solubility of the gradient copolymers were found to be dependent on the length of the gradient block.

Structural Thermodynamics of myr-Src(2-19) Binding to Phospholipid Membranes

Many proteins are anchored to lipid bilayer membranes through a combination of hydrophobic and electrostatic interactions. In the case of the membrane-bound nonreceptor tyrosine kinase Src from Rous sarcoma virus, these interactions are mediated by an N-terminal myristoyl chain and an adjacent cluster of six basic amino-acid residues, respectively. In contrast with the acyl modifications of other lipid-anchored proteins, the myristoyl chain of Src does not match the host lipid bilayer in terms of chain conformation and dynamics, which is attributed to a tradeoff between hydrophobic burial of the myristoyl chain and repulsion of the peptidic moiety from the phospholipid headgroup region. Here, we combine thermodynamic information obtained from isothermal titration calorimetry with structural data derived from (2)H, (13)C, and (31)P solid-state nuclear magnetic resonance spectroscopy to decipher the hydrophobic and electrostatic contributions governing the interactions of a myristoylated Src peptide with zwitterionic and anionic membranes made from lauroyl (C12:0) or myristoyl (C14:0) lipids. Although the latter are expected to enable better hydrophobic matching, the Src peptide partitions more avidly into the shorter-chain lipid analog because this does not require the myristoyl chain to stretch extensively to avoid unfavorable peptide/headgroup interactions. Moreover, we find that Coulombic and intrinsic contributions to membrane binding are not additive, because the presence of anionic lipids enhances membrane binding more strongly than would be expected on the basis of simple Coulombic attraction.

The molecular shape of poly(propylenimine) dendrimer amphiphiles has a profound effect on their self assembly

The shape of dendrimer amphiphiles has an unexpected effect on their self-assembly. A series of diaminobutane poly(propylenimine) generation 3 dendrimer (DAB-dendr-(NH(2))(16)) amphiphiles has been synthesized, bearing an average of five (PD5), three (PD3) and one (PD1) palmitoyl group(s) per dendrimer molecule. Additionally DAB-dendr-(NH(2))(16) was derivatized with a layer of poly(ethylene glycol) (PEG, degree of polymerization = 12) groups and conjugated to an average of 1 palmitoyl group at the PEG end (PPD1). A final amphiphile resulted from the conjugation of DAB-dendr-(NH(2))(16) with 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-succinimidylpropionate (DSPE-PEG(3400)-SPA), i.e.: DPD5 (with 4 DSPE-PEG arms). The critical micellar concentration in aqueous media followed the trend: DPD5 < PD5 = PD3 < PD1 < PPD1 and amphiphiles eventually formed 10-20 nm monomolecular or multimolecular micelles and/or 200 nm spheres or tubules. Aggregation was entropy driven, as expected, for DPD5, PD5 and PD1 and enthalpy driven with the most hydrophilic compound PPD1, but was unexpectedly enthalpy driven for PD3. PD3 aggregates formed low capacity hydrophobic domains with a limited capacity for encapsulation of cyclosporine A; encapsulation levels (mole drug per mole polymer) were 0.099, 0.014, 0.099, and 0.735 for PD1, PD3, PD5, and DPD5 and, respectively. We conclude that star shaped amphiphiles such as PD3 are sterically hindered from self-assembling into high capacity hydrophobic domains in aqueous media. Amphiphile-membrane interactions were promoted by hydrophobic groups, but diminished by PEG moieties. DPD5 is the most suitable amphiphile for biomedical applications.

Novel behavior of heat of micellization of pluronics F68 and F88 in aqueous solutions

It is well understood that the heat of micellization for surfactants is monotonically decreased along with an increase in temperature. However, this behavior for polymeric surfactants has never been carefully examined. In this study, the heat of micellization of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronics F68 and F88) in water as a function of temperature is carefully examined by using a high-sensitivity differential scanning calorimeter (HSDSC). The critical micelle temperature (CMT) decreases along with an increase in the concentration of Pluronic F68 (or F88). The heat of micellization decreases along with an increase in the temperature, as expected, when the CMT is higher than 55 and 42 degrees C for Pluronics F68 and F88, respectively. It is interesting to observe that the heat of micellization increases along with the temperature while the temperature is below 55 and 42 degrees C for Pluronics F68 and F88, respectively. The enthalpy-entropy compensation phenomenon for the micellization of Pluronics F68 and F88 in connection with the hydrophobicity is discussed.

Molecular simulation study of peptide amphiphile self-assembly

We study the self-assembly of peptide amphiphile (PA) molecules, which is governed by hydrophobic interactions between alkyl tails and a network of hydrogen bonds between peptide blocks. We demonstrate that the interplay between these two interactions results in the formation of assemblies of different morphology, in particular, single beta-sheets connected laterally by hydrogen bonds, stacks of parallel beta-sheets, spherical micelles, micelles with beta-sheets in the corona, and long cylindrical fibers. We characterize the size distribution of the aggregates as a function of the molecular interactions. Our results suggest that the formation of nanofibers of peptide amphiphiles obeys an open association model, which resembles living polymerization.

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.