Surface cationic compounds

Of the four classes of surface-active agents, however, the cationic compounds arguably play the most important role in an antimicrobial context. 

Anionic and cationic products generally tend to interact with each other, usually diminishing the surface-active properties of both and often resulting in precipitation of the complex formed. Amphoteric compounds can also be incompatible with anionics in acid solution but are generally compatible with cationics and nonionics. Interaction between anionic and cationic agents can sometimes be prevented by addition of a nonionic. In some cases, if an ethoxylated sulphate or phosphate is used as the anionic component a cationic compound produces no obvious precipitation, since the oxyethylene chain acts as dispersant for any complex that may be formed. 

Surfactants are surface-active compounds, which are used in industrial processes as well as in trade and household products. They have one of the highest production rates of all organic chemicals. Commercial mixtures of surfactants consist of several tens to hundreds of homologues, oligomers and isomers of anionic, non-ionic, cationic and amphoteric compounds. Therefore, their identification and quantification in the environment is complicated and cumbersome. Detection, identification and quantification of these compounds in aqueous solutions, even in the form of matrix-free standards, still poses the analyst considerable problems. 

The same disciission may apply to the anodic dissolution of semiconductor electrodes of covalently bonded compounds such as gallium arsenide. In general, covalent compoimd semiconductors contain varying ionic polarity, in which the component atoms of positive polarity re likely to become surface cations and the component atoms of negative polarity are likely to become surface radicals. For such compound semiconductors in anodic dissolution, the valence band mechanism predominates over the conduction band mechanism with increasing band gap and increasing polarity of the compounds. 

Fig. 9-13. Reaction rate of simultaneous dissolution of surface cations and anions from a semiconductor electrode of ionic compound as a iimction of potential of a compact layer 4 )=potmitial of acorn-...

Semiconductor electrodes of ionic compounds can also dissolve with the oxidation of surface anions or with the reduction of surface cations as shown schematically in Fig. 9-15. 

Fig. 9-15. Oxidative and reductive dissolution reactions of semiconductor electrodes of ionic compounds (a) cation dissolution coupled with anodic hole oxidation of surface anions, (b) anion dissolution coupled with cathodic electron reduction of surface cations.

An attempt to combine electrochemical and micellar-catalytic methods is interesting from the point of view of the mechanism of anode nitration of 1,4-dimethoxybenzene with sodinm nitrite (Laurent et al. 1984). The reaction was performed in a mixture of water in the presence of 2% surface-active compounds of cationic, anionic, or neutral nature. It was established that 1,4-dimethoxy-2-nitrobenzene (the product) was formed only in the region of potentials corresponding to simultaneous electrooxidation of the substrate to the cation-radical and the nitrite ion to the nitrogen dioxide radical (1.5 V versus saturated calomel electrode). At potentials of oxidation of the sole nitrite ion (0.8 V), no nitration was observed. Consequently, radical substitution in the neutral substrate does not take place. Two feasible mechanisms remain for addition to the cation-radical form, as follows ... 

In MEKC, mainly anionic surface-active compounds, in particular SDS, are used. SDS and all other anionic surfactants have a net negative charge over a wide range of pH values, and therefore the micelles have a corresponding electrophoretic mobility toward the anode (opposite the direction of electro-osmotic flow). Anionic species do not interact with the negatively charged surface of the capillary, which is favorable in common CZE but especially in ACE. Therefore, SDS is the best-studied tenside in MEKC. Long-chain cationic ammonium species have also been employed for mainly anionic and neutral solutes (16). Bile salts as representatives of anionic surfactants have been used for the analysis of ionic and nonionic compounds and also for the separation of optical isomers (17-19). 

Polymyxin B. Polymyxin antibiotics are cationic compounds that are attracted to negatively charged phospholipids in the bacterial cell membrane. These drugs penetrate and disrupt the architecture and integrity of the surface membrane. Essentially, polymyxins act as detergents that break apart the phospholipid bilayer, which creates gaps in the bacterial cell wall, leading to the subsequent destruction of the bacteria.31... 

The uncertainty concerning the identification of the stabilization mechanism on polar ZnO surfaces is partly due to the lack of atomically resolved STM images. Such images are possible for the nonpolar (1010) and (1120) surfaces [40,41] but have, to our knowledge, not been reported for polar surfaces. The polar cation terminated (111) surface of zincblende compounds typically displays a 2 x 2 reconstruction associated with removal of every fourth surface cation [43,50-52]. This structure is ideally suited to match the charging condition for surface stabilization for this particular surface orientation. The 2x2 reconstruction and the missing surface atoms can directly be observed by STM [52]. In contrast to literature [53], a 2 x 2 reconstruction is also frequently observed in our group for the (0001) surface of wurtzite CdS.4 The reconstruction on the anion terminated (III) surfaces of III—V and II-VI zincblende compounds are considerably more complex. These surfaces... 

Cationic compounds are highly surface-active but are used less frequently as emulsifiers. The cation portion of the molecule is usually a quaternary ammonium... 

Divalent cations (e.g. Mg2+, Ca2+) present in hard water may also interact with the microbial cell surface and block disinfectant adsorption sites necessary for activity. On the other hand, cationic compounds may disrupt the outer membrane of Gram-negative bacteria and facilitate their own entry. 

The cation-active surface-active compounds, which are usually quaternary ammonium or pyridinium derivatives, form another important group of textile auxiliaries. An example is Sapamine CH, which is diethylethylene diamine condensed with oleyl chloride and converted to the corresponding hydrochloride ... 

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