Diffusion behavior of rhodamine 6G in single octadecylsilyl-functionalized silica particle revealed by fluorescence correlation spectroscopy

We investigated the diffusion behavior of rhodamine 6G (Rh6G) within single octadecylsilyl-functionalized (ODS) silica particle in an acetonitrile (ACN)/water system using fluorescence correlation spectroscopy (FCS). FCS measurements were conducted at the center of the particle to exclusively determine the intraparticle diffusion coefficient (D). The obtained D values were analyzed based on a pore and surface diffusion model, the results of which indicate that surface diffusion primarily governs the intraparticle diffusion of Rh6G. Furthermore, an increase in the concentration of ACN (CACN) resulted in a corresponding increase in the surface diffusion coefficient (Ds), whereas the addition of NaCl did not significantly affect the Ds values. We attributed this dependence of Ds to the dielectric constant change in the interfacial liquid phase formed on the ODS layer. Specially, Ds values of (4.0 ± 0.5) × 10-7, (7.7 ± 1.1) × 10-7, (1.0 ± 0.3) × 10-6, and (1.1 ± 0.2) × 10-6 cm2 s-1 were obtained for CACN = 20, 30, 40, and 50 vol%, respectively. We anticipate that this approach will contribute to advancing research on intraparticle mass transfer mechanisms.

Dynamic Interfacial Tension Measurement for the Kinetic Study of Eu(III) Extraction in the NaNO3/Tributyl Phosphate System Based on an Increase in Interfacial Tension

We demonstrated for the first time that interfacial tension measurements can be used to evaluate the kinetics of the solvent extraction of metal ions. The Eu(III) extraction mechanism in the nitrate ion/tributyl phosphate (TBP) system was investigated on the basis of dynamic interfacial tension. Interestingly, the interfacial tension of the TBP droplet (γ) increased with Eu(III) extraction. This behavior can be explained by the electrocapillary effect. The time dependence of γ was kinetically analyzed, and we demonstrated that the rate-determining process was the interfacial reaction of Eu(III). Furthermore, the dependence of the mass transfer rate constant upon the concentration of the nitrate ions revealed that two nitrate ions were involved in the interfacial reaction during Eu(III) extraction. However, no change in the rate constant upon TBP concentration was observed, because the change in the TBP concentration did not affect the electrocapillary effect. We determined the forward and backward reaction rates to be k1 = (1.5 ± 0.7) × 10-6 m M-2 s-1 and k-MT = (6.9 ± 3.9) × 10-7 m s-1. Therefore, for the first time, we demonstrated that dynamic interfacial tension, which is involved in the electrocapillary effect, can be used to elucidate the kinetics of Eu(III) extraction. We expect that this study will attract the attention of researchers in several fields, including physical and analytical chemistry.

Kinetics and mechanism of Eu(III) transfer in tributyl phosphate microdroplet/HNO3 aqueous solution system revealed by fluorescence microspectroscopy

In this study, we reveal an Eu(III) extraction mechanism at the interface between HNO₃ and tributyl phosphate (TBP) solutions using fluorescence microspectroscopy. The mass transfer rate constant at the interface is obtained from the analysis of fluorescence intensity changes during the forward and backward extractions at various HNO₃ and TBP concentrations to investigate the reaction mechanism. This result indicates that one nitrate ion reacts with Eu(III) at the interface, whereas TBP molecules are not involved in the interfacial reaction, which is different from the results obtained using the NaNO₃ solution in our previous study. We demonstrate that the chemical species of Eu(III) complex with nitrate ion and TBP in the aqueous solution play an important role for the extraction mechanism. The rate constants of the interfacial reactions in the forward and backward extractions are (4.0-5.0) × 10^-7 m M^-1 s^-1 and (3.2-3.3) × 10^-6 m s^-1, respectively. We expect that our revealed mechanism provides useful and fundamental knowledge for actual solvent extraction.

Microscope measurements for the transient formation of W/O emulsions of sodium bis(2-ethylhexyl) sulfosuccinate in the dodecane/water interfacial region

The present study investigated the transient formation of water-in-oil (W/O) emulsions of sodium bis(2-ethylhexyl) sulfosuccinate (aerosol OT, AOT) in a dodecane/water interfacial region and the anomalous uptake of water in the dodecane phase by in situ bright-field optical microscopy and water concentration measurements in detail. The hydrodynamic radius of the individual W/O emulsions in the dodecane phase was determined to be 0.1-1.2 μm from the analysis of their diffusion behavior; they are much larger than common W/O microemulsions (a few nanometers in radius). At first, they were formed spontaneously in the dodecane/water interfacial region without shaking, and they diffused away into the dodecane phase. Then, almost all of them vanished at the interface by fusion. Their number and the water concentration in the dodecane phase increased first and then decreased gradually. The formation mechanism was discussed with estimated concentration profiles of AOT and water molecules, which suggests that larger W/O emulsions of 0.01-0.44 μm in radius can be formed in the dodecane phase near the interface (within 2 μm) because the concentration of AOT becomes lower than that of water there.

Controllable adsorption and ideal H-aggregation behaviors of phenothiazine dyes on the tungsten oxide nanocolloid surface

The monomer-aggregate equilibrium of four phenothiazine (PN) dyes, containing thionine (TH), methylene blue (MB), new methylene blue (NMB), and 1,9-dimethylmethylene blue (DMB), in the tungsten(VI) oxide (WO(3)) nanocolloid solution has been investigated by means of UV-vis spectroscopy. Addition of PN dye into the WO(3) nanocolloid solution brought about significant changes in the absorption spectrum, suggesting the formation of H-type (face-to-face fashion) trimer on the WO(3) nanocolloid surface. The adsorptivity of PN dyes onto the WO(3) nanocolloid surface was diminished by the raising the ionic strength, indicating the evidence of the electrostatic interaction between cationic PN dye and negatively charged WO(3) nanocolloids. The detail analysis of each spectral data provided insight into the effect of molecular structure of PN dyes on the adsorption and aggregation behaviors on the WO(3) nanocolloid surface. Moreover, in situ measurement of PN dye aggregation using the centrifugal liquid membrane (CLM) technique revealed that the aggregation of PN dyes on the WO(3) nanocolloid surface proceeded in a two-step three-stage (monomer --> dimer --> trimer) formation. The aggregation mechanism of PN dyes on the WO(3) nanocolloid surface was discussed on the basis of Kasha's exciton theory.

Use of porous anodic alumina membranes as a nanometre-diameter column for high performance liquid chromatography

The possibility of using porous anodic alumina membranes as a column for normal-phase high performance liquid chromatography was evaluated using phenol and toluene with mobile phases having different solvent compositions.

Site-Selective Formation of Optically Active Inclusion Complexes of Alkoxo-Subphthalocyanines with β-Cyclodextrin at the Toluene/Water Interface

Several subphthalocyanine derivatives that contain an alkoxo substituent as an axial ligand (RO-Subpc, R = 9-anthracenemethyl, benzyl, phenyl, 3,5-dimethylbenzyl, 3,5-dimethylphenyl, 4-methylbenzyl, and 4-methylphenyl) were synthesized. The formation of inclusion complexes of RO-Subpc with beta-CD in DMSO and at the toluene/water interface was investigated by UV/Vis absorption spectroscopy, induced circular dichroism (ICD), and nuclear magnetic resonance (NMR) measurements. Interfacial tension measurements suggested that beta-CD adsorbed as a monolayer at the toluene/water interface and probably orientated towards the toluene phase with its primary face. The 1:1 composition of beta-CD.RO-Subpc inclusion complexes was confirmed in DMSO and at the toluene/water interface for BzO-Subpc, PhO-Subpc, MeBzO-Subpc, and MePhO-Subpc. A 2:1 inclusion complex of AnO-Subpc formed in DMSO. The observed ICD spectra of beta-CDRO-Subpc inclusion complexes are discussed with respect to molecular modeling and the simulation based on Tinoco-Kirkwood theory. Interestingly, the ICD spectra of beta-CD.BzO-Subpc and beta-CD.MeBzO-Subpc inclusion complexes exhibited a negative sign in DMSO and a positive sign at the toluene/water interface. This reversal of the ICD sign strongly suggests a difference in the structure of the inclusion complexes: beta-CD at the interface formed the inclusion complex with its primary face, whereas the secondary face of beta-CD bound favorably to RO-Subpc in DMSO.

Formation of helical J-aggregate of chiral thioether-derivatized phthalocyanine bound by palladium(II) at the toluene/water interface

A chiral thioether-substituted phthalocyanine ((2,3,9,10,16,17,23,24-octakis-1-phenylethylthiophthalocyaninato)magnesium(II) [MgPc(SEtPh)8]) has been synthesized, which can be bound by soft-metal ions such as palladium(II) ion. Aggregates formed from MgPc(SEtPh)8 and Pd(II) in toluene and at the toluene/water interface were characterized by means of UV-vis absorption and circular dichroism (CD) spectrometries using centrifugal liquid-membrane (CLM) cell. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF/MS) and scanning electron microscope (SEM) were used as complementary techniques. The toluene solution of MgPc(SEtPh)8 had no optical chirality. However, the addition of PdCl2 into the toluene solution of MgPc(SEtPh)8 induced optical chirality, which indicated the formation of a twisted H-type dimer (face-to-face fashion) of MgPc(SEtPh)8 bound by four PdCl2 in the bulk toluene solution. On the other hand, in the toluene/water interface, helical J-aggregate (head-to-tail fashion) of MgPc(SEtPh)8 bound by PdCl2 was formed, in which one J-aggregate unit contained five 1:2 complexes of MgPc(SEtPh)8-Pd(II) on average. On the basis of the experimental results and the exciton theory for optical chirality, a possible mechanism for the chiral aggregation of MgPc(SEtPh)8-Pd(II) complexes at the interface was proposed. In the present study, we demonstrated a novel strategy for the design of helical phthalocyanine aggregates using the liquid/liquid interface as a template.

Recent analytical methodologies on equilibrium, kinetics, and dynamics at liquid/liquid interface

The form of liquid/liquid interface is flexible and it cannot be fixed at a spatial position. Also the interface is prevented from any physical contact by the organic phase and aqueous phase. In addition, analytical methods operated in vacuo cannot be applied. These restrictions depressed the development of liquid/liquid interfacial chemistry. However, the modification of liquid/liquid interfacial form and original analytical methods have been invented interdependently. The present review classifies the forms of liquid/liquid interface first, and it arrays the related analytical methods with brief explanations. It dominantly deals with recent reports of analytical methodologies, which were published in 2001-2004, on equilibrium, kinetics, and dynamics of substances at liquid/liquid interface, but it also includes historically important studies.

Measurement of ordered associate of protonated tetraphenylporphine formed at toluene/aqueous H2SO4 interface by attenuated total internal reflection spectroscopy with polarized light

Attenuated total internal reflection (ATR) spectroscopy with an s- or p-polarized visible light was examined for some species of protonated 5,10,15,20-tetraphenylporphine (tpp) at toluene/aqueous H2SO4 (3-6 mol dm(-3)) interface. Tpp initially dissolved in the toluene phase was diprotonated at the interface to form monomeric H2tpp2+, the absorption peak of which was 438nm. At the same time, a long H2tpp2+ oligomer was formed, the absorption peak of which was 448 nm. The two interfacial species were transient. Just after their disappearance, a rod-shaped H2tpp2+ associate was formed at the interface, the absorption peak of which was 417 and 478 nm. The former and latter wavelengths corresponded to H- and J-bands of the associate, respectively. Theoretical calculation of the strength of electric field of light at the interface allowed one to estimate the interfacial concentration of the three species with measured reflection absorbance (A(R)). The monomeric H2tpp2+ and its oligomer were at sub-monolayer levels, whereas the associate was at a multilayer level. Reflection absorption anisotropy (K(R)), which was calculated from A(R) with the s- and p-polarized lights, was adopted for the evaluation of out-of-plane orientation of the interfacial species for the first time. The K(R) value suggested that the rod-shaped associate lay at the interface.

Aggregation of thioether-substituted subphthalocyanines with palladium(ii) at the toluene-water interface

Novel chloro-subphthalocyaninatoboron()(Subpc) derivatives with six thioether at the 2 and 3 positions of peripheral benzene (Subpc(SR)) such as chloro-[2,3,9,10,16,17-hexakis-ethylthio-subphthalocyaninato]boron()(Subpc(SEt)) and chloro-[2,3,9,10,16,17-hexakis-benzylthio-subphthalocyaninato]boron()(Subpc(SBz)) were synthesized. The interfacial adsorption of Subpc and Subpc(SR), and the interfacial aggregation of Subpc(SR)-Pd() complexes in the toluene-water system have been investigated by means of high-speed stirring (HSS) spectrometry, centrifugal liquid-membrane (CLM) spectrometry, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/MS). Characteristically orientated structures of Subpc(SR) molecules and the aggregates of Subpc(SR)-Pd() complexes at the toluene-water interface have been suggested depending on the structures of Et and Bz at the periphery. The interfacial aggregation was observed only in the presence of palladium(), in which one aggregate unit contained at least seven 1 : 2 complexes of Subpc(SR)-Pd(). Interestingly, the absorption spectra of the interfacial aggregates of Subpc(SEt)-Pd() and Subpc(SBz)-Pd() complexes were very similar to the previously reported spectra of μ-oxo dimer and binuclear molecule of subphthalocyanine, respectively, suggesting the formation of "face-to-face" or H-aggregate for Subpc(SEt)-Pd() and "head-to-tail" or J-aggregate for Subpc(SBz)-Pd(), respectively, at the toluene-water interface.

Chemical oscillation with periodic adsorption and desorption of surfactant ions at a water/nitrobenzene interface

Chemical oscillations with periodic adsorption and desorption of surfactant ions, alkyl sulfate ions, at a water/nitrobenzene interface have been investigated. The interfacial tension was measured with a quasi elastic laser scattering (QELS) method and the interfacial electrical potential was obtained. We found that this oscillation consists of a series of abrupt adsorptions of ions, followed by a gradual desorption. In addition, we observed that each abrupt adsorption was always accompanied by a small waving motion of the liquid interface. From the analysis of the video images of the liquid interface or bulk phase, we could conclude that each abrupt adsorption is caused by nonlinear amplification of mass transfer of ions from the bulk phase to the liquid interface by a Marangoni convection, which was generated due to local adsorption of the surfactant ions at the liquid interface that resulted in the heterogeneity of the interfacial tension. In the present paper, we describe the mechanism of the chemical oscillation in terms of the hydrodynamic effect on the ion adsorption processes, and we also show the interfacial chemical reaction with ion exchange during the ion desorption process.

Interfacial aggregate growth process of Fe(II) and Fe(III) complexes with pyridylazophenol in solvent extraction system

The complexation mechanism and aggregate formation of bis[2-(5-bromo-2-pyridylazo)-5-diethylaminophenolate] iron(II) and iron(III) complexes at the heptane-water interface were studied spectrophotometrically by the high-speed stirring method and the centrifugal liquid membrane method. Furthermore, the reduction process of the Fe(III) complex with ascorbic acid at the interface was spectrophotometrically observed. The chemical compositions of the interfacial aggregate of complexes have been proved by the X-ray photoelectron spectroscopy. The aggregation of the complex at the interface was observed as a red-shifted, very strong and narrower absorption band with respect to the absorption band of the monomer complex. The aggregate of Fe(III) complex showed more shifted spectrum than that of Fe(II) complex, which proposed the larger aggregation number of Fe(III) aggregate (n = 8) than that of Fe(II) aggregate (n = 3). The obtained rate constants of interfacial aggregation were smaller than rate constants of interfacial monomer complexation, because the formation of aggregate required the assembly of the monomers.

Complex Formation of Copper(II) and Iron(II) with Octadecyloxythiazolylazophenol at the Heptane-Water Interface

The complex formation of Cu(II) and Fe(II) with a hydrophobic ligand, 5-(octadecyloxy)-2-(2-thiazolylazo)phenol (TARC18), was investigated in the heptane/water system by the high-speed stirring spectrometry and the micro-two phase flow ESI/MS method. At first, the dissociation constant of TARC18 at the heptane/water interface was determined as pKa = 7.11. The interfacial complexation of the ligand with Cu(II) and Fe(II) under stirred conditions progressed with an increase of the pH. The experimental results showed that a 1:1 complex of Cu(II) and TARC18 was formed at the interface, but was hardly extracted into the heptane phase. On the other hand, the 1:2 complex of Fe(II) with TARC18 formed at the interface was significantly extracted into the heptane phase. The extraction constants and interfacial complex formation constants were estimated for the two systems from the experimental results, and all of the reaction schemes, including the interfacial reactions, were elucidated.

Photoinduced Electron Transfer of 5,10,15,20-Tetraphenylporphyrinato Zinc(II) at the Polarized Water/1,2-Dichloroethane Interface

The photocurrent at the polarized water/1,2-dichloroethane (DCE) interface was successfully observed in the presence of a lipophilic sensitizer, 5,10,15,20-tetraphenylporphyrinato zinc (ZnTPP), in the organic phase. The photocurrent transient responses were apparently affected by the employed organic supporting electrolyte: tetrapenthylammonium tetraphenylborate (TPnATPB) or tris(tetraoctylammonium)tungstophosphate ((TOcA)3PW12O40). The photocurrent measured in the TPnATPB system exhibited rather slow responses associated with the ion transfer of photoproducts. On the other hand, the photoinduced heterogeneous electron transfer could be observed in the use of (TOcA)3PW12O40. The photocurrent intensity in the (TOcA)3PW12O40 system exhibited an apparent pH dependence and the photoreduction of hydrogen ions probably took place at the water/DCE interface. By analyzing the real and imaginary components of the photocurrent depending on the photoexcitation frequency, we roughly estimated the phenomenological rate constants of the product separation (k(ps)) and recombination (k(rec)) processes as log(k(ps)/s(-1)) = 1.5 +/- 0.2 and log(k(rec)/s(-1)) = 1.8 +/- 0.1, respectively.

Measurement of Circular Dichroism Spectra of Liquid/Liquid Interface by Centrifugal Liquid Membrane Method

The direct measurement of the circular dichroism (CD) spectra of liquid/liquid interface has been achieved for the first time by the centrifugal liquid membrane (CLM) method combined with a conventional CD spectropolarimetry. In the sample chamber of the CD spectropolarimeter, a cylindrical glass cell containing small amounts of organic and aqueous phases was rotated at ca. 7000 rpm to generate a two-phase liquid membrane with a high specific interfacial area. The CD spectra of the J-aggregate of protonated 5,10,15,20-tetraphenylporphyrin formed at the toluene/sulfuric acid interface in the rotating cell have been measured. The results demonstrated the novelty and advantages of this method.

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.

Interfacial Hydrolysis in the .MU.-oxo Dimer Formation of Tetraphenylporphinatomanganese(III) in the Toluene/Water System

The interfacial adsorption and dimerization behavior of tetraphenylporphinatomanganese(III)chroride (Mn(TPP)Cl) in the toluene/water two phase system was studied. The absorption spectra of the porphyrin complex in the organic phase were blue shifted with increasing of pH in the aqueous phase, suggesting the formation of the mu-oxo dimer. The interfacial species in these systems were examined by means of UV/vis external reflection (ER) spectrophotometry. ER spectral maxima were also blue shifted with the increase in pH, whereas the shape of the ER spectra in the basic conditions was different from that of the mu-oxo dimer. The observed results suggest that the hydroxide-coordinated monomer, Mn(TPP)OH, was generated from the Mn(TPP)(H2O)+ at the interface under the basic conditions and that was further converted to the mu-oxo dimer, which was soluble into the toluene phase.

Magnetic Field Enhanced Microextraction Rate of Europium(III) with 2-Thenoyltrifluoroacetone and Oxalate at Dodecane-Water Interface

A microscopic system for the observation of reactions at a liquid-liquid interface was established, to which strong magnetic fields (0-0.4 T) could be applied with permanent magnets. In situ observation for the interfacial extraction of fluorescent and paramagnetic Eu(III) ion with 2-thenoyltrifluoroacetone (Htta) in dodecane was carried out. In the presence of oxalate (ox2-), micro-aggregates of Eu(III)-tta-ox complexes were generated in the aqueous phase before its extraction. When the micro-aggregates diffused to the dodecane-water interface, Eu(tta)3 was extracted with excess Htta in the dodecane phase. The microextraction process of the aggregates was observed as random flashes of Eu(tta)3 fluorescence at the dodecane-water interface. The single flash contained about 10(-16)-10(-14) mol of Eu(III). An application of magnetic fields made the flash frequency increase, which corresponded to an enhancement of interfacial Eu(III) extraction rate. The enhancement effect was attributable to the magnetophoresis of the paramagnetic microaggregates to the dodecane-water interface.