Micellar Aggregation Number Research Articles

A systematic coarse-graining strategy for semi-dilute copolymer solutions: from monomersto micelles

A systematic coarse-graining procedure is proposed for the description and simulation ofAB diblock copolymers in selective solvents. Each block is represented by a small number,nA ornB, of effective segments or blobs, containing a large number of microscopic monomers.nA andnB are unequivocally determined by imposing that blobs do not, on average, overlap,even if complete copolymer coils interpenetrate (semi-dilute regime). Ultra-softeffective interactions between blobs are determined by a rigorous inversionprocedure in the low concentration limit. The methodology is applied to anathermal copolymer model where A blocks are ideal (theta solvent), B blocksself-avoiding (good solvent), while A and B blocks are mutually avoiding. Themodel leads to aggregation into polydisperse spherical micelles beyond a criticalmicellar concentration determined by Monte Carlo simulations for several size ratiosf of the two blocks. The simulations also provide accurate estimates of the osmotic pressureand of the free energy of the copolymer solutions over a wide range of concentrations. Themean micellar aggregation numbers are found to be significantly lower than those predictedby an earlier, minimal two-blob representation (Capone et al 2009 J. Phys. Chem. B 113 3629).

Intrinsic Parameters for the Structure Control of Nonionic Reverse Micelles in Styrene: SAXS and Rheometry Studies

Shape, size, and internal structure of nonionic reverse micelle in styrene depending on surfactant chain length, concentration, temperature, and water addition have been investigated using a small-angle X-ray scattering (SAXS) technique. The generalized indirect Fourier transformation (GIFT) method has been employed to deduce real-space structural information. The consistency of the GIFT method has been tested by the geometrical model fittings, and the micellar aggregation number (N(agg)) has been determined. It was found that diglycerol monocaprate (C(10)G(2)), diglycerol monolaurate (C(12)G(2)), and diglycerol monomyristate (C(14)G(2)), spontaneously self-assemble into reverse micelles in organic solvent styrene under ambient conditions. The micellar size and the N(agg) decrease with an increase in surfactant chain length, a scenario that could be understood from the modification of the critical packing parameter (cpp). A clear picture of one-dimensional (1-D) micellar growth was observed with an increase in surfactant weight fraction (W(s)) in the C(10)G(2) system, which eventually formed rodlike micelles at W(s) ≥ 15%. On the other hand, micelles shrunk favoring a rod-to-sphere type transition upon heating. Reverse micelles swelled with water, forming a water pool at the micellar core; the size of water-incorporated reverse micelles was much bigger than that of the empty micelles. Model fittings showed that water addition not only increase the micellar size but also increase the N(agg). Zero-shear viscosity was found to decrease with surfactant chain but increase with W(s), supporting the results derived from SAXS.

Fluorescence investigation of a specific structure formed by aggregation of transglycosylated stevias: Solubilizing effect of poorly water-soluble drugs

Spray-dried particles (SDPs) with indomethacin (IND) and alpha-glycosyl transferase-treated stevia (Stevia-G) indicated extremely high dissolution rates and apparent solubility compared to particles of a ground mixture and a physical mixture of IND/Stevia-G. The apparent solubility of IND from SDPs was 5-fold higher than that of untreated IND. We investigated the solubility-enhancing effect of Stevia-G towards hydrophobic materials by using fluorescence spectroscopy. 1-Naphthoic acid (1-NPA) and pyrene were used as hydrophobic fluorescence probes. Although both the ground mixture and SDPs of 1-NPA/Stevia-G exhibited halo patterns in powder X-ray diffraction, the fluorescence spectra of 1-NPA solely in SDPs indicated a monomolecularly dispersed state of 1-NPA. The plot of the pyrene I 1/ I 3 ratio versus the Stevia-G concentration showed a sigmoidal curve as a function of the Stevia-G concentration, suggesting the existence of a hydrophobic environment around pyrene molecules under high Stevia-G concentrations. The critical micelle concentration calculated from the pyrene I 1/ I 3 plot was about 16 mg/mL. Based on results from the static quenching plots, the micellar aggregation number of Stevia-G was estimated as ca.15. Therefore, the hydrophobic steviol-skeleton of Stevia-G made a hydrophobic core around a hydrophobic molecule. This specific structure formed by Stevia-G molecules led to an enhancement of the apparent solubility of poorly water-soluble drugs.

Surface adsorption and aggregate formation of nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF 6)

Surface adsorption and aggregation behavior of polyoxyethylene (POE)-type nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF 6), were investigated by means of surface tension, 1H NMR, and dynamic light-scattering (DLS) measurements. The maximum surface excess in bmimPF 6 is much lesser than those observed for aqueous solution, reflecting a much weaker solvophobicity exhibited by the surfactant hydrocarbon chain in the ionic liquid compared with the hydrophobicity experienced in water. Compared with another ionic liquid with different anion species, 1-butyl-3-methylimidazolium terafluoroborate (bmimBF 4), the surfactant solution in bmimPF 6 brings about higher critical micelle concentration (cmc), smaller micellar aggregation number, and weaker dependence of cmc on the surfactant hydrocarbon chain length. This indicates that the solvophobic interaction between surfactant hydrocarbon chains acts more weakly in bmimPF 6 than in bmimBF 4. The weaker solvophobic effect in bmimPF 6 is also appreciable in the thermodynamic parameters for micelle formation derived from temperature dependence of cmc, and would be attributed to weaker attractive interaction between bmim cation and PF 6 - ion compared with that between bmim cation and BF 4 - ion. Present results give an insight into a better understanding of the importance of anion species for the properties of ionic liquids as a solvent to support a self-assembly of amphiphilic molecules.

Quantitative analysis and subsequent effects of partitioning of a mono- and dihydric C 4 alcohols into the micelles of cationic surfactants

Surfactant–alcohol systems are not uncommon in colloidal chemistry, what is rare is a specific emphasis on the environmental sensitive behaviour of 1-butanol (C 4OH) in different surfactant solutions. Intermediate solubility of C 4OH (in between totally soluble short chain alcohol and partially miscible long chain alcohol) has prompted us to study its partitioning between bulk and micellar phase for monomeric and dimeric surfactants and consequences on micellar parameters. Micellar aggregation number ( N agg) and micelle size has been evaluated by using SANS. Partitioning of C 4OH between bulk and micellar phase was evaluated from 2D NMR NOESY and DOSY experiments. The effect of the structure of C 4 alcohol was evaluated by taking all the above studies for 1,4-butanediol (1,4-BTD), a non-partitioning alcohol by virtue of its terminal hydroxyl groups. Finally, results from SANS and NMR are supported by the routine study of micellar behaviour from conductivity and viscosity. It is revealed that C 4 alcohol plays substantial role in micellization and the two alcohols contribute differently in controlling the overall micellization and micellar parameters.

Implicit-Solvent Models for Micellization: Nonionic Surfactants and Temperature-Dependent Properties

We have investigated micellization properties of surfactants using a recently developed implicit-solvent model and grand canonical Monte Carlo simulations. The original model had been parametrized for ionic surfactants at a single temperature; it is extended here to aqueous solutions of nonionic surfactants and given an explicit temperature dependence. Specifically, we have developed an implicit-solvent model of polyethylene glycol (PEG) surfactants and obtained the critical micelle concentrations (cmc's) and micellar aggregation numbers at low surfactant loadings. Various combinations of ethoxy and hydrocarbon tail segments were investigated in order to explore the predictive capabilities of the model. For the temperature dependence of the micellization properties, we have utilized thermodynamic approaches to quantify the hydrophobic attraction at temperatures ranging from 280 to 365 K. The temperature dependence of the cmc and the aggregate sizes were obtained for various ionic and nonionic surfactants, specifically sodium dodecyl sulfate, dodecyltrimethylammonium bromide and chloride, and PEG surfactants. For all systems studied, the model yields cmc and aggregation sizes that are in near-quantitative agreement with experimental results.

SAXS and Rheometry Studies of Diglycerol Monolurate Reverse Micelles in Styrene

We have investigated the structure and rheology of diglycerol monolaurate (C(12)G(2)) reverse micelles in styrene using small-angle X-ray scattering (SAXS) and rheometry techniques. The SAXS data have been evaluated by generalized indirect Fourier transformation (GIFT) method and further supported by geometrical model fittings. We found that the C(12)G(2) when added into styrene spontaneously self-assemble into ellipsoidal prolate type reverse micelles under ambient conditions. Micelles grew and micellar aggregation number (N(agg)) increased with the increase in surfactant concentration demonstrating concentration induced one dimensional micellar growth, which was further supported by rheology data; the relative viscosity, η(r), for the C(12)G(2)/styrene mixtures exhibit far steeper concentration dependence behavior than those predicted for a dispersion of spherical particles based on the Krieger-Dougherty relation, due to the elongated micellar structures. On the other hand, a distinct modulation in the shape and size of micelles favoring ellipsoidal prolate-to-sphere type transition was observed upon heating, whose scheme could be attributed to the enhanced penetration tendency of oil into the lipophilic shell of the surfactant at the higher temperatures. As anticipated reverse micelles swelled with water causing two dimensional micellar growth; both the maximum dimension and maximum core radius increase with water.

Polyelectrolyte Multilayers of Diblock Copolymer Micelles with Temperature-Responsive Cores

We report on assembly and stimuli-response behavior of layer-by-layer (LbL) films of pH- and temperature-responsive cationic diblock copolymer micelles (BCMs) of poly(2-(dimethylamino)ethyl methacrylate)-block-poly(N-isopropylacrylamide) (PDMA-b-PNIPAM) and a linear polyanion polystyrene sulfonate (PSS). As a function of solution pH at temperatures above lower critical solution temperature (LCST) of PNIPAM, PDMA-b-PNIPAM micelles have been demonstrated earlier to exhibit an abrupt change in micellar aggregation number and hydrodynamic size between larger and smaller BCMs (LBCMs and SBCMs, respectively). Here, LBCMs or SBCMs were included within LbL films through self-assembly with a polyanion, and film pH and temperature responses were studied using ellipsometry and atomic force microscopy (AFM). Both types of micelle preserved their micellar morphology when adsorbed at the surface of oxidized silicon wafers coated with PSS-terminated precursor layer at a constant pH. Response of adsorbed BCMs to temperature and pH variations was strongly dependent on whether or not BCMs were coated with the PSS layer. While monolayers of LBCMs lost their original dry morphology in response to pH or temperature variations, depositing a PSS layer atop LBCMs inhibited such irreversible restructuring. As a result of wrapping around and strong binding of PSS chains with LBCM micelles, BCM/PSS assemblies preserved their original dry state morphology despite the application of pH and temperature triggers. However, the wet-state film response to pH and temperature stimuli was drastically different. Swelling of BCM/PSS multilayers was strongly affected by temperature but was almost independent of pH due to neutralization of BCM PDMA's coronal charge with PSS. Cycling the temperature below and above PNIPAM's LCST caused PNIPAM chains within BCM cores to swell or collapse, resulting in reversible swelling transitions in the entire BCM/PSS assemblies. Temperature-controlled switching between the hydrophobic and hydrophilic state of assembled micellar cores was also used to regulate the release of a micelle-loaded hydrophobic pyrene dye, whose release rate increased at temperatures below PNIPAM's LCST.

Synthesis of a New Zwitterionic Surfactant Containing an Imidazolium Ring. Evaluating the Chameleon-like Behavior of Zwitterionic Micelles

Synthesis of a new zwitterionic surfactant containing the imidazolium ring 3-(1-tetradecyl-3-imidazolio)propanesulfonate (ImS3-14) is described. The solubility of ImS3-14 is very low but increases on addition of a salt which helps to stabilize the micellized surfactant. Fluorescence quenching and electrophoretic evidence for ImS3-14 shows that the micellar aggregation number is only slightly sensitive to added salts, as is the critical micelle concentration, but NaClO(4) markedly increases zeta potentials of ImS3-14 in a similar way as in N-tetradecyl-N,N-dimethylammonio-1-propanesulfonate (SB3-14) micelles. The rate of specific hydrogen ion catalyzed hydrolysis of 2-(p-heptoxyphenyl)-1,3-dioxolane and equilibrium protonation of 1-hydroxy-2-naphthoate ion in zwitterionic micelles of ImS3-14 and SB3-14 are increased markedly by NaClO(4) which induces anionoid character and uptake of H(3)O(+), but NaCl is much less effective in this respect. Comparison of ImS3-14 with SB3-14 is based on experimental evidence, and computational calculations indicate similarities and differences in structures of both compounds.

Location of ethanol in sodium dodecyl sulfate aggregates

The hexagonal liquid crystalline phase of SDS (Sodium dodecyl sulfate)/H2O system changes into lamellar liquid crystal and the effective length of surfactant molecule d0/2 in the lamellar liquid crystal decreases with the addition of ethanol. The micellar aggregation number N of SDS decreases and the micellar diffusion coefficient increases with the added ethanol. Under a constant concentration of SDS, the molecule number ratio of ethanol to SDS in the micelle increases with the concentration of ethanol and even exceeds 10 when ethanol concentration is 1. 085 mol/L. All these results show that ethanol, even though a short chain alcohol and soluble in water, can partly exist in the interphase of the amphiphilic aggregates showing some properties of co-surfactant.

Micelle formation in aqueous solutions of dodecylbenzene sulfonic acid studied by small-angle neutron scattering

Structure and interaction parameters of micelles in solutions of dodecylbenzene sulfonic acid (DBSA) in deuterated water are studied by small-angle neutron scattering. The dependences of micellar aggregation number, fractional charge, charge per micelle and surface potential on surfactant concentration are analyzed. A typical increase in the micelle size with the growth in the acid content is found, which can be related to the transition from spherical to rod-like micelles. The obtained data are used for estimating surfactant micelle concentration in water-based ferrofluids stabilized with DBSA.

Rheological and Fluorescence Investigation of Interaction between Hexadecyltrimethylammonium Bromide and Methylcellulose in the Presence of Hydrophobic Salts

The interaction of hexadecyltrimethylammonium bromide (HTAB) and methylcellulose (MC) has been investigated in aqueous medium employing rheological and fluorescence probing experiments. The associated physicochemical properties of the polymer-surfactant mixed system such as critical aggregation concentration (cac), critical micelle concentration (cmc), micellar aggregation number (N), micellar micropolarity index (pyrene I(1)/I(3) values), and viscosity curve are studied as a function of HTAB concentration. The effects of sodium benzoate (NaBz), sodium hexanoate (NaHx) and sodium chloride (NaCl) on the behavior of the polymer-surfactant solution have been followed systematically. The obtained experimental findings unfold significant information with respect to the effects of the salts on the solution behavior of the mixed system in addition to gelation characteristics.

Monte Carlo simulations of self-assembling star-block copolymers in dilute solutions

Computer simulations have been performed to analyze the aggregation behavior in dilute solutions of star-block copolymers of the type ( A B ) n in a selective solvent for the B block. We found spontaneous aggregation of single stars and formation of roughly spherical aggregates. By changing the solvophobic/solvophilic length ratio of the two blocks, and keeping the total arm length constant, we observed significant changes in the resulting micellar properties, such as the critical micellar concentration (CMC) and aggregation number. More specifically, by increasing the length of the solvophobic A block, we observe micellization at higher temperatures; whereas by increasing the length of the solvophilic B block, we observe micellization at very low temperatures. We also found a dependence of the CMC on the temperature which is in very good agreement with a recent theoretical description based on a simple thermodynamic framework. We compare our results with this theory and predict the enthalpy and entropy of micellization as a function of the temperature.

Synthesis and comparative behaviour study of fluorocarbon and hydrocarbon cationic surfactants in aqueous media

A series of perfluorinated cationic surfactants and their corresponding hydrocarbon ones whose general formula is C n X 2 n+1 –C(O)NH–(CH 2) 3–N +Me 3, I −, with X = F, H and n = 9, 11, have been synthesized via two steps. Their aggregative and surface-active properties were studied in aqueous solution using tensiometry and conductimetry. The critical micelle concentrations and the molecular areas at the air/water interface of fluorinated surfactants are lower than those of their hydrocarbon homologues. Micellar aggregation numbers and geometric packing parameters have been investigated. The results indicated that fluorocarbon surfactants tend to form lamellar aggregates while the hydrocarbon ones associate into spherical aggregates.

Surfactant Aggregates in Ionic Liquids and Reactivity in Media

In this paper the association of different families of amphiphilic compounds in room temperature ionic liquids in micelles, vesicles and reverse microemulsions was demonstrated. First the formation of nonionic polyoxyethylene-type surfactant micelles in the ionic liquid 1-butyl-3-methyl-imidazolium tetrafluoroborate is clearly evidenced using tensiometry and dynamic light scattering measurements. Critical micellar concentrations, object sizes and aggregation numbers are given and compared to aqueous and formamide systems. Then, vesicular aggregation of glycerophospholipid DPPC in two ionic liquids bmimBF4 and emimNTf2 was evidenced. Lastly, the phase diagram and microstructure of the ternary system ionic liquid (benzylpyridinium bis(trifluoromethanesulfonyl) imide)/nonionic surfactant (octylphenol ethoxylate)/toluene were studied. Phase stability and small-angle neutron scattering data show the formation of ionic liquid-in-oil microemulsion droplets. These reverse microemulsions were used as nanoreactors to perform a Matsuda-Heck reaction. The reaction yields obtained were greater in microemulsions (67%) than in bulk IL (33%) highlighting a strong effect of confinement. The reactivity in these confined media led to promising results.

Molecular Dynamics Simulations of Glycocholate−Oleic Acid Mixed Micelle Assembly

We have applied a molecular dynamics (MD) method to investigate the aggregation behavior and physicochemical properties of bile salt as well as bile salt/fatty acid mixed micelles. Local atomic density profiles from the center of the micelles confirm that the self-assembly of the trihydroxy bile salt, glycocholate, is largely driven by hydrophobic aggregation of the nonpolar beta-faces of the steroid backbones. Additional association occurs between neighboring monomers through hydrogen-bonding interactions. The average micellar aggregation number for glycocholate at 37 degrees C with a background salt concentration of 150 mM is shown to be 8.5 molecules per micelle, while the critical micelle concentration (cmc) is 3.1 mM. The good agreement of these results with experimental values illustrates that a MD approach is useful to study mixed micelles of bile salts and fatty acids, critical to the understanding of oral lipid-based formulations. The aggregation behavior and colloidal structure of such micelles are simulated and presented in this article.

Application of Microelectrode Voltammetry to Study the Properties of Surfactant Solutions: Alkyltrimethylammonium Bromides

Microelectrode cyclic voltammetry (MV) has been employed to investigate the micellar properties of solutions of homologous alkyltrimethylammonium bromides, RMe(3)ABr, R = C(10), C(12), and C(14), in water and in the presence of added NaBr. The micellar self-diffusion coefficient was calculated from the limiting current for the reversible electron transfer of micelle-bound ferrocene. From the values of this property, other parameters were calculated, including the micellar hydrodynamic radius, R(H), and aggregation number, N(agg); the latter was also theoretically calculated. We determined the values of the diffusion coefficient as a function of various experimental variables and observed the following trends: The diffusion coefficient decreases as a function of increasing surfactant concentration (no additional electrolyte added); it decreases as a function of increasing surfactant concentration at fixed NaBr concentration; and it shows a complex dependence (increase then decrease) on the NaBr concentration at a fixed RMe(3)ABr concentration. The value of the intermicellar interaction parameter decreases and then increases as a function of increasing NaBr concentration. These results are discussed in terms of intermicellar interactions and the effect of NaBr on the micellar surface charge density and sphere-to-rod geometry change. The NaBr concentration required to induce the latter change increases rapidly as a function of decreasing the length of R: no geometry change was detected for C(10)Me(3)ABr. Values of N(agg) increase as a function of increasing the length of R and are in good agreement with both literature values and values that were calculated theoretically. Thus, MV is a convenient and simple technique for obtaining fundamental properties of surfactant solutions, including additive-induced changes of micellar parameters (N(agg)) and morphology changes.

Towards a structural characterization of charge-driven polymer micelles

Light scattering and small-angle neutron scattering experiments were performed on comicelles of several combinations of oppositely charged (block co)polymers in aqueous solutions. Fundamental differences between the internal structure of this novel type of micelle --termed complex coacervate core micelle (C3Ms), polyion complex (PIC) micelle, block ionomer complex (BIC), or interpolyelectrolyte complex (IPEC)-- and its traditional counterpart, i.e., a micelle formed via self-assembly of polymeric amphiphiles, give rise to differences in scaling behaviour. Indeed, the observed dependencies of micellar size and aggregation number on corona block length, N (corona) , are inconsistent with scaling predictions developed for polymeric micelles in the star-like and crew-cut regime. Generic C3M characteristics, such as the relatively high core solvent fraction, the low core-corona interfacial tension, and the high solubility of the coronal chains, are causing the deviations. A recently proposed scaling theory for the cross-over regime, as well as a primitive first-order self-consistent field (SCF) theory for obligatory co-assembly, follow our data more closely.

Self-Aggregation Behavior of Fluorescent Carbazole-Tailed Imidazolium Ionic Liquids in Aqueous Solutions

Three amphiphilic imidazolium ionic liquids (ILs), 1-[n-(N-carbazole)alkyl]-3-methylimidazolium bromide [carbazoleC(n)mim]Br (n = 6, 10, and 12), that incorporate a fluorescent carbazole moiety at the terminal of the hydrocarbon chain, were designed and synthesized. Their surface activity, aggregation behavior, and thermodynamics of micelle formation in aqueous solutions were systematically investigated by surface tension, electrical conductivity, and (1)H NMR spectroscopy. The incorporation of bulky carbazole groups led to the loose arrangement of [carbazoleC(n)mim]Br molecules at the air-water interface. Both the enhanced hydrophobicity and the pi-pi interaction due to the incorporation of carbazole groups contributed to the lower critical micelle concentration (cmc) and higher micellar aggregation number, reflecting a strong capability of the designed carbazole-tailed imidazolium ILs for the formation of micelle. The analysis of (1)H NMR spectra revealed that the carbazole moieties may overlap with the middle methylenes of the alkyl chains. The amphiphilic carbazole-tailed imidazolium ILs demonstrated strong and stable fluorescence properties, indicating their potential application in the combined field of surface chemistry and photochemistry.

Micellization and micellar growth of alkanediyl-α,ω-bis(dimethyldodecylammonium bromide) surfactants in the presence of medium-chain linear alcohols

Micellization and micellar growth of cationic dimeric surfactants of the alkanedyil-α,ω-bis(dimethyldodecylammonium) bromide type, 12- s-12,2Br − ( s = 3, 4, 6), in the presence of various amounts of 1-butanol, 1-pentanol, and 1-hexanol have been investigated. The influence of the nature and concentration of alcohol on the cmc, on the micellar ionization degree, on the average micellar aggregation number, and on the polarity of the micellar interfacial region was investigated by using conductivity and fluorescence measurements. Subsequently, effects of alcohol addition on the surfactant concentration range where sphere to rod transitions occur were examined and information about changes in the micropolarity and in the microviscosity accompanying the morphological transition was obtained. The experimental results were explained by considering the variations in the different contributions to the Gibbs energy of micellization caused by the presence of alcohols. The study of the reaction methyl naphthalene-2-sulfonate + Br − in some water–alcohol 12-6-12,2Br − micellar solutions provided information about the characteristics of the dimeric micelles as microreactors and show the complexity of the microheterogeneous systems studied.