Quaternary ammonium salts interface

Sodium cyanide does not dissolve m butyl bromide The two reactants contact each other only at the surface of the solid sodium cyanide and the rate of reaction under these con ditions IS too slow to be of synthetic value Dissolving the sodium cyanide m water is of little help because butyl bromide is not soluble m water and reaction can occur only at the interface between the two phases Adding a small amount of benzyltrimethyl ammonium chlonde however causes pentanemtnle to form rapidly even at room temper ature The quaternary ammonium salt is acting as a catalyst it increases the reaction rate How7... 

In some cases, the Q ions have such a low solubility in water that virtually all remain in the organic phase. ° In such cases, the exchange of ions (equilibrium 3) takes place across the interface. Still another mechanism the interfacial mechanism) can operate where OH extracts a proton from an organic substrate. In this mechanism, the OH ions remain in the aqueous phase and the substrate in the organic phase the deprotonation takes place at the interface. Thermal stability of the quaternary ammonium salt is a problem, limiting the use of some catalysts. The trialkylacyl ammonium halide 95 is thermally stable, however, even at high reaction temperatures." The use of molten quaternary ammonium salts as ionic reaction media for substitution reactions has also been reported. " " ... 

The interfacial mechanism provides an acceptable explanation for the effect of the more lipophilic quaternary ammonium salts, such as tetra-n-butylammonium salts, Aliquat 336 and Adogen 464, on the majority of base-initiated nucleophilic substitution reactions which require the initial deprotonation of the substrate. Subsequent to the interfacial deprotonation of the methylene system, for example the soft quaternary ammonium cation preferentially forms a stable ion-pair with the soft carbanion, rather than with the hard hydroxide anion (Scheme 1.8). Strong evidence for the competing interface mechanism comes from the observation that, even in the absence of a catalyst, phenylacetonitrile is alkylated under two-phase conditions using concentrated sodium hydroxide [51],... 

Nitroarenes are reduced to anilines (>85%) under the influence of metal carbonyl complexes. In a two-phase system, the complex hydridoiron complex [HFe,(CO)u]2-is produced from tri-iron dodecacarbonyl at the interface between the organic phase and the basic aqueous phase [7], The generation of the active hydridoiron complex is catalysed by a range of quaternary ammonium salts and an analogous hydrido-manganese complex is obtained from dimanganese decacarbonyl under similar conditions [8], Virtually no reduction occurs in the absence of the quaternary ammonium salt, and the reduction is also suppressed by the presence of carbon monoxide [9], In contrast, dicobalt octacarbonyl reacts with quaternary ammonium fluorides to form complexes which do not reduce nitroarenes. 

A degree of stereoselective control of the course of a reaction, which is absent or different from that prevalent when the reaction is conducted in the absence of quaternary ammonium salts, may be achieved under standard phase-transfer catalysed reaction conditions. The reactions, which are influenced most by the phase-transfer catalyst, are those involving anionic intermediates whose preferred conformations or configurations can be controlled by the cationic species across the interface of the two-phase system. For example, in the base-catalysed Darzens condensation of aromatic aldehydes with a-chloroacetonitriles to produce oxiranes (Section 6.3), the intermediate anion may adopt either of the two conformations, (la) or (lb) which are stabilized by interaction across the interface by the cations (Scheme 12.1) [1-4]. 

With soluble quaternary ammonium salts as catalysts the reaction is thought to take place at the aqueous/organic interface because a) the solubilities of quaternary ammonium hydroxides in organic solvents are too low to account for the observed reaction rates, and b) the most active catalysts are benzyltriethylammonium and... 

Monolayers are best formed from water-insoluble molecules. This is expressed well by the title of Gaines s classic book Insoluble Monolayers at Liquid-Gas Interfaces [104]. Carboxylic acids (7-13 in Table 1, for example), sulfates, quaternary ammonium salts, alcohols, amides, and nitriles with carbon chains of 12 or longer meet this requirement well. Similarly, well-behaved monolayers have been formed from naturally occurring phospholipids (14-17 in Table 1, for example), as well as from their synthetic analogs (18,19 in Table 1, for example). More recently, polymerizable surfactants (1-4, 20, 21 in Table 1, for example) [55, 68, 72, 121], preformed polymers [68, 70, 72,122-127], liquid crystalline polymers [128], buckyballs [129, 130], gramicidin [131], and even silica beads [132] have been demonstrated to undergo monolayer formation on aqueous solutions. 

As mentioned above, the hydrophilic head group may be unionised [e.g., alcohols or poly(ethylene oxide) (PEO) alkane or alkyl phenol compounds], weakly ionised (e.g., carboxylic acids), or strongly ionised (e.g., sulphates, sulphonates, and quaternary ammonium salts). The adsorption of these different surfactants at the A/W and O/W interfaces depends on the nature of the head group. With nonionic surfactants, repulsion between the head groups is small and these surfactants are usually strongly adsorbed at the surface of water from very dilute solutions. Nonionic surfactants have much lower cmc values when compared to ionic surfactants with the same alkyl chain length typically, the cmc is in the region of... 

A peculiar type of adsorption [26] may take place while a glass slide is raised out of a solution of a substituted quaternary ammonium salt, which is a dewetting agent for glass. Adsorbed molecules at the solution-air interface are transferred to the solid surface and are added to the molecules already adsorbed on the solid directly from the solution. Tenebre [26] has called this transfer complementary adsorption. The densities of molecules adsorbed from the bulk of the solution and those transferred from the free surface of the solution were measured... 

Polyethylene glycols (PEGs) can also be used as the catalyst in SLPTC. Chu [207] reported the kinetics for etherification of sodium phenoxide with benzyl bromide using quaternary ammonium salts and PEG as the catalyst in SLPTC. When PEG is used as the catalyst, formation of the complex PEG-Na PhO mainly occurs at the solid-liquid interface. The phenoxide anion carried by PEG can dissolve much more than its original... 

The versatile properties and manufacturability of polymers has evoked immense interest in developing a class of biomaterials with the potential to interface with biological systems [1]. However, polymers are prone to pathogenic attack resulting in deterioration of properties, malfunction and so on. Various methods such as the ionic binding technique, incorporation of metal particles/metal oxides/nanoparticles (NP) and physico-chemical modification via, e.g., the addition of quaternary ammonium salts and blending with antimicrobial polymers, have been explored for the fabrication of bactericidal materials [2], However, these methods can result in reduced biocompatibility, cytotoxicity and eco-toxicity. 

Cationic surfactants, characterized by their amphiphilic properties, contain an alkyl hydrophobe and a hydrophilic positively charged head-group. Among cationic surfactants, quaternary ammonium salts are notable for their ability to reduce surface and interfacial tensions by ready adsorption to a surface or interface, such as hair and skin. This ability to adsorb on to substrates makes the use of cationic surfactants extremely important in the personal care industry. 

CE-MS has been used in analyses of anions and cations as a method simultaneously providing positive identification and quantitation. CE-ESI MS is particularly well suited for analyses of quaternary ammonium salts. Speciation analysis of As can be carried out using a CE-ICP-MS. Problems in interfacing the ICP-MS detection to CE are associated with low flow rates and small samples analyzed. Detection limits in ppb region can be attained, using postcapillary hybridization prior to ICP-MS. 

Although adsorbed molecules of extractants are well oriented at the interface, the interfacial reactions occur more slowly, as recalculated for the same volume, than reactions in the aqueous phase. The extraction of palladium(II) from HCl solutions with dialkyl sulfides is a classic example of a very slow process. Modifying the extractant structure by adding a hydrophilic hydroxyl group and/or a phase transfer catalyst, e.g., trialkylamine or quaternary ammonium salt, increases the rate of extraction. The addition of sulfonic and phosphoric acids to hydroxyoximes may enhance the rate of copper extraction due to the formation of reverse micelles and the development of the microscopic interface. The a-acyloin oximes that form an intermediate complex that has a five-member ring with copper(II) increase the rate of extraction of aromatic hydroxyoximes. The addition of surfactants may cause both retardation and acceleration of extraction. If the interfacial tension is decreased, surfactants cause an increase in the interfacial surface area in dispersed systems. When they adsorb at the interface, they cause an additional... 

Aliquat 336, a quaternary ammonium salt, has been used as a phase transfer catalyst for a solid-liquid interface. A comparison of its catalytic ability with that of 18-crown-6-ether and tetramethylethylenediamine shows that the quaternary ammonium salt is equivalent and sometimes superior in catalysing anion displacement reactions of acetate, fluoride and adeninyl anions. ... 

---