Electrical properties and surface orientation of some salicylic acid derivatives at water/air interface

Surface activity, effective dipole moments and orientation of molecules at the interface were determined from surface tension () and surface potential (V), measured on aqueous solutions of salicylic acid, acetylsalicylic acid and salicylaldoxime.Applying the Demchak and Fort formula [1], the components of the effective dipole moments were calculated, which are connected to the reorientation of water molecules, hydrophilic and hydrophobic groups and local dielectric permittivities. From the effective dipole moments the orientation angles of the molecules at the water/air interface were estimated.     

Amphiphilic derivatives of dextran: adsorption at air/water and oil/water interfaces

Ionic amphiphilic dextran derivatives were synthesized by the attachment of sodium sulfopropyl and phenoxy groups on the native polysaccharide. A family of dextran derivatives was thus obtained with varying hydrophobic content and charge density in the polymer chains. The surface-active properties of polymers were studied at the air-water and dodecane-water interfaces using dynamic surface/interfacial tension measurements. The adsorption was shown to begin in a diffusion-limited regime at low polymer concentrations, that is to say, with the diffusion of macromolecules in the bulk solution. In contrast, at long times the interfacial adsorption is limited by interfacial phenomena: adsorption kinetics or transfer into the adsorbed layer. A semiempirical equation developed by Filippov was shown to correctly fit the experimental curves over the whole time range. The presence of ionic groups in the chains strongly lowers the adsorption kinetics. This effect can be interpreted by electrostatic interactions between the free molecules and the already adsorbed ones. The adsorption kinetics at air-water and oil-water interfaces are compared.     

Equilibrium surface properties of lipid mixtures of retinal,phosphatidylcholine and fatty acid derivatives at the air/water interface

The miscibilities of phosphatidylcholine, retinal and saturated fatty acid derivatives in surface phases at the air/water interface are investigated on the basis of the thermodynamic two-dimensional phase rule. The latter is applied to the ‘collapse’ pressure and the equilibrium surface pressure characteristics of binary lipid monolayers or spread amphiphilic mixtures, respectively. The equilibrium surface pressures (ESPs), at which insoluble lipid monolayers are in equilibrium with three-dimensional lipid phases, are determined by spreading of single-component or binary solutions of lipids in organic solvent up to supersaturation at the air/water interface. The kinetics of establishment of steady surface pressure values at supersaturation is followed depending on the nature of the lipid samples. ESPs and ‘collapse’ pressures of mixtures of dilaur-oylphosphatidylcholine (DLPC), all-trans retinal (t-R) and lauric acid (LA) are studied at various lipid molar ratios. The compositional phase diagrams of the ESPs and ‘collapse’ pressures, obtained at a constant temperature, indicate that the interfacial miscibilities of both DLPC and t-R and DLPC and LA are non-ideal. Owing to its ‘bulky’ molecular structure and the tendency towards self-aggregation, dominated by intermolecular π-π interactions, the t-R component could be accommodated in the hydrophobic portion of the phospholipid membrane at mole fractions less than 0.5. The accommodation of the other neutral, rod-like fatty acid component in the DLPC matrix is probably favoured by the formation of intermolecular hydrogen bonding. Phase separation between DLPC and LA is evident from the thermodynamic results at high LA mole fractions (> 0.75) in the surface mixtures.     

Effect of amino acids on aggregation behaviors of sodium deoxycholate at air/water surface: surface tension and oscillating bubble studies

The aggregation behaviors of sodium deoxycholate (NaDC) at the air/water surface were investigated via surface tension and oscillating bubble measurements in the absence and presence of three alkaline amino acids, namely, L-Lysine (L-Lys), L-Arginine (L-Arg), and L-Histidine (L-His). The results of surface tension measurements show that NaDC has a lower ability to reduce the surface tension of water, because NaDC molecules orient at the surface in an oblique direction and tend to aggregate together, which is approved by molecular dynamics (MD) simulation. L-Lys is the most efficient of the three amino acids in reducing the critical aggregation concentration (cac) of NaDC in aqueous solution. The influence of amino acids on the dilational rheological properties of NaDC was studied using the drop shape analysis method in the frequency range from 0.02 to 0.5 Hz. The results reveal that the absolute modulus passes through a maximum value with increasing NaDC concentration. The addition of amino acids increases the absolute modulus of NaDC, and the maximum value is observed at much lower concentration. From the perspective of structures of amino acids, the performance of L-Arg is similar to that of L-His, and both of them bring out a smaller effect on the absolute modulus than that of L-Lys. From the above results, it may be presumed that electrostatic and hydrophobic effects are important impetus during the interaction between amino acids and NaDC at the air/water surface. Hydrogen bonding is so ubiquitous in the system that the difference of hydrogen bonding between NaDC and amino acid is ignored.     

On the adsorption properties of surface chemically pure CHAPS at the air/water interface

CHAPS, a surface-active derivative of the steroids' basic structure of the cholic acid [3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate] has become a very important material in biological and pharmaceutical application. Investigations of the adsorption properties of aqueous, surface-chemically pure CHAPS solutions at the air/water interface were performed using surface tension and surface potential measurements. Unlike ordinary extended-chain surfactants, the amphiphilic structure of CHAPS is prone to adopt different concentration-dependent surface states of the adsorption layer. These are well reflected in the adsorption isotherm and in the electric surface properties. They are explained by changes in the adsorbate molecule's orientation and/or conformation as a result of the latter's different surface area demand. The versatile favorable application properties of the CHAPS molecule are obviously due to its complicated molecular structure, which enables it to comply with rather different interfacial and colloidal challenges.     

Adsorption of polyprotic acid at the water/air interface

A rigorous thermodynamic treatment appropriate for surface adsorption from mixed aqueous solution of alkali and polyprotic acid was derived. Those equations were applied to mixed aqueous solution/air systems of alkali metal hydroxide and Fe^III complex with ethylenediamine- N, N, N′,N′-tetraacetate (Fe-EDTA). Surface density of each species arising from Fe-EDTA was separately evaluated, and thus, surface activity of Fe-EDTA was studied, especially its dependence on pH and how it is influenced by the counter cations. Fe-EDTA was positively adsorbed at the water/air interface at very low pHs and negatively at high pHs. The pH range of positive adsorption of Fe-EDTA with potassium ion, as a counter ion, was wider than that with sodium ion. Thus, potassium ion, a structure breaker, tended to smooth surface adsorption of Fe-EDTA at the water/air interface, whereas sodium ion, a structure maker, tended to withdraw Fe-EDTA from the interfacial region.     

Ozonolysis of uric acid at the air/water interface

Uric acid (UA) epoxide, peroxide, and ozonide species produced in aqueous UA microdroplets exposed to O(3)(g) are detected by online mass spectrometry within approximately 1 ms. UA conversions are independent of its initial concentration below approximately 0.1 mM and are unaffected by addition of excess H(2)O(2) or t-butanol. UA reactivity increases approximately 380 times from pH 4 to 7, which is at variance with the pH-independent rates reported for the UA + O(3)(aq) reaction in bulk water. At pH approximately 7, UA and ascorbic acid (AH(2)) microdroplets react with O(3)(g) at similar rates, although UA is approximately 40 times more reactive than AH(2) toward O(3)(aq) in bulk water. Only the UA epoxide, plus traces of UA peroxide, are formed upon mixing UA(aq) and O(3)(aq) solutions. We infer that the gas-liquid ozonolysis of UA proceeds in an interfacial aqueous medium quite distinct from bulk water. Thus, UA, a component of the pulmonary epithelial lining fluid that scavenges atmospheric O(3)(g) into less deleterious species (similar to AH(2)), is rendered inactive below pH approximately 5. The potential implications of these findings on synergistic health effects between tropospheric ozone and acidic particulates are briefly analyzed.     

Amphiphilic polymer supports for the asymmetric hydrogenation of amino acid precursors in water

This paper describes the synthesis and characterization of a new class of amphiphilic, water-soluble diblock copolymers based on 2oxazoline derivatives with pendent (2S,4S)-4-diphenylphosphino-2-(diphenylphosphinomethyl)pyrrolidine (PPM) units in the hydrophobic block. The synthetic strategy involves the preparation of a diblock copolymer precursor with ester functionalities in the side chain; which were converted into carboxylic acids in a polymer-analogous step and finally reacted with the PPM ligand. The structures of the copolymers were characterized by (1)H and (31)P NMR spectroscopy and GPC measurements. Subsequently, these polymers were successfully utilized as a polymeric support for the asymmetric hydrogenation of 1) (Z)-alpha-acetamido cinnamic acid and 2) methyl (Z)-alpha-acetamido cinnamate in water, showing 90 % substrate conversion at 25 degrees C within 20 minutes at atmospheric H(2) pressure (1 bar) for methyl (Z)-alpha-acetamido cinnamate.     

Amphiphilic siloxane phosphonate macromolecule monolayers at the air/water interface: effects of structure and temperature

A comprehensive study is reported of Langmuir-Blodgett (LB) films (spread at the air/water interface using the Langmuir balance technique) composed of surface active, nonionic, and OH-free amphiphilic siloxane phosphonate ester macromolecules. Analysis is made on three molecular structures in the form of linear polymer poly(diethylphosphono-benzyl-alphabeta-ethyl methylsiloxane) (PPEMS), cyclic oligomer methylphosphonobenzyl-alphabeta-ethyl cyclosiloxane (MPECS), and copolymer poly(PEMS-co-DMS). The surface pressure-surface area (pi -A) isotherms of homopolymer at 3-40 degrees C show a clear temperature-induced phase transition (plateaus at pit approximately 17-19 mN/m) below 10 degrees C. The magnitude of the transition substantially increases upon lowering the temperature (partial differential DeltaAt/ partial differential T approximately -0.1 nm2 unit(-1) deg(-1) and partial differential pi t / partial differential T approximately -0.25 mN m(-1) deg(-1)). The positive entropy and enthalpy gain infers that strong coupling with the subphase and excess hydration attributed to hydrogen bonding between the P=O bond and the subphase prevails at low temperatures. The cyclic oligomer MPECS forms a condensed monolayer at the air/water interface that does not display a similar transition in the experimental temperature range. The temperature sensitivity of MPECS film is observed only in the collapsed region. The nature of the interaction with the subphase is similar for MPECS and PPEMS, indicating that the size and thermal mobility are the controlling factors in these processes. The elasticity plot reveals two distinct states (above and below transition). This observation is supported by BAM images that show irregular spiral structures below 10 degrees C. The transition occurring in the copolymer at 20 degrees C is due to relaxation of the PDMS component. The two maxima shown in the elasticity plot indicate additive fractions of PPEMS and PDMS. The surface areas of these macromolecules in the relaxed (1.48 nm2/unit) and packed (0.45 nm2/unit) forms obtained by PM3 modeling agree well with the experimental data and seem to indicate that the siloxane chain is being lifted off the subphase by the hydrophobic phenylic part of the molecule.     

Viscoelastic properties of insoluble amphiphiles at the air/water interface

The effects of the presence of a molecular monolayer on the dilatational properties of the air/water interface have been investigated. Two water insoluble amphiphiles, dipalmitoyl phosphatidyl choline and quercetin 3-O-palmitate, were spread onto a pendant drop and the dynamic surface pressure was measured by means of drop shape analysis. The surface dilatational elasticity and viscosity of the spread monolayers were also determined by the oscillating drop technique. Constraints on the range of measuring conditions were investigated and we demonstrated that the pressure-area isotherms derived from oscillatory dynamic measurements display phase behaviour similar to that found in equilibrium measurements, albeit at reduced resolution. Both the amphiphiles formed purely elastic films that were characterised by a dilatational modulus that depended on the surface concentration and obeyed a power scaling law. The exponent of the relationship could be related to the thermodynamic conditions prevailing at the interface. The phospholipid monolayer scaling exponent was 2.8 in a temperature range of 20-26 degrees C indicates a favourable solvency of molecules in the bidimensional matrix. A very high scaling exponent (11.8 at 7 degrees C) for quercetin palmitate was interpreted assuming that molecules self-organise in fibre-like structures. This interface structure and the phase behaviour was found consistent with observations of the surface film obtained by Brewster angle microscopy. The structured quercetin 3-O-palmitate monolayers are disrupted by temperature increase or by adding a 0.2 molar fraction of the immiscible dipalmitoyl phosphatidyl choline.     

Adsorption and surface rheology of n-dodecanol at the water/air interface

Adsorption layers of n-dodecanol at the water/air interface show phase transitions at low temperatures [Vollhardt, Fainerman, Emrich, J. Phys. Chem. B 104 (2000) 8536]. Using a drop shape technique it is shown that the dilational elasticity disappears in the coexistence region of the adsorption layer. The relaxation time between the condensed and liquid-like surface states is in the sub-second time range.     

Effect of interactions between the adsorbed species on the properties of single and mixed-surfactant monolayers at the air/water interface

Experimental data on surface tension available from the literature and generated in the present study are analyzed to estimate the applicability of adsorption models, based on the Frumkin equation, to nonionic and ionic surfactants and their mixtures. Optimization programs based on the least-squares method in media of Delphi V and Pascal VII are used. The effect of interactions between the adsorbed species on surface tension is considered in all cases. The results are compared to those obtained with the simpler Szyszkowski equation, employed in numerous studies of nonionic surfactants, when interactions are neglected. Cases where the Frumkin model can be successfully employed with ionic surfactants and mixtures are presented and the conditions of its applicability are analyzed. Related characteristic quantities (maximum adsorption, standard free energy of surfactant adsorption, energy of interaction between adsorbed species, standard free energy of counterion adsorption, degree of coverage by surfactant/counterion associates) are established as a function of: The properties of an adsorption layer from a mixture of nonionic and ionic surface-active species are compared to those of the single surfactants.     

Evaluation of protein quantification using standard peptides containing single conservative amino acid replacements

Structural analogs are evaluated as peptide internal standards for protein quantification with liquid chromatography‐multiple reaction monitoring mass spectrometry (LC‐MRM); specifically, single conservative amino acid replacements (SCAR) are performed to create tagged standards that differ by the addition or subtraction of a single methylene group in one amino acid side chain. Because the performance of stable isotope‐labeled standards (SIS) has been shown to be superior to structural analogs, differences in both development and quantitative performance between assays based on SIS and SCAR peptides are explored. To establish an assay using the structural analogs, analysis of endogenous, SCAR and SIS peptides was performed to examine their ion signal, fragmentation patterns and response in LC‐MRM. Performance of SCAR and SIS peptides was compared for quantification of epidermal growth factor receptor from lung cancer cell lysates and immunoglobulin M in the serum of multiple myeloma patients. Copyright © 2012 John Wiley & Sons, Ltd.     

Adsorption behavior of surface chemically pure N-cycloalkylaldonamides at the air/water interface

Equilibrium surface tension (sigma(e)) and electric surface potential (DeltaV(e)) versus concentration isotherms of the homologous series of N-cycloalkylaldonamides synthesized from cycloalkylamines (from cyclopentyl- to cyclododecylamine) and D-glucono-1,5-lactone (c-C(n)GA) or D-glucoheptono-1,4-lactone (c-C(n)GHA) (c-n(C) = 5-12) were investigated at the air/water interface. The measurements were performed with aqueous, surface chemically pure surfactant solutions. Equilibrium surface tension vs concentration isotherms were evaluated to get the adsorption parameters, i.e., standard free energy of adsorption, DeltaG degrees (ads), saturation surface concentration, Gamma(infinity), minimum surface area demand per molecule adsorbed, A(min), and interaction parameter, H(s). Increasing the size of the cycloalkyl moiety leads to a significant increase of the minimum surface area demand per molecule adsorbed. This fact, together with a decrease of the intermolecular interaction parameter suggests that the introduction of a more bulky cycloalkyl ring (c-n(C) = 7 and 8) causes an attenuation of the hydrogen-bond network. This goes in line with the exceptional finding that the higher homologues revealed improved solubility in water. In addition, surface tension investigations suggest occurrence of a phase transition for the N-cyclooctylaldonamides at relatively small surface coverage. This observation is well supported by the surface potential measurements, for which the effect of possible changes in the molecules' surface orientation is even more pronounced. Moreover, the concentration intervals of N-cyclooctylaldonamide in which the change in orientation is observed for either the surface tension or the surface potential isotherms are in very good agreement.     

Design of surface active soluble peptide molecules at the air/water interface

Surface active molecules collect at interfaces and have the potential to be used for water evaporation reduction. The objective of this work is to design surface active soluble peptides that collect at the air/water interface using molecular simulations. Rotational isomeric state Monte Carlo (RISMC) sampling together with a solvation model that we recently invented, the AAD solvation model [Gu, C.; Lustig, S.; Trout, B. J. Phys. Chem. B 2006, 110 (3), 1476-1484] was applied to calculate the adsorption free energy of the peptide molecule at the air/water interface. The results were validated by both molecular dynamics simulations with an explicit solvent model and surface tension measurements on synthesized peptides. It was demonstrated that this approach is able to give a reasonable prediction of surface activity with an approximately 50% hit rate in terms of designed surface active molecules actually being surface active. The relationship between the chemical composition and the surface morphology is also discussed.     

Aggregation behavior of block polyethers with branched structure at air/water surface

The block polyethers with various branch structure, such as TEPA[(PO)₃₆(EO)₁₀₀]₇, TEPA[(PO)₃₆(EO)₁₀₀(PO)₃₆]₇, and TEPA[(PO)₃₆(EO)₁₀₀(PO)₅₆]₇ were synthesized. Moreover, the aggregation behavior was investigated via the measurements of equilibrium surface tension, dynamic surface tension, and surface dilational viscoelasticity, in order to probe the effect of the block structure on the property of the branched block polyethers. The surface tension results show that the efficiency and effectiveness of the block polyethers to lower surface tension increase with the increase of the PO group numbers. The maximum surface excess concentration (Γ_max) values and the minimum occupied area per molecule at the air/water interface (A_min) values of the branched block polyethers obtained from Gibbs adsorption equations increase and decrease with the increases of the PO group numbers, respectively. The dynamic parameters n and t^* representing the diffusion speed of the polyether molecules from bulky solution to the subsurface and from the subsurface to the air/water surface are obtained according to the equation proposed by Rosen. The results show that the n values firstly increase and then decrease and t^* values decrease with the increase of the polyether concentrations. The results of surface dilational viscoelasticity show that the dilational modulus of TEPA[(PO)₃₆(EO)₁₀₀(PO)₅₆]₇ is the largest among the three block copolymers at the low concentration (<1 mg L⁻¹) but that of TEPA[(PO)₃₆(EO)₁₀₀]₇ is the largest at the high concentration (>1 mg L⁻¹).     

Interfacial dilational properties of tri-substituted alkyl benzene sulfonates at air/water and decane/water interfaces

The dilational rheological properties of absorbed film of three pairs of structural isomers, tri-substituted alkyl benzene sulfonates, at the air-water and decane-water interfaces have been investigated by drop shape analysis method. The influences of bulk concentration on dilational elasticity and viscosity were expounded. Interfacial tension relaxation method was employed to obtain dilational parameters in a reasonably broad frequency range. The experimental results showed that the meta-alkyl to sulfonate group plays a crucial role in the interfacial dilational properties: the longer meta-alkyl will lead to higher dilational parameters for air-water interface and lower ones for decane-water interface when the total alkyl carbon numbers are equal. For alkyl benzene sulfonates with shorter meta-alkyl, the surface dilational properties are similar to interfacial dilational properties, whereas the surface dilational parameters are obviously higher than the interfacial dilational parameters for alkyl benzene sulfonates with longer meta-alkyl in general. The possible mechanism has been proposed and ensured by Cole-Cole plots.     

Analysis of dynamic surface tension of tetraethyleneglycol monooctyl ether at air/water interface

The dynamic surface tension of the aqueous solutions of tetraethyleneglycol monooctyl ether (C₈E₄), a nonionic surfactant, was measured at different concentrations and temperatures. Present data at 298.15 K clearly indicate that the mechanism of adsorption is purely diffusion controlled at low concentrations (0.1~0.4 mmol/kg), and there is a switchover in adsorption mechanism to the mixed diffusion-kinetic control at higher concentrations. The calculated activation energies increase with concentration, and thus, with surface density, but decrease with temperature. The magnitude of activation energy and its increase with surface density suggest that the barrier is due to the free surface site formation by overcoming mainly the attractive van der Waals forces between the chain of adsorbed C₈E₄ molecules.     

Complete immiscibility in binary cholesteryl-n-carboxylic acid ester/stearic acid monolayers at the water/air interface

Applying different surface and spreading techniques to form binary monolayers in a different mixing state, the mixing behavior of the three binary systems cholesteryl formiate/stearic acid, cholesteryl acetate/stearic acid, and cholesteryl-n-propionate/stearic acid were investigated and compared.Analyzing the force ()/area (a) isotherms and the equilibrium spreading pressures (^e of the binary monolayers, it can be concluded that the components of the three binary systems do not mix within the whole concentration range. The lipids in the binary monolayers are completely immiscible.