Small angle neutron scattering study of temperature-independent formulation of mixed micellar structures

SANS measurements have been performed on mixed systems of ionic surfactant sodium dodecyl sulphate (SDS) and nonionic surfactant polyoxyethylene 10 lauryl ether (C12E10). The total concentration of the mixed system was kept fixed (10 wt%) and the ionic to nonionic surfactant ratio varied in the range 0 to 1. The temperature effect on the structures of mixed micelles has been studied for temperatures between 30 and 75°C. Micelles of pure ionic and nonionic surfactants show opposite trends when the temperature is increased. Sizes of pure ionic micelles decrease and those of nonionic micelles increase with increase in temperature. We show a formulation balancing these two effects which is temperature-independent and consists of about 25% of ionic surfactants in the mixed system. Contrast variation SANS measurements by contrast matching one of the surfactant components to the solvent suggest homogeneous single mixed micelles of the two components in the mixed systems.      

Premicellar and micelle formation behavior of aqueous anionic surfactants in the presence of triphenylmethane dyes: protonation of dye in ion pair micelles

The premicellar and micelle formation behaviors of four cationic triphenylmethane dyes, viz, Pararosaniline (RN), Crystal violet (CV), Ethyl violet (EV), and Malachite green (MG), in aqueous anionic surfactant solutions of sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), and sodium dodecyl sulfonate (SDSN) have been studied by spectral and surface tension measurements. The study was carried out within a pH range where the dyes are stable in their quinoid forms. The dyes have been found to form dye–surfactant ion pairs (DSIPs) with the surfactants, at the surfactant concentrations well below their critical micelle concentration, CMC*. The DSIPs behave like nonionic surfactants and form an air–water interfacial monolayer. The DSIPs have a lower critical micelle concentration (CMC_IP), greater efficiency, and lower effectiveness than the corresponding pure surfactants. As the surfactant concentration is increased below the CMC*, the DSIPs start forming micelles of their own where the dye gets protonated and exists as a protonated dye–surfactant ion pair (PDSIP) in the ion pair micelles. As the concentration of the surfactant exceeds the CMC* of the pure surfactant, the protonation reverses gradually with the dye remaining in the micelles in solubilized form and the DSIPs in the air–water interfacial monolayer are replaced by pure surfactants. The distorted helical isomeric form (isomer B) of the dyes is favored in the PDSIPs. Copyright © 2009 John Wiley & Sons, Ltd.     

Small angle neutron scattering study of mixed micelles of oppositely charged surfactants

Structures of mixed micelles of oppositely charged surfactants dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulphate (SDS) have been studied using small angle neutron scattering. The concentration of one of the components was kept fixed (0.3 M) and that of another varied in the range 0 to 0.1 M. The aggregation number and micellar size increase and fractional charge decreases dramatically with the addition of small amount of oppositely charged surfactant. The effect of addition of SDS on DTAB is significantly different from that of the addition of DTAB on SDS. The contrast variation SANS experiments using deuterated surfactant suggests the homogeneous mixing of two components in mixed micellar system.      

Spectroscopic investigations on the interaction of an anionic probe with nonionic micelles of Igepal surfactants in aqueous media

The behaviour of the anionic dye 8-anilino-1-napthalenesulfonic acid ammonium salt, or ANS, in aqueous solutions containing the Igepal series of polyoxyethylene nonionic surfactants was investigated using fluorescence spectroscopic technique. The interactions of the dye with the nonionic surfactants were examined in micellar media, to prevent dye aggregate formation and to ensure maximum dye and surfactant interaction. From the relative fluorescence enhancements, binding constants of the dye to the surfactant micelles and aggregation numbers of the micelles were determined. The aggregation numbers were also separately determined by static fluorescence quenching of pyrene by cetylpyridinium chloride in aqueous surfactant mixtures at a fixed concentration of surfactant, and compared with the value obtained from the present investigation of the interaction of the micelles with the ANS probe. The values of binding constants, micropolarity values sensed by pyrene and the Stern–Volmer constants for quenching of pyrene fluorescence by cetylpyridinium chloride were correlated with the number of ethylene oxide groups in the Igepal series.     

Photophysical and light scattering studies on the aggregation behaviour of Triton X-100 in formamide—water mixed solvents

Micelle formation and structure of the non-ionic surfactant p-tert-octyl-phenoxy (9.5) polyethylene ether (Triton X-100) in mixed solvents consisting of water and formamide have been investigated. Changes in the critical micelle concentration of the surfactant upon the addition of formamide were examined by using the pyrene 1:3 ratio method. The observed increase in the critical micelle concentration was attributed to a rise in the solubility of the surfactant as the formamide content increased in the solvent system. Micelle structure parameters were obtained as a function of the co-solvent concentration by using combined static and dynamic light scattering measurements. It was found that the decrease in the micelle size, produced by the addition of formamide, is mainly due to a reduction in the mean aggregation number rather than to changes in the magnitude of the whole solvation of micelles. This fact was also supported by the observed trend in the partial specific volume of the TX-100 micelles, obtained by complementary density measurements. However, the postulated changes in the composition of the solvation layer of micelles were supported by the rise in both the surface area per head group and the cloud point of the surfactant. From the photophysical response of different fluorescent probes incorporated in the micellar phase, we obtained information on the changes in the microstructure of Triton X-100 micelles upon the addition of formamide. The pyrene 1:3 ratio index revealed an increasing micropolarity as the formamide content increases in the solvent system. On the other hand, studies based on both fluorescence polarization of coumarin 6 and intermolecular pyrene excimer formation have shown that the microviscosity of Triton X-100 decreases with the presence of co-solvent. These results were interpreted on the basis of considerable contact of the co-solvent with the inner region of the micelles.     

Aggregation and microenvironmental properties of gemini and conventional mixed surfactants systems: A fluorometric study

Aggregation behavior of cationic gemini (hexanediyl-1,5-bis(dimethylcettylammonium bromide) (16-5-16)) surfactant with conventional single chain surfactants cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB) were studied with the help of fluorescence measurements. Fluorescence probe is a proficient technique for examining the surfactant-surfactant interaction and aggregation. The micelle aggregation number (N _agg) was measured using steady-state fluorescence quenching method. The micelle aggregation numbers of binary combinations fall between those of constituent surfactants. The micropolarity (I ₁/I ₃), binding constant (K _sv) and dielectric constant (D _exp) of mixed systems were determined from the ratio of peaks intensity in the pyrene fluorescence spectrum. The I ₁/I ₃ values were found to be more than >1, showing more polar environment around pyrene in the mixed micelle as compared to the pure micelles.     

Monitoring Micelle Formation of Nonionic Polyurethane in Water by Fluorescence Spectrophotometry

A series of nonionic polyurethanes were dissolved in water and their miceller self-assembly behaviors were characterized by aid of a fluorescence spectrophotometer. Fluorescence and resonance light scattering techniques were carried out for characterizing the micelles, and information such as aggregation number, critical micelle concentration, micropolarity, and micelle volume change was obtained. The micropolarity decreased as the aggregation number increased. The nonionic polyurethane micelle volume with two long ethoxyl chains per polyurethane molecule was smaller than the one with short chains, even with the same aggregation number, and was less polar. The micropolarity depended on both the aggregation number and the volume of a single micelle.     

Microstructure of Polyelectrolyte Nanoaggregates Studied by Fluorescence Probe Method

The microstructure of water soluble nanoaggregates based on polyelectrolyte complex formed by the cationic comb-type copolymer poly(acrylamide -co-[3- (methacryloyl-amino)propyl] trimethylammonium chloride)-graft- polyacrylamide [P(AM-co-MAPTAC)-g-PAM] and the anionic linear polyelectrolyte sodium polyacrylate (NaPA) was investigated using the fluorescence probe technique. The fluorescence probe were 1-anilinonaphthalene-8-sulfonic acid (ANS), pyrene (Py) and 1,10-bis(1-pyrene) decane (PD). The fluorescence properties in polyelectrolyte complex solutions, which are sensitive to either micropolarity (ANS, Py) or microviscosity (PD), were related to the quantities obtained in different pure or mixed solvents. Micropolarities were quantified utilizing the polarity common index (Reichardt) E _T(30). ANS and Py showed a variation of the micropolarity with the charge ratio of the two polymers, with the lowest polarity reached at the complex neutralization. The PD probe, by its excimer-to-monomer fluorescence intensities ratio, enabled us to evidence the effect of the composition and the comb-type copolymer grafting density on the microviscosity of the interpolyelectrolytes aggregates. It has been found that the microviscosity increased with the density of the grafting PAM chains.     

Protonated dye-surfactant ion pair formation between neutral red and anionic surfactants in aqueous submicellar solutions

Spectral and surface tension behavior of aqueous neutral red in the presence of sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfonate (SDSN) have been studied to understand the nature of the interactions in their submicellar concentration ranges. The variations in spectra and surface tension with variation in the concentrations of the surfactants suggest the formation of a 1:1 close-packed dye-surfactant ion pair, HNR⁺S⁻ between the acid form, HNR⁺ of the dye and the surfactant anion at very low concentrations of the surfactant below critical micelle concentration (cmc) of the pure surfactant. The dye-surfactant ion pair behaves like a nonionic surfactant having higher efficiency and lower cmc than that of the corresponding pure anionic surfactant. The ion pairs are adsorbed on the air/water interface at very low concentrations of the surfactant. As the concentration of the surfactant increases and the ion pairs form micelles of their own, the dye in the ion pair is protonated to form H₂NR²⁺S⁻. As the cmc of the pure surfactant is approached, the protonation equilibrium gradually reverses and pure surfactant ions gradually replace the ion pairs at the interface. Finally, a homogeneous monolayer of pure surfactant anions exists at the air/water interface and the dye remain solubilized in pure micelles above the cmc of the pure surfactant. The equilibrium constants, K_c for the close-packed protonated dye-surfactant ion pair (PDSIP) formation have been determined at varying pH. The submicellar interaction has been found to be stronger with SDS than SDBS. The plots of logarithm of K_c vs. pH have been found to be quite linear which consolidates the assumption of formation of the species, H₂NR²⁺S⁻. The interaction is driven by enthalpy as well as entropy.     

Nano-control of self-assembled biomolecular structures

We have controlled the structure of self-assembled systems by introducing charges (charge effect) and polymeric tails (steric effects) on a spherical–cylindrical shape of nonionic surfactant micelles. In detail, we studied the effects of a phospholipid (DL-α-phosphatidycholine dimyristol: DMPC) on the shape of nonionic surfactant micelles (penta-ethyleneglycol mono-n-dodecyl ether: C₁₂E₅), which has been studied in terms of an aggregation number, critical micellization concentration (CMC), second virial coefficient (A₂), and hydrodynamic diameter (D_H) by laser light scattering. DMPC, DOPC (DL-α-phosphatidylcholine dioleoyl), and DMPE (DL-α-phosphoethanolamin dimyristol) molecules added in C₁₂E₅ micelle solutions decrease the spontaneous curvatures, leading to an increase of the end-cap energy E_c that favors micellar growth. Based on the CMC values, the total free energy per micelle of C₁₂E₅/DMPC mixtures is estimated. The free energy per micelle of C₁₂E₅/DMPC mixtures decreases as DMPC is added. This is consistent with the decrease of A₂ and the strong hydrophobicity of DMPC compared with C₁₂E₅. The average contour length, the diffusion coefficient, and the end-cap energy of mixed micelles are estimated based on the CMC and molecular specific volumes of the moiety. The end-cap energy of the mixed micelles and the average contour length increase as DMPC is added, which is also reasonable considering the molecular structure of DMPC. Furthermore, the diffusion coefficients obtained from dynamic laser light scattering are in excellent agreement with the estimated diffusion coefficients obtained from a one-dimensional growth model based on static light scattering measurements.Charged lipid (1,2-dioleoyl-3-trimethylammonium-propane) and polymer lipid (1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-[poly(ethylene glycol)]) increase the spontaneous curvatures, resulting in breaking micelles into small size. When the lipids are added, the hydrodynamic diameter of the micelles of C₁₂E₅/lipids is nearly temperature independent due to the strong charge or steric repulsion.     

Confinement of 4,4-Diaminodiphenyl Sulfone by γ –CD in Micellar Environment: A Spectroscopic Investigation

This paper reports the double confinement of 4,4-diaminodiphenyl sulfone (Dapsone) inside γ–cyclodextrin (CD) in presence of surfactants (cationic, anionic and nonionic) using steady-state and time-resolved fluorescence spectroscopy. Interpretation of fluorescence spectra, fluorescence anisotropy and time resolved fluorescence decay of the γ-CD?Dapsone?micellar system hints at lesser microviscosity and the partial release of the probe molecule from the supramolecular host–guest complex in ionic micelles, of which greater in cationic micelles, but due to greater restriction and rigidity in presence of non-ionic micelle makes the probe more rigidly inside CD. Changes in computed rotational decay also corroborate the above findings.

An isothermal titration calorimetry study of the interactions between an oxyphenylethylene/oxyethylene diblock copolymer and sodium dodecyl sulfate

Interactions between the diblock copolymer S₁₅E₆₃ and the surfactant sodium dodecyl sulfate (SDS) have been investigated by isothermal titration calorimetry (ITC) in the temperature range 10–40°C. At 20°C, the block copolymer is associated into micelles with a hydrodynamic radius of 11.6?nm, which is composed of a hydrophobic styrene oxide (S) core and a water-swollen oxypolyethylene (PEO) corona. The copolymer/surfactant system has been studied at a constant copolymer concentration of 0.25?wt% and over a wide range of surfactant concentration, from 7.5?×?10^?6 up to 0.3?M. The titration calorimetric data for SDS in the temperature range 10–20°C presents a first endothermic increase indicating the formation of mixed copolymer rich-surfactant micelles. From that point, important differences in the ITC plots for surfactant titrations in the presence and in the absence of the copolymer are present. A shallow second endothermic peak is assigned to the interaction between SDS molecules and copolymer molecules resulting from the beginning of micelle disruption. An exothermic peak indicates the end of this disruption where only SDS micelles attached to single copolymer monomers are present, as shown by DLS in a previous paper. At higher temperatures in the range 25–40°C, the first endothermic maximum is not totally shown because interactions between surfactant and block copolymer start at very low SDS concentrations. Moreover, the second endothermic peak is absent and the exothermic minimum is less pronounced as a consequence of the increased micellization of the block copolymer.     

Co-adsorption of surfactants and propyl gallate on the hydrophilic oxide surfaces

Propyl gallate (PG) adsolubilisation in the cationic, anionic and nonionic surfactant micelles formed in the bulk solution and at the silica/solution interface has been investigated. It was found that in the absence of surfactant, propyl gallate does not adsorb on the silica surface from aqueous solution. However, in the presence of hexyltrimethylammonium bromide (CTAB), its uptake by silica significantly increases. Alumina is quite an effective adsorbent for SDS and propyl gallate and does not adsorb nonionic TX-100. The addition of PG promotes adsorption of SDS and TX-100.     

Micelles of poly(isoprene-b-2-vinylpyridine-b-ethylene oxide) terpolymers in aqueous media and their interaction with surfactants

Well-defined poly(isoprene-b-2-vinylpyridine-b-ethylene oxide) (PI-P2VP-PEO) triblock terpolymers were synthesized by anionic polymerization high-vacuum techniques. The terpolymers formed spherical three-layer (onion-type) micelles in neutral and acidic pH aqueous media as evidenced by static and dynamic light scattering. In pure water, kinetically frozen micelles with a core composed of a soft PI inner part and a hard P2VP outer shell and protected by a neutral PEO corona were formed. In acidic media the core was formed by the soft PI hydrophobic segment, whereas the corona consisted of an inner cationic polyelectrolyte P2VPH⁺ part and an outer PEO shell. The aggregation numbers were found to be high in all cases, due to the high hydrophobicity of the core-forming blocks. In the latter case an increase in size was observed due to the electrostatic repulsions between the P2VPH⁺ chains in the inner part of the corona, which is also responsible for the lower aggregation numbers observed in the acidic solutions. The interaction of these onion-type micelles with cationic (DTMAB) and anionic (SDS) surfactants led to the formation of mixed polymer/surfactant aggregates. Their structural characteristics could be varied by combining changes in surfactant type and concentration, solution pH and type of electrostatic interaction, leading to interesting, block-copolymer-based, environmentally responsive colloidal systems.     

Light scattering and fluorescence probe studies on micellar properties of Triton X-100 in KCl solutions

The effect of KCl on micelle formation and structure of Triton X-100 (TX-100) was investigated by using combined static and dynamic light scattering measurements, together with the fluorescence probe technique. An analysis of the light scattering data, including hydrodynamic radius and micellar aggregation number, accounted for both micelle growth and hydration. Fluorescence studies using pyrene as a probe were carried out to determine the critical micelle concentration (CMC) as a function of solution composition. In addition, with the aim of gaining information on the possible changes in the micro-environmental properties of TX-100 micelles, fluorescence probe studies, including intermolecular pyrene excimer formation and fluorescence polarization of coumarin 6 associated with micelles, were carried out. It was found that the addition of electrolyte induces a decrease in the CMC and an increase in both aggregation number and hydration. However, complementary data of partial specific volume and cloud point of the surfactant suggested that the main contribution to micellar hydration is due to water mechanically trapped in the micelle. Fluorescence measurements do not indicate changes in the micellar micropolarity, probably due to modifications of the solubilization site of the probe caused by the micellar growth. Both pyrene excimer formation and fluorescence polarization of coumarin 6 revealed an increase in microviscosity with electrolyte addition, which is consistent with increased micellar hydration.     

Non-ideal behavior of mixed micelles of cationic gemini surfactants with varying spacer length and anionic surfactants: A conductimetric study

Mixtures of cationic and anionic surfactants (catanionic mixtures) are often highly non-ideal, exhibiting strong synergism in their interfacial properties, manifested for instance in significant reduction of the mixture critical micelle concentration (cmc) and enhanced adsorption onto surfaces. The magnitude of such effects is of fundamental interest and has important application-related uses (e.g. in detergent formulation). In this work, the micellization process of mixtures of cationic gemini surfactants of the alkanediyl-α,ω-bis(alkyl dimethylammonium bromide) type, denoted by 12–n–12 (where n is the spacer length), with several common anionic surfactants has been investigated by electric conductivity. For the purpose of comparison, cationic–cationic mixtures, where dodecyltrimethylammonium bromide is the second cationic surfactant, have also been investigated. The cationic/anionic mixtures show relatively significant deviations from ideal behavior, depending on the structure of the gemini surfactant and the anionic surfactant. The interaction parameter β₁₂, within Rubingh's non-ideal model for mixed micelles, has been calculated for each mixture, as well as the mixed micelle composition as a function of mixture composition. The observed synergism in the different mixtures is interpreted in terms of the molecular structure of the surfactants and corresponding head–head and chain–chain interactions.     

Proton Relaxation and Spin Label Studies of Papaverine Localization in Ionic Micelles

The localization of papaverine (PAV) in micelles of zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS), cationic cetyltrimethylammonium chloride (CTAC), and anionic sodium dodecyl sulfate (SDS) in D₂O was studied by ¹H NMR and ESR in the presence and absence of 5-doxyl- or 12-doxyl-stearic acid. PAV, surfactants, and spin probes are characterized by restricted anisotropic motion in micelles. The rotational correlation time of doxyl fragment was in the range of 0.2 to 0.5 nanoseconds. Binding of PAV to micelles decreases the mobility of both probes, suggesting the localization of PAV inside the hydrophobic part of micelles near the micelle-water interface. According to the NOE data, the methoxy groups of PAV are located in the vicinity of the nitrogen atom in CTAC and HPS micelles, the methoxy groups of the PAV heterocycle being immersed slightly deeper inside the micelle. The T₁ relaxation enhancements by two different spin probes show that the H5 and methoxy substituents of the PAV heterocycle are in close proximity to the α-CH₂ of acyl chains in all types of micelles, whereas H3 and H12 are the most distant from the α-CH₂. No significant differences were found for the protonated and neutral PAV in SDS micelles at pD 4.9 and 11.2. These data show that the geometry of the PAV-micelle complex is practically independent of the PAV charge and surfactant headgroup.     

Laser synthesis of aluminium nanoparticles in biocompatible polymer solutions

Pulsed laser ablation of Aluminium (Al) in pure water rapidly forms a thin alumina (Al₂O₃) layer which drastically modifies surface plasmon resonance (SPR) absorption characteristics in deep-UV region. Initially, pure aluminium nanoparticles (NPs) are generated in water without any stabilizers or surfactants at low laser fluence which gradually transform to stable Al-Al₂O₃ core-shell nanostructure with increasing either residency time or fluence. The role of laser wavelength and fluence on the SPR properties and oxidation characteristics of Al NPs has been investigated in detail. We also present a one-step in situ synthesis of oxide-free stable Al NPs in biocompatible polymer solutions using laser ablation in liquid method. We have used nonionic polymers (PVP, PVA and PEG) and anionic surfactant (SDS) stabilizer to suppress the Al₂O₃ formation and studied the effect of polymer functional group, polymeric chain length, polymer concentration and anionic surfactant on the incipient embryonic aluminium particles and their sizes. The different functional groups of polymers resulted in different oxidation states of Al. PVP and PVA polymers resulted in pure Al NPs; however, PEG and SDS resulted in alumina-modified Al NPs. The Al nanoparticles capped with PVP, PVA, and PEG show a good correlation between nanoparticle stability and monomeric length of the polymer chain.     

Mixtures of antidepressant amphiphilic drug imipramine hydrochloride and anionic surfactant: Micellar and thermodynamic investigation

Aggregation as well as thermodynamic behavior of amphiphilic imipramine hydrochloride (IMH) drug (antidepressant) and anionic surfactant (sodium dodecyl sulfate [NaDS] as well as sodium dodecylbenzenesulfonate [NaDBS]) mixtures as a function of solution composition in aqueous solutions have been evaluated by conductometry method at different temperatures. Surfactant (NaDS and NaDBS) employed in the current study is anionic in nature. Various theoretical models such as Clint, Rubingh, Motomura, and Rodenas were employed to gain information regarding the type of interaction between the components in the solution mixtures. The value of micellar mole fraction evaluated by different utilized models is found to be more for IMH‐NaDBS mixtures in comparison with IMH‐NaDS mixtures, signifying that participation of NaDBS is more in mixed micelles as compared with NaDS. Owing to the different charge of employed drug and surfactant, which sources high synergistic results in the mixed system comparative to the possessions of their pure compounds, means here anionic surfactants were elected in favor of the quantitation of cationic drug. The value of interaction parameters (β) was also evaluated by employing the Rubingh's model. The values of Gibbs free energy (?G⁰_m) for all systems attained to be negative in all studied systems showing that the systems are spontaneous in nature.     

Characterization of mixed micelles of sodium cumene sulfonate with sodium dodecyl sulfate and cetyl trimethylammonium bromide by SANS,FTIR spectroscopy and NMR spectroscopy

The aqueous solutions of sodium cumene sulfonate (NaCS) and its mixtures with cetyl trimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) are studied by Small Angle Neutron Scattering (SANS), Fourier Transform Infrared (FTIR) spectroscopy and Nuclear Magnetic Resonance (NMR) spectroscopy. The compositions of mixed micelles are determined using Rubingh's Regular Solution Theory. NaCS when added to CTAB solution leads to the formation of long rod shaped micelles with dramatic increase in the CTAB aggregation number. Its addition to SDS on the other hand results in the formation of smaller mixed micelles where parts of SDS molecules in the micelle are replaced by NaCS molecules. NaCS–SDS mixed micelles prefer elongated ellipsoidal geometry in order to accommodate short NaCS molecules. The FTIR spectroscopy results indicate enhanced ordering of CTAB tails inside the NaCS–CTAB mixed micelles with reduction in the gauche/trans conformer ratio. Addition of NaCS to SDS on the other hand results in decreased ordering of SDS tails, as compared to SDS micelles alone. The chemical shifts observed in ¹H NMR spectra of NaCS–SDS and NaCS–CTAB mixture indicate that NaCS resides near the surface of the SDS micelle.