Potential-dependent structure of the interfacial water on the gold electrode

Doubly tunable sum frequency generation (SFG) spectra demonstrate that the water molecules at gold/electrolyte interface change their orientation with applied potential. At negative potentials, water molecules in the double layer align with their oxygen atom pointing to the solution. As potential became positive to be close to the potential of zero charge (PZC), the SFG signal decreased, suggesting the OH groups of the water molecule are either in random orientation or parallel to the electrode. As potential became more positive than the PZC, the SFG signal increased again with the oxygen-up orientation as same as in the negative potential region, indicating that water molecules interact with the adsorbed sulfate anions. The peak position of the SFG spectra indicates a relatively disordered state of water molecules at the gold electrode surface, in contrast to the previously observed ice-like structure of water at electrolyte/oxide interfaces.     

Molecular structure of polystyrene at Air/Polymer and Solid/Polymer interfaces

IR-visible sum-frequency generation (SFG) spectroscopy has been used in a total internal reflection geometry to study the molecular structure of polystyrene (PS) at PS/sapphire and PS/air interfaces, simultaneously. The symmetric vibrational modes of the phenyl rings dominate the SFG spectra at the PS/air interface as compared to the antisymmetric vibrational modes at the PS/sapphire interface. This indicates approximately parallel orientation of the phenyl rings at the PS/air interface while nearly perpendicular orientation at the PS/sapphire interface, with respect to the surface normal.     

Impact of intermolecular vibrational coupling effects on the sum-frequency generation spectra of the water/air interface

ABSTRACT

We have examined the impact of intermolecular vibrational coupling effects of the O-H stretch modes, as obtained by the surface-specific velocity-velocity correlation function approach, on the simulated sum-frequency generation spectra of the water/air interface. Our study shows that the inclusion of intermolecular coupling effects within the first three water layers, i.e. from the water/air interface up to a distance of 6?Å towards the bulk, is essential to reproduce the experimental SFG spectra. In particular, we find that these intermolecular vibrational contributions to the SFG spectra of the water/air interface are dominated by the coupling between the SFG active interfacial and SFG inactive bulk water molecules. Moreover, we find that most of the intermolecular vibrational contributions to the spectra originate from the coupling between double-donor water molecules only, whereas the remaining contributions originate mainly from the coupling between single-donor and double-donor water molecules.     

Sum frequency generation (SFG) – surface vibrational spectroscopy studies of buried interfaces: catalytic reaction intermediates on transition metal crystal surfaces at high reactant pressures; polymer surface structures at the solid–gas and solid–liquid interfaces

SFG spectra of polyethylene and polypropylene show monolayer sensitivity and reveal temperature-dependent changes of surface structure. For polymer blends, the hydrophobic component segregates to the solid–air interface, and the hydrophilic component segregates at the solid–water interface. Changes in SFG spectra of polymer blends as a function of bulk concentration correlate with changes of contact angle. SFG is an excellent probe of surface-structure and surface-composition changes as the polymer interface is altered. Received: 20 September 1998     

空气/水界面水分子的取向和运动

对四种不同的实验构型下空气／水界面自由O-H键在3700cm~(-1)的和频振动光谱的分析表明,水分子在空气／水界面的取向运动只可能是在平衡位置附近有限角度之内的受限转动。界面水分子的自由O-H键取向距界面法线约33°,而取向分布或运动的宽度不超过15°。这一图像显著地不同于Wei等人(Phys. Rev. Lett.86,4799(2001))只通过单一的SFG实验构型所得出结论,即：空气／水界面的水分子在超快的振动弛豫时间内在很大的角度范围内运动。     

Optical sum-frequency emission from langmuir-blodgett films of variable thickness: effects of the substrate and polar orientation of fatty acids in the films

The nonlinear optical response arising from a model multilayer structure, i.e., Langmuir-Blodgett (LB) films comprised of different numbers of per-protonated (H) and per-deuterated (D) fatty acid layers on solid substrates, has been evaluated by sum-frequency generation (SFG) spectroscopy. The SFG signals depend significantly on the absolute polar orientation of the fatty acids in the individual layers and on the nonlinear optical response of the substrate. The SFG spectra on gold and fused quartz substrates demonstrate a totally different dependence on the number of the contributing H layers, which it is possible to analyze quantitatively. The results provide important information for understanding the origin of the nonlinear optical responses from ordered systems composed of multiple interfaces and therefore for extracting exact structural information about each interface from the observed SFG signals.     

Melting at alkyl side chain comb polymer interfaces

Infrared visible sum frequency generation (SFG) spectroscopy has been used to study structure and melting transition temperatures of alkyl-side chain-acrylate comb polymers at air and solid interfaces. At the air interface, the SFG spectra show methyl bands and two transitions are observed: the first, near the bulk melting temperature, T(m), and the second 10-20 degrees C higher than T(m). The shorter the alkyl side chain, the larger the difference between the two transition temperatures. In contrast, methylene bands are observed at sapphire interface with a single transition near T(m) (C18).     

二氧化硅-十八烷基三氯硅烷界面的和频光谱相位测量及其测量精度分析

相位测量和频振动光谱（SFG）可以获得物质表面分子取向等信息,但在实验重复性、实验设计和界面分析等方面仍有一些关键问题没有解决。相位误差会引起光谱变化并误导界面结构分析,因此分析并准确控制误差是相位测量SFG的关键技术。使用z-切石英作为相位标准,测量了修饰在熔融石英基底上的十八烷基三氯硅烷（OTS）在C-H振动波段的和频振动光谱,对OTS的相位光谱进行了解析,结果表明OTS虚部光谱中,2 878和2 936 cm-1处的两个正峰分别是末端CH₃的对称振动（CH₃ss）和费米共振（CH₃FR）,2 960 cm-1处的负峰为CH₃的反对称伸缩振动（CH₃as）,这三个峰的光谱特征和指认与文献一致。2 910 cm-1附近的负峰为CH₂反对称伸缩（CH₂as）,与文献比较,约有20 cm-1的偏移,且在2 850 cm-1附近还观察到一个负峰,归属为CH₂对称伸缩（CH₂ss）,分析认为与文献的差异可能是因为样品制备时间影响了OTS的分子排列结构。通过建立OTS虚部谱与CH₃取向角的关系,发现CH₃的三种振动模式的c轴与表面法线的夹角均小于90°,其H更多为向上取向且排列有序,表明相位测量相较于强度测量可以获得更丰富的表面信息。同时,讨论了待测样品和参考样品位置的非一致性对相位测量精度的影响。通过测量OTS在三个不同位置（12.1,12.3和12.73 mm）的虚部谱,并与模拟相位误差的引入对虚部谱的影响对比,发现待测样品与参考样品测量位置间2.5 μm的位移对应于1°的相位误差,20°相位的偏移会导致零点位置移动约6 cm-1,从而引起振动峰位置和符号等的改变,导致对光谱的错误解析。为了获得界面分子稳定可靠的相位信息,需要严格控制两次样品测量位置一致。实验研究结果为提高和频振动光谱相位测量的精度与准确性提供了指导,为界面分子表面态的检测与分析、及微小信号的探测提供了有效手段。     

Fourier Transform Infrared Spectroscopic Investigation on Langmuir-Blodgett Films of Octadecanoyl Ester of 1-(2-Carboxyethyl) Thymine: Molecular Orientation,Molecular Recognition to Complementary Base of Nucleic Acid and Order-Disorder Transition

Fourier transform infrared (FTIR) transmission, polarized transmission, reflection absorption (RA) and Attenuated total reflection (ATR) spectroscopy were employed to investigate the molecular orientation and molecular recognition to the complementary base at the air / water interface of a novel nucleolipid amphiphile, octadecanoyl ester of 1-(2-carboxyethyl) thymine. It has been found that the molecules are biaxially oriented and the hydrocarbon chain titled considerably from the substrate normal as well as the chromophoric part in the LB film Molecular recognition through complementary bases pairing takes place at the air / water interface between the thymine moiety in the head group of the amphiphile and adenosine in the subphase, the hydrogen involved in the host-guest interactions has been detected by using FTIR-ATR technique. FTIR transmission spectra measured at elevated temperatures revealed that the order-disorder transition of the LB film which contains the complementary base occurs at lower temperature. To our best knowledge, investigation of the order-disorder transitions in nucleolipid amphiphile LB films and comparative studies of the order-disorder transitions between nucleolipid amphiphile LB film that contains and does not contain the complementary bases have been virtually nonexistent, so far.     

离子液体阳离子在空气/[bmim][BF4]水溶液界面的取向和结构

采用和频振动光谱研究了空气/[bmim][BF₄]低浓度水溶液界面的取向结构. 研究发现,在体相浓度非常低时,丁基链具有较大的旁式扭曲,表明此时的取向比较无序;阳离子咪唑环则采取一个较小的取向角. 随着浓度的升高,阳离子咪唑环趋向平躺在界面. 由于链-链相互作用,此时丁基链的旁式扭曲也减小,说明界面分子的取向变得有序. 进一步研究发现,PPP和SPS光谱上甲基反对称的峰存在位移,表明界面丁基链上的甲基存在不同取向或具有不同的化学环境.结果有助于从微观层次理解水溶性离子液体和基于咪唑的表面活性剂在界面上的物理化学行为.     

Ftir Studies on LB Film of Amphiphile with Schiff Base as Headgroup and Its Copper Complex

Two different ways to form monolayers and LB films (surface film and subphase film) of the complex have been used, where a novel amphiphile containing Schiff base as a headgroup was used as a ligand. the monolayer behavior at the air/water interface was characterized by π-A isotherms and two-dimensional molecular orientation of alkyl chains in LB films and thermal stability were measured by polarized and variable temperature FTIR transmission spectra, indicating that the LB film of the novel amphiphile and its copper(II) complex are very stable as well as stearic acid. Because incorporating the metal ion into the monolayer makes it more condensed, thermal stability of the LB film was enhanced. as can be compared from their structure and properties, subphase films are superior to surface films.     

红外扫描和频振动光谱系统的激光线宽与光谱分辨率

本文报道了一个简化的利用可见光和红外光带宽来计算和频光谱分辨率的公式. 公式显示和频振动光谱的Voigt线宽可以通过振动模式的均匀线宽(洛伦兹线宽)、非均匀线宽(高斯线宽)、红外光与可见光的高斯线宽计算获得. 利用本实验室新搭建的频率分辨及偏振分辨的皮秒和频光谱系统验证了该公式的准确性. 实验结果显示,本激光系统获取的红外光的高斯线宽为1.5 cm^-1. 本激光系统的光谱分辨率约为4.6 cm^-1,结果与胆固醇单层膜光谱获取的光谱分辨率(3.5~5 cm^-1)基本一致.     

EPR Spectra of Spin-labeled Fatty Acids in Detergent Solutions: “Immobilized Species” in the Absence of Protein

Electron paramagnetic resonance spectra of 5-doxyl-stearic acid, 3-(2-Dodecyl-1-oxyl-5,5-dimethylpyrrolidin-2-yl)propanoic acid radical (4-proxyl-palmitic acid) and of 3-(1-Oxyl-2,5-dimethyl-5-dodecylpyrrolidine-2-yl)propanoic acid radical (4-azetoxyl-stearic acid) in lipid micelles of nonaethyleneglycol-monododecyl ether (C₁₂E₉) and pentaethyleneglycol-monooctyl ether (C₈E₅) can reveal immobilized components typical of spin-labeled protein-bound fatty acids as well as phospholipids. The occurrence of such species depends on the detergent, the relative orientation of the nitroxide group with respect to the longitudinal axis and the molecular structure of the fatty acid as well as on polarity and ionic strength of the aqueous phase. Whereas 5-doxyl-stearic acid and 4-proxyl-palmitic acid exhibited these highly immobilized components in C₁₂E₉ micelles but not in C₈E₅, only a single relatively mobile component was observed with 4-azetoxyl-stearic acid in both detergents. The immobilized component is lost by the addition of isopropanol but not as much by ethanol and also when distilled water is substituted for the buffer.     

Motional effect in surface sum-frequency vibrational spectroscopy

Fast orientational motion of molecules at a surface can affect the sum-frequency vibrational spectra of the surface. Calculation shows that the effect is significant if the molecular orientation varies over a broad range within the vibrational relaxation time. The stretch vibration of the free OH bonds at the vapor/water interface is used to illustrate the importance of the effect.     

一种液晶化合物LB膜中的分子取向及膜结构

"利用LB技术制备出MPUOPM的Langmuir和LB膜．随着表面压力的升高,在?-A曲线上的可观察到一个明显的折点．通过与分子刚性基团的理论截面积相比较,发现该折点反映的是单层膜向多层膜的转变．紫外可见、偏振红外等光谱被用来研究膜的结构,发现烷基链与基片法线夹角为48o,C＝N基团互相平行,与基片法线呈51o夹角排列．"     

Studies on behaviors of dipalmitoylposphatidylcholine and bilirubin in mixed monolayer at the air/water interface

Mixed monolayers of dipalmitoylposphatidylcholine (DPPC) and bilirubin (BR) were prepared on different subphases. The properties of DPPC/BR monolayer, such as collapse pressure (π_coll), limiting area per molecule (A_lim), surface compressibility modulus, free energy (ΔG_mix) and excess free energy (ΔG_ex), were investigated based on the analysis of the surface pressure-area isotherms on pure water. The results showed that DPPC and BR were miscible and formed non-ideal mixed monolayers at the air/water interface. With the molar fraction of BR (X_BR) increasing, the LE-LC coexistence region of DPPC monolayer was eliminated gradually. The DPPC/BR complex (M_D-B) of 1:2 stoichiometry formed as a result of the strong hydrogen bonds between the polar groups of DPPC and BR. The studies of effects of pH values and calcium ions in subphase on the DPPC/BR monolayers showed that the mixed monolayer became expanded on alkali aqueous solution and on 1 mmol/L CaCl₂ aqueous solution. The orientation of DPPC and BR at air/water interface was also discussed.     

Total Internal Reflection Fluorescence Spectroscopy for Investigating the Adsorption of a Porphyrin at the Glass/Water Interface in the Presence of a Cationic Surfactant Below the Critical Micelle Concentration

Total internal reflection fluorescence (TIRF) spectroscopy was used to investigate the adsorption behavior of meso-tetrakis(p-sulfonatophenyl)porphyrin (TPPS) at the glass/water interface in the presence of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) far below the critical micelle concentration. The adsorption model of TPPS at the glass/water interface in the presence of low concentration of CTAB was proposed, which was different from the adsorption of TPPS in the presence of micelles of CTAB at the glass/water interface. TPPS and CTAB did not form stable complex at the interface in dilute system. The interfacial species of TPPS were analyzed by comparing the spectra of TPPS at the glass/water interface and in the aqueous phase. The influences of the TPPS concentration, the CTAB concentration, and the pH values on the interfacial fluorescence spectra and intensities were studied. It was demonstrated that electrostatic interaction and hydrophobicity performed an important role on the adsorption of TPPS in the presence of CTAB. The different effects of TPPS concentration on the adsorption behaviour of TPPS at different pH were observed for the first time. It was found that the adsorption isotherms of TPPS at glass/water interface could fit Freundlich equation at pH 7.1.     

羟基对界面水分子的扰动与其对非离子型氢氟烃杂化表面活性剂聚集行为的改变

本文合成并比较了具有和不具有羟基的两种非离子型氢氟烃杂化表面活性剂，它们均表现出良好的热稳定性和优异的表面活性. 实验观察到羟基对改变其溶液的表面张力和所形成胶束的形态具有较大的影响. 该作用可归因于烷烃基团从空气/水表面上方到其下方的重排以及由羟基诱导的界面水结构的扰动. 本工作提供了一种通过修改界面处的取向结构来弱化碳氢链和碳氟链之间的不混溶性，从而利于设计具有不同界面性质表面活性剂的策略.     

二次谐波光谱和布鲁斯特角显微镜研究二棕榈酰磷脂酰胆碱和维生素B2的相互作用

二棕榈酰磷脂酰胆碱（DPPC）分子在气液界面上形成的Langmuir膜是一种重要的生物膜模拟体系,其手性结构及其与外来物质的相互作用一直是相关学科研究的前沿问题。维生素B₂（VB₂）是人体中一种重要的营养物质,它在代谢障碍引起的脂质沉积性类疾病中有大量的实例应用,经常在一些特殊的临床症状中有出乎意料的治疗奇效。目前,VB₂如何参与到膜上生物事件的过程和细胞乃至生命的作用过程中的研究报道较少,特别是VB₂分子与磷脂分子靶标的立体相互作用,其可能发生的手性分子识别现象会在许多生物事件中起着关键作用。综合二次谐波-线二色光谱（SHG-LD）、Langmuir膜天平和布鲁斯特角显微镜(BAM)技术初步研究了VB₂和DPPC分子在气液界面上的相互作用,分别从气液界面上介观水平和宏观水平上互补表征脂质分子在气液界面上的分子骨架自组装的结构。压缩等温线发现纯水界面L-DPPC和D-DPPC液态扩展相/液态凝聚相(LE/LC)共存阶段的膜压几乎不变,race-DPPC的共存相膜压区域稍微缩短,VB₂水溶液界面上race-DPPC的LE/LC共存相消失。此外,弹性模量研究表明VB₂分子可以提高L-DPPC单分子层膜的弹性模量,但降低D-DPPC和race-DPPC单层膜的弹性模量。结合SHG-LD研究发现,在膜压13 mN·m-1下,L-DPPC在纯水和VB₂水溶液界面上表面手性过量值（DCE）保持不变。与纯水界面相比较,D-DPPC在VB₂水溶液上DCE值出现反转,而race-DPPC的DCE值则不随亚相改变而变化。相同膜压下,BAM观察到单一手性相互作用使得L-DPPC和D-DPPC在纯水界面上各自组装成不同枝臂弯曲方向的手性三叶草微畴(microdomain)。VB₂诱导D-DPPC微畴,使其直径增大1～2倍。同时,VB₂也诱导了race-DPPC单层膜上近似圆形状的微畴伸展,并长出了三条有曲率的枝臂。对此可以解释为VB₂降低了非单一手性相互作用的能量,使得race-DPPC出现手性相分离。与此同时,VB₂也诱导了race-DPPC单层膜微畴的手性结构发生变化。该研究有助于理解VB₂调节磷脂膜横向组织结构的分子机理,在细胞膜界面发生的过程中,脂层单层的二维特性和生物分子之间的相互作用可能决定了生物分子的亲和力。