Micellar solution gels

Product recoveiy from reversed micellar solutions can often be attained by simple back extrac tion, by contacting with an aqueous solution having salt concentration and pH that disfavors protein solu-bihzation, but this is not always a reliable method. Addition of cosolvents such as ethyl acetate or alcohols can lead to a disruption of the micelles and expulsion of the protein species, but this may also lead to protein denaturation. These additives must be removed by distillation, for example, to enable reconstitution of the micellar phase. Temperature increases can similarly lead to product release as a concentrated aqueous solution. Removal of the water from the reversed micelles by molecular sieves or sihca gel has also been found to cause a precipitation of the protein from the organic phase. 

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

This chapter is concerned with experiments and theory for semidilute and concentrated block copolymer solutions.The focus is on the thermodynamics, i.e. the phase behaviour of both micellar solutions and non-micellar (e.g. swollen lamellar) phases. The chapter is organized very simply Section 4.2 contains a general account of gelation in block copolymer solutions. Section 4.3 is concerned with the solution phase behaviour of poly(oxyethylene)-containing diblocks and tri-blocks. The phase behaviour of styrenic block copolymers in selective solvents is discussed in Section 4.4. Section 4.5 is then concerned with theories for ordered block copolymer solutions, including both non-micellar phases in semidilute solutions and micellar gels. There has been little work on the dynamics of semidilute and concentrated block copolymer solutions, and this is reflected by the limited discussion of this subject in this chapter. 

It is now well established that formation of hard or stiff gels is the result of association of micelles into cubic phases. The notation hard gel follows Hvidt and co-workers (Almgren et al 1995 Hvidt et al. 1994) and refers to a micellar solution with a dynamic elastic shear modulus G > 103Pa. The correlation between the formation of a cubic phase and the onset of plastic flow (i.e. formation of a gel with a finite yield stress) was first made for PS-PI solutions in... 

The phase behaviour established for concentrated aqueous solutions of PEO-PPO-PEO copolymers has its counterpart in PEO/PBO copolymer solutions. A phase diagram for PE058PB0i7PE0M based on tube inversion experiments is shown in Fig. 4.14 (Luo et al. 1992). The hard gel is isotropic under the polarizing microscope and can be characterized as a cubic phase formed from spherical micelles of a similar size to those in the dilute micellar solution. 

The confinement of a relatively large number of dye molecules in the small volume of a nanoparticle may trigger collective phenomena otherwise not observable in bulk solution. This has been demonstrated by Prasad and coworkers in the case of an ORMOSIL pH sensor.69 The PEBBLEs contain a naphthalenylvinylpyridine derivative (NVP) as pH-sensitive fluorescent dye which has been functionalized with a triethoxysilane anchor by reaction with an excess of (3-isocyanatopropyl)triethoxysi-lane (ICTES). The sol-gel polymerization in aqueous micellar solution of the NVP-ICTES derivative with VTES gives spherically shaped 33 nm silica nanoparticles in which the dye is covalently linked to the silica matrix and uniformly distributed in the nanoparticle volume. The NVP dye responds ratiometrically to protons, with a... 

Wolff and Muller30 were among the first to report the ability to switch between sol and gel states by the conformational changes that can be brought about by irradiation with light. Their studies concerned the selective production of the unstable 9-methyl-anthracene by preorganization in micellar solutions of cetyltrimethylammonium bromide and subsequent irradiation with light (Fig. 23.3). 

Fig. 14 Cryo-TEM (a), AFM- (b), and TEM images of polyelectrolyte block copolymer vesicles (PB-P2VP.MeI). The image (c) is taken from a silica-template which was obtained by a sol/gel-process of a concentrated micellar solution [47, 56, 64]...

Gel Filtration. Micellar solutions have also been utilized in gel permeation (filtration) chromatography ( ] ). In fact, the first example of a separation which used a micellar mobile phase was in this area of exclusion liquid chromatography (ELC) ( 86). The last six entries in Table XI summarize some of the separations/work reported concerning micellar mobile phases in ELC. In most of these applications, the work was conducted with stationary phases of relatively small pore size. With these type phases, the relatively large micellar aggregates are confined to the excluded volume of the column and elute rapidly whereas smaller solute molecules in a mixture... 

Nonideal Behavior. The discussion of phase behavior up to this point represents the ideal case. A number of factors cause deviation from ideality. The phases present may include liquid crystals, gels, or solid precipitates in addition to the oil, brine, and microemulsion phases (39, 40). The high viscosities of these phases are detrimental to oil recovery. To control the formation of these phases, the practice has been to add low-molecular-weight alcohols to the micellar solution these alcohols act as cosolvents or in some cases as cosurfactants. 

Cetyltrimethylammonium bromide (CTMABr) was dissolved in water at 40°C, a clear micellar solution was obtained. Then sodium silicate was added to this solution and the pH value was adjusted with sulfuric acid. pH value and surfactant/silicium molar ratio were fixed at 10 and 0.62 respectively according to literature [8], After stirring for several hours at room temperature, the homogenous gel with the molar composition of 1 CTMABr 0.63 SiOz 102 HzO was sealed in Teflon autoclaves and heated. The crystallization temperature and time varied respectively from 80°C to 140°C and from 1 day to 11 days. The obtained solid phases after ethanol extraction with a Soxhlet apparatus were dried in vacuum at 100 °C overnight. 

An important common feature of macroion solutions is that they are characterized by at least two distinct length scales determined by the size of macroions (an order up to lOnm in the case of ionic micellar solutions) and size of the species of primary solvent (water molecules and salt ions, i.e. few Angstroms). Considering practical colloidal macro-dispersions, like foams, gels, emulsions, etc., usually we are dealing with as many as four distinct length scales molecular scale (up to lnm) that characterizes the species of the primary solvent (water or simple electrolytes) submicroscopic or nano scale (up to lOOnm) that characterizes nanoparticles or surfactant aggregates called micelles microscopic or mesoscopic scale (up to lOO m) that encompasses liquid droplets or bubbles in emulsion and foam systems as well as other colloidal suspensions, and macroscopic scale (the walls of container etc). 

Two unusual approaches to TLC of amoxicillin and other penicillins are the use of silica gel impregnated with tricaprylmethylammonium chloride with a methanol/water mobile phase [111], and the use of micellar solutions as mobile phase [112]. The latter was reported to give better separation of penicillins and their degradation products than organic mobile phases. 

Crystallization-induced phase separation can occur for concentrated solutions (gels) of diblocks [58,59]. SAXS/WAXS experiments on short PM-PEO [PM=poly(methylene) i.e. alkyl chain] diblocks revealed that crystallization of PEO occurs at low temperature in sufficiently concentrated gels (>ca. 50% copolymer). This led to a semicrystalline lamellar structure coexisting with the cubic micellar phase which can be supercooled from high temperatures where PEO is molten. These experiments on oligomeric amphiphilic diblocks establish a connection to the crystallization behaviour of related nonionic surfactants. 

A 50 wt.% micellar solution of Cu(EO)6 was prepared by dissolving the surfactant at room temperature in an aqueous solution during 3 hours. The obtained medium was further stirred for three hours at room temperature before adding drop by drop the inorganic source zirconium propoxide [Zr(OC3H7)4]. The surfactant / zirconia molar ratio was varied from 0.5 to 10. The obtained gel was sealed in Teflon autoclaves. Hydrothermal treatment was performed during 2 days at 60°C, in a first time. The surfactant was removed by ethanol extraction. 

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