Activation Energy of Ion Desorption at Ionic Liquid/Pt Electrode Interfaces: A Sum-Frequency Generation Vibrational Spectroscopic Study.

Electrolyte/electrode interfaces of room-temperature ionic liquids (RTILs) exhibit hysteretic responses to different applied potentials owing to the differences in the ion adsorption/desorption processes; the ion desorption requires excess potential, which reflects the activation energy of ion desorption. Thus far, the contributions of the ion adsorption energy and the activation barrier for ion desorption toward the ion-dependent excess potential have not been quantified. Herein, we report on our infrared-visible sum-frequency generation vibrational spectroscopy study of the hysteretic responses of the anion adsorption/desorption at Pt electrode interfaces using neat, binary, and diluted RTILs composed of 1-butyl-3-methylimidazolium cations ([C4mim]+) and bis(trifluoromethanesulfonyl)amide ([TFSA]-) and trifluoromethanesulfonate ([OTf]-) anions. Experimental results are compared to the theoretical calculations for the electric double layer model. The hysteretic response of the RTIL/Pt interface derives predominantly from the activation energy of anion desorption, which causes the negative excess potential required for anion desorption. A comparison of the anion adsorption/desorption behaviors of neat RTILs with those of binary and diluted RTILs reveals that the large activation energy of anion desorption at the neat RTIL/Pt interface originates largely from the activation barrier for restructuring ionic layering in the diffuse layer.

Effect of pH and salt on the protonation state of fatty aniline investigated by sum-frequency vibrational spectroscopy.

Langmuir monolayers of fatty aniline (C16-aniline) were investigated using sum-frequency vibrational spectroscopy at various pH levels and NaCl concentrations. To analyze the sum-frequency generation (SFG) spectra of aniline, a multi-peak Lorentzian model, in accordance with the theory of SFG for a charged interface, was applied. First, SFG spectra of C16-aniline from pH 2 to 6 at a constant ionic strength of 10 mM (where the phase of the complex potential of the dc-induced signal was suppressed to a few degrees) were fitted with the above-mentioned method. The mean-field theory that considers the chemical equilibrium of the aniline headgroup was used to analyze the fitting results to find that the pKa of aniline is 4.4 ± 0.3. The protonation fraction of the aniline headgroup was estimated to be less than 5% at pH 6 and NaCl concentrations were up to 1M. The generalized Poisson-Boltzmann equation in the Gouy-Chapmann model effectively explained the observed SFG spectra in the OH region for fatty aniline at pH as low as 2, even for the systems without addition of any salt.

Sensitive Detection of Biomolecular Adsorption by a Low-Density Surfactant Layer Using Sum-Frequency Vibrational Spectroscopy.

Small molecules or proteins interact with a biomembrane in various ways for molecular recognition, structure stabilization, and transmembrane signaling. In this study, 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP), having a choline group, was used to investigate this interaction by using sum-frequency vibrational spectroscopy. The sum-frequency spectrum characteristic of a neat monolayer changed to that of a bare air/water interface at a larger molecular area of the DPTAP molecules due to local laser heating. Upon introduction of the aromatic molecules in the subphase at around 120 Å2 per molecule, the sum-frequency signal suddenly reappeared due to molecular adhesion, and this was utilized to probe the adsorption of the aromatic ring molecules in the water subphase to the choline headgroup of the DPTAP by cation-π interaction. The onset concentrations of this sum-frequency signal change allowed a comparison of the relative interaction strengths between different aromatic molecules. A zwitterionic surfactant molecule (DPPC) was found to interact weakly compared to the cationic DPTAP molecule.

Systematic investigation of coupling between symmetric and antisymmetric stretches of D2O in CHCl3 by 2D IR.

The coupling between the symmetric (νs) and antisymmetric (νa) OD stretch modes of monomeric D2O in CHCl3 is investigated using polarization-dependent two-dimensional infrared (2D IR) spectroscopy supported by numerical 2D IR simulations based on the exciton-band theory. The relationship between the local modes' and the exciton states' parameters is systematically studied, including center frequencies, diagonal anharmonicities, coupling, and off-diagonal anharmonicity. The mean coupling between νs and νa is accurately evaluated to be -49.96 ± 0.14 cm-1. The degree of relaxation in the harmonic approximation is quantified, and the angle between the exciton-state dipoles is accurately evaluated to be 101.4° ± 3.6°. In addition, the effect of the local-mode frequency correlation on the resulting exciton-state frequency correlation and the spectral shape of the linear and 2D IR spectra are also investigated.

Sum-Frequency Vibrational Spectroscopic Study of the Cation-π Interaction: Amine and Guanidine.

The cation-π interaction is an interaction between a positively charged cation and π electrons in an aromatic group of a molecule. It is considered to play key roles in signal transduction, stabilization of the protein structure, enzyme catalysis in biology, and wet adhesion and biomolecular condensation. In this study, octadecylguanidine hydrochloride (ODG) and octadecylamine (ODA) having guanidine and amine headgroups, respectively, are found to interact with π molecules (phenol or indole) as investigated by sum-frequency vibrational spectroscopy. ODG is unstable and does not form a neat monolayer on the water surface. However, after adding π molecules into subphase water, it becomes more stable against dissolution as evidenced by the appearance of its CHx peaks and a CH peak of the aromatic ring in the sum-frequency spectrum. Unlike ODG, ODA forms a stable monolayer on the water surface at a neutral pH. After adding π molecules into the solution, the amine-π interaction promotes the protonation of the amine headgroup and the penetration of the π molecules makes the ODA monolayer more disordered. Indole is found to be more effective in binding with the ODG as compared to phenol.

Characterization of Noise in a Single-Molecule Fluorescence Signal.

Single-molecule fluorescence experiments allow monitoring of the structural change and dynamics of a single biomolecule in real time using dye molecules attached to the molecule. Often, the molecules are immobilized on the surface to observe a longer molecular dynamics, yet the finite photon budget available from an individual dye molecule before photobleaching sets the limit to the relatively poor signal-to-noise level. To increase the accuracy of these single-molecule experiments, it is necessary to study the cause of noise in the fluorescence signal from the single molecules. To find the origin of this noise, the lifetime of the fluorescent dye molecules labeled on surface-immobilized DNA was measured by using time-correlation single photon counting. The standard deviation of the fluorescence lifetimes obtained from repeated measurements of a single dye molecule with the total photon number N decreased as 1/N, thus following a shot noise of the Poisson statistics. On the other hand, an additional constant noise source, which is independent of the photon number, was observed from the lifetime uncertainties from many molecules and became more dominant after a certain photon number N. This trend was also followed in the uncertainties of the single-molecule FRET signals obtained from single and many molecules. This additional noise is considered to come from the inhomogeneous environment of each DNA immobilized on the surface.

Recovery of Fatty Acid Monolayers by Salts Investigated by Sum-Frequency Generation Spectroscopy.

Langmuir monolayers consisting of fatty acids with relatively short alkyl chains (C14H29COOH (pentadecanoic acid), C15H31COOH (palmitic acid), and C16H33COOH (heptadecanoic acid)) are stable at a neutral pH (pH ≈ 6) but become unstable at a high pH (pH ≈ 11). Further addition of a small amount of divalent salt in subphase water was found to recover the monolayer at a high pH because binding of the divalent cations to the carboxylic headgroups renders the molecule more stable against dissolution in subphase water. This revival of the monolayer was observed via a pressure-area isotherm measurement and sum-frequency generation spectrum in the CHx and OH ranges. Fatty acids with longer alkyl chains needed less amount of MgCl2 to recover the monolayer at a high pH. A much lower concentration of Mg2+ as compared to Ca2+ is required to revive fatty acid molecules to the surface. Monovalent and trivalent salts were compared with the above divalent salts on the ability to recover the fatty acid monolayers.

Kinetics of DNA looping by Anabaena sensory rhodopsin transducer (ASRT) by using DNA cyclization assay.

DNA cyclization assay together with single-molecule FRET was employed to monitor protein-mediated bending of a short dsDNA (~ 100 bp). This method provides a simple and easy way to monitor the structural change of DNA in real-time without necessitating prior knowledge of the molecular structures for the optimal dye-labeling. This assay was applied to study how Anabaena sensory rhodopsin transducer (ASRT) facilitates loop formation of DNA as a possible mechanism for gene regulation. The ASRT-induced DNA looping was maximized at 50 mM of Na+, while Mg2+ also played an essential role in the loop formation.

Structure of Electric Double Layer under Cationic Langmuir Monolayer: Charge Condensation Effect.

Langmuir monolayers consisting of mixtures of 1-hexadecanol (HD) and 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP) (having quaternary amine headgroup) at different molar ratios were prepared to investigate the effect of the surface charge density on the structure of an electric double layer. The fatty alcohol molecules worked as passive spacers to widen the distance between the amine groups in the monolayer, to vary the surface charge density of the monolayer, and these mixture monolayer systems were probed by surface-sensitive sum-frequency vibrational spectroscopy. A strong sum-frequency signal in the OH range for a pure DPTAP monolayer (with a surface charge density of ∼0.4 C/m2) hardly decreased as the surface charge density was reduced up to ∼0.12 C/m2 (1 e per 140 Å2) and afterward decreased monotonically as more HD occupied the monolayer. The Gouy-Chapman theory incorporating a charged-condensed layer in which the counterion concentration is limited by a close packing of the counterions could account for the above saturation behavior in the sum-frequency spectra.

Reply to the 'Comment on "Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- and molecular dynamics simulation-based study"' by M. Deutsch, O. M. Magnussen, J. Haddad, D. Pontoni, B. M. Murphy and B. M. Ocko, Phys. Chem. Chem. Phys., 2021, DOI.

In our recent paper titled "Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- and molecular dynamics simulation-based study" co-authored by T. Iwahashi, T. Ishiyama, Y. Sakai, A. Morita, D. Kim, and Y. Ouchi, Phys. Chem. Chem. Phys., 2020, 22, 12565 (hereafter referred to as IW), the sum-frequency (SF) spectra for a homologous series of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Cnmim][TFSA] n = 4, 6, 8, 10, and 12) were reported. In particular, a clear decrease in the SF signals from the [TFSA]- anions with increasing chain length of the [Cnmim]+ cation (Fig. 5 of IW) was explained in terms of "head-to-head" bi-layer formation at the air/ionic liquid (IL) interface. A comment by M. Deutsch et al. (hereafter referred to as DE) questioned this report, claiming that our proposed structure is not consistent with a multilayered electron density (ED) profile obtained by X-ray reflectivity (XR).

Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- and molecular dynamics simulation-based study.

Room-temperature ionic liquids (RTILs) are being increasingly employed as novel solvents in several fields, including chemical engineering, electrochemistry, and synthetic chemistry. To further increase their usage potential, a better understanding of the structure of their surface layer is essential. Bi-layering at the surfaces of RTILs consisting of 1-alkyl-3-methylimidazolium ([Cnmim]+; n = 4, 6, 8, 10, and 12) cations and bis(trifluoromethanesulfonyl)amide ([TFSA]-) anions was demonstrated via infrared-visible sum-frequency generation (IV-SFG) vibrational spectroscopy and molecular dynamics (MD) simulations. It was found that the sum-frequency (SF) signal from the [TFSA]- anions decreases as the alkyl chain length increases, whereas the SF signal from the r+ mode (the terminal CH3 group) of the [Cnmim]+ cations is almost the same regardless of chain length. MD simulations show the formation of a bi-layered structure consisting of the outermost first layer and a submerged second layer in a "head-to-head" molecular arrangement. The decrease in the SF signals of the normal modes of the [TFSA]- anions is caused by destructive and out-of-phase interference of vibrations of corresponding molecular moieties oriented toward each other in the first and second layers. In contrast, the r+ mode of [Cnmim]+ does not experience destructive interference because the peak position of the r+ mode differs marginally at the surface and in the bulk. Our conclusions are not limited to the system presented here. Similar bi-layered structures can be expected for the surfaces of conventional RTILs, which necessitates the consideration of bi-layering in the design and application.

Suppression of the Growth of Intermetallic Compound Layers with the Addition of Graphene Nano-Sheets to an Epoxy Sn⁻Ag⁻Cu Solder on a Cu Substrate.

This study investigated the suppression of the growth of the intermetallic compound (IMC) layer that forms between epoxy solder joints and the substrate in electronic packaging by adding graphene nano-sheets (GNSs) to 96.5Sn⁻3.0Ag⁻0.5Cu (wt %, SAC305) solder whose bonding characteristics had been strengthened with a polymer. IMC growth was induced in isothermal aging tests at 150 °C, 125 °C and 85 °C for 504 h (21 days). Activation energies were calculated based on the IMC layer thickness, temperature, and time. The activation energy required for the formation of IMCs was 45.5 KJ/mol for the plain epoxy solder, 52.8 KJ/mol for the 0.01%-GNS solder, 62.5 KJ/mol for the 0.05%-GNS solder, and 68.7 KJ/mol for the 0.1%-GNS solder. Thus, the preventive effects were higher for increasing concentrations of GNS in the epoxy solder. In addition, shear tests were employed on the solder joints to analyze the relationship between the addition of GNSs and the bonding characteristics of the solder joints. It was found that the addition of GNSs to epoxy solder weakened the bonding characteristics of the solder, but not critically so because the shear force was higher than for normal solder (i.e., without the addition of epoxy). Thus, the addition of a small amount of GNSs to epoxy solder can suppress the formation of an IMC layer during isothermal aging without significantly weakening the bonding characteristics of the epoxy solder paste.

The Effect of Epoxy Polymer Addition in Sn-Ag-Cu and Sn-Bi Solder Joints.

To analyze the reinforcement effect of adding polymer to solder paste, epoxies were mixed with two currently available Sn-3.0Ag-0.5Cu (wt.% SAC305) and Sn-59Bi (wt.%) solder pastes and specimens prepared by bonding chip resistors to a printed circuit board. The effect of repetitive thermal stress on the solder joints was then analyzed experimentally using thermal shock testing (-40 °C to 125 °C) over 2000 cycles. The viscoplastic stress⁻strain curves generated in the solder were simulated using finite element analysis, and the hysteresis loop was calculated. The growth and propagation of cracks in the solder were also predicted using strain energy formulas. It was confirmed that the epoxy paste dispersed the stress inside the solder joint by externally supporting the solder fillet, and crack formation was suppressed, improving the lifetime of the solder joint.

Concentration Dependence of Ion Pairing in Imidazolium-Based Ionic Liquid Solutions.

Infrared vibrational spectroscopy was used to probe concentration-dependent ion pair dissociation of imidazolium-based ionic liquids with three different halide anions (I- , Br- , and Cl- ) in deuterated chloroform. Dissociation of the ion pairs at low concentrations of ionic liquids was found to be the easiest for ionic liquid with Cl- anion, the most electronegative anion among the three investigated. This anomalous trend of ion pair dissociation was explained in terms of varying interaction strength between the solvent (CDCl3 ) and the anions investigated.

Binding of trivalent ions on fatty acid Langmuir monolayer: Fe3+ versus La3.

Langmuir monolayers consisting of fatty acid molecules were prepared on solutions of FeCl3 and LaCl3 to investigate adsorption of trivalent metal ions on carboxylic headgroups by using sum-frequency vibrational spectroscopy. Fe3+ ions bound to the fatty acid headgroups only in the form of hydroxide complexes (Fe(OH)x +3-x), and sum-frequency intensity of water stretch modes increased markedly upon adsorption of ion hydroxide. On the other hand, La3+ ions bound to the charged anionic headgroup as bare trivalent ions. Upon Fe(OH)x +3-x adsorption, the sum-frequency spectrum of carboxyl headgroups showed significant redshift which is opposite to the case of La3+ as well as those for alkali (Na+, K+) and alkali earth metal (Ca2+, Mg2+) ions, which also supports that Fe3+ binding is by covalent metal-ligand bonding, while La3+ binding is by Coulomb attraction.

Rotation and translation dynamics of coumarin 153 in choline chloride-based deep eutectic solvents.

The equilibrium and dynamic solvation responses of coumarin 153 (C153) in a range of deep eutectic solvents (DESs) based on choline chloride with either urea (molar ratio 1:2, ChCl:U), glycerol (1:2, ChCl:G), ethylene glycol (1:2, ChCl:E), or malonic acid (1:1, ChCl:Mal) were investigated using both steady-state and time-resolved fluorescence emission spectroscopy at room temperature (298 K). From steady-state fluorescence data, "red-edge effects" were observed in all the DESs studied, attributed to spatial heterogeneity of the DES matrix. Time-resolved Stokes shifts were used to quantify dynamic solvation with the solvation response function in DES found to be a biexponential function of time, which were used to obtain average solvation times (⟨τ s ⟩) which are generally faster in DES than in ionic liquids of comparable viscosity. Average solvation times showed a partial correlation with viscosity between different DESs. The choline chloride-glycerol DES showed deviation from the viscosity trend observed in the other DES for both dynamic and steady-state results. Rotational reorientation times obtained from dynamic anisotropy (r(t)) measured for these DESs showed a partial correlation with viscosity between different DESs. Determination of the DES rotational coupling with C153 showed more "slip"-like behavior than the previously reported ionic liquids and dipolar solvents.

Change of hydrogen bonding structure in ionic liquid mixtures by anion type.

Ionic liquid mixtures have gained attention as a way of tuning material properties continuously with composition changes. For some mixture systems, physicochemical properties such as excess molar volume have been found to be significantly different from the value expected by linear interpolation, but the origin of this deviation is not well understood yet. The microstructure of the mixture, which can range from an ideal mixture of two initial consisting ionic liquids to a different structure from those of pure materials, has been suggested as the origin of the observed deviation. The structures of several different ionic liquid mixtures are studied by IR spectroscopy to confirm this suggestion, as a particular IR absorption band (νC(2)-D) for the moiety participating in the hydrogen bonding changes sensitively with the change of the anion in the ionic liquid. The absorbance of νC(2)-D changes proportionally with the composition, and a relatively small excess molar volume is observed for the mixtures containing an electronegative halide anion. By contrast, the absorbance changes nonlinearly, and the excess molar volumes are larger for the mixtures of which one of the anions has multiple interaction sites.

Turn-on and Turn-off Fluorescent Probes for Carbon Monoxide Detection and Blood Carboxyhemoglobin Determination.

Water-soluble, carbazole-based two-photon excitable fluorescent probes MPVC-I ("turn-on") and MPVC-II ("turn-off") are rationally designed and synthesized for the selective monitoring of carbon monoxide (CO). Both probes can effectively measure carboxyhemoglobin (HbCO) in the blood of the animals exposed to a CO dose as low as 100 ppm for 10 min. The palladium catalyzed azidocarbonylation reaction was optimized to improve the sensing efficiency.

RecA filament maintains structural integrity using ATP-driven internal dynamics.

At the core of homologous DNA repair, RecA catalyzes the strand exchange reaction. This process is initiated by a RecA loading protein, which nucleates clusters of RecA proteins on single-stranded DNA. Each cluster grows to cover the single-stranded DNA but may leave 1- to 2-nucleotide (nt) gaps between the clusters due to three different structural phases of the nucleoprotein filaments. It remains to be revealed how RecA proteins eliminate the gaps to make a seamless kilobase-long filament. We develop a single-molecule fluorescence assay to observe the novel internal dynamics of the RecA filament. We directly observe the structural phases of individual RecA filaments and find that RecA proteins move their positions along the substrate DNA to change the phase of the filament. This reorganization process, which is a prerequisite step for interjoining of two adjacent clusters, requires adenosine triphosphate hydrolysis and is tightly regulated by the recombination hotspot, Chi. Furthermore, RecA proteins recognize and self-align to a 3-nt-period sequence pattern of TGG. This sequence-dependent phase bias may help the RecA filament to maintain structural integrity within the kilobase-long filament for accurate homology search and strand exchange reaction.

Salt Promotes Protonation of Amine Groups at Air/Water Interface.

Interfacial water reorientation caused by charged Langmuir monolayers consisting of primary fatty amine (ODA) and cationic lipid having quaternary amine headgroup (DPTAP) were investigated by interface-selective vibrational sum-frequency generation spectroscopy. For DPTAP monolayer, initially large sum-frequency intensity from interfacial water OH band decreased steadily by increasing monovalent salt (NaCl, NaI) concentration due to counterion adsorption. On the other hand, ODA/water exhibited significantly smaller sum-frequency intensity than DPTAP/water, implying only small portion of protonated amine group (-NH3+) initially existed. By increasing the ionic strength, however, SF intensity of water OH band was enhanced markedly up to ∼1 mM, and then decreased in both NaCl and NaI solutions. By measuring the phase of the sum-frequency spectra, it was found that water dipoles under the ODA headgroup point downward, indicating that the surfaces were always positively charged. This demonstrated that increasing ionic strength facilitates protonation of primary amine headgroups. A simple model based on Poisson-Boltzmann (PB) theory explained this protonation behavior of primary amines.