Competitive adsorption of amphiphilic molecules and the stability of water-swollen micelles in oil

Motivated by recent attempts to confine biochemical processes inside water-in-oil microemulsions, we studied the composition and stability of mixed-amphiphile water-swollen micelles in oil from a theoretical point of view. A novel adsorption model demonstrates how the micellar contents (DNA, proteins, etc.) can dramatically affect the composition of the amphiphilic film and the resulting distribution of micelles. Special attention is given to the effect of electrostatic interactions within the micelles as well as between different ones. Since in a low dielectric medium charge fluctuations can lead to long-range intermicellar attractions, we suggest that the presence of amphiphilic polymers in the surfactant film may be needed to stabilize such microemulsions.     

Static and dynamic structures of spherical nonionic surfactant micelles during the disorder-order transition

We have investigated the static and dynamic structures of nonionic surfactant micelles, a C(12)E(8)/water binary system, during the disorder-order transition using small angle x-ray scattering, static light scattering, and dynamic light scattering techniques. In the disordered phase, the micelles have spherical shape and intermicellar interactions are governed by the hard core and weak long ranged attractive potentials. With increase of the micellar concentration, the disordered micelles transform to the three characteristic ordered micellar phases, a hexagonally close packed lattice, a body centered cubic lattice, and an A15 lattice having area-minimizing structure. The stability of these phases is well explained by balance of a close packing rule and a minimal-area rule proposed by Ziherl and Kamien [Phys. Rev. Lett. 85, 3528 (2000)]. The role of hydrodynamic interactions in surfactant micellar solutions was compared with that in hard sphere colloidal particle suspensions.     

Structure and interactions in micellar solutions: molecular simulations of pluronic L64 aqueous solutions

Coarse-grained, implicit solvent molecular simulations have been conducted to investigate the structure and interactions of L64 Pluronic micelles in aqueous solutions. Simulations of an L64 solution beginning with monodisperse micelles (aggregation number Nagg = 40 chains) resulted in a narrow Gaussian distribution of Nagg centered around 40. While not fully equilibrated, this distribution supports the supposition that L64 micelles with Nagg = 40 are representative of the conditions considered and model employed. Detailed analysis of intramicellar monomers distribution and micelle shapes revealed that L64 micelles have a scalene ellipsoidal shape. Additional simulations of solutions containing 125 micelles constrained to have Nagg = 40 at polymer volume fractions of 0.024 and 0.110 were performed to study micelle-micelle structure factor, single micelle form factor, and total scattering intensity. The ability of various models utilized in analysis of scattering profiles in micellar solutions to describe the structure of the model L64 solutions was investigated. Investigation of the potential of mean force between two micelles reveals that the interactions between micelles are repulsive but on a length scale smaller than the mean micelle diameter, indicating that the micellar shape fluctuations are important in determining intermicellar interactions.     

Selective transport of amino acids into the gas phase: driving forces for amino acid solubilization in gas-phase reverse micelles

We report a study on encapsulation of various amino acids into gas-phase sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) reverse micelles, using electrospray ionization guided-ion-beam tandem mass spectrometry. Collision-induced dissociation of mass-selected reverse micellar ions with Xe was performed to probe structures of gas-phase micellar assemblies, identify solute-surfactant interactions, and determine preferential incorporation sites of amino acids. Integration into gas-phase reverse micelles depends upon amino acid hydrophobicity and charge state. For examples, glycine and protonated amino acids (such as protonated tryptophan) are encapsulated within the micellar core via electrostatic interactions; while neutral tryptophan is adsorbed in the surfactant layer. As verified using model polar hydrophobic compounds, the hydrophobic effect and solute-interface hydrogen-bonding do not provide sufficient driving force needed for interfacial solubilization of neutral tryptophan. Neutral tryptophan, with a zwitterionic structure, is intercalated at the micellar interface between surfactant molecules through complementary effects of electrostatic interactions between tryptophan backbone and AOT polar heads, and hydrophobic interactions between tryptophan side chain and AOT alkyl tails. Protonation of tryptophan could significantly improve its incorporation capacity into gas-phase reverse micelles, and displace its incorporation site from the micellar interfacial zone to the core; protonation of glycine, on the other hand, has little effect on its encapsulation capacity. Another interesting observation is that amino acids of different isoelectric points could be selectively encapsulated into, and transported by, reverse micelles from solution to the gas phase, based upon their competition for protonation and subsequent encapsulation within the micellar core.     

荧光和动态光散射方法研究两性表面活性剂胶束的聚集和相互作用

以芘为荧光探针、二苯酮为猝灭剂,利用荧光方法测定了两性表面活性剂N-十二烷基-N,N-二甲基氨基丙磺酸盐(DDAPS)胶束在不同温度和不同NaCl浓度下的聚集数。利用动态光散射方法得到了胶束的水力半径Rh.结果表明,DDAPS的胶束聚集数和Rh值随NaCl浓度的升高略有增大;随温度的升高而稍有下降。DDAPS胶束之间的作用力以排斥力为主。     

Structure of BRIJ-35 nonionic surfactant in water: a reverse Monte Carlo study

There are some contradictions in the literature on the structure of micelles formed by the BRIJ-35 surfactant in water. One can find reasonable differences in the aggregation numbers and micellar sizes, but there is a lack of data on the intermicellar structure. In this study, we reevaluated the small-angle neutron scattering experiments performed previously on the BRIJ-35 surfactant in the concentration range of 5-200 g/dm3 at 20, 40, and 60 degrees C. The data were analyzed with a reverse Monte Carlo-type method developed recently for colloids. The micelles were modeled as spherical cores representing the hydrophobic parts and number of balls put on the cores to mimic the hydrated hydrophilic chains. The simulations provided data on the mean aggregation number and on the extent of hydration of the hydrophilic shell of the micelles. We obtained intermicellar pair-correlation functions indicating different micelle-micelle interactions from the usually assumed hard-sphere ones.     

Environmentally responsive micelles from polystyrene–poly[bis(potassium carboxylatophenoxy)phosphazene] block copolymers

Amphiphilic diblock copolymers that contained hydrophilic poly[bis(potassium carboxylatophenoxy)phosphazene] segments and hydrophobic polystyrene sections were synthesized via the controlled cationic polymerization of Cl₃P?NSiMe₃ with a polystyrenyl–phosphoranimine as a macromolecular terminator. These block copolymers self‐associated in aqueous media to form micellar structures which were investigated by fluorescence spectroscopy, dynamic light scattering, and transmission electron microscopy. The size and shape of the micelles were not affected by the introduction of different monovalent cations (Li⁺, K⁺, Na⁺, and Cs⁺) into the stable micellar solutions. However, exposure to divalent cations induced intermicellar crosslinking through carboxylate groups, which caused precipitation of the ionically crosslinked aggregates from solution. This micelle‐coupling behavior was reversible: the subsequent addition of monovalent cations caused the redispersion of the polystyrene‐block‐poly[bis(potassium carboxylatophenoxy)phosphazene] (PS–KPCPP) block copolymers into a stable micellar solution. Aqueous micellar solutions of PS–KPCPP copolymers also showed pH‐dependent behavior. These attributes make PS–KPCPP block copolymers suitable for studies of guest retention and release in response to ion charge and pH. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2912–2920, 2005     

Nanoparticle-reinforced associative network hydrogels

ABA triblock copolymers in solvents selective for the midblock are known to form associative micellar gels. We have modified the structure and rheology of ABA triblock copolymer gels comprising poly(lactide)-poly(ethylene oxide)-poly(lactide) (PLA-PEO-PLA) through addition of a clay nanoparticle, laponite. Addition of laponite particles resulted in additional junction points in the gel via adsorption of the PEO corona chains onto the clay surfaces. Rheological measurements showed that this strategy led to a significant enhancement of the gel elastic modulus with small amounts of nanoparticles. Further characterization using small-angle X-ray scattering and dynamic light scattering confirmed that nanoparticles increase the intermicellar attraction and result in aggregation of PLA-PEO-PLA micelles.     

Aqueous surfactant-alcohol systems: A review

The formation and properties of aggregates in aqueous surfactant + alcohol mixtures are reviewed, with particular emphasis on: (i) alcohol partition coefficient in micellar solutions; (ii) critical micelle concentration and micelle ionization degree (iii) micelle size and shape and intermicellar interactions; (iv) theoretical aspects; (v) dynamics of the mixed surfactant + alcohol micelles; (vi) phase diagrams of, and microstructure in, selected mixtures; (vii) role of alcohols in microemulsions.     

连接基长度对Gemini表面活性剂胶团间相互作用的影响

用电导率法和动态光散射法测定十二烷基三甲基溴化铵和季铵盐型Gemini表面活性剂胶团的电离度和扩散系数,并结合DLVO理论研究联接基长度和电解质浓度对胶团间相互作用的影响.实验结果表明,联接基团长度会改变胶团电离度和胶团表面电荷密度,从而影响胶团间的相互作用,其影响程度主要取决于联接基的吸电子能力和Gemini表面活性剂分子中两个带电基团的电荷重叠程度;电解质浓度对胶团间相互作用的影响可分为两种情况:在低电解质浓度时,胶团间的相互作用以排斥力为主,不利于胶团的生长;而在高电解质浓度时,胶团间的相互作用以吸引力为主,有利于胶团的生长.     

Incorporating intermicellar interactions in the fitting of SANS data from cationic wormlike micelles

Small-angle neutron scattering (SANS) from cationic wormlike micellar solutions composed of hexadecyltrimethylammonium bromide (CTABr) and hexadecylpyridinium bromide (CPyBr) in deuterated water was studied at 40 degrees C as a function of surfactant and salt concentrations. Two scattering functions of semiflexible chains incorporating excluded volume effects, with and without the intermicellar interactions, were used in SANS data model fitting. Two needed changes were made in the well-accepted models. Extensive and systematic SANS data analysis suggests the robustness of these corrected scattering functions when the intermicellar interactions are included. The influence of the headgroups and ionic strength on the contour length and micellar flexibility of these two systems was demonstrated on the basis of the quantitative structural information obtained from the model fitting. Micellar flexibility was found to depend on surfactant concentration, even when intermicellar interactions were taken into account, despite predictions to the contrary.     

Coexistence and growth of micellar species in a sugar-based surfactant/phenol mixture studied by analytical ultracentrifugation

Knowledge of the shape and size of surfactant micelles in the presence of small organic molecules is important for understanding the solubilization properties of micellar phases. In this work, structural information on micelles of mixed n-dodecyl-beta-d-maltoside (DM) and phenol, including the aggregation number, diffusion coefficient, and effective radius, was obtained using an analytical ultracentrifugation technique. The micelles were found to increase in size and undergo shape transition from quasispherical to cylindrical with an increase in the surfactant and phenol concentrations in the micellar phase. Importantly, the coexistence of different micellar species was observed in certain cases with the larger species double the size of the smaller one. Based on the results obtained, a two-step micellar growth model is proposed to describe the micelles shape transition in the system. In the first step, the micelles expand continuously, whereas in the second step, it undergoes a sudden shift from the existing micellar species to a larger species causing the coexistence of two micellar species. This micellar growth is attributed to molecular packing and intermicellar interaction energy parameters. The mechanism proposed can be applied to other mixed systems and utilized for devising chemicals for the efficient removal of pollutants.     

Interaction of amphiphilic block copolymer micelles with surfactants

An out line and summary of literature studies on interactions between different types of amphiphilic copolymer micelles with surfactants has been given. This field of research is still emerging and it is difficult presently to make generalisations on the effects of surfactants on the copolymer association. The effects are found to be varied depending upon the nature and type of hydrophobic (hp) core and molecular architecture of the copolymers and the hydrocarbon chain length and head group of surfactants. The information available on limited studies shows that both anionic and cationic surfactants (in micellar or molecular form) equally interact strongly with the associated and unassociated forms of copolymers. The beginning of the interaction is typically displayed as critical aggregation concentration (CAC), which lies always below the critical micelle concentration of the respective surfactant. The surfactants first bind to the hydrophobic core of the copolymer micelles followed by their interaction with the hydrophilic (hl) corona parts. The extent of binding highly depends upon the nature, hydropobicity of the copolymer molecules, length of the hydrocarbon tail and nature of the head group of the surfactant. The micellization of poly(ethylene oxide) (PEO)–poly(propylene oxide) (PPO)–poly(ethylene oxide) was found to be suppressed by the added surfactants and at higher surfactant concentrations, the block copolymer micelles get completely demicellized. This effect was manifested itself in the melting of liquid crystalline phases in the high copolymer concentrations. However, no such destabilization was found for the micelles of polystyrene (PS)–poly(ethylene oxide) copolymers in water. On the contrary, the presence of micellar bound surfactant associates resulted in to large super micellar aggregates through induced intra micellar interactions. But with the change in the hydrophobic part from polystyrene to poly(butadiene) (PB) in the copolymer, the added surfactants not only reduced the micellar size but also transformed cylindrical micelles to spherical ones. The mixtures in general exhibited synergistic effects. So varied association responses were noted in the mixed solutions of surfactants and copolymers.     

Redox-gated potential micellar carriers based on electrostatic assembly of soft coordination suprapolymers

We report in this paper the release and uptake of charged payloads in redox responsive electrostatic micellar systems composed of negatively charged soft iron coordination suprapolymers and positively charged block copolymers. This micellar system was reported in our previous work (Yan, Y.; Lan, Y. R.; de Keizer, A.; Drechsler, M.; Van As, H.; Stuart, M. A. C.; Besseling, N. A. M. Redox responsive molecular assemblies based on metallic coordination polymers. Soft Matter, 2010, 6, 3244-3248), where we proposed that the system can be used as a redox-triggered release and uptake system. In this paper, we successfully selected a negatively charged fluorescent dye, eosin B, as a model cargo to track the release and upload process. Upon being compacted in the mixed micelles of coordination polymers and diblock copolymers, the fluorescence of eosin B was effectively quenched. Once reduction was conducted, excess negative charges were introduced to the mixed micelles so that the negatively charged eosin B was expelled out which was accompanied by the recovery of the fluorescence. The free negatively charged eosin B was able to be taken up by the Fe(II) micelles again if oxidation of Fe(II) was carried out since excess positive charges were produced. Beside eosin B, other charged species, such as various charged macromolecules, were tested to be capable of uptake and release by this micellar system. We suppose this system can be potentially used as a redox-gated micellar carrier for uptake and release of charged cargos.     

Mixed micelles based on cationic and anionic amphiphilic diblock copolymers containing identical hydrophobic blocks

Through the use of the methods of turbidimetry, UV spectrophotometry, fluorescence spectroscopy, dynamic light scattering, and ultracentrifugation, micelle formation is studied for cationic (polysty-rene-poly-N-ethyl-4-vinylpyridium bromide) and anionic (polystyrene-sodium polyacrylate) diblock copolymers containing identical polystyrene blocks in dilute aqueous saline solutions. Mixing of aqueous dispersions of individual micelles is accompanied by the formation of only insoluble products, which likely are intermicellar interpolyelectrolyte complexes. At the same time, mixing of diblock copolymers in a nonselective solvent and its subsequent gradient replacement with water during suppressed interpolyelectrolyte interactions yields mixed diblock copolymer micelles, which are found to be dispersionally stable in an excess of charged units of any polymer component. The micelles are composed of an insoluble polystyrene core and a mixed interpolyelectrolyte corona, and their hydrodynamic characteristics are controlled by the ratio of charged units in the mixed diblock copolymers. The mixed micelles are found to be able to interact with the macromolecules of a homopolyelectrolyte, sodium poly(styrene sulfonate), in aqueous solutions and form ternary complexes. In this case, depending on the composition of the mixed micelles, ternary complexes can be dispersionally stable or can aggregate and precipitate.     

Discrete π‐Stacks from Self‐Assembled Perylenediimide Analogues

The formation of well‐defined finite‐sized aggregates represents an attractive goal in supramolecular chemistry. In particular, construction of discrete π‐stacked dye assemblies remains a challenge. Reported here is the design and synthesis of a novel type of discrete π‐stacked aggregate from two comparable perylenediimide (PDI) dyads ( PEP and PBP ). The criss‐cross PEP ‐ PBP dimers in solution and ( PBP ‐ PEP )‐( PEP ‐ PBP ) tetramers in the solid state are well elucidated using single‐crystal X‐ray diffraction, dynamic light scattering, and diffusion‐ordered NMR spectroscopy. Extensive π–π stacking between the PDI units of PEP and PBP as well as repulsive interactions of swallow‐tailed alkyl substituents are responsible for the selective formation of discrete dimer and tetramer stacks. Our results reveal a new approach to preparing discrete π stacks that are appealing for making assemblies with well‐defined optoelectronic properties.     

Structure and dynamics of concentrated micellar solutions of sodium dodecyl sulfate

In micellar solutions of sodium dodecyl sulfate, as the concentration of surfactants increases, the spheroid shape of the micelles changes from almost spherical to ellipsoidal with increasing ratio of half-axes ratio, and further the transition to cylindrical micelles occurs. The micelles in an aqueous solution can directly contact (compact aggregates) or be separated from one another by layers of intermicellar medium (periodical colloid structures). In the latter case, the thickness of the layer can significantly exceed the micelle size, and then no mutual correlation in micelle arrangement is observed. According to the data of small-angle X-ray scattering, the relationship between the surfactant concentration and formation of “quasi-crystalline” micellar structure is nonlinear, which can be due to both micelle aggregation processes and nonuniformity of their structure. The possible influence of ordered micellar structures on the diffusion mobility of micelles is shown.     

Time-dependent shrinkage of polymeric micelles of amphiphilic block copolymers containing semirigid oligocholate hydrophobes

Alkyne-derivatized poly(ethylene glycol) (M.W. 5000) was coupled to several azide-terminated oligocholates by the click reaction to form amphiphilic block copolymers. A copolymer with a cholate hexamer as the hydrophobic block formed polymeric micelles that shrank by ~50% over a period of 10 h at 25°C. Shrinkage was faster and more dramatic at 35°C. Shortening the oligocholate by two units or inserting a 4-aminobutyroyl spacer in the hexacholate eliminated or diminished the shrinkage. Metastable aggregates were proposed to form when the block copolymers began to aggregate in water. The large hydrophobic surface, awkward shape, rigidity, and facial amphiphilicity of the cholate repeat unit and the long chain made it difficult for the oligocholates to adjust within the micellar core. As the oligocholates rearranged to maximize hydrophobic interactions and hydrogen-bonding while minimizing conformational strain, the polymeric micelles became more compact over time.     

Evidence for micellar structure in the gas phase

We have compared micelles, reverse micelles, and reverse micelles encapsulating myoglobin using electrospray mass spectrometry. To enable a direct comparison, the same surfactant (cetyltrimethylammonium bromide (CTAB)) was used in each case and micelle formation was controlled by manipulating the aqueous and organic phases. Tandem mass spectra of the resulting micelle preparations reveal differences in the ions that dissociate: those that dissociate from regular micelles have undergone >90% exchange of bromide ions from the headgroup with acetate ions from bulk solvent. By contrast, for reverse micelles, ions are detected without exchange of bromide ions from the headgroup, consistent with their protection in the core of the micellar structure. Tandem mass spectra of micelles and reverse micelles reveal polydispersed assemblies containing several hundred CTAB molecules, indicating the coalescence of the micellar systems to form large assemblies. For reverse micelles incorporating myoglobin, spectra are consistent with one holo myogolobin molecule in association with approximately 270 CTAB molecules. Overall, therefore, our results show that the solution-phase orientation of surfactants is preserved during electrospray and are consistent with interactions being maintained between surfactants and an encapsulated protein.     

Dynamic light scattering from non-entangled wormlike micellar solutions

The fuzzy cylinder theory, originally proposed for conventional polymer solutions, was applied to wormlike micellar solutions to take into account effects of the intermicellar collision and hydrodynamic interaction on the self-diffusion of wormlike micelles in solution at finite concentrations. Previously reported apparent hydrodynamic radius data obtained by dynamic light scattering for non-entangled wormlike micelles formed in aqueous solution by non-ionic surfactants, polyoxyethylene monoalkyl ethers C(i)E(j), were analyzed by this theory to estimate the persistence length q of the wormlike micelles. The results of q estimated were consistent with those obtained from radius of gyration data obtained by static light scattering.