Cationic surfactant absorption

Surfactants have a unique long-chain molecular structure composed of a hydrophilic head and hydrophobic tail. Based on the nature of the hydrophilic part surfactants are generally categorized as anionic, non-ionic, cationic, and zwitter-ionic. They all have a natural tendency to adsorb at surfaces and interfaces when added in low concentration in water. Surfactant absorption/desorption at the vapor-liquid interface alters the surface tension, which decreases continually with increasing concentrations until the critical micelle concentration (CMC), at which micelles (colloid-sized clusters or aggregates of monomers) start to form is reached (Manglik et al. 2001 Hetsroni et al. 2003c). 

There have been many recent studies in support of this mechanistic approach. Stepwise reductive formation of Ag3+ and Ag4+ clusters has been followed using spectroscopic methods by Henglein [33], Reduction of copper (II) to colloidal Cu protected by cationic surfactants (NR4+) through the intermediate Cu+ prior to nucleation of the particles [36] as monitored by in situ x-ray absorption spectroscopy is another example. The seed-mediated synthesis also serves as evidence in support of this mechanism [38-41],... 

Agents that decrease the viscoelasticity of mucus, for example anionic and cationic surfactants and bile salts, have been shown to increase absorption. 

The use of the cationic micellar agent CTAB (50 mM) in phosphate-borate buffer (10 mM of each salt), pH 8.6, with 10% acetonitrile was preferred because of a faster separation (about 15 min) of heroin and related substances. Because the cationic surfactant, which coats the capillary silica wall with a positively charged layer, reverses the electroosmotic flow (EOF), the voltage (-15 kV) must be applied with a reversed polarity (with the cathode at the injection point). Detection was by UV absorption at 280 nm. 

Some studies have examined the effects of surfactants on intestinal absorption of insulin, with variable results. Both rectal and jejunal absorption of insulin was increased by anionic and cationic surfactants. However, in humans, oral polyoxyethylene-20-oleyl ether resulted in poor and variable insulin absorption. " ... 

Any enhancing effect of surfactants on drug absorption appears to be related to increased drug solubilization, modification of mucosal permeability, or reduction of resistance of the unstirred water layer at the GI membrane surface. In general, unionic surfactants have little effect on membrane structure but cationic surfactants have been associated with reversible cell loss and loss of goblet cells. These effects must limit consideration of surfactants as absorption promoters, particularly for long term treatment. 

Large, hard, transparent, mesoporous silica spheres1282 (see Figure 8.55) were synthesized in one step by using oil-in-water emulsion chemistry under basic conditions with cationic surfactants and TBOS. The pores are shown by TEM and nitrogen-absorption studies to be monodispersed in size, with total surface area over 1000 m2/g. 

Eriochrome Cyanine R (ECR) (formula 4.17) reacts with beryllium ions [4,9,10,16,30] similarly to Chrome Azurol S (see Section 9.2.1). At pH 9.7, A-max of ECR is 435 nm and that of its water-soluble beryllium complex is 525 nm. The molar absorptivity of the complex is 1.5 10 . EDTA, tartrate and cyanide are used as the main masking agents for interfering metals. In the presence of cationic surfactants, the sensitivity is increased several times, and significant bathochromic shifts are observed. In the case of CTA, e = 8.7-1 O at 590 nm (pH 7) [31,32]. Beryllium was also sorbed on anion exchange resins impregnated with ECR [33]. 

Indium ions and Eriochrome Cyanine R (ECR) (formula 4.17) (in excess), in the pH range 3-7, form a complex with Xmax = 565 nm [27]. In the presence of CTA (cationic surfactant) in a sufficiently large excess, an intensely coloured ternary complex is formed [28,29]. Its maximum absorption is obtained at pH 5.2 0.1 after 5 min in the presence of acetate buffer. In the absence of acetate, the reaction proceeds more slowly. The concentrations of ECR and CTA must be in appropriate excesses (as given below in the procedure). The molar absorptivity e is 1.0-10 (a = 0.87) at 585 nm. 

Other triphenylmethane reagents, besides the Eriochrome Cyanine R and Chrome Azurol S discussed above, have been applied for determination of Fe(III). A large increase in sensitivity and significant bathochromic shifts are observed when the reactions are performed in the presence of cationic surfactants, CTA, CP, Zephiramine, or dimethyllaurylbenzylammonium ions [70]. Systems of Fe(III) with Pyrocatechol Violet and CTA [71], Chromal Blue G and CTA [72], and Sulphochrome and CP [73] have been applied. The molar absorptivities of these systems are within 1.3-10 -1.7-10. The methods based on Pyrogallol Red alone or with the use of surfactants are less sensitive (e within 5.2-10 -7.5-10 ) [74-76]. 

The reaction of Bromopyrogallol Red (BPR) (formula 4.21) with Mo(Vl) is not suitable for analytical use because the absorption maxima of the binary complex and of the reagent itself are too close. However, in the presence of cationic surfactants, bathochromic and hyperchromic effects are observed. These ternary systems allow a sensitive determination of molybdenum the absorbance of the reagent at the absorbance maximum of the ternary complex is insignificant [63-65]. The best results are obtained with the use of CTA ions. 

The following triphenylmethane dyes have been employed for determination of Sc similarly to Xylenol Orange Methylthymol Blue [33], Chrome Azurol S [34,35], Chromal Blue G [36], and Eriochrome Brilliant Violet B [37]. Much higher sensitivities have been obtained in the presence of some cationic surfactants [38 0]. In the method with Chrome Azurol S and Zephiramine, the e value is 1.5-10 at 610 nm, and in the method with Eriochrome Cyanine R and CP, e = 9.2-10" at 600 nm [40]. When o-hydroxy-quinonephthalein and CP are used, the molar absorptivity is 1.1-10 at 555 nm [41], Scandium has been determined with the use of Nile Blue in a poly(vinyl alcohol) medium [42]. 

Cationic surfactants (quaternary ammonium group) and amphoteric surfactants with a long alkyl chain present a poor absorption in UV. 

A number of sorbents have been proposed to clean water surfaces from oil [318]. The use of hydrophobic aerosil was proposed for this purpose, which, however, can hardly be accomplished for economic reasons. More promising seems to be the proposal to use natural materials for oil absorption, such as turf, diatomite, vermiculite, swelled perlite. A method has been proposed for the modification of perlite by a consequent treatment with cationic surfactants and higher carboxylic acid salts. Such modification of swelled perlite increases its oil capacity up to 600%, the water absorption decreases 10 -100-fold, and the sinkability decreases considerably. The degree of oil removal from the water surface is, according to in vitro tests data, 98 - 99%. Methods have been found to use oil-saturated sorbents. 

You ll notice this when you dip a soap pad into water containing dishwashing detergent. Such detergents are anionics, and, as the manufacturer of SOS pads confirms, steel wool pads infused with soap use cationic surfactants. Why they use cationics, which are generally less effective cleaners, isn t clear, but it likely has to do with steel wool s absorption capabilities. 

Based on the principles of precipitate flotation, a rapid and convenient separation technique has been developed for the determination of toxic heavy metals adsorbed on suspended solids in freshwater. Because suspended solids are negatively charged species, they are rendered hydrophobic and coagulate to form bulky floes with a cationic surfactant and sodium chloride (to increase the ionic strength). The floes are easily floated by bubbling and are then treated in nitric acid to determine the desorbed heavy metals (e.g., chromium, manganese, copper, cadmium, and lead) by graphite furnace atomic absorption spectrometry. 

Most of the results reported herein on the sorption of polycations by natural keratins have been obtained on simplified model systems. The rationale behind such an approach was to seek to determine the fate of sorbing polymer under well-defined conditions and so to obtain mechanistic insight. The results have clearly indicated that both adsorption and absorption processes can occur. They have also shown that the nature, extent, and consequences of polycation sorption can all be influenced by the presence of surfactant. In general, nonionic surfactants have a small effect on sorption, because of low interaction while cationic surfactants can have a very large effect because of competition for the sorption sites. Because of interaction and complex formation with cationic polyelectrolytes anionic surfactants can exercise an intermediate, but potentially very important, influence. 

Using steady-state absorption studies, several other authors examined the micropolarity of confined IL in microemulsions stabilized by ionic surfactants [64,85,87], For example, Sarkar and coworkers examined [bmim][BF ]/benzene mixtures stabilized by the anionic SAIL surfactant [bmim][AOT] and observed that, within the studied range, the A for solubilized MO continued to undergo redshift with increasing R [85, 87], In another work, Falcone and coworkers compared the micropolarities of [bmim][BF4]/benzene mixtures stabilized by cationic BHDC and nonionic TX-lOO surfactants using l-methyl-8-oxyquinolinium betaine (QB), a dye that locates mainly at the surfactant interfacial layer [64]. When [bmim][BF ] was added to both BHDC/benzene and TX-lOO/benzene systems, a larger hypsochromic shift was sensed by the probe in the former. This implies that the local environments in BHDC/benzene system are more polar. The authors ascribed this phenomenon to the strong electrostatic interactions between the [BFJ anion and the BHD moiety of the cationic surfactant. 

---