Differences between tethered polyelectrolyte chains on bare mica and hydrophobically modified mica

This study investigates the structures of layers of amphiphilic diblock copolymers of poly(t-butyl styrene)-poly(styrene sulfonate) (PtBS-PSS) adsorbed on both the bare mica surface (hydrophilic) and an octadecyltriethoxysilane (OTE)-modified mica surface (hydrophobic). When the surface is rendered hydrophobic, the nonsoluble block exhibits stronger interaction with the surface and higher adsorbed masses are achieved. Interaction forces between two such adsorbed layers on both substrates were measured using the surface forces apparatus. The effect of salt concentration (Cs) and molecular weight (N) on the height of the self-assembled layers (L0) was examined in each case. The resulting scaling relationship is in good agreement with predictions of the brush model, L0 proportional to N(1.0) in the low-salt limit and L0N(-1) proportional to (Cs/sigma)(-0.32) in the salted regime, when adsorption takes place onto the hydrophobized mica surface. For adsorption on the bare mica surface, L0N(-0.7) proportional, variant Cs(-0.17) agrees with the scaling prediction of the sparse tethering model. The results suggest that, on the hydrophilic bare mica surface, the adsorbed amount is not high enough to form a brush structure and only very little intermolecular stretching of the tethered chains occurs; in contrast, the presence of the hydrophobic OTE layer increases the tethering density such that the polyelectrolyte chains adopt a brush conformation.     

Ellipsometry and Infrared Reflection Absorption Spectroscopy of Adsorbed Layers of Soluble Surfactants at the Air-Water Interface

Optical techniques play an increasingly important role in the characterization of microstructure and surface densities of thin films at various interfaces. In this study, ellipsometry and infrared reflection absorption spectroscopy (IRRAS) were used for determining the surface densities of adsorbed layers of cationic surfactants in situ at the air-water interface. The surfactants were N(alpha)-lauroyl-arginine methyl ester (LAM) and N(alpha), N(omega)-bis(N(alpha)-lauroyl-arginine)-alpha,omega-alkylidenediamide (C(6)(LA)(2)). In ellipsometry, the ellipsometric phase angle Delta was obtained at various surfactant concentrations and was referenced to that of the solvent. Three algorithms were used for analyzing the data. The surface densities are 3.3+/-0.3x10(-6) mol/m(2) at 1 mM for LAM and 1.5+/-0.3x10(-6) mol/m(2) at 0.1 mM for C(6)(LA)(2) by using an algorithm for which the monolayer thickness was estimated from molecular modeling. The corresponding surface densities from literature surface tension data and the Gibbs adsorption isotherm procedure are 2.2+/-0.4x10(-6) mol/m(2) and 1.2+/-0.2x10(-6) mol/m(2), respectively. In addition, IRRAS spectra were obtained from monolayers of LAM and C(6)(LA)(2) at the air-water interface. The frequencies of the methylene stretching vibration bands indicate that the monolayers are liquid-like. The surface densities were determined from the reflectance-absorbance data by using the model of either an isotropic film or an anisotropic film on the aqueous subphase. The IRRAS-based surface densities from either model, by using DPPC monolayers for calibration, are 2.4+/-0.7x10(-6) mol/m(2) at 1 mM for LAM and 1.5+/-0.6x10(-6) mol/m(2) at 0.1 mM for C(6)(LA)(2), which are in fair agreement with the ellipsometry- and the surface-tension-based surface densities. Copyright 2001 Academic Press.     

等离子体处理云母粉对其表面特性及对其与聚合物界面特性的影响

In this work mica powder was treated with n-butyl aldehyde plasma of radio frequency. Then the apparent contact angle of different liquids on the powder was determined by measuring the penetration rate of the liquid through the packed powder. The surface tension of mica powder and interface tension between the powder and typical linear macromolecules were calculated from the data of contact angle obtained under Fowkes' assumption. Rationalized conclusions were as follows: (i) For polar liquids, wetting of treated mica is more difficult than the untreated one and for nonpolar liquids, no difference has been observed. (ii) The surface tension of mica powder drops from 41.34 (N·m~(-1)·10~(-3)) to 31.51 after 5 min plasma treatment and to 25.59 (N·m~(-1)·10~(-3)) after 30 min plasma treatment. (iii) The polar component of interface tension between the mica powder and any linear macromolecule monotonically decreases with the time of plasma treating. However, the whole interface tension changes after plasma treatment, depending on the polymer used.     

Surface stress, thickness, and mass of the first few layers of polyelectrolyte

The effects of surface stress and mass loading upon the adsorption of polyelectrolytes onto flexible silicon micromechanical cantilever sensors (MCSs) were studied in situ. A self-assembled monolayer of 2-mercaptoethylamine chloride (2-MEA) on gold was used to achieve single-side adsorption on the MCS. Such a preparation gave a positive surface potential, whereas a bare SiOx surface gave a negative surface potential. Wide scan X-ray photoelectron spectroscopy confirmed that the adsorption of polystyrenesulfonate (PSS) and polyallylamine hydrochloride (PAH) followed the general rule expected from the electrostatic interaction between the substrate and the polyelectrolyte, whereas the adsorption polyethyleneimine (PEI) did not. The adsorption of PAH on SiO(x) from a 3 mM water solution containing 1 M NaCl was associated with a deflection of the MCS toward the polyelectrolyte monolayer (tensile surface stress) owing to the hydrogen bonding between neighboring amino groups. Here, a surface stress change of 1.4 +/- 0.1 N/m was estimated. The adsorption of PSS from a 3 mM water solution containing 1 M NaCl on a 2-MEA surface induced a deflection of the MCS away from the polyelectrolyte layer (compressive stress), toward the SiO(x) side. Here, a surface stress change of 3.1 +/- 0.3 N/m was determined. The formation of a PAH layer on top of the PSS layer resulted in a deflection of the MCS toward the PAH layer. This indicated that the adjacent PSS layer was deswelling, corresponding to a surface stress change of 0.5 +/- 0.1 N/m.     

A molecular dynamics study of the thermodynamic properties of calcium apatites. 1. Hexagonal phases

Structural and thermodynamic properties of crystal hexagonal calcium apatites, Ca10(PO4)6(X)2 (X = OH, F, Cl, Br), were investigated using an all-atom Born-Huggins-Mayer potential by a molecular dynamics technique. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, with maximum deviations of ca. 4% for the haloapatites and 8% for hydroxyapatite. The standard molar lattice enthalpy, delta(lat)H298(o), of the apatites was calculated and compared with previously published experimental results, the agreement being better than 2%. The molar heat capacity at constant pressure, C(p,m), in the range 298-1298 K, was estimated from the plot of the molar enthalpy of the crystal as a function of temperature, H(m) = (H(m,298) - 298C(p,m)) + C(p,m)T, yielding C(p,m) = 694 +/- 68 J x mol(-1) x K(-1), C(p,m) = 646 +/- 26 J x mol(-1) x K(-1), C(p,m) = 530 +/- 34 J x mol(-1) x K(-1), and C(p,m) = 811 +/- 42 J x mol(-1) x K(-1) for hydroxy-, fluor-, chlor-, and bromapatite, respectively. High-pressure simulation runs, in the range 0.5-75 kbar, were performed in order to estimate the isothermal compressibility coefficient, kappaT, of those compounds. The deformation of the compressed solids is always elastically anisotropic, with BrAp exhibiting a markedly different behavior from those displayed by HOAp and ClAp. High-pressure p-V data were fitted to the Parsafar-Mason equation of state with an accuracy better than 1%.     

Gibbs monolayers at the air/water interface: surface partitioning and lateral mobility of an electrochemically active surfactant

Monolayer films of a water-soluble surfactant, 4-octaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C8-TEMPO) were investigated at the air/water interface. An electrochemical, horizontal touch method was developed to measure the equilibrium surface concentrations (gamma) of C8TEMPO. The dependence of gamma on the solution concentration followed a Langmuir isotherm and yielded the partition constant K = (2.3+/-0.2) x 10(4) M(-1). These results were verified by surface tension measurements and Brewster angle microscopy. Within experimental error, the same K values were obtained. The lateral diffusion constants vs surface concentration of this molecule were measured by 2D voltammetry. In these experiments, the component of the oxidation current due to C8TEMPO in the bulk of the solution was subtracted from the total measured current to obtain the component due to the lateral surface diffusion. In the ange of mean molecular areas from 120 to 400 A2/molecule, the lateral diffusion constant of C8TEMPO increased from 1.0 x 10(-6) to 1.0 x 10(-5) cm2/s. The latter value is about 2.5 times larger than the C8TEMPO diffusion constant in bulk water. Comparison of the lateral mobilities of C8TEMPO and two longer alkane chain, water-insoluble homologues, C14TEMPO and C18TEMPO, showed no statistically significant differences.     

Surface stress, kinetics, and structure of alkanethiol self-assembled monolayers

The surface stress induced during the formation of alkanethiol self-assembled monolayers (SAMs) on gold from the vapor phase was measured using a micromechanical cantilever-based chemical sensor. Simultaneous in situ thickness measurements were carried out using ellipsometry. Ex situ scanning tunneling microscopy was performed in air to ascertain the final monolayer structure. The evolution of the surface stress induced during coverage-dependent structural phase transitions reveals features not apparent in average ellipsometric thickness measurements. These results show that both the kinetics of SAM formation and the resulting SAM structure are strongly influenced both by the surface structure of the underlying gold substrate and by the impingement rate of the alkanethiol onto the gold surface. In particular, the adsorption onto gold surfaces having large, flat grains produces high-quality self-assembled monolayers. An induced compressive surface stress of 15.9 +/- 0.6 N/m results when a c(4x2) dodecanethiol SAM forms on gold. However, the SAMs formed on small-grained gold are incomplete and an induced surface stress of only 0.51 +/- 0.02 N/m results. The progression to a fully formed SAM whose alkyl chains adopt a vertical (standing-up) orientation is clearly inhibited in the case of a small-grained gold substrate and is promoted in the case of a large-grained gold substrate.     

Elasticity of crystalline beta-sheet monolayers

Designed amphiphilic beta-sheet peptides with the sequence Pro-Glu-(Phe-Glu)(n)-Pro (n = 2-7) were previously shown by grazing incidence X-ray diffraction (GIXD), to form ordered two-dimensional (2-D) monolayer structures at interfaces induced by the proline residues at peptide termini. The GIXD diffraction pattern was modeled with two coexisting lattice arrangements, suggesting structural flexibility exhibited in the multiple ways by which beta-strands and their amino acid side chains pack into ordered 2-D structures. Here, we find by in-situ GIXD measurements that the ordered beta-sheet assemblies may undergo a quasi-reversible compression and expansion cycle at the air-water interface. The diffraction measurements indicate that on compression the repeat distance that corresponds to the long axes of the peptide strands may decrease by up to 37% in length. Upon expansion the compressed beta-sheet assemblies revert elastically to their original conformation. The interstrand repeat distance along the peptide hydrogen bonds apparently does not change along the film compression and expansion. Based on the GIXD data, at surface pressures higher than approximately 3 mN/m, beyond the peptide limiting area per molecule, the compressibility is 7.4 +/- 0.6 m/N. The out-of-plane Bragg rod diffraction patterns imply that in the compressed state the beta-strands buckle up in reaction to the increase in surface pressure. At low surface pressure, the 2-D compressibility of the crystalline beta-sheet was estimated at approximately 32 m/N attributed to interdomain rearrangements.     

Cationic Gemini surfactant at the air/water interface

The surface properties and structures of a cationic Gemini surfactant with a rigid spacer, p-xylyl-bis(dimethyloctadecylammonium bromide) ([C(18)H(37)(CH(3))(2)N(+)CH(2)C(6)H(4)CH(2)N(+)(CH(3))(2)C(18)H(37)],2Br(-), abbreviated as 18-Ar-18,2Br(-1)), at the air/water interface were investigated. It is found that the surface pressure-molecular area isotherms observed at different temperatures do not exhibit a plateau region but display an unusual "kink" before collapse. The range of the corresponding minimum compressibility and maximum compressibility modulus indicates that the monolayer is in the liquid-expanded state. The monolayers were transferred onto mica and quartz plates by the Langmuir-Blodgett (LB) technique. The structures of monolayers at various surface pressures were studied by atomic force microscopy (AFM) and UV-vis spectroscopy, respectively. AFM measurements show that at lower surface pressures, unlike the structures of complex or hybrid films formed by Gemini amphiphiles with DNA, dye, or inorganic materials or the Langmuir film formed by the nonionic Gemini surfactant, in this case network-like labyrinthine interconnected ridges are formed. The formation of the structures can be interpreted in terms of the spinodal decomposition mechanism. With the increase of the surface pressure up to 35 mN/m, surface micelles dispersed in the network-like ridges gradually appear which might be caused by both the spinodal decomposition and dewetting. The UV-vis adsorption shows that over the whole range of surface pressures, the molecules form a J-aggregate in LB films, which implies that the spacers construct a pi-pi aromatic stacking. This pi-pi interaction between spacers and the van der Waals interaction between hydrophobic chains lead to the formation of both networks and micelles. The labyrinthine interconnected ridges are formed first because of the rapid evaporation of solvent during the spreading processes; with increasing surface pressure, some of the alkyl chains reorient from tilting to vertical, forming surface micelles dispersed in the network-like ridges due to the strong interaction among film molecules.     

Urea-amide preferential interactions in water: quantitative comparison of model compound data with biopolymer results using water accessible surface areas

Two fundamentally different thermodynamic approaches are in use to interpret or predict the effects of urea on biopolymer processes: one is a synthesis of transfer free energies obtained from measurements of the effects of urea on the solubilities of small, model compounds; the other is an analysis of preferential interactions of urea with a range of folded and unfolded biopolymer surfaces. Here, we compare the predictions of these two approaches for the contribution of urea-amide (peptide) interactions to destabilization of folded proteins by urea. For these comparisons, we develop independent thermodynamic analyses of osmometric and solubility data characterizing interactions of a model compound with urea (or any other solute) and apply them to all five model compounds (glycine, alanine, diglycine, glycylalanine, and triglycine) where both isopiestic distillation (ID) and solubility data in aqueous urea solutions are available. We use model-independent expressions to calculate mu ex 23, the derivative of the "excess" chemical potential of solute "2" (either a model compound or a biopolymer) with respect to the molality of solute "3" (urea). Analyses of ID data for these systems reveal significant dependences of mu ex 23 on both m2 and m3, which must be taken into account in making comparisons with values of mu ex 23 obtained from solubility studies or from analyses of urea-biopolymer preferential interactions. Values of mu ex 23 calculated from model compound ID data at low m2 and m3 are directly proportional to the amount of polar amide (N, O) surface area, and not to any other type of surface. The proportionality constant in this limit, mu ex 23 /(RT x ASA) = (1.0 +/- 0.1) x 10(-3) A(-2), is very similar to that previously obtained by analysis of urea-biopolymer preferential interactions ((1.4 +/- 0.3) x 10(-3) A(-2)). This level of agreement for amide surface in the low concentration limit, as well as the absence of any significant preferential interaction of urea with Gly and Ala, reinforces the conclusion that the primary preferential interaction of urea with protein surface is a favorable interaction (resulting in local accumulation of urea) at polar amide surface, located mostly on the peptide backbone. However, mu ex 23 for interactions of urea with these model amides is found from both ID and solubility data to be urea concentration-dependent, in contrast to the urea concentration independence of the analogous quantity for protein unfolding.     

Interfacial tension of the lipid membrane formed from lipid-fatty acid and lipid-amine systems

Interfacial tension has been determined for phosphatidylcholine-stearic acid and phosphatidylcholine-stearylamine membranes. Phosphatidylcholine, stearic acid and stearylamine were used in the experimental. The interfacial tension values of the pure components are 1.62x10(-3) N/m, - 1.54x10(-2) N/m and 4.40x10(-3) N/m (hypothetical values), respectively. The 1:1 complexes were formed during formation of phosphatidylcholine-stearic acid and phosphatidylcholine-stearylamine membranes. The following parameters describing the complexes were determined: the surface concentrations of the lipid membranes formed from these complexes, A(3)(-1), the interfacial tensions of such membranes, gamma(3) and the stability constants of these complexes, K.     

Free radical chemistry of disinfection-byproducts. 1. Kinetics of hydrated electron and hydroxyl radical reactions with halonitromethanes in water

Halonitromethanes are disinfection-byproducts formed during ozonation and chlorine/chloramine treatment of waters that contain bromide ion and natural organic matter. In this study, the chemical kinetics of the free-radical-induced degradations of a series of halonitromethanes were determined. Absolute rate constants for hydroxyl radical, *OH, and hydrated electron, e(aq)-, reaction with both chlorinated and brominated halonitromethanes were measured using the techniques of electron pulse radiolysis and transient absorption spectroscopy. The bimolecular rate constants obtained, k (M(-1) s(-1)), for e(aq)-/*OH, respectively, were the following: chloronitromethane (3.01 +/- 0.40) x 10(10)/(1.94 +/- 0.32) x 10(8); dichloronitromethane (3.21 +/- 0.17) x 10(10)/(5.12 +/- 0.77) x 10(8); bromonitromethane (3.13 +/- 0.06) x 10(10)/(8.36 +/- 0.57) x 10(7); dibromonitromethane (3.07 +/- 0.40) x 10(10)/(4.75 +/- 0.98) x 10(8); tribromonitromethane (2.29 +/- 0.39) x 10(10)/(3.25 +/- 0.67) x 10(8); bromochloronitromethane (2.93 +/- 0.47) x 10(10)/(4.2 +/- 1.1) x 10(8); bromodichloronitromethane (2.68 +/- 0.13) x 10(10)/(1.02 +/- 0.15) x 10(8); and dibromochloronitromethane (2.95 +/- 0.43) x 10(10) / (1.80 +/- 0.31) x 10(8) at room temperature and pH approximately 7. Comparison data were also obtained for hydroxyl radical reaction with bromoform (1.50 +/- 0.05) x 10(8), bromodichloromethane (7.11 +/- 0.26) x 10(7), and chlorodibromomethane (8.31 +/- 0.25) x 10(7) M(-1) s(-1), respectively. These rate constants are compared to recently obtained data for trichloronitromethane and bromonitromethane, as well as to other established literature data for analogous compounds.     

Kinetics and mechanism of the reaction of chlorine atoms with n-pentanal

The kinetics and mechanism of the reaction Cl + CH3(CH2)3CHO was investigated using absolute (PLP-LIF) and relative rate techniques in 8 Torr of argon or 800-950 Torr of N2 at 295 +/- 2 K. The absolute rate experiments gave k[Cl+CH3(CH2)3CHO] = (2.31 +/- 0.35) x 10(-10) in 8 Torr of argon, while relative rate experiments gave k[Cl+CH3(CH2)3CHO] = (2.24 +/- 0.20) x 10(-10) cm3 molecule(-1) s(-1) in 800-950 Torr of N2. Additional relative rate experiments gave k[Cl+CH3(CH2)3C(O)Cl] = (8.74 +/- 1.38) x 10(-11) cm3 molecule-1 s(-1) in 700 Torr of N2. Smog chamber Fourier transform infrared (FTIR) techniques indicated that the acyl-forming channel accounts for 42 +/- 3% of the reaction. The results are discussed with respect to the literature data and the importance of long range (greater than or equal to two carbon atoms along the aliphatic chain) effects in determining the reactivity of organic molecules toward chlorine atoms.     

Calculation of the crystal-melt interfacial free energy of succinonitrile from molecular simulation

The crystal-metal interfacial free energy for a six-site model of succinonitrile [N triple bond C-(CH(2))(2)-C triple bond N] has been calculated using molecular-dynamics simulation from the power spectrum of capillary fluctuations in interface position. The orientationally averaged magnitude of the interfacial free energy is determined to be (7.0+/-0.4)x10(-3) J m(-2). This value is in agreement (within the error bars) with the experimental value [(7.9+/-0.8)x10(-3) J m(-2)] of Marasli et al. [J. Cryst. Growth 247, 613 (2003)], but is about 20% lower than the earlier experimental value [(8.9+/-0.5)x10(-3) J m(-2)] obtained by Schaefer et al. [Philos. Mag. 32, 725 (1975)]. In agreement with the experiment, the calculated anisotropy of the interfacial free energy of this body-centered-cubic material is small. In addition, the Turnbull coefficient from our simulation is also in agreement with the experiment. This work demonstrates that the capillary fluctuation method of Hoyt et al. [Phys. Rev. Lett. 86, 5530 (2001)] can be successfully applied to determine the crystal-melt interfacial free energy of molecular materials.     

Potentiometric studies of indium(III) azide complexes in aqueous medium

The stability constants of indium-azide complexes were determined by the potentiometric method (glass electrode). The effect monitored was the change in pH of a solution of azide and hydrazoic acid (N(-)(3)/HN(3)) when indium(III) cations were added. The azide concentration was varied from close to zero to 90mM, the ionic strength being kept at 2.000 M with sodium perchlorate and the temperature at 25.0 degrees . Evaluation of experimental data showed only mononuclear species, and the global constants found were beta(1) = (2.0 +/- 0.1) x 10(3), beta(2) = (7 +/- 2) x 10(5), beta(3) = (5 +/- 1) x 10(7) and beta(4) = (7 +/- 3) x 10(8).     

Generation of a peroxynitrato metal complex from nitrogen dioxide and coordinated superoxide

The reaction between photogenerated NO(2) radicals and a superoxochromium(III) complex, Cr(aq)OO(2+), occurs with rate constants k(Cr)(20) = (2.8 +/- 0.2) x 10(8) M(-)(1) s(-)(1) (20 vol % acetonitrile in water) and k(Cr)(40) = (2.6 +/- 0.5) x 10(8) M(-)(1) s(-)(1) (40 vol % acetonitrile) in aerated acidic solutions and ambient temperature. The product was deduced to be a peroxynitrato complex, Cr(aq)OONO(2)(2+), which undergoes homolytic cleavage of an N-O bond to return to the starting materials, the rate constants in the two solvent mixtures being k(H)(20) = 172 +/- 4 s(-)(1) and k(H)(40) = 197 +/- 7 s(-)(1). NO(2) reacts rapidly with 10-methyl-9,10-dihydroacridine, k(A)(20) = 2.2 x 10(7) M(-)(1) s(-)(1), k(A)(40) = (9.4 +/- 0.2) x 10(6) M(-)(1) s(-)(1), and with N,N,N',N'-tetramethylphenylenediamine, k(T)(40) = (1.84 +/- 0.03) x 10(8) M(-)(1) s(-)(1).     

Reflected shock tube studies of high-temperature rate constants for OH + C2H2 and OH + C2H4

The reflected shock tube technique with multi-pass absorption spectrometric detection of OH-radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the reactions, OH + C(2)H(2)--> products (1) and OH + C(2)H(4)--> C(2)H(3) + H(2)O (2). The present optical configuration gives a S/N ratio of approximately 1 at approximately 0.5-1.0 x 10(12) radicals cm(-3). Hence, kinetics experiments could be performed at [OH](0) = approximately 4-20 ppm thereby minimizing secondary reactions. OH was produced rapidly from the dissociations of either CH(3)OH or NH(2)OH (hydroxylamine). A mechanism was then used to obtain profile fits that agreed with the experiment to within <+/-5%. The derived Arrhenius expressions, in units of cm(3) molecule(-1) s(-1) are: k(1) = (1.03 +/- 0.24) x 10(-10) exp(-7212 +/- 417 K/T) for 1509-2362 K and k(2) = (10.2 +/- 5.8) x 10(-10) exp(-7411 +/- 871 K/T) for 1463-1931 K. The present study is the first ever direct measurement for reaction (1) at temperatures >1275 K while the present results extend the temperature range for (2) by approximately 700 K. These values are compared with earlier determinations and with recent theoretical calculations. The calculations agree with the present data for both reactions to within +/-10% over the entire T-range.     

Heterogeneous chemistry of the NO3 free radical and N2O5 on decane flame soot at ambient temperature: reaction products and kinetics

The interaction of NO3 free radical and N2O5 with laboratory flame soot was investigated in a Knudsen flow reactor at T = 298 K equipped with beam-sampling mass spectrometry and in situ REMPI detection of NO2 and NO. Decane (C10H22) has been used as a fuel in a co-flow device for the generation of gray and black soot from a rich and a lean diffusion flame, respectively. The gas-phase reaction products of NO3 reacting with gray soot were NO, N2O5, HONO, and HNO3 with HONO being absent on black soot. The major loss of NO3 is adsorption on gray and black soot at yields of 65 and 59%, respectively, and the main gas-phase reaction product is N2O5 owing to heterogeneous recombination of NO3 with NO2 and NO according to NO3 + {C} --> NO + products. HONO was quantitatively accounted for by the interaction of NO2 with gray soot in agreement with previous work. Product N2O5 was generated through heterogeneous recombination of NO3 with excess NO2, and the small quantity of HNO3 was explained by heterogeneous hydrolysis of N2O5. The reaction products of N2O5 on both types of soot were equimolar amounts of NO and NO2, which suggest the reaction N2O5 + {C} --> N2O3(ads) + products with N2O3(ads) decomposing into NO + NO2. The initial and steady-state uptake coefficients gamma 0 and gamma ss of both NO3 and N2O5 based on the geometric surface area continuously increase with decreasing concentration at a concentration threshold for both types of soot. gamma ss of NO3 extrapolated to [NO3] --> 0 is independent of the type of soot and is 0.33 +/- 0.06 whereas gamma ss for [N2O5] --> 0 is (2.7 +/- 1.0) x 10(-2) and (5.2 +/- 0.2) x 10(-2) for gray and black soot, respectively. Above the concentration threshold of both NO3 and N2O5, gamma ss is independent of concentration with gamma ss(NO3) = 5.0 x 10(-2) and gamma ss(N2O5) = 5.0 x 10(-3). The inverse concentration dependence of gamma below the concentration threshold reveals a complex reaction mechanism for both NO3 and N2O5. The atmospheric significance of these results is briefly discussed.     

Uptake of SO2 on HOBr-ice surfaces

The uptake of SO2 on HOBr-treated ice surfaces has been studied using a flow reactor coupled with a differentially pumped quadrupole mass spectrometer at 190-240 K. The initial uptake coefficient was determined as a function of HOBr surface coverage, theta(HOBr), on the ice. The uptake coefficients increase as the HOBr coverage increases. The uptake coefficient can be expressed as gamma(t) = k(h)theta(HOBr), where k(h) = 1.5 x 10(-19) molecules(-1) cm(-2) at 191 K and k(h) = 6.4 x 10(-21) molecules(-1) cm(-2) at 210 K and theta(HOBr) is in the range of 8 x 10(13) to 1.2 x 10(15) molecules cm(-2). The effects of temperature and film thickness on the uptake coefficients of SO2 by the HOBr-treated ice films were also studied. The activation energy E(a) of SO(2) on HOBr-ice surfaces is approximately -81 +/- 8 kJ/mol in the 190-215 K range. Kinetic results were interpreted in terms of the Eley-Rideal mechanism. This study suggests that the uptake of SO2 on ice/snow surfaces is enhanced by the presence of HOBr near the ice surface. The implication for atmospheric chemistry is that HOBr-ice surfaces may not provide a significant pathway to oxide S(IV) in the boundary layer due to both lower uptake coefficient and smaller HOBr surface coverage at T > 220 K.     

Structure of adsorbed layers of nitrophenoxy-tailed quaternary ammonium surfactants at the air/water interface studied by neutron reflection

The adsorbed layers of N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (PhiC(10)TAB) and N,N,N('),N(')-tetramethyl-N,N(')-bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide [(PhiC(10))(2)C(6)] at the air/water interface have been studied by neutron reflection. The coverage of the surfactants was obtained over the concentration range from critical micelle concentration (CMC) to CMC/100. The area per PhiC(10)TAB molecule changes from 50+/-3 to 390+/-60 A(2) over this concentration range and the area per (PhiC(10))(2)C(6) molecule changes from 139+/-3 to 288+/-10 A(2). The overall thicknesses (single uniform layer) of the surfactant layers at CMC are about 19 and 16 A for PhiC(10)TAB and (PhiC(10))(2)C(6) respectively. The distributions of the C(10) chains show that the chains of both surfactants are tilted away from surface normal, with the tilt increasing in the outer part of the layer. The distribution of C(10) chains in (PhiC(10))(2)C(6) is narrower than that in PhiC(10)TAB, indicating that the alkyl chains of (PhiC(10))(2)C(6) are more tilted. For both surfactants, the broad nitrophenoxy distribution may indicate significant positional disorder of the nitrophenoxy groups along the surface normal direction and their intermixing with alkyl chains in the adsorbed layer.