Ethoxylate number determination

The above chemical shift assignments and integrations also allow us to determine the average ethoxylate number for this surfactant. Given that the ethoxylate peak ( c ) is at 3.7 ppm ... 

Either HPLC or TLC analysis is suitable for determining noncarboxylated ethoxylate impurities, as described in Chapters 7 and 9. These impurities can also be determined on a preparative scale by passing an ethanolic solution through an anion exchange column the nonionics pass through unretained (142). A simple hydroxyl number determination, as described for the analysis of nonionics in Chapter 2, is used for quality control determination of residual nonionic surfactant and PEG (145). 

Two different instrumental measurements were used to test the oxidation hypothesis GC and atomic absorption (AA). GC was used to determine the chain length distribution of the EO. From the GC results, an average chain length number was calculated. The gas chromatogram is shown in Fig. 21.1. In addition, headspace GC measurement of the gaseous compounds dissolved in the liquid ethoxylate was made and compared to typical or good product. Atomic absorption spectroscopy was used to determine the levels of several trace metals known to catalyze oxidation of fatty chemicals. 

Ethoxylated sorbitan ester surfactant mixtures like Tween 20 (cf. Fig. 2.9.38) were often used in biochemical applications. Detergents of this type were analysed by MALDI MS. The aim was to compare the separation results of TLC and RP-LC and to detect impurities within these ethoxylated sorbitan esters [30], Tween 20, the ethoxylated sorbitan carboxylate was ionised resulting in [M + Na]+ and [M + K]+ ions. The Tween 20 isomeric and homologue molecules contained a varying number of ethoxylate units. The number of EO units (-CH2CH2O-) was determined from 18 to 34 resulting in Am/z 44 equally spaced signals [30]. 

While fast atom bombardment (FAB) [66] and TSI [25] built up the basis for a substance-specific analysis of the low-volatile surfactants within the late 1980s and early 1990s, these techniques nowadays have been replaced successfully by the API methods [22], ESI and APCI, and matrix assisted laser desorption ionisation (MALDI). In the analyses of anionic surfactants, the negative ionisation mode can be applied in FIA-MS and LC-MS providing a more selective determination for these types of compounds than other analytical approaches. Application of positive ionisation to anionics of ethoxylate type compounds led to the abstraction of the anionic moiety in the molecule while the alkyl or alkylaryl ethoxylate moiety is ionised in the form of AE or APEO ions. Identification of most anionic surfactants by MS-MS was observed to be more complicated than the identification of non-ionic surfactants. Product ion spectra often suffer from a reduced number of negative product ions and, in addition, product ions that are observed are less characteristic than positively generated product ions of non-ionics. The most important obstacle in the identification and quantification of surfactants and their metabolites, however, is the lack of commercially available standards. The problems with identification will be aggravated by an absence of universally applicable product ion libraries. 

Having established that bilayer flexibility and bilayer interaction are the mesoscopic determinants, the next question is whether these determinants can be coupled to molecular parameters. In fact, this has been done to quite some extent. In general, bilayer flexibility can be shown (both experimentally as well as theoretically by simulation methods) to be directly related to bilayer thickness, lateral interaction between heads and tails of the surfactants, type of head group (ethoxylate, sugar, etc.), type of tail (saturated, unsaturated) and specific molecular mixes (e.g. SDS with or without pen-tanol). The bilayer interaction is known to be related to characteristics such as classical electrostatics. Van der Waals, Helfrich undulation forces (stemming from shape fluctuations), steric hindrance, number, density of bilayers, ionic strength, and type of salt. Two examples will be dicussed. 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

The structural architecture of silicone polymers, such as the number of D, T, and Q sites and the number and type of cross-link sites, can be determined by a degradative analysis technique in which the polymer is allowed to react with a laige excess of a capping agent, such as hexamethyldisiloxane, in the presence of a suitable equilibration catalyst (eq. 38). Triflic acid is often used as a catalyst because it promotes the depolymerization process at ambient temperature (444). A related process employs the KOH- or KOC2H5-catalyzed reaction of silicones with excess Si(OC2H5)4 (eq. 39) to produce ethoxylated methylsilicon species, which are quantitatively determined by gc (445). 

For the evaluation of the foamability of a surfactant the bulk concentration is used at which the relative rate of foam collapse is equal to 50% of its formation (cw °). The cw ° values determined from foam formation isotherms of a number of products are given in Table 6.1. As it is seen, typical representatives of anionics (sodium dodecyl sulphate), cationics (cetyl trimethyl ammonium bromide) and nonionics (ethoxylated alkylphenols) give bubble foams at very low concentrations, and the foam stability of ionic surfactants does not differ much from that of nonionics. For anionics, the highest concentrations are required for soaps of higher carboxylic acids. 

Only the apphcation of LC-MS analysis can be conceived of as reliable surfactant analysis. To elaborate determination methods for the analysis of the anionic surfactant mixture of alkyl ethoxysuUates (AES) APCI and ESI-MS(-t/-) studies combined with in-source MS/MS examinations were performed and results were compared (cf. Pig. 15.4 and 15.5 and 15.3.3.2 ESI, surfactants). APCI fragment ion spectra revealed the aUcyl chain length and the number of ethoxylate moieties therefore APCI was found to be the method of choice [326]. To confirm determination methods apphed in surfactant analyses an inter-laboratory comparison study of LC-MS techniques and enzyme-hnked immunosorbent assay for the determination of surfactants in wastewaters was performed in seven laboratories. The quantitative determination of the non-ionic NPEOs, AEOs, coconut diethanol amides (CDEAs) and the anionic LAS, NPEO-sulfates and the secondary alkane sulfonates (SAS) was performed under APCI or ESI-interfacing conditions in positive or negative... 

It has been known for a number of years that FD-MS is an effective analytical method for direct analysis of many rubber and plastic additives. Major components and impurities in commercial additives can be assessed quickly, and the FD-MS data can be used to help determine what (if any) additional analytical characterization is needed. Lattimer and Welch showed that FD-MS gives excellent molecular ion spectra for a number of polymer additives, including rubber accelerators (diWocarbamates, guanidines, benzothiazyl, and thiuram derivatives) antioxidants (hindered phenols, aromatic amines) p-phenylenediamine-based antiozonants, processing oils, and phthalate plasticizers. Zhu and Su characterized alkylphenol ethoxylate surfactants by FD-MS. Jackson et al. analyzed some plastic additives (hindered phenol antioxidants and a benzotriazole UV stabilizer) by FD-MS. ... 

The number of different emulsifiers is in the thousands. Even restricting selection to nonylphenol ethoxylates leaves many choices. Some parameter is necessary to compare emulsifiers. Hydrophilic/Lipophilic balance (HLB number) was developed by W.C. Griffin (Griffin, 1949, 1954) in the 40 s and describes emulsifying properties of a surfactant. Values vary between 0 and 20. A low HLB, such as 4, indicates an oil soluble emulsifier useful for dissolving small amounts of water into oil. A large number, such as 16, indicates a water soluble emulsifier useful for dissolving small amounts of oil into water. The HLB number can be determined experimentally or calculated for alkyl and aryl ethoxylates from Equation 1. 

Davies [93] has shown that the agreement between HLB numbers calculated from the above equation and those determined experimentally is quite satisfactory. Various other procedures were developed to obtain a rough estimate of the HLB number. Griffin found good correlation between the cloud point of 5 % solution of various ethoxylated surfactants and their HLB number. 

The determination of alkylphenols and nonylphenol ethoxylates is challenging due to the complexity of the mixtures, comprised of various isomers with different branching of the alkyl moiety and oligomers (with different numbers of ethoxylate units) [82-85]. 

The procedure for aldehyde determination is described above in the section on EO, PO, dioxane, and acetaldehyde. Generally, HPLC is preferred for determination of all aldehydes but acetaldehyde. Formic and acetic acid can be determined by a number of procedures, including gas chromatography and ion chromatography. A method for HPLC determination of these acids as their 2-nitrophenylhydrazone derivatives has been worked out specifically for ethoxylated esters of sorbitan (93). 

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