Aqueous Surfactant-Based Methods

In a similar synthesis, cetyltrimethylammonium tosylate (CTAT) has been used to produce pentagonal silver nanorods [79]. It is worthy of mention here that only those seed particles with a multiple twinned decahedral structure could grow to 

STEP i PREFERENTIAL SURFACTANT OMINNeTO SPECtRC CRYSTAL FACES 

Positively charged CTAB bilayer stabilizes the nanorods 

Figu re 3.9 (a) Typical TEM image of a multiple twinned decahedral particle (b) Preferential binding of the CTAT to the lateral 100 plane of the nanorod. Adapted with permission from Ref [79] 2005 The American Chemical 

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Do L, Sabatini D. 2010. Aqueous Extended-Surfactant Based Method for Vegetable Oil Extraction Proof of Concept. J. Am. Oil Chem. Soc. 87(10) 1211-1220. 

Favretto and co-workers [198,206-208] have described direct spectrophoto-metric methods for non-ionic surfactants based on the formation of a sodium picrate surfactant adduct. This method has been applied to seawater. A mean value of 93 1% was obtained in recovery experiments on C12E9 (at an aqueous concentration of 0.10 mg/1) extracted from synthetic sea water by means of this... 

When the hydrogen atom of the hydroxyl group on C6 of cellulose is partially substituted with a hydroxyethyl (-CH CH OH) group in a reaction with ethylene oxide under alkaline condition, hydroxyethyl cellulose (HEC) is produced. So far there are no known testing methods for HEC detection. However, if one wants to distinguish CMC from HEC, an ion tolerance test can be conducted. CMC is anionic and can be precipitated from an aqueous solution with a cationic surfactant. Since HEC is non-ionic, its aqueous solution is compatible with cationic surfactants. Based on the same ionic tolerance principle, a high salt concentration can precipitate CMC, not HEC. 

Leon-Gonzalez et al.[31] proposed an FI spectrophotometric method for the determination of Triton-type non-ionic surfactants based on their reaction with alizarin fluorine blue. An on-line ion-exchange column was incorporated in the system to eliminate interferences from ionic and amphoteric surfactants. In case of interferences from non-ionic surfactants, an on-line Amberlite XAD-4 adsorption column was used to retain selectively the Triton-type surfactant, which was subsequently eluted by ethanol. However, no information was given regarding interferences from refractive index effects at the ethanol/aqueous interface and their elimination. 

Removal of traces of organics from aqueous solutions Separation of organic solvent from water containing 0.1%-0.2% styrene, toluene, chloroform, butyl acetate, diethyl ether Separation methods for environmental technologies Removal of tetrachloroethylene from surfactant-based soil remediation fluid 1-Methoxy propanol and water... 

Two procedures have been adopted to resolve the preceding problem. Vitha et al. have used a modified curve resolution method [14,15]. This method is based on singular value decomposition (SVD) of a matrix of spectra recorded at several concentrations of surfactant. The method assumes that only two components, namely, the indicator in the aqueous and micellar phase, contribute to the observed spectra. A knowledge of the molar volume of the surfactant and the aggregation number is also required. The curve resolution method gives three parameters the distribution coefficient and the spectra of the indicator solute in the two phases. The value of the spectroscopic parameter of interest (e.g., the position of band maximum) was then determined from the spectrum in the micellar phase. 

In circulating methods, the aqueous surfactant solution is pumped and sprayed in a shower or is allowed to fall onto the solution surface (fall/recycle principle). An AFNOR norm based on this fall/recycle principle is presently in draft form [25] In a water-jacketed glass tube, a defined volume of aqueous surfactant solution (minimum 1 g of surfactant/ liter) is pumped and dropped freely from a fixed distance onto a bed of the same surfactant solution. The tested foaming solution is recycled continuously at a constant flow rate. This method is useful for assessing the foaming profile of surfactants and for the study of efficacy of foam-control agents like EOPO (ethylene oxide-propylene oxide) block copolymers [26]. 

Centrifugation leads to the preferential removal of the densest constituents of the multi-component systems constituted by aqueous surfactant-CNT dispersions. The nature and the quantity of species removed are strongly related to the CNT type and batch [as already illustrated in the previous paragraph] and to the surfactant types. Regarding this last point, it has been foreseen and/or demonstrated many times that all surfactants do not necessary have the same ability to exfoliate CNTs [see refs. 13, 14, 189, among others]. In particular. Tan and Resasco developed a method based on UV-Vis spectroscopy for quantifying the [individual CNT]/[CNT bundle] ratio. This method leads to the ranking of the relative ability of surfactants to exfoliate SWCNTs. 

A new one phase method for the synthesis of uniform monodisperse crystalline Ag nanoparticles in aqueous systems was developed by using newly synthesized mono and dihydroxylated ionic liquids and cationic surfactants based on 1,3-disubstituted imidazolium cations and halogens anions. The hydroxyl functionalized ionic liquids and hydroxyl functionalized cationic surfactants simultaneously act both as the reducing and protective agent. By changing the carbon chain length, alcohol structure and anion of the hydoxy-functionalized 1,3-imidazolium based ionic liquids and the hydroxyl functionalized cationic surfactants the particle size, uniformity and dispersibility of nanoparticles in aqueous solvents could be controlled (Dorjnamjin et al, 2008). 

Sodium sulfate is the main inorganic salt expected in sulfonated surfactants. It may also be deliberately added to formulations as a filler or processing aid. Classical methods for sulfate determination are gravimetric or based on BaCl2 titration. It has been found that non-aqueous acid-base titration permits sulfate to be differentiated from sulfonates (18). Sulfate ion has been titrated with lead chloride solution using dithizone as indicator and acetone/water as solvent (19). Alternatively, it may be titrated with barium perchlorate with Sulfonazo III as indicator (9). 

Other detection methods are based on optical transmittance [228-231], Alcohol sulfates have been determined by spectrophotometric titration with barium chloride in aqueous acetone at pH 3 and an indicator [232] or by titration with Septonex (carbethoxypentadecyltrimethylammonium bromide) and neutral red as indicator at pH 8.2-8.4 and 540 nm [233]. In a modified two-phase back-titration method, the anionic surfactant solution is treated with hyamine solution, methylene blue, and chloroform and then titrated with standard sodium dodecyl sulfate. The chloroform passing through a porous PTFE membrane is circulated through a spectrometer and the surfactant is analyzed by determining the absorbance at 655 nm [234]. The use of a stirred titration vessel combined with spectrophotometric measurement has also been suggested [235]. Alternative endpoint detections are based on physical methods, such as stalag-mometry [236] and nonfaradaic potentiometry [237]. 

Extraction of protein from aqueous solution by surfactant-containing lipophilic organic solvent (phase transfer method, or equivalently, w/o-ME-based liquid-liquid extracting, LEE). 

Dissolution test data will be required in all cases (and for all strengths of product) for development and routine control and should be based on the most suitable discriminatory conditions. The method should discriminate between acceptable and unacceptable batches based on in vivo performance. Wherever possible Ph Eur test methods should be used (or alternatives justified). Test media and other conditions (e.g., flow through rate or rate of rotation) should be stated and justified. Aqueous media should be used where possible and sink conditions should be maintained. A small amount of surfactant may be added where necessary to control surface tension or for active ingredients of very low solubility. Buffer solutions should be used to span the physiologically relevant range—the current advice is over pH 1 6.8 or perhaps up to pH 8 if necessary. Ionic strength of media should be reported. The test procedure should employ six dosage forms (individually) with the mean data and a measure of variability reported. 

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