Thermodynamics of Adsorption of L-Amino Acids at Oil–Water Interfaces

Lowering of the interfacial tension of heptane–water, benzene–water, and nitrobenzene–water interfaces due to addition of 20 different amino acids to the aqueous phase has been measured. From the plot of surface pressure against molar concentration of amino acids, the initial slope and the surface excess ₂ ¹ for different amino acids have been calculated using the Gibbs adsorption equation. ₂ ¹ for most amino acids at benzene–water and heptane–water interfaces was found to be positive, with only a few being negative. At the nitrobenzene–water interface, both positive and negative ₂ ¹ values were observed. The area per adsorbed molecule at surface saturation A _m was found to vary widely, indicating different orientations of amino acid molecules at the interfaces. Using the integrated form of the Gibbs adsorption equation, the standard Gibbs energy change G ^o in kJ-m² of the adsorbed surface have been calculated for various interfaces. G ^o was found to vary linearly with the ₂ ¹ of different amino acids and the slope of the line, designated as –G _B ⁰ was found to be 22 kJ-mol^–1 for heptane–water, 23.2 kJ-mol^–1 for benzene–water, and 19.3 kJ-mol^–1 for nitrobenzene–water interfaces, irrespective of the nature of the amino acid. The origin of the linear scale of the Gibbs energy for heptane–water, benzene–water and nitrobenzene–water interfaces has been discussed in terms of hydrophobic and other interactions.     

Hydrogen Bonds at Silica – Polyvinylpyrrolidone – Water Interfaces

The effect of the adsorption of polyvinylpyrrolidone on the surface of highly dispersed silica on the state of interfacial water was studied by ¹H NMR spectroscopy and thermally stimulated depolarization with freezing out of the bulk water.     

Structure and Orientation of Tetracarboxylic Acids at Oil–Water Interfaces

Fouling caused by tetracarboxylic acids in transport and separation process chains involving petroemulsions occurs when the interfacial concentration of tetraacids becomes large enough for calcium ions in the water phase to “crosslink” the adsorbed tetraacid molecules and form a precipitate. At present, the structure and orientation of tetraacid molecules at oil–water interfaces, which influences the precipitation behavior, has not been studied in detail. In this work, molecular dynamics simulations of indigenous and synthetic tetracarboxylic acid compounds are presented to describe the structure and spatial orientation of tetraacid molecules at oil–water interfaces. Molecular distributions relative to the oil–water dividing surface along with the length and orientation angle distributions of the acidic arm groups are presented. The probability distributions determined here that describe the tetraacids at an oil–water interface can be employed to reconstruct the density of carboxylic acid groups at the oil–water interface. The interfacial carboxylic acid density can be employed to determine the fraction of adsorbed tetraacid molecules that are “crosslinked” with calcium ions based on the distances between carboxylic acid groups. The simulations presented also form a basis to calculate interfacial molecular areas and virial coefficients to employ in molecular mixed monolayer adsorption isotherms.     

Dynamic and Rheological Properties of Span 80 at Liquid–Liquid Interfaces

The adsorption characteristics of Span 80 at liquid/liquid interfaces were investigated. The equilibrium interfacial tension values were successfully fitted with a Langmuir isotherm resulting in the determination of a mean molecular area from 25 to 35 Å²/mol. The measured interfacial tension values and deduced adsorption parameters depend on the experimental technique used to obtain them, either Du Noüy ring or profile analysis tensiometry. Two possible explanations to such phenomenon are provided. Adsorption kinetics of Span 80 at liquid/liquid interfaces were studied, and it was concluded that the diffusion of Span 80 molecules from the bulk is the rate determining step of the adsorption. Finally the interfacial rheology properties were investigated and compared to the Lucassen–van den Tempel model. A good match was obtained when the isotherm parameters determined by profile analysis tensiometry were used.      

Interfacial Dilational Properties of Two Different Structure Demulsifiers at Oil–Water Interfaces

The dilational properties of demulsifiers PE1 and PE2 at the decane-water interface were investigated. Meanwhile, the influence of demulsifiers on interfacial dilational modulus of 5% crude oil also was explored. The experimental results indicate that the diffusion process is still not the main factors to control the properties of interfacial film even at large concentration. The dilational modulus of demulsifier PE2 passes through a maximum with the increasing concentration, and that of demulsifier PE1 changes a little with the increasing concentration at the decane-water interface. The dilational moduli of crude oil inerfaces decrease with the addition of both demulsifiers. At low concentration, the effect on reducing the dilational modulus of crude oil is stronger for PE1, which maybe caused by its higher substitution ability. For PE2, the ability to decrease interfacial dilational modulus gets stronger with increasing bulk concentration, which might be explained by that the adsorption increases with increasing concentration. The results of relaxation experiments support those obtain by oscillating barriers method.     

Voltammetric Probing of Molecular Assemblies of Ubiquinone-10 at the Air–Water Interfaces

The redox properties of ubiquinone 10 (UQ10) placed at the air–water interface were studied using the horizontal touching method with a thin mercury film (TMFE) working electrode and cyclic voltammetry. Changes of pH of the subphase affected the formal potential of the ubiquinone/ubiquinol system exhibiting the participation of protons in the overall reduction of UQ10. The protonation of the semiquinone transition product was found to be the rate determining step. This explains the dependence of the rate constant value on pH. The highest values of rate constants were found at pH over 13. Under these conditions the mechanism of the process is different. The concentration of protons is small, and the availability of the counter ions (i.e., K⁺) becomes crucial for the kinetics of reduction. Their role is to neutralize the negative charge of the redox group following its reduction. The logarithm of rate constants was found to decrease linearly with the increase of surface concentration of ubiquinone. This reflects the influence of intermolecular interactions in the monolayer on the kinetics of the electrode process.     

Thermodynamics of Adsorption in a Cellulose–Water System

Russian Journal of Physical Chemistry A - The sorption properties of native cellulose and the state of water adsorbed by its fibers are studied by measuring 1H NMR relaxation. A scheme of the...     

Dilational Rheological Properties of Non-Ionic Surfactants at the Water–Decane Interface: Effect of Unsaturated Hydrophobic Group

The interfacial tensions and dilational properties of adsorbed films of two non-ionic surfactants with different hydrophobic groups, polyoxy-ethylene sorbitan stearate (Tween 60) and polyoxy-ethylene sorbitan monooleate (Tween 80), at the water–decane interface have been investigated by the drop-shape analysis method. The effects of dilational frequency and bulk concentration on the interfacial properties were expounded. The influence of low temperature on the interfacial tensions and dilational properties have also been researched. The experiment results show that the interfacial activity of Tween 80 is rather large compared with Tween 60. The minimum area per molecule at the water–decane interface (A_min) value of Tween 80 is little large than that of Tween 60, which is due to the steric effect of unsaturated double bond in Tween 80 molecule. The dilational data show that the ethylene oxide groups of non-ionic surfactant form a stable sub-layer, which results in the increase of modulus and the decrease of phase angle for both Tween 60 and Tween 80 than those of common ionic surfactants. Moreover, the unsaturated hydrophobic group of Tween 80 is much flexible, which is easily crosslinked and entangled. Therefore, dilational modulus of Tween 80 is higher and phase angle is lower than that of Tween 60. Low temperature decreases the flexibility of unsaturated hydrophobic group and lessens the influence on the interaction of saturated hydrophobic group. Saturated surfactant molecules of Tween 60 almost lose temperature response.     

Adsorption of Unstructured Protein β-Casein to Hydrophobic and Charged Surfaces

In this Monte Carlo simulation study we use mesoscopic modeling to show that β-casein, an unstructured milk protein, adsorbs to surfaces not only due to direct electrostatic and hydrophobic interactions but also due to structural rearrangement and charge regulation due to proton uptake and release. β-casein acts as an amphiphilic chameleon, changing properties according to the chemical environment, and binding is observed to both positively and negatively charged surfaces. The binding mechanisms, however, are fundamentally different. A detailed, per-residue-level analysis shows that the adsorption process is controlled by a few very specific regions of the protein and that these change dramatically with pH. Caseins, being the most abundant proteins in milk, are crucial for the properties of fermented dairy products, such as nutrition, texture, and viscosity, but may also influence adhesion to packaging materials. The latter leads to product losses of about 10%, leading to economical and environmental problems.     

Nonlinear Behavior and Domain Feature at Oil／Water Interface

Studies on nonlinear behavior at oil/water interface membrane were performed. This system showed rhythmic oscillations and chaos of electrical potential in a given concentration domain. The nonlinear behavior response at the liquid membrane apparently resembled that of biological chemoreceptive membrane. The possibility of developing a new type of chemical sensor with the ability to simulate substance equilibrium in living organisms was suggested in the paper.     

Effect of Demulsifiers on Dilatational Properties of Crude Oil–Water Interfaces

The dilatational properties of polyether demulsifiers PEA, PEB, PEC, PED, PEF, and PEG at the decane-water interface were investigated. Meanwhile, the effect of demulsifiers with different structures on interfacial dilatational modulus of diluted crude oil also was explored. The properties of demulsifiers are compared and analyzed in combine with the dilatational parameters at decane-water interface and at 5% crude oil-water interface. The results show that interfacial dilatational viscoelasticity could characterize the interfacial behavior of demulsifiers. The demulsifiers, which have different kinds or structures, have different effects on destroying the interfacial film of crude oil with increasing bulk concentration. Therefore, the dosage of demulsifier is a very important role in controlling nature of crude oil film.     

Direct Electrochemical Evidence of the Dissociation and Adsorption Behavior of Acetonitrile at Gold Electrodes by Ultrafast Voltammetry

Ultrafast cyclic voltammetry was used to study the redox behavior of a gold electrode in acetonitrile. The direct electrochemical evidence of the dissociation and adsorption behavior of acetonitrile at gold electrodes was found. It could be stated that two consecutive redox paths are involved, each with a special adsorption state acting as the reaction intermediate. The mean value, obtained of the electron-transfer rate constant of the second path, was 1.3 × 105 s-1 with a standard deviation of 0.24 × 105 s-1.     

Isolation and Characterization of Naphthenic Acids from a Metal Naphthenate Deposit: Molecular Properties at Oil‐Water and Air‐Water Interfaces

Naphthenic acids from a West African metal naphthenate deposit have been isolated and characterized by infrared (IR), nuclear magnetic resonance (NMR), and Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS). The sample has been shown to comprise a narrow group of 4‐protic naphthenic acids of molecular weight ～1230 Da. The determined mass of 1230.0627 Da suggests a compound with the elemental composition C₈₀H₁₄₂O₈. The NMR data show no sign of carbon‐carbon multiple bonds. Hence, the elemental composition indicates the presence of six saturated hydrocarbon rings. The naphthenic acids have proved to be highly oil‐water (o/w) interfacially active. On elevation of the pH from 5.6 to 9.0, interfacial activity increases gradually due to a higher degree of dissociation of the carboxylic groups. At pH 9.0, the interfacial tension (IFT) between water and toluene‐hexadecane (1–9 vol.) is lowered by ～40 mN/m at concentrations of only 0.0050–0.010 mM naphthenic acid. The time rate of decrease of the IFT (dγ/dt) is also concentration‐dependent, and a well‐defined IFT is attained at long observation periods.

The C₈₀ naphthenic acids form relatively unstable Langmuir monolayers. The stability decreases further with increasing pH as more monomers become dissociated and dissolve into the aqueous phase. The stability is altered upon addition of calcium ions into the subphase due to formation of calcium naphthenate at the surface. In the undissociated state, the acids have a molecular area of ～160 Å²/molecule in the noninteracting region. The high area reflects an extended molecular structure comprising four carboxylic head groups, which are likely to be separated by hydrocarbon chains.     

Synthesis and Characterization of a Hydrophilic/Hydrophobic IPN Composed of Poly（vinyl alcohol） and Polystyrene

A hydrophilic/hydrophobic interpenetrating polymer network （IPN） of poly （vinyl alcohol） / polystyrene was prepared by conversion of the IPN of poly （vinyl acetate）/polystyrene. The hydrophilic/hydrophobic IPN was characterized by FT-IR and DSC, and the swelling ratios of the IPN in different solvents were measured.     

Versatile Sugar Derivatives for the Synthesis of Potential Degradable Hydrophilic‐Hydrophobic Polyurethanes and Polyureas

Biodegradable polymers obtained from renewable natural sources are currently receiving increasing attention because they are an alternative to the traditional petroleum‐based plastics. In the present communication we describe the synthesis of the diol monomers 2,3,4‐tri‐O‐benzyl‐L‐arabinitol (ABnOH) and 2,3,4‐tri‐O‐benzylxylitol (XBnOH), and the diamino monomers 1,5‐diamino‐1,5‐dideoxy‐2,3,4‐tri‐O‐benzyl‐L‐arabinitol (ABnNH ₂ ) and 1,5‐diamino‐1,5‐dideoxy‐2,3,4‐tri‐O‐benzylxylitol (XBnNH ₂ ), which can be used in the preparation of new potentially biodegradable sugar‐based polymers. As an example, we describe the synthesis and characterization of a polyurethane [PU‐(ABnOH‐HMDI)] and a polyurea [PUR‐(ABnNH₂‐HMDI)] by poly addition reaction of ABnOH and ABnNH ₂ with 1,6‐hexamethylene diisocyanate.      

Hydrophobic Silica Sol Coatings on Textiles—the Influence of Solvent and Sol Concentration

Hydrophobic silica sol coatings on textiles were investigated with respect to the influence of the solvents and the concentration of the sol. For this purpose, two silica sols, prepared with the hydrophobic additives octyltriethoxysilane and perfluoroctyltriethoxysilane were diluted by different solvents: water, ethanol and aceton.In case of using pure water for dilution, the hydrophobicity of coated textiles decreases drastically with increasing dilution of the applied sol. For coatings on polyester fabrics or mixed fabrics made from polyester and cotton, the use of the organic solvents ethanol or aceton leads to significant hydrophobicity even in case of strong dilution down to a sol concentration < 1%. The hydrophobic effect of coated polyamide textile is less. The reason for different hydrophobicity of coated textiles resulting from the use of water instead of organic solvents is explained by different surface morphologies of the coatings deposited on the textile fibres, as observed by REM. In case of using organic solvents the coatings contain a more flat morphology which covers the fibres completely. In contrast, sols with higher water content lead to less adhesive coatings with crack formation.The use of a combination of water with less inflammable organic solvents such as di(propylene glycol) n-propyl ether (Dowanol^TM DPnP) in hydrophobic silica sols yields textile coatings with good hydrophobicity, even in case of low sol concentration. For practical application of textile coatings, especially silica sols with high water content are of interest, due to less risk of inflammation and lower ecological impact. Therefore, the use of water diluted hydrophobic silica sols with small amounts of DPnP offers a chance for textile refinement by the sol–gel technique.     

Ovalbumin at oil–water interfaces: Adsorption and emulsification

In this article, we study the adsorption of protein ovalbumin (OVA) at corn oil (CO), soybean oil (SBO), olive oil (OO), and water interfaces along with the emulsification of these oils in water. The dynamic interfacial tension (IFT) measurements show a reduction in IFT in the order SBO–water?～?CO–water?>?OO–water, with OVA adsorption being dominated by the free diffusion of OVA at the interfaces. CO–water, OO–water, and SBO–water emulsions cream with time. The cream phase consists of jammed closed-packed oil droplets due to depletion-induced inter-droplet attractions with higher G′ and G″ (～700?Pa) for emulsions with 1?wt% OVA.     

Adsorption of cationic multicharged surfactants at liquid–gas interface

We present the review of the existing methods for describing adsorption at fluid interfaces that may be applicable for multiple charge cationic surfactant structures of various architectures—multiheaded (e.g., dicephalic) structures and multimeric surfactants (e.g., dimeric (so-called gemini), trimeric, tetrameric, etc.). We discuss the role of the prefactor the in the Gibbs adsorption equation originating from the number of charges present in the adsorbed layer and the reasons for the deviation from its thermodynamic value. We present several models of adsorption that can be used to describe surface tension isotherms and appreciate the role of prof. Peter Kralchevski in their development. Application of molecular dynamics methods combined with the approach based on the thermodynamic models of multicharged surfactant adsorption may lead to an explanation of some experimentally observed phenomena and can provide the basis for the development of novel materials designated for a variety of applications.     

Volumetric Studies of 2,2,2-Cryptand in Aqueous and Aqueous KBr Solutions at 298.15 K: An Example Involving Solvent-Induced Hydrophilic and Hydrophobic Interactions

Density measurements of good precision are reported for aqueous and aqueous salt (KBr) solutions containing 2,2,2-cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) (~0.009 to ~0.24 mol·kg^?1) for the binary systems and for the ternary system with ~0.1 mol·kg^?1 2,2,2-cryptand and varying KBr concentrations (~0.06 to ~ 0.16 mol·kg^?1) at 298.15 K. The density data have been used to study the variation of apparent molar volume (\( \varphi_{V} \)) of 2,2,2-cryptand and of KBr as a function of concentration. 2,2,2-Cryptand is a diamine and hence it is hydrolyzed in aqueous solutions and needs an appropriate methodology to obtain meaningful thermodynamic properties. We have adopted a method of hydrolysis correction developed initially by Cabani et al. and later by Kaulgud et al. to analyze our volumetric data for the aqueous solutions. The method is described and we were successful in obtaining the limiting partial molar volume of the bare (free) cryptand in water at 298.15 K. Volumes of ionization as well as volumes of complexation (with KBr) are calculated. Estimations of the apparent molar volume of 2,2,2-cryptand in CCl₄ are also reported. There is a loss in volume for the cryptand on transferring it from CCl₄ to water. The volume changes due to ionization for the cryptand in water are calculated to be –20.5 and –0.6 cm³·mol^?1 for the mono- and di-protonation equilibria respectively, while the volume of complexation for K⁺ is +24.5 cm³·mol^?1. The results are discussed in terms of conformation, protonation equilibria and selective encapsulation of K⁺ ions in cryptand cavities. The solution volume properties seem to depend upon water–solute interaction as well on the solute–solute association because of hydrophobic interactions caused by lowering of the charge density on formation of cryptand-K⁺ species in solution.     

Comparative Analysis of the Properties of Tween‐20, Tween‐60, Tween‐80, Arlacel‐60, and Arlacel‐80

Foamability and foam stability, emulsifying power, surface tension, and interfacial tension were investigated for Tween‐20 (polyoxyethylene sorbitan monolaurate), Tween‐60 (polyoxyethylene sorbitan monostearate), Tween‐80 (polyoxyethylene sorbitan monooleate), Arlacel‐60 (Sorbitan stearate), and Arlacel‐80 (Sorbitan oleate). Among all the surfactants tested for their foaming power and foamabilty, Arlacel‐60 and Arlacel‐80 showed the best results; the foaming power and foamability was found to be 100%. The surfactants having foam stability more than 50% can be considered as metastable and those less than 50% are considered as low‐stability foams. In case of surface tension and interfacial tension property measurements, Arlacel‐80 showed the best results. At 1% surfactant concentration, the surface tension and interfacial tension of Arlacel‐80 was found to be 29.9 dynes/cm and 1.1 dynes/cm at 30°C ambient temperature. Also, Arlacel‐60 was found to exhibit the best emulsifying power among all the surfactants tested. At 30°C, the emulsifying property of Arlacel‐60 was 6 hours.