Negative Values Of Interaction Parameter Research Articles

Use of Flory-Huggins Interaction Parameter and Contact Angle Values to Predict the Suitability of the Drug-Polymer System for the Production and Stability of Nanosuspensions.

Use of Flory-Huggins interaction parameter and contact angle values to predict the suitability of the drug-polymer system for the production and stability of nanosuspensions. Melting point depression of the drug was measured using differential scanning calorimetry. Interaction parameter, χ, was calculated using the melting point depression data to elucidate the drug-polymer interaction strength to predict the suitability of the drug-polymer system for the production and stability of nanosuspensions. Contact angle of the drug films were measured with purified water and 0.1%w/w polymer solutions to predict polymer's suitability for the production and stability of nanosuspension. Nanosuspensions were manufactured to validate the application of the melting point depression approach along with surface property information. All three polymers, HPMC, Soluplus®, and poloxamer exhibited a negative interaction parameter with naproxen and budesonide. Higher negative interaction parameter values for the naproxen-polymer system indicated stronger drug-polymer interactions, while smaller negative interaction parameter values for the budesonide-polymer system indicated weaker drug-polymer interactions. Interaction parameter was not obtained for fenofibrate with HPMC and Soluplus®, and similarly, no interaction parameter was obtained for carvedilol with HPMC, most likely due to weaker drug-polymer interactions. All three polymers provided lower equilibrium contact angle values when compared to purified water, indicating an affinity for polymers. Successful production and stability of several nanosuspensions were correlated with Flory-Huggins's interaction parameter and contact angle values. In the absence of melting point depression, contact angle values can also be used predict the agglomeration tendencies as we have shown for this study.

Mixed Micellization, Thermodynamic and Adsorption Behavior of Tetracaine Hydrochloride in the Presence of Cationic Gemini/Conventional Surfactants.

In this approach, tensiometry and UV-visible techniques are used to determine the effect of cationic gemini and conventional surfactants on tetracaine hydrochloride (TCH), an anesthetic drug. We have estimated micellar, interfacial, and energetic constraints. To gain a deep understanding of their mixed association behavior, the outputs were examined using different theoretical models. The critical micelle concentration for single and mixed amphiphiles was estimated. The cmc values of mixed amphiphiles were found between the individual amphiphiles due to strong attractive interaction (synergism) between the components after mixing. The non-ideal behavior of mixtures was confirmed by the larger values of ideal cmc than the experimental cmc values. The negative values of interaction parameter (β) and values of activity coefficients less than unity indicate strong synergistic interaction between drug and surfactant. The stability of the mixed systems is demonstrated by the negative Gibbs free energy of micellization and excess free energy of micellization. In contrast to a single chain surfactant, a double chain surfactant (gemini) exhibits better interactions with the drug. Spectral measurements (UV-visible spectra) were used to monitor the binding of the drug with surfactant (conventional as well as gemini). Studying these mixed aggregates could help to optimize their compositions and find synergistic properties between TCH monomers and surfactants.

Influence of hydrotrope on micellization behaviour of zwitterionic dimeric surfactants carrying ammonium and phosphodiester headgroups at different temperatures in aqueous medium

Analogous novel zwitterionic dimeric surfactants in the sequence of 2-dodecylphosphate,1-(N,N-dimethyl,N-dodecylammonium)ethane abbreviated as C12(-)–2–C12(+); 2-dodecylphosphate,1-(N,N-dimethyl,N-tetradecylammonium)ethane abbreviated as C12(-)–2–C14(+); and 2-dodecylphosphate,1-(N,N-dimethyl,N-hexadecylammonium)ethane abbreviated as C12(-)–2–C16(+); with phosphodiester anion and quaternary ammonium salt as hydrophilic and the methylene chains as hydrophobic groups, two methylene groups separate the two headgroups, were prepared, and the effect of aromatic hydrotropes aniline hydrochloride (AHC) and sodium salicylate (NaSal) on the mode of micellization at four unlike temperatures with a difference of 5 K in aqueous medium by the method of conductivity were studied. The decrease in cmc of zwitterionic surfactants with the addition/increase in bulk mole fraction of NaSal and AHC is almost linear. But anionic NaSal is more productive as compared to the cationic hydrotrope AHC in reducing the critical micelle concentration (cmc). Negative values of interaction parameter (βm) were found to decline with extension in one of the hydrophobic chain lengths keeping the other hydrophobic chain length identical. Symmetrical zwitterionic surfactants interact more strongly via headgroups and hydrocarbon tails with aromatic hydrotropes and decrease the cmc effectively by concealing of headgroups and hydrophobic forces operating mutually thus promotes micellar shape transition by alteration of micelle surface curvature. The enthalpy of micellization ΔmicH° suggest an endothermic process and the negative value of free energy of micellization ΔmicG° due to encouraging augmentation in entropy of micellization ΔmicS°. The doping of surfactants with organic salts or hydrotropes amplifies the solubilizing power, viscosity and subdue the cmc value. Such systems promise to improve the performance used in separation methods, drug permeation, and delivery as these surfactants are less toxic and less sensitive to changes in pH and ionic strength.

Mixed micelles of sodium perfluorooctanoate and imidazolium based ionic liquids in aqueous solution: A SANS and Tensiometric study

Micellization and microstructure of mixed micelles from mixtures of an anionic fluorocarbon surfactant sodium perfluorooctanoate (NaPFO) and cationic alkylmethyl-imidazolium based non-amphiphilic and amphiphilic ionic liquids (ILs) with different alkyl chain length (C4, C6, C8, C10 and C12) and anions viz. chloride (Cl−), tetrafluoroborate (BF4−), hexaflurophosphate (PF6−), octylsulphate (C8SO4−) and trifluoromethane-sulphonate (CF3SO3−) were examined extensively by surface tension and small-angle neutron scattering (SANS) measurements. Low critical micelle concentration (CMC) and large negative value of interaction parameter (β) showed synergism in all the mixed systems and it depends on the molecular characteristics of ILs. Thermodynamic parameters for mixed monolayers and mixed micelles were evaluated and discussed. Micellar growth, shape changes and micelle to vesicle transitions obtained from these oppositely charged mixed systems depend on the surface activity of ILs (alkyl chain and counter ions). Influence of ILs which differ in counter ions with similar chain length and similar counter ions with different chain lengths on aggregation patterns of NaPFO micelles have been examined using SANS. The data for size, shape and aggregation number for aggregates formed by these mixed systems evaluated from the fitted SANS data are reported and discussed in terms of their distinct interaction with NaPFO micelles. The present study is a significant addition to existing knowledge on ionic liquid induced morphological changes of sodium perfluorooctanoate micelles.

Resolving Liquid-Liquid Phase Separation for a Peptide Fused Monoclonal Antibody by Formulation Optimization.

Liquid-liquid phase separation (LLPS) of protein solutions has been usually related to strong protein-protein interactions (PPI) under certain conditions. For the first time, we observed the LLPS phenomenon for a novel protein modality, peptide-fused monoclonal antibody (pmAb). LLPS emerged within hours between pH 6.0 to 7.0 and disappeared when solution pH values decreased to pH 5.0 or lower. Negative values of interaction parameter (kD) and close to zero values of zeta potential (ζ) were correlated to LLPS appearance. However, between pH 6.0 to 7.0, a strong electrostatic repulsion force was expected to potentially avoid LLPS based on the sequence predicted pI value, 8.35. Surprisingly, this is significantly away from experimentally determined pI, 6.25, which readily attributes the LLPS appearances of pmAb to the attenuated electrostatic repulsion force. Such discrepancy between experiment and prediction reminds the necessity of actual measurement for a complicated modality like pmAb. Furthermore, significant protein degradation took place upon thermal stress at pH 5.0 or lower. Therefore, the effects of pH and selected excipients on the thermal stability of pmAb were further assessed. A formulation consisting of arginine at pH 6.5 successfully prevented the appearance of LLPS and enhanced its thermal stability at 40°C for pmAb. In conclusion, we have reported LLPS for a pmAb and successfully resolved the issue by optimizing formulation with aids from PPI characterization.

Catanionic mixtures of surface-active ionic liquids and N-lauroyl sarcosinate: Surface adsorption, aggregation behavior and microbial toxicity

The surface activity and aggregation behavior of catanionic mixtures of imidazolium- or pyridinium-based surface-active ionic liquids (SAILs) and sodium N-lauroyl sarcosinate (Na-LS) in aqueous solution were investigated. The effects of the alkyl chain length, the polar head group and functional groups in the SAIL molecule on the interfacial properties and self-assembly of the catanionic mixtures were evaluated by surface tension and dynamic light scattering measurements. In addition, the toxicity of the catanionic surfactant mixtures against bacteria and fungi was studied. The SAIL-LS mixed systems reduced water surface tension with high effectiveness and efficiency, and had a lower critical aggregation concentration compared to the individual components. In all the catanionic systems investigated, synergistic effects caused by strong electrostatic and hydrophobic interactions resulted in negative interaction parameter values, which increased with the alkyl chain length and in the order of non-functionalized < amide-functionalized < ester-functionalized SAILs. The structure of the mixed aggregates in the SAIL-LS catanionic systems depended on the hydrophobicity of the SAIL, the micelle-vesicle transition being driven by the alkyl chain length. The catanionic systems exhibited similar microbial toxicity to that of individual SAIL components and behaved like broad-spectrum antimicrobials.

Concentration-dependent physicochemical behaviors and micellar interactions in polyalkoxylated fatty alcohol-based binary surfactant systems

The concentration-dependent physicochemical behaviors of the binary mixtures of polyalkoxylated fatty alcohol (PAFA)-AS (alkyl sulfonate), PAFA-CB (cocamidopropyl betaine) and PAFA-APG (alkyl polyglucosides) and their micellar interactions were determined at the ratios of 9:1, 8:2, 7:3 and 6:4 (w/w). At concentrated level, the rheological study of all the PAFA-mixed surfactants display a Newtonian behavior at low viscosities. Prominent discrepancies among the mixed surfactant systems were observed after stepwise dilution of the samples. In tensiometry study, the types of mutual interaction in the mixed surfactant systems were evaluated by regular solution theory (RST). The CMCexp curves of PAFA-AS and PAFA-CB systems demonstrate lower position than their CMCtheo curves signifying that these nonionic–anionic and nonionic–zwitterionic samples behave nonideal showing synergistic interaction between individual components in the mixed micelles, as further verified by the negative values of interaction parameter β and activity coefficients f1 and f2 are less than 1. Meanwhile, the PAFA-APG system shows no significant mutual interaction between the nonionic–nonionic components in the mixed micelles. TEM micrographs depict the formations of branched wormlike micelles and multiconnected threadlike networks for the PAFA-AS and PAFA-CB systems, respectively, while gigantic multilamellar vesicles for the PAFA-APG system. Upon dilution of these mixed surfactants to CMC, the mixed micelles are in monodisperse sizes of less than 15 nm and in various magnitudes of zeta potential ranging from –2.92 to –21.87 mV.

Mixed micellization behavior of benzyldimethylhexadecylammonium chloride with gemini surfactants and their applications in the synthesis of silver nanoparticles

Aggregation and adsorption properties of benzyldimethylhexadecylammonium chloride (BDHAC) with three cationic alkanediyl-1,s-bis(tetradecyldimethyl-ammonium bromide) gemini surfactants, viz. butanediyl-1,4-bis(tetradecyldimethylammonium bromide), pentanediyl-1,5-bis(tetradecyldimethylammonium bromide) and hexanediyl-1,6-bis(tetradecyldimethylammonium bromide) respectively referred to as 14-4-14, 14-5-14 and 14-6-14 (in general 14-s-14, where s = 4, 5, 6) have been studied by tensiometric, conductometric, fluorimetric and dynamic light scattering experimental methods at 298.15 K. The studies were conducted at various compositions of the mixed systems ranging between 0 and 1. The parameters determined include different physicochemical and interaction parameters. The negative values of interaction parameters obtained for mixed micelle formation point out the attractive interaction among the components and lead to non-ideality which has been theoretically treated by Clint’s and Rubingh’s models. The BDHAC + 14-s-14 mixtures in different mole fractions were used in the synthesis of very stable silver nanoparticles (Ag NPs) in aqueous media, which were characterized by UV-visible spectroscopy. Dynamic light scattering measurements revealed that the size of micelles as well as Ag NPs increases with the increase in mole fraction of BDHAC in the mixtures and the size of Ag NPs can be very much controlled by using mixtures of surfactants.

Interaction study of mixed micellar system of isoquinoline based surface active ionic liquids and cationic surfactant in aqueous medium

Surface-active ionic liquids (SAILs) can modify the physiological properties of conventional surfactants. Mixed micelles of isoquinoline-based SAILs, dodecylisoquinolinium bromide [C12iQuin][Br], tetradecylisoquinolinium bromide [C14iQuin][Br] and cationic surfactant, dodecyltrimethylammonium bromide (DTAB) in aqueous medium and their interactions have been studied by employing conductometry and 1H NMR technique. The critical micelle concentration (cmc) and various thermodynamic parameters have been calculated from conductometry measurements. Mixed micellar parameters such as ideal cmc (cmc*), micellar mole fraction ( $$ {X}_1^m $$ ), micellar interaction parameter (βm) and activity coefficients (f1) and (f2) of component 1 (SAIL) and component 2 (DTAB) have been evaluated by applying Clint, Rubingh and Motomura theoretical models. The interaction between SAILs and surfactant has been found synergistic and non-ideal. The peak merging and broadening of protons suggests the growth of micelles. Use of SAILs as an additive effectively reduce the cmc that is indicated by negative value of interaction parameter (βm).

Experimental and theoretical investigation of micellization behavior of sodium dodecyl sulfate with cetyltrimethylammonium bromide in aqueous/urea solution at various temperatures

Mixed micelle formation behavior of cationic surfactant-cetyltrimethylammonium bromide (CTAB) and anionic surfactant sodium dodecyl sulfate (SDS) in aqueous as well as in urea medium from 303.15 K to 323.15 K at 5 K interval was carried out by conductometric method. The differences between the experimental values of critical micelle concentrations (cmc) and ideal critical micelle concentrations (cmcid) illustrate the interaction between the amphiphiles studied. The values of micellar mole fraction ( $$\text{X}_1^{Rub}$$ (Rubingh), $$\text{X}_1^{M}$$ (Motomura), $$\text{X}_1^{Rod}$$ (Rodenas) and $$\text{X}_1^{id}$$ (ideal) of surfactant CTAB determined by different proposed models and outcome indicate high involvement of CTAB in SDS-CTAB mixed micellization, which enhance by means of the augment of mole fraction of CTAB. The negative value of interaction parameter (β) showed an attractive interaction involving CTAB and SDS. Activity coefficients were less than unity in all case, which also reveals the presence of interaction between CTAB & SDS. The negative $$\Delta{\text{G}}_m^0$$ values imply the spontaneous mixed micellization phenomenon. The attained values of $$\Delta{\text{H}}_m^0$$ were positive at inferior temperature, while negative at superior temperature. The negative $$\Delta{\text{H}}_m^0$$ values in urea (NH2CONH2) medium illustrate exothermic micellization process. The magnitudes of $$\Delta{\text{S}}_m^0$$ were positive in almost all cases. The excess free energy of mixed micelle formation (ΔGex) was found to be negative, which indicates the stability of mixed micelle as compared to the individual’s components micelles.

Enhanced foamability and foam stability of polyoxyethylene cholesteryl ether in occurrence of ionic surfactants

The micellization and foam studies of binary surfactant mixtures assembled of nonionic polyoxyethylene cholesteryl ether (ChEOn; where n = 30), cationic hexadecyltrimethylammonium bromide (CTAB) and anionic sodium dodecylbenzenesulfonate (SDBS) was investigated by surface tension, foaming behavior and viscosity measurements. As an exceptional outcome it was observed that foam stability and foamability of nonionic surfactant ChEO30 in mixtures with ionic surfactants is much superior to the individual ChEO30, CTAB or SDBS components. Moreover, the mixed surfactant systems (ChEO30+CTAB) and (ChEO30+SDBS) have much lower critical micellar concentrations (CMCs) than corresponding sole surfactants. The negative values of interaction parameter (β) estimated from Rubingh’s theory specify synergism amongst mixed ChEO30+CTAB and ChEO30+SDBS surfactants systems.

Mixed micellization of novel cationic ester-bonded gemini surfactants: Investigations by conductometric and tensiometric measurements

The micellization and interfacial properties of three homologous dicationic ester-bonded cleavable and biodegradable gemini surfactants, ethane-1,2-diyl bis(N,N-dimethyl-N-alkylammoniumacetoxy) dichlorides, referred as m-E2-m (m=12, 14, 16), and their binary mixtures were investigated in aqueous solution by performing conductivity and surface tension measurements. The decrease in critical micelle concentration (CMC) indicates nonideality of the mixed systems of different compositions. The unequal hydrophobic chain length might be playing a significant role in the reduction of CMC values. Several theoretical treatments were used to analyze and compare the mutual interaction parameters, counter ion binding, surface parameters, excess free energy of micellization and standard free energies for the mixed surfactant systems as well. The negative values of interaction parameters show an overall attractive force, i.e., synergistic effect in the mixed state. The excess free energy of mixing has negative values for all the systems.

Interaction of an Amphiphilic Drug and Sodium Bis(2-ethylhexyl)sulfosuccinate at Low Concentrations in the Absence and Presence of Sodium Chloride

In the present study mixed micelles of the amphiphilic phenothiazine drug promazine hydrochloride (PMZ) and the anionic surfactant sodium bis(2-ethylhexyl)sulfosuccinate (AOT) have been studied using conductometry at different compositions and temperatures in aqueous as well as aqueous electrolyte (25 mmol·kg−1 NaCl) solutions to determine various physicochemical properties of the solutions. The experimental and ideal critical micelle concentration (CMC and CMCid) values suggest attractive interactions in PMZ–AOT mixed systems. The negative values of interaction parameter (β) obtained from regular solution theory indicate synergistic interactions; the negative values of β decrease further with the addition of salt. From the CMC values obtained as a function of temperature, various other thermodynamic properties have been evaluated.

Micellization behavior of mixtures of amphiphilic promazine hydrochloride and cationic aniline hydrochloride in aqueous and electrolyte solutions

We studied the influence of cationic hydrotrope aniline hydrochloride on the micellization behavior of cationic amphiphilic phenothiazine drug promazine hydrochloride in the presence and absence of 50mmol kg−1 NaCl. The experimental critical micelle concentration (CMC) values came out to be lower than ideal CMC (CMC id ) values, signifying attractive interactions between the two components in mixed micelles. NaCl further decreases the CMC of pure PMZ and aniline hydrochloride as well as their mixture due to screening of the electrostatic repulsion among the polar head groups. The bulk properties of solution were examined by using different theoretical models for justification and comparison of results. The micellar mole fraction of aniline hydrochloride (X1 Rub , X1 M , X1 Rod and X1 id ) was evaluated by different proposed models, showing greater contribution of hydrotrope in mixed micelle. The negative values of interaction parameter (β) indicate synergistic interactions and negative values of β further decrease by the addition of salt in mixed systems. From the CMC values as a function of temperature, various thermodynamic properties have been evaluated and discussed in detail.

Study of Mixed Micelles of Promethazine Hydrochloride (PMT) and Nonionic Surfactant (TX-100) Mixtures at Different Temperatures and Compositions

Abstract Herein, we have investigated the interaction between a phenothiazine drug promethazine hydrochloride (PMT) and nonionic surfactant t-octylphenoxypolyethoxyethanol (TX-100) in aqueous solutions using a conductometric technique at different temperatures and compositions. The evaluated critical micelle concentration (cmc) values are lower than cmc id values suggesting attractive interactions of mixed micelles components. It is observed that with the increase in temperature, the cmc value increases first and after the value decreases at higher temperature. At 298.15 K, the maximum cmc values were attained in presence or absence of TX-100. The bulk properties of solution were studied by means of different theoretical models reported in the literature such as Clint, Rubingh, Motomura, and Rodenas for explanation and comparison of results of different binary mixtures of the drug and TX-100. The negative values of interaction parameter (β) obtained from the regular solution theory (RST) recommend synergistic interactions. Activity coefficients (f 1 and f 2) calculated by all theoretical models used herein were obtained to be always below unity indicating nonideality in all binary mixtures. Thermodynamic parameters (like standard Gibbs energy (ΔG°m), enthalpy (ΔH°m), and entropy (ΔS°m)) are also evaluated which propose dehydration of hydrophobic portion of the drug at higher temperatures. The above results obtained may be helpful in model drug delivery systems.

Micellization and Surface Properties of Alkanediyl-1,4-bis(tetradecyldimethylammonium Bromide) Gemini Surfactants with Dodecylethyldimethylammonium Bromide

Mixed micellization and surface properties of three cationic gemini surfactants, viz., (butanediyl-), (pentanediyl-), (hexanediyl-)-1,4-bis(tetradecyldimethylammonium bromide) (14-s-14; s = 4–6) with dodecylethyldimethylammonium bromide have been studied by conductometric, tensiometric, and fluorimetric techniques at 298 K. Values of cmc, ideal cmc(cmc*), degree of counter ion dissociation (g), micellar mole fractions of the components, and interaction parameters have been determined. Further, activity coefficients of the components, surface excess concentration (Γmax), minimum surface area per molecule (Amin), surface pressure at the cmc (πcmc), average aggregation numbers of the micelles (Nagg), excess Gibbs free energy (), and Gibbs free energy of micellization () were also calculated and discussed. The compositions of the mixed systems were varied between 0 and 1. It is observed that the values of cmc, g, , , and Γmax decreases whereas Amin increases with augmentation of 14-s-14 (s = 4–6) mole fraction in the mixed systems. The negative values of interaction parameters (βσ and βm) obtained for mixed micelle formation indicate attractive interaction between the components leading to nonideality. The nonideal behavior has been theoretically treated by Clint's and Rubingh's model in light of regular solution theory.

Interacting behavior between amino sulfonate amphoteric surfactant and octylphenol polyoxyethylene ether (7) in aqueous solution and pH effect

The micellization properties of binary mixtures of sodium n-dodecyl diamine sulfonate (C12AS) and octylphenol polyoxyethylene ether (7), OP-7, in aqueous solution at the pH of 3.5, 6.0, and 8.5 have been characterized mainly employing both the UV–vis spectroscopy method, using pyrene as a probe, and the tensiometric method. The interaction parameters between two surfactants, the activity coefficients, the molar fractions of components in mixed micelles, and thermodynamic parameters obtained using some theoretical treatments including the regular solution theory (RST), the phase separation model, the Rubingh model, Maeda's treatment, and so on, were evaluated and the effect of pH on the above parameters was also discussed. Reduction of the mixed critical micelle concentration and negative values of interaction parameter indicate synergistic effect between two surfactants for all mixed systems. The charge density and steric effect of head group of C12AS may play a vital role on the formation of mixed micelle, and the values of pH in aqueous solution can influence the ionic form of C12AS and this steric effect, thus resulting in different interacting behaviors between two surfactants. Thermodynamic data show that the process of micellization for all mixed systems is spontaneous, the stability of mixed micelle is mainly dependent on the components in bulk solution, and the more stable mixed micelle can form in neutral or weak acidic solution.

Interaction Between Cationic Gemini and Monomeric Surfactants: Micellar and Surface Properties

Attraction effects in the mixed micelle of gemini (C16-12-C16, 2Br− and monomeric surfactants i.e., cetyltrimethylammonium bromide (CTAB), cetyltriphenylphosphonium bromide (CTPB) and cetyltributylphosphonium bromide (CTBuPB) mixtures in aqueous solution were investigated by conductivity and surface tension measurements. The attractive interaction of surfactants in the mixed micelle leading to non-ideality has been theoretically treated by Clint’s and Rubingh’s model in the light of regular solution theory. The experimental critical micelle concentration (cmc) values were lower than predicted from ideal mixture theory. The negative values of interaction parameters (m and for mixed micelle formation indicate the attractive interaction for all the systems. The Motomura’s theory was used to determine the composition of each compound in mixed micelles (Xideal. The excess free energy of the mixed systems was also calculated and the obtained negative values for all the mixed systems studied give an indication that the mixed micelles are thermodynamically more stable relative to the individual component

Micellar and Interfacial Behavior of Cationic Benzalkonium Chloride and Nonionic Polyoxyethylene Alkyl Ether Based Mixed Surfactant Systems

AbstractMicellar and interfacial properties of mixed surfactant systems comprising benzalkonium chloride, a cationic surfactant and nonionic polyoxyethylene alkyl ether surfactants (POE: C10E7, C10E8, C10E9, C10E10) have been investigated by surface tension, fluorescence and dynamic light scattering techniques. Critical micelle concentration (CMC) for different mixing mole fractions has been investigated by surface tension and fluorescence measurements. Ideal CMC, mixed micellar composition (X1m, X1σ), interaction parameters for mixed micelles (βm) and adsorption monolayer (βσ), surface excess concentration (Гmax), minimum area per molecule (Amin) and related thermodynamic properties have also been determined. Lowering of the CMC and negative interaction parameter values indicate synergism in the mixed micelle and monolayer formed, whereas, thermodynamic parameters evaluated for the proposed mixed systems indicate stability of the resulting micelles and monolayer. Micellar aggregation number (Nagg) and hydrodynamic diameter (Dh) computed from fluorescence and dynamic light scattering measurements respectively illustrate micellar growth in the mixed state. Results obtained for the proposed mixed systems can be helpful in designing smart materials for industrial surfactant based formulations.

Self-assembly of onion-like vesicles induced by charge and rheological properties in anionic–nonionic surfactant solutions

Surface and bulk properties of mixed nonionic tetraethylene glycol monododecyl ether (C12EO4) and dialkyl anionic sodium bis(2-ethyl hexyl)sulfosuccinate (AOT) mixtures in aqueous solution were studied and self-assembly of onions (multilamellar vesicles) with uncountable bilayers induced by charge were obtained. An attractive net interaction exists between the two surfactant molecules and a negative interaction parameter values (βm and βs) indicate a synergy in the non-ideal mixing behavior. The onion-like structures, i.e., the densely packed multilamellar vesicles with uncountable bilayers are directly observed in a bluish sample solution by charging the swollen nonionic lamellar phases with addition of anionic surfactant, AOT. The addition of ionic surfactants into the nonionic lamellar phase, through electrostatic repulsion of the ionic headgroups, will suppress the Helfrich undulation and induce the transition from planar lamellae to vesicles. Similarly, in an AOT-rich region, upon addition of the nonionic ones, the planar lamellar gel phases of AOT–H2O mixture can also be transformed into closed vesicle gel-phases due to the reduction of the Coulombic force. Rheological measurements show that both of the storage modulus (G′) and the loss modulus (G′′) increase with the increase of membrane charge density. Interestingly, the addition of NaCl into the mixed systems causes the phase transition from micelles to vesicles. On increasing the amount of salts, the bilayers of the lamellar phase exhibit more flexibility due to the electrostatic shielding of electrolyte, along with a decrease of the solution viscosity, the G′, and the G′′. It is believed that the ionic charges play crucial roles in the flexibility of bilayer membranes. The obtained multilamellar architectures are excellent for the long-term storage of multiple components and can effectively slow their release.