α-Sulfonato palmitic acid methyl ester-hexaoxyethylene monododecyl ether mixed surfactant system: Interfacial,thermodynamic, and performance property study

Interfacial, thermodynamic, and performance properties of aqueous binary mixtures of α-sulfonato palmitic acid methyl ester, C₁₄H₂₉CH(SO₃Na)COOCH₃(PES), and hexaoxyethylene monododecyl ether, CH₃(CH₂)₁₁(OCH₂CH₂)₆OH (C₁₂E₆), were investigated with tensiometric, conductometric, fluorimetric, and viscometric techniques. The critical micelle concentration (CMC), maximum surface excess, minimum area per molecule of surfactant at the air/water interface, and the thermodynamics of micellization and adsorption were determined. The CMC was very low for mixed systems, indicating probable use as a detergent with less effect on the environment because of surfactant biodegradability and less amount in the environment. The interaction parameter β^m, computed by using the theory of Rubingh and Maeda, indicated an attractive interaction (synergism) between the surfactant molecules, which was also confirmed by proton nuclear magnetic resonance studies in the mixed micelle. The micellar aggregation number (N _agg), determined by using a steady-state fluorescence quenching method at a total surfactant concentration of about ∼10 mM at 25°C, was almost independent of the surfactant mixture composition. The micropolarity and the binding constant (K _sv) for the C₁₂E₆/PES mixed system were determined by the ratio of the intensities (I ₁/I ₃) of the pyrene fluorescence emission spectrum, and the local microenvironment inside the micelle was found to be polar. The viscosity of the mixed system at all mole fractions suggested that mixed micelles are nonspherical in nature. The cloud point of oxyethylene group-containing surfactants was increased by the addition of PES. Foaming was temperature dependent, and a 1∶1 mixed system showed minimum foaming. All performance properties were composition dependent.     

Effects of sodium glycocholate and sodium taurocholate on the mixed micelles of bile salts and nonionic surfactant

Effects of sodium glycocholate (NaGC) and sodium taurocholate (NaTC) on the mixed micelles for two systems consisting of NaGC-octaoxyethylene glycol monon-decyl ether (C₁₀E₈) and NaTC-C₁₀E₈ have been studied as a function of the mixed micelles’ compositions, polarities of the micelles’ interior and mean aggregation numbers. The compositions of the mixed micelles are calculated from critical micelle concentration (CMC) data by using excess thermodynamic quantities. The polarities and mean aggregation numbers are determined from pyrene fluorescence in the mixed micelles. Both mixed systems were nonideal, and the mole fraction of NaGC or NaTC in a mixed micelle near the CMC was less than that in the aqueous mixed solution. However, the mixed micelle of the NaTC-C₁₀E₈ system contained more bile salt molecules than that of the NaGC-C₁₀E₈ system because of a good miscibility of NaTC and C₁₀E₈ molecules. The pyrene fluorescence results suggested that the mixed micelles changed from C₁₀E₈-rich micelles to NaGC- or NaTC-rich micelles, and mean aggregation numbers of the mixed micelles decreased abruptly with increasing mole fraction of bile salts. In the low mole fraction range of bile salts, however, both the polarities and the mean aggregation numbers for the NaTC-C₁₀E₈ system are lower than those for the NaGC-C₁₀E₈ system because of the high mole fraction of NaTC in a mixed micelle, and also because of the different effect of the conjugated group between NaTC and NaGC molecules in the mixed micelles.     

Synthesis and characterization of amphiphilic graft copolymers with poly(ethylene glycol) and cholesterol side chains

Amphiphilic graft copolymers comprising monomeric units of methoxy poly(ethylene glycol) (mPEG)-acrylate, 2-hydroxyethyl methacrylate (HEMA)–cholesterol conjugates and HEMA were synthesized and their properties characterized. The value of the critical micelle concentration (CMC) for these copolymers is linearly proportional to the ratio of the number of mPEG–acrylates to that of the HEMA–cholesterol conjugates per macromolecule (N_PEG/N_c), which is the most important parameter which influences the formation of polymeric micelles. The latter show excellent colloidal stability and their sizes decrease with increasing CMC. Based on the quenching of pyrene fluorescence, the relatively high levels of the loading capacity of pyrene are attributed to the elevated hydrophobicity of the micelle core. The loading capacity of pyrene decreases with increasing CMC. The weight-average partition coefficient for pyrene in polymeric micelles increases with increasing polymer concentration because more micelles are available for accommodating pyrene. Copyright © 2004 Society of Chemical Industry     

Synergistic Behavior Between Zwitterionic Surfactant α-Decylbetaine and Anionic Surfactant Sodium Dodecyl Sulfate

Here, we present experimental surface tension isotherms of mixed solutions of a zwitterionic surfactant α-decylbetaine (DB) and an anionic surfactant sodium dodecyl sulfate (SDS) in different molar ratios. These mixed solutions show a composition dependency with respect to both surface tension effectiveness and critical micelle concentration. The pseudo-regular solution theory has been used to evaluate the interaction parameters in the micelle, β ^m and at the surface, β ^s. The results revealed that the mixed solutions of DB/SDS behave synergistically in both surface tension reduction effectiveness and mixed micelle formation at all mole fractions investigated. The values of adsorption area per surfactant molecule at air/solution interface were estimated, which provides some useful information on evaluating the interaction between DB and SDS in mixed adsorbed monolayers. The solubilization behaviors of toluene in DB/SDS mixed solutions were also investigated to help in understanding the structure of mixed micelles of DB and SDS.     

Interaction between bisphenol A and micelles of ionic surfactants

Interactions between bisphenol A (BPA) and ionic surfactants—cationic hexadecyltrimethylammonium bromide (HTAB) and anionic sodium dodecylsulfate (SDS)—were studied by measuring interfacial tensions and the intensities of pyrene fluorescence. The critical micellar concentrations (CMC) decreased with an increase in the BPA concentration, and the degree of that decrease was greater in HTAB than in SDS. In those micelles, BPA interacted more strongly with HTAB than with SDS. Conversely, BPA adsorbed on the air-water interface cooperatively with the surfactants, even though almost no adsorption of BPA itself was observed. This cooperative adsorption was more enhanced with HTAB than with SDS. Thus, BPA worked on the surfactants to stabilize the micelles and interfacial adsorption. The stability gained by the addition of BPA was greater on the interface than in the micelle. This was evidence of decreased hydrophilicity of the head group of the ionic surfactant, which interacted with BPA, because this decrease acted on the surface activities of the surfactants more directly than on the micelle stabilities. Pyrene fluorescence measurements yielded identical results for the effect on micelle stabilities. It is noteworthy that the fluorescence intensity of peak 1, l ₁, decreased with an increase in BPA concentrations at constant concentrations of surfactant greater than the CMC, but the peak ratio, l ₁/l ₃, remained almost unchanged. This fact was also related to the interaction of BPA with the hydrophilic head groups in the surfactant micelle.     

Interaction Between Ionic Liquids and Gemini Surfactant: A Detailed Investigation into the Role of Ionic Liquids in Modifying Properties of Aqueous Gemini Surfactant

Tuning physicochemical properties of aqueous surfactant solutions comprised of normal or reverse micelles by external additives is of utmost importance due to the enormous application potential of surfactant‐based systems. Unusual and interesting properties of environmentally benign ionic liquids (IL) make them suitable candidates for this purpose. To understand and establish the role of IL in modifying properties of aqueous gemini surfactants, we studied the effect of the IL, 1‐hexyl‐3‐methylimidazolium bromide ([Hmim][Br]) and 1‐octyl‐3‐methylimidazolium bromide ([Omim][Br]) on the properties of the aqueous cationic gemini surfactant 1,6‐hexanediyl‐α,ω‐bis(dimethyltetradecyl)ammonium bromide (14‐6‐14,2Br^?). The behavioral changes were investigated by measuring the critical micelle concentration (CMC) using electrical conductance, surface tension, dye solubilization and fluorescence probe measurements at 298.15 K. It was observed that the CMC of 14‐6‐14,2Br^? gemini surfactant decreases with addition of IL, thus favoring the micellization process. An increase in micellar size was observed at lower IL concentration using dynamic light scattering, with a decrease in aggregation number (N_agg) determined from fluorescence probe quenching measurements. It is noteworthy that the extent of modulation of the micellar properties is different for both the IL due to their structural differences. IL behave like electrolytes at lower concentrations and cosurfactants at higher concentrations and form mixed micelles with the cationic gemini surfactant showing an increase in N_agg.     

Cationic Micelles Modulated in the Presence of α,ω-Alkanediols: A SANS,NMR and Conductometric Study

The solution behavior of a typical cationic surfactant, tetradecyltrimethylammonium bromide, in mixed solvent systems composed of water and varying concentrations of α,ω-alkanediols; 1,2-ethanediol (ED), 1,4-butanediol (BD), 1,6-hexanediol (HD) and 1,8-octanediol (OD) was examined via electrical conductance measurements, ¹³C-NMR spectroscopy and small angle neutron scattering (SANS) measurements. The critical micelle concentration (CMC) values and degree of counterion dissociation (α) indicate that both ED and BD oppose micellization, whereas HD and OD enhance micelle formation. Changes in the ¹³C-NMR chemical shifts (∆δ values) reveal that the short chain diols reside almost exclusively in the bulk phase and hence, affect the formation of micelles by altering the solvent properties in the bulk of the solution, whereas HD and OD partition between the pseudomicellar phase and the bulk phase. SANS studies indicated that both the micellar size and aggregation number (N _agg) decrease in the presence of all diols. ED and BD behave like cosolvents and increase the α and CMC values and decrease N _agg. We note that the effect of HD and OD on the properties of the micelles is concentration dependent; at low concentrations, these diols interact with the micelles and behave as cosurfactants (as evidenced by the trends in the micellar properties), while at higher concentrations, they enhance the surfactant solubility and behave as a cosolvent.     

CMC and dynamic properties of poly(VA-<Emphasis Type="Italic">b</Emphasis>-St) copolymer micelles for drug delivery

The polymeric micelles from amphiphilic block copolymer poly(vinyl alcohol-b-styrene) (poly(VA-b-St)) with different syndiotacticity of poly(vinyl alcohol) (PVA) block were prepared by dialysis against water. Critical micelle concentration (CMC) and dynamic properties of poly(VA-b-St) copolymeric micelles were investigated by fluorescence techniques. From the fluorescence emission spectrum measurements using pyrene as a fluorescence probe, the observed CMC value was in the range of 0.125–4.47 mg/L. The CMC value increased with decreasing the weight ratio of PS to PVA block and with increasing the syndiotacticity of PVA block. The rate of pyrene release was very slow for block copolymers containing PVA block with higher syndiotacticity, which indicates that their micelles have increased kinetic stability. This work was presented at 13 ^th YABEC symposium held at Seoul, Korea, October 20–22, 2007.     

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

Micellar 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 (X ₁ ^m , X ₁ ^σ ), interaction parameters for mixed micelles (β ^m) and adsorption monolayer (β ^σ ), surface excess concentration (Г_max), minimum area per molecule (A _min) 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 (N _agg) and hydrodynamic diameter (D _h) 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.     

Micellization Behavior of Mixtures of Sodium Dioctylsulfosuccinate with Sodium Dodecylsulfate in Water

Mixtures of sodium dioctylsulfosuccinate (AOT) and sodium dodecylsulfate (SDS) that were studied in water at 25 °C by using surface tension, conductance, emf and fluorescence emission methods exhibit synergism in the region where the mole fraction of AOT in the bulk solution (α ₁) is less than 0.7 and ideality in the region where α ₁ ≥ 0.7. The molal conductance versus the concentration behavior of an aqueous solution of AOT is found to be different from that of other ionic surfactants with the exception of bile salts. Composition of the mixed micelle was evaluated and discussed using the Rubingh’s and the Rodenas–Valiente–Villafruela (RVV) treatments. The values of the counter ion binding constant determined from the emf data show that the counter ion binding behavior of the mixed micelle is controlled entirely by AOT. The free energy for mixed micelle formation was calculated using a modified equation. The aggregation number determined by the fluorescence quenching method indicated that in the mixed micelle, as α ₁ increases, the number of molecules of AOT remains constant and that of SDS decreases. Characteristics of the adsorption layer of the mixed surfactant system were also examined using the theoretical treatment of Rosen and Hua.

Aggregation Behavior of Perfluorononanoic Acid/Sodium Dodecyl Sulfate Mixtures

The mixed system of an anionic hydrocarbon surfactant, sodium dodecyl sulfate, and a perfluorinated surfactant, perfluorononanoic acid (PFNA), was investigated by a combination of methods. The critical micellar concentrations (CMC) were determined over a wide range of sample compositions by surface tension, conductivity and UV–visible spectrophotometry using N-(4-nitrophenyl)perfluorononanamide as a molecular probe. The values of CMC obtained by different techniques were in good agreement. In addition, the aggregation numbers were determined in the mixtures with a low content of hydrocarbon surfactant, by measuring the fluorescence quenching of pyrene. The hydrodynamic radii of the aggregates were estimated through the diffusion ordered ¹⁹F- and ¹H-NMR spectroscopy. The values obtained are in agreement with those expected according to the measured aggregation numbers. The analysis of the data with different aggregation models suggests the formation of a non-ideal mixed micelle that is enriched in the perfluorinated surfactant when its mole fraction increases, and that is practically formed by PFNA only at mole fractions higher than 0.8.     

Physiochemical insight into the solution behavior of cationic gemini surfactant in water and ethanol–water systems

Surfactants in water and both alcohol-water mixed solutions are used extensively in a host of industrial applications. This work presents the solution behavior and micellar transition of a cationic gemini surfactant (GS): N,N′-dihexadecyl-N,N,N′,N′-tetramethyl-N,N′-ethanediyl-diammonium dibromide (16-2-16) in water and mixed water-ethanol media. Phase behavior for 16-2-16 in the ethanol–water system was investigated at ambient temperature. The rheological data obtained for these systems at varying alcohol concentrations showed that the system viscosity (η) decreased with as the ethanol concentration increased. Small-angle neutron scattering (SANS) was used to probe the structural details of the cationic micelles as a function of ethanol concentration and temperature. The scattering data inferred a structural transition from unilamellar vesicles (ULV) through rod-like micelles to ellipsoidal micelles occurs that is dependent on the solvent composition and temperature indicating the behavior of ethanol molecules as a cosolvent in the process of micelle breaking. The plausible physicochemical interactions in the 16-2-16-ethanol mixed system were further investigated using a computational simulation study employing density functional theory (DFT)/B3LYP (Gauss View 5.0.9) utilizing a 3-21G basis set.     

Interactions Between an Anionic Fluorosurfactant and a PEO-PPO-PEO Triblock Copolymer in Aqueous Solutions

Surface tensions were determined for a mixture of an anionic fluorinated surfactant and a PEO-PPO-PEO triblock copolymer. The interactions between the two surfactant molecules in the mixed monolayer and the mixed micelle were studied through molecular interaction parameters (β ^σ, β ^M) and the molecule exchange energy (ε, ε _m). It was noted that synergism and strong attractive interactions took place between the anionic fluorinated surfactant and the triblock copolymer molecules in both mixed micelles and mixed monolayers, reflected by the interaction parameter values of between −10 and −18 for all mixtures investigated. Moreover, it can be seen from the value of (ε − ε _m) that when the mixture has a small amount of triblock copolymer, the formation of mixed micelle results in a greater decrease in energy than does the formation of a mixed monolayer. With an increase in the mole fraction of the triblock copolymer in the mixture, in order to obtain the lowest surface energy, surfactants tend to form mixed monolayers first, and then form mixed micelles.     

Mixed micelles of some metal dodecyl sulfates and triton X-100 in aqueous media

Tensiometric studies on several binary surfactant mixtures containing anionic surfactants, viz., metal (lithium, sodium, potassium, copper, cobalt, and magnesium) dodecyl sulfates and a nonionic surfactant (Triton X-100) in water at different mole fractions (0–1) provide critical micelle concentration (CMC) values. The composition of mixed micelles and the interaction parameter, β, evaluated from the CMC data for different systems using Rubingh's theory, are discussed. Marked interaction is observed with monovalent dodecyl sulfates. The influence of counter-ion valence on the formation of mixed micelles was investigated for anionic-nonionic systems, and results indicated that mixed systems with bivalent counter-ions in metal dodecyl sulfate resembled nonionic-nonionic systems where weak/negligible interaction has been reported. Salt addition revealed the weakening of interaction in the mixed systems, which is attributed to the head group charge neutralization and the dehydration of the ethylene oxide units of the nonionic surfactants. A few cloud point and viscosity data are also reported.     

Sodium Lauryl Ether Sulfate Micellization and Water Solubility Enhancement Towards Naphthalene and Pyrene: Effect of the Degree of Ethoxylation

The effect of ethoxylation on self-aggregation behavior in aqueous media of sodium lauryl ether sulfate (SLES), C₁₂H₂₅ (OCH₂CH₂)_xOSO₃Na, where x = 1 (SLE1S), x = 2 (SLE2S), and x = (SLE3S) was investigated. CMCs were determined from the effect of surfactant concentration on (2,2′-bipyridine dichloro-ruthenium(II) hexahydrate (Ru(bipy)₃²⁺,2Cl⁻) fluorescence emission. Aggregation numbers and micelle concentrations were estimated from the fluorescent quenching of (Ru(bipy)₃²⁺,2Cl⁻) by 9-methylanthracene. Interestingly, critical micelle concentrations (0.80 mM), average aggregation numbers (N = 43), and micelle concentrations ([M] = 40 μM) were found to be independent of the degree of ethoxylation. Water solubilization enhancement of naphthalene and pyrene by SLES was also investigated. Water solubility enhancement for naphthalene and pyrene was observed with increasing degrees of ethoxylation and follow the order SLE3S > SLE2S > SLE1S. The higher impact of oxyethylene (OE) groups on solubilization enhancement of naphthalene over pyrene was taken as an indication for a preferential distribution of the relatively polar organic naphthalene molecule at the outer layer of the micelle in close contact with the hydrated oxyethylene units, thereby allowing for a strong polycyclic aromatic hydrocarbon (PAH)—micelle interaction. On the other hand, pyrene molecule is preferentially distributed in the core of the micelle, relatively far away from the OE units located at the micelle-water interface for PAH-micelle interaction to occur. Water solubility enhancement for both PAHs was also was associated with an increase in SLES micelle size with the degree of ethoxylation as well as to the accessibility of the locus solubilization for the solute.     

The Clouding of an Anionic Surfactant in Acid Solution: Mechanistic and Analytical Implications

The clouding of sodium dodecyl sulfate (SDS) in strongly acidic solutions has seen analytical use, but its mechanism has generally been misinterpreted. In the present work it was found that as SDS slowly hydrolyzes to form dodecanol, the solution passes through a series of compositions at which the aggregation of surfactant is promoted by nucleation onto traces of insoluble dodecanol. This occurred at concentrations well below the critical micelle concentration of SDS and resulted in mixed aggregates that grew to macroscopic size, giving the solution a cloudy appearance. The increasing dodecanol content eventually caused coalescence into a coacervate phase which evolved into a solid layer of dehydrated dodecanol. The process, which continued over an extended period, depended on the temperature and the concentration and type of acid used. The early stages of SDS aggregation were monitored through the I ₁/I ₃ ratio of pyrene fluorescence, which confirmed the existence of micelle-like aggregates at low surfactant concentration. The mixed SDS/dodecanol systems formed in acid hydrolysis were mimicked in neutral solution by combining the appropriate amounts of SDS, dodecanol, and NaCl. Clouding and the formation of a coacervate phase generally proceeded in a similar manner in these solutions.     

Selection of surfactant pairs for optimization of interfacial properties

Guidelines are provided for the selection of surfactant pairs when synergism in various interfacial properties in aqueous media is desired. To maximize the reduction of the critical micelle concentration, the two surfactants should show strong attractive interaction in the mixed micelle; in order to maximize efficiency in surface tension reduction, strong interaction in the mixed monolayer at the aqueous solution/air interface (large negativeβ values is needed). The more surface-active material should predominate in the mixture. When interaction is not strong, the two surface-active materials used should have approximately equal surface activities and should be used at equimolar concentration in the aqueous phase. To minimize the surface tension (γ) of the solution, the surfactant-surfactant attractive interaction in the mixed monolayer at the aqueous solution/air interface must exceed that in the mixed micelle. Optimization can be achieved by using two surfactants with approximately equal γ values at their respective critical micelle concentrations (CMC’s). When these γ values are not equal, the surfactant with the higher γ value at its CMC should have the smaller area/molecule at the surface. The greater the difference between attractive interaction at the interface and in the micelle, the lower the value of the surface tension.     

Interaction Between an Anionic and an Amphoteric Surfactant. Part I: Monomer–Micelle Equilibrium

A mixture of anionic and amphoteric surfactants is composed of three components at intermediate pH levels: anionic, cationic (protonated amphoteric), and zwitterionic (unprotonated amphoteric). Knowledge of the composition of each surfactant in both monomer and micellar forms (monomer–micelle equilibrium) is important in applications using this mixture. Hydrogen ion titration of the mixed surfactant solution as a function of surfactant composition is combined with the pseudophase separation model and regular solution theory for the three-surfactant mixture to calculate the concentration of each surfactant in monomer and in micelle forms at different pH levels. The specific systems studied here contain sodium dodecyl sulfate (SDS) and dimethyldodecylamine oxide (DDAO), which are used in a wide range of consumer products. The degree of protonation of monomeric DDAO is not affected by the presence of SDS, indicating an insignificant formation of ion pairs between these monomers. However, the presence of SDS in micelles shifts the micellar pK _a of DDAO protonation significantly and the method used here allows the quantification of partial fugacities of each individual surfactant in micelle form. The composition in the monomer phase at each pH will aid in understanding and predicting solution compositions corresponding to anionic/amphoteric surfactant precipitation boundaries, which is the focus of the subsequent paper in this series.

Small angle neutron scattering study of structural aspects of nonionic surfactants (Tween 20 and Tween 80) in the presence of polyethylene glycols and triblock polymers

Small angle neutron scattering (SANS) technique has been used to study the micellar behavior of nonionic surfactants, Tween 20 and Tween 80 with additives like polyethylene glycols (PEG with molecular mass 400, 6000, and 15,000) and triblock polymers (TBPs) of varying composition. Surfactant‐additive interactions have been explained on the basis of parameters like aggregation number (N_agg), core radius (R_c), hard sphere radius (R_hs), volume fraction (ϕ) and axial ratio (b/a). The SANS analysis indicate the reduction in values of N_agg of Tween on addition of PEG additive. Shape of Tweens (3 wt %) micelles in the presence of PEG (10 wt %) is found to oblate ellipsoidal. Similarly, the shape of Tween (3 wt %) micelles is oblate ellipsoidal at low concentration of TBPs (1 wt %); however, they become spherical as the concentration of TBP increases to 10 wt %. The shape of micelles of pure TBPs also comes out to be spherical. Results reflects that at low concentration of TBP shape is controlled by surfactant (Tween 20 and Tween 80) while at high concentration of TBP shape of mixed micelle is controlled by TBP. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010