Krister Holmberg

N,N‐di(3‐aminopropyl)dodecylamine (C12‐Y‐amine), N‐dodecyl‐N,N‐di[(3‐d‐gluconylamido)propyl]amine (C12‐DGA), N‐dodecyl‐N,N‐bis[(3‐lactobionylamido)propyl]amine (C12‐DLA), N‐dodecyl‐N‐[(3‐lactobionylamido)propyl]amine (C12‐LA), and ethoxylated N,N‐di(3‐aminopropyl)dodecylamine [C12‐Y‐amine with 4 and 8 ethylene oxide (EO)] have been synthesized, and their physical‐chemical properties have been studied. Adsorption was measured at the air–water interface by the du Noüy ring method and at solid surfaces consisting of self‐assembled monolayers (SAMs) of alkanethiols on gold using surface plasmon resonance. Results from surface tension measurements showed that adsorption at the air–water interface is pH dependent. At low pH, reduction in surface tension is less pronounced as a result of protonation of the amino groups of the surfactants. At the SAM model surfaces, generated by adsorption of alkanethiols or mixtures of alkanethiols on gold presenting methyl, hydroxyl, and carboxyl groups, the highest amount of adsorbed surfactant was obtained on a surface composed of a mixture of methyl and carboxyl groups. In general, the sugar‐derived surfactants, DGA, DLA, and LA, adsorbed less than the ethoxylates. Surfactant biodegradation was investigated by the closed bottle test. Only C12‐Y‐amine was found to be readily biodegradable. However, the polyol surfactants were more biodegradable than the ethoxylates.

Reactant incompatibility is often a problem in organic synthesis. Here, we report an investigation of a nucleophilic substitution reaction, involving incompatible substrate and nucleophile, performed in three different self‐assembly systems, two liquid crystalline phases and one microemulsion, and also in slurries of finely divided mesoporous silica with either hexagonal or cubic symmetry. The reactants used were 4‐tert‐butylbenzyl bromide and potassium iodide. The reaction turned out to be extremely fast in the two liquid crystalline phases, intermediate in the mesoporous materials and somewhat slower—although still fast compared to the two‐phase system—in the microemulsion. The very high reactivity in the liquid crystalline phases is believed to be associated with the very high interfacial area of these systems. This work shows that surfactant liquid crystals can be used as minireactors for organic reactions. Removal of the surfactant after completed reaction is a practical problem, however, that needs to be taken into account.

Microemulsions, i.e., thermodynamically stable, one‐phase mixtures of oil, water and surfactants, are useful as reaction media as a way to overcome the reactant incompatibility problem that one frequently encounters in organic synthesis. The use of a microemulsion can be seen as an alternative to phase‐transfer catalysis, i.e., use of a two‐phase system with added phase‐transfer agent. The microemulsion approach and phase‐transfer catalysis can also be combined, i.e., the reaction can be performed in a microemulsion in the presence of a small amount of a phase‐transfer agent. A very high reaction rate may then be obtained. The reaction rate in a microemulsion is often influenced by the charge at the interface and this charge depends on the surfactant used. For instance, reactions involving negatively charged species will be accelerated by the use of a cationic surfactant. This effect is analogous to micellar catalysis and may be termed microemulsion catalysis. The acceleration is more pronounced when the surfactant counterion is small and weakly polarizable. Microemulsions can also be used to induce regiospecificity of organic reactions. Organic molecules with one more polar and one less polar end will accumulate at the oil‐water interface of microemulsions. They will orient at the interface so that the polar part of the molecule extends into the water domain and the nonpolar part extends into the hydrocarbon domain. A water‐soluble reagent will react from the “water side”, i.e., attack the polar part of the amphiphilic molecule, and a reagent soluble in hydrocarbon will react at the other end of the amphiphilic molecule. This review demonstrates that microemulsions can be a useful for (i) overcoming reactant incompatibility, (ii) speeding up reactions of one polar and one apolar reactant (microemulsion catalysis), and (iii) inducing regiospecificity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The cover picture shows how a lipophilic reactant, 4‐tert‐butylbenzyl bromide, meets a hydrophilic reactant, an iodide ion, at the oil‐water interface of a microemulsion. The interface is covered by a monolayer of nonionic and cationic surfactants. Details are presented in the Microreview by K. Holmberg on p. 731 ff. (Mr. Tomasz Witula is acknowledged for drawing the figure.)

Mesoporous silica materials made by the use of self-assembled surfactants as templates have attracted a lot of interest in recent years. The number of publications on the topic has increased by a factor of 100 between 1993 and 2003. The vast majority of the papers deal with syntheses from organosilicates, such as tetramethylorthosilicate (TMOS) and tetraethylorthosilicate (TEOS), as the silica source. These alkoxides are convenient as a starting material but they are expensive and the products made from them can only find use in applications where price is not a major issue. This review summarizes the published work on the preparation of mesoporous silica from inorganic sources, such as water glass and silica sol, still using self-assembled surfactants as the template. The use of such raw materials instead of the organosilicates will radically change the production price of the products and may open up completely new fields of application. Much less work has been put into the synthesis of mesoporous silica from these starting materials and relatively little is known about the reaction mechanism. The current understanding is summarized and discussed and comparisons are made with the conventional route that originates from the organosilicates.

A stopped‐flow technique combined with pulsed‐field‐gradient spin‐echo nuclear magnetic resonance (NMR) measurements was used to study the kinetics of exchange, size, and shapes in micellar systems of cationic surfactant dimers of the alkanediyl‐α‐ω‐bis(dodecyldimethylammonium bromide) type, with alkanediyl being 1,2‐ethylene, 1,3‐propylene, and 1,4‐butylene. By measuring the slow relaxation time for micelles, τ₂, the micelle lifetime as a function of spacer length was obtained and was further confirmed by micelle exchange measurements by NMR diffusometry. The micelle lifetimes for the gemini surfactants were found to be in orders of magnitude longer than for the corresponding conventional surfactants. All three cationic surfactant dimers showed an increase in micelle size in one direction, i.e., became prolates, as the concentration was increased. The growth of the micelles was most pronounced for the gemini surfactants with the shortest linker unit, i.e., ethylene.

A series of surface‐active betaine esters of medium‐ to long‐chain alcohols has been synthesized. The hydrophobic parts used were decyl, dodecyl, tetradecyl, and oleyl chains. Ethyl betainate was prepared as a non‐surface‐active reference compound. The surfactants were characterized by critical micellization concentration (CMC), and base‐catalyzed hydrolysis was studied by using ¹H nuclear magnetic resonance (NMR). Micellar catalysis was shown to have a strong influence on the hydrolysis. This means that the hydrolysis rates are concentration dependent. For dodecyl betainate, the effect of the degradation products (dodecanol and betaine) on the micellar shape was investigated by NMR diffusion experiments. The degradation products were shown to induce micellar growth.