Self-aggregation Of Surfactants Research Articles

Effect of different micelles on charging and discharging behavior of phase change material

Phase change material (PCM) offers high-density thermal energy storage, making it attractive for thermal management applications of electronic circuitry and thermal energy storage. However, PCM, like paraffin wax, ideal for such low-temperature applications, has very low thermal conductivity. The present study focused on the improvement of the thermal conductivity of paraffin wax using surfactants. The surfactants used as thermal conductivity enhancers (TCE) are cetyltrimethyl ammonium bromide (CTAB), Dioctyl sodium sulfosuccinate (known as AOT) and sodium dodecyl sulfate (SDS). The surfactant self-aggregation called a micelle, acts as conducting medium inside the paraffin wax, providing better thermal conductivity. The highest heat transfer rate with a peak temperature of 71 °C was observed in the case of AOT micelle paraffin wax. Adding SDS, CTAB, and AOT surfactants increased the highest temperatures by 4%, 8.4%, and 18.33% compared to pure PCM.

One-step synthesis of Janus hydrogel via heterogeneous distribution of sodium α-linoleate driven by surfactant self-aggregation.

Janus adhesive hydrogels have one-sided adhesive properties and hold promising applications in the health care field. However, a simple method for synthesizing Janus hydrogels is still lacking. In this study, we introduce an innovative method to prepare Janus hydrogels by harnessing a fundamental phenomenon: the self-aggregation of surfactants at high concentrations at the water-air interface. By combining a small amount [0.8 to 3.2 weight %, relative to mass of acrylamide (AM)] of sodium α-linoleate (LAS) with AM through free radical polymerization, we have synthesized Janus adhesive hydrogels. The Janus hydrogels exhibit remarkable adhesive strength and adhesive differences, with the top side (84 J m-2) being 21 times stronger than the bottom side, also an excellent elongation rate. Through comprehensive experiments, including chemical composition, surface morphology, and molecular dynamics (MD) simulations, we thoroughly investigate the mechanisms of the hydrogel's heterogeneous adhesion. This study presents an easy, efficient, and innovative method for preparing one-sided adhesive hydrogels.

Constituent- and Composition-Dependent Surfactant Aggregation in (Lanthanide Salt + Urea) Deep Eutectic Solvents

Due to the ease of tailoring the physicochemical properties by simply changing a constituent or composition, deep eutectic solvents (DESs) possess widely varying capabilities for surfactant self-assembly that could depend on the surfactant headgroup charge. The self-aggregation process of three surfactants, sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB), and Triton X-100 (TX-100), dissolved in DESs composed of a lanthanide salt (Ln) and urea (U) is investigated. The role of the identity of the metal salt is assessed by using [La(NO3)3·6H2O] (La) and [Ce(NO3)3·6H2O] (Ce) and that of the composition is deciphered by systematically changing the mole ratio of the metal salt and urea in (La/U) DESs. The response to a fluorescence probe pyrene-1-carboxaldehyde along with electrical conductance and surface tension measurements is used to obtain the critical aggregation concentration (CAC). While the CACs in 1:3.5 (Ln/U) for SDS are significantly lower than that in water, the values are marginally higher for CTAB and TX-100. The CACs for all three surfactants are similar in 1:3.5 (La/U) and (Ce/U) DESs, implying that the identity of the metal in the salt is not so important. Increasing the urea composition in (La/U) DESs results in increased CAC for SDS and CTAB; however, a minimal decrease in CAC is observed for TX-100. From the temperature dependence of CAC, thermodynamic parameters, ΔGagg0, ΔHagg0, and ΔSagg0, of the surfactant self-aggregation process are estimated. These parameters reveal that while at a lower urea content, the SDS/CTAB self-assembly process is enthalpically driven, it becomes entropically favored at higher urea concentrations. The TX-100 self-aggregation in these DESs is found to be strongly enthalpically favored and entropically un-favored. These parameters are explained as a combination of passage of the solvophobic surfactant chain from the bulk DES to the aggregate pseudo-phase and differential orientation/organization of DES constituents around surfactant monomers and/or aggregates.

Mixed micelle formation by sodium dodecylsulfate and dodecyltrimethylammonium bromide in aqueous ionic liquid media

Surfactant self-aggregation within ionic liquid (IL) - based media has been investigated to compare and contrast the role of such media with that of water. Behavior of a surfactant mixture constituting of two surfactants with oppositely charged head groups, namely anionic sodium dodecylsulfate (SDS) and cationic n-dodecyltrimethylammonium bromide (DTAB), is assessed in aqueous IL media. Contrary to what is observed in water, it is shown earlier that effective charge screening of the surfactant headgroups by IL ions in a typical IL 1-ethyl-3-methylimidazolium ethylsulfate ([C2C1im][EtSO4]) results in formation of (SDS + DTAB) mixed micelles with critical micelle concentrations (CMCs) ranging between those of SDS and DTAB, respectively. The miscibility of (SDS + DTAB) mixture in aqueous [C2C1im][EtSO4] reveals that as the content of IL [C2C1im][EtSO4] is increased, the miscibility of the surfactant mixture improves indicating increased charge screening involving ions of the IL and the surfactant headgroups. For water mole fraction (XW) ≤ 0.90, (SDS + DTAB) surfactant mixture is miscible in aqueous [C2C1im][EtSO4] system in all proportions. For 1.00 ≥ XW≥ 0.90, while synergism persists due to the presence of attractive electrostatic attraction between SDS and DTAB headgroups leading to mixture CMCs below the CMCs of SDS and DTAB, respectively, the extent of synergism diminishes with increase in IL [C2C1im][EtSO4] in the solution. For XW = 0.80, the (SDS + DTAB) mixture exhibits CMC values that are closest to the values estimated assuming ideal behavior. For 0.70 ≥ XW ≥ 0.30, the (SDS + DTAB) exhibits antagonistic trend with CMC values higher than those estimated for ideal solution; the antagonistic trend gradually decreases with increase in [C2C1im][EtSO4] and again attains close to ideal behavior in neat IL [C2C1im][EtSO4]. This trend is subsequently manifested in the composition of the mixed micelles as well as in the net interaction parameter, β. Addition of small proportion of IL to water helps in solubilization of surfactant mixture of oppositely charged surfactants due to effective charge screening of the surfactant headgroups and results in formation of mixed micelles in aqueous IL media over complete composition range.

Rhamnolipid–SLES aqueous mixtures: From the molecular self-aggregation to the functional and ecotoxicological properties

Biosurfactants represent an ecosustainable alternative to conventional surfactants, allowing to reduce the environmental impact arising from the increasing production of industrial surfactant-containing formulations. In the present work, the physico-chemical properties of the aqueous solutions of rhamnolipids (Rha) are compared to those of sodium lauryl ether sulfate (SLES), a widely used surfactant in many applications. Moreover, in view of a possible partial replacement, mixtures of these surfactants are analyzed. The surfactant self-aggregation in bulk solution is investigated by surface tension, dynamic light scattering (DLS) and electron paramagnetic resonance (EPR), finding that Rha and SLES form mixed aggregates whose morphology, dimension and internal structure depend on the composition of the mixture. An increase of the Rha content induces the formation of large micelles, with a compact inner core. A nonideal behavior of Rha-SLES mixtures is found, indicating antagonistic interactions between the two surfactants. Contact angle, emulsification and foaming measurements are carried out to investigate the functional behavior of the surfactant aqueous mixtures at the interface with solids, oil and air. Rha improves the wettability of hydrophobic surfaces but decreases the emulsion stability and the foamability. The results of the different experimental techniques are rationalized in a framework based on the molecular architectures, highlighting that the balance between the flexibility of the SLES headgroups and the bulkiness of the Rha ones allow tuning the mixture behavior, both in bulk and at the interfaces. Understanding the relationship between the physico-chemical properties of these mixtures and their functional behavior is strategic to steer the rational exploitation of Rha in green chemical formulations. In this direction, a comparative ecotoxicological assay is finally presented, whose results demonstrate the better profile of Rha compared to SLES.

Evaluation of the influence of calcium chloride on the behavior of phases of nanosystems applied in the larvicidal control of Aedes aegypti

Systems stabilized by surfactants in order to incorporate different substances at their interfaces, such as hydrophilic polymers and active compounds, are systems capable of establishing vector control of Aedes aegypti. The system was obtained through the surface tension of PPG-5-CETETH-20/calcium chloride dispersion at concentrations of 0.2%, 0.4%, 0.8%, 1% and 2%, determining the CMC of each sample. Then, a binary study was performed and the system structures were evaluated by Polarized Light Microscopy (PLM) and rheological analysis. Biological analysis was performed to assess the mean lethal concentration (LC50) against Aedes aegypti larvae and toxicity to non-target organisms in Artemia salina. In this sense, the objective of this work was to evaluate the influence of calcium chloride on the behavior of phases and on the biological activity of formulations based on the self-aggregation of surfactants, an alternative larval insecticide to be used as a replacement for larval insecticides already on the market.

Molecular modeling of ionic aggregates at several concentrations of SDS in aqueous solution

The results of molecular modeling of ionic surfactant self-aggregation in sodium dodecyl sulphate (SDS) aqueous solutions, both salt-free and with addition of NaCl, are reported. The modeling has been based on all-atom molecular dynamics simulations at several SDS concentrations. Formation of one or three ionic aggregates of different size has been observed in the molecular dynamics runs. The computed trajectories of molecules and ions have been used to study the effects of aggregation on local densities of water molecules and counterions, and on the diffusivities of the aggregates themselves. With finding the mean force potential, the degree of counterion binding for aggregates with different aggregation numbers has been estimated. The results have been compared with the classical square-gradient density functional computations for a single ionic micelle in polar solvent. In addition, the influence of size of the simulation box on the transport properties of SDS aggregates has been investigated. In particular, we have found how the diffisivity of an ionic aggregate with a given aggregation number depends on the total surfactant concentration. The diffusivities of aggregates with aggregation numbers 16, 32, 48, and 64 at fixed total surfactant concentration have been computed. With the help of the Stokes-Einstein relation, the viscosity of the micellar solution at different total surfactant concentrations has been calculated. Time dependencies of the number of aggregates in the simulation box and of the average aggregation number during the self-assembly of surface active ions have been studied.

Platonic Micelles: Monodisperse Micelles with Discrete Aggregation Numbers Corresponding to Regular Polyhedra

The concept of micelles was first proposed in 1913 by McBain and has rationalized numerous experimental results of the self-aggregation of surfactants. It is generally agreed that the aggregation number (Nagg) for spherical micelles has no exact value and a certain distribution. However, our studies of calix[4]arene surfactants showed that they were monodisperse with a defined Nagg whose values are chosen from 6, 8, 12, 20, and 32. Interestingly, some of these numbers coincide with the face numbers of Platonic solids, thus we named them “Platonic micelles”. The preferred Nagg values were explained in relation to the mathematical Tammes problem: how to obtain the best coverage of a sphere surface with multiple identical circles. The coverage ratio D(N) can be calculated and produces maxima at N = 6, 12, 20, and 32, coinciding with the observed Nagg values. We presume that this “Platonic nature” may hold for any spherical micelles when Nagg is sufficiently small.

Micellization thermodynamics and the nature of enthalpy–entropy compensation

Abstract Surfactants self-assemble to form micelles guided by thermodynamic rules. The observed free energy, enthalpy and entropy changes are system specific. According to enthalpy–entropy compensation (EEC) phenomenon, for similar chemical systems (or processes) at a fixed temperature if the individual enthalpy and entropy changes (ΔH and ΔS, respectively) are plotted then a linear relation can be found in many occasions. The slope of the line provides a temperature which is different from the experimental temperature, and it is called the compensation temperature (Tcomp). There are several reasons proposed for the effect but none of them are still accepted, and the EEC remains to be an empirical, extra-thermodynamic and measurement error related phenomenon. However, from classical thermodynamics a linear relation between enthalpy and entropy changes can be shown. This is generally true for many chemical processes including surfactant self-aggregation (micelle formation), etc. Of the three essential thermodynamic parameters (ΔG, ΔH and ΔS), ΔG is independently determined whereas ΔH is found from temperature dependence of ΔG (in terms of van’t Hoff equation), and ΔS then follows from the relation, ΔG = ΔH − TΔS. Thus, the latter two parameters are indirectly obtained. This raises a question on the EEC phenomenon; measurement uncertainties may also influence it. From isothermal titration calorimetry measurements of both free energy and enthalpy of micellization can be determined from a single run, only the entropy of micellization is indirectly found from the above stated relation. The uncertainties of the measurements can thus be reduced. We shall discuss such results on the self-aggregation of different amphiphiles under different conditions to examine the nature of the EEC phenomenon. We have also found anti-EEC (AEEC) phenomenon of some micellar systems in support to such observations on non micelle forming processes. These findings and their characteristic features are presented and discussed. This is a comprehensive study on EEC not done in the past.

Chitosan macroporous foams obtained in highly concentrated emulsions as templates

Emulsion templating is an effective route for the preparation of macroporous polymer foams, with well-defined pore structures. This kind of material is usually obtained by polymerization or crosslinking in the external phase of highly concentrated emulsions. The present article describes the synthesis of macroporous foams based on a cationic polymer, chitosan, crosslinked with genipin, a natural crosslinker. The phase behavior was used to study the influence of chitosan on surfactant self-aggregation. Hexagonal and lamellar liquid crystalline structures could be obtained in the presence of chitosan, and polymer did not greatly influence the geometric lattice parameters of these self-aggregates. O/W highly concentrated emulsions were obtained in the presence of chitosan in the continuous phase, which allowed reducing both droplet size and polydispersity. The emulsions were stable during the time required for crosslinking, obtaining macroporous foams with high pore volume and degree of crosslinking.

Mutual and Intradiffusion Coefficients for the Binary System n-Decyl Dimethyl Phosphine Oxide + Water at 25 °C

Diffusion coefficients for the binary system n-decyl dimethyl phosphine oxide + water have been studied at 25 °C, with the aim to quantitatively describe the surfactant micellization process in aqueous medium. Accurate mutual diffusion coefficients have been measured by the Taylor dispersion technique. Intradiffusion coefficients of both components have been measured by the pulsed gradient spin echo (PGSE)−FT NMR technique. The data have been compared and interpreted in the framework of the equilibrium model of surfactant self-aggregation. Interpolation of the mutual diffusion coefficients has allowed us to estimate reliable values of the average number of surfactant molecules involved in the formation of a micellar aggregate and the constant of the equilibrium between these aggregates and surfactant unimers. Interpolation of water intradiffusion coefficients has allowed us to estimate the number of water molecules involved in the hydration of surfactants in unimeric and micellized form.

A Study of the Microstructural and Diffusion Properties of Poly(vinyl alcohol) Cryogels Containing Surfactant Supramolecular Aggregates

Surfactant-containing poly(vinyl alcohol) (PVA) cryogels have been prepared by drying and reswelling hydrogel patches, previously obtained by the freeze/thaw procedure, in decyltrimethylammonium bromide (C10TAB) aqueous solutions. The microstructural and diffusive properties of the resulting material have been characterized by a combined experimental strategy. Gravimetric measurements show that the cryogel maximum swelling is not affected by the surfactant. The surfactant concentration within the cryogel, measured by ion chromatography, is the same as that in the rehydrating surfactant solution. Electron paramagnetic resonance (EPR) spin-probe and small-angle neutron scattering (SANS) measurements show that surfactant self-aggregation in the gel is similar to that in water, occurring at the same critical concentration and resulting in the formation of micellar aggregates whose structure is not affected by the cryogel polymeric scaffold. However, both the micelle intradiffusion coefficients, measured by PGSE-NMR, and the spin-probe correlation times, measured by EPR, indicate that dynamic processes in the hydrogel are much slower than in bulk water. A quantitative analysis of these results suggests that the cryogel polymer-poor domains, in which surfactant molecules are solubilized, have an average dimension of approximately 0.1 microm. Interestingly the experimental data also show that the polymer-poor phase contains more polymer than expected, suggesting that the spinodal decomposition, which occurs during the freezing step of cryogel preparation, is not complete or prevented by ice formation.

XPS study of the surfactant film adsorbed onto growing titania nanoparticles

TiO2 particles, prepared by following a sol–gel preparative route, were submitted to hydrothermal growing stages in the presence of an anionic surfactant, sodium dodecyl-sulfate (SDS), at solution pH values corresponding, respectively, to positive surface charges and to the isoelectric point of the oxide. The concentration of the surfactant in the aqueous solution was varied in order to produce different conditions of self-aggregation between the amphiphilic molecules. XPS analyses were performed on the aged and dried precursors to characterize the surfactant films adsorbed onto the oxide. The regions of Ti 2p, O 1s, and C 1s were specifically investigated. The samples, calcined at 600°C, were characterized for phase composition-crystallinity, by X-ray diffraction, and for surface area. The role played by the oxide–surfactant interactions and by the surfactant self-aggregation phenomena in affecting the physico-chemical properties of the powders is discussed.

Protein transfer through polyacrylamide hydrogel membranes polymerized in lyotropic phases.

A way to control the average pore size in cross-linked polyacrylamide-based membranes is by altering the ratio of cross-linker to acylamide monomer. Larger pore sizes are prepared with a minimum amount of cross-linker, resulting in membranes that are mechanically weak and have short lifetimes. The aim of this study was to prepare cross-linked polyacrylamide membranes with large pore sizes and with good mechanical integrity. The methodology was to carry out the polymerization in a template, formed from the self-aggregation of surfactant. Two surfactant templates were used, and their pore size was examined with proteins of different sizes. The surfactants chosen for this study were sodium dodecyl sulfate (SDS, ionic surfactant) and TERIC BL8 (nonionic surfactant), both of which have very different aggregation properties. The data showed that at 10% and greater of TERIC BL8, a very different and open gel structure is formed, in which the pore size was significantly increased. SDS seemed to have little effect on the pore size. The data suggests that the gel structures for both surfactants up to 4% (w/v) are similar and micellular, because SDS is known to favor a micelle structure. Above 4% (w/v), TERIC BL8 then goes through a change in its lyotropic phase, thus, producing membranes of a large pore size. In conclusion, the pore size and gel structure of polyacrylamide hydrogel membranes can be significantly increased using TERIC BL8 (nonionic) surfactant. This allows large-pore-size membranes with a high cross-link density and consequently high mechanical strength to be prepared for the separation of large biomolecules.

Transport Properties of Aqueous Solutions of Alkyltrimethylammonium Bromide Surfactants at 25°C

Intradiffusion coefficients, D, of n-alkyltrimethylammonium bromides [CH3–(CH2)n−1–N(CH3)3Br, CnTAB] (n=6, 8, 10, 12) in mixtures with heavy water were measured by the PGSE–NMR technique at 25°C. The experimental data permitted evaluation of the influence of the alkyl chain length on the surfactant self-aggregation process. For all the surfactants considered, the D trend showed a slope change corresponding to the critical micellar composition (cmc). In the premicellar composition range, D decreased linearly with the square root of the surfactant molality. The D values extrapolated at infinite dilution were related to the limiting mutual diffusion coefficients, determined through the Taylor dispersion technique. In the micellar composition range, solubilized tetramethylsilane (TMS) molecules were used to determine the micelle intradiffusion coefficient, DM, from which the aggregate radii and the aggregation numbers were obtained. The decreasing trend of DM with increasing surfactant molality was interpreted in terms of interparticle electrostatic repulsion. DM values allowed evaluation of the Gouy–Chapman layer thickness. The solvent intradiffusion coefficient in the heavy water–CnTAB mixtures, Dw, was also measured. It decreased with increasing surfactant molality. For n=8, 10, 12 the Dw trend presented a slope change at the cmc, which could be ascribed to the strong decrease in hydration of surfactant molecules upon micellization. Because of its short hydrophobic tail, C6TAB exhibited peculiar aggregation behavior. Its cmc, which is poorly marked, is lower than the value predicted by extrapolating the cmc values obtained for the other terms of the series. The C6TAB aggregates do not solubilize TMS molecules; the estimated aggregation number is extremely low (∼3). Finally, no abrupt slope change in the solvent intradiffusion coefficient trend was detected. This evidence suggests that C6TAB molecules do not micellize in aqueous solution, but form trimers in which the surfactant hydrophobic tails are not hidden from contact with water molecules.

Self-aggregation of surfactants in water solution by NMR

Dynamic behavior of surfactant molecules, sodium dodecyl sulfonate (SDSN), cetyl trimethyl ammonium bromide (CTAB) and triton X-100, and their micelles in water solution at various concentrations were studied by chemical shift, self diffusion coefficient (D) and nuclear magnetic resonance relaxation (T1 and T2) measurements. Experimental results show that there exist turning points at surfactant concentration (C) nearby the critical micelle concentration (cmc) for all of the above-mentioned parameters, which approach the constant values when C equals 5 to 10 times cmc. T1 and T2 measurements show that when C<cmc, Triton X-100 molecules exhibit fast isotropic motion in water solution. After micellization, the motion of hydrophobic chains is far away from the extreme narrowing condition, however, the long hydrophilic oxyethylene chains move relatively freely, although their T1 and T2 become shorter in comparison with those of the molecules in mono-molecular state. The behavior of the hydrophobic chains within the micellar core of SDSN and CTAB is similar to that of Triton X-100 to some extent.

Supramolecular Structure of the LiPFN−PVP System: 19F NMR Studies

19F NMR measurements on lithium perfluorononanoate [LiPFN] in D2O and in solutions of D2O with poly(vinylpyrrolidone) [PVP] at different molecular weights have been carried out. A careful study of literature data and COSY maps allowed the full assignment of the 19F resonances as a function of concentration and composition of the systems. For the LiPFN/D2O system, analysis of the chemical shifts of the fluorine atoms bound to the different carbons of the LiPFN backbone reveals significant variations correlated to the LiPFN concentration. Such a finding has been explained in terms of the changes in the local dielectric constant caused by the surfactant self-aggregation process. The presence of PVP in solution strongly modifies the trend of the 19F chemical shifts observed in the binary LiPFN/D2O system. Such a feature, depending both on the position of the fluorine atoms along the surfactant backbone and on the LiPFN concentration, indicates the presence of strong polymer−surfactant interactions. The line w...

Formation of liquid-crystal phases in the binary system cetylpyridinium bromide/N-methylsydnone study by light microscopy and small-angle x-ray diffraction

Liquid-crystal phases were detected in the binary system cetylpyridinium bromide/N-methylsydnone by optical microscopy and small-angle X-ray diffraction. Surfactant self-aggregation can thus occur in aprotic solvents. Hydrogen bonding in solvents is thus not an essential requirement for molecular aggregation