The Interaction between PEO-PPO-PEO Triblock Copolymers and Ionic Surfactants in Aqueous Solution Studied Using Light Scattering and Calorimetry

Long-term high-resolution imaging and culture of C. elegans in chip-gel hybrid microfluidic device for developmental studies

Developmental studies in multicellular model organisms such as Caernohabditis elegans rely extensively on the ability to cultivate and image animals repeatedly at the cell or subcellular level. However, standard high-resolution imaging techniques require the use of anaesthetics for immobilization, and may have undesirable side effects on development. Thus such techniques are not ideal in allowing the same animals to grow and be imaged throughout development to observe specific developmental processes. In this paper, we present a microfluidic system designed to overcome these difficulties. The system allows for long-term culture of C. elegans starting at L1 larval stage and repeated high-resolution imaging at physiological temperatures without using anaesthetics. We use a commercially available biocompatible polymer, Pluronic F127 for immobilization; this polymer is capable of a reversible thermo-sensitive sol-gel transition within approximately 2 degrees C, which is well-controlled in the microfluidic chip. The gel phase is sufficient to immobilize the animals. While animals are not imaged, they are cultured in individual chambers in media containing nutrients required for development. We show here that this method facilitates time-lapse studies of single animals at high-resolution and lends itself to live imaging experiments on developmental processes and dynamic events.

Study of the pluronic-silica interaction in synthesis of mesoporous silica under mild acidic conditions

The interaction between silica and poly(ethylene oxide) (PEO) in water may appear trivial and it is generally stated that hydrogen bonding is responsible for the attraction. However, a literature search shows that there is not a consensus with respect to the mechanism behind the attractive interaction. Several papers claim that only hydrogen bonding is not sufficient to explain the binding. The silica-PEO interaction is interesting from an academic perspective and it is also exploited in the preparation of mesoporous silica, a material of considerable current interest. This study concerns the very early stage of synthesis of mesoporous silica under mild acidic conditions, pH 2-5, and the aim is to shed light on the interaction between silica and the PEO-containing structure directing agent. The synthesis comprises two steps. An organic silica source, tetraethylorthosilicate (TEOS), is first hydrolyzed and Pluronic P123, a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer, is subsequently added at different time periods following the hydrolysis of TEOS. It is shown that the interaction between the silica and the Pluronic is dependent both on the temperature and on the time between onset of TEOS hydrolysis and addition of the copolymer. The results show that the interaction is mainly driven by entropy. The effect of the synthesis temperature and of the time between hydrolysis and addition of the copolymer on the final material is also studied. The material with the highest degree of mesoorder was obtained when the reaction was performed at 20 degrees C and the copolymer was added 40 h after the start of TEOS hydrolysis. It is claimed that the reason for the good ordering of the silica is that whereas particle formation under these conditions is fast, the rate of silica condensation is relatively low.

Aqueous gels of mixtures of ionic surfactant SDS with pluronic copolymers P123 or F127

Gel diagrams based on tube inversion and oscillatory rheometry are reported for Pluronic copolymers F127 (E(98)P(67)E(98)) and P123 (E(21)P(67)E(21)) in mixtures with anionic surfactant sodium dodecyl sulfate (SDS). Total concentrations (c, SDS+copolymer) were as high as 50 wt % with mole ratios SDS/copolymer (mr) in the ranges 1-5 (F127) and 1-7 (P123). Temperatures were as high as 90 degrees C. Determination of the temperature dependences of the dynamic moduli served to confirm the gel boundaries from tube inversion and to reveal the high elastic moduli of the gels, e.g., compared at comparable positions in the gel phase, a 50 wt % SDS/P123 with mr = 7 had G' three times that of a corresponding gel of P123 alone. Small-angle X-ray scattering (SAXS) was used to show that the structures of all the SDS/F127 gels were bcc and that the structures of the SDS/P123 gels with mr = 1 were either fcc (c = 30 wt %) or hex (c = 40 wt %). Assignment of structures to SDS/P123 gels with values of mr in the range 3-7 was more difficult, as high-order scattering peaks could be very weak, and at the higher values of c and mr, the SAXS peaks included multiple reflections.

Surface deposition and phase behavior of oppositely charged polyion/surfactant ion complexes. 1. Cationic guar versus cationic hydroxyethylcellulose in mixtures with anionic surfactants

Mixtures of cationic guar (cat-guar) or cationic hydroxyethylcellulose (cat-HEC) with the anionic surfactants sodium dodecyl sulfate or sodium lauryl ether-3 sulfate have been investigated by a wide range of complementary techniques (phase studies, turbidity measurements, dynamic light scattering, gel-swelling experiments, and in situ null ellipsometry), with the following objectives in mind: (1) to establish the relationship between the bulk phase behavior (precipitation and redissolution) of the polyion/surfactant ion complexes and formation/deposition of such complexes at silica surfaces and (2) to obtain molecular interpretations of the large, previously unresolved, quantitative differences between the various investigated mixtures. There were clear similarities, for each studied system, between the bulk phase behavior, gel swelling, and surface deposition on increasing surfactant concentration. This is because all phenomena reflect the polyion/surfactant ion binding isotherm: an initial binding step at a low critical association concentration (cac) of the surfactant and a second more-or-less cooperative binding step beginning at a second cac, the cac(2). The details of the interactions are system-specific, however, and cat-guar/surfactant mixtures generally had larger precipitation regions and gave rise to larger adsorbed amounts on silica compared to mixtures with cat-HEC of a similar charge density. The observed quantitative differences are attributed to a difference in the hydrophobicity of the polyions. For cat-guar, the comparatively weak hydrophobic polyion/surfactant attraction is seen as a very gradual binding commencing at the cac(2) and continuing past the bulk critical micelle concentration of the surfactant, resulting in an unusually large phase-separation region. For cat-HEC, the dissolution of the precipitate takes place at lower surfactant concentrations because of a stronger hydrophobic interaction between the surfactant and the polyion. The results have implications for the successful design of oppositely charged polyelectrolyte/surfactant formulations for surface deposition applications.

Thermodynamics of surfactants, block copolymers and their mixtures in water: the role of the isothermal calorimetry

The thermodynamics of conventional surfactants, block copolymers and their mixtures in water was described to the light of the enthalpy function. The two methodologies, i.e. the van’t Hoff approach and the isothermal calorimetry, used to determine the enthalpy of micellization of pure surfactants and block copolymers were described. The van’t Hoff method was critically discussed. The aqueous copolymer+surfactant mixtures were analyzed by means of the isothermal titration calorimetry and the enthalpy of transfer of the copolymer from the water to the aqueous surfactant solutions. Thermodynamic models were presented to show the procedure to extract straightforward molecular insights from the bulk properties.

Fluorescence spectroscopic investigation to identify the micelle to gel transition of aqueous triblock copolymer solutions

Steady-state and time-resolved fluorescence anisotropy measurements using probes coumarin 153 (C153) and 4-heptadecylumbelliferon (HUF) have been carried out to understand the micelle to gel transition of an aqueous triblock copolymer P123 ((EO)(20)-(PO)(70)-(EO)(20)) (EO = ethylene oxide; PO = propylene oxide) solution. Anisotropy results with a normal fluorescent probe, C153, do not show a characteristic change due to the micelle to gel transition. However, the probe HUF having a long hydrocarbon chain that helps its strong association with the micelle shows an increase in anisotropy above the sol-gel transition point. This difference has been explained as invoking a substantial contribution from the micellar structural fluctuations to the depolarization of HUF as its hydrocarbon chain is embedded in the micellar structure, which is not sensed significantly by the normal probe C153. That the extent of change in anisotropy for HUF upon gelation is not that large is possibly caused by the collective motion of the physically interconnected nodes, as observed from the dynamic light scattering studies, which acts in favor of a relatively faster depolarization in the gel phase. Similar studies in other copolymers, such as P85 ((EO)(26)-(PO)(40)-(EO)(26)) and F127 ((EO)(100)-(PO)(65)-(EO)(100)), further demonstrate the potential of probes latched with hydrocarbon chains in displaying a characteristic change for the micelle to gel transition which otherwise remains obscured for normal fluorescent probes.

Structural characterization of nonionic mixed micelles formed by C12EO6 surfactant and P123 triblock copolymer

A structural characterization of mixed micelles formed in aqueous solution by the PEO-PPO-PEO triblock copolymer P123 and the nonionic surfactant C(12)EO(6) was carried out using various techniques, including ultralow shear viscosimetry, depolarized dynamic light scattering (VH-DLS), depolarized static light scattering (VH-SLS), and small-angle X-ray scattering (SAXS). The sphere-to-rod transition of the mixed micelles was studied in a diluted regime (P123 concentrations ranging from 0.5 to 10 wt %) at C(12)EO(6)/P123 molar ratios of 2.2, 3.2, 6.0, and 11 as well as for the pure C(12)EO(6). The data from VH-SLS and viscosimetry displayed a sharp increase in the intensity and viscosity, respectively, at the sphere-to-rod transition, and the results from the two methods were in accordance. In both techniques, an increased transition temperature with increasing content of C(12)EO(6) (in the molar ratio regime from 2.2 to 11) was observed. SAXS was used as the main technique, and a thorough structural characterization was performed, where indirect Fourier transformation (IFT) and generalized indirect Fourier transformation (GIFT) were employed in the analysis procedure of the SAXS data. The p(r) functions obtained from the IFT (employed at low P123 concentrations, i.e., 1.0 and 2.0 wt %) and GIFT (employed above 2.0 wt %) analyses revealed increased inhomogeneities in the mixed micelles when the molar ratio was increased. This suggested that the C(12)EO(6) organized themselves at the interface between the PPO core and the PEO corona of the P123 micelles, with the C(12) alkyl chain stretching into the hydrophobic core and the EO(6) part residing in the hydrophilic corona. The structure factor parameters obtained with GIFT for a molar ratio of 2.2 at a P123 concentration of 5.0 wt % showed radius values smaller than what was estimated from the p(r) functions. This was explained by an interpenetration of the PEO chains from one mixed micelle into a neighboring one. VH-DLS was performed on the mixed micelles at a temperature 3 degrees C above the transition temperature and at a molar ratio of 2.2. From the analyzed data, the average length L of the rods was estimated to be 102 nm.

Micelle formation of amphiphilic polystyrene-b-poly(N-vinylpyrrolidone) diblock copolymer in methanol and water-methanol binary mixtures

The micelle formation by the amphiphilic polystyrene-block-poly(N-vinylpyrrolidone) (PS48-b-PNVP99) copolymer is investigated in methanol and water-methanol binary mixtures of various compositions using 1H NMR, fluorescence spectroscopy, static/dynamic light scattering (SLS/DLS), and transmission electron microscopy (TEM). Critical micelle concentrations (cmc) are determined by employing fluorescence spectroscopy and DLS measurements. The cmc of the PS48-b-PNVP99 block copolymer increases with increasing methanol content in the water-methanol binary mixtures, suggesting that methanol is a better solvent for the PS48-b-PNVP99 block copolymer than water-methanol mixtures or pure water. The amphiphilic PS48-b-PNVP99 diblock copolymer forms spherical micelles of Rh approximately 16 nm in pure methanol solution as revealed by DLS measurements. In contrast, significantly larger micelles having higher aggregation numbers are formed in water-methanol binary mixtures. Temperature dependent data reveal an increase in aggregation number and radius of gyration (Rg) concomitantly with temperature (10-40 degrees C). In contrast, the overall size (Rh) of the micelles remains almost constant over the same temperature range. An explanation is tendered that PNVP coronas dehydrate/desolvate at higher temperatures counteracting the increase in micelle size (Rh) caused by increased aggregation numbers (Nagg).

Solubilization of poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} in water by nonionic amphiphiles

In the presence of the nonionic alkyloxyethylene surfactant n-dodecylpentaoxyethylene glycol ether (C12E5), the anionic conjugated polyelectrolyte (CPE) poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) dissolves in water, leading to a blue shift in fluorescence and dramatic increases in fluorescence quantum yields above the surfactant critical micelle concentration (cmc). No significant changes were seen with a poly(ethylene oxide) of similar size to the surfactant headgroup, confirming that specific surfactant-polyelectrolyte interactions are important. From UV-visible and fluorescence spectroscopy, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM), and electrical conductivity, together with our published NMR and small-angle neutron scattering (SANS) results, we provide a coherent model for this behavior in terms of breakup of PBS-PFP clusters through polymer-surfactant association leading to cylindrical aggregates containing isolated polymer chains. This is supported by molecular dynamics simulations, which indicate stable polymer-surfactant structures and also provide indications of the tendency of C12E5 to break up polymer clusters to form these mixed polymer-surfactant aggregates. Radial electron density profiles of the cylindrical cross section obtained from SAXS results reveal the internal structure of such inhomogeneous species. DLS and cryo-TEM results show that at higher surfactant concentrations the micelles start to grow, possibly partially due to formation of long, threadlike species. Other alkyloxyethylene surfactants, together with poly(propylene glycol) and hydrophobically modified poly(ethylene glycol), also solubilize this polymer in water, and it is suggested that this results from a balance between electrostatic (or ion-dipole), hydrophilic, and hydrophobic interactions. There is a small, but significant, dependence of the emission maximum on the local environment.

Association of a hydrophobically modified polyelectrolyte and a block copolymer followed by fluorescence techniques

By using absorption and fluorescence (steady-state and time-resolved) techniques, the interaction between a poly(acrylic acid) (PAA), randomly grafted with pyrene (Py) units (PAAMePy55), and a triblock copolymer of poly(ethylene oxide) and poly(propylene oxide) (EO(20)PO(68)EO(20), P123) was investigated. From the fluorescence data, it is shown that upon addition of P123 a decrease of the (pyrene-pyrene, Py-Py) intramolecular association, i.e., a decrease of dynamic and static excimer formation, is observed. Time-resolved fluorescence data reveal the existence of two types of monomers (monomers that are able to form excimer, MAGRE, and isolated monomers) and two excimers. Addition of P123 causes also an increase of the amount of isolated Py monomers. The overall fluorescence data suggest that the PAAMePy55 and the P123 block copolymer associate strongly at low pH, leading to the formation of P123 micelles surrounded by one PAAMePy55 chain, where the pyrene groups are located at the PPO/PEO interface of the P123 micelles. Steady-state fluorescence results also showed that an excess of P123 micelles in solution is required for the association to occur. At high pH (pH 9 and above) the situation is less clear. The steady-state (particularly in the I(1)/I(3) ratio) and time-resolved fluorescence results indicate a contact between the pyrene groups and PEO, which then would imply that there may be an interaction, but much weaker than at low pH.

Effect of ethanol on the micellization and gelation of pluronic p123

In certain applications copolymer P123 (E21P67E21) is dissolved in water-ethanol mixtures, initially to form micellar solutions and eventually to gel. For P123 in 10, 20, and 30 wt % aqueous ethanol we used dynamic light scattering from dilute solutions to confirm micellization, oscillatory rheometry, and visual observation of mobility (tube inversion) to determine gel formation in concentrated solutions and small-angle X-ray scattering (SAXS) to determine gel structure. Except for solutions in 30 wt % aqueous ethanol, a clear-turbid transition was encountered on heating dilute and concentrated micellar solutions alike, and as for solutions in water alone (Chaibundit et al. Langmuir 2007, 23, 9229) this could be ascribed to formation of wormlike micelles. Dense clouding, typical of phase separation, was observed at higher temperatures. Regions of isotropic and birefringent gel were defined for concentrated solutions and shown (by SAXS) to have cubic (fcc and hcp) and hexagonal structures, consistent with packed spherical and elongated micelles, respectively. The cubic gels (0, 10, and 20 wt % ethanol) were clear, while the hex gels were either turbid (0 and 10 wt % ethanol), turbid enclosing a clear region (20 wt % ethanol), or entirely clear (30 wt % ethanol). The SAXS profile was unchanged between turbid and clear regions of the 20 wt % ethanol gel. Temperature scans of dynamic moduli showed (as expected) a clear distinction between high-modulus cubic gels (G'max approximately 20-30 kPa) and lower modulus hex gels (G'max<10 kPa).

Initial stages of SBA-15 synthesis: an overview

This work presents an overview of the data obtained for SBA-15 synthesis under the reaction conditions using synchrotron based small angle X-ray scattering and small angle neutron scattering. Three major stages in the synthesis of SBA-15 materials proceeding according to the cooperative self-assembly mechanism have been identified, and the structures of the intermediates species have been established. Our in situ time-resolved neutron scattering experiments demonstrate that only spherical micelles of the templating agent are present in the synthesis mixture during the first stage of the reaction. According to the neutron scattering and X-ray scattering data, in the second stage of the reaction the formation of hybrid organic-inorganic micelles is accompanied with the transformation from spherical to cylindrical micelles, which takes place before the precipitation of the ordered SBA-15 phase. During the third stage, these micelles aggregate into a two-dimensional hexagonal structure, confirming that the precipitation takes place as the result of self-assembly of the hybrid cylindrical micelles. As the synthesis proceeds, the voids between the cylinders are filled with the silicate species which undergo condensation reactions resulting in cross-linking and covalent bonding, leading to the formation of highly ordered SBA-15 mesostructure. This work demonstrates that valuable structural information can be obtained from X-ray and neutron scattering characterisation of complex systems containing periodic phases with d-spacing values up to 30 nm, and that both techniques are powerful means for in situ monitoring of the formation of nanostructured materials.

Chemical analysis and aqueous solution properties of charged amphiphilic block copolymers PBA-b-PAA synthesized by MADIX®

We have linked the structural and dynamic properties in aqueous solution of amphiphilic charged diblock copolymers poly(butyl acrylate)-b-poly(acrylic acid), PBA-b-PAA, synthesized by controlled radical polymerization, with the physico-chemical characteristics of the samples. Despite product imperfections, the samples self-assemble in melt and aqueous solutions as predicted by monodisperse microphase separation theory. However, the PBA core are abnormally large; the swelling of PBA cores is not due to AA (the Flory parameter chi(PBA/PAA), determined at 0.25, means strong segregation), but to h-PBA homopolymers (content determined by liquid chromatography at the point of exclusion and adsorption transition, LC-PEAT). Beside the dominant population of micelles detected by scattering experiments, capillary electrophoresis CE analysis permitted detection of two other populations, one of h-PAA, and the other of free PBA-b-PAA chains, that have very short PBA blocks and never self-assemble. Despite the presence of these free unimers, the self-assembly in solution was found out of equilibrium: the aggregation state is history dependant and no unimer exchange between micelles occurs over months (time-evolution SANS). The high PBA/water interfacial tension, measured at 20 mN/m, prohibits unimer exchange between micelles. PBA-b-PAA solution systems are neither at thermal equilibrium nor completely frozen systems: internal fractionation of individual aggregates can occur.

Preparation parameter development for layer-by-layer assembly of Keggin-type polyoxometalates

Polyoxometalates possess many useful properties for electrochemical catalysis. These molecule-size clusters can be assembled into thin films through the layer-by-layer method. In this study, we determined a cluster concentration range within which layer-by-layer (LbL) films have been successfully fabricated. We also find the influence of salt added to the deposition solutions. In an attempt to understand the self-assembly process at the molecular level, thermodynamic arguments, derived from complexation between nanoscale particles and oppositely charged polyelectrolyte chains, have been employed to interpret the adsorption of polyoxometalate clusters onto a cationic polymer layer. The scaling results describe the contact mode between a polymer chain and a cluster. The assembly can be visualized with assistance by understanding the contact between the polymer chain and the cluster.

Rheological study of the shape transition of block copolymer-nonionic surfactant mixed micelles

A rheological study of mixed micelles formed by PEO-PPO-PEO triblock copolymer P123 and nonionic surfactant C12EO6 in aqueous solutions has been carried out with the purpose of investigating the time dependence of a shape transition of the mixed micelles and characterizing the shape before and after the transition. The rheology results presented in this report give clear evidence that the P123-C12EO6 mixed micelle grows and changes gradually in shape from spherical to elongated (rodlike) geometry with increasing temperature. These results are in accordance with the results found in the parallel dynamic and static light scattering and calorimetrical investigation.1,2 By using steady-state rheology, the time dependence of the sphere-to-rod transition of the mixed micelle system was carefully followed with time and temperature as simultaneously recorded variables in the experiments. This was performed by a designed novel experimental procedure. A temperature ramp was applied at a rate of 2.6 degrees C/min from a temperature below to a temperature above the shape transition at a constant shear rate while the viscosity of the solution was measured. The investigation was limited to two different compositions, surfactant-to-copolymer molar ratios (MR=nC12EO6/nP123) of 2.2 and 6.0 with varying total concentration from 1.5 to 21 wt % in comparison with the neat component. At low concentration, a slow transition was observed, which indicated that the mixed micelles are still growing into rods for several minutes after reaching the final temperature. At a total concentration of 4.0 wt % and above, the system reached equilibrium quickly. A concentration-dependent kinetic process is therefore anticipated, which was also found in the time-resolved static light scattering experiments previously performed (Löf, D.; Schillén, K.; Olofsson, G.; Niemiec, A.; Loh, W. J. Phys. Chem. B 2007, 111, 5911). At concentrations above 10 wt %, shear-thinning behavior was observed for the mixed solutions, which strongly suggests the extended shape of the mixed micelles after the shape transition. The obtained zero-shear viscosity at the investigated molar ratios was found to be lower with higher molar ratios, which indicates that the mixed micelles both in the spherical and in the rodlike state becomes smaller with higher content of C12EO6. These results correlate well with the obtained results from the previous dynamic light scattering measurements on the same system (Löf, D.; Schillén, K.; Olofsson, G.; Niemiec, A.; Loh, W. J. Phys. Chem. B 2007, 111, 5911).

Effect of ionic surfactants on the hydration behavior of triblock copolymer micelles: a solvation dynamics study of coumarin 153

Dynamic fluorescence Stokes shift measurements of coumarin 153 (C153) have been carried out to study the influence of ionic surfactants (sodium dodecyl sulfate, SDS and hexadecyltrimethylammonium chloride, CTAC) on the hydration behavior of aqueous poly(ethylene oxide)(20)-poly(propylene oxide)(70)-poly(ethylene oxide)20 (P123) block copolymer micelles. Increase in SDS or CTAC concentration at a fixed P123 concentration induces the steady-state emission spectra of C153 to shift gradually toward lower energy. This is attributed to an increase in polarity (due to enhanced hydration) experienced by the probe as a consequence of incorporation of ionic head groups in the Corona region. The observed dynamic fluorescence Stokes shift value decreases more in mixed micellar systems than in pure copolymer micelles and the trends are quite similar in the presence of SDS and CTAC. The spectral shift correlation functions were observed to be nonexponential in nature. Critical analysis of the spectral shift correlation function indicates a fast solvation component (<0.2 ns) in P123 micelles, which was absent in the presence of ionic surfactants. Due to increased hydration in the presence of ionic surfactants, the initial fast solvation event was elusive in mixed copolymer-surfactant systems, reflecting the absence of faster solvation component and reduced observed Stokes shift in mixed systems. It has been argued that in the low surfactant concentration region, increase in hydration with the incorporation of ionic head groups in the Corona region is mainly due to increase in mechanically trapped water content. However, at higher surfactant concentrations, bound water content dominates and leads to slower solvation dynamics. The present results also indicate that though CTAC alters the Corona hydration more efficiently than SDS, the overall influence of ionic surfactants on the Corona hydration is grossly similar irrespective of the cationic or anionic nature of the surfactants. Interaction of SDS and CTAC with poly(ethylene oxide)(100)-poly(propylene oxide)(70)-poly(ethylene oxide)(100) (F127) block copolymer micelles has also been studied to comprehend the effect of copolymer composition. The overall trends in dynamic fluorescence Stokes shift and solvation times are similar in both the copolymer micelles.

Controlling the melting of kinetically frozen poly(butyl acrylate-b-acrylic acid) micelles via addition of surfactant

We have studied the melting of polymeric amphiphilic micelles induced by small-molecule surfactant and explained the results by experimental determination of the interfacial tension between the core of the micelles and the surfactant solutions. Poly(n-butyl acrylate-b-acrylic acid) (PBA-b-PAA) amphiphilic diblock copolymers form kinetically frozen micelles in aqueous solutions. Strong interactions with surfactants, either neutral or anionic [C12E6, C6E4, sodium dodecyl sulfate (SDS)], were revealed by critical micelle concentration (cmc) shifts in specific electrode and surface tension measurements. Since both polymer and surfactant are either neutral or bear negative charges, the attractive interactions are not due to electrostatic interactions. Light scattering, neutron scattering, and capillary electrophoresis experiments showed important structural changes in mixed PBA-b-PAA/surfactant systems. Kinetically frozen micelles of PBA-b-PAA, that are hardly perturbed by concentration, ionization, ionic strength, and temperature stresses, can be disintegrated by addition of small-molecule surfactants. The interfacial energy of the PBA in surfactant solutions was measured by drop shape analysis with h-PBA homopolymer drops immersed in small-molecule surfactant solutions. The PBA/water interfacial energy gammaPBA/H2O of 20 mN/m induces a high energy cost for the extraction of unimers from micelles so that PBA-b-PAA micelles are kinetically frozen. Small-molecule surfactants can reduce the interfacial energy gammaPBA/solution to 5 mN/m. This induces a shift of the micelle-unimer equilibrium toward unimers and leads, in some cases, to the apparent disintegration of PBA-b-PAA micelles. Before total disintegration, polymer/surfactant mixtures are dispersions of polydisperse mixed micelles. Based on core interfacial energy arguments, the disintegration of kinetically frozen polymeric micelles was interpreted by gradual fractionation of objects (polydisperse dispersion mechanism), whereas the disintegration of polymeric micelles in a thermodynamically stable state was interpreted by an exchange between a population of large polymer-rich micelles and a population of small surfactant-rich micelles (bidisperse dispersion mechanism). Finally, in our system and other systems from the literature, interfacial energy arguments could explain why the disintegration of polymer micelles is either partial or total as a function of the surfactant type and concentration and the hydrophobic block molar mass of the polymer.

Peptides and Gd complexes containing colloidal assemblies as tumor-specific contrast agents in MRI: physicochemical characterization

The aggregation behavior of an amphiphilic supramolecular system, with potential application as a tumor-specific magnetic resonance imaging contrast agent, has been studied in detail by dynamic light scattering, small-angle neutron scattering and cryotransmission electron microscopy. The system was constituted of mixed aggregates formed by an anionic unimer containing the DTPAGlu, a chelating agent for the paramagnetic Gd(3+) ion, and an uncharged unimer containing the bioactive peptide CCK8, capable of directing the assembly toward tumor cells. Mixed aggregates formed by both unimers, and in the case of the DTPAGlu unimer with the chelating agent as free base or as Gd(3+) complex, have been investigated. A number of interesting features of the aggregation behavior were revealed: at physiological pH, micelles and bilayer structures were present, whereas upon decreasing solution pH or increasing ionic strength, the formation of bilayer structures was favored. On the basis of the above observations, the aggregating mechanism has been elucidated by considering the screening effect on intra- and interaggregate electrostatic repulsions.

Dynamic and static light scattering analysis of DNA ejection from the phage lambda

With the aid of time-resolved dynamic light scattering (DLS) and static light scattering (SLS), we have analyzed the ejection kinetics from the bacterial virus bacteriophage (or phage) lambda , triggered in vitro by its receptor. We have used DLS to investigate the kinetics in such a system. Furthermore, we have shown that both SLS and DLS can be interchangeably used to study the process of phage DNA release. DLS is superior to SLS in that it also allows the change in the light scattering arising from each of the components in the system to be monitored under conditions such that the relaxation times are separable. With help of these two methods we present a model explaining the reason for the observed decrease in the scattering intensity accompanying DNA ejection from phage. We emphasize that ejection from phage capsid occurs through a very long tail (which is nearly three times longer than the capsid diameter), which significantly separates ejected DNA from the scattering volume of the capsid. The scattering intensity recorded during the DNA ejection process is the result of a change in the form factor of the phage particle, i.e., the change in the interference effects between the phage capsid and the DNA confined in the phage particle. When the DNA molecule is completely ejected it remains in the proximity of the phage for some time, thus contributing to the scattering signal as it diffuses away from the phage capsid, into the scattering volume and returns to its unperturbed chain conformation in bulk solution. The free DNA chain does not contribute to the scattered intensity, when measured at a large angle, due to the DNA form factor and the low concentration. Although the final diffusion-controlled step can lead to overestimation of the real ejection time, we can still use both scattering methods to estimate the initial DNA ejection rates, which are mainly dependent on the pressure-driven DNA ejection from the phage, allowing studies of the effects of various parameters affecting the ejection.

Plum-Pudding Gels as a Platform for Drug Delivery: Understanding the Effects of the Different Components on the Diffusion Behavior of Solutes

The internal structure of composite gels made of responsive microgel particles inserted into a bulk hydrogel (N-isopropylacrylamide microgel particles in a cross-linked dimethylacrylamide matrix) has been investigated from the diffusion behavior of poly(ethylene glycol) (PEG) probes through the network, in the absence of specific interactions between the diffusing molecules and the system. The effect of the different components has been examined, for example, the size of the probe, the bulk structure, and the microgel nature. Particles were characterized prior to their insertion into the hydrogel in order to describe their properties as a function of size and cross-linker content, thus revealing different swelling behaviors. The biggest effects on the diffusion of the PEG probes were related to the bulk structure, and no major effects were registered by the addition of different microgels into the hydrogel network. We attempt to rationalize this behavior in terms of the composite gel structure and discuss the results in terms of their meaning for controlled drug delivery strategies.

Rotational Diffusion of Organic Solutes in Surfactant−Block Copolymer Micelles: Role of Electrostatic Interactions and Micellar Hydration

Rotational diffusion of a cationic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole, DMDPP has been examined in the surfactant-block copolymer system of sodium dodecyl sulfate (SDS) and poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123). In this study, the mole ratio of SDS to P123 was varied from 0 to 5 in steps of one unit, to investigate the role of electrostatic interactions and micellar hydration on solute rotation. It has been noticed that there is a significant enhancement in the average reorientation time of rhodamine 110, when [SDS]/[P123] increased from 0 to 1. This has been rationalized on the basis of migration of rhodamine 110 from the interfacial region of P123 micelles to the palisade layer (corona region) due to the electrostatic interaction with negatively charged head groups of SDS, whose tails are embedded in the polypropylene oxide core. Further increase in the mole ratio of SDS to P123 has resulted in only a marginal decrease in the average reorientation time of rhodamine 110, which is probably due to the solute molecule experiencing a microenvironment similar to the interfacial region of SDS micelles. In contrast, a gradual decrease has been observed in the average reorientation time of DMDPP with [SDS]/[P123], which is due to the increase in hydration levels in the palisade layer (corona region) of the micelle. These explanations are consistent with the structure of the SDS-P123 micellar system that has been deduced from neutron scattering and viscosity measurements recently.