ON ESTIMATION OF THE AREA PER A SURFACTANT MOLECULE IN A 2D MONOLAYER DURING THE LIQUID-EXPANDED-LIQUID-CONDENSED PHASE TRANSITION AT THE AIR/WATER INTERFACE

Предложен метод оценки площади A, приходящейся на молекулу монослоя ПАВ, вначале перехода от жидко-растянутой к жидко-конденсированной фазе на основе двух теоретических моделей. Термодинамическая модель поведения дифильных монослоев с учетом неидеальности энтропии смешения позволяет оценить величину энергий Гиббса кластеризации ПАВ на основе П-А-изотерм, полученных при различных температурах. С другой стороны квантово-химический подход также дает возможность рассчитать данный термодинамический параметр и оценить структурные особенности получаемых монослоев. Величины энергий Гиббса кластеризации малых ассоциатов ПАВ и геометрических параметров элементарных ячеек монослоев были рассчитаны ранее с помощью квантово-химического полуэмпирического метода РМ3 для восьми классов дифильных соединений: насыщенные и этоксилированные спирты, насыщенные и цис-моноеновые карбоновые кислоты, α -гидрокси- и α -аминокислоты, N -ацилпроизводные аланина и диалкилзамещенные меламина. Эти параметры были использованы в термодинамической модели с учетом неидеальности энтропии смешения для расчета величин A. Оцененные значения A адекватно отражают экспериментальную температурную зависимость для рассматриваемого фазового перехода: с ростом температуры площадь, приходящаяся на молекулу ПАВ фиксированной длины цепи, уменьшается, и, наоборот, с ростом длины цепи ПАВ при фиксированной температуре величина A увеличивается. Это позволяет использовать предложенный подход в прогностических целях. A method is proposed to estimate the area per molecule of a surfactant monolayer A at the transition onset of the liquid-expanded to a liquid-condensed phase based on two theoretical models. A thermodynamic model with account for nonideality of the mixing entropy makes it possible to estimate the Gibbs energy of surfactant clusterization using the П-A isotherms obtained at different temperatures. On the other hand, the quantum-chemical approach also makes it possible to calculate this thermodynamic parameter and assess the structural features of the obtained monolayers. The values of the Gibbs clusterization energies of small surfactant associates and the geometric parameters of the monolayer unit cells were previously calculated using the quantum-chemical semiempirical method PM3 for eight classes of amphiphilic compounds: saturated and ethoxylated alcohols, saturated and cis-monoenic carboxylic acids, α-hydroxylic and α-amino acids, N -acyl-substituted alanines and dialkyl-substituted melamine. These parameters are used in the thermodynamic model with account for nonideality of the mixing entropy to calculate A. The estimated values Aadequately reflect the experimental temperature dependence for the considered phase transition: with an increase in temperature the area per surfactant molecule of a fixed chain length decreases, and vice versa, with an increase in the surfactant chain length at a fixed temperature, the value A increases. This makes it possible to use the proposed approach for prognostic purposes.

Systematic Studies of Sodium γ-Octyl Aspartate to evaluate its Biodegradation, Surface Parameters, Micellization Aspects and Thermodynamic Properties of its Binary Mixture using Electrolytes

γ-octyl aspartate has been synthesized with 46.2% yield. The optimized conditions for esterification of aspartic acid and n-octanol are 24 hours duration and 1:1 molar ratio of reactants (aspartic acid: n-octanol). FT-IR, 1H NMR and CHNS element analysis techniques are used for structure elucidation of γ -octyl aspartate. Surface/ interfacial studies, micellization aspects, thermodynamic parameters, biodegradability and binary salt behaviour of sodium γ-octyl aspartate have been studied. The CMC of synthesized surfactants was 0.0078 mol/L, whereas minimum surface area per surfactant molecule was calculated as 29.49 Å2m2; calculated aggregation number was 34 and micelle radius (r) found to be 1.40 nm. The degree of counter-ion dissociation (α) was determined to be 0.86. Thermodynamic parameter - standard free energy of adsorption (ΔG0ads) was calculated as -18.70 kJ/mol. The shape of micelle of synthesized surfactant was noticed to be spherical but during binary studies in some cases, it was found to be cylindrical. OECD and Winkler’s method were used to determine biodegradation of sodium γ-octyl aspartate found to be 95.1 %.

Salicylic acid stabilizes Staphylococcus aureus biofilm by impairing the agr quorum-sensing system

Salicylic acid (SAL) has recently been shown to induce biofilm formation in Staphylococcus aureus and to affect the expression of virulence factors. This study was aimed to investigate the effect of SAL on the regulatory agr system and its impact on S. aureus biofilm formation. The agr quorum-sensing system, which is a central regulator in S. aureus pathogenicity, plays a pivotal role in the dispersal of S. aureus mature biofilms and contributes to the creation of new colonization sites. Here, we demonstrate that SAL impairs biofilm dispersal by interfering with agr expression. As revealed by our work, protease and surfactant molecule production is diminished, and bacterial cell autolysis is also negatively affected by SAL. Furthermore, as a consequence of SAL treatment, the S. aureus biofilm matrix revealed the lack of extracellular DNA. In silico docking and simulation of molecular dynamics provided evidence for a potential interaction of AgrA and SAL, resulting in reduced activity of the agr system. In conclusion, SAL stabilized the mature S. aureus biofilms, which may prevent bacterial cell dissemination. However, it may foster the establishment of infections locally and consequently increase bacterial persistence leading to therapeutic failure.

DIAMETER OF SODIUM DI-(2-ETHYLHEXYL)PHOSPHATE AND SODIUM DODECYLSULFATE REVERSE MICROEMULSION DROPLETS: EXPERIMENTAL DATA AND CALCULATION METHODS

The article is concerned with study of diameters of reverse microemulsion droplets depending on the water content in sodium bis-(2-ethylhexyl)phosphate – decane – water and sodium dodecylsulfate – butanol-1 – decane – water systems. Hydrodinamic diameters of microemulsion droplets were determined by the method of dynamic light scattering. It increases from 5.9 to 9.7 nm with an increase in the molar ratio of water and sodium bis-(2-ethylhexyl)phosphate from 5.0 to 20.0 in the sodium bis-(2-ethylhexyl)phosphate – decane – water microemulsion. Diameter of microemulsion droplets in the sodium dodecylsulfate – butanol-1 – decane – water system increases from 2.4 to 8.6 nm with an increase in the molar ratio of water and dodecylsulfate from 7.5 to 30.0. It was shown that the dependence of the diameter (d) on the water content (W) is linear. The applicability of the experimental data on the area per surfactant molecule at the "water-oil" interface and literature data on the values of molar volumes of microemulsion components to calculate the diameter of microemulsion droplets was analyzed. It was shown that the equation of the form d = kW + b is suitable for describing the dependence of the diameter of microemulsion droplets on the water content in the sodium dodecyl sulfate-butanol – 1-decane-water system. Parameters k and b of this equation are calculated using the value of the area per surfactant molecule at the "water-oil" interface, which can be obtained from experimental data on interfacial tension. Values of the diameter of microemulsion droplets calculated using the proposed equation differ by no more than 1 nm from the values obtained by dynamic light scattering. Values of the droplet diameter of microemulsion in the DEHPA – decane – water system calculated using this equation are underestimated by 1.0-2.5 nm relative to the experimental values, while the slope of the lines obtained from the experimental data and during the calculation is almost the same.

Interfacial and Micellization Behavior of Cetyltrimethylammonium Bromide (CTAB) in Water and Methanol-Water Mixture at 298.15 to 323.15 K

The micellization behavior of cetyltrimethylammonium bromide (CTAB) in water , 0.1, 0.2, 0.3, and 0.4 volume fractions of methanol at 298.15, 308.15, 318.15, and 323.15 K were investigated by surface tension measurements. The effect of methanol on values of critical micelle concentration (cmc), free energies of micellization ΔGmo, and surface properties viz. maximum surface excess concentration Γmax, area occupied by per surfactant molecule Amin, surface pressure πcmc, solution surface tension γcmc, solvent surface tension (γo), free energies of adsorption ΔGadso, the efficiency of adsorption (pC20), effective Gibbs free energy ΔGeffo, and free energy of surface at equilibrium (Gmin) were investigated using surface tension values. Other parameters such as the packing parameter (P), aggregation number (N), concentration of surfactant in the bulk phase (C20), relation between Amin and πcmc, and correlation of slopes dγ/d log C, γo/γcmc, Γ/Γmax, cmc/C20, ΔGadso/ΔGmo, and cmc/pC20 with the volume fraction of methanol are calculated and discussed in the light of the experiment done.

A multifunctional surfactant catalyst inspired by hydrolases

The remarkable power of enzymes to undertake catalysis frequently stems from their grouping of multiple, complementary chemical units within close proximity around the enzyme active site. Motivated by this, we report here a bioinspired surfactant catalyst that incorporates a variety of chemical functionalities common to hydrolytic enzymes. The textbook hydrolase active site, the catalytic triad, is modeled by positioning the three groups of the triad (-OH, -imidazole, and -CO2H) on a single, trifunctional surfactant molecule. To support this, we recreate the hydrogen bond donating arrangement of the oxyanion hole by imparting surfactant functionality to a guanidinium headgroup. Self-assembly of these amphiphiles in solution drives the collection of functional headgroups into close proximity around a hydrophobic nano-environment, affording hydrolysis of a model ester at rates that challenge α-chymotrypsin. Structural assessment via NMR and XRD, paired with MD simulation and QM calculation, reveals marked similarities of the co-micelle catalyst to native enzymes.

Mixed adsorption of hexadecylpyridinium bromide and Triton X surfactants at graphitized carbon black

Adsorption of cationic 1-hexadecylpyridinium bromide (HDPB) and non-ionic p-(1,1,3,3-tetramethylbutyl)-phenoxypolyoxyethylene glycols surfactants of the Triton X series (Triton X-45, Triton X-100 and Triton X-305) from their single and mixed aqueous solutions at thermally graphitized carbon black (CB) was studied. It was shown that the adsorption of the non-ionic surfactant from its individual solution decreased when a number of ethylene oxide units in the surfactant molecule increased. Addition of the non-ionic surfactants increased the amount of HDPB adsorbed from HDPB/Triton X mixtures. At low solution concentrations it was found that in HDPB/Triton X mixtures, the experimental values of total surfactants adsorption are higher than the adsorption values calculated for the ideal surfactant mixtures. The composition of the mixed HDPB/Triton X adsorption layer and the parameters of the intermolecular interaction (?s), between the components in this layer, were calculated using the Rubingh-Rosen approach. It was shown that ?s parameters have negative values, which indicate notable interactions between Triton X molecules and HDPB ions in the mixed adsorption layer. It was found that the composition of the mixed adsorption layer at CB surface is notably different from the surfactants composition in the bulk solution. The mixed HDPB/Triton X adsorption layer is enriched with Triton X surfactant and the mole fraction of Triton X increases with decreasing of ethoxylation degree of its molecules.