Logo: to the web site of Uppsala University

Contrast variation SANS and (19)F chemical shifts were measured for three mixed equimolar micelle systems: sodium perfluorooctanoate (SPFO) and sodiumdecylsulfate (SDeS) in 200 mM NaCl, lithium perfluorononanate (LiPFN) and lithium dodecylsulfate (LiDS) in 200 mM LiCl, and a nonionic system C(8)F(17)C(2)H(4)(OC(2)H(4))(9) and C(12)H(25)(OC(2)H(4))(8) in water, all at 25 degrees C. The chemical shift measurements allow the calculation of the average fraction of nearest neighbors of each kind around the reporter group (the trifluoromethyl group). A preference for like neighbors were found in all systems, smallest in the SDeS/SPFO system and largest in the nonionic system, but in all cases substantially smaller than expected at critical conditions. From the SANS measurements the width of the micelle composition distribution was obtained. For the ionic systems similar values were obtained, showing a broadening compared to ideal mixtures, but not broad enough for demixing or clearly bimodal distributions. In the nonionic system the width was estimated as sigma = 0.18 and 0.22 using two different evaluation methods. These values suggest that the system is close to critical conditions. The lower value refers to a direct modeling of the system, assuming an ellipsoidal shape and a Gaussian composition distribution. The modeling showed the nonionic mixed micelles to be prolate ellipsoids with axial ratio 2.2 and an aggregation number larger than 100, whereas the two ionic systems fitted best to oblate shapes (axial ratios 0.8 and 0.65 for SDeS/SPFO and LiDS/LiPFN, respectively) and aggregation numbers of 60 for both.

Stomatosomes and related structures formed by perforated fluid bilayer have been identified in a number of systems comprising surfactants and/or lipids. The types of system show a strong variation, but at the same time the conditions allowing such structures to evolve seem rather restrictive. Small changes in the nature of the surfactant can make the perforated bilayers disappear, and also give a major change of the phase behavior. This behavior is exemplified with phase diagrams for systems with C_xTAC (alkyltrimethylammonium chloride, x is number of carbons in the alkyl chain; x = 12,14,16) and lipids in brine. On an increase of the spontaneous curvature the perforated bilayers can be found between lamellar bilayers (or smooth vesicles in dilute samples) and cylindrical micelles. The formation of perforations may be understood as a cooperative process starting at a critical concentration of a surfactant in the bilayer. The surfactant promotes the formation of toroidal holes rimmed by hemi cylindrical micelles. With increasing surfactant concentration the surfactant-rich edges increase in length and finally results in bands or cylindrical micelles. Some theoretical calculations on these types of structures are discussed briefly. Observations and theoretical calculations suggest that usually different structures may give similar free-energies, resulting in coexistence of structures and sensitivity to minor effects.

Cryogenic transmission electron microscopy is an important technique for investigation of nano-structures formed by amphiphilic molecules in dilute aqueous solution. The basic principle and applicability of the method as well as sample preparation, interpretation of images and some artefacts are briefly outlined. The introductory section is followed by structural characterization of aqueous dispersions prepared from lipids with lamellar and non-lamellar propensities that form, respectively, liposomes and nano-size particles with a dense, frequently regular, internal structure. Eggphosphatidylcholine, soybean phosphatidylcholine, distearoyl phosphatidylcholine, dipalmitoyl phosphatidylcholine, and dioleoyl phosphatidylethanolamine are amongst the lipids with lamellar propensity, whereas glycerylmonooleate represents those of non-lamellar propensity. Steric stabilization of the nano-structures is achieved by using non-ionic block copolymers based on poly(ethylene glycol) that bear short blocks of lipid-mimetic units. Other physicochemical and biopharmaceutical techniques are employed to complement the structural investigations.

Amphiphilic, nonionic diblock copolymers based on poly(ethylene glycol) (PEG 2000-5000), comprising short blocks of lipid-mimetic units, where tested for their ability to afford steric stabilization of distearoylphosphathydilcholine:cholesterol liposomes. The copolymers bear 1-4 lipid-mimetic anchors per copolymer chain. Effects on liposomes size depend on copolymer type and content. Cryo-TEM reveals well-sepd., intact, predominantly spherical liposomes at copolymer contents up to 5 mol%. A "flat" liposomes fraction occurs upon incorporation of above 7.5 mol% of copolymers bearing 2 or 4 lipid anchors. 5,6-carboxyfluorescein assay indicates lower leakage of stabilized vs. plain liposomes up to concn. 7.5 mol%. Leakage from liposomes with higher copolymer concn. is insignificantly greater.

Small angle neutron scattering measurements have been performed on three systems (HFDeP-d5-C (N-1(1,1,2,2-tetrahydroperfluorodecanoyl)pyridinium-d5 chloride)/C16PC in 63 mM NaCl; HFDeP-d5-C/C12PC in 200 mM NaCl, and as an example of an ideally mixed system, SDS/SDS-d25 in 200 mM NaCl) contg. micelles formed in a binary mixt. of surfactants, in order to investigate the compn. distribution of the mixed micelles. The exptl. data were collected varying the contrast between the av. scattering length d. of micelles and aq. solvent by changing the H2O/D2O ratio. Anal. of data includes a model-independent approach-the indirect Fourier transformation method and direct modeling-simultaneous fit at all contrasts by the scattering from micelles of equal size and shape with compn. distribution and an effective interaction. It has earlier been shown (Almgren, M.; Garamus, V. M. J. Phys. Chem. B 2005, 109, 11348) that for micelles of equal size, independent of the compn., and with negligible intermicellar interactions, the scattered intensity at zero angle varies quadratically with the contrast, with the min. intensity at the nominal match point proportional to s2, the variance of the micelle compn. distribution. Within the regular soln. framework, the compn. distribution and its variance are uniquely defined by the value of the interaction parameter and the micelle aggregation no. At 25 DegC, the first system gave s = 0.37, corresponding to a broad, bimodal compn. distribution, the second s = 0.22, a broad distribution with a shallow min. at the midpoint. For SDS/SDS-d25, we found s = 0.006 +- 0.030, which is a smaller value than that of the binominal compn. distribution expected for an ideally mixed system.

Aqueous dispersions of monoolein (MO) with a commercial hydrophobically modified ethyl hydroxyethyl cellulose ether (HMEHEC) have been investigated with respect to the morphologies of the liquid crystalline nanoparticles. Only very low proportions of HMEHEC are accepted in the cubic and lamellar phases of the monoolein−water system. Due to the broad variation of composition and size of the commercial polymer, no other single-phase regions were found in the quasi-ternary system. Interactions of MO with different fractions of the HMEHEC sample induced the formation of lamellar and reversed hexagonal phases, identified from SAXD, polarization microscopy, and cryogenic TEM examinations. In excess water (more than 90 wt %) coarse dispersions are formed more or less spontaneously, containing particles of cubic phase from a size visible by the naked eye to small particles observed by cryoTEM. At high polymer/MO ratios, vesicles were frequently observed, often oligo-lamellar with inter-lamellar connections. After homogenization of the coarse dispersions in a microfluidizer, the large particles disappeared, apparently replaced by smaller cubic particles, often with vesicular attachments on the surfaces, and by vesicles or vesicular particles with a disordered interior. At the largest polymer contents no proper cubic particles were found directly after homogenization but mainly single-walled defected vesicles with a peculiar edgy appearance. During storage for 2 weeks, the dispersed particles changed toward more well-shaped cubic particles, even in dispersions with the highest polymer contents. In some of the samples with low polymer/MO ratio, dispersed particles of the reversed hexagonal type were found. A few of the homogenized samples were freeze-dried and rehydrated. Particles of essentially the same types, but with a less well-developed cubic character, were found after this treatment.

The synthesis and physicochem. characterization of a series of polymethylene-1,w-bis(phosphocholines) with even-numbered chain lengths between 22 and 32 carbon atoms is described. Two new synthetic strategies for the prepn. of long-chain 1,wo-diols as hydrocarbon building blocks are presented. The temp.-dependent self-assembly of the single-chain bolaamphiphiles was investigated by cryo transmission electron microscopy (cryo-TEM), differential scanning calorimetry (DSC), and Fourier transform IR spectroscopy (FTIR).

In this paper we parametrize nanostructures formed in water by a series of macromol. surfactants based on polyglycidol (PG). They are considered analogs of Pluronic, that is poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers, in which the blocks of PEO are substituted by PG. PG is structurally similar to PEO and differs in that each monomer unit bears a hydroxyl group. The investigated PG-PPO-PG copolymers are of a const. d.p. of the middle PPO block of 34; they cover a mol. wt. range from 2900 to 12400 and PG content from 30 to 84 wt %. The major techniques used to examine the behavior at the dil. (but invariably above the crit. aggregation concn.) concn. limit were static and dynamic light scattering; the aggregates were visualized by cryogenic transmission electron microscopy. The wt.-av. mol. wts., radii of gyration, second virial coeffs., diffusion coeffs., and hydrodynamic radii were detd. at different temps. The light scattering results revealed that the particles were considerably larger in size and aggregation no. than the micelles of the Pluronic copolymers. Hydrophobic interactions and interactions via hydrogen bonding are equally involved in formation of well-sepd. spherical particles with discontinuous hydrophobic PPO domains randomly distributed in a strongly hydrogen-bonded continuous medium consisting of PG and water.

LCNP, liquid crystalline nanoparticles, are dispersions of liquid crystalline phases in excess solvent, usually water. Vesicles are the ones derived from the lamellar phase, and are characterized by a spontaneous monolayer curvature close to zero, or a surfactant packing parameter close to one. By changing the composition of amphiphile mixtures, the spontaneous curvature may be changed. This results in a change of the phase behavior as well as in the stability and morphology of the dispersed particles. This article reviews such changes mainly for systems based on polar lipid amphiphiles such as lecithins and glycerylmonooleate mixed with surfactants or amphiphilic block copolymers. Perforated vesicles, stomatosomes, are paid particular attention, not because of their importance as LCNP, but simply because they are least well known. As a tribute to Per Ekwall, some cryoTEM micrographs of vesicles from the “holy system,” sodium caprylate-decanol-water, are presented. Stomatosomes were not found in that system.