Formation Of Surfactant Aggregates Research Articles

Photo-Isomerization Kinetics of Azobenzene Containing Surfactant Conjugated with Polyelectrolyte.

Ionic complexation of azobenzene-containing surfactants with any type of oppositely charged soft objects allows for making them photo-responsive in terms of their size, shape and surface energy. Investigation of the photo-isomerization kinetic and isomer composition at a photo-stationary state of the photo-sensitive surfactant conjugated with charged objects is a necessary prerequisite for understanding the structural response of photo-sensitive complexes. Here, we report on photo-isomerization kinetics of a photo-sensitive surfactant in the presence of poly(acrylic acid, sodium salt). We show that the photo-isomerization of the azobenzene-containing cationic surfactant is slower in a polymer complex compared to being purely dissolved in aqueous solution. In a photo-stationary state, the ratio between the trans and cis isomers is shifted to a higher trans-isomer concentration for all irradiation wavelengths. This is explained by the formation of surfactant aggregates near the polyelectrolyte chains at concentrations much lower than the bulk critical micelle concentration and inhibition of the photo-isomerization kinetics due to steric hindrance within the densely packed aggregates.

Structure and dynamics of aqueous solutions containing poly-(acrylic acid) and non-ionic surfactant pentaethylene glycol n-octyl ether (C8E5): A molecular simulations study

Self-assembly of non-ionic surfactant pentaethylene glycol n-octyl ether (C8E5) in aqueous solution containing anionic polyelectrolyte poly (acrylic acid) PAA was investigated via atomistic molecular dynamics (MD) simulations. The concentration of surfactant (Cs) varied in the range (Cs = 9.6 × 10−4–0.3 M) of dilute to concentrated systems while polymer concentration was in dilute solution condition (i.e. Cp = 0.01 M). Binary and ternary systems show formation of nearly spherical micellar aggregate at low Cs and lamellar aggregates at high Cs. Strong interaction between PAA and C8E5 is observed at low Cs, in agreement with experimental data. PAA chain adsorbs onto the micellar aggregate and wraps around the aggregate. For lamellar phase aggregates PAA chain shows polar interactions with hydrophilic moieties of surfactant, originating from interactions between COOH groups of PAA and CO groups of C8E5. The concentration at which free-micelles form in ternary (polymer, surfactant and water) solution is greater than CMC of the binary surfactant-water system, in qualitative agreement with experimental data. At low Cs the interaction between surfactants and polymer chain is weak and does not perturb chain conformations. The radius-of-gyration Rg of PAA increases with Cs. Interaction of surfactant molecules with water is more cooperative at higher values of surfactant concentration, as evident from the values of solvation enthalpy. Significant hydrogen bonds were observed to occur between polar oxygens and hydroxyl oxygen (O2HA) of PAA. The number of h-bonds between polar oxygens and O2HA of PAA increase with Cs. Hydrogen bonds of PAA with surfactant molecules increases with Cs as the polymer expands at high Cs. Formation of surfactant aggregate is enthalpically favored across the entire range of concentration in binary and ternary solution. Binding of PAA to surfactant aggregate is favorable at higher surfactant concentration. The enthalpy of formation of ternary solution exhibits a transition from unfavorable to favorable which interestingly is driven by the structural transition from spherical micellar to the lamellar phase. This signature for this transition is also seen by conformation change of PAA upon binding to surfactant aggregate.

Subsea dispersants injection (SSDI), effectiveness of different dispersant injection techniques – An experimental approach

The main objective with this study has been to study injection techniques for subsea dispersant injection (SSDI) to recommend techniques relevant for both laboratory studies and operational response equipment.The most significant factor was the injection point of the dispersant in relation to the release of the oil. The dispersant should be injected immediately before or after the oil is released. Then the dispersant will mix into the oil and reduce IFT before the oil enters the turbulent zone where initial droplet formation occurs.All injection techniques tested gave significant reductions in oil droplet sizes. However, due to the rapid oil droplet formation in turbulent jets and possible formation of surfactant aggregates in the oil, premixing of dispersants should not be used for experimental studies of subsea dispersant injection. This could underestimate dispersant effectiveness and produce results that might not be representative for up-scaled field conditions.

Effect of TritonX-100 on the aggregation behavior of SDS in water

ABSTRACTThe phase behavior patterns and microstructures of surfactant systems containing sodium dodecyl sulfate (SDS)/TritonX-100/H2O were investigated under a polarizing microscope. Ternary mixtures of SDS/TritonX-100/H2O form two types of well-ordered lyotropic liquid crystals: a hexagonal phase linked to the SDS/water axis and a lamellar phase located at the corner of their phase diagram. Surface tension measurements were conducted for TritonX-100 in the SDS solutions. In the SDS/TritonX-100/H2O system, transition points were found in the surface tension concentration curves at different quality ratios. On the basis of the critical micelle concentration values, the thermodynamic parameters of the aggregate formation, namely, , , and , were estimated. Results show that the addition of TritonX-100 is beneficial to the formation of surfactant aggregates, and thermodynamic data indicate that the micellization of binary mixture of SDS/TritonX-100 in aqueous solution is an entropy-driven spontaneous process. Therefore, the addition of TritonX-100 in SDS aqueous solutions may have potential applications in nanomaterial preparation.

Impact of Alkyl Chain Length on the Transition of Hexagonal Liquid Crystal-Wormlike Micelle-Gel in Ionic Liquid-Type Surfactant Aqueous Solutions without Any Additive.

The search for functional supramolecular aggregations with different structure has attracted interest of chemists because they have the potential in industrial and technological application. Hydrophobic interaction has great influence on the formation of these aggregations, such as hexagonal liquid crystals, wormlike micelles, hydrogels, etc. So a systematical investigation was done to investigate the influence of alkyl chain length of surfactants on the aggregation behavior in water. The aggregation behavior of 1-hexadecyl-3-alkyl imidazolium bromide and water has been systematically investigated. These ionic liquid surfactants are denoted as C16-Cn (n = 2, 3, 4, 6, 8, 9, 10, 12, 14, 16). The rheological behavior and microstructure were characterized via a combination of rheology, cryo-etch scanning electron microscopy, polarization optical microscopy, and X-ray crystallography. The alkyl chain has great influence on the formation of surfactant aggregates in water at the molecular level. With increasing alkyl chain length, different aggregates, such as hexagonal liquid crystals, wormlike micelles, and hydrogels can be fabricated: C16-C2 aqueous solution only forms hexagonal liquid crystal; C16-C3 aqueous solution forms wormlike micelle and hexagonal liquid crystal; C16-C4, C16-C6 and C16-C8 aqueous solutions only form wormlike micelle; C16-C9 aqueous solution experiences a transition between wormlike micelle and hydrogel; C16-C10, C16-C12, C16-C14 and C16-C16 only form hydrogel. The mechanism of the transition of different aggregation with increasing alkyl chain length was also proposed.

Characterization of the Liquid-Gas Interface of Aqueous Systems Containing a Derivative from Castor Oil

An anionic surfactant, synthesized with ricinoleic acid from castor oil, was obtained and its behavior in terms of microemulsion formation (via pseudo-ternary diagram analysis) and liquid-gas surface tension (both for microemulsions and pure surfactant-water systems) was determined as a function of temperature and NaCl concentration in the aqueous phase. Microemulsions were formed by using butanol as co-surfactant and kerosene as the oil phase. Concerning the pseudoternary diagrams, the increase in NaCl concentration resulted in a decrease in the Winsor IV region, which was correlated to a possible occurrence of nonmicellar aggregates, induced by the high concentration of NaCl in the aqueous phase. Surface tension measurements also indicated that at the very high NaCl concentrations used there could be the formation of surfactant aggregates. The oil phase in microemulsionated systems decreased surface tension (but increased CMC): Possible interactions between isolated surfactant molecules and molecules from the oil phase were used to explain these results.

Surfactant Self-Assembly in Cylindrical Silica Nanopores

The aggregative adsorption of a nonionic surfactant (C12E5) in the cylindrical pores of SBA-15 ordered mesoporous silica (pore diameter 8 nm) was studied by small-angle neutron scattering (SANS). Scattering profiles obtained at contrast-matching conditions between the aqueous solvent and the silica matrix can be represented quantitatively by an analytical scattering function for the diffuse small-angle scattering superimposed with Bragg reflections from the pore lattice. These two contributions provide complementary information about the self-assembly of the surfactant in the pores: diffuse scattering indicates the formation of surfactant aggregates at preferred distances from each other, and analysis of the Bragg peaks shows that a layer of surfactant is formed at the pore walls. These findings suggest that adsorption of the surfactant starts by formation of discrete surface aggregates, which increase in number and later merge to interconnected patches as the plateau of the adsorption isotherm is approached.

The adsorption of cationic surfactants on photoresist surfaces and its effect on the pattern collapse in high aspect ratio patterning

The collapse of photoresist patterns in high aspect ratio patterning is a defect that may reduce the production yield drastically. Recently, a novel method to reduce the pattern collapse was successfully applied: the rinse of the photoresist patterns with a cationic surfactant solution. In a previous study it was shown that one chosen surfactant adsorbs on both untreated photoresist surfaces and photoresist films that had undergone the whole photolithographic process. For this surfactant best pattern collapse reduction coincides with the formation of surfactant aggregates on the photoresist surface and with a maximum receding contact angle on processed photoresist. The present study investigates the influence of the hydrocarbon chain length of the surfactant on the pattern collapse reduction. The adsorption of surfactants of various chain length on unexposed and exposed photoresist was observed using null-ellipsometry and zeta potential measurements. Increasing the surfactant chain length shifts the adsorption domains to lower concentrations. It leads to an increased adsorbed amount and maximum contact angle as well as to a better pattern collapse reduction.

Reusable adsorbents for dilute solution separation 3. Sorption dynamics of phenanthrene on surfactant-modified alumina

The formation of surfactant aggregates (hemi-micelles) on alumina substantially increased the adsorption and retarded the breakthrough of a hydrophobic organic compound (phenanthrene) in a laboratory-scale column through adsolubilization. The adsorption waves of both surfactant (sodium dodecylsulfate, SDS) and phenanthrene were predicted adequately by existing models. However, the desorption wave was not predicted satisfactorily. The maximum capacities of the alumina bed for both SDS and phenanthrene were determined and compared with batch adsorption isotherm data; the values were in good agreement. The regeneration of the alumina bed was done by adjusting the feed solution pH to higher values than during the adsolubilization step; three regeneration cycles were studied. The capacities of alumina for phenanthrene and SDS sorption were found to be the same in all three cases, indicating no significant deterioration of the alumina surface. These characteristics make the process of adsolubilization attractive for concentration of dilute aqueous waste streams.

Binding of sodium dodecylsulfate to poly( N-isopropylacrylamide) microgels at different temperatures

The binding of sodium dodecylsulfate (SDS) to poly ( N-isopropylacrylamide), (PNIPAM) microgels at different temperatures (below and above the transition temperature from swollen to shrunken particles, T s) has been investigated by fluorescence probe methods complemented with surface tension and dynamic light scattering measurements. Pyrene and 1-anilinonaphthalene-8-sulfonate (ANS) were employed as fluorescence probes. At 21°C, the association of the surfactant with the particles occurs at a critical aggregation concentration (cac). Values obtained were (cac)=1 mM form surface tension data and (cac)≅0.75 mM from the dependence of the fine structure of pyrene fluorescence spectra with SDS addition. The binding isotherm derived from surface tension data at 21°C is highly cooperative, implying that the binding takes place with formation of surfactant aggregates. At 42°C (a temperature above T s), the (cac) shifts to 1.7 mM indicating that the interaction of SDS with shrunken PNIPAM microgels is weaker than with the swollen species. The microenvironment sensed by pyrene probe in both, the SDS aggregates formed onto PNIPAM swollen (21°C) and shrunken (42°C) species is less polar than that corresponding to SDS micelles in pure water. The T s of the particles and the effect of SDS on it can be determined from the temperature dependence of ANS fluorescence intensity or emission band position. At temperatures higher than T s, coulombic repulsion effects (probably due to the higher charge density of the collapsed particles) are evidenced by the fluorescence behaviour of both probes employed.

Reusable adsorbents for dilute solution separation. 1. Adsorption of phenanthrene on surfactant-modified alumina Kalliat

Activated γ-alumina surface was modified by adsorption of an anionic surfactant, sodium dodecylsulfate (SDS) from the aqueous phase. Typical S-shaped isotherms of surfactants on mineral oxides were observed for the adsorption of SDS on alumina. The formation of surfactant aggregates (hemi-micelles) on the surface made the alumina hydrophobic and increased the capacity of the oxide surface for an organic compound, namely, phenanthrene (PHE). The sorption of phenanthrene was directly related to the concentration of surfactant adsorbed. The partitioning of phenanthrene normalized to the adsorbed surfactant concentration was independent of pH. The linear sorption constant for a number of organic compounds was correlated to the octanol-water partition constant and activity coefficient in water, which are indicators of compound hydrophobicity. It is suggested that the ability to easily regenerate a modified alumina surface could be exploited in using it for wastewater treatment of contaminants at dilute concentrations.

Coadsorption of Steroids and Nonionic Surfactants on Polystyrene Latex Particles from Aqueous Solutions

The effect of the adsorption of two nonionic surfactants, Triton X-100 and a copolymer, poloxamer 188, on the coadsorption (adsolubilization) of three steroids, hydrocortisone, testosterone, and progesterone, on latex polystyrene nanoparticles has been investigated. It is shown that the formation of surfactant aggregates on the nanoparticles induces the uptake from the aqueous solution of up to 50 wt% of the total concentration of those steroids which are not adsorbed at the solid/liquid interface in the absence of surfactant. Above the equilibrium surfactant critical micelle concentration, the solutes are distributed between the adsorbed surfactant aggregates at the solid surface and the free micelles. The most hydrophobic molecules, progesterone, at all concentrations, and testosterone, at higher concentrations, are adsorbed on the particles in the absence of surfactant. The influence of steroid concentration, the hydrophobicity of the solutes, and the effect of added ethanol were investigated, especially in the case of hydrocortisone. Although, in absolute values, the steroid coadsorption is larger at higher total concentrations, the relative solute uptake is larger at lower concentrations. The addition of ethanol has a small effect on the adsorption of Triton X-100 but a large effect on the coadsorption of hydrocortisone. An attempt has been made to evaluate the solutes partition coefficient between the adsorbed surfactant aggregates and the aqueous solution. Adsorption of Triton X-100 or poloxamer 188 aggregates onto the polystyrene latex particles induces solute partition coefficient values on the same order of magnitude when expressed on the molar scale. For all systems studied, the coadsorption partition coefficients are larger than the corresponding values for the classical micellar solubilization effect.

Interaction of ionic surfactants with polymers in acqueous solution

A quantitative model based on the hydrophobic interaction concept has been presented for the polymer—surfactant complex formation and checked by potentiometric measurements using Au/Hg/Hg lDS and sodium-ion selective glass electrodes. The interaction of sodium dodecyl sulfate (NaDS) with poly-(vinyl alcohol) (PVA). poly-(vinylpyrrolidone) (PVP) and poly-(ethylene oxide) (PEO) has been investigated. The polymer surfactant complex formation is characterized by the cooperativity of the process, i.e., by the formation of surfactant aggregates (subunits), which are smaller than the ordinary micelles, being distributed along the polymer chain. The aggregation number of subunits for the PVA-NaDs complex is 12 and it increased with the counter-ion concentration. In the presence of 0.1 M excess NaNO 3 the aggregation numbers are between 40 and 50 for all the polymer—NaDS complexes concerned here. The standard free energy change of the transfer of DS-ions into surfactant complex subunits are very close to that of the micelle formation, which confirms that the main driving force of the polymer—surfactant complex formation is the micelle-like aggregation of the surfactant ions. The energy contribution arising from the interaction between the subaggregates being in the same polymer chain seems to be insignificant comparing to that of the sub-aggregate formation. The binding of the surfactants to the polymer (or the increase of the complex concentration) with increasing amount of surfactants is interrupted by the free micelle formation, because the surfactant monomer activity reaches its upper limit. In the case of the PVA—NaDS system the degree of binding is far from saturation but, at the beginning of the micelle formation, the PVP—NaDS and PEO—NaDS system approach it. The presenteòi treatment explains adequately the shape of binding isotherms of ionic surfactants by polymers, e.g. as observed by equilibrium dialysis and surface tension measurements.