Regular ArticleAdsorption of Dodecyl Sulfate Surfactants with Monovalent Metal Counterions at the Air-Water Interface Studied by Neutron Reflection and Surface Tension
Abstract
Neutron reflection and surface tension have been used to study the adsorption at the air-water interface of the dodecyl sulfates of the alkali metals (MDS). The critical micelle concentration (cmc) at 33°C was found to vary from 8.5 × 10-3M for LiDS to 5.9 × 10-3M for CsDS, in agreement with the values in the literature. Neutron reflection was used to measure the adsorption of LiDS, NaDS, and CsDS at their cmc's at a temperature of 25°C for LiDS and NaDS, and 33°C for CsDS. The area per molecule(Acmc) was found to be 50, 45, and 38 ± Å2 respectively. Surface tension measurements were made for LiDS, NaDS, KDS, RbDS, and CsDS at varying temperatures and Acmc has also been calculated using the Gibbs equation. The values corresponding to the neutron reflection measurements are 51, 48, and 39 ± 3 Å2 when the counterions are Li+, Na+, and Cs+, respectively. The effect of temperature on Acmc was found to be smaller than the change of counterion. The variation of the surface tension with temperature was used to determine the thermodynamic parameters ΔGads, ΔHads, and ΔSads and ΔGmic, ΔHmic, and ΔSmic for adsorption and micellization respectively. The most noteworthy feature is that an increase in temperature decreases the surface tension of LiDS but increases it for all the other species. The structure of the adsorbed layer was determined by neutron reflection. Two methods of analysis of the data were used. One is the optical matrix method which fits a single structural model to the reflectivity profiles from a set of isotopic species at a given concentration, and the other by a more direct approach based on the kinematic theory. Both methods give identical structures for all three systems. The thickness of the layers at their corresponding cmc's are 18 Å for LiDS, 18 Å for NaDS, and 19 ± 1 Å for CsDS and the number of water molecules associated with each surfactant head group region is respectively eight, seven, and four. The distance from the center of the alkyl chain distribution to that of the solvent is found to be 6.5 Å for LiDS, 7.5 Å for NaDS, and 8.0 ± 1 Å for CsDS. The extent to which the alkyl chain is immersed in water decreases with the area per molecule and is about 35% for LiDS and below 30% for CsDS at their cmc's.
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2024, Journal of Molecular LiquidsThe hydrophilicity of SDS surfactant was evaluated using different charge distributions derived from atomic charges obtained by quantum mechanical calculations. The atomic charges were determined using the Hirshfeld, CM5, CHELPG, and Merz-Kollman methods. The results indicate that the correct charge distribution in the hydrophilic group has an impact on the description of the surfactant hydrophilicity, which allows us to obtain a good estimation of the interfacial properties of the vacuum/SDS/water system. Two models of charge distribution based on the point charge model were proposed for SDS surfactant, using atomic charges computed through Quantum Mechanical calculations. The charge distributions derived from CM5, Merz-Kollman, and CHELPG atomic charges consistently predict the interfacial properties such as: area occupied per molecule, interfacial thicknesses, and radial distribution functions. Using Molecular Dynamics simulations, the ability of the charge distributions combined with nonbonded parameters of the GROMOS53A6 force field to predict the interfacial properties has the following order: CM5 > Merz-Kollman ≈ CHELPG > Hirshfeld. In contrast, the charge distributions overestimated the reduction of the interfacial tension of the vacuum/SDS/water system. Interfacial properties such as interfacial tension, area occupied per SDS molecule, and molecular interaction between the hydrophilic headgroup and water molecules were evaluated. Simulations demonstrated that the high interfacial activity of the SDS surfactant is associated with the high negative charge distribution defined for the hydrophilic headgroup of the surfactant, which enhances the ability to attract water molecules and the solvation of the hydrophilic region.
Organic pollutant collection and electrochemical CO<inf>2</inf> reduction promoted by pH-Responsive surfactants
2022, Journal of Molecular LiquidsPH-responsive surfactants with controllable self-assembly properties are expected to become recyclable sources for collecting organic pollutants and capturing CO2 in aqueous solutions. In this work, F9-N2-mor exhibits remarkable pollutant-collection and CO2-capture properties, which are confirmed by the saturation concentrations of Mes-Acr-Me+ and pyrene dyes increasing 78.7-fold and 54.1-fold, respectively, and the electrochemical CO2 reduction current being enhanced 3.22-fold. The promoted pollutant collection and electrochemical CO2 reduction in these pH-responsive surfactants are outstanding compared to the commercial surfactants SDS and SDBS. More interestingly, these pH-responsive surfactants defoam at pH values below 4, and are easily recycled. Overall, pH-responsive surfactants provide a new inspiration for pollutant collection, oil removal and CO2 conversion.
How do chain lengths of acyl-L-carnitines affect their surface adsorption and solution aggregation?
2022, Journal of Colloid and Interface Sciencel-carnitines in our body systems can be readily converted into acyl-l-carnitines which have a prominent place in cellular energy generation by supporting the transport of long-chain fatty acids into mitochondria. As biocompatible surfactants, acyl-l-carnitines have potential to be useful in technical, personal care and healthcare applications. However, the lack of understanding of the effects of their molecular structures on their physical properties has constrained their potential use.
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C12LC is fully soluble over a wide temperature and concentration range. There is however a strong decline of solubility with increasing acyl chain length. The adsorption and aggregation behavior of C14LC was therefore studied at 30 °C and C16LC at 45 °C. The solubility boundaries displayed distinct hysteresis with respect to heating and cooling. The CMCs of C12LC, C14LC and C16LC at pH 7 were 1.1 ± 0.1, 0.10 ± 0.02 and 0.010 ± 0.005 mM, respectively, with the limiting values of the area per molecule at the CMC being 45.4 ± 2, 47.5 ± 2 and 48.8 ± 2 Å2 and the thicknesses of the adsorbed CnLC layers at the air/water interface increasing from 21.5 ± 2 to 22.6 ± 2 to 24.2 ± 2 Å, respectively. All three surfactants formed core–shell spherical micelles with comparable dimensional parameters apart from an increase in core radius with acyl chain length. This study outlines the effects of acyl chain length on the physicochemical properties of CnLCs under different environmental conditions, serving as a useful basis for developing their potential applications.
Facile synthesis and marked pH-responsive behavior of novel foaming agents based on amide- and ester-linked morpholine fluorosurfactants
2021, Journal of Molecular LiquidsThere is an increasing demand to solve the environmental dissemination problems of fluorosurfactants and improve their responsive behavior, surface properties, chemical stability and foaming performance. In this work, three pH-responsive fluorosurfactants, named F9-es-mor, F9-N3-mor and F9-N2-mor, were synthesized by facile one-step esterification or amidation. The F9-N2-mor efficiently reduced the water surface tension to 14.56 mN·m−1 with a low critical micelle concentration (CMC) of 0.20 mM. More importantly, both F9-N2-mor and F9-N3-mor obviously displayed reproducible pH-response foaming behaviors at an optimal pH of 6.0 and 8.0, respectively, probably owing to their rigid molecular structure, strong intermolecular H-bond and high membrane elasticity. Besides, the morpholine fluorosurfactants defoam at pH below 4.0, indicating that these fluorosurfactants could be recycled and their residue could be easily removed. Overall, the novel fluorosurfactants F9-N2-mor might provide new inspiration to recycle fluorosurfactants as well as construct pH-responsive foams.
Surface adsorption and solution aggregation of a novel lauroyl-L-carnitine surfactant
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This work evaluated the surface adsorption of lauroyl-l-carnitine (C12LC) and its aggregation behavior. The size and shape of the aggregates of C12LC surfactant were studied at different temperatures, concentrations, pH and ionic strength by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Surface tension measurements were carried out to determine the critical micellar concentration (CMC) of C12LC. Combining with the Gibbs equation, the surface excess at different concentrations could be determined. Neutron reflection (NR) was used to determine the structure of the adsorbed layer at the air/water interface with the help of isotopic contrast variations.
At pH 7, the limiting area per molecule (ACMC) of the zwitterionic C12LC adsorbed layer at the air/water interface was found to be 46 Å2 from surface tension and neutron reflection, smaller than the values of C12PC, C12E5, DTAB, C12C4betaine and C12C8betaine but close to that of SDS. A pronounced surface tension minimum at pH 2 at the low ionic strength was linked to a minimum value of area per molecule of about 30 Å2, indicating the competitive adsorption from traces of lauric acid produced by hydrolysis of C12LC. As the concentration increased, area per molecule reached a plateau of 37–39 Å2, indicating the dissolution of the more surface-active lauric acid into the micelles of C12LC. DLS and SANS showed that the size and shape of micelles had little response to temperature, concentration, ionic strength or pH. The SANS profiles measured under 3 isotopic contrasts could be well fitted by the core–shell model, giving a spherical core radius of 15.7 Å and a shell thickness of 10.5 Å. The decrease of pH led to more protonated carboxyl groups and more positively charged micelles, but the micellar structures remained unchanged, in spite of their stronger interaction. These features make C12LC potentially attractive as a solubilizing agent.
Salt-tolerance of alkyl-glyceryl ether carboxylates hydrotropes and surfactants. Dramatic effect of the methylation of the glyceryl spacer
2021, Journal of Colloid and Interface ScienceThe insertion of polyether spacers between the anionic head and the alkyl chain of ionic surfactants significantly improves their salt-tolerance. The aim of this work is to study whether the petro-based polyethoxy spacer can be replaced by a glyceryl ether group for high salinity applications.
A series of amphiphilic sodium salts of alkyl glyceryl ether carboxylates are synthesized with different alkyl chain lengths from 4 to 12 and various spacers between the glyceryl and the carboxylate groups. Their aggregation behavior is studied by tensiometry and their amphiphilicities are assessed by the PIT-slope method. The dramatic effect of the methylation of the glyceryl spacer on the salt-tolerance is highlighted, and rationalized by DFT calculations and molecular dynamics.
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