Surfactant Head Groups Research Articles

Synthesis, physicochemical properties and biological activities of novel alkylphosphocholines with foscarnet moiety

A series of alkylphosphocholines with foscarnet moiety was synthesized. The structure of these zwitterionic amphiphiles was modified in both polar and non-polar parts of surfactant molecule. Investigations of physicochemical properties are represented by the determination of critical micelle concentration, the surface tension value at the cmc and the surface area per surfactant head group utilising surface tension measurements. Hydrodynamic diameter of surfactant micelles was determined using the dynamic light scattering technique. Alkylphosphocholines exhibit significant cytotoxic, anticandidal (Candida albicans) and antiamoebal (Acanthamoeba spp. T4 genotype) activity. The relationship between the structure, physicochemical properties and biological activity of the tested compounds revealed that lipophilicity has a significant influence on biological activity of the investigated surfactants. More lipophilic alkylphosphocholines with octadecyl chains show cytotoxic activity against cancer cells which is higher than that of the compounds with shorter alkyl chains. The opposite situation was observed in case of anticandidal and antiamoebal activity of these surfactants. The most active compounds were found to have pentadecyl chains. The foscarnet analogue of miltefosine C15-PFA-C showed the highest anticandidal activity. The minimum value of anticandidal activity of this compound is 1,4 μM thus representing the highest anticandidal activity found within the group of alkylphosphocholines.

Pure and mixed aqueous micellar solutions of Sodium Dodecyl sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO): Role of temperature and composition

Aqueous mixtures of anionic and nonionic/cationic surfactants can form non-trivial self-assemblies in solution and exhibit macroscopic responses. Here, we investigate the micellar phase of pure and mixed aqueous solutions of Sodium Dodecyl Sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO) using a combination of Small Angle Neutron Scattering (SANS), Fourier-Transform Infrared Spectroscopy (FTIR) and rheological measurements. We examine the effect of temperature (0–60 °C), on the 20 wt% SDS micellar solutions with varying DDAO (⩽5 wt%), and seek to correlate micellar structure with zero-shear solution viscosity. SANS establishes the formation of prolate ellipsoidal micelles in aqueous solutions of pure SDS, DDAO and SDS/DDAO mixtures, whose axial ratio is found to increase upon cooling. Elongation of the ellipsoidal micelles of pure SDS is also induced by the introduction of the non-anionic DDAO, which effectively reduces the repulsive interactions between the anionic SDS head-groups. In FTIR measurements, the formation of elongated mixed ellipsoidal micelles is confirmed by the increase of ordering in the hydrocarbon chain tails and interaction between surfactant head-groups. We find that the zero-shear viscosity of the mixed surfactant solutions increases exponentially with decreasing temperature and increasing DDAO content. Significantly, a master curve for solution viscosity can be obtained in terms of micellar aspect ratio, subsuming the effects of both temperature and DDAO composition in the experimental range investigated. The intrinsic viscosity of mixed micellar solutions is significantly larger than the analytical and numerical predictions for Brownian suspensions of ellipsoidal colloids, highlighting the need to consider interactions of soft micelles under shear, especially at high concentrations.

Thermodynamics of micellization of nonionic surfactants – The effect of incorporating CO2 moieties into the head group

HypothesisThe micellization behavior of nonionic surfactants is significantly influenced by substituting ethylene oxide (EO) units with CO2 in the head group of nonionic C12EOj surfactants. Incorporating hydrophobic units has a major effect on the driving forces of the micellization process by a reduced hydration affinity. Hence, the incorporation of CO2 moieties is favoring micellization and thereby adding a further tuning parameter. ExperimentsThe micellization of the surfactants was characterized in terms of thermodynamics and the assembly properties on the water/air interface by isothermal titration calorimetry (ITC) and surface tension measurements. The incorporation of CO2 moieties was compared to the effect of propylene oxide (PO) and propiolactone (PL) over the temperature range of 25–50 °C. From ITC measurements and the van’t Hoff relation, we determined the thermodynamic parameters of micellization: enthalpy (ΔHmic), entropy (ΔSmic), and Gibbs free energy (ΔGmic). FindingsThe incorporation of CO2 moieties reduces the critical micellization concentration (cmc) and a transfer energy of −0.36 kT/CO2 unit quantifies favored micellization for CO2 surfactants. The presence of PO or CO2 in the head group has a similar effect on the cmc, but for CO2 ΔHmic is substantially decreased, resulting in a largely reduced temperature sensitivity of the micellization process and indicating a reduced hydration affinity. This thermodynamic analysis reveals that CO2 and PO behave very differently concerning their effect on the micellization process.

Phosphonium surfactant stabilised silver nanoparticles. Correlation of surfactant structure with physical properties and biological activity of silver nanoparticles

Silver nanoparticles were prepared by chemical reduction method and stabilised with cationic single-chain phosphonium surfactants of variable alkyl chain length and a different number of methyl and phenyl substituents in the hydrophilic headgroup part of surfactant molecule. Physical properties of silver nanoparticles were studied utilizing several experimental methods, such as visible spectroscopy, dynamic light scattering and scanning electron microscopy for the nanoparticle size determination and zeta potential measurements for the characterization of the charge of nanoparticles. As the physical measurements indicate, zeta potential of phosphonium surfactant stabilised silver nanodispersions was found to be in the range +27 mV to +50 mV depending on the alkyl chain length of the stabilising surfactant. Hydrodynamic diameter values of phosphonium surfactant stabilised silver nanoparticles were found to be between 40 nm and 109 nm with the smallest diameter values being around 40 nm for the nanoparticles stabilised with hydrophobic triphenyl-substituted phosphonium surfactants. Investigations of biological activity of both silver nanodispersions and silver-free aqueous solutions of surfactants are represented by the determination of antimicrobial activity against Gram-negative, Gram-positive bacterial strains and against yeast. Cytotoxicity of the systems mentioned above was determined on human carcinoma HepG2 cell line. Based on the obtained results, the relationship between phosphonium surfactant structure and physical and biological properties of nanoparticles is analysed. The results indicate that the increasing number of phenyl substituents in the surfactant headgroup and the increasing alkyl chain length play a dominant role in the improvement of biological activity of phosphonium surfactant stabilised silver nanoparticles as well as result in the formation of stable silver nanodispersions with small hydrodynamic diameter and a highly positive zeta potential.

Antagonistic mixing in micelles of amphiphilic polyoxometalates and hexaethylene glycol monododecyl ether

HypothesisPolyoxometalates (POMs) are metal oxygen clusters with a range of interesting magnetic and catalytic properties. POMs with attached hydrocarbon chains show amphiphilic behaviour so we hypothesised that mixtures of a nonionic surfactant and anionic surfactants with a polyoxometalate cluster as headgroup would form mixed micelles, giving control of the POM density in the micelle, and which would differ in size and shape from micelles formed by the individual surfactants. Due to the high charge and large size of the POM, we suggested that these would be nonideal mixtures due to the complex interactions between the two types of surfactants. The nonideality and the micellar composition may be quantified using regular solution theory. With supplementary information provided by small-angle neutron scattering (SANS), an understanding of this unusual binary surfactant system can be established. ExperimentsA systematic study was performed on mixed surfactant systems containing polyoxometalate-headed amphiphiles (K10[P2W17O61OSi2(CnH2n+1)2], abbreviated as P2W17-2Cn, where n = 12, 14 or 16) and hexaethylene glycol monododecyl ether (C12EO6). Critical micelle concentrations (CMCs) of these mixtures were measured and used to calculate the interaction parameters based on regular solution theory, enabling prediction of micellar composition. Predictions were compared to micelle structures obtained from SANS. A phase diagram was also established. FindingsThe CMCs of these mixtures suggest unusual unfavourable interactions between the two species, despite formation of mixed micelles. Micellar compositions obtained from SANS concurred with those calculated using the averaged interaction parameters for P2W17-2Cn/C12EO6 (n = 12 and 14). We attribute the unfavourable interactions to a combination of different phenomena: counterion-mediated interactions between P2W17 units and the unfolding of the ethylene oxide headgroups of the nonionic surfactant, yet micelles still form in these systems due to the hydrophobic interactions between surfactant tails.

Synthesis, Characteristics and Application of Novel Non-Ionic Gemini Surfactants as Reverse Micellar Systems for Encapsulation of Some Aromatic α-Amino Acids in n-Hexane

Abstract Non-ionic carbohydrate based gemini surfactants with rigid aromatic spacer CH2-Ar-CH2, which carry two hydrophobic tails of different tail lengths (C12, C14 and C16) and two sugar moiety polar head groups were synthesized and their reverse micellar behavior for solubilization of some aromatic α-amino acids viz. histidine (His), phenylalanine (Phy), tyrosine (Tyr) and tryptophan (Trp) in n-hexane were studied by spectroscopic analysis. The head group of these gemini surfactants consists of sugar moiety connected to C-6 of tertiary amines. Amino acids form complexes in order of His > Phy > Tyr > Trp, and in all cases it was found that the D-enantiomers solubilize better in comparison to the L-enantiomers. Moreover, more hydrophobic surfactants i.-e. surfactants with longer hydrocarbon tails show greater complex formation tendency towards D- and L-enantiomers of aromatic α-amino acids.

The effect of polar head group of dodecyl surfactants on the growth of wheat and cucumber

The increasing application of various surfactants nowadays, may lead to the contamination of the natural environment and represent potential threat to terrestrial higher plants. In this article, the effect of 13 surfactants, with dodecyl alkyl chain and various aromatic (imidazolium, pyridinium, thiazolium) and aliphatic (guanidinium, ammonium, thiosemicarbazidium) polar heads, on germination, development and growth of wheat and cucumber was investigated. The study aimed to prove how changes in lipophilicity of surfactants and their various structural modifications (existence of the aliphatic or aromatic polar group, the introduction of oxygen and sulfur) influence toxicity towards investigated plants. The calculated lipophilic parameter (AlogP) is shown to be a useful parameter for predicting potential toxicity of the compound. The strategy of using surfactants with aliphatic polar heads instead of aromatic prove to be a promising strategy in reducing harmful effect, as well as the introduction of polar groups in the structure of cation. From all investigated compounds, surfactants with imidazolium polar head displayed the most harmful effect towards wheat and cucumber. The cucumber seeds were more sensitive to the addition of surfactants comparing to wheat. All obtained experimental results were additionally investigated using computational methods, simulating the transport of surfactants through a lipid bilayer. The influence of cation tendency to fit in lipid bilayer structure was correlated with toxicity. For the first time, it is concluded that cation ability to mimic the structure of bilayer have less harmful effect on plant development.

Capping action of ionic surfactants on the nucleation of lawsone-Ag+ redox system

Cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were used a stabilizing agent to determine the rate of nucleation and growth process for the reduction of Ag+ ions by lawsone. Critical micellar concentration (CMC) were calculated by using surface tension method and found to be 6.3 × 10−3 and 9.2 × 10−4 mol dm−3 for SDS and CTAB, respectively in presence of Ag+ ions. Reaction rates increases with increasing the surfactant concentrations. SDS aggregates formed complex with Ag+ ions due to electrostatic interaction with negative head group as well as solubilized the lawsone into the aqueous micellar phase through hydrophobically. CTAB effect might be due to the solubilization of lawsone into the Stern layer. The rate constants were found to be 3.2 × 10−4 and 8.1 × 10−5 s−1 for CTAB and SDS, respectively. SDS exhibited inhibition effect at post-micellar concentrations. Menger-portnoy model modified by Bunton and his coworkers was used to determine the micelles assisted parameters. For CTAB micellized reaction, binding constant (Ks = 42 mol−1 dm3 and KL = 86 mol−1 dm3), micellar phase rate constant (km = 3.5 × 10−3 s−1) were calculated with pseudo-phase kinetic model. Morphology of AgNPs strongly depends on nature of surfactant head group.

Role of the Counterions in the Surface Tension of Aqueous Surfactant Solutions. A Computer Simulation Study of Alkali Dodecyl Sulfate Systems

We have investigated the surface tension contributions of the counterions, surfactant headgroups and tails, and water molecules in aqueous alkali dodecyl sulfate (DS) solutions close to the saturated surface concentration by analyzing the lateral pressure profile contribution of these components using molecular dynamics simulations. For this purpose, we have used the combination of two popular force fields, namely KBFF for the counterions and GROMOS96 for the surfactant, which are both parameterized for the SPC/E water model. Except for the system containing Na+ counterions, the surface tension of the surfactant solutions has turned out to be larger rather than smaller than that of neat water, showing a severe shortcoming of the combination of the two force fields. We have traced back this failure of the potential model combination to the unphysically strong attraction of the KBFF counterions, except for Na+, to the anionic head of the surfactants. Despite this failure of the model, we have observed a clear relation between the soft/hard character (in the sense of the Hofmeister series) and the surface tension contribution of the counterions, which, given the above limitations of the model, can only be regarded as an indicative result. We emphasize that the obtained results, although in a twisted way, clearly stress the crucial role the counterions of ionic surfactants play in determining the surface tension of the aqueous surfactant solutions.

An elaborate investigation on the transition of rod-like micelle of cetyltrimethylammonium p-toluenesulfonate in presence of different additives

Abstract The noticeable change in self-aggregation nature (rod-like micelle) of cetyl trimethylammonium p-toluene sulfonate (CTAT) in presence of various electrolytes and non-electrolytes was probed using tensiometry, conductometry, isothermal titration calorimetry and fluorimetry. The impact of electrolytes and non-electrolytes on the critical micelle concentration (CMC) of the surfactant was found to be opposite of each other. The non-electrolytes having lower dielectric constants than water increased the repulsion between the surfactant head groups, thereby increasing the CMC. Less negative value of ΔG0m with increase in solvent concentration indicated delayed process of micellization. On the contrary, electrolytes decreased the CMC by a large extent due to suppression of the repulsion between the charged head groups by supplying more counter ions from the salts. The aggregation number of CTAT solution in presence of electrolytes increased with increase in salt concentration but decreased in presence of non-electrolytes, with increase of its concentration. The packing parameter value indicated the transition towards spherical shape with increase in concentration of non-electrolytes and increase in more non-spherical shape with increase in concentration of electrolytes for rod-like micelles of CTAT.

Effect of head group of surfactant on the self-assembly structures and aggregation transitions in a mixture of cationic surfactant and anionic surfactant-like ionic liquid

Abstract The effect of head group of surfactant on the self-assembly structures and aggregation transitions, which had rarely been reported in mixed aqueous solutions of cationic surfactant and anionic surfactant-like ionic liquid molecules, was studied in three different cases. We have investigated the effect of the methyl, ethyl and 2-phenoxy-ethyl on the aggregation by the methods of turbidity, dynamic light scattering (DLS), transmission electron microscopy (TEM), rheological measurements, and steady-state fluorescence measurements. It is found that the head group has an important effect on the self-assembly structures and phase transition behaviors of mixed aqueous solutions. The ethyl group introduces a more considerable steric hindrance compared with the methyl group, which leads to the smaller packing parameter. Therefore, the system containing the methyl group has a slightly larger vesicle phase range than the system containing the ethyl group. However, compared with the ethyl group, 2-phenoxy-ethyl introduces the non-covalent pi-pi interaction between the imidazolium ring and benzene ring. This interaction easily eliminates the steric hindrance. Therefore, it is more prone to form the wormlike micelles and vesicles for the system with the 2-phenoxy-ethyl group. We also find intriguing rheological behavior from the non-spherical vesicles. Moreover, for the wormlike micelles, we find it only form in aqueous surfactant mixtures with strong interactions.

Improvement of Foaming Ability of Surfactant Solutions by Water-Soluble Polymers: Experiment and Molecular Dynamics Simulation.

Aqueous foam is widely used in fire extinguishing and dust suppression technologies. Improving the foaming ability is the key to reducing the added concentration of foaming agents as well as the economic cost. In this work, the effect of a water-soluble polymer (polyvinyl alcohol, PVA) on the foaming ability of anionic surfactant (sodium dodecyl ether sulfate, SDES) was studied by an experiment and molecular dynamics simulation. The experimental results showed that PVA greatly improves the foaming ability of SDES solutions when the surfactant concentration is less than 0.1%, which is attributed to the fact that the polymer can enhance the stability of bubble films and reduce the bubble rupture rate during the foam generation process. The simulation results indicate that PVA can enhance the hydration of surfactant head groups and contribute to the formation of a three-dimensional hydrogen bond network between surfactants, polymers, and water molecules, thus greatly improving the stability of bubble liquid films. The above results suggest that water-soluble polymers can be used to improve the foaming ability of surfactant solutions by enhancing the bubble film stability, which is beneficial as it reduces the added concentration of foaming agents in aqueous foam applications.

Oil solubilization in sodium dodecylbenzenesulfonate micelles: New insights into surfactant enhanced oil recovery

HypothesisThe current mechanism of surfactant enhanced oil recovery (EOR) mainly relies on forming middle-phase microemulsions to get ultra-low oil-water interfacial tension. However, residual oil can also be recovered using low concentration surfactant solutions without microemulsion formation, and the interaction between the surfactant solution and crude oil at very early contact has not been studied yet. We hypothesize micelle solubilization of oil as an alternative EOR mechanism. ExperimentsSodium dodecylbenzenesulfonate (SDBS), anisole and 1-hexene were used as a model surfactant and model polar and nonpolar compounds in crude oil, respectively. The interaction between SDBS micelles and these two additives was investigated with dynamic light scattering, UV–Vis spectroscopy, 1H NMR spectroscopy, cryogenic transmission electron microscopy, confocal microscope and small angle neutron scattering. FindingsSDBS micelles become larger upon increasing additive concentration to transfer into swollen micelles. 1-Hexene is localized in the micelle core, and retains the spherical micelle shape, while anisole resides in the palisade layer and weakens the electrostatic repulsions among surfactant headgroups, inducing a sphere–rod transition. No emulsion droplets were observed for 0.2 wt% SDBS solution until 1.5 wt% anisole or 1-hexene was introduced. These findings help understanding the role surfactant micelles in EOR and propose a new mechanism for surfactant EOR processes.

Experimental Depth Profiles of Surfactants, Ions, and Solvent at the Angstrom Scale: Studies of Cationic and Anionic Surfactants and Their Salting Out.

Neutral impact ion scattering spectroscopy (NICISS) is used to measure the depth profiles of ionic surfactants, counterions, and solvent molecules on the angstrom scale. The chosen surfactants are 0.010 m tetrahexylammonium bromide (THA+/Br-) and 0.0050 m sodium dodecyl sulfate (Na+/DS-) in the absence and presence of 0.30 m NaBr in liquid glycerol. NICISS determines the depth profiles of the elements C, O, Na, S, and Br through the loss in energy of 5 keV He atoms that travel into and out of the liquid, which is then converted into depth. In the absence of NaBr, we find that THA+ and its Br- counterion segregate together because of charge attraction, forming a narrow double layer that is 10 Å wide and 150 times more concentrated than in the bulk. With the addition of NaBr, THA+ is "salted out" to the surface, increasing the interfacial Br- concentration by 3-fold and spreading the anions over a ∼30 Å depth. Added NaBr similarly increases the interfacial concentration of DS- ions and broadens their positions. Conversely, the dissolved Br- ions are significantly depleted over a depth of 0-40 Å from the surface because of charge repulsion from DS- ions within the interfacial region. These different interfacial Br- propensities correlate with previously measured gas-liquid reactivities: gaseous Cl2 readily reacts with Br- ions in the presence of THA+ but drops 70-fold in the presence of DS-, demonstrating that surfactant headgroup charge controls the reactivity of Br- through changes in its depth profile.

Curcumin degradation kinetics in micellar solutions: Enhanced stability in the presence of cationic surfactants

In this work, the rate of curcumin degradation in the presence of various surfactants (anionic, cationic, and nonionic) and salts (i.e., NaCl, NaHCO3, Na2CO3, and NaOH) was investigated by photodiode-array UV–vis spectroscopy. The curcumin degradation at neutral pH or in the presence of NaCl during the first 16 h of the study was negligible (i.e., zero rate constant). Depending on the type and concentration of added alkaline salt, the color of the curcumin surfactant solution changed from yellow (λmax. = 420−430 nm) to orange (λmax. = 450−500 nm). The degradation rate constants and the final degradation percentage were obtained and reported for various conditions. The maximum curcumin degradation for the investigated surfactants was observed in the presence of 28.5 mM Na2CO3 at pH ∼10.8. The curcumin degradation rate was much lower in the presence of cationic surfactants (monomeric and gemini type). The gemini cationic surfactant (12-4-12) was even more efficient than the single chain cationic surfactant (DTAB) to suppress the curcumin degradation indicating the importance of cationic charge density of the surfactant head group on the electrostatic interaction with curcumin. Curcumin is deprotonated in alkaline conditions and carries negative charges; therefore, cationic surfactants can accommodate and protect it much better than anionic or nonionic surfactants against further degradation.

Cleaning Ability of Mixed Solutions of Sulfonated Fatty Acid Methyl Esters

AbstractHere, we present results from a systematic study on cleaning of oily deposits from solid surfaces (porcelain and stainless steel) by solutions of fatty acid sulfonated methyl esters (SME), sodium salts. The zwitterionic dodecyldimethylamine oxide (DDAO) has been used as a cosurfactant. As representatives of the vegetable and mineral oils, sunflower seed oil and light mineral oil have been used. The process of oil drop detachment from the solid substrates (roll‐up mechanism) has been monitored. In the case of porcelain, excellent cleaning of oil is achieved by mixed solutions of SME and DDAO. In the case of stainless steel, excellent cleaning (superior than that by linear alkylbenzene sulfonate and sodium lauryl ether sulfate) is provided by binary and ternary mixtures of SME, which may contain also DDAO. For the studied systems, the good cleaning correlates neither with the oil/water interfacial tension, nor with the surfactant chainlength and headgroup type. The data imply that governing factors might be the thickness and morphology of admicelle layers formed on the solid/water interface. The results indicate that the SME mixtures represent a promising system for formulations in house‐hold detergency, having in mind also other useful properties of SME, such as biodegradability, skin compatibility, and hard water tolerance.

Effect of CTAB on the surface resonance plasmon intensity of silver nanoparticles: Stability and oxidative dissolution

Stable silver nanoparticles (AgNPs) were prepared from the reduction of silver ions by sodium borohydride in presence of shape controlling cationic cetyltrimethylammonium bromide (CTAB) at room temperature. The intensity and position of surface plasmon resonance (SRP), morphology depends on the concentrations of CTAB. The complex forming tendency of CTAB with positive head group of surfactant and borohydride anion (BH4−) was responsible for pre-, and post-micellar effect. The values of BH4−-CTAB binding constant (Ks = 33.6 × 104 mol−1 dm3) and rate constant in micellar pseudo phase (km = 5.5 × 10−5 s−1) were calculated. Catalytic role of CTAB discussed in term of pseudo-phase model. The silver soles were unstable at higher [BH4−] (≥14.0 × 10−5 mol/L). Transmission electron microscopic images indicate that the AgNPs are roughly spherical and poly dispersed. The oxidative dissolution of silver nanoparticles was very slow. The [BH4−] has significant impact on the sensing properties of AgNPs. The dissolution rate increases drastically by adding hydrogen peroxide. Scavengers quenched the oxygen radical reactive species (hydroxy radical and superoxide) and decreased the hydrogen peroxide catalyses rate of AgNPs.

Quantitative analysis of micellar effect on the reaction rate of cationic triphenylmethine dyes with water according to Berezin’s model

Several approaches quantitatively describe the effect of surfactant micellar solution on the reaction rate. The most used among them are Piszkiewicz’s, Berezin’s, and Pseudophase Ion-Exchange (PIE) models. The last-named was developed by Bunton and Romsted. Piszkiewicz’s model is based on representations of the micellization according to the mass action law with the formation of a catalytic micelle, which consists of some surfactant molecules and a substrate. In our previously paper, this model was used to explain the kinetic micellar effect on the reaction of cationic triphenylmethine dyes with water once again showed the main disadvantages of this approach. Berezin’s model is based on another model of micelle formation viz. the pseudophase model, and the binding of reagents by micelles is considered as the distribution of a substance between two phases. In this work, we aim to consider the applicability of Berezin’s approach for the interaction of malachite green and brilliant green cations with water molecule as a nucleophile in aqueous systems of nonionic, anionic, cationic, and zwitterionic surfactants. On the whole, Berezin's model performed well when applied to the description of the micellar effect on the reaction of similar dye with the hydroxide ion. However, it was revealed that this model does not take into account the change in the local concentration of the HO– ions due to a compression of the double electric layer upon addition of reacting ions to the system, as well as the constant of association of the HO– ions with cationic head groups of surfactant. In this case, when water is used as a nucleophile, the question of the degree of nucleophile binding can be solved differently. The PIE model is also based on a pseudophase model of micellization, but a substrate binding by micelles is considered as an association in a stoichiometric ratio of 1:1, and a nucleophile concentration is expressed in a local concentration based on the neutralization degree of micelles. Given the latter, its approach cannot be applied to the kinetic micellar influence on the reaction of cationic triphenylmethine dyes with water.

Nitroxyl spin probe in ionic micelles: A molecular dynamics study

The compounds containing nitroxyl radical (NO˙) are actively used as spin probes to examine colloid systems, including lipid membranes and micelles. Their electron paramagnetic resonance spectrum provides information about the composition of the medium, in particular, the content of water there. Yet, the proper treatment of the measurement results demands understanding the microscopic characteristics of the molecular probe. In the present paper, we extend our previous studies on the microscopic state of acid-base and solvatochromic probes in surfactant micelles to the field of spin probes. We report the results of molecular dynamics simulation of a common spin probe, methyl-5-doxylstearate, in micelles of anionic (sodium n-dodecyl sulfate, SDS) and cationic (n-dodecyltrimethylammonium bromide, DTAB) surfactants. The localization of the molecule within the micelles, its shape, composition of the local environment, hydration were quantified and compared with the available relevant experimental data. No significant dissimilarity was found in the characteristics of the probe molecule in both kinds of micelles. However, the characteristics of the O˙ atom carrying the unpaired electron are pronouncedly different, namely, in DTAB micelles it is less hydrated and forms less hydrogen bonds with water. Similar situation was observed for the COO group. The main reason was found to be the interactions with cationic surfactant headgroups, which screen the O˙ atom and COO group from water. These findings allowed revisit the point of view that the surface layer of DTAB micelles as a whole is less hydrated in comparison to that of the SDS ones.

Role of Ionic Headgroups on the Thermal, Rheological, and Foaming Properties of Novel Betaine-Based Polyoxyethylene Zwitterionic Surfactants for Enhanced Oil Recovery

Long-term thermal stability of surfactants under harsh reservoir conditions is one of the main challenges for surfactant injection. Most of the commercially available surfactants thermally degrade or precipitate when exposed to high-temperature and high-salinity conditions. In this work, we designed and synthesized three novel betaine-based polyoxyethylene zwitterionic surfactants containing different head groups (carboxybetaine, sulfobetaine, and hydroxysulfobetaine) and bearing an unsaturated tail. The impact of the surfactant head group on the long-term thermal stability, foam stability, and surfactant–polymer interactions were examined. The thermal stability of the surfactants was assessed by monitoring the structural changes when exposed at high temperature (90 °C) for three months using 1H-NMR, 13C-NMR, and FTIR analysis. All surfactants were found thermally stable regardless of the headgroup and no structural changes were evidenced. The surfactant–polymer interactions were dominant in deionized water. However, in seawater, the surfactant addition had no effect on the rheological properties. Similarly, changing the headgroup of polyoxyethylene zwitterionic surfactants had no major effect on the foamability and foam stability. The findings of the present study reveal that the betaine-based polyoxyethylene zwitterionic surfactant can be a good choice for enhanced oil recovery application and the nature of the headgroup has no major impact on the thermal, rheological, and foaming properties of the surfactant in typical harsh reservoir conditions (high salinity, high temperature).