Total Internal Reflection Fluorescence Dynamic Anisotropy of Sulforhodamine 101 at a Liquid/Liquid Interface: Rotational Reorientation Times and Interfacial Structures

An electrochemical perspective on the interfacial width between two immiscible liquid phases

Molecular dynamics simulations and vibrational sum-frequency spectroscopy are historically the main techniques applied to the description of the molecular structure and dynamics of the immiscible liquid/liquid interface. A molecular sharpness is estimated for oil/water interfaces, with an interfacial width that extends from hundreds of Å to 1 nm. However, electrochemical studies have elucidated a deeper liquid/liquid interface on the order of several micrometers. The breaking down of single-entity electrochemistry to simpler systems and the combination of high-resolution microscopies is confirming a larger extension of the interface. What can be the role of the electrochemist in clarifying this fundamental question? We try to give a suggestion at the end of a brief historical overview of the liquid/liquid interface studies.

Tracking Molecular Transport Across Oil/Aqueous Interfaces: Insight into "Antagonistic" Binding in Solvent Extraction

Liquid/liquid (L/L) interfaces play a key, yet poorly understood, role in a range of complex chemical phenomena where time-evolving interfacial structures and transient supramolecular assemblies act as gatekeepers to function. Here, we employ surface-specific vibrational sum frequency generation combined with neutron and X-ray scattering methods to track the transport of dioctyl phosphoric acid (DOP) and di-(2-ethylhexyl) phosphoric acid (DEHPA) ligands used in solvent extraction at buried oil/aqueous interfaces away from equilibrium. Our results show evidence for a dynamic interfacial restructuring at low ligand concentrations in contrast to expectation. These time-varying interfaces arise from the transport of sparingly soluble interfacial ligands into the neighboring aqueous phase. These results support a proposed "antagonistic" role of ligand complexation in the aqueous phase that could serve as a holdback mechanism in kinetic liquid extractions. These findings provide new insights into interfacially controlled chemical transport at L/L interfaces and how these interfaces vary chemically, structurally, and temporally in a concentration-dependent manner and present potential avenues to design selective kinetic separations.

Surface Modification of Porous Polyethylene Implants with an Albumin-Based Nanocarrier-Release System

BACKGROUND

Porous polyethylene (PPE) implants are used for the reconstruction of tissue defects but have a risk of rejection in case of insufficient ingrowth into the host tissue. Various growth factors can promote implant ingrowth, yet a long-term gradient is a prerequisite for the mediation of these effects. As modification of the implant surface with nanocarriers may facilitate a long-term gradient by sustained factor release, implants modified with crosslinked albumin nanocarriers were evaluated in vivo.

METHODS

Nanocarriers from murine serum albumin (MSA) were prepared by an inverse miniemulsion technique encapsulating either a low- or high-molar mass fluorescent cargo. PPE implants were subsequently coated with these nanocarriers. In control cohorts, the implant was coated with the homologue non-encapsulated cargo substance by dip coating. Implants were consequently analyzed in vivo using repetitive fluorescence microscopy utilizing the dorsal skinfold chamber in mice for ten days post implantation.

RESULTS

Implant-modification with MSA nanocarriers significantly prolonged the presence of the encapsulated small molecules while macromolecules were detectable during the investigated timeframe regardless of the form of application.

CONCLUSIONS

Surface modification of PPE implants with MSA nanocarriers results in the alternation of release kinetics especially when small molecular substances are used and therefore allows a prolonged factor release for the promotion of implant integration.

How Viscous Is the Solidlike Structure at the Interface of Ionic Liquids? A Study Using Total Internal Reflection Fluorescence Spectroscopy with a Fluorescent Molecular Probe Sensitive to High Viscosity

Aiming at the evaluation of the viscosity of the interfacial solidlike structure of ionic liquids (ILs), we performed total internal reflection fluorescence (TIRF) spectroscopy for N,N-diethyl-N'-phenyl-rhodamine (Ph-DER), a fluorescent probe that is sensitive to viscosity in a high-viscosity range. TIRF spectra at the glass interface of trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide (TOMAC4C4N), a hydrophobic IL, showed that the fluorescence intensity of Ph-DER increases with the decrease of the evanescence penetration depth, suggesting that there exists a high-viscosity region at the interface. In contrast, glycerol, which is a molecular liquid with a bulk viscosity similar to that of TOMAC4C4N, did not show such a fluorescence increase, supporting that the formation of a highly viscous solidlike structure at the interface is intrinsic to ILs. A model analysis suggested that the high viscous region at the glass interface of TOMAC4C4N is at least twice thicker than the ionic multilayers at the air interface, implying that the solid substrate enhances the ordering of the interfacial structure of ILs. The viscosity at the glass interface of TOMAC4C4N was found to be at least 40 times higher than that of the liquid bulk.

Anomalous effect of flow rate on the electrochemical behavior at a liquid|liquid interface under microfluidic conditions

We have investigated the oxidation of ferrocene at a flowing organic solvent|aqueous electrolyte|solid electrode junction in a microfluidic setup using cyclic voltammetry and fluorescent laser scanning confocal microscopy. At low flow rates the oxidation current decreases with increasing flow, contrary to the Levich equation, but at higher flow rates the current increases linearly with the cube root of the flow rate. This behavior is explained using a simple model postulating a smallest effective width of the three-phase junction, which after fitting to the data comes to be ca. 20 μm. The fluorescence microscopy reveals mixing of the two phases close to the PDMS cover, but the liquid|liquid junction is stable close to the glass support. This study shows the importance of the solid|liquid|liquid junctions for the behavior of multiphase systems under microfluidic conditions.

Ultrafast Photoinduced Dynamics at Air/Liquid and Liquid/Liquid Interfaces

Although liquid/liquid and air/liquid interfaces are omnipresent, very little is known up to now about the dynamics of processes occurring at such interfaces. As a detailed understanding of these processes could be of invaluable technological, environmental, and medical importance, considerable effort has been invested over the last two decades in developing new interface-selective techniques that allow for gaining further insight into the dynamics of these processes. Whereas several major results have been achieved that helped to contribute to a deeper understanding, there are still many aspects concerning the properties of liquid interfaces that are not yet fully understood. In this Perspective, the work that has been carried out so far on photoinduced interfacial dynamics will be reviewed and the current challenges in this still emerging field of research discussed.

A thermodynamic study on the complexation between riboflavin and a diaminotriazine derivative mediated by triple hydrogen bonds at water/oil interfaces

The changes in Gibbs free energy (DeltaG (int)), enthalpy (DeltaH (int)) and entropy (TDeltaS (int)) upon complexation between riboflavin (RF) and N,N-dioctadecyl-[1,3,5]triazine-2,4,6-triamine (DTT), mediated by triple hydrogen bonds at water/carbon tetrachloride, trichloroethylene and chloroform interfaces, were determined via temperature-controlled interfacial tension measurements. It was shown that hydrogen bonding interactions between RF and DTT were best characterized by large and negative DeltaH (int) values, unlike those predicted from either the polarity in each phase or the arithmetic average of the polarities in the two phases. Furthermore, the DeltaH (int) values became more positive as the dielectric constant of the oil phase was increased. These results strongly indicate that DeltaH (int) is governed by the dielectric properties of the oil phase. Adsorption of RF, DTT and the RF-DTT complex at the water/oil interface gave rise to restrictions on the translational and rotational motions of these species, as demonstrated by the DeltaS (int) values observed, which is another characteristic of interfacial complexation. The thermodynamic parameters evaluated in the present study revealed the characteristic complexation behavior that occurs at a water/oil interface, as mediated by hydrogen bonding.

Interfacial behavior of sulforhodamine 101 at the polarized water/1,2-dichloroethane interface studied by spectroelectrochemical techniques

The transfer mechanism of an amphoteric rhodamine, sulforhodamine 101 (SR101), across the polarized water/1,2-dichloroethane (DCE) interface was investigated using cyclic voltammetry, differential voltfluorometry and potential-modulated fluorescence (PMF) spectroscopy. The voltammetric response for the ion transfer of SR101 monoanion from water to DCE was observed as the diffusion-controlled transfer process. An unusual voltammetric response was found at 0.15 V more negative than the formal transfer potential of SR101(-) (deltaW(O)phi degrees') in the cyclic voltammogram and voltfluorogram. The frequency dependence of the PMF responses confirmed the presence of the adsorption processes at negative potentials. In addition, a further transient adsorption step was uncovered at deltaW(O)phi degrees'. The interfacial mechanism of SR101 is discussed by comparing the results obtained from each technique.

Critical coagulation concentration for a suspension of cation-absorptive biocolloids

Critical coagulation concentration (CCC) of a biocolloidal suspension is investigated theoretically by taking into account the influences of cationic absorption in particulate membrane phase, variation in dielectric constant, size of charged species and nonuniform distribution of fixed membrane groups. Here, an increase in both valence and effective radius of the original functional group (OFG) via absorption of electrolyte cation(s) is especially considered. The simulated results indicate that stronger membrane electricity yields a larger electrostatic repulsion and a higher potential energy, which generates a higher CCC. A lower CCC can be resulted from a larger (1) cation-functional group complex (CFGC) for a fixed difference between the radius of CFGC and that of OFG, Delta, (2) number of OFG(s) involved in the formation of a CFGC, (3) Delta for a positively charged CFGC, (4) dielectric constant of main membrane phase, in general, (5) membrane thickness for a constant amount of space-average functional groups, and (6) effective radius of anions. CCC decreases with the following parameters: (1) Delta for a negatively charged CFGC, (2) equilibrium constant of the reaction of cationic absorption, (3) nonuniform feature index of fixed groups, and (4) effective radius of cations.

Computer Simulation Investigation of the Water−Benzene Interface in a Broad Range of Thermodynamic States from Ambient to Supercritical Conditions

The dependence of the properties of the water-benzene system on the thermodynamic conditions in a broad range of temperatures and pressures has been investigated by computer simulation methods. For this purpose, Monte Carlo simulations have been performed at 23 different thermodynamic states, ranging from ambient to supercritical conditions. The density profiles of the water and benzene molecules have been determined at each of the thermodynamic states investigated. Information on the dependence of the mutual solubility of the two components in each other as well as of the width of the interface on the temperature and pressure has been extracted from these profiles. The width of the interface has been found to increase with increasing temperature up to a certain point, where it diverges. The temperature of this divergence corresponds to the mixing of the two phases. The determination of the critical mixing temperature at various pressures allowed us to estimate the upper critical curve, separating the two-phase and one-phase liquid systems, of the phase diagram of the simulated water-benzene system. In analyzing the preferential orientation of the interfacial molecules relative to the interface, it has been found that the main orientational preference of the benzene molecules is to lie parallel with the plane of the interface, and the water molecules penetrated deepest into the benzene phase prefer to stay perpendicular to the interface, pointing by one of their O-H bonds almost straight toward the benzene phase, whereas the waters located at the aqueous side of the interface are preferentially aligned parallel with the interfacial plane. Although the strength of the observed orientational preferences decreases rapidly with increasing temperature, the preferred orientations themselves are found to be independent of the thermodynamic conditions. Remains of the orientational preferences of the molecules are found to be present up to temperatures as high as 650 K. The analysis of the relative orientation of the neighboring water-benzene pairs has revealed that the radius of the first hydration shell of the benzene molecules is independent of the thermodynamic conditions, even if the system consists of one single phase. It has been found that the nearest water neighbors of the benzene molecules are preferentially located above and below the benzene ring, whereas more distant water neighbors, belonging still to the first hydration shell, prefer to stay within the plane of the benzene molecule. In the two-phase systems the dipole vector of the nearest waters has been found to be preferentially perpendicular to the vector pointing from the center of the benzene molecule to the water O atom.

A spectroscopic and photophysical study on molecular recognition via hydrogen-bonding and π–π stacking interactions

Spectroscopic and photophysical properties of a Kemp's tricarboxylic acid derivative having an anthracene chromophore (I) upon recognition of 9-butyladenine (BA) in chloroform were studied in detail. Molecular recognition of BA by I via hydrogen-bonding and pi-pi stacking interactions were sensed successfully on the basis of absorption and fluorescence spectroscopies, by which the binding constant of the I:BA complex was determined to be 240 M(-1). The fluorescence quantum yield and lifetime of I in the absence of BA were 0.24 and 5.6 ns, respectively, while those in the presence of an enough amount of BA increased to 0.35 and 13 ns, respectively. These values demonstrated that the nonradiative decay rate constant of I decreased from 13.6 x 10(7) to 5.0 x 10(7) s(-1) upon binding with BA. Such changes in the photophysical properties of I before and after complexation with BA were discussed in terms of hydrogen-bonding and pi-pi stacking interactions between I and BA. In particular, intramolecular hydrogen-bonding between the amide and imide groups in was shown to play important roles in determining the photophysical characteristics of I before complexation, while intermolecular hydrogen-bonding between I and BA governed the excited-state properties of the I:BA complex. The change in the hydrodynamic diameter of I before and after complexation with BA was also discussed on the basis of the results by fluorescence dynamic anisotropy measurements.

Adsorption behavior of lauric acid at heptane/water interface as studied by second harmonic generation spectroscopy and interfacial tensiometry

Interfacial tensiometry and second harmonic generation (SHG) spectroscopy were applied to examine the adsorption behavior of lauric acid (LA) at a heptane/water interface. From interfacial tensiometry measurements, the adsorption kinetics of LA was revealed to be diffusion-controlled, and the adsorption constant of LA was estimated to be 9.6 x 10(4) M(-1). The adsorption isotherms obtained by SHG measurements were analyzed by taking account of both the molecular orientation of LA at the interface and a surface electric field generated by the adsorbed LA layer. It was confirmed that the carboxylic groups of adsorbed LA molecules were well ordered at the heptane/water interface and the orientation of the carboxylate group was invariant during the adsorption process.

Development of a total internal reflection ultrafast transient lens method for studying molecular dynamics on an interface

We have developed the total internal reflection ultrafast transient lens (TIR-UTL) method to detect nonradiative chemical processes at interfaces and surfaces with subpicosecond time resolution. In the TIR-UTL measurements, the evanescent field of a pump beam irradiated under the TIR condition generates a refractive index change. The refractive index change is attributed to changes of the molecular electronic state, of density by molecular orientation/structure change, and of temperature by vibrational relaxation processes. The refractive index change is detected as a change of the power intensity of the probe beam adjusted coaxially with the pump beam. At first, we discuss a theoretical principle of a coaxial configuration in the TIR-UTL measurement. This configuration has an advantage of versatility over the established TIR configuration. Then, we evaluate time resolution of TIR-UTL and obtain a value of less than 400 fs. We measure the ultrafast molecular dynamics of the cationic chromophore Auramine O (AuO) at a silica/water interface. Two slow time constants originating from AuO adsorbed on the silica surface are detected by TIR-UTL. These are attributed to AuO, whose twisting motion is strongly hindered by adsorption on a silica surface.

Facilitated Sulfate Transfer across the Nitrobenzene-Water Interface as Mediated by Hydrogen-Bonding Ionophores

Facilitated SO4(2-) transfers by hydrogen bond-forming ionophores are investigated across the nitrobenzene (NB)-water interface by using polarography with a dropping electrolyte electrode. Bis-thiourea 1, alpha,alpha'-bis(N'-p-nitrophenylthioureylene)-m-xylene, is found to significantly facilitate the transfer of the highly hydrophilic SO4(2-) whereas its counterpart, N-(p-nitrophenyl)-N'-propylthiourea (ionophore 2), cannot. In contrast to the predominant formation of a 1:1 complex with SO4(2-) in the bulk NB phase, the SO4(2-) transfer assisted by 1 is indeed based on the formation of a 1:2 complex between SO4(2-) and ionophore, even under the condition of [SO4(2-)]aq >> [1]org. Such an exclusive formation of the 1:2 (SO4(2-) to ionophore) complex at the NB-water interface is not observed with structurally similar bis-thiourea 3, alpha,alpha'-bis(N'-phenylthioureylene)-m-xylene, where p-nitrophenyl moietes of bis-thiourea 1 are simply replaced by phenyl groups. The facilitated transfer of SO4(2-) with bis-thiourea 1 is further compared to that of HPO4(2-) and H2PO4- across the NB-water interface, which was previously shown to be assisted by 1 through the formation of the 1:1 and 2:1 (anion to ionophore) complexes, respectively. On the basis of these examinations, unique binding behaviors of hydrogen bond-forming ionophores at the NB-water interface are discussed, with a view towards development of ionophore-based anion-selective chemical sensors.

Total-internal-reflection broad-bandwidth sum frequency generation spectroscopy of hexadecanethiol adsorbed on thin gold film deposited on CaF2

Sum frequency vibrational spectra for hexadecanethiol (HDT) adsorbed on thin gold film deposited on the surface of a CaF2 prism have been measured using total-internal reflection broad-bandwidth sum frequency generation (TIR-BBSFG) spectroscopy. The bands attributed to the CH3 symmetric and asymmetric stretching vibrational modes were observed in the sum frequency vibrational spectra. The orientation of the methyl groups was analyzed using the ratio of the intensities of the two modes. The methyl groups of HDT on the thin gold film were much more randomly orientated than those on Au( 111).