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To sensitively detect the electrode surface roughness change in the initial process of the lithium electrodeposition/dissolution processes in ionic liquids (ILs), electrochemical surface plasmon resonance (ESPR) measurements were performed in an in situ manner on a gold electrode in a glyme-Li salt solvate IL, tetraglyme/lithium bis(fluorosulfonyl)amide ([Li(G4)⁺][FSA⁻]), and an IL, 1-butyl-3-methylimidazolium bis(fluorosulfonyl)amide ([C₄mim⁺][FSA⁻]), containing 100 mM Li⁺[FSA⁻]. The SPR angle shifts (Δθ[SPR]) were tracked simultaneously with the repetitively recorded cyclic voltammograms (CVs) of the Li underpotential deposition (UPD)/underpotential stripping (UPS). Δθ[SPR] increased/decreased in the UPD/UPS processes, sensitively responding to the refractive index change at the IL/electrode interface. The time derivative Δθ[SPR] curves basically reproduced CVs but were significantly less influenced by residual current, indicating that ESPR was an effective in situ method to track the Li UPD/UPS processes. In [Li(G4)⁺][FSA⁻], the shift in Δθ[SPR]per deposited Li amount did not change as the CV scan was repeated, indicating no change in surface roughness. In contrast, in [C₄mim⁺][FSA⁻], the same parameter increased with an increase in the scan number, reflecting the increase in surface roughness as confirmed by Fresnel reflectivity simulations. The comparison of ESPR results with the simulations suggests that for both ILs, the surface of the deposited Li layer was smoothened during the period after the Li UPD and before the Li UPS in CVs.

Reductive metal deposition at the interface between two immiscible electrolyte solutions (ITIES), the oil/water interface, is an attractive methodology to produce metal nanostructures. However, the metals that can be deposited are limited to noble metals; otherwise, water molecules are reduced. Herein, a method to overcome this limitation by utilizing a novel water-free ITIES between a hydrophilic ionic liquid and oil was introduced. As proof-of-concept experiments, the reductive deposition of zinc, a base metal with a standard redox potential more negative than that of water, was successfully realized at the ionic liquid/oil interface. The morphology of the zinc nanostructures was investigated, and the reaction mechanism was discussed, in which the electron transfer and ion transfer reactions across the interface simultaneously occurred to maintain the electroneutrality of the liquids.

Neutron reflectometry (NR) has been utilized to study the electric double layer (EDL) of ionic liquids (ILs), however, further improvement of the sensitivity toward interfacial structure would be desirable. We recently proposed two ways to improve the NR sensitivity toward the EDL structure at the IL/electrode interface (J. Phys. Chem. C, 123 (2019) 9223). First, as the electrode, a thin film of metal (Nb) was used with the scattering length density (SLD) and thickness controlled to sensitively analyze the potential dependent EDL structure. Second, the IL cation and anion were chosen so that they have large size and large SLD difference, both of which also increase the sensitivity. In the present study, we have further explored this rational material design for the sensitivity enhancement, by changing the film metal from Nb to Bi whose SLD is closer to those for two bulk materials: Si and the IL used, trihexyltetradecylphosphonium bis(nonafluorobutanesulfonyl)amide. We successfully observed not only the first ionic layer in the EDL but also the overlayers, revealing that the IL cation is specifically adsorbed on the electrode and that the cation-rich first layer induces overscreening in the overlayers up to the third ionic layer.

Molecular dynamics simulations were performed to investigate the interfacial liquid structure at the solid interface of a ethanol-cyclohexane mixture, a typical system with micro-heterogeneity in t...

The Nelder-Mead (NM) method is a popular derivative-free optimization algorithm owing to its fast convergence and robustness. However, it is known that the method often fails to converge or costs a long time for a large-scale optimization. In the present study, the NM method has been improved using direct inversion in iterative subspace (DIIS). DIIS is a technique to accelerate an optimization method, extrapolating a better intermediate solution from linear-combination of the known ones. We compared runtimes of the new method (NM-DIIS) and the conventional NM method using unimodal test functions with various dimensions. The NM-DIIS method showed better results than the original NM on average when the dimension of the objective function is high. Long tails of the runtime distributions in the NM method have disappeared when DIIS was applied. DIIS has also been implemented in the quasi-gradient method, which is an improved version of the NM method developed by Pham et al. [IEEE Trans. Ind. Informatics, 7 (2011) 592]. The combined method also performed well especially in an upwardly convex test function. The present study proposes a practical optimization strategy and proves the versatility of DIIS.

The detection capability of long-pulse laser-induced breakdown spectroscopy (LIBS) for the simultaneous analysis of submerged solids and bulk water has been investigated with a Cu target immersed in a CaCl2 solution. It is shown that a short pulse leads to the formation of Fraunhofer-type absorption lines at shorter delays and severe self-reversal structures at longer delays for Cu. In contrast, a long ns pulse leads to high-intensity clear emission lines free from self-reversal structures. Especially, the signal enhancement for the spectral lines of Ca by the long-pulse irradiation is more significant than that of Cu, with the line intensity being increased by more than 10 times, even though the breakdown threshold of the liquid is not reached. Also, the use of a long pulse broadens the variety of detectable elements. We found that the atomic lines of H and O in water can be observed, and the relative standard deviation (RSD) of the LIBS signal is reduced by nearly 3 times by employing O as an internal standard. The obtained results suggest that the simultaneous detection of submerged solids and bulk water can be well achieved by using a long pulse with a low laser pulse energy, enabling long-pulse LIBS to be more advantageous in underwater in situ applications.

The effects of pulse duration on spectral emission of H and O obtained by laser-induced breakdown spectroscopy (LIBS) in bulk water have been investigated. We found distinct atomic lines of H and O by irradiating with a laser pulse of 100 ns duration or longer. The time-resolved emission spectra show that higher emission intensity and longer plasma persistence time can be obtained with the increase of pulse duration. The quantification performance by using O as an internal standard demonstrates better linearity, higher stability and lower limit of detection (LOD). The present results suggest that the use of a long ns pulse is essential for the efficient detection of atomic lines of H and O in underwater single-pulse LIBS, which can serve as a good internal standard for quantitative analysis.

Molecular dynamic (MD) simulations have been performed for 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim+][TFSA-]), an ionic liquid (IL), on a charged graphene electrode to achieve the quantitative analysis of the static differential capacitance using the electrochemical surface plasmon resonance (ESPR). The MD simulations have provided the surface charge density on the electrode and ionic distributions in the electric double layer, both of which are indispensable for the evaluation of static differential capacitance using ESPR but are difficult to be measured by experimental techniques. This approach has allowed the quantitative analysis and explanation of the SPR angle shift in ESPR. The major contribution to the SPR angle shift is found to be the change in ionic concentrations of the first ionic layer on the electrode, owing to higher polarizabilities of ions in the first ionic layer than those in the overlayers. Moreover, the ionic orientation on the electrode and ionic multilayer structure have also been investigated in detail. The butyl group of C4mim+ in the first ionic layer is found to provide extra room for C4mim+ in the second ionic layer but exclude TFSA-, which affects the interval and regularity of ionic multilayers.

Long ns pulses have been proven to be effective in enhancing the LIBS signals of submerged solids. However, the enhancement effect on liquid detection has yet to be further revealed, which is also of great significance for the application of long-pulse LIBS in underwater chemical analysis. In this work, we investigated the temporal emission characteristics of underwater laser-induced plasma under different pulse duration conditions by using time-resolved spectroscopy and imaging techniques. It is shown that short pulses lead to the formation of an elongated and short-lived plasma with weak emission due to multi-site underwater breakdown, and a rapid decrease of line intensity with delay time. Long pulses with a pulse duration of 180 ns, on the other hand, produce a compact plasma with strong emission and a longer lifetime. Such an enhancement is more pronounced for the spectral line with a high-energy transition level, whose line intensity can be increased by more than 6 times. For this reason, the clear atomic lines of O in water were also detected, which are difficult to be observed with short pulses. This is also related to the changes in the physical properties of the plasma, i.e., the long pulses enable the plasma to maintain higher electron density and plasma temperature for a longer period of time. The present results suggest that long ns pulses can be of great advantage in underwater LIBS detection as well as capable of performing important roles in in situ chemical applications.

To investigate the potential dependence of the dynamics of the interface structure of an ionic liquid (IL), electrochemical surface plasmon resonance (ESPR) has been combined with normal pulse and differential pulse techniques at the gold electrode interface of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide. The fast response of SPR angle in 0.1 ∼ 0.2 s, originating from the orientation, distortion, and electronic polarization of ions in the first ionic layer, has been clearly observed, which is opposite to both the direction of the change in potential and of the slow response. This separate detection of the fast and slow responses has been realized based on the fact that the ionic rearrangement, which results in the slow response, does not proceed in a short potential pulse, especially for the positive pulse. The fast response, which reflects the interfacial dielectric constant, exhibits the potential-dependent dielectric saturation, i.e., a bell shape (or camel shape) potential dependence with a maximum around the potential of zero charge (PZC) and steep decrease at the potentials far from PZC. Molecular dynamics simulation explains that the decrease is caused by the strong electric field and ionic crowding in the first ionic layer which hinders the orientation and distortion of ions.

The interfacial dielectric relaxation of ionic liquids (ILs) at the gold electrode interface has been investigated using electrochemical surface plasmon resonance (SPR), by analyzing the SPR response to the potential step for four ILs with the two cations, trioctylmethylammonium and 1-butyl-3-methylimidazolium, and three amide anions, bis(fluorosulfonyl)amide, bis(trifluoromethanesulfonyl)amide, and bis(nonafluorobutanesulfonyl)amide. For all the four ILs, the SPR response to the potential step exhibits a fast relaxation process mainly ascribable to the ionic reorientation, followed by an ultraslow relaxation process ascribable to the ionic translation, with the opposite directions of SPR shift to each other. The ultraslow relaxations by the positive potential steps are always significantly slower than those by the negative steps. The average time constants of the ultraslow relaxation process and the amplitudes of the fast relaxation process are evaluated by the fitting with a multiple exponential function and are compared with the measured bulk viscosity and the dipole moments of each ion. The interfacial viscosity, estimated from the time constants of the ultraslow relaxation process, is several orders of magnitude higher than the bulk viscosity, and the viscosity increase is larger for the ILs composed of smaller ions. The amplitude of the fast relaxation, reflecting the ionic reorientation, is moderately correlated with the dipole moments of counter-ions on the electrode at the pre-step potential.

Small-angle X-ray and neutron scatterings can measure the structure factor of dispersed particles (macromolecules or colloidal particles), which can estimate the pair distribution function and the ...

In this study, one- and two-dimensional (1D and 2D) Au nanostructure/reduced graphene oxide (rGO) composites are simultaneously prepared via a redox-active ionic liquid (RAIL)|water (W) interfacial method without the use of a capping agent. The RAIL (ferrocenylmethyl)dodecyldimethylammonium tetrakis[3, 5-bis(trifluoromethyl)phenyl]borate ([FcMDDA]⁺][TFPB⁻]) plays the dual role of a hydrophobic liquid supporting the interfacial system as well as a reducing agent for the metal precursor and graphene oxide (GO). The RAIL|W interfacial system provides two kinds of composites in the growth regions with no interference between them, namely, Au nanofiber/rGO composites at the RAIL|W interface and Au nanosheet/rGO composites in the bulk of W, which can be easily separated after synthesis. The spontaneous formation mechanism of the two Au-rGO composites has been discussed in detail. Thus, an efficient strategy to prepare 1D and 2D Au-rGO composites simultaneously is developed by employing an interfacial system, which provides a new direction in the synthesis of metal–GO composites.

Highly correlated positioning of ions underlies Coulomb interactions between ions and electrified interfaces within dense ionic fluids such as biological cells and ionic liquids. Recent work has shown that highly correlated ionic systems behave differently than dilute electrolyte solutions, and interest is focused upon characterizing the electrical and structural properties of the dense electrical double layers (EDLs) formed at internal interfaces. It has been a challenge for experiments to characterize the progressive development of the EDL on the nanoscale as the interfacial electric potential is varied over a range of positive and negative values. Here we address this challenge by measuring X-ray reflectivity from the interface between an ionic liquid (IL) and a dilute aqueous electrolyte solution over a range of interfacial potentials from -450 to 350 mV. The growth of alternately charged cation-rich and anion-rich layers was observed along with a polarity reversal of the layers as the potential changed sign. These data show that the structural development of an ionic multilayer-like EDL with increasing potential is similar to that suggested by phenomenological theories and MD simulations, although our data also reveal that the excess charge beyond the first ionic layer decays more rapidly than predicted.

Electric double layer structure at the electrode interface has been studied by using molecular dynamics simulation on four quaternary ammonium-based ionic liquids (QaILs) to investigate the effect ...

A molecular dynamics simulation at the electrode interface of a quaternary ammonium ionic liquid, tributylmethylammonium bis(trifluoromethanesulfonyl)amide ([N1444+][TFSA−]), has been performed. Unlike the commonly used cations, such as 1-alkyl-3-methylimidazolium and 1,1-alkylmethylpyrrolidinium cations, N1444+ has multiple long-alkyl groups (three butyl groups). The behavior of ions at the electrode interface, especially these butyl groups, has been investigated. N1444+ at the first layer mainly has two types of orientations, lying and standing. The lying orientation is dominant at moderately negative potentials. However, the standing one becomes dominant at the more negative potentials. Due to this orientational change, the number of N1444+ increases at the first layer as the potential becomes negative even at the potentials where the anions are completely depleted there. The change in orientation results in the upward deviation of the differential capacitance from the theoretical prediction at the negative potentials. The results suggest that the orientational preference caused by the steric constraint between alkyl groups plays an important role in the behavior of the electric double layer of the ionic liquids.

In this study, we present {11 2 ¯ 2} InGaN underlayers of high In composition and multi-quantum wells (MQWs) grown using the m-plane sapphire substrate. Fully relaxed InGaN templates with high In composition (In: ∼18%) were investigated. We used an InGaN template with an In composition of 16.5%. The photoluminescence (PL) spectrum of MQWs on the InGaN template exhibited higher emission intensity and longer emission wavelength than that on a GaN template. Moreover, an InGaN barrier was found to be more suitable than a GaN barrier for the long-wavelength (red) region in the InGaN underlayer. The intensity of the PL spectrum of InGaN/InGaN MQWs was twice as high as that of InGaN/GaN MQWs in the red emission region. Finally, we analyzed the optical polarization of the MQWs on {11 2 ¯ 2} InGaN underlayers and its characteristics were discussed using theoretical equations.

Au nanofiber/carbon nanotube (CNT) ₁D/₁D composites and Janus-type Au/CNT composites have been prepared by utilizing the liquid/liquid interface between water (W) and a hydrophobic ionic liquid (IL) as a redox reaction site. AuCl₄- in W is reduced at the IL/W interface where CNTs are adsorbed, by a reducing agent in the IL, leading to the formation of the Au/CNT composites. The Au/CNT composites are Janus-type in which Au microurchins and Au nanofibers are deposited on the W side and the IL side of the CNTs on the IL/W interface, respectively. Reversing the order of the CNT adsorption and AuCl₄- reduction results in the formation of the Au nanofiber/CNT composites, which are ₁D/₁D metal/carbon composites.

Au nanoshells have been synthesized at the liquid-liquid interface between primary ammonium-based ionic liquids and water. Ionic liquid-in-water emulsions, which are spontaneously formed on the water side of the interface, function as a template for the synthesis of Au nanoshells. Selective adsorption of the primary amine group on the Au surface plays an important role in the formation of Au nanoshells.

The surface structure of four quaternary ammonium-based ionic liquids (QaILs) at the QaIL|vacuum interface has been analyzed using molecular dynamics simulation to investigate the effect of switching the length of alkyl chains (k) of the quaternary ammonium cations on the surface structure. These four QaILs are composed of a common anion, bis(trifluoromethanesulfonyl)amide (TFSA⁻), and different cations: butyltrimethylammonium (N₁₁₁₄⁺, k = 1), dibutyldimethylammonium (N₁₁₄₄⁺, k = 2), tributylmethylammonium (N₁₄₄₄⁺, k = 3), and tetrabutylammonium (N₄₄₄₄⁺, k = 4), where k represents the number of butyl chains. All the QaILs show the same features as well-studied imidazolium-based ionic liquids (ILs): the formation of the interfacial ionic layers and the orientational preference that nonpolar parts of ions point to the vacuum phase. The thickness of the first ionic layer decreases with increasing k. This results from two-dimensional nanosegregation between polar and nonpolar parts of ions, where the state of the polar parts changes from the continuous phase for small k to dispersed one for large k because of the enlargement of the nonpolar domain with increasing k. Orientational distributions of the butyl chains of the Qa cations indicate that the orientational preference of the butyl chains pointing to the vacuum phase is weakened with increasing k, especially significantly from k = 1 to 2. Even for k = 4, N₄₄₄₄⁺ still shows the orientational preference in spite of its symmetric structure. A linear relation is found between the interfacial potential differences and the surface densities of the Qa cations, suggesting a possibility to control surface absorptivity of dipolar gas molecules in ILs by changing the cation size.

Neutron reflectometry (NR) has been performed at the electrochemical interface between a Nb film and an ionic liquid, trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate ([THTDP+][PFPB–...

Molecular dynamic (MD) simulations have been performed for 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C

The structure at the electrochemical liquid/liquid interface between water (W) and trioctylmethylammonium bis (nonafluorobutanesulfonyl)amide, a hydrophobic ionic liquid (IL), was studied using molecular dynamics (MD) simulation in which the interfacial potential difference was controlled. On the IL side of the IL|W interface, ionic multilayers were found in the number density distribution of IL ions whereas monolayer-thick charge accumulation was found in the charge density distribution. This suggests that the potential screening is completed within the first ionic layer and the effect of overlayers on the potential is marginal. The W side of the interface showed the diffuse electric double layer as expected, and also unveiled a density depletion layer, indicating that the IL surface is hydrophobic enough to be repelled by water. The IL ions in the first ionic layer showed anisotropic orientation even at the potential of zero charge, in which the polar moieties were oriented to the W side and the non-polar moieties preferred parallel to the interface. When an electric field is applied across the interface so that the IL ions are more accumulated, the non-polar moieties changed the parallel preference to more oriented to the IL side due to the dipolar nature of the IL ions. The ionic orientations at the IL|W interface were compared with those at other two IL interfaces, the vacuum and graphene interfaces of the IL. The parallel preference of the non-polar moieties was similar at the IL|graphene interface but different from the perpendicular orientation toward the vacuum side at the IL|vacuum interface. The comparison suggests that water behaves like a wall repelling IL ions like a solid electrode.

An electrochemical system at the ionic liquid (IL) | oil (O) interface has been constructed and utilized as electrochemical reaction field for reductive deposition of metal nanostructures. The interface between 1-(3-hydroxypropyl)-3-methylimidazolium chloride (C3OHmimCl), a hydrophilic IL, and 1,6-dichlorohexane (containing an organic electrolyte) exhibits a polarized potential window of 150 mV, which is limited by the ion transfer (IT) of the IL cation and anion at the positive and negative edges, respectively. The polarizable IL | O interface has allowed to record voltammograms for the electron transfer (ET) and IT processes across the IL | O interface that are involved in the reductive deposition of gold at the IL | O interface. The ET between AuCl4− in the IL phase and decamethylferrocene in the O phase proceeds without applying external voltage by coupling with the IT of AuCl4−, spontaneously forming Au nanostructures at the IL | O interface.

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.

Silver chain-like nanobelts (NBs) have been prepared by a redox-active ionic liquid (RAIL)|water (W) interfacial method with utilizing the soft interface as a template. A highly hydrophobic RAIL (ferrocenylmethyl)dodecyldimethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([FcMDDA+][TFPB−]) works as both the reducing agent and the liquid substrate in the RAIL|W interfacial system. The morphology of Ag chain-like NBs, which possesses flat surfaces, sharp corners on the edges, and large aspect ratio, combines the features of one-dimensional (1D) and 2D nanostructures. The formation of Ag chain-like NBs is realized by the linkage of small nanounits through an oriented growth along the RAIL|W interface. The size and shape of the Ag deposited at the RAIL|W interface can be controlled by regulating the concentration of Ag precursor in W, where higher (lower) concentration of AgNO3 induces the formation of Ag NBs with smaller (larger) size. The prepared Ag chain-like NBs show good surface enhanced-Raman scattering activity which can be used as the substrate for plasmonic applications.

Pt nanofibers have been prepared with a redox reaction occurring spacio-selectively at the liquid-liquid interface between a highly hydrophobic ionic liquid (IL) and water (W), which has been utilized as two-dimensional reaction field. The preparation is conducted based on the concept of the coupling between ion transfer (IT) and electron transfer (ET) across the IL|W interface, which previously enabled direct formation of dendritic Au nanofibers at the IL|W interface (Chem. Commun., 2015, 51, 13638). The successful preparation of Pt nanofibers has manifested the generality of the IL|W interface method for the formation of metal nanofibers. Electrochemical measurements at the IL|W interface have confirmed that IT of PtCl42− from W to IL is actually coupled with ET between PtCl42− initially dissolved in W and decamethylferrocene in IL, leading to the spontaneous formation of Pt nanofibers at the IL|W interface. Herein, IT of PtCl42− ions from W to IL not merely neutralizes the excess charges generated in each liquid phase by ET, but also provides PtCl42− into IL as reactants for the growth of Pt nanofibers that takes place at the IL side of the IL|W interface. The electrocatalytic activity of the Pt nanofibers in oxygen reduction reaction (ORR) has also been evaluated.

Electrochemical surface plasmon resonance (ESPR) has been employed as a probe of redox reactions at the interface between gold (Au) and ionic liquid (IL). Two redox couples, ferrocene (Fc)/Fc+ and (ferrocenylmethyl)trimethylammonium (FTA+)/FTA2+, and two ILs, trioctylmethylammonium bis(trifluoromethanesulfonyl)amide ([TOMA+][C1C1N−]) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim+][C1C1N−]) have been studied. In the ESPR measurements, the shift of the SPR angle has been recorded simultaneously with cyclic voltammogram (CV). It has been revealed that the SPR angle shift reproduces CV when the former is semi-differentiated or the latter is semi-integrated, and, therefore, that the ESPR response probes the surface concentration of redox couples at the IL|Au interface. Among the four combination of the two redox couples and the two ILs, the case for Fc/Fc+ in [TOMA+][C1C1N−] shows significantly greater ESPR response than the other three cases. A model has been established for the relationship between the SPR angle shift and the surface concentration of redox species. The model predicts that the SPR angle shift becomes pronounced with increasing DR/DO, the diffusion coefficient ratio of the reduced (R) and oxidized (O) species when the reduced species is initially dissolved in the IL as is the case for the present study. Significantly greater DR/DO for Fc/Fc+ in [TOMA+][C1C1N−] than the three other cases has been confirmed from the DR and DO measurements by CV with a microdisk electrode. The trend of the measured DR and DO values agrees with recent findings by researchers that small neutral solutes in ILs composed of large ions diffuse fast beyond the prediction of the Stokes-Einstein relation.

We calculate potential of mean force (PMF) between the spherical particles in an ionic liquid by using an integral equation theory, a statistical mechanical theory for liquids. The PMFs between like-charged particles, unlike-charged particles, charged-uncharged particles, and uncharged particles are calculated. Effect of the particle diameter on the PMF is also studied. It is found that the pairs of the like charged particles, unlike charged particles, and uncharged particles are stabilized when they are closely coupled, whereas the charged-uncharged particles are destabilized by the contact. To investigate roles of the energy and entropy in the PMF, it is decomposed into the energetic and entropic components by a temperature differentiation. It is found that the energetic component has relatively high dependency on the surface charges of the immersed particles. On the other hand, the entropic component does not change much upon change in the surface charges of the immersed particles. Generally, an interaction between two particles in an ionic liquid is often explained by the electrostatic and van der Waals interactions, which are classified as the energetic component. However, we found that the entropic component also plays an important role in the interaction.

Colloidal probe atomic force microscopy (CP-AFM) can be used for measuring force curves between the colloidal probe and the substrate in a colloidal suspension. In the experiment, an oscillatory force curve reflecting the layer structure of the colloidal particles on the substrate is usually obtained. However, the force curve is not equivalent to the interfacial structure of the colloidal particles. In this paper, the force curve is transformed into the number density distribution of the colloidal particles as a function of the distance from the substrate surface using our newly developed transform theory. It is found by the transform theory that the interfacial stratification is enhanced by an increase in an absolute value of the surface potential of the colloidal particle, despite a simultaneous increase in a repulsive electrostatic interaction between the substrate and the colloidal particle. To elucidate the mechanism of the stratification, an integral equation theory is employed. It is found that crowding of the colloidal particles in the bulk due to the increase in the absolute value of the surface potential of the colloidal particle leads to pushing out some colloidal particles to the wall. The combined method of CP-AFM and the transform theory (the experimental-theoretical study of the interfacial stratification) is related to colloidal crystallization, glass transition, and aggregation on a surface. Thus, the combined method is important for developments of colloidal nanotechnologies.

Static differential capacitance (Cdc) at the liquid-liquid interface between ionic liquids (ILs) and eutectic Ga-In alloy (EGaIn) has been measured using the pendant drop method for two ILs: 1-ethyl-3-methylimidazolium tetrafluoroborate ([C₂mim⁺]BF₄⁻) and 1-octyl-3-methylimidazolium bis(nonafluorobutanesulfonyl)amide ([C₈mim⁺][C₄C₄N⁻]). The potentials of zero charge for the IL|EGaIn interfaces are shifted compared with the IL|Hg interfaces with an amount that can be considered by the difference in the work functions of EGaIn and Hg. The measured Cdc at the [C₂mim⁺]BF₄⁻|EGaIn interface has well reproduced the camel-shape potential dependence of Cdc at the Hg interface of the same IL at the negatively charged potential region. This suggests that there are little specific interaction between the IL ions with EGaIn and Hg. The [C₈mim⁺][C₄C₄N⁻]|EGaIn has been compared with the [C₈mim⁺]BF₄⁻|Hg interface where IL-cation is the same but IL-anion is different. Also in that case, Cdc is similar to each other at the negatively charged potential region, which means that accumulated C₈mim⁺ ions at the interface mainly govern the Cdc behavior.

We study the pulse-to-pulse variation of the emission spectral intensity of underwater laser-induced breakdown spectroscopy. Emission spectral intensity and its dispersion were measured as a function of the fluence of the laser pulse at a metal target surface immersed in water. The coefficient of variation, which is an index of the dispersion, showed a minimum at a certain fluence. The dispersion at the low fluences was attributed to the variations of the population density and the atomic excitation temperature, according to the error propagation analysis of the theoretical spectral line intensity based on the Boltzmann distribution. The population density and the temperature showed a negative correlation, which is consistently explained by the unstable division of pulse energy into two parts, i.e., the early part of a pulse is attributed to materials ablation and hence to the population density, while the other part to plasma heating and hence to the temperature.

The effect of two leveling additives, thiourea (TU) and coumarin (CM), on cobalt electrodeposition process in an ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (C4mimTFSA), at the gold surface has been analyzed in situ using electrochemical surface plasmon resonance (ESPR), in which the SPR resonance angle (Δθ) has been recorded simultaneously with cyclic voltammograms. The two additives show Δθ behaviors different from the additive-free case and from each other. In the case of TU, the positive Δθ shift due to Co cathodic deposition is larger than the additive-free case. Even after the subsequent negative Δθ shift due to Co anodic dissolution, Δθ remains positive not returning to the original value. This indicates that the dissolution of gold facilitated by TU roughens the surface of the gold electrode. In contrast, in the case of CM, the positive Δθ shift due to Co cathodic deposition is smaller than the additive-free case. A model analysis using Fresnel reflectivity has revealed that CM actually smooths the surface of the Co film even in the initial process of the electrodeposition. These findings illustrate that ESPR can sense in situ the surface roughness change during electrodeposition on the order of Å.

We study resonant frequencies of water sessile drops in various oils. The resonant frequency is experimentally obtained as the frequency shift of the scattered laser light, using an optical heterodyne technique. The resonant frequency is explained by the capillary wave propagating along the meridian curve of the drop with the fixed-end boundary condition at the three-phase contact line, although the sessile drop in air considerably deviates from the fixed-end boundary condition. We study this point further by observing the vibration directly with a fast video camera, and by analyzing the contour of the snapshot near the contact line. The results agree with the resonant frequency analysis results, where the number of half waves in the meridian curve of the water sessile drop is an integer when the drop is in oil, while in air, it deviates considerably from an integer.