Using a water-in-oil microemulsion system, silica nanoparticles containing superparamagnetic iron oxide (SPIO) crystals have been prepared and characterized. With this method, the loading of iron oxide crystals, the thickness of the silica shells, and the overall particle sizes are tunable. Moving from low to high water concentration, within the microemulsion region, resulted in a gradual shift from larger particles, ca. 100 nm and fully loaded with SPIOs, to smaller particles, ca. 30 nm containing only one or a few SPIOs. By varying the amount of silica precursor, the thickness of the silica shell was altered. Field dependent magnetization measurements showed the magnetic properties of the SPIOs were preserved after the synthesis.

A series of four homologous pure nonionic surfactants, all monoesters of tetra(ethylene glycol), were synthesized. The ester surfactants varied in the degree of substitution on the α-carbon of the acyl chain, from no substitution to 2-methyl, to 2-ethyl, and on to 2,2-dimethyl. All surfactants were based on C 8 -acids except the 2-methyl-substituted, which was based on a C 7 -acid. The ester surfactants were characterized by critical micelle concentration (CMC) and cloud point. Base-catalyzed hydrolysis was investigated by using 1 H NMR and tensiometry. The surfactants showed a pronounced difference in hydrolytic reactivity; the nonsubstituted surfactant was 90 times more reactive than the disubstituted, and the reactivity of the methyl-substituted surfactant was 14 times more reactive than the ethyl-substituted. Hydrolysis studies above the CMC revealed that the ester bond of the aggregated surfactant is protected from attack by hydroxide ions; thus, only surfactants in monomeric form are being cleaved.Surfactants with built-in weak bonds, so-called cleavable surfactants, are attracting attention as a result of the increasing legislative pressure on surfactant producers to develop products that break down rapidly in the environment (1,2). Esterbased surfactants are one natural choice of cleavable surfactants because (i) such products are relatively easy to make, (ii) their rate of chemical hydrolysis can be adjusted by the choice of pH, and (iii) hydrolysis of lipophilic esters is catalyzed by lipases; thus, the biodegradation characteristics can be expected to be good.Ester-based surfactants, in particular poly(ethylene glycol) (PEG) esters of fatty acids, have been around for a long time. They are usually made by KOH-initiated ethoxylation of a fatty acid. Owing to transesterification reactions occurring during the ethoxylation, the product obtained is a complex mixture of a PEG monoester, a PEG diester, and free PEG. A typical ratio of the three components, PEG monoester, PEG diester, and free PEG, is 2:1:1. In addition, the polyoxyethylene chains have the broad homolog distribution typical of all ethoxylated products (3,4).Although surface-active PEG esters are established surfactants, little has been published about the kinetics of their alkaline hydrolysis. In this paper we present studies on the hydrolysis of well-defined, homolog-pure PEG esters, made by esterification of preformed oligo(ethylene glycol) instead of ethoxylation; see Figure 1. The hydrolysis is investigated both below the critical micelle concentration (CMC), where the surfactant is present as free monomers in solution, and above the CMC, where there are both aggregated and free surfactants in solution. We have been particularly interested in the effect of substituents on the carbon atom adjacent to the carbonyl carbon of the ester bond on the hydrolysis rate, and the investigation was performed with a series of four monoesters of tetra(ethylene glycol) with different alkyl substituents in the α-position of the acyl chain.Work on enzymati...

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Unexpected colloidal assemblies form in aqueous mixtures of sodium dodecylbenzenesulfonate (SDBS) with the following imidazoline compounds: 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (V-44, which is a commonly used free-radical initiator), 2,2'-tetramethylenedi-2-imidazoline (TMI), and the main recombination product (RP) from the decomposition of V-44. All of these imidazoline compounds act as hydrotropes. As the molar ratio of imidazoline to SDBS increases, a gradual transition from micelles to vesicles to bilayers to precipitate is observed. V-44 decomposes slowly at 25 degrees C, and the phase diagrams of V-44/SDBS and RP/SDBS are similar. The vesicular region observed in mixtures of TMI/SDBS is larger in composition than that of V-44/SDBS and RP/SDBS mixtures. At equimolar compositions of SDBS and RP, a novel colloidal structure with multiple closely packed bilayers is observed. In these mixtures, small unilamellar vesicles (<80 nm in diameter) form spontaneously, although with time they coexist with a small amount of precipitate and their size increases steadily. The self-assembly of vesicles occurs over a wide range of compositions when the solution pH is lower than the pK(a) of the imidazoline moiety. Quasi-elastic light scattering, cryogenic transmission electron microscopy, nuclear magnetic resonance, and small-angle neutron scattering were used to determine the characteristic length scales and properties of the assemblies.

Base-catalyzed hydrolysis, biodegradation, and enzyme-catalyzed hydrolysis of a series of four monoesters of tetra(ethylene glycol) have been investigated. The surfactants varied in substitution on the α-carbon of the acyl chain, from no substitution, to 2-methyl, to 2-ethyl, and on to 2,2-dimethyl. All surfactants were based on C 8 -acids except the methyl-substituted, which was based on a C 7 -acid. The hydrolysis was investigated using 1 H nuclear magnetic resonance. The surfactants showed a pronounced difference in stability with respect to type of substitution in the vicinity of the ester bond. In alkaline hydrolysis the most significant difference in reactivity lay between the surfactant with an ethyl group and the surfactant with a methyl group in the α-position of the acyl chain. However, in the biodegradation studies these surfactants broke down at almost exactly the same rate as the nonsubstituted surfactant. In the biodegradation test, the disubstituted surfactant deviated considerably. Two lipases, from Mucor miehei (MML) and Candida antarctica B (CALB), were used in the enzyme-catalyzed hydrolysis. The surfactant with no substitution was found to hydrolyze much faster than the other surfactants, and the hydrolytic activity of MML, but not CALB, increased in the presence of surfactant micelles.Environmental considerations are a major driving force in the development of new surfactants. As a result of increasing legislative pressure, more environmentally benign products are continually replacing conventional surfactants. The most important property of these surfactants is that they break down rapidly in the environment, i.e., have a good biodegradability profile. One way to improve biodegradability is to build weak bonds into the surfactants to make socalled cleavable surfactants (1,2). Ester-based surfactants are a natural choice as cleavable surfactants because (i) such products are relatively easy to make, (ii) the rate of chemical hydrolysis can be adjusted by the choice of pH, and (iii) hydrolysis of lipophilic esters is catalyzed by, for example, lipases; thus, the biodegradation characteristics can be expected to be good.In a previous paper (3) four homolog-pure monoesters of tetra(ethylene glycol) with different alkyl substituents in the α-position of the acyl chain (see Fig. 1) were synthesized and characterized with respect to physicochemical properties. In that work, alkaline hydrolysis above and below the critical micelle concentration (CMC) was investigated, and it was found that below the CMC, the hydrolysis rate was greatly affected by the degree and type of alkyl substitution. An unexpected result was that the ester surfactants, when present in micelles, were stable to alkaline hydrolysis even when exposed to strong basic conditions. The ester bond seemed to be embedded deep enough in the hydrophobic core of the micelles to be protected from attack by hydroxide ions. Besides, a fraction of the negatively charged carboxylate that formed during hydrolysis entered the micelle, making it ne...