Aqueous Solutions Of Amphiphiles Research Articles

Surface tension and adsorption kinetics of amphiphiles in aqueous solutions: The role of carbon chain length and temperature

The effects of carbon chain length and temperature were investigated on adsorption kinetics and surface tension of a group of slightly volatile, short carbon chain molecules: 1-octanol, 1-hexanol, and 1-butanol. Experiments were performed in a closed chamber where simultaneous adsorption from both sides of the vapor/liquid interface was considered. The dynamic (time dependent) and steady-state surface tensions were found to decrease with temperature ranging from 10°C to 35°C. It was shown that, at the final steady-state, the effect of adsorption from the vapor phase was much more important than that from the liquid phase especially for short carbon chain molecules (e.g., 1-butanol). The modified Langmuir equation of state and modified kinetic transfer equation, which account for adsorption from both sides of a vapor/liquid interface, were used to model the experimental data of the steady-state and dynamic surface tension, respectively. Modeling results showed that the equilibrium constants and adsorption rate constants were increased with temperature and carbon chain length. The maximum surface concentration showed a decrease with temperature and an increase with carbon chain length. Some variations in the fitting parameters were observed in the dynamic modeling. These variations may be due to the experimental errors or the limitations of the proposed model.

Micellization of Monomeric and Poly-ω-methacryloyloxyundecyltrimethylammonium Surfactants

We have used small-angle neutron scattering to study how micelle morphology of the tail-polymerizable surfactants MUTAB and MUTAC (ω-methacryloyloxyundecyltrimethylammonium bromide and chloride) is affected by classic self-assembly modifiers such as temperature changes, salt addition, and counterion exchange, as a function of their conversion from monomer into polymer amphiphile in aqueous solution. Contrary to common assumptions about polymerized surfactants, these systems remain in dynamic equilibrium under all conditions examined and at all conversions (except for a small amount of high-molecular-weight precipitation by MUTAC). Counterintuitively, the polymerized methacrylate backbone has little influence on aggregate morphology, except for the formation of rod-like mixed micelles of polymerized and unpolymerized surfactant at intermediate conversions. The addition of salt produces a transition to rod-like micelles at all conversions except in the unpolymerized surfactant, which has some characteristics of an asymmetric bolaform surfactant and retains its spheroidal geometry under almost all conditions.

Modeling the Tetraphenylalanine-PEG Hybrid Amphiphile: From DFT Calculations on the Peptide to Molecular Dynamics Simulations on the Conjugate

The conformational properties of the hybrid amphiphile formed by the conjugation of a hydrophobic peptide with four phenylalanine (Phe) residues and hydrophilic poly(ethylene glycol), have been investigated using quantum mechanical calculations and atomistic molecular dynamics simulations. The intrinsic conformational preferences of the peptide were examined using the building-up search procedure combined with B3LYP/6-31G(d) geometry optimizations, which led to the identification of 78, 78, and 92 minimum energy structures for the peptides containing one, two, and four Phe residues. These peptides tend to adopt regular organizations involving turn-like motifs that define ribbon or helical-like arrangements. Furthermore, calculations indicate that backbone···side chain interactions involving the N-H of the amide groups and the π clouds of the aromatic rings play a crucial role in Phe-containing peptides. On the other hand, MD simulations on the complete amphiphile in aqueous solution showed that the polymer fragment rapidly unfolds maximizing the contacts with the polar solvent, even though the hydrophobic peptide reduce the number of waters of hydration with respect to an individual polymer chain of equivalent molecular weight. In spite of the small effect of the peptide in the hydrodynamic properties of the polymer, we conclude that the two counterparts of the amphiphile tend to organize as independent modules.

Catanionic Mixtures Forming Gemini-like Amphiphiles

The properties of aqueous mixtures of cationic species with alkyl dicarboxylic acid compounds have been studied. The cationic compounds used in this study were tertiary amine-type N-methyl-N-(2,3-dioxypropyl)hexadecylamine (C16amine) and quaternary ammonium-type N,N-dimethyl-N-(2,3-dioxypropyl)hexadecylammonium chloride (C16Q). The alkyl dicarboxylic acid compounds used were HOOC(CH(2))(10)COOH (C12H) and its sodium salt (C12Na). Three aqueous mixtures were examined in this study: (System I) C16amine + C12H, (System II) C16Q + C12Na, and (System III) C16Q + C12H. The solution pH was set at 12 for System III. The combination of (1)H-NMR and mass spectroscopy data has suggested that a stoichiometric complex is formed in the aqueous solutions at a mole fraction of C12H (or C12Na) = 0.33. Here, the C12H (or C12Na) molecule added to the system bridges two cationic molecules, like a spacer of gemini surfactants. In fact, the static surface tensiometry has demonstrated that the stoichiometric complex behaves as gemini-like amphiphiles in aqueous solutions. Our current study offers a possible way for easily preparing gemini surfactant systems.

Concentration and pH-dependent aggregation behavior of an l-histidine based amphiphile in aqueous solution

The surface activity and self-assembly behavior of zwitterionic amphiphile N-(2-hydroxydodecyl)- l-histidine (C 12HHis) were studied in phosphate buffers of pH 2 and 13 using surface tension and fluorescence probe techniques, respectively. Transmission electron microscopic images of the aggregates have revealed existence of nano-size vesicles in dilute solutions of both acidic and basic pH. In basic medium, the vesicles are converted to tubular aggregates upon increase of surfactant concentration. The nanotubes undergo phase transition to form elongated or small rod-like micelles at a much higher concentration of the amphiphile. The vesicles and nanotubes were found to become more stable upon addition of 10 mol% of cholesterol.

Amphiphile self-aggregation: An attempt to reconcile the agreement–disagreement between the enthalpies of micellization determined by the van’t Hoff and Calorimetry methods

In this article, discrepancies between the enthalpies of micellization of amphiphiles in aqueous solution determined by the methods of van’t Hoff (VH) and calorimetry have been addressed. The contributions of the hydrophobic interaction, electrostatic interaction and the micellar size effect have been considered to assess the total picture of the amphiphile self-association process and related energetic parameters, especially the enthalpy and the specific heat capacity. Literature results on 23 amphiphile systems (six nonionics, five anionics, and twelve cationics) have been analyzed, and the assessed enthalpies by VH method and direct calorimetry have been presented and compared. VH results considering participation of 5% of total amphiphile monomer to form micelle at cmc have been also compared. In addition to this, the changes in the standard specific heat of micellization for all the amphiphile aggregation processes evaluated by the VH and calorimetry procedures have been presented. The differences between the standard enthalpy of micellization Δ H m ° by the methods of VH and calorimetry are minor for nonionic surfactants but major for ionics, whereas the standard specific heat capacities of micellization Δ C p m ° by both the procedures fairly agree for all types of surfactants. Like Δ H m ° - Δ S m ° compensation observed in kinetic and equilibrium processes, a linear correlation between Lt T → 0 Δ H m ° and Δ C p m ° has been observed with no distinction between the VH and calorimetry derived results for all the surfactant systems herein dealt with.

Water Mediates the Metastable Crystal-to-Stable Crystal Phase Transition Process in Phospholipid Aqueous Dispersion

The metastable crystal usually appears first during crystallization from solution, and it will then transform into the stable form. This metastable crystal-to-stable crystal phase transition process is important in the field of crystal growth and engineering. Crystallization behaviors of amphiphiles such as phospholipids are special as they have preassembled structures prior to crystallization, and the fundamental mechanisms regarding the transformation details of their polymorphic forms at the molecular level are not well understood. In this work, by using Fourier transform infrared spectroscopy combined with the hydrogen/deuterium (H/D) exchange method, we have demonstrated that water plays a critical role in mediating the metastable crystal-to-stable crystal phase transition process of a representative phospholipid, dilauroylphosphoethanolamine (DLPE). The H/D exchange method applied here is a powerful and simple approach to probe if water is involved in the crystal-to-crystal transformation process. The results indicate that water works like a catalyst for the physical process. Furthermore, the conformational changes of the lipid molecules during the crystal-to-crystal transition process have been examined. Understanding the details of the transition from a metastable crystal to a stable crystal will shed light on the crystal growth theory and engineering of the self-assembled amphiphiles in aqueous solutions.

Self-Assembly and Polymerization of Diacetylene-Containing Peptide Amphiphiles in Aqueous Solution

A systematic study was performed of the fiber forming properties and polymerization characteristics of two peptide amphiphiles containing a diacetylene functionality in the alkyl tail comprising 23 and 25 C atoms, respectively. Both diyne containing peptide amphiphiles were able to form stable beta-sheet fibers of micrometers length in an aqueous solution. However, there was a large difference between the stability of the two amphiphiles. This was shown by a large difference in assembly and disassembly temperature and by different behavior during polymerization. Because the monomers were preorganized with a tight molecular packing, the polymerization could be carried out using wavelengths up to 532 nm. For both amphiphiles, the fiber structure did not change when the polymerization was carried out at an elevated temperature. The degree of polymerization, however, barely decreased for the longer amphiphile (2) but showed a gradual decline for the shorter one (1) when the temperature was raised from room temperature to the melting temperature of the fibers. Furthermore, the pH did not influence the fiber assembly for 2, but hampered it for 1 at alkaline pH. The fiber structure was, for both of the amphiphiles, not dependent on the pH. After polymerization, the molecular packing of the amphiphiles was only slightly influenced by an increase in temperature, as indicated by the small color change of polymerized fibers, which was also reversible. Additionally, pH had no influence on the assembly structure, as indicated by the color of the polymer which was the same at all pH values. Thus, both fibers increased in stability upon polymerization. The large difference in assembly and polymerization behavior of the two similar-looking amphiphiles 1 and 2, with a 23 or 25 carbon tail, is indicative of the subtlety of the assembly and disassembly processes in these fibrous architectures.

Fluorescent Nanoparticles Comprising Amphiphilic Rod−Coil Graft Copolymers

Three fluorescent amphiphilic rod−coil grafted copolymers comprising fluorene-based backbones and PEO side chains have been synthesized through the Suzuki coupling reaction. Stable and uniform fluorescent micelles were formed from the fluorescent amphiphiles in aqueous solution. The micelle size was tuned from 85 to 178 nm through controlling the hydrophilic/hydrophobic ratio and molecular weight of the amphiphiles. Preliminary biocompatibility and bioimaging investigation with BV-2 cells indicates that the fluorescent micelles are noncytotoxic, and BV-2 cells can be uniformly labeled by the micelles. Because of the intense fluorescence, biocompatibility, and noncytotoxicity of the amphiphiles, the fluorescent micelles may find potential applications in visualization of microscopic structures and drug delivery tracing.

Self-assembly and supramolecular transition of poly(amidoamine) dendrons focally modified with aromatic chromophores

Three novel series of amphiphiles based on poly(amidoamine) dendrons (from G1 to G3) and having different aromatic chromophores (Cz I, Cz II, and Py) at the focal point were synthesized and studied for their self-assembly behavior in aqueous solution by using electronic microscopies (i.e., SEM and TEM), UV–vis, fluorescence, IR, and 1H NMR spectroscopy. It was found that the generation of dendrons affected significantly the self-assembly of these amphiphiles in aqueous solution and the morphological structures of the resulting assemblies depended greatly on the architecture of the focal chromophores. As a result, the first generation of dendrons assembled readily into vesicles at low concentrations. These vesicular structures subsequently fused to form a stable tubular structure. Similar tubular structures could also be directly obtained through self-assembly of these amphiphilic dendrons at high concentrations. X-ray investigations showed that the resulting tubules possessed a lamellar structure. A head-to-head packing model of amphiphilic dendrons in the assemblies was proposed.

Supramolecular Fluorophores for Biological Studies: Phenylene Vinylene-Amino Acid Amphiphiles

We report here on a family of self-assembling fluorescent organic amphiphiles with a biomolecular L-lysine hydrophile and a photonically active phenylene vinylene hydrophobe. Unlike conventional amphiphiles, these segmented dendrimers feature a rigid, branched hydrophobe, and have packing characteristics controlled by the ratio of cross-sectional areas of the hydrophobe and hydrophile. In dilute solution, the amphiphiles form supramolecular aggregates, which are easily taken in by cells through an endocytic pathway, and have no discernible effect on cell proliferation or morphology. An analogous pyrene-based amphiphile was cytotoxic, suggesting that cell survival may be linked either to the self-assembling nature of the amphiphiles, or to the specific properties of the phenylene vinylene segment. The combination of photonic and biological components in these amphiphiles provides great potential for applications in sensing or delivery of molecules to intracellular targets.

Self-assembling of ion-containing polymers and surfactants in aqueous solutions studied by ESR and fluorescence probes

We present a review of spectroscopic probe studies on self-assembling of ionic amphiphiles in aqueous solutions. The potential of electron spin resonance (X-band electron spin resonance (ESR)) and fluorescence methods is illustrated by the examples of experiments performed in our laboratories for the systems containing ionomers, short-chain surfactants and polyelectrolytes. In particular, the use of protiated probes to examine fluorinated hosts is discussed.

Self-micellization of gemfibrozil 1-O-beta acyl glucuronide in aqueous solution.

Phase II metabolism involves the conjugation of a polar moiety, such as sulfate or glucuronic acid, to a (relatively) nonpolar xenobiotic. Although it might be expected that such conjugates may exhibit amphiphilic character (e.g., surface activity and potential to form micelles), no detailed study of the micellization characteristics of any drug-glucuronide conjugates has yet been reported. Therefore, the aim of this study was to investigate the solution behavior and amphiphilic characteristics of gemfibrozil 1-O-beta glucuronide (GG), a model drug-glucuronide conjugate. Crude GG was extracted from the urine of volunteers dosed with 600 mg of gemfibrozil, and this material was then purified by reversed-phase high-performance liquid chromatography to yield a white solid. The amphiphilic properties of GG within the bulk aqueous phase were studied by isothermal titration microcalorimetry and 1H-NMR spectrometry, whereas those at the aqueous/air interface were studied by surface tensiometry. The results of each independent analytical technique were consistent with GG in aqueous solution exhibiting amphiphilic properties typical of a hydrophilic surfactant. The titration microcalorimetry and 1H-NMR spectrometry data were in excellent agreement with each other, yielding critical micellization concentrations (cmc) for GG in 0.1 M acetate buffer of 18.1 +/- 0.4 mM and 18.3 +/- 0.3 mM, respectively. The profile and results of the surface tension measurements were consistent with GG localizing at the aqueous/air interface. These results confirm the hypothesis that a glucuronide conjugate of a relatively nonpolar xenobiotic, such as gemfibrozil, behaves as an amphiphile in aqueous solution. The implications of this observation include a likely basis for the previously observed concentration-dependence in the degradation rate of the acyl glucuronides of 2-phenylpropionic acid, as well as identifying a possible broader contributory effect to the structural dependencies in biliary choleresis of different glucuronide conjugates of xenobiotics.

Interactions of an Azobenzene-Functionalized Anionic Amphiphile with Cationic Amphiphiles in Aqueous Solution

Interactions between the azobenzene-containing amphiphile methyl octyl orange (MOO) and n-alkyltrimethylammonium bromides (CnTAB, n = 1, 6, 8, 10, 12, 16) in aqueous solution have been studied using UV−vis spectroscopy. Interactions occur at concentrations below the critical micelle concentrations of the individual cationic surfactants (n = 8, 10, 12, 16) as indicated by a ca. 80 nm blue shift of the main absorption band of MOO. In addition, aggregation of MOO with C6TAB, which is a hydrotrope rather than a surfactant, was also observed. The critical aggregation concentration (cac) of MOO determined by surface tension experiments was 0.68 mM. The cac's of aqueous mixtures of CnTAB and MOO (1:1) were considerably lower than those of the individual surfactants. The absorption spectrum of MOO at concentrations above the cac in the 1:1 mixtures was also blue shifted with respect to the absorption spectrum of MOO in aqueous solution. Vesicular aggregates formed in equimolar aqueous mixtures of CnTAB and MOO were characterized by means of cryo- and negative staining transmission electron microscopy. Vesicles are small and range in diameter from 8 to 15 nm although some vesicles having a diameter between 30 and 80 nm were also observed. Vesicle size distributions were confirmed using dynamic light scattering. Vesicle sizes decreased upon increasing the chain mismatch, and ultimately, vesicles transformed into micelles. Micelles were also formed in a 1:1 aqueous mixture of C6TAB and MOO.

Aggregative processes in aqueous solutions of isomeric 2-butoxyethanols at 298.15 K

Ultrasound speed measurements across the entire composition range of aqueous mixtures of both isobutoxyethanol (iC4E1) and tert-butoxyethanol (tC4E1) have been made at 298.15 K with a sonic solution monitor that employs a “pulse–echo–overlap” technique. In addition, densities of aqueous mixtures of tC4E1 were determined using a vibrating tube densimeter. These new data were complemented with literature values for densities of aqueous mixtures of tC4E1 and for densities and sound speeds of aqueous mixtures of n-butoxyethanol (nC4E1). In all cases, density values were converted to molar volumes, Vm, and excess molar volumes, VmE. Estimates of the isentropic molar compression, KS,m [= −(∂Vm/∂p)S], and of its excess counterpart, KS,mE, were obtained from the combination of the ultrasound speeds and density values. Data reduction procedures were used to generate consistent sets of values for thermodynamic properties of isomeric amphiphiles with increasing degree of alkyl branching. The graphs for the composition dependence of excess partial molar volumes and isentropic compressions of water show enhanced visual impact. These graphs are used for presenting evidence for identifying the prevailing patterns of molecular aggregation. Segmented-composition models, including a version onto which a mass action component has been grafted, are employed, together with a simplified pseudo-phase model, to analyse the various excess molar quantities. The experimental evidence thus obtained is used to relate the effect of chain branching with the degree of self-aggregation of amphiphiles in aqueous solution. An unexpectedly low self-aggregation among iC4E1 molecules is found and discussed in terms of vicinity to the lower critical solution temperature.

Comment on "Spin-1 aggregation model in one dimension".

Girardi and Figueiredo have proposed a simple model of aggregation in one dimension to mimic the self assembly of amphiphiles in aqueous solution [Phys. Rev. E 62, 8344 (2000)]. We point out that interesting results can be obtained if a different set of interactions is considered, instead of their choice (the s=1 Ising model).

An examination of the vapour-liquid interface of associating fluids using a SAFT-DFT approach

With a realistic description of the free energy of bulk fluids, it is now possible to make accurate predictions at the molecular level for the phase behaviour of systems as complex as aqueous solutions of amphiphiles, reacting and associating fluids, polymers, and electrolytes (e.g. using the statistical associating fluid theory SAFT). A quantitative molecular description of the interfacial properties of inhomogeneous fluids, including surface tension and adsorption is much less common. In this work we first hope to improve the general understanding of the effect of association on the vapour-liquid interface. The vapour-liquid interface of the inhomogenous associating fluid is examined by combining the SAFT and density functional theory (DFT) approaches. A simple SAFT-HS representation is employed as it incorporates all of the essential physics of associating fluids and provides a good representation of the vapour pressure and coexisting phases. In this simplified SAFT approach the bulk fluid is represented as a hard-core reference, the association is treated with Wertheim's first order perturbation theory (TPTI), and a van der Waals mean-field approximation is used for the dispersive attractive interactions. In order to keep the representation of the bulk fluid and interface at the same level of approximation we use the van der Waals theory for non-uniform fluids, which is a DFT at the level of a local density approximation (LDA); the correlations are neglected in the attractive non-local term. The vapour-liquid interface of model systems with one, two and four bonding sites are examined for varying degrees of association. As expected, a stronger site-site interaction is generally found to sharpen the interface (decrease the interfacial thickness) and increase the surface tension. In the case of a dimerizing (single site) fluid a limiting behaviour is reached for full association (saturation) where the molecular species are dimers. After an in depth analysis of the effect of association on the vapour-liquid interface, we highlight the strengths of our simple SAFT-DFT approach by making some quantitative comparisons with experimental surface tensions for selected systems including water and replacement refrigerants.

Preparation of amphiphilic lignin derivative as a cellulase stabilizer

A polymeric amphiphile, PE-AL, was prepared from acetic acid lignin (AL) obtained by acetic acid pulping of birch under atmospheric pressure with polyethylene glycol diglycidyl ether (PE) as the crosslinker. The behavior of PE-AL solutions and the complex formation of PE-AL with protein were investigated to clarify the function of this novel lignin derivative. The reduced viscosity of the amphiphile in aqueous solution was low (<0.3dl/g), and it decreased with increasing concentration in dilute solution. This suggested that the PE-AL in aqueous solution has a structure similar to that of Einstein's sphere and shrinks upon hydrophobic interaction among the structural moieties in AL and the exclusive volume effect. The amphiphilic PE-AL obviously formed a complex with bovine serum albumin (BSA) at 4°C with a reaction time of about 1 week. After complex formation with cellulase for 1 week, the cellulase activity of the resulting complex is significantly enhanced and is preserved after recycling the complex for hydrolysis of cellulosic materials several times.

Aggregation Behavior of Urocanic Acid Bolaamphiphiles

Symmetrical bolaamphiphiles derived from (E) urocanic acid (3-[1H-imidazolyl-(4)-yl]-propenoic acid) were prepared and their self-assembling properties examined. The morphologies of the aggregates formed by these amphiphiles in aqueous solutions have been found to be strongly dependent on three parameters (pH, structure, and position of the connecting links). For the N-alkylated derivatives, vesicles are obtained irrespective of the pH. For the O-alkylated derivative with an ester linkage, the hydration of the imidazolium headgroup led to the formation of spherical aggregates. With an amide linkage, amphiphiles can self-assemble to give interesting aggregates that can be tuned by changing the pH. In acidic media, two kinds of aggregates are observed: rigid rods and flexible water-filled tubules. These investigations reveal that the number and the position of the functions generating hydrogen bonding play a key role in the type of aggregate formed. Hydrogen bonding encourages the formation of fibers.

Fibrous Assemblies Made of Amphiphilic Metallophthalocyanines

A family of copper and zinc phthalocyanine-based amphiphililes possessing racemic and optically active diol units, (rac)-ZnPc(OH)16, (rac)-CuPc(OH)16, and (S)-CuPc(OH)16, have been synthesized. The self-assembling properties of these amphiphiles in aqueous solution have been studied by UV−vis, Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopies, X-ray diffraction (XRD) patterns, and transmission electron microscopy (TEM). Only the copper complexes produced fibrous assemblies from aqueous solutions through two noncovalent bondings: π−π interaction among phthalocyanine rings and hydrogen bonds among diol units. The formation of fibrous assemblies strongly depends on the central metal of the phthalocyanine complex. The optically active (S)-CuPc(OH)16 is stacked and arranged in a left-handed helix. The chirality of diol units in (S)-CuPc(OH)16 also affects the intercolumnar lattice of phthalocyanine stacks.