Molecular Packing of Amphiphiles with Crown Polar Heads at the Air−Water Interface

Effect of Sodium Dodecyl Sulfonate on the Foam Stability and Adsorption Configuration of Dodecylamine at the Gas-Liquid Interface

In this study, the effect of sodium dodecyl sulfonate (SDS) on the foam stability of dodecylamine (DDA) and on its adsorption configuration at the gas-liquid interface was investigated. Froth stability experiments, surface tension measurements, time-of-flight secondary-ion mass spectrometry measurements, and molecular dynamics simulation calculations were performed in this investigation. The results revealed that the foam stability of DDA solution was extremely strong, and the addition of SDS could decrease the foam stability when the concentration of DDA was less than a certain value. The decrease in foam stability could be ascribed to several reasons, namely, the big cross-sectional area of SDS at the gas-liquid interface and low adsorption capacity of surfactants at the gas-liquid interface, the high surface tension, the change in the double-layer structure, the small thickness of the gas-liquid interfacial layer, the weak interaction intensity between the head groups of the surfactants and the water molecules, the strong movement ability of the water molecules around the head groups, and the sparse and less upright arrangement configuration of molecules at the gas-liquid interface. These findings can greatly help in solving the strong foam stability problem in DDA flotation and provide a method for reducing foam stability.

Chiral supramolecular organization from a sheet-like achiral gel: a study of chiral photoinduction

Chiral photoinduction in a photoresponsive gel based on an achiral 2D architecture with high geometric anisotropy and low roughness has been investigated. Circularly polarized light (CPL) was used as a chiral source and an azobenzene chromophore was employed as a chiral trigger. The chiral photoinduction was studied by evaluating the preferential excitation of enantiomeric conformers of the azobenzene units. Crystallographic data and density functional theory (DFT) calculations show how chirality is transferred to the achiral azomaterials as a result of the combination of chiral photochemistry and supramolecular interactions. This procedure could be applied to predict and estimate chirality transfer from a chiral physical source to a supramolecular organization using different light-responsive units.

Gold nanoparticles grown on star-shaped block copolymer monolayers

We report on the growth of gold nanoparticles in polystyrene/poly(2-vinyl pyridine) (PS/P2VP) star-shaped block copolymer monolayers. These amphiphilic PS(n)P2VP(n) heteroarm star copolymers differ in molecular weight (149,000 and 529,000 Da) and the number of arms (9 and 28). Langmuir-Blodgett (LB) deposition was utilized to control the spatial arrangement of P2VP arms and their ability to reduce gold nanoparticles. The PS(n)P2VP(n) monolayer acted as a template for gold nanoparticle growth because of the monolayer's high micellar stability at the liquid-solid interface, uniform domain morphology, and ability to adsorb Au ions from the water subphase. UV-vis spectra and AFM and TEM images confirmed the formation of individual gold nanoparticles with an average size of 6 ± 1 nm in the P2VP-rich outer phase. This facile strategy is critical to the formation of ultrathin polymer-gold nanocomposite layers over large surface areas with confined, one-sided positioning of gold nanoparticles in an outer P2VP phase at polymer-silicon interfaces.

Molecular dynamics study of the effect of calcium ions on the monolayer of SDC and SDSn surfactants at the vapor/liquid interface

The effect of Ca(2+) ions on the hydration shell of sodium dodecyl carboxylate (SDC) and sodium dodecyl sulfonate (SDSn) monolayer at vapor/liquid interfaces was studied using molecular dynamics simulations. For each surfactant, two different surface concentrations were used to perform the simulations, and the aggregation morphologies and structural details have been reported. The results showed that the aggregation structures relate to both the surface coverage and the calcium ions. The divalent ions can screen the interaction between the polar head and Na(+) ions. Thus, Ca(2+) ions locate near the vapor/liquid interface to bind to the headgroup, making the aggregations much more compact via the salt bridge. The potential of mean force (PMF) between Ca(2+) and the headgroups shows that the interaction is decided by a stabilizing solvent-separated minimum in the PMF. To bind to the headgroup, Ca(2+) should overcome the energy barrier. Among contributions to the PMF, the major repulsive interaction was due to the rearrangement of the hydration shell after the calcium ions entered into the hydration shell of the headgroup. The PMFs between the headgroup and Ca(2+) in the SDSn systems showed higher energy barriers than those in the SDC systems. This result indicated that SDSn binds the divalent ions with more difficulty compared with SDC, so the ions have a strong effect on the hydration shell of SDC. That is why sulfonate surfactants have better efficiency in salt solutions with Ca(2+) ions for enhanced oil recovery.

Molecular dynamics study of alkyl benzene sulfonate at air/water interface: effect of inorganic salts

Molecular dynamics simulations have been performed to investigate the effect of inorganic salts on the structural and dynamic properties of alkyl benzene sulfonate monolayer formed at the air/water interface. The alkyl benzene sulfonates are two surfactant isomers in the family of sodium hexadecane benzene sulfonates defined by 1C16 and 5C16, indicating a benzene sulfonate group attached to the first and fifth carbon atom in hexadecane backbone. It has been observed that both benzene ring groups and headgroups (-SO(3)(-)) are hydrated due to their polar nature. Water molecules can form stable hydrogen bonds with headgroups of surfactants, and the counterions (Na(+), Mg(2+), or Ca(2+)) are distributed close to the air/water interface. The stronger electrostatic repulsion drives the 1C16 monolayer arranged in disorder in comparison with 5C16, and the presence of inorganic salts may screen electrostatic repulsions between headgroups and decrease the thickness of the interfacial water layer, which follows the series Ca(2+) > Mg(2+) > Na(+). The order of inorganic salt tolerance of two surfactants is 5C16 > 1C16. The counterions may penetrate into the hydration shell of the surfactant headgroups and restrict the mobility of the water molecules situated in this area.

Structure and properties of functionalized bithiophenesilane monodendrons

This study reports a focal group modification of bithiophenesilane monodendrons and its effect on their molecular ordering in solution, bulk, and surface. We investigated hydrophobic MDn monodendrons and COOH-functionalized MDn-COOH monodendrons with generations, n=0, 1, 2, and 3. We observed that increasing the number of branches led to the progressive blue shift, indicating distorted packing of branched thiophene fragments of MDn. In contrast, MDn-COOH monodendrons showed a progressive red shift with the increasing generation number, indicating gradual domination of sigma-pi interactions. Moreover, the introduction of a focal carboxylic group resulted in the formation of a highly crystalline state for the linear MD0-COOH compound with separated alkyl tail-thiophene packing, which limits pi-pi interactions. Increasing branching in the COOH-containing monodendrons resulted in a hydrophobic-hybrophilic balance sufficient to form stable and uniform Langmuir monolayers at the air-water at a modest surface pressure (<10 mN/m), easily transferrable to a solid substrate. However, a further increase in the thickness of the surface layers from tens to hundreds of nanometers via Langmuir-Blodgett (LB) deposition or spin casting is limited by the formation of globular surface aggregates because of strong intermolecular interactions. A modest red shift observed for condensed Langmuir monolayers indicates densification of thiophene branches and limited intramonolayer crystallization, which preserves photoluminescence. In contrast, thicker surface films showed a significant red shift, confirming a dense molecular packing with strong pi-pi interactions, which results in photoluminescence quenching.

Toroid morphology by ABC-type amphiphilic rod-coil molecules at the air-water interface

The interfacial and aggregation behavior of the ABC-type amphiphilic molecules with semirigid dumbbell-shaped core and variable length of hydrophobic branched tails (R=(CH2)nCH3 with n=5 (1), 9 (2), 13 (3)) were investigated. At low surface pressure, smooth, uniform monolayers were formed at the air-water interface by molecules 1 and 2, whereas for molecule 3 unique 2D toroid aggregates have been formed. These aggregates were relatively stable within a range of surface pressure and spreading solution concentration. Upon compression, the 2D toroid aggregates collapsed into large, round 3D aggregates. Finally, the choice of spreading solvent has a great influence on aggregation formation into 2D or 3D micelles as a result of the variable balance of the hydrophobic interactions of branched tails and the pi-pi stacking interaction between aromatic segments.

Molecular reorganization of paired assemblies of T-shaped rod-coil amphiphilic molecule at the air-water interface

A T-shaped aromatic amphiphilic molecule based on linear oligo(ethylene oxide) was synthesized. We suggest that its peculiar interfacial behavior at the air-water interface and the structure of the Langmuir-Blodgett monolayer are associated with its peculiar T-shape and competing steric and amphiphilic interactions at different surface pressures. At low surface pressure, uniform and smooth monolayers were formed. Upon compression, the molecular reorganization from spherical to cylindrical transformation occurred, as caused by the submerging of the oligo(ethylene oxide) chains, providing for efficient pi-pi interactions of the central core. At the highest surface pressure, the monolayer collapses into bilayer domains, following a bicontinuous network formation which tends to transform into a perforated film. The unique shape of T-like rigid aromatic cores makes their structural reorganization very peculiar with paired, dimerlike molecular packing dominating in gas and solid states. This paired aggregation is so strong that it is preserved in the course of flipping and formation of vertically oriented backbones.

Formation of silver nanoparticles at the air-water interface mediated by a monolayer of functionalized hyperbranched molecules

Nanofibrillar micellar structures formed by the amphiphilic hyperbranched molecules within a Langmuir monolayer were utilized as matter for silver nanoparticle formation from the ion-containing water subphase. We observed that silver nanoparticles were formed within the multifunctional amphiphilic hyperbranched molecules. The diameter of nanoparticles varied from 2-4 nm and was controlled by the core dimensions and the interfibrillar free surface area. Furthermore, upon addition of potassium nitrate to the subphase, the Langmuir monolayer templated the nanoparticles' formation along the nanofibrillar structures. The suggested mechanism of nanoparticle formation involves the oxidation of primary amino groups by silver catalysis facilitated by "caging" of silver ions within surface areas dominated by multibranched cores. This system provides an example of a one-step process in which hyperbranched molecules with outer alkyl tails and compressed amine-hydroxyl cores mediated the formation of stable nanoparticles placed along/among/beneath the nanofibrillar micelles.

Langmuir and Grafted Monolayers of Photochromic Amphiphilic Monodendrons of Low Generations

Four generations of monodendrons with multiple dodecyl alkyl tails (AA-N, N representing number of alkyl tails from 1 to 8), an azobenzene spacer group, and a carboxylic acid polar head have been studied at the air-water and air-solid interface using AFM, GIXD, X-ray reflectivity, and UV-vis spectrometry. The one and two tail molecules formed orthorhombic lateral packing with long-range intramonolayer ordering. Good agreement between molecular models and thickness measurements indicated that the one and two tail molecules orient along the surface normal. The increase in the cross-sectional mismatch caused by the presence of the multiple chains for the higher generations disrupted the long-range ordering and forced the alkyl tails to adopt quasi-hexagonal structure. The higher generations (AA-4 and AA-8) formed a kinked structure with the alkyl tails oriented perpendicular to the surface with the azobenzene group tilted at a large degree toward the surface. The photoisomerization behavior in dilute solutions, at the air-water interface, and for grafted layers demonstrated that lower generation monodendrons maintained the photochromic behavior after chemical grafting to the silicon substrates, although the confinement of the molecules in monolayers significantly increased the reorganization time.

Interfacial micellar structures from novel amphiphilic star polymers

An amphiphilic heteroarm star polymer containing 12 alternating hydrophobic/hydrophilic arms of polystyrene (PS) and poly(acrylic acid) (PAA) connected to a well-defined rigid aromatic core was studied at the air-water and the air-solid interfaces. At the air-water interface, the molecules spontaneously form pancakelike micellar aggregates which measure up to several microns in diameter and 5 nm in thickness. Upon reduction of the surface area per molecule to 7 nm2, the two-dimensional micelles merged into a dense monolayer. We suggest that confined phase separation of dissimilar polymer arms occurred upon their segregation on the opposite sides of the rigid disklike aromatic core, forcing the rigid cores to adopt a face-on orientation with respect to the interface. Upon transfer onto solid supports the PS chains face the air-film interface making it completely hydrophobic, and the PAA chains were found to collapse and form a thin flattened underlayer. This study points toward new strategies to create large 2D microstructures with facial amphiphilicity and suggests a profound influence of star molecular architecture on the self-assembly of amphiphiles at the air-water interface.

Assembling of Amphiphilic Highly Branched Molecules in Supramolecular Nanofibers

We found that the amplification of weak multiple interactions between numerous peripheral branches of irregular, flexible, polydisperse, and highly branched molecules can facilitate their self-assembly into nanofibrillar micellar structures at solid surfaces and the formation of perfect long microfibers in the course of crystallization from solution. The core-shell architecture of the amphiphilic dendritic molecules provides exceptional stability of one-dimensional nanofibrillar structures. The critical condition for the formation of the nanofibrillar structures is the presence of both alkyl tails in the outer shell and amine groups in the core/inner shell. The multiple intermolecular hydrogen bonding and polar interactions between flexible cores stabilize these nanofibers and make them robust albeit flexible. This example demonstrates that one-dimensional supramolecular assembling at different spatial scales (both nanofibers and microfibers) can be achieved without a tedious, multistep synthesis of shape-persistent molecules.

Grazing-incidence x-ray diffraction study of Langmuir films of amphiphilic monodendrons

We have used pressure-area isotherms and grazing-incidence x-ray diffraction to study structures of Langmuir films of first-generation monodendrons with two or three peripheral alkyl chains. Unlike the structures observed in their bulk liquid crystalline mesophases, these multichain monodendrons form either a centered rectangular lattice with molecular axes tilted toward nearest neighbors or an oblique lattice with molecular axes tilted in low-symmetry directions.