Mean Aggregation Number Research Articles

Behaviour of Pluronic® P84 block copolymer micelles above the gelification temperature as probed by positron annihilation lifetime spectroscopy

The new method based on positron annihilation lifetime spectroscopy (PALS) to determine both the mean core radius, R(core), and aggregation number, N(ag), of micelles is applied to the study of aqueous solutions of the triblock Pluronic P84 copolymer as a function of temperature (T), beyond the gelification point (334 K). Two long-lived components appear in the PALS spectra, ascribed to triplet positronium in the water bulk (o-Ps(aq)) and in the organic core of the micelles (o-Ps(org)). Of the various fitting parameters, only the lifetime of the latter species, tau4, and the micellar parameters, R(core) and N(ag), disclose the occurrence of gelification by first increasing up to 334 K, then decreasing. By contrast to what is known in case of phase transition, none of the parameters shows any abrupt change at 334 K, whereas the macroscopic viscosity of the solutions suffers a drastic increase. This is attributed to the fact that positronium is sensitive to the microviscosity of the solutions. At the transition point, the properties of the polyoxipropylene aggregates forming the organic core of the P84 micelles are not greatly affected. Furthermore, the fact that the experimental N(ag) values coincide with those calculated for spheres, from the R(core) values, indicates that the shape of the P84 cores does not change significantly after gelification. The onset of gelification results from a decrease in the hydrogen bonding interactions in the solution with an ensuing relative increase in the interactions between the polyoxipropylene (PPO) groups, initially forming the corona of the P84 micelles, in an intermicellar mode. This increased solicitation of the PPO groups outside their initial location would result in depletion in the number of surfactant molecules forming the micelles, viz. a decrease in both R(core) and N(ag) above 334 K. From the data, additional information can be gained regarding the local viscosity and surface tension in the micellar cores.

The Interaction Between Polyvinylpyrrolidone and Cationic Surfactant: Surface Tension and Steady‐State Fluorescence Methods

The interaction between a new synthetic cationic surfactant, 3‐cetyloxy‐2‐hydroxypropyltrimethyl ammonium chloride (CHPTAC), and polyvinylpyrrolidone (PVP) has been investigated by surface tension and steady‐state fluorescence methods. The results indicate that PVP and CHPTAC have synergistic surface activity by forming a mixed monolayer. In bulk phase CHPTAC molecules associates with PVP to form assemblies driven by the hydrophobic effect and intermolecular hydrogen bonding. Although each PVP molecule can bind 70 CHPTAC molecules, the mean aggregation number of CHPTAC varies with concentrations.

Preparation and Characterization of Self-Assembled Nanoparticles of Heparin-Deoxycholic Acid Conjugates

Various deoxycholic acid (DOCA) bearing heparin (HD) amphiphilic conjugates with different degrees of substitution (DS) with DOCA were synthesized using heparin as a hydrophilic segment and DOCA as a hydrophobic segment. Structural characteristics of these HD conjugates were investigated using 1H NMR, dynamic light scattering, zeta potential, transmission electron microscopy (TEM), and fluorescence spectroscopy. HD conjugates provided monodispersed self-aggregates in water, with mean diameters decreasing with increasing DOCA DS in the range of 120-200 nm. HD aggregates were covered with negatively charged heparin shells, exhibiting xi potentials near -56 mV. The critical aggregation concentration (cac) of the HD (0.02-0.003 mg/mL) depended upon DOCA DS. TEM images demonstrated that the shape of the self-aggregates was spherical. Partition equilibrium constants, Kv, for pyrene in the self-aggregates in water indicated that increasing DS enhanced the hydrophobicity of the self-aggregate inner core. The mean aggregation number of DOCA per hydrophobic microdomain, estimated by the fluorescence quenching methods using cetylpyridinium chloride, indicated that five to nine of HD chains comprised a hydrophobic domain in the conjugates.

Diffusion of alpha-tocopherol in membrane models: probing the kinetics of vitamin E antioxidant action by fluorescence in real time.

The new fluorescent membrane probe Fluorazophore-L, a lipophilic derivative of the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene, is employed to study the quenching of alpha-tocopherol (alpha-Toc) by time-resolved fluorescence in the microheterogeneous environments of Triton XR-100 and SDS micelles, as well as POPC liposomes. Fluorazophore-L has a small nonaromatic fluorescent polar headgroup and an exceedingly long-lived fluorescence (e.g., 140 ns in aerated SDS micelles), which is efficiently quenched by alpha-Toc (3.9 x 10(9) M(-1) s(-1) in benzene). Based on solvatochromic effects and the accessibility by water-soluble quenchers, the reactive headgroup of Fluorazophore-L, along with the chromanol group of alpha-Toc, resides at the water-lipid interface, which allows for a diffusion-controlled quenching in the lipidic environments. The quenching experiments represent an immobile or stationary case; that is, interparticle probe or quencher exchange during the excited-state lifetime is insignificant. Different quenching models are used to characterize the dynamics and antioxidant action of alpha-Toc in terms of diffusion coefficients or, where applicable, rate constants. The ideal micellar quenching model is suitable to describe the fluorescence quenching in SDS micelles and affords a pseudo-unimolecular quenching rate constant of 2.4 (+/- 0.4) x 10(7) s(-1) for a single quencher per micelle along with a mean aggregation number of 63 +/- 3. In Triton micelles as well as in unilamellar POPC liposomes, a two-dimensional (lateral) diffusion model is most appropriate. The mutual lateral diffusion coefficient D(L) for alpha-Toc and Fluorazophore-L in POPC liposomes is found to be 1.8 (+/- 0.1) x 10(-7) cm(2) s(-1), about a factor of 2 larger than for mutual diffusion of POPC, but more than 1 order of magnitude lower than a previously reported value. The comparison of the different environments suggests a quenching efficiency in the order benzene >> SDS micelles > Triton micelles > POPC liposomes, in line with expectations from microviscosity. The kinetic measurements provide important benchmark values for the modeling of oxidative stress in membranes and other lipidic assemblies. The special case of small lipidic assemblies (SDS micelles), for which the net antioxidant efficacy of alpha-Toc may be lower than expected on the grounds of its diffusional behavior, is discussed.

Synergistic Sphere-to-Rod Micelle Transition in Mixed Solutions of Sodium Dodecyl Sulfate and Cocoamidopropyl Betaine

Static and dynamic light scattering experiments show that the mixed micelles of sodium dodecyl sulfate (SDS) and cocoamidopropyl betaine (CAPB) undergo a sphere-to-rod transition at unexpectedly low total surfactant concentrations, about 10 mM. The lowest transition concentration is observed at molar fraction 0.8 of CAPB in the surfactant mixture. The transition brings about a sharp increase in the viscosity of the respective surfactant solutions due to the growth of rodlike micelles. Parallel experiments with mixed solutions of CAPB and sodium laureth sulfate (sodium dodecyl-trioxyethylene sulfate, SDP3S) showed that the sphere-to-rod transition in SDP3S/CAPB mixtures occurs at higher surfactant concentrations, above 40 mM. The observed difference in the transition concentrations for SDS and SDP3S can be explained by the bulkier SDP3S headgroup. The latter should lead to larger mean area per molecule in the micelles containing SDP3S and, hence, to smaller spontaneous radius of curvature of the micelles (i.e., less favored transition from spherical to rodlike micelles). The static light scattering data are used to determine the mean aggregation number and the effective size of the spherical mixed SDS/CAPB micelles. From the dependence of the aggregation number on the surfactant concentration, the mean energy for transfer of a surfactant molecule from a spherical into a rodlike micelle is estimated.

Micelle formation of sodium chenodeoxycholate and solubilization into the micelles: comparison with other unconjugated bile salts

Micellization of sodium chenodeoxycholate (NaCDC) was studied for the critical micelle concentration (CMC), the micelle aggregation number, and the degree of counterion binding to micelle at 288.2, 298.2, 308.2, and 318.2 K. They were compared with those of three other unconjugated bile salts; sodium cholate (NaC), sodium deoxycholate (NaDC), and sodium ursodeoxycholate (NaUDC). The I 1/ I 3 ratio of pyrene fluorescence and the solubility dependence of solution pH were employed to determine the CMC values. As the results, a certain concentration range for the CMC and a stepwise molecular aggregation for micellization were found reasonable. Using a stepwise association model of the bile salt anions, the mean aggregation number ( n̄) of NaCDC micelles was found to increase with the total anion concentration, while the n̄ values decreased with increasing temperature; 9.1, 8.1, 7.4, and 6.3 at 288.2, 298.2, 308.2, and 318.2 K, respectively, at 50 mmol dm −3. The results from four unconjugated bile salts indicate that the number, location, and orientation of hydroxyl groups in the steroid nucleus are quite important for growth of the micelles. Activity of the counterion (Na +) was determined by a sodium ion selective electrode in order to confirm the low counterion binding to micelles. The solubilized amount of cholesterol into the aqueous bile salt solutions increased in the order of NaUDC<NaC<NaCDC<NaDC. The first stepwise association or solubilization constants ( K̄ 1) between a cholesterol monomer and a vacant micelle were evaluated at different bile salt concentrations. The constants were also determined for polycyclic aromatic compounds (benzene, naphthalene, anthracene, and pyrene). The corresponding Δ G 0 value was most negative for cholesterol among the solubilizates studied, which indicated that cholesterol was thermodynamically stabilized most by solubilization into the bile salt micelles.

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.

Sulfonamide based pH-sensitive polymeric micelles: physicochemical characteristics and pH-dependent aggregation

Polymeric micelles composed of block copolymers of poly(l-lactide) (PLLA)/poly(ethylene glycol) (PEG) of which end was modified with a novel pH sensitive element of a sulfonamide, sulfadimethoxine (SD; pKa 6.1), or a polymer (PSD; Mw: 3000) of its derivative were investigated. PSD was synthesized by radical polymerization of a vinylated SD (SD monomer; SDM) using 2-aminoethanethiol as a chain transfer agent. SD or PSD was coupled to carboxylic PEG (Mw=2000)/PLLA (Mw=3500) block copolymer. The polymeric micelles were prepared by a diafiltration method and their physicochemical properties in aqueous media were characterized by dynamic light scattering (DLS) and fluorescence spectroscopy. The mean diameter of all samples was smaller than 60 nm with a unimodal size distribution. The critical micelle concentration (CMC) and mean aggregation number of PLLA block in one hydrophobic domain of PLLA/PEG–PSD micelle in borate buffer (pH 9.0) were slightly higher than those of PLLA/PEG and PLLA/PEG–SD micelles due to the enhanced hydrophilic nature of PSD when ionized. The pH-sensitive properties were examined by the measurement of pH-dependent solution turbidity and the size of micellar aggregates. The PLLA/PEG–PSD micelle aggregated below pH 7.0 due to deionized SDM on the surface, while the other micelles were stable. The polymeric micelle prepared in this study sharply responded to the change in pH around the physiological pH.

Modeling the hydrodynamic fluctuations of self-associating fluids: An application to the Brillouin scattering of 1-octanol

A theoretical model, encompassing the concentration fluctuations of associated and unassociated species, has been developed in order to explain the non-Landau behavior observed in self-associating fluids in the hydrodynamic regime. The complex dynamics for the formation of different species is depicted as a pseudo two-components system formed by unbound molecules coexisting with a unique associated specie, the mean aggregation number of which is dictated by thermodynamic requirements. The model has been tested in the case of a hydrogen-bonding system, 1-octanol, over a wide range of temperatures and wave vectors. The model explains, albeit qualitatively, most of the observed deviations from the Landau behavior observed in the Rayleigh–Brillouin spectra at different values of the k-vector.

Thermodynamic and Aggregation Properties of Gemini Surfactants with Hydroxyl Substituted Spacers in Aqueous Solution

Critical micelle concentration (cmc), degree of micelle ionization (α), headgroup area (a0), mean aggregation number, enthalpy of micellization (ΔHM), and volume of micellization (ΔVφ,M) properties are reported for a series of gemini surfactants, 12−4(OH)n−12 (n = 0, 1, or 2), with increasing hydroxyl substitution within the spacer group. The cmc and mean aggregation numbers are observed to decrease with increased hydroxyl substitution consistent with increasing hydrophilicity of the headgroup. Both a0 and α are unaffected by hydroxyl substitution. ΔHM° is observed to decrease (become more exothermic) with increased substitution of the spacer group, while ΔVφ,M is observed to increase. These observations are rationalized in terms of changes in the solvation and conformation of the spacer group at the micelle−bulk solution interface. Free energies of micellization, ΔGM°, and entropies of micellization, ΔSM°, are calculated from the ΔHM°, cmc, and α values. The results indicate that the micellization process for all surfactants is entropy-driven.

Studies of the Aggregation Behavior of Cyclic Gemini Surfactants

The specific conductance, surface tension, mean aggregation number, and apparent molar volume properties of aqueous solutions of a novel series of N,N′-bis(cyclododecyldimethyl)-α,ω-alkanediammonium dibromide (c12-s-c12) surfactants, where s is the spacer chain length, are reported. Surfactants with s=3, 4, and 6 have been prepared and characterized in terms of their Krafft temperature (TKr), critical micelle concentration (cmc), surfactant head group area (a) at the air–water interface, mean aggregation number (Nagg), and the volume change upon micelle formation (ΔVφ,M). The c12-3-c12 shows little evidence of aggregate formation, while the results obtained for the c12-4-c12 and c12-6-c12 homologues suggest the formation of small, poorly defined micellar aggregates in aqueous solution.

Solubilization of cholesterol and polycyclic aromatic compounds into sodium bile salt micelles (Part 2)

The aqueous solubility of cholesterol was determined over the temperature range from 288.2 to 318.2 K with intervals of 5 K by the enzymatic method. The solubility was (3.7±0.3)×10 −8 mol dm −3 (average±S.D.) at 308.2 K. The maximum additive concentrations of cholesterol into the aqueous micellar solutions of sodium deoxycholate (NaDC), sodium ursodeoxycholate (NaUDC), and sodium cholate (NaC) were spectrophotometrically determined at different temperatures. The cholesterol solubility increased in the order of NaUDC<NaC<NaDC; for example, 0.10 for NaUDC, 0.61 for NaC, and 2.99 mmol dm −3 for NaDC at the concentration of 60 mmol dm −3 and at 308.2 K. The same solubilization experiments were made at 308.2 K using polycyclic aromatic compounds (benzene, naphthalene, anthracene, pyrene) as a reference. Their solubility increase for the bile salts was in the same order as above. Thermodynamic analysis was made for the solubilization, where a micelle was regarded as a chemical species. The average number of solubilizate per micelle was less than unity throughout the experiments. From the Gibbs energy change for solubilization at the different mean aggregation numbers, the function of bile salt micelles was discussed from the viewpoint of molecular structure of solubilizates. The Δ G 0 value for cholesterol was most negative among the solubilizates studied, which reflected that solubilization of cholesterol into bile salt micelles brought about largest thermodynamic stabilization.

Solvent Effect on Chiral Aggregate Formation of Acylamino Acids.

We studied formation of chiral aggregates of optical active N-lauroyl-L-valine (L-LVA), N-lauroyl-L-glutamic acid (L-LGA) and methylester of L-LVA (L-LVMe) in various solvents such as acetonitrile, 1,4-dioxane, tetrahydrofuran and 1,2-dichloroethane through the measurements of circular dichroism (CD), NMR chemical shift of N-H proton and vapor-pressure osmometry (VPO). The above acylamino acids and L-LVMe showed CD bands at 212-215 nm, whose intensities depended on temperature. The CD bands have been believed to come from the formation of chiral aggregates. However, by means of NMR and VPO analyses, the acylamino acids and L-LVMe were found to be present mostly as monomers in relatively polar solvents such as acetonitrile, 1,4-dioxane and tetrahydrofuran. On the other hand, L-LVA in 1,2-dichloroethane forms chiral aggregates; the mean aggregation number of L-LVA increases with increasing in its concentration.

Quenching of Pyrene Derivatives' Fluorescence by Nitroxide Radicals in Sodium Dodecyl Sulfate Micellar Solutions

Dynamic fluorescence quenching measurements have been performed on pyrene derivatives (pyrene (Py), 1-pyrenebutanoic acid (PBA), and 1-pyrenedodecanoic acid (PDA)), using as quenchers nitroxide free radicals (2,2,6,6-tetramethyl-1,1-piperidinyloxyl, 4-hydroxy-2,2,6,6-tetramethyl-1,1-piperidinyloxyl, and 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy (TEMN)) in aqueous solutions of sodium dodecyl sulfate. The mean aggregation number values are comparable with the literature data only when the partition coefficient of the quencher is higher than 1100 M−1. It is shown that the dynamic fluorescence quenching for the PBA/TEMN pair cannot be described by the Infelta—Tachiya model owing to the fact that the intramicellar quenching rate constant is lower than the exit rate constant of the quencher from the micelle. The average location of the fluorescent probes is also discussed, Py and PDA having the pyrenyl moieties located at approximately the same depth in the micellar core, while in the case of PBA the pyrenyl moiety is buried deeper.

SANS and SAXS study on aqueous mixtures of fullerene-based star ionomers and sodium dodecyl sulfate

Fullerene-based star ionomer C 60[ CO( CH 2) 5 O( CH 2) 4 SO 3 Na] 6 ( FC 10 S) , with six sodium-dodecyl-sulfate-like arms randomly bonded on the fullerene, has been synthesized. This novel molecule has a high water solubility and a structural shape resembling the skeleton of a micelle of sodium dodecyl sulfate (SDS). We study aqueous mixtures of the C 60-based ionomers and SDS using small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS). With a selected deuteration of SDS, we have identified the formation of complex aggregates of FC 10S with SDS in the mixtures. The structural information of the complex aggregates is extracted from the SANS and SAXS data measured for the mixtures. On the average, the complex aggregates observed have a cylinder-like shape with a radius of 19 A ̊ and a length of ≈100 A ̊ for the sample mixtures containing 2.3 or 1.2 mM of FC 10S and 6 mM of SDS. The mean aggregation numbers of FC 10S and SDS in each complex aggregate are 15 and 11, respectively.

The Remarkable “Flip−Flop” Self-Assembly of a Diblock Copolymer in Aqueous Solution

In this paper, it is shown that tertiary amine methacrylate-based diblock copolymers can form both micelles and reverse micelles in aqueous solution at room temperature. This remarkable behavior is due to subtle variations in the hydrophilic−hydrophobic balance of the two block sequences, which are based on 2-(diethylamino)ethyl methacrylate (DEA) and 2-(N-morpholino)ethyl methacrylate (MEMA). 1H NMR spectroscopy studies indicate that these DEA−MEMA copolymers dissolve molecularly in water at pH 6−7 at 20 °C. DEA−core micelles are formed at higher pH, and MEMA−core micelles are formed at pH 6.5 in the presence of 1.0 M Na2SO4. Both types of micelles were characterized in terms of their intensity-average diameters and mean aggregation numbers using dynamic and static light scattering, respectively. In addition, one of the copolymers (60 mol % MEMA, Mw = 30 000, Mw/Mn = 1.05) was selected for a detailed small-angle neutron scattering (SANS) study. SANS data for the micelles were analyzed in terms of a weakl...

Model for the aggregation state of living anionic polymers

To better understand the aggregation of living polymers formed in anionic polymerization, we develop a theoretical model for the aggregation state of living polymers with polar headgroups in hydrocarbon solvents. In particular, we have examined polymers with butadienyllithium headgroups in benzene. The system is modeled as a population of star-like micelles coexisting with a broadly polydisperse population of long cylindrical micelles. We use numerical solutions of the self-consistent mean-field equations to quantitatively calculate the stretching energy of the polymer chains in the micelles, and semiempirical and ab initio quantum chemistry methods to calculate headgroup geometries and binding energies. We find that the general aggregation state of the model consists of nearly Gaussian-distributed spherical micelles for long polymer chain lengths, which cross over to exponentially-distributed cylindrical micelles at short chain lengths, with a region of coexistence of the two types of micelles in a narrow intermediate range of chain length. For physically reasonable values of the model parameters, the predicted aggregation state is consistent with experimental features observed for this system by Stellbrink et al., and in particular with the formation of star-like micelles with mean aggregation numbers of 10 or larger.

Effect of 1-Pentanol on Size and Shape of Sodium Dodecyl Sulfate Micelles as Studied by Positron Annihilation Lifetime Spectroscopy

The changes in size and shape of micelles formed in a 0.2 mol dm-3 sodium dodecyl sulfate (SDS) aqueous solution are studied as a function of added 1-pentanol concentration at 294 K, by using positron annihilation lifetime spectroscopy (LS). As in a previous work, the trapping of ortho-positronium (o-Ps) from the aqueous to the organic micellar subphase results in the appearance of two o-Ps states with distinct lifetimes. The LS data are analyzed on the basis of a time-dependent trapping rate coefficient, which involves two parameters characteristic of the micellar system: the mean core radius, Rcore, and the mean aggregation number, Nag. Setting the Nag values from a previous experimental work, the LS data give thus access to Rcore. In parallel to the variation of Nag, Rcore is found to decrease slightly, then to increase steadily as the alcohol concentration is increased. A quantitative agreement is found between the experimental values of Rcore and those expected on the basis of the volume of the hydrocarbon tails of both SDS and 1-pentanol with different geometries, strongly suggesting that the micelles evolve from spherical to rod shaped. In addition, the lifetime (τ4) of those o-Ps atoms trapped in the micelles shows an initial increase, at low alcohol concentration, which is attributed to a decrease in the local surface tension inside the dodecane pools due to the presence of the shorter 1-pentanol hydrocarbon tail. On adding more alcohol, τ4 shows a somewhat linear decrease, indicating a deeper penetration of the pentanol molecules inside the micelles with a resulting increase in their surface tension.

Self-Association Behavior of Hydrophobically Modified Poly(aspartic acid) in Water Studied by Fluorescence and Dynamic Light Scattering Techniques

Poly(succinimide) (PSI), derived from l-aspartic acid by polycondensation, is a reactive precursor for synthesis of various derivatives of poly(aspartic acid) (PASP), a commercially interesting polyelectrolyte because of its biodegradable properties. Hydrophobically modified PASP was synthesized by reacting PSI with dodecylamine, followed by hydrolysis. The self-association behavior of PASP having pendant dodecyl groups (DDA-PASP) in water was characterized by fluorescence and quasielastic light scattering (QELS) techniques as a function of the mole percent content of dodecyl groups in the polymer (fDod). For fluorescence studies, the polymers were labeled singly with pyrene (Py-DDA-PASP) or doubly with naphthalene and pyrene (Np-Py-DDA-PASP). The vibrational fine structure of pyrene fluorescence spectra and the pyrene fluorescence lifetime for an aqueous solution of Py-DDA-PASP at pH = 9 indicated that associations of polymer-bound dodecyl groups started to occur at a low fDod (near 3 mol %), and a significant fraction of pyrene labels were incorporated in hydrophobic microdomains formed. Intrapolymer nonradiative energy transfer from naphthalene to pyrene in Np-Py-DDA-PASP indicated that a significant contraction of the conformational dimension occurred in the region 20 < fDod < 50 mol % at pH = 9, this fDod region shifting to 10−30 mol % upon decrease in pH to 4 or upon addition of 0.05 M NaCl at pH = 9. QELS results at pH = 9 in the presence of 0.05 M NaCl indicated that DDA-PASP with fDod ≤ 31 mol % forms two kinds of aggregates with well-defined hydrodynamic radii (Rh), Rh ranging 3−4 nm for one type of the aggregates and 60−90 nm for the other, and they are coexisting together. The fraction of the larger aggregates increases with polymer concentration. In contrast, DDA-PASP with fDod = 48 mol % forms only small aggregates with Rh on the order of 4 nm independent of the polymer concentration studied (0.25−1.0 g/dL). The mean aggregation number of dodecyl groups in this small aggregate was determined to be ca. 68 using a time-resolved fluorescence method. Since this aggregation number agrees well with the number of dodecyl groups per polymer chain (ca. 64) calculated from the degree of polymerization along with fDod, this small aggregate is suggested to be a unimolecular micelle. Accordingly, all the small aggregates with Rh = 3−4 nm found for the polymers with fDod ≤ 31 mol % are also likely to be unimolecular micelles. Consequently, DDA-PASP exhibits a strong preference for intrapolymer hydrophobe self-associations to form a unimolecular micelle if the hydrophobe content is as high as 48 mol %.

Aggregate Structure in Aqueous Solutions of Brij-35 Nonionic Surfactant Studied by Small-Angle Neutron Scattering

The aggregate formation in aqueous solutions of Brij-35 (C12EO23) nonionic surfactant has been studied in the concentration range from 5 to 200 g/L at 20, 40, and 60 °C. The structure of micelles was modeled by a spherical core with tethered polymer chains to the surface, while the intermicellar interaction was described by a hard sphere structure factor. The mean aggregation number was found to increase substantially with both concentration and temperature from 34 (5 g/L, 20 °C) to 64 (200 g/L, 60 °C). The average core radii obtained are between 18 and 19 A. On the basis of the above two parameters, 35−50% of the core volume is occupied by hydrated poly(ethylene oxide) (PEO) chains. The radius of gyration of tethered chains, Rgc, is in agreement with the theoretical value for Gaussian conformation (11.6 A) at the smaller concentrations measured but increases significantly with increasing concentration. It reveals considerable stretching of the chains that is attributed to sterical constraints. At higher ...