Low-surface energy surfactants with branched hydrocarbon architectures

Molecular Interactions of Perfluorinated and Branched Fluorine-Free Surfactants at Interfaces: Insights from a New Reliable Force Field

Per- and polyfluoroalkyl substances (PFAS) constitute a class of synthetic compounds with exceptional interfacial properties. Their widespread use in many industrial applications and consumer products, combined with their remarkable chemical and thermal stability, has led to their ubiquitous presence in environmental matrices, including surface water and groundwater. To replace PFAS with fluorine-free surfactants, it is necessary first to develop a deep molecular-level understanding of the mechanisms responsible for the exceptional properties of PFAS. For instance, it has been shown that fluorine-free surfactants with highly branched or methylated chains can achieve low surface tensions at air-water interfaces and can provide highly hydrophobic surface coatings. Although molecular simulations combined with experiments are promising for uncovering these mechanisms, the reliability of simulation results depends strongly on the accuracy of the force fields implemented. At the moment, atomistic force fields are not available to describe PFAS in a variety of environments. Ab initio methods could help fill this knowledge gap, but they are computationally demanding. As an alternative, ab initio calculations could be used to develop accurate force fields for atomistic simulations. In this work, a new algorithm is proposed, which, built from accurate ab initio calculations, yields force fields for perfluorinated sulfonic and perfluoroalkyl acids. The accuracy of the new force field was benchmarked against solvation free energy and interfacial tension data. The new force fields were then used to probe the interfacial behavior of the PFAS surfactants. The interfacial properties observed in our simulations were compared with those manifested by two branched fluorine-free surfactants. The good agreement achieved with experiments and ab initio calculations suggests that the proposed protocol could be implemented to study other perfluorinated substances and help in the design of fluorine-free surfactants for targeted applications.

Coal-Based Branched Vicinal Diol Ethoxylates Versus Guerbet Alcohol Ethoxylates: Role of Tertiary Hydroxyl Groups

Branched surfactants exhibit a lower surface tension, excellent low defoaming performance, and better wetting ability compared with linear surfactants, making them promising for applications in industrial cleaning. In this study, 2-hexyl-1-decene (C8 olefin dimer), obtained from the dimerization of 1-octene, was used as the hydrophobe to synthesize branched nonionic surfactants via hydroxylation and ethoxylation. The hydroxylation of the C8 olefin dimer to synthesize 2-hexyldecane-1,2-diol (C8 BD) using H2O2 and HCOOH was investigated systematically. Under the optimal reaction conditions (H2O2/C8 olefin dimer molar ratio: 1.5, HCOOH/C8 olefin dimer molar ratio: 4.0, reaction time: 10 h, reaction temperature: 50 °C), the conversion of the C8 olefin dimer and selectivity toward C8 BD were found to reach 99.96 and 79.89%, respectively. Further, branched nonionic surfactants (C8 BDEn) were synthesized via ethoxylation of C8 BD with ethylene oxide and characterized using FTIR, LCMS, 1H NMR, and 13C NMR techniques. The presence of a tertiary hydroxyl group in C8 BD increases the reactivity of the primary hydroxyl group, leading to a narrower range of products and lower residual substrate content. The physicochemical properties and surface properties of C8 BDEn with different degrees of ethoxylation at various concentrations were investigated and compared with those of commercially available Guerbet alcohol polyoxyethylene ethers (C8 GAEO9 and C6 GAEO9). The results show that, compared with C8 GAEO9 and C6 GAEO9, C8 BDE6 displayed a higher surface activity with a lower equilibrium surface tension (27.14 mN·m-1), superior wettability with a smaller contact angle (39.2°), better emulsification performance with a longer emulsification time of 548 s, and excellent foaming properties (initial foam volume of 11.6 mL). This strategy of utilizing coal-based α-olefins for the synthesis of branched nonionic surfactants presents a route to prepare value-added fine chemicals from coal-based resources.

Small-Angle Neutron Scattering Insights into 2-Ethylhexyl Laurate: A Remarkable Bioester

Commercial (protiated) samples of the "green" and biodegradable bioester 2-ethylhexyl laurate (2-EHL) were mixed with D-2-EHL synthesized by hydrothermal deuteration, with the mixtures demonstrating bulk structuring in small-angle neutron scattering measurements. Analysis in a polymer scattering framework yielded a radius of gyration (Rg) of 6.5 Å and a Kuhn length (alternatively described as the persistence length or average segment length) of 11.2 Å. Samples of 2-EHL dispersed in acetonitrile formed self-assembled structures exceeding the molecular dimensions of the 2-EHL, with a mean aggregation number (Nagg) of 3.5 ± 0.2 molecules across the tested concentrations. We therefore present structural evidence that this ester can function as a nonionic (co)surfactant. The available surfactant-like conformations appear to enable performance beyond the low calculated hydrophilic-lipophilic balance value of 2.9. Overall, our data offer an explanation for 2-EHL's interfacial adsorption properties via self-assembly, resulting in strong emolliency and lubricity for this sustainable ester-based bio-oil.

High drug loading hydrophobic cross-linked dextran microspheres as novel drug delivery systems for the treatment of osteoarthritis

Drug delivery via intra-articular (IA) injection has proved to be effective in osteoarthritis (OA) therapy, limited by the drug efficiency and short retention time of the drug delivery systems (DDSs). Herein, a series of modified cross-linked dextran (Sephadex, S0) was fabricated by respectively grafting with linear alkyl chains, branched alkyl chains or aromatic chain, and acted as DDSs after ibuprofen (Ibu) loading for OA therapy. This DDSs expressed sustained drug release, excellent anti-inflammatory and chondroprotective effects both in IL-1β induced chondrocytes and OA joints. Specifically, the introduction of a longer hydrophobic chain, particularly an aromatic chain, distinctly improved the hydrophobicity of S0, increased Ibu loading efficiency, and further led to significantly improving OA therapeutic effects. Therefore, hydrophobic microspheres with greatly improved drug loading ratio and prolonged degradation rates show great potential to act as DDSs for OA therapy.

Association of exposure to perfluoroalkyl substances and risk of the acute coronary syndrome: A case-control study in Shijiazhuang Hebei Province

Exposures to perfluoroalkyl substances (PFAS) have been reported to increase the risk of atherosclerosis. Therefore, PFAS exposure may be linked to the risk of acute coronary syndrome (ACS), but this association remains uncertain. The objective of the present study was to investigate the association between PFAS exposure and ACS risk through a case-control study. The study included 355 newly diagnosed ACS cases and 355 controls matched by age (within 5 years) and sex. Twelve PFAS were measured in plasma by ultra-high-performance liquid chromatography-tandem mass spectrometry. The conditional logistic regression models were performed to investigate the association between the single and multiple PFAS and ACS risk. Furthermore, we investigated the association of PFAS mixture exposure with ACS risk using a quantile-based g-computation (qgcomp) approach. A mediating effect model was used to assess the mediating effect of platelet indices on the association between PFAS and ACS risk. The results showed that perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) were significantly positively associated with ACS risk in the multiple-PFAS model 2, and this effect was not significant in females. The odds ratios (95% confidence intervals) for PFAS (z-score PFAS) and ACS risk were 1.51 (1.07, 2.15) for PFOA and 1.77 (1.15, 2.72) for PFOS. The dose-response relationships revealed an increasing trend for ACS risk with PFOA and PFOS and decreasing trend for perfluorohexane sulfonic acid (PFHxS) and perfluorodecanoic acid (PFDA). There was no significant correlation between PFAS mixture exposure and ACS risk. Analysis of mediation indicated that platelet count mediated the relationship between PFOS and ACS risk. Our study suggests that higher levels of PFOA and PFOS, and lower levels of PFHxS and PFDA may increase the risk of ACS. However, the reported negative associations should not be considered as protective, and uncertain unresolved confounding may contribute to this result.

Synthesis of a branched surfactant from the castor derivative and its surface properties

A new class of ricinoleic acid-derived branched surfactant with a Y-shaped structure (ethoxylated monohydroxy stearic acid methyl ester, 12-HMEE_n) was synthesized and characterized by introducing a polyoxyethylene head group in the hydroxyl position inside the molecule. The physicochemical properties and surface activities of 12-HMEE_n with different degrees of ethoxylation at various concentrations were studied. The typical Y-shaped structure of the molecule facilitates its adsorption at the interface, which provides an excellent surface activity and affects its surfactant properties significantly. The dynamic contact angle, wettability, foaming properties, and compatibility tests of 12-HMEE_n showed that it has good wetting performance, low foaming and fast defoaming properties, and good compatibility in formulation applications, indicating that the surfactant has potential application in industrial cleaning.

Hydrocarbon-Based Statistical Copolymers Outperform Block Copolymers for Stabilization of Ethanol-Water Foams

Well-defined block copolymers have been widely used as emulsifiers, stabilizers, and dispersants in the chemical industry for at least 50 years. In contrast, nature employs amphiphilic proteins as polymeric surfactants whereby the spatial distribution of hydrophilic and hydrophobic amino acids within the polypeptide chains is optimized for surface activity. Herein, we report that polydisperse statistical copolymers prepared by conventional free-radical copolymerization can provide superior foaming performance compared to the analogous diblock copolymers. A series of predominantly (meth)acrylic comonomers are screened to identify optimal surface activity for foam stabilization of aqueous ethanol solutions. In particular, all-acrylic statistical copolymers comprising trimethylhexyl acrylate and poly(ethylene glycol) acrylate, P(TMHA-stat-PEGA), confer strong foamability and also lower the surface tension of a range of ethanol-water mixtures to a greater extent than the analogous block copolymers. For ethanol-rich hand sanitizer formulations, foam stabilization is normally achieved using environmentally persistent silicone-based copolymers or fluorinated surfactants. Herein, the best-performing fully hydrocarbon-based copolymer surfactants effectively stabilize ethanol-rich foams by a mechanism that resembles that of naturally-occurring proteins. This ability to reduce the surface tension of low-surface-energy liquids suggests a wide range of potential commercial applications.

Information Requirements under the Essential-Use Concept: PFAS Case Studies

Per- and polyfluoroalkyl substances (PFAS) are a class of substances for which there are widespread concerns about their extreme persistence in combination with toxic effects. It has been argued that PFAS should only be employed in those uses that are necessary for health or safety or are critical for the functioning of society and where no alternatives are available ("essential-use concept"). Implementing the essential-use concept requires a sufficient understanding of the current uses of PFAS and of the availability, suitability, and hazardous properties of alternatives. To illustrate the information requirements under the essential-use concept, we investigate seven different PFAS uses, three in consumer products and four industrial applications. We investigate how much information is available on the types and functions of PFAS in these uses, how much information is available on alternatives, their performance and hazardous properties and, finally, whether this information is sufficient as a basis for deciding on the essentiality of a PFAS use. The results show (i) the uses of PFAS are highly diverse and information on alternatives is often limited or lacking; (ii) PFAS in consumer products often are relatively easy to replace; (iii) PFAS uses in industrial processes can be highly complex and a thorough evaluation of the technical function of each PFAS and of the suitability of alternatives is needed; (iv) more coordination among PFAS manufacturers, manufacturers of alternatives to PFAS, users of these materials, government authorities, and other stakeholders is needed to make the process of phasing out PFAS more transparent and coherent.

Ionic Liquids and Deep Eutectics as a Transformative Platform for the Synthesis of Nanomaterials

Ionic liquids (ILs) are becoming a revolutionary synthesis medium for inorganic nanomaterials, permitting more efficient, safer and environmentally benign preparation of high quality products. A smart combination of ILs and...

Surface Activity of Ethoxylate Surfactants with Different Hydrophobic Architectures: The Effect of Layer Substructure on Surface Tension and Adsorption

A series of nonionic ethoxylate surfactants containing different combinations of alkyl, phenyl, and adamantyl units in nine different arrangements, each combined with penta- and hexa-ethylene glycol groups, were synthesized and purified. The surface properties of all of the surfactants were investigated at the air-water (A-W) interface using surface tension (ST) to determine the limiting surface excess (Γ_lim), the limiting surface tension (σ_lim), and the critical micelle concentration (CMC). A smaller selection was investigated at the hydrophilic silica-water interface by neutron reflectometry to obtain the thickness of the adsorbed layer and the total adsorption at the CMC. An unusual and largely unrecognized feature of the ethoxylate group is that it is both hydrophilic and hydrophobic. It was found possible to account for the variation of σ_lim and Γ_lim of all of the adsorbed layers in terms of a balance of the estimated STs of the sublayers forming the overall adsorbed layer, including that of the underlying ethoxylate layer. The values of σ_lim were found to be highest for phenyl- and adamantyl-capped surfactants and lowest mainly when there was more than one methyl group at the surface. However, in terms of the concentration required to reach a given low ST, increasing the number of attached methyl groups was found to be less effective than using a smaller number of better-placed methyl groups. At the solid-liquid interface, adsorption at or above the CMC was in all cases in the form of a fragmented bilayer whose coverage varied approximately linearly with the packing parameter. However, results on the phenyl-capped surfactants showed that the high ST exhibited by these surfactants at the A-W interface becomes a high cohesion energy in the interior of the bilayer and they exhibited significantly higher adsorption than expected from simple packing arguments.

Design of Surfactant Tails for Effective Surface Tension Reduction and Micellization in Water and/or Supercritical CO2

The interfacial properties and water-in-CO₂ (W/CO₂) microemulsion (μE) formation with double- and novel triple-tail surfactants bearing trimethylsilyl (TMS) groups in the tails are investigated. Comparisons of these properties are made with those for analogous hydrocarbon (HC) and fluorocarbon (FC) tail surfactants. Surface tension measurements allowed for critical micelle concentrations (CMC) and surface tensions at the CMC (γ_CMC) to be determined, resulting in the following trend in surface activity FC > TMS > HC. Addition of a third surfactant tail gave rise to increased surface activity, and very low γ_CMC values were recorded for the double/triple-tail TMS and HC surfactants. Comparing effective tail group densities (ρ_layer) of the respective surfactants allowed for an understanding of how γ_CMC is affected by both the number of surfactant tails and the chemistry of the tails. These results highlight the important role of tail group chemical structure on ρ_layer for double-tail surfactants. For triple-tail surfactants, however, the degree to which ρ_layer is affected by tail group architecture is harder to discern due to formation of highly dense layers. Stable W/CO₂ μEs were formed by both the double- and the triple-tail TMS surfactants. High-pressure small-angle neutron scattering (HP-SANS) has been used to characterize the nanostructures of W/CO₂ μEs formed by the double- and triple-tail surfactants, and at constant pressure and temperature, the aqueous cores of the microemulsions were found to swell with increasing water-to-surfactant ratio (W₀). A maximum W₀ value of 25 was recorded for the triple-tail TMS surfactant, which is very rare for nonfluorinated surfactants. These data therefore highlight important parameters required to design fluorine-free environmentally responsible surfactants for stabilizing W/CO₂ μEs.

An overview of the uses of per- and polyfluoroalkyl substances (PFAS)

Per- and polyfluoroalkyl substances (PFAS) are of concern because of their high persistence (or that of their degradation products) and their impacts on human and environmental health that are known or can be deduced from some well-studied PFAS. Currently, many different PFAS (on the order of several thousands) are used in a wide range of applications, and there is no comprehensive source of information on the many individual substances and their functions in different applications. Here we provide a broad overview of many use categories where PFAS have been employed and for which function; we also specify which PFAS have been used and discuss the magnitude of the uses. Despite being non-exhaustive, our study clearly demonstrates that PFAS are used in almost all industry branches and many consumer products. In total, more than 200 use categories and subcategories are identified for more than 1400 individual PFAS. In addition to well-known categories such as textile impregnation, fire-fighting foam, and electroplating, the identified use categories also include many categories not described in the scientific literature, including PFAS in ammunition, climbing ropes, guitar strings, artificial turf, and soil remediation. We further discuss several use categories that may be prioritised for finding PFAS-free alternatives. Besides the detailed description of use categories, the present study also provides a list of the identified PFAS per use category, including their exact masses for future analytical studies aiming to identify additional PFAS.

An overview of the uses of per- and polyfluoroalkyl substances (PFAS)

Per- and polyfluoroalkyl substances (PFAS) are of concern because of their high persistence (or that of their degradation products) and their impacts on human and environmental health that are known or can be deduced from some well-studied PFAS. Currently, many different PFAS (on the order of several thousands) are used in a wide range of applications, and there is no comprehensive source of information on the many individual substances and their functions in different applications. Here we provide a broad overview of many use categories where PFAS have been employed and for which function; we also specify which PFAS have been used and discuss the magnitude of the uses. Despite being non-exhaustive, our study clearly demonstrates that PFAS are used in almost all industry branches and many consumer products. In total, more than 200 use categories and subcategories are identified for more than 1400 individual PFAS. In addition to well-known categories such as textile impregnation, fire-fighting foam, and electroplating, the identified use categories also include many categories not described in the scientific literature, including PFAS in ammunition, climbing ropes, guitar strings, artificial turf, and soil remediation. We further discuss several use categories that may be prioritised for finding PFAS-free alternatives. Besides the detailed description of use categories, the present study also provides a list of the identified PFAS per use category, including their exact masses for future analytical studies aiming to identify additional PFAS.

New insights into the interactions between asphaltene and a low surface energy anionic surfactant under low and high brine salinity

HYPOTHESIS

The hyperbranched chains on the tail of low surface energy surfactants (LSES) causes lowering of surface free energy and rock wettability alteration, offering significant improvement in oil recovery in asphaltene oil reservoirs.

EXPERIMENTS

Oil sweep efficiency was determined by fluid displacement in pure brine and LSES-brine solutions in a microfluidic pattern that was representative of a sandstone cross-section. Interfacial tension (IFT), wettability alteration, Raman and X-ray photoelectron spectroscopy (XPS) were used to measure the changes of asphaltene interactions with oil-aged substrate after surface treating with brine and surfactant-brine solutions.

FINDINGS

The hyperbranched LSES yielded a significant increase in the original-oil-in-place (OOIP) recovery (58%) relative to brine flooding (25%), even in the presence of asphaltene. Raman spectra showed the LSES-brine solutions to be capable of causing change to the asphaltene aggregate size after centrifugation treatment.

The Synthesis, Self-Assembled Structures, and Microbicidal Activity of Cationic Gemini Surfactants with Branched Tridecyl Chains

Cationic gemini surfactants with polymethylene spacer and linear alkyl chains containing an even number of carbon atoms have been extensively studied in the recent past, with the emphasis put on the determination of their aggregation behaviour in aqueous solution and their biological properties. However, the information on the aggregation of branched gemini surfactants with an odd number of carbon atoms in their alkyl chains is only sparsely reported in the literature. To help cover this gap in the research of cationic gemini surfactants, a series of branched bisammonium cationic gemini surfactants with an odd number of carbon atoms in alkyl chains (tridecane-2-yl chains) and a polymethylene spacer with a variable length ranging from 3 to 12 carbon atoms have been synthesized and investigated. Critical micelle concentration, which was determined by three methods, was found to be in the order 10^-4 mol/L. A comparison of the obtained data of the novel series of tridecyl chain geminis with those of gemini surfactants with dodecyl chains and an identical spacer structure revealed that structural differences between both series of gemini surfactants result in different aggregation and surface properties for surfactants with 6 and 8 methylene groups in the spacer (N,N'-bis(tridecane-2-yl)-N,N,N',N'-tetramethylhexane-1,6-diaminium dibromide and N,N'-bis(tridecane-2-yl)-N,N,N',N'-tetramethyloctane-1,8-diaminium dibromide) with the cmc values 8.2 × 10^-4 mol/L and 6.5 × 10^-4 mol/L, respectively, as determined by surface tension measurements. Particle size analysis showed the formation of small stable spherical micelles in the interval between 2.8 and 5 nm and with zeta potential around +50 mV, which are independent of surfactant concentration and increase with the increasing spacer length. Microbicidal activity of 13-s-13 gemini surfactants was found to be efficient against Gram-positive, Gram-negative bacteria and yeast.

Branched alkyldimethylamine oxide surfactants: An effective strategy for the design of high concentration/low viscosity surfactant formulations

HYPOTHESIS

The rational design of branched-tail surfactants is a suitable strategy to obtain low-viscosity surfactant-rich isotropic aqueous mixtures with negligible effects on biodegradability. This opens a way to the design of concentrated ("water-free") surfactant formulations, highly attractive for their ecological and economic benefits.

EXPERIMENTS

The aggregation behaviour of N,N-dimethyl-2-propylheptan-1-amine oxide (C₁₀DAO-branched) in aqueous mixtures is investigated across the entire composition range by polarized optical microscopy, small angle X-ray and neutron scattering, electron paramagnetic resonance, and pulse-gradient stimulated echo nuclear magnetic resonance. The humidity scanning quartz crystal microbalance with dissipation monitoring technique is validated as a tool for the fast screening of surfactants phase behaviour. Furthermore, the shear viscosities and viscoelastic moduli of the systems are determined by rheological measurements.

FINDINGS

With respect to the linear isomer, C₁₀DAO-branched presents a much lower tendency to form lyotropic liquid crystalline phases. Except for a narrow composition and temperature range in which a lamellar structure is observed, C₁₀DAO-branched aqueous mixtures are isotropic liquids whose microstructure changes, with increasing concentration, from micellar solutions to unstructured dispersions of hydrated surfactant molecules. Low-viscosity was found for all these mixtures, including the most concentrated ones. Thus, the introduction of a single short side-chain in the tail is demonstrated to be an effective approach to increase the active concentration in surfactant formulations.

New bio-based monomers: tuneable polyester properties using branched diols from biomass

A family of monomers, including 2,5-hexandiol, 2,7-octandiol, 2,5-furandicarboxylic acid (FDCA), terephthalic acid (TA), and branched-chain adipic and pimelic acid derivatives, all find a common derivation in the biomass-derived platform molecule 5-(chloromethyl)furfural (CMF). The diol monomers, previously little known to polymer chemistry, have been combined with FDCA and TA derivatives to produce a range of novel polyesters. It is shown that the use of secondary diols leads to polymers with higher glass transition temperatures (T_g) than those prepared from their primary diol equivalents. Two methods of polymerisation were investigated, the first employing activation of the aromatic diacids via the corresponding diacid chlorides and the second using a transesterification procedure. Longer chain diols were found to be more reactive than the shorter chain alternatives, generally giving rise to higher molecular weight polymers, an effect shown to be most pronounced when using the transesterification route. Finally, novel diesters with high degrees of branching in their hydrocarbon chains are introduced as potential monomers for possible low surface energy materials applications.

A Novel Synthetic UV-Curable Fluorinated Siloxane Resin for Low Surface Energy Coating

Low surface energy materials have attracted much attention due to their properties and various applications. In this work, we synthesized and characterized a series of ultraviolet (UV)-curable fluorinated siloxane polymers with various fluorinated acrylates-hexafluorobutyl acrylate, dodecafluoroheptyl acrylate, and trifluorooctyl methacrylate-grafted onto a hydrogen-containing poly(dimethylsiloxane) backbone. The structures of the fluorinated siloxane polymers were measured and confirmed by proton nuclear magnetic resonance and Fourier transform infrared spectroscopy. Then the polymers were used as surface modifiers of UV-curable commercial polyurethane (DR-U356) at different concentrations (1, 2, 3, 4, 5, and 10 wt %). Among three formulations of these fluorinated siloxane polymers modified with DR-U356, hydrophobic states (91°, 92°, and 98°) were obtained at low concentrations (1 wt %). The DR-U356 resin is only in the hydrophilic state at 59.41°. The fluorine and siloxane element contents were investigated by X-ray photoelectron spectroscopy and the results indicated that the fluorinated and siloxane elements were liable to migrate to the surface of resins. The results of the friction recovering assays showed that the recorded contact angles of the series of fluorinated siloxane resins were higher than the original values after the friction-annealing progressing.

Self-assembly, phase behaviour and structural behaviour as observed by scattering for classical and non-classical microemulsions

In this review, we discuss the conditions for forming microemulsions, systems which are thermodynamically stable mixtures of oil and water made stable by the presence of an interfacial film containing surface active molecules. There are several types of microemulsions, depending largely on the stiffness of the amphiphilic monolayer that separates the oily and the aqueous micro-domain. We first discuss and compare the phase behaviour of these different types, starting from the classical microemulsion made from a flexible surfactant film but then also moving on to less classical situations: this occurs when the interfacial film is stiff or when microemulsions are formed in the absence of a classical surfactant. In the second part, we relate these different microemulsion types to the structural features as can be determined via different methodologies by small angle scattering (SAS). Using absolute scaling, general theorems as well as fitting under constraints or to pre-supposed shapes in real space or correlation functions in reciprocal space allows to classify all microemulsions into classical flexible, rigid or ultra-flexible microemulsions with either globular, connected cylinder of locally flat interfaces, with the corresponding conductivity and phase stability properties.

Tunable Surface Properties of Aluminum Oxide Nanoparticles from Highly Hydrophobic to Highly Hydrophilic

The formation of materials with tunable wettability is important for applications ranging from antifouling to waterproofing surfaces. We report the use of various low-cost and nonhazardous hydrocarbon materials to tune the surface properties of aluminum oxide nanoparticles (NPs) from superhydrophilic to superhydrophobic through covalent functionalization. The hydrocarbon surfaces are compared with a fluorinated surface for wettability and surface energy properties. The role of NPs' hydrophobicity on their dynamic interfacial behavior at the oil-water interface and their ability to form stable emulsions is also explored. The spray-coated NPs provide textured surfaces (regardless of functionality), with water contact angles (θ) of 10-150° based on their surface functionality. The superhydrophobic NPs are able to reduce the interfacial tension of various oil-water interfaces by behaving as surfactants.

Surfactant-Mediated Resistance to Surface Oxidation in MnO Nanostructures

The intrinsic physical properties of nanostructures of metals and their oxides are altered when they are prone to surface oxidation in ambient atmosphere. To overcome this limitation, novel synthesis methodologies are required. In this study, solid octahedral shapes of MnO limit the inward oxygen diffusion compared to that of the MnO-nanoparticle-assembled octahedra. In addition to morphology control, which restricts the thickness of the Mn₃O₄ surface layer, the binding chemistry of the surfactants plays an essential role. For example, the Mn₃O₄ surface layer is 0.4 nm thinner with trioctylphosphine oxide than with trioctylamine as the surfactant. The nanostructures were prepared by varying the surfactants, surfactant-to-precursor molar ratio, accelerating agent, and reaction heating rate. The surface oxidation of MnO nano-octahedra was probed by Rietveld analysis of X-ray diffraction patterns and X-ray photoelectron spectroscopy and characterized by magnetic measurements, as the presence of ferrimagnetic Mn₃O₄ shell on the antiferromagnetic MnO core provides an exchange coupling at the core-shell interface. Thicker the Mn₃O₄ shell, higher is the exchange-biased hysteresis loop shift.

Molecular-Scale Design of Hydrocarbon Surfactant Self-Assembly in Supercritical CO2

Forming wormlike reverse micelles (RMs) by hydrocarbon surfactant self-assembly is an economic and environmental strategy to improve the physicochemical properties of supercritical carbon dioxide (scCO₂), but it remains challenging. Introducing cosurfactant in hydrocarbon surfactant self-assembly system is a potential method to generate wormlike RMs. Here, adopting molecular dynamics simulations, we performed hydrocarbon surfactant (TC14) self-assembly with introducing cosurfactants (C₈Benz). It is found that adding the C₈Benz molecules will induce the spherical RMs to a short rodlike form. In this case, the microstructure of the short rodlike RMs shows a dumbbell-like form that is composed by three parts including a middle part of C₈Benz and two parts of TC14 aggregation at both ends of rodlike RMs, which is regarded as the origin of RMs shape transition. Further, the analysis of free energy for RMs fusion indicates that the high fusion ability of C₈Benz aggregation drives the formation of the dumbbell-like RMs. Accordingly, enhancing the affinity of the C₈Benz is found to be effective strategy to further fusion of rodlike RMs in end-to-end manner, yielding a wormlike RMs with a beads-on-a-string structure. It is expected that this work will provide a valuable information for design the hydrocarbon wormlike RMs and facilitate the potential application of scCO₂.

Subtle Effects of Aliphatic Alcohol Structure on Water Extraction and Solute Aggregation in Biphasic Water/n-Dodecane

Organic phase aggregation behavior of 1-octanol and its structural isomer, 2-ethylhexanol, in a biphasic n-dodecane-water system is studied with a combination of physical measurement, small-angle X-ray scattering (SAXS), and atomistic molecular dynamic simulations. Physical properties of the organic phases are probed following their mixing and equilibration with immiscible water phases. Studies reveal that the interfacial tension decreases as a function of increasing alcohol concentration over the solubility range of the alcohol with no evidence for a critical aggregate concentration (cac). An uptake of water into the organic phases is quantified, as a function of alcohol content, by Karl Fischer titrations. The extraction of water into dodecane was further assessed as a function of alcohol concentration via the slope-analysis method sometimes employed in chemical separations. This method provides a qualitative understanding of solute (water/alcohol) aggregation in the organic phase. The physical results are supported by analyses of SAXS data that reveals an emergence of aggregates in n-dodecane at elevated alcohol concentrations. The observed aggregate structure is dependent on the alcohol tail group geometry, consistent with surfactant packing parameter. The formation of these aggregates is discussed at a molecular level, where alcohol-alcohol and alcohol-water H-bonding interactions likely dominate the occurrence and morphology of the aggregates.

Surface Energy in Nanocrystalline Carbon Thin Films: Effect of Size Dependence and Atmospheric Exposure

Surface energy (SE) is the most sensitive and fundamental parameter for governing the interfacial interactions in nanoscale carbon materials. However, on account of the complexities involved of hybridization states and surface bonds, achieved SE values are often less in comparison with their theoretical counterparts and strongly influenced by stability aspects. Here, an advanced facing-target pulsed dc unbalanced magnetron-sputtering process is presented for the synthesis of undoped and H/N-doped nanocrystalline carbon thin films. The time-dependent surface properties of the undoped and H/N-doped nanocrystalline carbon thin films are systematically studied. The advanced plasma process induced the dominant deposition of high-energy neutral carbon species, consequently controlling the intercolumnar spacing of nanodomain morphology and surface anisotropy of electron density. As a result, significantly higher SE values (maximum = 79.24 mJ/m²) are achieved, with a possible window of 79.24-66.5 mJ/m² by controlling the experimental conditions. The intrinsic (size effects and functionality) and extrinsic factors (atmospheric exposure) are resolved and explained on the basis of size-dependent cohesive energy model and long-range van der Waals interactions between hydrocarbon molecules and the carbon surface. The findings anticipate the enhanced functionality of nanocrystalline carbon thin films in terms of selectivity, sensitivity, and stability.

Trimethylsilyl hedgehogs – a novel class of super-efficient hydrocarbon surfactants

Presented here are the results for a novel class of hydrocarbon surfactants, termed trimethylsilyl hedgehogs (TMS-hedgehogs), due to the presence of silicon in the tails.

Branched Hydrocarbon Low Surface Energy Materials for Superhydrophobic Nanoparticle Derived Surfaces

We present a new class of superhydrophobic surfaces created from low-cost and easily synthesized aluminum oxide nanoparticles functionalized carboxylic acids having highly branched hydrocarbon (HC) chains. These branched chains are new low surface energy materials (LSEMs) which can replace environmentally hazardous and expensive fluorocarbons (FCs). Regardless of coating method and curing temperature, the resulting textured surfaces develop water contact angles (θ) of ∼155° and root-mean-square roughnesses (Rq) ≈ 85 nm, being comparable with equivalent FC functionalized surfaces (θ = 157° and Rq = 100 nm). The functionalized nanoparticles may be coated onto a variety of substrates to generate different superhydrophobic materials.

Surfactants at the Design Limit

This article analyzes how the individual structural elements of surfactant molecules affect surface properties, in particular, the point of reference defined by the limiting surface tension at the aqueous cmc, γcmc. Particular emphasis is given to how the chemical nature and structure of the hydrophobic tails influence γcmc. By comparing the three different classes of surfactants, fluorocarbon, silicone, and hydrocarbon, a generalized surface packing index is introduced which is independent of the chemical nature of the surfactants. This parameter ϕcmc represents the volume fraction of surfactant chain fragments in a surface film at the aqueous cmc. It is shown that ϕcmc is a useful index for understanding the limiting surface tension of surfactants and can be useful for designing new superefficient surfactants.