Surfactant Tail Research Articles

X-Ray Reflectometry of the Adsorption Octadecanamide Film at the Toluene–Water Interface

The structure of an adsorption octadecanamide film at the planar toluene-water interface is studied by X-ray reflectometry using synchrotron radiation with photon energy of 15 keV. The electron density (polarizability) profiles, according to which the interface structure is determined by the pH level in the water subphase, are reconstructed from experimental data with the help of a model-independent approach. For a high pH~11, the adsorption film is a crystalline octadecanamide monolayer with a thickness of about 2.6 nm, in which aliphatic tails of surfactant are extended along the normal to the surface. For low pH ~ 2, the thickness of the surface structure consisting of the crystalline monolayer directly on the toluene-water interface and a thick layer of deposited octadecanamide micelles reaches about 50 nm. In our opinion, the condensation of nonionogenic surfactant micelles for which the surface concentration of the surfactant increases significantly is caused by a change in the polarization direction upon a decrease in the pH level in the electric double layer at the interface between the water subphase and the octadecanamide monolayer. The shape of the reconstructed electron density profiles also indicates the existence of a plane of the closest approach of surfactant micelles to the interface at a distance of about 7 nm from it.

Solvophobic effect of 1-alkyl-3-methylimidazolium chloride on the thermodynamic of complexation between β-cyclodextrin and dodecylpyridinium cation

Preferential solvation participate in various supramolecular self-assembly processes, whose thermodynamic properties can be modulated by the addition of ionic liquids (ILs). However, the effects of these liquids on the thermodynamics of the host-guest complexation process remain unexplored. In this study, the thermodynamic properties of the complexation between 1-dodecylpyridinium cations (C12Py+) and β-cyclodextrin (βCD) species in aqueous solutions with different concentrations of 1-alkyl-3-methylimidazolium halides (CnmimX) were investigated by isothermal titration calorimetry. In water, C12Py+ and βCD form a 1:1 inclusion complex, which is enthalpically (ΔHico= −9.2 ± 0.1 kJ mol−1) and entropically (TΔSico= 16.1 ± 0.2 kJ mol−1) favorable. However, in IL aqueous solutions, all the βCD―C12Py+ thermodynamic parameters of the complexation change and this IL effect is dependent on the carbon chain length of Cnmim+ cations. ILs with shorter alkyl chains (Cnmim+, n ≤ 4) decreases the system entropy, while ILs with longer alkyl chains (Cnmim+, n ≥ 6) reduce the enthalpy values. These effects are attributed to i) preferential solvation of surfactant tails by ILs; ii) ability of the ILs to modify the 3D water structure and iii) inclusion of IL molecules into the inner cavities of βCD.

Inhibitory effect of sodium carboxymethylcellulose and synergistic biodegradable gemini surfactants as effective inhibitors for MS corrosion in 1 M HCl

The aim of this paper is to study the anticorrosive performance of the polymer–surfactants system comprising of mixed solutions of varying concentrations (10–700 ppm) of sodium salt of carboxymethylcellulose (NaCMC) and fixed concentration (1 ppm) of synthesized oppositely charged biodegradable cationic di-ester bonded gemini surfactants i.e., ethane-1,2-diylbis(N,N-dimethyl-N-alkylammoniumacetoxy)dichloride, referred as m-E2-m, (where m = 12, 14, 16, the number of carbon atoms in alkyl chain and E2 is the di-ester group in gemini’s spacer part), for mild steel in 1 M HCl solution. The inhibition effect of mixed NaCMC - gemini surfactant system was examined using weight loss measurement, potentiodynamic polarization (PP) measurements, electrochemical impedance (EIS) measurements, surface morphological observation by scanning electron microscopy (), and molecular dynamics simulation study. NaCMC alone was observed to offer moderate inhibition efficiency (IE) of 57.3% at 500 ppm, which was synergistically increased in the presence of very small amount of synthesized gemini surfactants. The improvement in IE was highly dependent on the surfactant tail length; the maximum improvement in IE of 90.1% was observed in presence of 16-E2-16. The adsorption of the studied systems followed Langmuir adsorption isotherm. PP result showed that all the examined systems behave as mixed type inhibitors. EIS results supported the existence of protective layer by the adsorption of inhibitor molecules on the metal surface. The formation of protective layer of studied systems on the mild steel surface was confirmed by FT-IR and SEM results. Molecular dynamics simulation was further performed to provide molecular level insights into the adsorption mechanism with the structure of studied systems.

Self-assembled nanostructures in ionic liquids facilitate charge storage at electrified interfaces

Driven by the potential applications of ionic liquids (ILs) in many emerging electrochemical technologies, recent research efforts have been directed at understanding the complex ion ordering in these systems, to uncover novel energy storage mechanisms at IL-electrode interfaces. Here, we discover that surface-active ILs (SAILs), which contain amphiphilic structures inducing self-assembly, exhibit enhanced charge storage performance at electrified surfaces. Unlike conventional non-amphiphilic ILs, for which ion distribution is dominated by Coulombic interactions, SAILs exhibit significant and competing van der Waals interactions owing to the non-polar surfactant tails, leading to unusual interfacial ion distributions. We reveal that, at an intermediate degree of electrode polarization, SAILs display optimum performance, because the low-charge-density alkyl tails are effectively excluded from the electrode surfaces, whereas the formation of non-polar domains along the surface suppresses undesired overscreening effects. This work represents a crucial step towards understanding the unique interfacial behaviour and electrochemical properties of amphiphilic liquid systems showing long-range ordering, and offers insights into the design principles for high-energy-density electrolytes based on spontaneous self-assembly behaviour.

The Tail Effect of Some Prepared Cationic Surfactants on Silver Nanoparticle Preparation and Their Surface, Thermodynamic Parameters, and Antimicrobial Activity

AbstractThe surface parameters of some cationic surfactants having different hydrophobic alkyl chains were assessed in aqueous solution using different techniques; surface tension, ultraviolet‐Visible (UV–Vis) spectroscopy, and conductivity measurements. The obtained critical micelle concentration (CMC) for N‐(2‐((3,4‐dimethoxybenzylidene)amino)ethyl)‐N,N‐dimethyloctan‐1‐aminium bromide (DBAO), N‐(2‐((3,4‐dimethoxybenzylidene)amino)ethyl)‐N,N‐dimethyldodectan‐1‐aminium bromide (DBAD), and N‐(2‐((3,4‐dimethoxybenzylidene)amino)ethyl)‐N,N‐dimethylhexadectan‐1‐aminium bromide (DBAH) in aqueous solution using three techniques are nearly the same. Increasing the hydrophobic chain length enhances micelle formation. Raising the solution temperature from 25 to 65 °C also shows the same trend. The thermodynamic calculations outlined the adsorption propensity of the surfactants at the surface compared to their affinity to form micelles. Both micellization and adsorption processes are enhanced with both the hydrocarbon elongation and with raising the solution temperature. The effect of the surfactant tail on the preparation process of the silver nanoparticles (AgNP) was assessed and confirmed using transmission electron microscope (TEM), dynamic light scattering (DLS), and UV–Vis spectra. Increasing the surfactant tail leads to a smaller particle size with a narrow distribution. The stability of the prepared AgNP is enhanced with hydrophobic surfactant tail elongation as proved with increasing the zeta‐potential of the prepared AgNP colloid. The foaming power, interfacial tension, and emulsification stability of the DBAO, DBAD, and DBAH surfactants were determined. The DBAO, DBAD, and DBAH surfactants showed good antimicrobial activities against both bacteria (Gram positive and negative) and fungi, which have been enhanced because of incorporation of AgNP.

Meso- and molecular-scale modeling to provide new insights into interfacial and structural properties of hydrocarbon/water/surfactant systems

Water/oil emulsions become important in pharmaceutical, health, cosmetics, food, and oil and gas industries. The emulsion stability controls transport phenomena, transport in porous media, and surface science. Interfacial tension (IFT) is a vital property, which is related to emulsion stability. In this paper, dissipative particle dynamics (DPD) mesoscopic and molecular simulations are employed to study the impacts of temperature and oil type on the structural properties and IFT in order to obtain detailed insights into emulsion stability. Flory-Huggins chi parameter (χ) is determined as a function of temperature from molecular dynamics (MD) simulations; the DPD interaction parameter (aij) is then evaluated from the solubility parameter and energy of mixing, using Monte Carlo simulation framework. The predicted chi parameter is then compared to the experimental data. IFT is evaluated in the presence and absence of an emulsifier for different types of cyclic (cyclohexane and benzene) and aliphatic (octane and dodecane) oil molecules. The influence of temperature on the IFT is also investigated through the integration of DPD and MD approaches. The surfactant effect on the aggregation behaviour of the system is discussed by considering the structural characteristics of the surfactant molecules using the radius of gyration. Comparing the simulation snapshots taken at different simulation time steps, concertation profiles, and radius of gyration values, we find the surfactant to stretch more in the case of aliphatic hydrocarbon due to the linear structure of the oil molecules. For cyclic hydrocarbons, the radius of gyration is reported to be smaller since less space is available for the interactions of surfactant tail groups with oil molecules. It is also found that the IFT is a function of the molecular weight of hydrocarbon. Thus, the highest IFT is for dodecane (44.12 mN/m) and the smallest IFT is for benzene (28.68 mN/m), regardless of the structure and polarity of the molecules. By decreasing the IFT, the emulsion coalescence occurs faster and phases start to separate so that the mixture has no longer the characteristics of an emulsion. Constructing the force-field in the DPD simulation based on the molecular scale calculations of chi parameter improves the accuracy of simulation where the computational time and cost are reduced. The current study further highlights the importance of beading arrangement and the effect of oil molecules on the interfacial characteristic of hydrocarbon/surfactant/water systems.

Charging, aggregation, and aggregate strength of humic substances in the presence of cationic surfactants: Effects of humic substances hydrophobicity and surfactant tail length

The binding of natural organic matters (NOM) with organic cations and resulting changes in charging and aggregation of NOMs are important in water science and technology such as the enhanced settling by flocculation and the fate control of contaminants. We measured the electrophoretic mobility and aggregate strength of humic substances (HSs), Suwannee river fulvic acid (SRFA) and Leonardite humic acid (LHA), in the presence of cationic surfactants, cetylpyridinium chloride (CPC) and dodecylpyridinium chloride (DPC), to clarify the effects of carbon content, aromaticity of HSs, and the tail length of cationic surfactants on the charging and aggregation. Both of HSs showed charge reversal in the presence of 0.1 - 0.3 mM CPC and LHA in 1 and 2 mM DPC. The iso-electric point (IEP) pH of LHA-CPC and SRFA-CPC system was higher than that of LHA-DPC system, though more DPC concentration was needed for the charge reversal of LHA in DPC. We also observed pronounced aggregation of both HSs in CPC systems around IEP pH, though LHA in higher DPC concentration showed wider range of aggregation pH including IEP pH than the range at low DPC concentration. The maximum strength of LHA aggregate was higher, around 27.6 nN in LHA-CPC system and 19.1 nN in LHA-DPC system, than that of 5.2 nN in SRFA-CPC system. This higher value of aggregate strength in LHA-CPC system than others indicates more hydrophobic interaction in LHA-CPC system. The maximum strength around IEP pH in all the systems indicates the presence of electrostatic interactions along with hydrophobic and other non-DLVO forces.

Phase transition and release kinetics of polyphenols encapsulated lyotropic liquid crystals.

In this paper, the lyotropic liquid crystals formed in SL/IP/H2O system were prepared as a drug carrier system to encapsulate polyphenols. The components and introduction of drug had influences in the structure and rheological properties of lyotropic liquid crystals. The structure underwent a phase transition from lamellar phase (Lα) to Lα + HII (reverse hexagonal phase) mixed phases and micelle by increasing its IP/H2O mass ratio from 5/35 to 20/20 and 35/5, consistent with the transition from solid-like properties to viscous fluid properties. For the Lα + HII phase, the loading of DMY induced the structural transition to the HII phase, confirmed by SAXS results. This may be due to the penetration of DMY into the hydrophobic tails of surfactants. The in vitro release kinetic resulted that the release of drug in Lα was associated with diffusional mass transport and matrix swelling. While the release behavior of DMY in HII phase was predominantly controlled by concentration diffusion. These relationships among structure, rheology and release kinetics should be conductive to further design the drug sustained carrier system formed by lyotropic liquid crystals.

Molecular dynamics simulation of supercritical CO2 microemulsion with ionic liquid domains: Structures and properties

Abstract Supercritical carbon dioxide microemulsions are great medium to combine two immiscible substances through forming nanoscale polar cores in nonpolar continuous phase with the help of proper surfactants. The properties of microemulsions could be significantly affected by their constituents and structures. In this work, molecular dynamics simulation was implemented to study supercritical carbon dioxide microemulsions containing ionic liquid [bmim][PF6] and water by adding surfactant Ls-36. Results showed that the above components could form spherical aggregates in CO2 bulk phase with [bmim][PF6] and some water as the inner core, surfactant headgroups and water as the intermediate shell, and surfactant tails as the outer shell. The microstructure information about the outer shell was further investigated by defining an angle between the surfactant tail and the normal direction of the aggregate outer surface, which ranged from 78° to 125°. The influence of the ionic liquid content on the size and structure of microemulsions was explored and the best molar ratio between the ionic liquid and surfactant was around 1.25 for getting maximum water solubility.

Stabilization of 4FS(EO)2 constructed water-in-carbon dioxide microemulsions (W/C μEs) with nonfluorinated co-surfactants

Three nonfluorinated co-surfactants, sodium bis (2-(isopropoxy)-ethanol) sulfonate (di-IOESS), sodium bis (3,5,5-trimethyl-hexanol) sulfonate (di-TMHSS) and sodium bis (2-(pivaloyloxy)-ethanol) sulfonate (di-POESS) were synthesized to replace partial sodium bis (3,3,4,4,5,5,6,6,6-nonafluoro-hexanol) sulfonate (4FS(EO)2) to stabilize W/C microemulsions to decrease the fluorine content. The results showed that the stabilization ability of the co-surfactants at less than 55 °C was in the following order: di-TMHSS > di-POESS > di-IOESS. Results for MD simulations proved that the co-surfactants mainly changed the packing efficiency of the surfactant short chains. The flexible co-surfactant tails could maintain the bending fraction of the surfactant short tail at a low level and the appropriate length of co-surfactant tails helped to occupy more volume at the interface. Therefore, the nonfluorinated co-surfactants with flexible branched tails in an appropriate length relatively to the surfactant tails were conducive to replace partial fluorinated surfactants, which may help for designing nonfluorinated co-surfactants to decrease the fluorine content in W/C microemulsions.

Self-cleaning thin films with controlled optical properties based on WO3-rGO

The study investigates the self-cleaning and anti-reflectance properties of thin films based on tungsten oxide (WO3) and reduced graphene oxide (rGO) deposited by spraying stable dispersions of sol-gel powders. Two routes were employed: (Route 1) highly crystalline WO3 powder (annealed at 550 °C for 3 h) was mixed with commercial rGO powder in the dispersion and (Route 2) rGO was directly added to the WO3 precursor system to obtain a WO3-rGO sol-gel composite powder (further on annealed at 100 °C for 3 h). Both powders contain WO3 and rGO as confirmed by FTIR and XRD analyses. By milling the pristine WO3 (Route 1) with rGO, more stable dispersions could be obtained using sodium dodecylsulfate (SDS). The hydrophobic surfactant tail bonds to the rGO and the electrostatic repulsion between the anionic heads prevent the flakes agglomeration. In Route 2, the dispersions are not well stabilized by SDS as the anionic head and the WO3 “grafted” on the rGO are both negatively charged, thus repulsions can be expected. This leads to agglomerates formed on the thin films surface that affect the thin films’ UV-VIS transmittance and reflectance but increase their efficiency in methylene blue removal through adsorption. Overall, however, the films obtained following Route 1 proved more uniform, crystalline and superhydrophilic, while maintaining comparable stability in the photocatalysis conditions, and comparable efficiency in methylene blue removal: 25% after 8 h of irradiation with 10% UV + 90% VIS light, having the global irradiance of 34 W/m2.

Controlled Formation of Hydrated Micelles by the Intervention of Cyclodextrins.

The interaction between surfactants and cyclodextrins (CDs) is well known. Studies have focused mainly on destruction of micelles with CDs to release the encapsulated drugs. However, less emphasis has been given on understanding the formation of micelles with the CD encapsulated surfactants. We have used fluorescence spectroscopy to study the impact of CDs on micelles using a fluorophore that has been tactically designed as a reporter. This molecule has a pyrene moiety on one end and a cationic head group on the other so that the orientation of the compound can be prefixed on micelle formation in aqueous environment. We have observed that the CD encapsulated surfactants can form "hydrated micelles" that allow extensive penetration of water molecules toward the core. The mechanism for such a process involves inclusion of the hydrophobic surfactant tails within the CD core and participation of these inclusion complexes in micelle formation. The process could be controlled by tuning the concentration of CD. The degree of hydration varies as the micelles get more opened up due to the residence of the CDs inside them.

A novel fluorescent surfactant enhances the delivery of the Cas9 ribonucleoprotein and enables the identification of edited cells.

In this report, we designed and synthesized a novel fluorescent single tailed surfactant (termed FEDS), which can disrupt endosomes, complex lipofectamine, and can also identify cells that have been transfected. FEDS was able to increase the gene editing efficiency of lipofectamine/Cas9 ribonucleoprotein by 300% via a combination of fluorescent based enrichment and endosomal disruption.

Synthesis and Characterization of Polystyrene (PS)/Modified Ni‐Al LDH Nanocomposite: Role of Composition, Modifier and Synthesis Route of Layered Double Hydroxides (LDH)

Layered double hydroxides (LDH) are two‐dimensional nanostructured materials, which impart flame retardancy property and improvement in various properties on incorporation into the polymer matrix. In this study, Ni‐Al LDH with a molar ratio of 2:1 and 3:1 (2NiAl and 3NiAl) was prepared by co‐precipitation method and Ni‐Al LDH with a molar ratio of 2:1 (NiAl urea) was also synthesized by the urea hydrolysis method. As the d‐spacing of these as‐synthesized LDHs is very small (≈0.8 nm) as well as hydrophilic in nature, hence, the unmodified LDHs are unsuitable for polymer nanocomposite preparation. In order to enhance the d‐spacing of LDHs, a simple regeneration method was employed to organically modify LDHs using two anionic surfactants such as sodium dodecyl benzene sulfate (SDBS) and sodium dodecyl sulfate (SDS). The hydrophobic tail of these anionic surfactants replaced the nitrate ions present in the unmodified LDHs leading to higher d‐spacing value, which was evident from XRD analysis. The above prepared LDHs (3 wt%) were incorporated into PS matrix by the solvent blending method to make PS nanocomposites. The 003 peak of modified LDHs did not appear in the XRD pattern of PS nanocomposites due to the disorientation of LDH layers in the PS matrix that was confirmed by TEM analysis. The TGA results clearly demonstrated that the thermal stability of the PS nanocomposite samples was increased by 26, 22 and 22 °C (PS/2NiAl SDS, PS/3NiAl SDS, and PS/NiAl urea SDS) for SDS modified LDH fillers, while in the case of SDBS modified LDH fillers, the improvement was around 32 °C in comparison with PS, when 15% weight loss was taken as a reference. The degradation mechanism, thermal degradation kinetics, and integral procedural decomposition temperature (IPDT) for all the nanocomposite systems were also evaluated using Criado method, Coats‐Redfern method, and Doyle's method, respectively. The effect of dispersion of the modified LDH fillers into PS matrix was examined using melt rheological analysis.

Supramolecular Route for Enhancing Polymer Electrospinnability.

Electrospinning of polymers typically requires high solution concentrations necessitated by the requirement of sufficient chain overlaps to achieve the required viscoelastic properties. Here, we report on a novel supramolecular approach, involving polymer/surfactant complexes, which allows for a significant reduction in the solution concentration of polymer for electrospinning. The approach involved supramolecular complexation of poly(4-vinylpyridine) (P4VP) with a surfactant, dodecylbenzenesulfonic acid (DBSA), via ionic interactions. The supramolecular complexation of P4VP with DBSA led to a significant increase in the solution viscosity even at a DBSA/4VP molar ratio as low as 0.05. Furthermore, the solution viscosity of the P4VP/DBSA complex increased significantly with the DBSA/4VP molar ratio. The increase in the viscosity for the P4VP/DBSA complexes was plausibly due to the formation of physical cross-links between P4VP chains driven by hydrophobic interactions between the surfactant tails. The formation of such physical cross-links led to a significant decrease in the solution concentration needed for the onset of semidilute entangled regime. Thus, the P4VP/DBSA complexes could be electrospun at a much lower concentration. The critical solution concentration to obtain bead-free uniform nanofibers of P4VP/DBSA complexes in dimethylformamide was reduced to 12% (w/v), which was not possible for neat P4VP solution even up to approximately 35% (w/v). Furthermore, small-angle X-ray scattering and polarized optical microscopy results revealed that the electrospun nanofibers of P4VP/DBSA complexes self-assembled in lamellar mesomorphic structures similar to that observed in bulk. However, the electrospun nanofibers exhibited significantly improved lamellar order, which was plausibly facilitated by the preferred orientation of P4VP chains along the fiber axis.

Impact of Spacer and Hydrophobic Tail on Interfacial and Rheological Properties of Cationic Amido-Amine Gemini Surfactants for EOR Application

AbstractThe impact of spacer length, spacer rigidity, trans and cis conformation of spacer double bond, and hydrophobic tail length of amido-amine cationic gemini surfactants was investigated by means of interfacial and rheological properties. For the study, six cationic amido-amine gemini surfactants were synthesized and used a cationic polyacrylamide. The interfacial tension (IFT) of the surfactants between crude oil and water was measured as a function of the surfactant concentration. It has been observed that the nature of the spacer and hydrophobic moiety have a significant effect on the oil/water interfacial tension. Rheological measurements of the surfactant-polymer hybrid system were performed at various surfactant concentrations, shear rates and temperatures. All synthesized cationic gemini surfactants were compatible with the cationic polyacrylamide since no precipitates were observed. By increasing the surfactant concentration, the storage modulus, as well as viscous properties of the cationic polyacrylamide, decreased at all temperatures because of the effect of charge shielding. On the other hand, the effect of concentration of surfactant was only significant at low shear rates. The storage modulus and viscous properties of the cationic polyacrylamide were also decreased significantly upon increasing surfactant tail length (C12 to C18). However, spacer rigidity, length, and conformation of spacer double bond caused a slight decrease in the storage modulus and viscous properties of cationic polyacrylamide. IFT, rheological properties, and compatibility tests showed huge potential of investigated surfactant-polymer system for enhanced oil production.

Aggregation properties of sucrose fatty acid esters and some other sugar-based surfactants at different temperatures

Measurements of density, viscosity and light scattering of aqueous solutions of food additive antimicrobial surfactants: sucrose capric acid ester (SMD) and sucrose lauric acid ester (SML) and some other sugar-based ones (n‑octyl‑β‑d‑glucopyranoside (OGP), n‑dodecyl‑β‑d‑glucopyranoside (DDGP), n‑dodecyl‑β‑d‑maltoside (DM)) as well as the fluorescence intensity of 8‑Anilino‑1‑naphthalene-sulfonic acid (ANS) and pyrene (Py) in aqueous solutions of the studied surfactants were carried out at different temperatures. On the basis of the obtained results the critical micelle concentration, average aggregation number, apparent and partial molar volume as well as the standard Gibbs free energy, standard enthalpy and entropy of micellization were determined. From the obtained thermodynamic parameters of micellization of studied sugar surfactants under different conditions, the temperature and type of surfactant impact on their aggregation properties were deduced. From the changes of the standard Gibbs free energy of micellization it results that the tendency of studied surfactants molecules to form micelles comes down as the temperature and surfactant tail length decrease. The presence and strength of the hydrogen bonds between the water molecules and sugar moiety in a surfactant molecule plays a decisive role in the micellization process of the studied sugar-based surfactants.

Combining high performance fertiliser with surfactants to reduce the reverse solute flux in the fertiliser drawn forward osmosis process

Solutions to mitigate the reverse diffusion of solutes are critical to the successful commercialisation of the fertiliser drawn forward osmosis process. In this study, we proposed to combine a high performance fertiliser (i.e., ammonium sulfate or SOA) with surfactants as additives as an approach to reduce the reverse diffusion of ammonium ions. Results showed that combining SOA with both anionic and non-ionic surfactants can help in reducing the reverse salt diffusion by up to 67%. We hypothesised that, hydrophobic interactions between the surfactant tails and the membrane surface likely constricted membrane pores resulting in increased rejection of ions with large hydrated radii such as SO42−. By electroneutrality, the rejection of the counter ions (i.e., NH4+) also therefore subsequently improved. Anionic surfactant was found to further decrease the reverse salt diffusion due to electrostatic repulsions between the surfactant negatively-charged heads and SO42−. However, when the feed solution contains cations with small hydrated radii (e.g., Na+); it was found that NH4+ ions can be substituted in the DS to maintain its electroneutrality and thus the diffusion of NH4+ to the feed solution was increased.

Molecular-Level Structure and Packing in Phase-Separated Arachidic Acid-Perfluorotetradecanoic Acid Monolayer Films.

Synchrotron-based X-ray scattering measurements of phase-separated surfactant monolayers at the air-water interface provide molecular-level structural information about the packing and ordering of film components. In this work, grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR) measurements were used to collect crystallographic structural information for binary mixed monolayers of arachidic acid (AA, C19H39COOH) with perfluorotetradecanoic acid (PA, C13F27COOH), a system that has previously been investigated using a variety of thermodynamic and micron-scale structural characterization methods. GIXD measurements at surface pressures of π = 5, 15, and 30 mN/m indicated that AA in pure and mixed films forms a rectangular lattice at π = 5 and 15 mN/m but a hexagonal lattice at π = 30 mN/m. PA formed hexagonal lattices under all conditions, with films being highly ordered and crystalline (as determined by Bragg peak width) at even the lowest surface pressures investigated. Phase separation occurred for all mixed monolayer film compositions and surface pressures, manifesting as diffraction peaks characteristic of the individual components appearing at different in-plane scattering vector qxy. For both pure and mixed films, the molecular tilt angle of the AA hydrocarbon chain toward the nearest-neighbor was substantial at low pressures but decreased with increasing pressure. The PA fluorocarbon chain showed negligible molecular tilt under all conditions, and was oriented normal to the subphase surface regardless of surface pressure or the presence of AA in the films. In all cases, the two components in the mixed film behaved entirely independently of film composition, which is exactly the expected result for a fully phase-separated, immiscible system. XR measurements of film thickness at the air-water interface supported these results; overall film thickness approached the calculated ideal surfactant tail lengths with increasing surface pressure, indicating nearly normal oriented surfactants. The overall surfactant packing and crystallographic features of the mixed monolayers are discussed in terms of the lipophobic nature of the perfluorinated surfactant as well as in context of thermodynamic miscibility and domain structure formation reported elsewhere in the literature for these mixed monolayer systems.

Thermal response of a non-ionic surfactant layer at the water/oil interface during microwave heating

This study measured in-situ the interfacial tension of the decane/water interface during microwave radiation, in the presence of the non-ionic surfactants Triton X-100 and X-405. It has been found that the two surfactants, with different hydrophilicity, displayed contrasting responses to the microwave heating. For X-100, the interfacial tension increased with the solution temperature. However, for X-105, the tension decreased as the solution temperature rose. The tension-temperature trends were consistent for a range of different microwave pulsing patterns. The results can be explained by considering the molecular origins of the interfacial forces. In particular, a semi-quantitative analysis verified that the hydrophobic interaction between water in the bulk and water associated with the surfactant tails increases with rising temperature. This thermal response is opposite to the relationship between temperature and hydrophilic interaction. The results demonstrated that rigorous thermodynamic analysis must be applied in combination with the conventional HLB in designing targeted surfactant systems.