Adsorption and reaction of CO2 and SO2 at a water surface

The orientation and hydrogen bonding of water molecules in the vapor/water interfacial region in the presence of SO2 and CO2 gas are examined using vibrational sum-frequency spectroscopy (VSFS) to gain insight into the adsorption and reactions of these gases in atmospheric aerosols. The results show that an SO2 surface complex forms when the water surface is exposed to an atmosphere of SO2 gas. Reaction of SO2 with interfacial water leads to other spectral changes that are examined by studying the VSF spectra and surface tension isotherms of several salts added to the aqueous phase, specifically NaHSO3, NaHCO3, Na2SO3, Na2CO3, Na2SO4, and NaHSO4. The results are compared with similar studies of CO2 adsorption and reaction at the surface. A weakly bound surface complex is not observed with CO2.     

Study of the adsorption of sodium dodecyl sulfate (SDS) at the air/water interface: targeting the sulfate headgroup using vibrational sum frequency spectroscopy

The surface sensitive technique vibrational sum frequency spectroscopy (VSFS), has been used to study the adsorption behaviour of SDS to the liquid/vapour interface of aqueous solutions, specifically targeting the sulfate headgroup stretches. In the spectral region extending from 980 to 1850 cm(-1), only the vibrations due to the SO(3) group were detectable. The fitted amplitudes for the symmetric SO(3) stretch observed at 1070 cm(-1) for the polarization combinations ssp and ppp, were seen to follow the adsorption isotherm calculated from surface tension measurements. The orientation of the sulfate headgroup in the concentration range spanning from 1.0 mM to above the critical micellar concentration (c.m.c.) was observed to remain constant within experimental error, with the pseudo-C(3) axis close to the surface normal. Furthermore, the effect of increasing amounts of sodium chloride at SDS concentrations above c.m.c. was also studied, showing an increase of approximately 12% in the fitted amplitude for the symmetric SO(3) stretch when increasing the ionic strength from 0 to 300 mM NaCl. Interestingly, the orientation of the SDS headgroup was also observed to remain constant within this concentration range and identical to the case without NaCl.     

Kinetic study of adsorption of some biocompounds at the oil/water interface

The adsorption kinetics of some local anesthetics, like dibucaine and tetracaine, and of stearic acid from bulk solutions at the oil/water interface was studied by using the pendent drop and ring methods. The anesthetics were dissolved in aqueous solutions (pH 2), and the fatty acid was dissolved in benzene, each biocompound at several different concentrations in bulk solutions. Kinetic equations for Langmuir mechanism of adsorption at oil/water interface were tested. The kinetic analysis shows that Langmuir kinetic approach describes the dynamic interfacial pressures within the limits of the experimental errors over a wide range of time and for different surfactant concentrations in bulk solutions. It is also concluded that this approach allows the calculation of the ratio of the adsorption and desorption rate constants of these biocompounds at the oil/water interface. Obtained results are in substantial agreement with earlier reported data for the surfactant adsorption as, well as with their molecular structure.     

Nonlinear vibrational spectroscopic studies of the adsorption and speciation of nitric acid at the vapor/acid solution interface

Nitric acid plays an important role in the heterogeneous chemistry of the atmosphere. Reactions involving HNO(3) at aqueous interfaces in the stratosphere and troposphere depend on the state of nitric acid at these surfaces. The vapor/liquid interface of HNO(3)-H2O binary solutions and HNO(3)-H(2)SO(4)-H2O ternary solutions are examined here using vibrational sum frequency spectroscopy (VSFS). Spectra of the NO2 group at different HNO(3) mole fractions and under different polarization combinations are used to develop a detailed picture of these atmospherically important systems. Consistent with surface tension and spectroscopic measurements from other laboratories, molecular nitric acid is identified at the surface of concentrated solutions. However, the data here reveal the adsorption of two different hydrogen-bonded species of undissociated HNO(3) in the interfacial region that differ in their degree of solvation of the nitro group. The adsorption of these undissociated nitric acid species is shown to be sensitive to the H2O:HNO(3) ratio as well as to the concentration of sulfuric acid.     

SO2:H2O surface complex found at the vapor/water interface

A weakly bonded SO2:H2O surface complex is found at the vapor/water interface prior to the reaction and dissolution of SO2 into the aqueous phase. The results have important implications for understanding the formation of atmospheric aerosols and understanding the atmospheric sulfur cycle.     

Uptake of SO2 to aqueous formaldehyde surfaces

Aqueous surfaces act as a gateway to absorption and aqueous-phase reaction of gases in the atmosphere. The composition of aerosols varies greatly and is expected to influence the structure of the interface. For example, aldehydes comprise a significant fraction of atmospheric organics and are likely to accumulate at aqueous surfaces. But it is difficult to anticipate their effect on the migration of gaseous species through the interfacial region. Surface organics may act as a barrier to absorption, or they may facilitate uptake via cooperative interactions with absorbing compounds. The surface spectroscopic studies presented here examine the nature of the vapor/water interface during uptake of SO(2) to aqueous formaldehyde solutions, elucidating the role of surface species in a multicomponent interfacial system. The results show that the product of the reaction between SO(2) and formaldehyde, hydroxymethanesulfonate, shows a surface affinity that is enhanced in the presence of SO(2).     

Effect of relative humidity on OH uptake by surfaces of atmospheric importance

An experimental study of the dependence of the OH uptake coefficient gamma OH over a relative humidity of 0-48% was carried out at 100 Torr and room temperature, using a differential bead-filled flow tube coupled to a high-pressure chemical ionization mass spectrometer. Various organic (paraffin wax, pyrene, glutaric acid, and soot) and inorganic (NaCl, KCl, MgCl2, CaCl2, Na2SO4, and sea salt) surfaces served as proxies for tropospheric aerosols. A virtual cylindrical flow tube approximation and a surface coating dilution technique were successfully employed in the study, which included measurements of high radical uptake with an initial probability of near unity. For inorganic salts, the effect of water vapor, enhancement or inhibition of gamma OH, was found to be dependent on the blocking of anions and changes in surface pH. Although OH uptake by NaCl, the major component of sea-salt aerosols, is weakly dependent on water vapor, it is enhanced by a factor of approximately 2 for MgCl2 and determines the net relative humidity dependence of the radical uptake on sea salt, which is enhanced by a factor of approximately 4. For the organic surfaces studied, the enhancement effect of a factor 4 was also observed only for a hydrophilic organic surface, namely, glutaric acid. Results of the uptake studies suggest that the effect of relative humidity is important and should be accounted for in atmospheric modeling of tropospheric aerosol chemistry.     

Solubilization of Terfenadine,Riboflavin, and Sudan III by Aqueous Multi-basic Organic Acids

ABSTRACT

The aqueous solubility of terfenadine, riboflavin, and Sudan III (water-insoluble compounds) was enhanced by the addition of multi-basic organic acids, including citric, glutaric, malic, malonic, and tartaric acids. The variations of physical properties (density, viscosity, electrical conductivity, pH, and surface tension) against acid concentration (0–3.6 M at 25°C) were measured in order to explore possible mechanisms of solubility enhancement. Apart from the partial molar volumes, the measured physical properties varied nonlinearly with acid concentration. Glutaric acid contributes to solubility enhancement of terfenadine and Sudan III more than citric acid, with the latter slightly more effective towards riboflavin. Tartaric acid is the least effective, while malic and malonic acids occupy an intermediate position. Among the organic acids examined, only glutaric acid solutions exhibit significant surface activity, which lends itself to solubility enhancement of the three hydrophobic drugs (interfacial packing of 55 ± 3 Å² at the air–water interface, critical aggregate concentrations (CAC) at 1.8 ± 0.4 M). In contrast, all five organic acid solutions of terfenadine demonstrate more effective lowering of the surface tension of water, with the terfenadinium acid salts exhibiting interfacial packing of 108 ± 9 Å² at the air–water interface. On the other hand, organic acid solutions of riboflavin and Sudan III exhibited essentially no surface activity, aside from the intrinsic contribution of the organic acids themselves. Thus, self-association of glutaric acid contributes to the solubility enhancement of the three hydrophobic drugs. This combined with the surface activity of terfenadinium acid salts explains the higher tendency of glutaric acid to solubilize terfenadine. Mixed micellization of terfenadinium glutarate and glutaric acid occurs with an interfacial packing of 166 ± 18 Å² at the air–water interface. The corresponding CAC were estimated at 3.1 ± 0.2 mM for terfenadinium glutarate and 8.0 ± 0.4 mM for glutaric acid. Intermolecular hydrogen bonding with the extensive hydroxyl group network of riboflavin reflects the higher affinity of citric acid than glutaric acid towards riboflavin. The variability in solubility enhancement exhibited by tartaric, malic, and malonic acids appears to be a result of the interplay between several factors including intra- vs. inter-molecular hydrogen bonding, slight organic acid surface activity, and acid hydration.     

Uptake and surface reaction of methanol by sulfuric acid solutions investigated by vibrational sum frequency generation and Raman spectroscopies

The uptake of methanol at the air-liquid interface of 0-96.5 wt % sulfuric acid (H2SO4) solutions has been observed directly using vibrational sum frequency generation (VSFG) spectroscopy. As the concentration of H2SO4 increases, the VSFG spectra reveal a surface reaction between methanol and H2SO4 to form methyl hydrogen sulfate. The surface is saturated with the methyl species after 15 min. The uptake of methyl species into the solutions by Raman spectroscopy was also observed and occurred on a much longer time scale. This suggests that uptake of methanol by sulfuric acid solutions is diffusion-limited.     

Freeze-out extraction of monocarboxylic acids from water into acetonitrile under the action of centrifugal forces

It is established that the efficiency of the freezing-out extraction of monocarboxylic acids С₃–С₈ and sorbic acid from water into acetonitrile increases under the action of centrifugal forces. The linear growth of the partition coefficient in the homologous series of С₂–С₈ acids with an increase in molecule length, and the difference between the efficiency of extracting sorbic and hexanoic acid, are discussed using a theoretical model proposed earlier and based on the adsorption–desorption equilibrium of the partition of dissolved organic compounds between the resulting surface of ice and the liquid phase of the extract. The advantages of the proposed technique with respect to the degree of concentration over the method of low-temperature liquid-liquid extraction are explained in light of the phase diagram for the water–acetonitrile mixture.     

Hydrophobic and electrostatic interactions in adsorption of surface-active substances. Tetraalkylammonium salts

A relationship between the standard free energies of adsorption from aqueous solution at the oil/water interface and the radii of organic cations as exemplified by symmetric tetraakylammonium salts has been studied. Hydrophobic effects are shown to be major contributors to the interaction of surfactants with the interface. An adsorption coefficient to quantitate the hydrophobic effects and to specify the changes of standard adsorption energy depending upon the cavity surface area of the detergent hydrocarbon radical in aqueous solution has been proposed. A new formulation of the Traube rule, taking into account the hydrophobic effects concomitant with a transfer of surfactants from the water bulk onto the interface, has also been given.Standard free energies for the adsorption of organic and inorganic ions from aqueous solution at the interface of immiscible liquids have been found. The proposed method is based on an extrapolation of the relationship between the standard adsorption energy of tetraalkylammonium salts and the square of cationic radius to zero ionic radius. The standard free energy of adsorption for an inorganic counter-ion is derived from an intercept on the y-axis cut off by a straight line. The experimental adsorption data on inorganic salts have been used to calculate the standard free energies of adsorption for a variety of ions.A method of estimating the difference in potential at the oil/water interface between the adsorption plane and the aqueous solution has been proposed. The sign of potential provides a clue to the orientation of water molecules at the interface between immiscible liquids.     

Adsorption of organic matter at mineral/water interfaces. IV. Adsorption of humic substances at boehmite/water interfaces and impact on boehmite dissolution

The adsorption of Suwannee River fulvic acid (SRFA) and Pahokee peat humic acid (PPHA) at the boehmite (gamma-AlOOH)/water interface and the impact of SRFA on boehmite dissolution have been examined over a wide range of solution pH conditions (pH 2-12), SRFA surface coverages (Gamma(SRFA), total SRFA binding site concentration normalized by the boehmite surface area) of 0.0-5.33 micromol m(-2), and PPHA surface coverages (Gamma(PPHA), PPHA binding site concentration normalized by boehmite surface area) of 0.0-4.0 micromol m(-2), using macroscopic adsorption and in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. At relatively high SRFA surface coverages (Gamma(SRFA) = 5.33 micromol m(-2)), in situ ATR-FTIR spectral features of adsorbed SRFA are very similar to those measured for SRFA in solution at approximately 1-3 pH units higher. At sub-monolayer surface coverages (Gamma(SRFA) = 1.20 and 2.20 micromol m(-2)), several new peaks and enhancements of the intensities of a number of existing peaks are observed. The latter spectral changes arise from several nonorganic extrinsic species (i.e., adsorbed carbonate and water, for alkaline solution conditions), partially protonated SRFA carboxyl functional groups (near-neutral pH conditions), and small quantities of inner-spherically adsorbed SRFA carboxyl groups and/or Al(III)-SRFA complexes (for acidic conditions). The spectra of PPHA adsorbed at boehmite/water interfaces also showed changes generally consistent with our observations for SRFA sorbed on boehmite. These observations confirm that SRFA and PPHA are predominantly adsorbed at the boehmite/water interface in an outer-sphere fashion, with minor inner-sphere adsorption complexes being formed only under quite acidic conditions. They also suggest that the positively charged boehmite/water interface stabilizes SRFA and PPHA carboxyl functional groups against protonation at lower pH. Measurements of the concentration of dissolved Al(III) ions in the absence and presence of SRFA showed that the boehmite dissolution process is clearly inhibited by the adsorption of SRFA, which is consistent with previous observations that outer-spherically adsorbed organic anions inhibit Al-(oxyhydr)oxide dissolution.     

Densities and apparent molar volumes of atmospherically important electrolyte solutions. 1. The solutes H2SO4, HNO3, HCl, Na2SO4, NaNO3, NaCl, (NH4)2SO4, NH4NO3, and NH4Cl from 0 to 50 °C, including extrapolations to very low temperature and to the pure liquid state, and NaHSO4, NaOH, and NH3 at 25 °C

Calculations of the size and density of atmospheric aerosols are complicated by the fact that they can exist at concentrations highly supersaturated with respect to dissolved salts and supercooled with respect to ice. Densities and apparent molar volumes of solutes in aqueous solutions containing the solutes H(2)SO(4), HNO(3), HCl, Na(2)SO(4), NaNO(3), NaCl, (NH(4))(2)SO(4), NH(4)NO(3), and NH(4)Cl have been critically evaluated and represented using fitted equations from 0 to 50 °C or greater and from infinite dilution to concentrations saturated or supersaturated with respect to the dissolved salts. Using extrapolated densities of high-temperature solutions and melts, the relationship between density and concentration is extended to the hypothetical pure liquid solutes. Above a given reference concentration of a few mol kg(-1), it is observed that density increases almost linearly with decreasing temperature, and comparisons with available data below 0 °C suggest that the fitted equations for density can be extrapolated to very low temperatures. As concentration is decreased below the reference concentration, the variation of density with temperature tends to that of water (which decreases as temperature is reduced below 3.98 °C). In this region below the reference concentration, and below 0 °C, densities are calculated using extrapolated apparent molar volumes which are constrained to agree at the reference concentrations with an equation for the directly fitted density. Calculated volume properties agree well with available data at low temperatures, for both concentrated and dilute solutions. Comparisons are made with literature data for temperatures of maximum density. Apparent molar volumes at infinite dilution are consistent, on a single ion basis, to better than ±0.1 cm(3) mol(-1) from 0 to 50 °C. Volume properties of aqueous NaHSO(4), NaOH, and NH(3) have also been evaluated, at 25 °C only. In part 2 of this work (ref 1 ) an ion interaction (Pitzer) model has been used to calculate apparent molar volumes of H(2)SO(4) in 0-3 mol kg(-1) aqueous solutions of the pure acid and to represent directly the effect of the HSO(4)(-) ? H(+) + SO(4)(2-) reaction. The results are incorporated into the treatment of aqueous H(2)SO(4) density described here. Densities and apparent molar volumes from -20 to 50 °C, and from 0 to 100 wt % of solute, are tabulated for the electrolytes listed in the title and have also been incorporated into the extended aerosol inorganics model (E-AIM, http://www.aim.env.uea.ac.uk/aim/aim.php) together with densities of the solid salts and hydrates.     

On the adsorption kinetics of surface-chemically pure n-dodecanoic acid at the air/water interface

The dynamic surface tension data for n-dodecanoic acid in 0.005 M hydrochloric acid, for as-received as well as for surface-chemically pure solutions, show the presence of a prolonged induction period, clearly indicating that the adsorption of this nonionic surfactant is not simply diffusion-controlled. A kinetic model for the reversible formation of monolayer islands, long known in the field of electrochemistry, is shown to also apply to the adsorption of n-dodecanoic acid at the air/water interface. The rate constant increases linearly with increasing bulk concentration, while the induction time decreases exponentially. The phenomenon of nucleation at the air/water interface is consistent with the direct experimental observation of the formation of solid-like patches as the interfacial region is drastically compressed.     

Enhanced copper surface protection in aqueous solutions containing short-chain alkanoic acid potassium salts

The ability of dissolved potassium monocarboxylate salts to produce surface passivation and to inhibit aqueous corrosion of copper was studied. The electrochemical measurements indicate that the inhibiting efficiency of these compounds, with a general formula Cn-1H2n-1COOK or CnK (n=3...12), is dependent on the hydrocarbon chain length. The inhibiting efficiency was higher for a longer hydrocarbon chain of n-alkanoic acid. The degree of copper protection was found to increase with an increase in n-alkanoic acid potassium salt concentration; the optimum concentration of potassium dodecanoate (C12K) in sulfate solutions was found to be 0.07 M. The protective layers formed at the copper surface subsequent to exposure in various n-alkanoic acid potassium salt solutions were characterized by contact angle measurements, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared reflection spectroscopy. Pronounced copper protection was attributed to the growth of a protective film on the copper surface, containing both copper oxides and copper carboxylate compounds. It is suggested that the organic molecules enhance copper protection by covering copper oxides with a thin and dense organic layer, which prevents water molecules or aggressive anions from interacting with the copper surface.     

Chilling out: a cool aqueous environment promotes the formation of gas-surface complexes

SO(2), an important atmospheric pollutant, has been implicated in environmental phenomena such as acid rain, climate change, and cloud formation. In addition, SO(2) is fundamentally interesting because it forms spectroscopically identifiable complexes with water at aqueous surfaces. Vibrational sum frequency spectroscopy (VSFS) is used here to further investigate the mechanism by which SO(2) adsorbs to water at tropospherically relevant temperatures (0-23 °C). The spectral results lead to two important conclusions. SO(2) surface affinity is enhanced at colder temperatures, with nearly all of the topmost water molecules showing evidence of binding to SO(2) at 0 °C as compared to a much lower fraction at room temperature. This surface adsorption results in significant changes in water orientation at the surface, but is reversible at the temperatures examined here. Second, the SO(2) complex formation at aqueous surfaces is independent of aqueous solution acidity. One challenge in previous uptake studies was the ability to distinguish between the effects of surface adsorption as compared to bulk accommodation. The surface and vibrational specificity of these studies make this distinction possible, allowing a selective study of how the aqueous properties temperature and pH influence SO(2) surface affinity.     

Flux decline in nanofiltration due to adsorption of dissolved organic compounds: model prediction of time dependency

Flux decline during nanofiltration of aqueous solutions containing dissolved organic compounds is mainly caused by adsorption of these compounds in the membrane pores and on the membrane surface. In this paper, flux decline is modeled by incorporating the loss of permeability due to adsorption in the Spiegler-Kedem equation. This results in a logarithmic relation between normalized flux decline and time until the adsorption equilibrium is reached and the flux reaches a constant value. In this way, the expected flux decline due to adsorption of organic compounds can be estimated. Two different parameters were used in the model: the time delay t(0), which corresponds to the time at which the adsorption process in the membrane pores and on the membrane surface sets in, and the corresponding reduction of free (pore) volume (b). Both parameters depend on the hydrophobicity of the compounds and on the feed concentration.     

Supramolecular structures of coronene and alkane acids at the Au(111)-solution interface: a scanning tunneling microscopy study

Scanning tunneling microscopy (STM) is utilized to study the solution-solid interface formed between Au(111) and solutions of coronene in hexanoic, heptanoic, and octanoic acids. In all three cases adsorbed coronene is observed and lays flat on the metal surface. Heptanoic and hexanoic acid solutions produce a hexagonal symmetry monolayer. For the heptanoic and hexanoic cases, dipole-image dipole interactions and H bonding stabilize a surface structure in which 12 acid molecules surround each coronene and produce a coronene spacing of 1.45 nm. In the case of octanoic acid as solvent, the incorporation of the solvent into the monolayer is not as strongly favored. The coronene spacing can range from close-packed (1.2 nm) with no solvent presumed present in the monolayer, to 1.50 nm with up to 12 solvent molecules surrounding each coronene. The close-packed regions have hexagonal symmetry, as do those with the largest (1.5 nm) spacing. Heptanoic acid solutions give the clearest STM images and are associated with the most stable two-component monolayer. The present paper demonstrates that non-covalent interactions at the solution-metal interface can lead to complex multicomponent monolayer structures.     

芳香反离子与离子表面活性剂胶团的相互作用

用紫外吸收光谱的方法研究了多种芳香反离子与离子表面活性剂胶团之间的相互作用。发现阳离子表面活性剂和阴离子表面活性剂胶团对芳香反离子的吸收光谱有着显著不同的影响,芳香反离子结构的细微变化可导致光谱特性显著不同。这被解释为阳离子表面活性剂胶团与芳香环之间的阳离子-π相互作用。阳离子-π相互作用需要阳离子表面活性剂分子与芳香反离子具有合适的相对位置与距离。通过测定各混合体系的吸附量证明,不同的芳香反离子在胶团表面有不同的排列方式。NMR的实验结果支持了上述解释。     

Untersuchungen über das Desorptionsverhalten einiger löslichern-Alkansäuren an der Wasser-Luft-Grenzfläche

Adsorption layers of aqueous hydrochloric solutions of normal pentanoic, hexanoic, heptanoic, octanoic and decanoic acid were compressed after having reached adsorption equilibrium. In most cases after a certain time the compressed surface layer reached a constant surface tension value which lies below the value for the adsorption equilibrium. The difference between both values can amount to some dynes/cm and depends on the kind and concentration of the surfactant and the compression ratio. The time intervals necessary for establishing desorption equlibrium for various systems have been calculated using several theoretical models. Special experiments and theoretical considerations give evidence for the presence of small amounts of strongly surface active impurities causing the surface tension discrepancies. The problem of characterizing the specific effect of a surfactant is discussed in connection with purity.