Aggregation Behavior Of Surfactants Research Articles

Study of thermodynamic properties of sodium dodecyl sulphate in aqueous solutions of alkoxyalkanols at different temperatures

Densities, ρ, speed of sound, u , for aqueous solutions of alkoxyalkanols like ethylene glycol mono methyl ether (EGMME), ethylene glycol mono ethyl ether (EGMEE) and ethylene glycol mono butyl ether (EGMBE) have been measured in aqueous solutions of surfactant sodium dodecyl sulphate (SDS) at temperatures T=(288.15, 298.15, 308.15 and 318.15)K have been measured. The different parameters such as apparent molar volume, limiting apparent molar volume, transfer volume, partial molar expansibility have been derived from density data to study the nature of interactions and also the aggregation behavior of surfactant with alkoxyalkanols. Experimental speeds of sound data were used to estimate apparent molar isentropic compression, limiting apparent molar isentropic compression, partial molar isentropic compression of transfer. The pair and triplet interaction coefficient have been calculated from both the properties. These parameters have been discussed in the light of ion-ion and ion-solvent interactions.

Numerical simulations of the effect of ionic surfactant/polymer on oil–water interface using dissipative particle dynamics

AbstractEffects of the ionic surfactant (cetyltrimethylammonium bromide) and polymer (styrene‐maleic acid copolymers) on the oil/water interface are studied using dissipative particle dynamics at the mesoscopic scale. Effects of the water content on equilibrated morphology of the oil/water emulsion are studied with the help of ionic surfactant. In addition, oil/water emulsion has the patterns of the compound emulsifier at the very low and very high water content. The interfacial tension increases, reaches maximum, and then decreases with an increase of water content. Both experiments and simulations show that the inorganic salts can improve the interfacial efficiency of the ionic surfactants, and lower the interfacial tension. The influence of polymer number is studied on their ability to reduce interfacial tension. The distributions of interfacial tension and mean interfacial density are predicted. The lower the interfacial tension and faster oil coalescence are, the more polymers are. In addition, the influence of the polymer on the surfactant aggregation behavior of the oil–water interface is discussed. © 2016 Curtin University of Technology and John Wiley & Sons, Ltd.

Adsolubilization phenomenon perceived in chitosan beads leading to a fast and enhanced malachite green removal

Chitosan (CS) is a well recognized cationic polyelectrolyte, widely used as an adsorbent. However, CS hydrogel bead lacks its ability to adsorb cationic dyes because of the obvious repulsion between cationic CS and the dye of same charge. In this work CS hydrogel beads are modified by sodium dodecyl sulfate (SDS), an anionic surfactant, under various conditions to improve its efficiency for the adsorption of malachite green (MG), a cationic dye. In the first part, the complex forming ability of surfactant (at <<CMC) with counter ion polyelectrolyte is utilized to fabricate modified chitosan beads, which are designated as surfactant–polyelectrolyte-complex (SPEC). In the second part of the adsorbent preparation, micelle aggregation behavior of surfactant (at >>CMC) is used to modify the surface charges and solubilization properties of the modified adsorbent. These beads are called as chitosan–surfactant–core–shell (CSCS). The mechanism of formation for both is discussed. Besides these modified beads, the CS–SDS composite material (CSC) is also prepared. The adsorptive removal of MG by all the prepared beads has been evaluated. The CSCS beads are found to be the best (adsorption capacity: 360mg/g) for MG removal. Such an enhanced adsorption is believed to be due to ‘adsolubilization’ of MG on surfactant bilayer, which has been realized for the first time in CS bead support. The kinetics of removal also is much faster compared to CS beads. The efficiency of CSC for MG removal is higher compared to that of CS beads. The adsorption behavior of CS beads in binary mixture of MG and SDS has also been examined.

Modulation of partition and localization of perfume molecules in sodium dodecyl sulfate micelles.

The influence of perfume molecules on the self-assembly of the anionic surfactant sodium dodecyl sulfate (SDS) and their localization in SDS micelles have been investigated by ζ potential, small angle X-ray scattering (SAXS), one- and two-dimensional NMR and isothermal titration microcalorimetry (ITC). A broad range of perfume molecules varying in octanol/water partition coefficients P are employed. The results indicate that the surface charge, size and aggregation number of the SDS micelles strongly depend on the hydrophobicity/hydrophilicity degree of perfume molecules. Three distinct regions along the log P values are identified. Hydrophilic perfumes (log P < 2.0) partially incorporate into the SDS micelles and do not lead to micelle swelling, whereas hydrophobic perfumes (log P > 3.5) are solubilized close to the end of the hydrophobic chains in the SDS micelles and enlarge the micelles with higher ζ potential and a larger aggregation number. The incorporated fraction and micelle properties show increasing tendency for the perfumes in the intermediate log P region (2.0 < log P < 3.5). Besides, the molecular conformation of perfume molecules also affects these properties. The perfumes with a linear chain structure or an aromatic group can penetrate into the palisade layer and closely pack with the SDS molecules. Furthermore, the thermodynamic parameters obtained from ITC show that the binding of the perfumes in the intermediate log P region is more spontaneous than those in the other two log P regions, and the micellization of SDS with the perfumes is driven by entropy.

Supramolecular self-assembly between an amino acid-based surfactant and a sulfonatocalixarene driven by electrostatic interactions

Abstract Complex self-assembled structures can be built up in aqueous media by making use of non-covalent interactions between small organic molecules and conventional amphiphiles. The macrocycle p -sulfonatocalix[4]arene (SC4) is a known receptor for organic ammonium cations in aqueous solution, showing strong binding ability for those guests and selectivity for several amino acids. Previous studies have shown that this type of water-soluble calixarenes are able to modify the aggregation behavior of surfactants. Here, we explore the interactions and morphologies present in mixtures of SC4 and a cationic serine-based surfactant, resorting to surface tension, light microscopy, cryo-scanning electron microscopy and nuclear magnetic resonance studies. Complexation of the amino acid-based surfactant by the calixarene leads initially to mixed micelle formation and a significant lowering of the critical micelle concentration with respect to the neat surfactant even for very low content of SC4 (up to 1.4 mol%). Interestingly, as the SC4 fraction in the system is gradually increased (about 2–6 mol%), highly flexible tubular structures and vesicles are assembled. The observed self-assembly is rationalized in terms of electrostatic complexation between the two co-solutes and the preferred packing of the supramolecular complexes formed.

Surface properties and aggregation behavior of cationic gemini surfactants with dipropylammonium head-groups

Abstract A series of cationic gemini surfactants, butane-α,δ-bis-(dipropyl alkyl ammonium bromide), [CnH2n+1(CH3CH2CH2)2N(CH2)4N(CH3CH2CH2)2CnH2n+1]Br2 (where n is the tail chain length, n = 10, 12, 14, and 16), referred to as CnC4Cn(Pr) were synthesized. The surface properties and aggregation behavior of these surfactants were investigated by measuring their surface tension, electrical conductivity, fluorescence, dynamic light scattering (DLS), and transmission electron microscopy (TEM). These gemini surfactants CnC4Cn(Pr) have a lower critical micelle concentration (CMC) than their corresponding monomer surfactants. Solution electric conductivity measurements indicated that premicellar aggregations were found in the C16C4C16(Pr) solution. Steady-state fluorescence measurements revealed that the C16C4C16(Pr) surfactant formed noncompact aggregate structures in solution over a wide concentration range. Interestingly, dynamic light scattering and transmission electron microscopy studies showed that the aggregations of the four surfactants were vesicle, which is not in accordance with the critical packing parameter (P) values obtained from Tanford's equation. To explain this unusual phenomenon, the volume of the component groups was used to calculate the hydrophobic volume and it was assumed that this phenomenon probably resulted from the face-to-face hydrophobic chains of these gemini surfactant molecules were inclined to be interdigitized in the aggregate.

Aggregation behavior of a gemini surfactant with a tripeptide spacer.

A peptide gemini surfactant, 12-G(NH2)LG(NH2)-12, has been constructed with two dodecyl chains separately attached to the two terminals of a glutamic acid-lysine-glutamic acid peptide and the aggregation behavior of the surfactant was studied in aqueous solution. The 12-G(NH2)LG(NH2)-12 molecules form fiber-like precipitates around pH 7.0, and the precipitation range is widened on increasing the concentration. At pHs 3.0 and 11.0, 12-G(NH2)LG(NH2)-12 forms soluble aggregates because each molecule carries two positively charged amino groups at the two ends of the peptide spacer at pH 3.0, while each molecule carries one negatively charged carboxyl group in the middle of the peptide spacer at pH 11.0. 12-G(NH2)LG(NH2)-12 displays a similar concentration-dependent process at these two pHs: forming small micelles above the critical micelle concentration and transferring to fibers at pH 3.0 or twisted ribbons at pH 11.0 above the second critical concentration. The fibers formed at pH 3.0 tend to aggregate into bundles with twisted structure. Both the twisted fibers at pH 3.0 and the twisted ribbons at pH 11.0 contain β-sheet structure formed by the peptide spacer.

The role of hydroxyethyl groups in the construction of wormlike micelles in the system of quaternary ammonium surfactant and sodium salicylate.

To understand the role of electrostatic interactions and hydrogen bonds in the formation of wormlike micelles with the aid of sodium salicylate, two quaternary ammonium surfactants with the headgroup decorated by one hydroxyethyl group N-cetyl-N-(2-hydroxyethyl)dimethylammonium bromide and two hydroxyethyl groups N-cetyl-N,N-di(2-hydroxyethyl)methylammonium bromide, abbreviated as CHEMAB and CDHAB, respectively, were synthesized in this work. Single crystal X-ray diffraction was used to study the intermolecular interactions of surfactants, and (1)H NMR and rheological measurements were employed to investigate the molecular arrangement and morphology of the wormlike micelles. The synergistic interactions of hydrogen bonding and more effective shielding of electrostatic repulsion contribute to the formation and viscoelastic behavior of wormlike micelles. The results also revealed the aggregation behavior of surfactants with hydroxyethyl headgroups in aqueous solutions.

Specific ion effects on the self-assembly of ionic surfactants: a molecular thermodynamic theory of micellization with dispersion forces.

The self-assembly of amphiphilic molecules is a key process in numerous biological and chemical systems. When salts are present, the formation and properties of molecular aggregates can be altered dramatically by the specific types of ions in the electrolyte solution. We present a molecular thermodynamic model for the micellization of ionic surfactants that incorporates quantum dispersion forces to account for specific ion effects explicitly through ionic polarizabilities and sizes. We assume that counterions are distributed in the diffuse region according to a modified Poisson-Boltzmann equation and can reach all the way to the micelle surface of charge. Stern layers of steric exclusion or distances of closest approach are not imposed externally; these are accounted for through the counterion radial distribution profiles due to the incorporation of dispersion potentials, resulting in a simple and straightforward treatment. There are no adjustable or fitted parameters in the model, which allows for a priori quantitative prediction of surfactant aggregation behavior based only on the initial composition of the system and the surfactant molecular structure. The theory is validated by accurately predicting the critical micelle concentration (CMC) for the well-studied sodium dodecyl sulfate (SDS) surfactant and its alkaline-counterion derivatives in mono- and divalent salts, as well as the molecular structure parameters of SDS micelles such as aggregation numbers and micelle surface potential.

Supercritical or Compressed CO2 as a Stimulus for Tuning Surfactant Aggregations

Surfactant assemblies have a wide range of applications in areas such as the chemical industry, material science, biology, and enhanced oil recovery. From both theoretical and practical perspectives, researchers have focused on tuning the aggregation behaviors of surfactants. Researchers commonly use solid and liquid compounds such as cosurfactants, acids, salts, and alcohols as stimuli for tuning the aggregation behaviors. However, these additives can present economic and environmental costs and can contaminate or modify the product. Therefore researchers would like to develop effective methods for tuning surfactant aggregation with easily removable, economical, and environmentally benign stimuli. Supercritical or compressed CO(2) is abundant, nontoxic, and nonflammable and can be recycled easily after use. Compressed CO(2) is quite soluble in many liquids, and the solubility depends on pressure and temperature. Therefore researchers can continuously influence the properties of liquid solvents by controlling the pressure or temperature of CO(2). In this Account, we briefly review our recent studies on tuning the aggregation behaviors of surfactants in different media using supercritical or compressed CO(2). Supercritical or compressed CO(2) serves as a versatile regulator of a variety of properties of surfactant assemblies. Using CO(2), we can switch the micellization of surfactants in water, adjust the properties of reverse micelles, enhance the stability of vesicles, and modify the switching transition between different surfactant assemblies. We can also tune the properties of emulsions, induce the formation of nanoemulsions, and construct novel microemulsions. With these CO(2)-responsive surfactant assemblies, we have synthesized functional materials, optimized chemical reaction conditions, and enhanced extraction and separation efficiencies. Compared with the conventional solid or liquid additives, CO(2) shows some obvious advantages as an agent for modifying surfactant aggregation. We can adjust the aggregation behaviors continuously by pressure and can easily remove CO(2) without contaminating the product, and the method is environmentally benign. We can explain the mechanisms for these effects on surfactant aggregation in terms of molecular interactions. These studies expand the areas of colloid and interface science, supercritical fluid science and technology, and chemical thermodynamics. We hope that the work will influence other fundamental and applied research in these areas.

Aggregation Behavior of Surfactants with Different Molecular Structures in Aqueous Solution: DPD Simulation Study

The aggregation behavior of surfactants with different molecular structures in aqueous solution was investigated by dissipative particle dynamics (DPD) coarse-grained model simulation. With a simple surfactant model, we investigated how variations in micelle size and structures of surfactants influence their aggregation behavior. The obvious effects on the aggregation properties of surfactants caused by the variation of the hydrophobic or hydrophilic groups and the branch structures were investigated by corresponding structure charts in aqueous solution. The properties such as critical micelle concentration (CMC), cluster size, and density distribution of surfactants were analyzed at the mesoscopic level. Simulation results showed that the CMC value decreases as the length of the hydrophobic tail increases, while the cluster size and the density of tail increase. Conversely, an increase in the size of the head groups was found to enhance the CMC. It is also found that the surfactants with branch in tails are more efficient in forming micelles than linear ones. This is because the stronger excluded volume interactions make branched surfactants require more space at the surface and get more efficient. Additionally, branch in hydrophilic parts, including bola types of surfactant, leads to higher concentration needed to form micelles in aqueous solution and the cluster size are greatly larger than linear ones because of their peculiar structures.

Aggregation of sodium dodecylsulfate in aqueous nitric acid medium

Nitric acid medium is invariably used for nitration of organic molecules. Although surfactants are known to influence reaction rates, little is known about the aggregation behavior of surfactants in nitric acid medium. Micellization characteristics of sodium dodecylsulfate (SDS) in aqueous nitric acid are investigated in this work by using the conductance method. The critical micelle concentration (cmc) and the aggregation number were also determined by the surface tension and the steady-state fluorescence methods, respectively. This study reveals that in acidic medium SDS exhibits both normal and unusual conductivity behaviors. Equations developed on the basis of the mixed electrolyte model, Debye–Hückel–Onsager approach, and the pseudophase ion-exchange model successfully simulate the conductivity data. The exchange of sodium and hydrogen counterions at the micellar surface has no significant effect on the cmc of SDS. Acid concentration, surfactant concentration, and cmc control the competitive binding of sodium and hydrogen counterions. Analysis of conductivity data revealed hydrolysis of about 12% SDS when [HNO3]⩾0.02moldm−3. Hydrolysis of SDS has been confirmed by nitrating some of the substituted phenols. It has been predicted that SDS+aqueous HNO3 medium with [HNO3]⩾0.02moldm−3 may be used as a green medium for nitration.

Mesoscopic Simulations on the Aggregation Behavior of Polymeric Surfactants in Aqueous Solutions

Polymeric surfactants are widely used in many fields.Their aggregation behavior is different from that of small molecule surfactants because of complex configurations and large molecular weights.The studies of aggregation behavior at the micro-level can guide applications and therefore,many researchers have focused on theoretical investigations.With the recent advances in computer simulations,the study of surfactant aggregation behavior in aqueous solutions at the micro-or meso-level has been successfully undertaken.Based on our recent work,we review the aggregation behavior of polymeric surfactants by dissipative particle dynamics (DPD) and mesoscopic dynamics (MesoDyn).This paper especially introduces research about the phase behavior of polymeric surfactants as well as their interactions with low molecular weight surfactants.This method can directly provide the process of phase separations and changes in the conformation of the aggregates,which are not observable in macro-experiments.This method has the potential to complement and guide experimental methods.

The influence of hydrophobically modified cyclodextrins (HM-CD) on the aggregation behaviour of surfactants

Cyclodextrins and hydrophobically modified β-cyclodextrins (HM-CD) form inclusion complexes with hydrophobic molecules, such as alkylcarboxylic acids or hydrocarbons. HM-CD are also able to form micelles above a critical concentration. In this work, an effect is described which is not based on this known behaviour. This work shows the influence of HM-CD on the aggregation behaviour of viscoelastic surfactant solutions in the L1-phase. For the measurements, several surfactants were used which show very strong viscoelastic properties. The zero shear viscosity of these solutions decreases by adding HM-CD at a molar ratio of surfactant/HM-CD of 100:2 from a very high value (103 Pa·s) nearly to the viscosity of water. The high ratio of surfactant/HM-CD shows that this effect cannot depend on the formation of inclusion complexes. Light scattering and small angle neutron scattering experiments showed that the addition of HM-CD causes a transition of worm-like micelles to spherical micelles in these solutions. The HM-CD is solubilized in the interior of the micelles. With the solubilization into the micelles, a single CD-molecule is able to influence the aggregation behaviour of many surfactant molecules and thereby the shape of the micelles. HM-CD have therefore the same effect on micellar solutions, like solubilized hydrocarbons.

Aggregation Behavior of Surfactants in Polymer Gel Networks

Aggregation behavior of anionic sodium dodecyl sulfate and a nonionic surfactant, polyoxyethylene (p = 10) nonylphenyl ether, in the inside and outside of polymer hydrogels has been studied through solubilization techniques of an oil-soluble dye, measurements of surfactant concentration in the aqueous phase of the gel interior, and binding isotherms of the surfactants onto polymer chains of the gel. The surfactant aggregation is strongly affected by the presence of polymer gel networks. Interestingly, it is found that the nonionic surfactant cannot form micelles inside the gel at concentrations greater than even the critical micelle concentration in the outer bulk solution. The anionic surfactant molecules adsorb onto the polymer chains of a relatively hydrophobic N-isopropylacrylamide gel but not onto the hydrophilic acrylamide (AAm) gel. In the AAm gel, the anionic surfactant forms micelles in the aqueous solution phase inside the gel, avoiding the exclusive space of the polymer chains.

Theory of Surfactant Aggregation in Water/Ethylene Glycol Mixed Solvents

The aggregation behavior of surfactants in mixed solvents composed of water and ethylene glycol is predicted using an extension to our theory of aqueous surfactant solutions. The extension accounts for the dependence of (i) the surfactant tail transfer free energy, (ii) the aggregate core-solvent interfacial free energy, and (iii) the headgroup interaction free energy on the composition of the mixed solvent. As the proportion of ethylene glycol in the mixed solvent increases, the model predicts an increase in the critical micelle concentration (cmc), a decrease in the average aggregate size, an increase in the aggregate polydispersity, and a stronger dependence of the average aggregation number on the total surfactant concentration. The model reveals that (a) large values for cmc originate primarily from the smaller magnitude of the surfactant tail transfer free energy in the mixed solvent, (b) the aggregation numbers are small mainly because of the smaller magnitude of the hydrocarbon-mixed solvent interfacial tension, and (c) neither the cmc nor the aggregate size are greatly affected by the lower dielectric constant of the mixed solvent. The predicted aggregation characteristics of cetyl pyridinium bromide, cetyl trimethylammonium bromide, and sodium dodecyl sulfate at various compositions of the mixed solvent are presented for illustrative purposes and are compared with available measurements.

Field-Dependent NMR Relaxation Study of Aggregation and Dynamics in Dilute to Concentrated Micellar Decylammonium Chloride Solutions

The surfactant aggregation behavior in aqueous decylammonium chloride solutions for concentrations between 5 and 29 wt % was studied by 2H nuclear magnetic relaxation dispersion in the 0.5−77-MHz frequency range. Below the second cmc the micelles can be described as polydisperse fairly small aggregates undergoing unhindered Brownian rotation. Above the second cmc the micelles are rod shaped and grow with increasing concentration, and as a consequence the tumbling of the micelles eventually becomes restricted by neighboring aggregates, which is demonstrated by the low-frequency relaxation data. The restricted reorientation of the micelles in the concentrated solution has been modeled as rotational diffusion in a hard-walled double cone potential of mean torque.

Physical−Chemical Properties of the n-Octyl β-d-Glucoside/Water System. A Phase Diagram, Self-Diffusion NMR, and SAXS Study

The n-octyl β-d-glucoside/water binary phase diagram (temperature vs concentration) and the aggregation parameters of the individual phases have been determined. n-Octyl β-d-glucoside forms four different phases together with water. At temperatures below 22 °C, there is an isotropic solution region from neat water extending up to almost 60 wt % n-octyl β-d-glucoside. As the concentration is further increased, three liquid crystalline phases form in the following order: hexagonal, cubic, and lamellar. At high surfactant concentrations (>93 wt %) the lamellar phase is in equilibrium with hydrated crystals. The hexagonal phase disappears at temperatures slightly higher than 20 °C. The solution region has been investigated with 1H-NMR to deduce the self-diffusion coefficients of both n-octyl β-d-glucoside and water. From these results it has been possible to draw conclusions about the surfactant aggregation behavior at dilute concentrations and when the n-octyl β-d-glucoside concentration is increased. The w...