Alkyl Ethoxylate Surfactants Research Articles

Unraveling How Membrane Nanostructure Changes Impact the Eye Irritation of Nonionic Alkyl Ethoxylate Surfactants.

Nonionic surfactants used in agri-spraying processes may cause varying degrees of corneal irritation when they come in direct contact with farmers' eyes, and the exact irritations are thought to be determined by how surfactants interact with corneal cell membranes. However, how nonionic surfactants interact with cell membranes at the molecular and nano levels remains largely unexplored. In this study, the interactions between nonionic surfactants (alkyl ethoxylate, C12Em) and lipid membranes were examined by membrane permeability measurement, quartz crystal microbalance with dissipation, dual polarization interferometry, confocal laser scanning microscopy, and neutron reflection, aiming to reveal complementary structural features at the molecular and nano levels. Apart from the extremely hydrophobic surfactant C12E2, all nonionic surfactants studied could penetrate the model cell membrane composed of a phosphocholine lipid bilayer. Nonionic surfactants with intermediate amphiphilicity (C12E6) rapidly fused into the lipid membrane and stimulated the formation of pores across the lipid bilayer, consistent with the cytoplasm leakage and fast cell necrosis observed from the cytotoxicity study of corneal cells. In comparison, while hydrophobic and hydrophilic surfactants [those with long and short ethoxylates (C12E4,12,23)] could cause mild structural alteration to the outer lipid layer of the membrane, these structural changes were insufficient to elicit large cytoplasmic leakage rapidly and instead cell death occurred over longer periods of time due to changes in the membrane permeability. These results reveal the strong link of surfactant-lipid membrane interactions to surfactant cytotoxicity and the association with amphiphilicity of nonionic surfactants.

Investigating oil solubilization into nonionic micelles by Raman multivariate curve resolution

AbstractHydrophobic hydration, whereby water spontaneously structures around hydrophobic and amphiphilic molecules, plays a key role in the process of surfactant micelle formation and micellar oil solubilization. Using vibrational Raman multivariate curve resolution spectroscopy, we characterized changes in the hydrophobic hydration occurring within nonionic alkylphenol ethoxylate surfactant Tergitol NP‐12 micelles as a function of oil solubilization. We report trends in the changes of hydrophobic hydration depending on the chain length of the oil as well as the presence of a halogen atom in the oil chemical structure. Changes in hydrophobic hydration directly correlate to changes in the physical properties of the micellar solution, including cloud point and micelle hydrodynamic diameter. We compare hydrophobic hydration of Tergitol NP‐12 to nonionic linear alkyl ethoxylate surfactant Makon TD‐12 and ionic sodium dodecyl sulfate and observe similar trends; the molecular structure of the oil has the largest impact on the hydrophobic hydration. We believe these studies contribute to a fundamental understanding of the importance of hydrophobic hydration in surfactant and oil aggregates, especially as it relates to micellar oil solubilization, and provide insight into how the molecular solubilizate can impact micellar structure, size, and stability.

Phase Behavior of Alkyl Ethoxylate Surfactants in a Dissipative Particle Dynamics Model.

We present a dissipative particle dynamics (DPD) model capable of capturing the liquid state phase behavior of nonionic surfactants from the alkyl ethoxylate (CnEm) family. The model is based upon our recent work [Anderson et al. J. Chem. Phys. 2017, 147, 094503] but adopts tighter control of the molecular structure by setting the bond angles with guidance from molecular dynamics simulations. Changes to the geometry of the surfactants were shown to have little effect on the predicted micelle properties of sampled surfactants, or the water-octanol partition coefficients of small molecules, when compared to the original work. With these modifications the model is capable of reproducing the binary water-surfactant phase behavior of nine surfactants (C8E4, C8E5, C8E6, C10E4, C10E6, C10E8, C12E6, C12E8, and C12E12) with a good degree of accuracy.

Impacts of chain and head lengths of nonionic alkyl ethoxylate surfactants on cytotoxicity to human corneal and skin cells in agri-spraying processes*

HypothesisNonionic surfactants are widely used as co-formulants in agrochemical sprays. During spraying, they may come into direct contact with humans and animals, causing irritation in different tissues. However, how the molecular structures of these surfactants affect their toxicity towards human eye and skin at the cellular level has not been well characterised. ExperimentIn this study, the cytotoxicities of two sets of nonionic surfactants (alkyl ethoxylate, CnEm) against human corneal and skin cell lines were examined, with one set composed of varied surfactant head length but fixed tail length (C12E4-23) and the other set oppositely composed (C10-16E6). The cell viability and morphology against different nonionic surfactants for varied exposure times were studied, followed by characterisation of their membrane-lytic ability. FindingNonionic surfactants with intermediate amphiphilicity killed cells rapidly due to their strong membrane-lytic power. Those with weak or strong hydrophobicity exhibited low cytotoxicity but had different modes of action depending on their hydrophobicity. Hydrophobic surfactants were found to adsorb on to cell membranes with no observed structural damage for 2 hr. Hydrophilic surfactants were also found to adsorb on to cell membranes but did cause mild structural changes. While the changes were not sufficient to elicit large cytoplasmic leakage over short periods of time, membrane associations did cause cell shrinkage which eventually resulted in cell death over longer exposure periods. These results revealed that the specific amphiphilic nature of nonionic surfactants played a crucial role in determining their cytotoxicity. This work provided a useful basis for the assessment of amphiphilicity of the nonionic surfactants used in agrochemical sprays by balancing their efficiency, toxicity and environmental impact.

Contrasting impacts of mixed nonionic surfactant micelles on plant growth in the delivery of fungicide and herbicide

HypothesisNonionic alkyl ethoxylate surfactants are widely used in agrochemicals to facilitate the permeation of systemic herbicides and fungicides across the plant waxy film. Industrial grade surfactants are often highly mixed and how the mixing affects their interactions with pesticides and wax films remains largely unexplored. A better understanding could enable design of mixed nonionic surfactants for herbicides and fungicides to maximize their efficiency and reduce wastage whilst controlling their impact on plant wax films. ExperimentIn this study, nonionic surfactants with general structure n-oxyethylene glycol monododecyl ether (C12En) were used to form surfactant mixtures with the same average ethoxylate numbers but different hydrophilic-lipophilic balance (HLB) values. Their mixed micellar systems were then used to solubilize a herbicide diuron (DN) and a fungicide cyprodinil (CP), followed by plant wax solubilization upon contact with wax films. These processes were monitored by 1H NMR and SANS. FindingPesticide solubilization made surfactant micelles effectively more hydrophobic but subsequent wax dissolution caused pesticide release and the restoration of the micellar amphiphilicity. Nonionic surfactants with lower HLBs form larger nanoaggregates, show enhanced wettability, and have better ability to solubilize and permeate pesticides across the wax film, but may cause significant damage to plant growth. These observations help explain why herbicides applied on weeds would benefit from surfactants with lower HLB values while fungicides require surfactants with HLBs to balance between delivery efficiency and potential phytotoxicity risks.

First Approach to Measure Interfacial Rheology at High-Pressure Conditions by the Oscillating Drop Technique

An oscillating drop rheometer capable of operating under conditions of high pressure and high temperature has been built. The oscillating drop mechanism was able to support pressures as high as 1300 bar and successfully performed oscillations at constant pressure. Apparent elastic and viscous complex moduli were measured for a system of CO2 and synthetic seawater containing 100 ppm of a linear alkyl ethoxylate surfactant for different pressures and temperatures. The moduli had strong dependencies on both pressure and temperature. At temperatures of 40 and 80 °C, the apparent elastic modulus passed through a maximum for pressures between 100 and 300 bar. The harmonic distortion of the oscillations was calculated for all measurements, and it was found that drop oscillations below ca. 2.6 µL caused distortions above 10% due to a mechanical backlash of the motor.

A Surface Site Interaction Point Method for Dissipative Particle Dynamics Parametrization: Application to Alkyl Ethoxylate Surfactant Self-Assembly

Dissipative particle dynamics (DPD) is a coarse-grained approach to the simulation of large supramolecular systems, but one limitation has been that the parameters required to describe the noncovalent interactions between beads are not readily accessible. A first-principles computational method has been developed so that bead interaction parameters can be calculated directly from ab initio gas-phase molecular electrostatic potential surfaces of the molecular fragments that represent the beads. A footprinting algorithm converts the molecular electrostatic potential surfaces into a discrete set of surface site interaction points (SSIPs), and these SSIPs are used in the SSIMPLE (surface site interaction model for the properties of liquids at equilibrium) algorithm to calculate the free energies of transfer of one bead into a solution of any other bead. The bead transfer free energies are then converted into the required DPD interaction parameters for all pairwise combinations of different beads. The reliability of the parameters was demonstrated using DPD simulations of a range of alkyl ethoxylate surfactants. The simulations reproduce the experimentally determined values of the critical micelle concentration and mean aggregation number well for all 22 surfactants studied.

Structure and Dynamics of Spherical and Rodlike Alkyl Ethoxylate Surfactant Micelles Investigated Using NMR Relaxation and Atomistic Molecular Dynamics Simulations.

Predicting and controlling the properties of amphiphile aggregate mixtures require understanding the arrangements and dynamics of the constituent molecules. To explore these topics, we study molecular arrangements and dynamics in alkyl ethoxylate nonionic surfactant micelles by combining NMR relaxation measurements with large-scale atomistic molecular dynamics simulations. We calculate parameters that determine relaxation rates directly from simulated trajectories, without introducing specific functional forms to describe the dynamics. NMR relaxation rates, which depend on relative motions of interacting atom pairs, are influenced by wide distributions of dynamic time scales. We find that relative motions of neighboring atom pairs are rapid and liquidlike but are subject to structural constraints imposed by micelle morphology. Relative motions of distant atom pairs are slower than nearby atom pairs because changes in distances and angles are smaller when the moving atoms are further apart. Large numbers of atom pairs undergoing these slow relative motions contribute to predominantly negative cross-relaxation rates. For spherical micelles, but not for cylindrical micelles, cross-relaxation rates are positive only for surfactant tail atoms connected to the hydrophilic headgroup. This effect is related to the lower packing density of these atoms at the hydrophilic-hydrophobic boundary in spherical vs cylindrical arrangements, with correspondingly rapid and less constrained motion of atoms at the boundary.

Formulating Nonionic Detergents via the Integrated Free Energy Model

AbstractThis work explores the optimum detergency conditions of alkyl ethoxylate (CXEOY) surfactants with the integrated free energy model (IFEM). IFEM is a molecular thermodynamic model that calculates the free energy of formation of oil‐swollen spherical micelles, with a core solubilization radius Ro, using surfactants from empty (oil‐free) micelles and oil molecules from a continuous oil phase. The described geometry allows for rapid calculations, using a personal laptop (3.4 GHz processor), where each solubilization energy profile (free energy vs. Ro curve) can be solved in 5 min or less. While previous work showed quantitative agreement between IFEM predictions and experimental solubilization of alkanes in CXEOY micelles, this work explores the possibility of using IFEM as a tool in surfactant selection. Experimental work has shown that detergency improved when operating near the phase inversion temperature (PIT) of the surfactant‐hexadecane system. The IFEM simulations in this work show, for the first time, that IFEM can be used to predict the PIT of surfactant‐oil systems, and that the surfactants selected via this method are consistent with the selection guided by experimental observations.

Search for the Source of an Apparent Interfacial Resistance To Mass Transfer of CnEm Surfactants To the Water/Oil Interface.

Experiments have shown that relaxation of oil/water interfacial tension by adsorption of alkyl ethoxylate surfactants from water onto an oil droplet is delayed relative to diffusion-controlled adsorption. We examine possible causes of this delay, and we show that several are implausible. We find that redissolution of the surfactant in the oil droplet cannot explain the apparent interfacial resistance at short times because the interface will preferentially fill before any such redissolution occurs. We also perform umbrella sampling with molecular dynamics simulation and do not find any evidence of a free-energy barrier or low-diffusivity zone near the interface. Nor do we find evidence from the simulation that premicellar aggregation slows diffusion enough to cause the observed resistance to interfacial adsorption. We are therefore unable to pinpoint the cause of the resistance, but we suggest that "dead time" associated with the experimental method could be responsible-specifically a local depletion of surfactant by the ejected droplet when creating the fresh interface between the oil and water.

Dissipative particle dynamics: Systematic parametrization using water-octanol partition coefficients.

We present a systematic, top-down, thermodynamic parametrization scheme for dissipative particle dynamics (DPD) using water-octanol partition coefficients, supplemented by water-octanol phase equilibria and pure liquid phase density data. We demonstrate the feasibility of computing the required partition coefficients in DPD using brute-force simulation, within an adaptive semi-automatic staged optimization scheme. We test the methodology by fitting to experimental partition coefficient data for twenty one small molecules in five classes comprising alcohols and poly-alcohols, amines, ethers and simple aromatics, and alkanes (i.e., hexane). Finally, we illustrate the transferability of a subset of the determined parameters by calculating the critical micelle concentrations and mean aggregation numbers of selected alkyl ethoxylate surfactants, in good agreement with reported experimental values.

A Simplified Methodology to Measure the Characteristic Curvature (Cc) of Alkyl Ethoxylate Nonionic Surfactants

AbstractThis work introduces a simplified methodology for measuring the characteristic curvature (Cc) of commercial alkyl ethoxylate nonionic surfactants using carefully selected reference surfactants and oils that produce rapid and well defined separations in salinity scans. The Cc of the commercial reference surfactants was calculated using optimal salinities (S*) obtained from solubilization parameter curves, from interfacial tensions (for a selected system), and from emulsion stability tests. The latter provided a fast detection of S*, in a matter of minutes. The calibrated Cc of the reference surfactants was subsequently used to measure the Cc of various commercial alkyl ethoxylate surfactants. The combination of mixtures of test and reference surfactants and emulsion stability tests produced reproducible Cc values that could be obtained with simple bottle tests and in a timely manner. The values obtained using this methodology were cross‐checked, and proved to be consistent, when using different combinations of reference surfactants and oils, and when conducted by different individuals. The standard deviation of Cc from these measurements was typically ±0.2 Cc units, but it could be as large as 25 % of the Cc value for highly hydrophilic surfactants. After comparing the values of Cc obtained experimentally with values calculated from nominal structures (via a group contribution model) it became clear that there are differences between these values, likely because of the polydispersity of alkyl ethoxylate surfactants.

Computer Simulation–Molecular-Thermodynamic Framework to Predict the Micellization Behavior of Mixtures of Surfactants: Application to Binary Surfactant Mixtures

We present a computer simulation-molecular-thermodynamic (CSMT) framework to model the micellization behavior of mixtures of surfactants in which hydration information from all-atomistic simulations of surfactant mixed micelles and monomers in aqueous solution is incorporated into a well-established molecular-thermodynamic framework for mixed surfactant micellization. In addition, we address the challenges associated with the practical implementation of the CSMT framework by formulating a simpler mixture CSMT model based on a composition-weighted average approach involving single-component micelle simulations of the mixture constituents. We show that the simpler mixture CSMT model works well for all of the binary surfactant mixtures considered, except for those containing alkyl ethoxylate surfactants, and rationalize this finding molecularly. The mixture CSMT model is then utilized to predict mixture CMCs, and we find that the predicted CMCs compare very well with the experimental CMCs for various binary mixtures of linear surfactants. This paper lays the foundation for the mixture CSMT framework, which can be used to predict the micellization properties of mixtures of surfactants that possess a complex chemical architecture, and are therefore not amenable to traditional molecular-thermodynamic modeling.

Structural Characterization of Surfactant Aggregates Adsorbed in Cylindrical Silica Nanopores

The self-assembly of nonionic surfactants in the cylindrical pores of SBA-15 silica with a pore diameter of 8 nm was studied by small-angle neutron scattering (SANS) at different solvent contrasts. The alkyl ethoxylate surfactants C(10)E(5) and C(12)E(5) exhibit strong aggregative adsorption in the pores as indicated by the sigmoidal shape of the adsorption isotherms. The SANS intensity profiles can be represented by a sum of two terms, one accounting for diffuse scattering from surfactant aggregates in the pores and the other for Bragg scattering from the pore lattice of the silica matrix. The Bragg reflections are analyzed with a form factor model in which the radial density profile of the surfactant in the pore is approximated by a two-step function. Diffuse scattering is represented by a Teubner-Strey-type scattering function which indicates a preferred distance between adsorbed surface aggregates in the pores. Our results suggest that adsorption starts with formation of discrete surface aggregates which increase in number and eventually merge to interconnected patches as the plateau value of the adsorption isotherm is approached. A grossly different behavior, viz. formation of micelles as in solution, is found for the maltoside surfactant C(10)G(2), in agreement with the observed weak adsorption of this surfactant in SBA-15.

Study of pH-triggered heteroaggregation and gel formation within mixed dispersions

This work involves an investigation of pH-triggered heteroaggregation and gelation within mixed dispersions of polystyrene (PS) and pigment particles. The PS particles were stabilised by a carboxylated alkyl ethoxylate surfactant which is pH-responsive. The pigment used was β-copper phthalocyanine. The pigment particles contained a co-surfactant system consisting of the carboxylated alkyl ethoxylate and a non-ionic surfactant. The latter was a β-naphthol ethoxylate. The PS and pigment particles were characterised using SEM, TEM, photon correlation spectroscopy and electrophoretic mobility measurements. The PS dispersions exhibited pH-triggered aggregation when the pH was decreased to below a critical value (pH crit), which was 1.9. Concentrated PS dispersions formed stable particle gels at pH values less than or equal to pH crit. Dilute pigment dispersions were found to have a pH crit of 3.45. However, concentrated pigment dispersions did not form gels when the pH was decreased to below pH crit. A phase diagram for the mixed dispersions was constructed which showed a gel phase existed at pH values between 2.0 and 3.0, which corresponds to a pH region higher than pH crit for the PS particles. This implicates PS–pigment inter-particle bonds in the gel structure. The heteroaggregate gels were investigated using dynamic rheological measurements and it was apparent that the highest elastic modulus values were obtained in the pH range of approximately 2 to 3. SEM images provided evidence of heteroaggregates with diameters of a few micrometers. These primary heteroaggregates are suggested to be the network forming unit for the gels formed in mixed dispersions. The data from the study are used to propose a conceptual model for the structure of the heteroaggregate gels.

Cationisation of oligomeric alkylethoxylate surfactants in ToF‐SIMS analysis

AbstractVariable cationisation of the alkylethoxylate surfactant Synperonic A7™ has been observed during a ToF‐SIMS study of its interaction with a leaf surface, either by itself or as a component in a simple herbicide formulation. Depending on the conditions, cationisation predominantly by K+, Na+ and H+ was found. By deliberate surfactant solution doping with alkali bromides, the cationisation channel could be manipulated. Comparison of the apparent molecular weight dependence, derived from the ToF‐SIMS spectra, with that measured by HPLC‐MS, showed a further dependence on the cationising species, with Na+ giving the best match. An understanding of these effects is critical to the correct interpretation of the spectra from this surfactant. Copyright © 2007 John Wiley & Sons, Ltd.

Three-component microemulsions formed using pH-degradable 1,3-dioxolane alkyl ethoxylate surfactants

A series of three nonionic surfactants which undergo acid-catalyzed hydrolysis, O-[(2,2-dialkyl-1,3-dioxolan-4-yl) methoxy] – O′-methoxy poly(ethylene glycol), or cyclic ketal (CK), were synthesized. The surfactants shared similar HLB values (9.2–9.8) but differed in the relative length of their two alkyl tails. Water/CK/isooctane microemulsion systems formed by each of the three surfactants also shared similar phase behavior, physical properties, and surfactant hydrolysis kinetics. The apparent critical microemulsion concentration for three CK surfactants, 0.060–0.075 g g −1 (in both water and isooctane), and the efficiency, the minimum amount of surfactant required to totally mix water and oil into single microemulsion phase, 0.35–0.40 g g −1, are significantly higher than values possessed by regular, linear, alkyl ethoxylate surfactants. The underlying causes for the high values were investigated. Due to the broad distribution of its ethoxylate head group size, CK surfactants were distributed among all phases within two- and three-phases systems, with CKs possessing shorter ethoxylate groups partitioning to the oil phase and CKs with larger ethoxylate groups to the aqueous phase. Small-angle neutron scattering experiments suggest that CK exists as monomers or very small pre-micellar aggregates in isooctane at 12% and as ellipsoidal aggregates in water at 2.5–25%. Surface pressure versus surface area measurements of CK surfactant monolayers at the water-air interface demonstrate two differences from linear alkyl ethoxylate monolayers: the occurrence of a collapse at moderately low surface pressure (30 mN m −1) and high surface area per molecule (0.59 nm 2) and the inability to undergo dissolution when subjected to a constant, moderately high surface pressure (25 mN m −1). The results suggest that CK molecules are not able to pack as efficiently as linear alkyl ethoxylates at interfaces, and are strongly anchored to monolayers in aqueous media.

Triacylglycerol microemulsions stabilized by alkyl ethoxylate surfactants—A basic study: Phase behavior, interfacial tension and microstructure

As triacylglycerols are the main components of natural fats and oils their solubilization in the form of emulsions or microemulsions was of great interest within the last years. However, systematic studies of their properties along the classical lines of complex fluids science are still missing. In the present paper we focus on the phase behavior, the interfacial tension and the microstructure of systems of type H 2O/NaCl–triacylglycerol–alkylpolyglycolether (C i E j ). The interfacial tension between water and oil σ ab is high in a microemulsion system containing triolein. Thus, one needs high surfactant mass fractions to formulate a single-phase microemulsion. We show that this is not only true for triolein, but also for saturated long-chained triacylglycerols. The determination of the amphiphilicity factor f a and the calculation of the bending rigidities of the amphiphilic film confirm that despite the fact that high surfactant mass fractions and high temperatures are needed to solubilize triolein in a system of type H 2O/NaCl–triacylglycerol–alkylpolyglycolether (C i E j ), this is still a microemulsion in the narrower sense.

Anti-foam action of oil vapours

Abstract Alkane vapours adsorb onto surfactant monolayers at the air–water surface to form mixed alkane–surfactant films. We have determined the effects of alkane vapours on the foamability and foam stability for aqueous foams stabilised by non-ionic alkyl ethoxylate surfactants of different head and tail chain lengths. Foamability was determined by measuring the volume of foam generated by bubbling nitrogen gas, with and without oil vapour, through the surfactant solution as a function of surfactant concentration. In the absence of oil vapour, the surfactant concentration C 1/2 (corresponding to the transition from non-foaming to foaming behaviour) is less than the critical micelle concentration (cmc) for short chain surfactants and greater than the cmc for long chain surfactants. The addition of alkane vapours within the gas stream during foam formation increases C 1/2 to concentrations in excess of the cmc, i.e. oil vapours inhibit foamability. In addition to the effects on foamability, oil vapours also accelerate the decay rates of foams derived from plots of foam volume versus time.

Alkylethoxylate surfactants for rehydration of roses and Bouvardia flowers

The rate of water uptake of cut rose and Bouvardia flowers, following a period of dry storage, is often low, and this results in a short vase life. Surfactants, applied prior to dry storage, drastically increase the rate of water uptake and the length of vase life. The slightly biodegradable Agral-LN has previously used in practice for pulsing. In Rosa, cultivar Sonia, and Bouvardia, cultivar Artemis Jowhite, we compared the effects of some rapidly biodegradable alkylethoxylates with Agral-LN, and with Atlas G 2130, a mixture of alkylethoxylates. The chemicals were applied as a 24 h pulse (at 20 °C). Flowers were then held dry for 24 h at 20 °C prior to placement in water at 20 °C. The alkylethoxylate apolar chain length was varied between 10 and 18 carbon atoms, and the polar chain between 2 and 8 ethoxy (EO) units. Good results were obtained with 10 or 14 carbon atoms in the apolar chain, combined with 5 or 8 EO units. Atlas G 2130 also had a good effect. The toxicological data are briefly discussed. It is concluded that some (mixtures of) alkylethoxylate surfactants are well suited for practical application.