Specifics of localization of carbon black at the interface between polymeric phases

The thermodynamic and kinetic factors that determine the localization of carbon black particles at interfaces in polymer blends were established. Substantial differences in particle localization conditions at the interfaces of two high-molecular-mass or low-molecular-mass liquids were revealed; these differences are manifested in the determining effect of the sequence of mixing of the filler with the polymeric components of the blend on the localization. The effect is explained in terms of nonequilibrium positions of carbon black particles at the interface. This nonequilibrium distribution is primarily due to a high energy of desorption of macromolecules from the solid surface.     

Accelerated reactions of amines with carbon dioxide driven by superacid at the microdroplet interface

Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk. Carbon dioxide undergoes C–N bond formation reactions with amines at the interface of droplets to form carbamic acids. Electrospray ionization mass spectrometry displays the reaction products in the form of the protonated and deprotonated carbamic acid. Electrosonic spray ionization (ESSI) utilizing carbon dioxide as nebulization gas, confines reaction to the gas–liquid interface where it proceeds much faster than in the bulk. Intriguingly, trace amounts of water accelerate the reaction, presumably by formation of superacid or superbase at the water interface. The suggested mechanism of protonation of CO₂ followed by nucleophilic attack by the amine is analogous to that previously advanced for imidazole formation from carboxylic acids and diamines.

Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk.     

Controlled preparation of carbon nanotube-conducting polymer composites at the polarisable organic/water interface

The electro-polymerisation of polypyrrole (PPy) at the interface between two immiscible electrolyte solutions (ITIES) is reported. The approach is used to demonstrate the formation of a carbon nanotube (SWCNT)-conducting polymer composite, by performing polymerisation in the presence of an assembly of SWCNT films. The morphology of the SWCNT/PPy nanocomposites was determined using probe and electron microscopy and complementary spectroscopic techniques (EDAX, Raman).     

Ions at the air/water interface

In a recent review of this topic [B.C. Garett, Science 303 (2004) 1146] the emphasis was on some recent experiments, in which it was found that some anions accumulate at the air/water interface and not in the bulk, as usually happens to the cations, and on some simulations which explained those positive surface adsorption excesses. Because a large number of these experiments could be explained on the basis of some simple physical models proposed by the authors for the interaction between the ions and the air/water interface [M. Manciu, E. Ruckenstein, Adv. Colloid Interface Sci. 105 (2003) 63; Adv. Colloid Interface Sci. 112 (2004) 109; Langmuir 21 (2005) 11312], those models are reviewed in the present note, the goal being to draw attention to them.     

Photoinduced redox cycle of riboflavin at a water/oil interface.

A photoinduced redox reaction cycle of Riboflavin (RF) at a water/CCl4 interface was studied directly by means of both steady-state and time-resolved total internal reflection (TIR) fluorescence spectroscopies. The TIR fluorescence spectrum of RF observed at the water/CCl4 interface with the maximum wavelength of 517 nm was assigned to the pi-pi* transition from the excited singlet-state of the isoalloxazine chromophore in RF. Upon prolonged laser irradiation (400 nm) in the presence of N,N-dioctadecyl-[1,3,5]triazine-2,4,6-triamine (DTT) as a guest for RF in the CCl4 phase, on the other hand, a new TIR fluorescence band appeared at around 480 nm. Furthermore, the fluorescence intensity at around 480 nm increased in the presence of acetic acid in the water phase. Detailed studies demonstrated that the new fluorescence band should be ascribed to 1,5-dihydoroflavin (RFH2). The present results indicated that RFH2 was produced through the photoreaction of the RF-DTT hydrogen-bonded complex formed at the water/CCl4 interface, whose reaction mechanisms were discussed on the basis of the results observed by fluorescence spectra/dynamics measurements under the TIR conditions as well as by transient absorption spectroscopy.     

Electronic states at the water/air interface

Using combined path integral-molecular dynamics simulation techniques, we analyze electronic solvation at the water/air interface. Superficial electrons present a considerable extent of spatial confinement, somewhat less marked but still comparable to that found in bulk. The characteristics of the interfacial polarization promote an overall structure for the solvated electron-polymer which looks flatter along the direction perpendicular to the interface. Spatial and orientational responses of different slabs in the close vicinity of the interface were also investigated. Solvent configurations obtained from the simulations have been used to analyze electronic excited states and the optical absorption spectrum of superficial electrons. Compared to bulk results, the distribution of bound electronic states at the surface presents similar characteristics, that is, a ground s-state and three, quasi-degenerate, p-like excited states. The reduction of the energy gap between the ground state and the rest of excited states leads to a approximately 0.52 eV red-shift in the position of the absorption maximum.     

Protein-lipid interactions at the air/water interface

Surface pressure measurements and external reflection FTIR spectroscopy have been used to probe protein-lipid interactions at the air/water interface. Spread monomolecular layers of stearic acid and phosphocholine were prepared and held at different compressed phase states prior to the introduction of protein to the buffered subphase. Contrasting interfacial behaviour of the proteins, albumin and lysozyme, was observed and revealed the role of both electrostatic and hydrophobic interactions in protein adsorption. The rate of adsorption of lysozyme to the air/water interface increased dramatically in the presence of stearic acid, due to strong electrostatic interactions between the negatively charged stearic acid head group and lysozyme, whose net charge at pH 7 is positive. Introduction of albumin to the subphase resulted in solubilisation of the stearic acid via the formation of an albumin-stearic acid complex and subsequent adsorption of albumin. This observation held for both human and bovine serum albumin. Protein adsorption to a PC layer held at low surface pressure revealed adsorption rates similar to adsorption to the bare air/water interface and suggested very little interaction between the protein and the lipid. For PC layers in their compressed phase state some adsorption of protein occurred after long adsorption times. Structural changes of both lysozyme and albumin were observed during adsorption, but these were dramatically reduced in the presence of a lipid layer compared to that of adsorption to the pure air/water interface.     

Nanoaggregate shapes at the air/water interface

Chiral interfaces and molecular recognition phenomena are of special interest not only for the understanding of biological recognition processes but also for the potential application in material science. Langmuir monolayers at the air-water interface have successfully been used as simple models to mimic biological phenomena. Recent experimental studies revealed that both chirality and molecular recognition processes of amphiphiles are controlling the features of the nano-aggregates at the air/water interface. The objective of experimental studies has been to gain information about the properties of mesoscopic length scale aggregates obtained on the basis of chiral discrimation effects and the formation of supramolecular entities by molecular recognition of non-surface active species dissolved in the aqueous subphase. Differences in the two-dimensional morphology and lattice structures of the nano-aggregates cannot be explained by macroscopic theories and needed information about the detailed orientation and distance dependence of the intermolecular interaction within the aggregates. First new bottom-up studies have been directed toward understanding the driving forces for the aggregation processes of monolayers. Different types of interactions have been successfully considered using semi-empirical quantum chemical methods. The possibilities of Langmuir-Blodgett (LB) patterning to be an alternative paradigm for large-area patterning with mesostructured features are discussed.     

Adsorption of dioctyldimethylammonium chloride at water/hexane interface

The adsorption behavior of dioctyldimethylammonium chloride at water/ hexane interface has been studied by measuring the interfacial tension as a function of temperature and pressure at various bulk concentrations. By applying the thermodynamics of adsorption at interfaces to the experimental results, the thermodynamic quantity changes associated with adsorption and the interfacial density of dioctyldimethylammonium chloride have been evaluated.The interfacial tension vs temperature and concentration curves have shown the breaks and it has been concluded that the first order phase transition takes place between a gaseous and an expanded state. The entropy and volume changes associated with adsorption have shown the remarkable dependence on temperature and pressure and have been found to decrease with increasing the molality. Also the energy change associated with adsorption has been evaluated and it has been concluded that the adsorption of dioctyldimethylammonium chloride at water/hexane interface is enhanced by negative values of the partial molar energy change. Further, all the thermodynamic quantities have been characterized by the discontinuous change attributable to the phase transition.     

Adsorption of crotonaldehyde at the mercury/water interface

The adsorption of crotonaldehyde from aqueous 1 M KCl has been studied by means of differential capacity, zero charge potential and maximum surface tension measurements. The adsorption has been found to obey a Frumkin isotherm with the interaction parameter depending on the electric field. Different possible molecular orientations are suggested depending on charge and coverage. The contribution of the molecular dipole moment and differences in polarizability between the adsorbate and the solvent are considered.     

Adsorption of tetramethylthiourea at the mercury/water interface

The adsorption of tetramethylthiourea was studied by means of differential capacity measurements. Various parameters have been obtained by back-integration. The extent of orientation of the organic molecule has been estimated from the experimental adsorption potential shift by subtracting the contribution due to adsorbed water molecules calculated according to the Bockris-Habib model. Adsorption of tetramethylthiourea has been found to obey a Frumkin isotherm with the interaction parameter depending on the electric field. A scenario is proposed where different possible orientations are suggested, depending on charge sign and coverage. Such a change in orientation has been interpreted in terms of the electronic polarization effect.     

Adsorption of oleic acid at sillimanite/water interface

The interaction of oleic acid at sillimanite-water interface was studied by adsorption, FT-IR, and zeta potential measurements. The isoelectric point (IEP) of sillimanite obtained at pH 8.0 was found to shift in the presence of oleic acid. This shift in IEP was attributed to chemisorption of oleic acid on sillimanite. Adsorption experiments were conducted at pH 8.0, where the sillimanite surface is neutral. The adsorption isotherm exhibited a plateau around 5 micromol/m2 that correspond to a monolayer formation. Adsorption of oleic acid on sillimanite, alumina, and aluminum hydroxide was studied by FT-IR. Chemisorption of oleic acid on the above substrates was confirmed by FT-IR studies. Hydroxylation of mineral surface was found to be essential for the adsorption of oleic acid molecules. These surface hydroxyl sites were observed to facilitate deprotonation of oleic acid and its subsequent adsorption. Thus protons from oleic acid react with surface hydroxyl groups and form water molecules. Based on the experimental results, the mechanism of oleic acid adsorption on mineral substrate was proposed. Free energy of adsorption was estimated using the Stern-Graham equation for a sillimanite-oleate system.     

Polymer/surfactant interactions at the air/water interface

The development of neutron reflectometry has transformed the study and understanding of polymer/surfactant mixtures at the air/water interface. A critical assessment of the results from this technique is made by comparing them with the information available from other techniques used to investigate adsorption at this interface. In the last few years, detailed information about the structure and composition of adsorbed layers has been obtained for a wide range of polymer/surfactant mixtures, including neutral polymers and synthetic and naturally occurring polyelectrolytes, with single surfactants or mixtures of surfactants. The use of neutron reflectometry together with surface tensiometry, has allowed the surface behaviour of these mixtures to be related directly to the bulk phase behaviour. We review the broad range of systems that have been studied, from neutral polymers with ionic surfactants to oppositely charged polyelectrolyte/ionic surfactant mixtures. A particular emphasis is placed upon the rich pattern of adsorption behaviour that is seen in oppositely charged polyelectrolyte/surfactant mixtures, much of which had not been reported previously. The strong surface interactions resulting from the electrostatic attractions in these systems have a very pronounced effect on both the surface tension behaviour and on adsorbed layers consisting of polymer/surfactant complexes, often giving rise to significant surface ordering.     

Ion-transfer voltammetry at a polarized room-temperature molten salt/water interface.

Tetraoctylammonium cation forms a room-temperature molten salt (RTMS) with 2,4,6-trinitrophenolate anion. The RTMS is immiscible with water (W) and forms a stable RTMS/W interface. It has been shown that the RTMS/W interface can be electrochemically polarized. A well-defined voltammetric wave due to the transfer of thiocyanate ion across the RTMS/W interface was observed within the potential window. This is the first example of a polarized RTMS/W interface.     

The interactions of saturated fatty acids at the dodecane/water interface and their sodium salts at the air/water interface

The interfacial tension of lauric, myristic and palmitic acids at the dodecane/water interface and of their sodium salts at the air/water interface were measured using the Du Noüy (ring) method. On the basis of these results the standard free energies of adsorption (ΔG⁰_ad) and of micellization (ΔG⁰_mic) of the above mentioned systems were calculated. According to deviations of the Langmuir isotherm, the corresponding interactions were discussed from the dependence of the standard free energy of adsorption on the interface coverage (Θ).     

Electrical double layers at the oil/water interface

This review presents the historical development and current status of the theory of the electrical double layer at a liquid/liquid interface. It gives rigorous thermodynamic definitions of all basic concepts related to liquid interfaces and to the electrical double layer. The difference between the surface of a solid electrode and the interface of two immiscible electrolyte solutions (ITIES) is analyzed in connection to their electrical properties. The most important classical relationships for the electrical double layer are presented and critically discussed. The generalized adsorption isotherm is derived. After a short review of the classical Gouy-Chapman and Verwey-Niessen models, more recent developments of the double layer theory are presented. These include effects of variable dielectric permittivity, nonlocal electrostatics, hydration forces, the modified Poisson-Boltzmann equation and the ion-dipole plasma. The relative merits of different theories are estimated by comparing them with computer simulation of the ITIES and electrical double layer. Special attention is given to the structure of ITIES and its variation due to adsorption of ions and amphiphilic molecules.     

Cationic Gemini surfactant at the air/water interface

The surface properties and structures of a cationic Gemini surfactant with a rigid spacer, p-xylyl-bis(dimethyloctadecylammonium bromide) ([C(18)H(37)(CH(3))(2)N(+)CH(2)C(6)H(4)CH(2)N(+)(CH(3))(2)C(18)H(37)],2Br(-), abbreviated as 18-Ar-18,2Br(-1)), at the air/water interface were investigated. It is found that the surface pressure-molecular area isotherms observed at different temperatures do not exhibit a plateau region but display an unusual "kink" before collapse. The range of the corresponding minimum compressibility and maximum compressibility modulus indicates that the monolayer is in the liquid-expanded state. The monolayers were transferred onto mica and quartz plates by the Langmuir-Blodgett (LB) technique. The structures of monolayers at various surface pressures were studied by atomic force microscopy (AFM) and UV-vis spectroscopy, respectively. AFM measurements show that at lower surface pressures, unlike the structures of complex or hybrid films formed by Gemini amphiphiles with DNA, dye, or inorganic materials or the Langmuir film formed by the nonionic Gemini surfactant, in this case network-like labyrinthine interconnected ridges are formed. The formation of the structures can be interpreted in terms of the spinodal decomposition mechanism. With the increase of the surface pressure up to 35 mN/m, surface micelles dispersed in the network-like ridges gradually appear which might be caused by both the spinodal decomposition and dewetting. The UV-vis adsorption shows that over the whole range of surface pressures, the molecules form a J-aggregate in LB films, which implies that the spacers construct a pi-pi aromatic stacking. This pi-pi interaction between spacers and the van der Waals interaction between hydrophobic chains lead to the formation of both networks and micelles. The labyrinthine interconnected ridges are formed first because of the rapid evaporation of solvent during the spreading processes; with increasing surface pressure, some of the alkyl chains reorient from tilting to vertical, forming surface micelles dispersed in the network-like ridges due to the strong interaction among film molecules.