Ethoxymer distribution

Aliphatic AEOs, considered as environmentally safe surfactants, are the most extensively used non-ionic surfactants. The commercial mixtures consist of homologues with an even number of carbon atoms ranging typically from 12 to 18 or of a mixture of even-odd linear and a-substituted alkyl chains with 11—15 carbons. Furthermore, each homologue shows an ethoxymer distribution accounting typically for 1—30 ethoxy units with an average ethoxylation number in the range 5—15. The separation of the AEO complex mixtures was achieved by reversed-phase and normal-phase chromatographic systems [74—76]. 

The ethoxymers distribution appeared similar in all sediments, with A9PEO1 the dominant ethoxymer, and smaller but significant contri-... 

As discussed earlier in this chapter, peaked or narrow-range ethoxylates are available which have peaked or narrow ethoxymer distributions. Peaking the distribution effectively concentrates certain ethoxymers. If these ethoxymers are key to performance, then the latter (soil removal, wetting, etc.) will be enhanced. However, the opposite is also true, which is why utilizing peaked ethoxylates requires optimization of EO content in order to obtain the benefit of peaking. 

The general relationship described above between hydrophobe structure and EO chain length of alcohol ethoxylates also applies to MEEs. However, since the catalysts used to prepare MEEs are often the same catalysts used to produce peaked ethoxylates, the ethoxymer distributions for MEEs are in fact already peaked . The degree of peaking, however, varies significantly depending on the catalyst used. 

Another important characteristic of methyl ester ethoxylate structure is the distribution of the ethoxymers (the relative concentration of unethoxylated feedstock, of 1-mol ethoxymer, of 2-mol ethoxymer, etc.). As discussed later in this chapter, the ethoxymer distribution of methyl ester ethoxylates, like that of their alcohol ethoxylate counterparts, can vary depending on the catalysts used to prepare them. 

Ferguson et al. [18] reported on the application of a mixed-mode HPLC separation, coupled with ESI-MS for the comprehensive analysis of NPEOs and nonylphenol (NP) concentrations and distributions in sediment and sewage samples. The mixed-mode separation, which operates with both size-exclusion and reversed-phase mechanisms, allows the resolution of NPEO ethoxymers prior to introduction to... 

Boyd-Boland and Pawliszyn [77] pioneered the SPME analysis of APEOs by SPME-HPLC using normal-phase gradient elution with detection by UV absorbance at 220 nm. The Carbowax-template resin (CW-TR) and Carbowax-divinylbenzene (CW-DVB) fibres allowed the analysis of APEO with a linear range of 0.1-100 mg L 1. The former coating produced the best agreement between the distribution of ethoxymers before and after extraction. This CW-TR fibre provided a limit of detection for individual AP ethoxamers at the low ppb level. The determination of NP in water by SPME-GC (FID) was accomplished by Chee et al. [78] using a polydimethylsiloxane (PDMS) fibre. The linear range was between 1 and 15 mg L 1 with an estimated detection limit of 0.1 mg L-1. 

APEO concentrations in suspended matter samples were similar to the sediment concentrations. From these values, in situ organic carbon corrected suspended matter/water distribution coefficients (log Koc) were calculated. For the separate ethoxymers NP, A9PE0i, A9PE02,... 

Sorption of APEO to sediments does occur, and once the surfactants have entered the sediment, further degradation will be very slow [28]. In general, once buried in sediments, ethoxymer (APEO) and isomer (AP, APEO) distributions probably do not change substantially. 

These materials, of course, contain no active hydrogen, so how does the reaction work The mechanism is complex and not fully understood but is thought to involve transesterification. The actual distribution of the ethoxymers depends on the catalyst used but... 

Figure 13.10. Distribution of ethoxymers for the sodium-hydroxide-catalysed ethoxylation of dodecanol (to 60% EO by weight)...

Although there hasn t been a great deal of research on the mechanisms involved in the ethoxylation of esters, one mechanism that has been proposed involves transesterification (7). As shown in Figure 13.12, it is the catalyst (in this case, a mixture of calcium and aluminium alkoxides) that first becomes ethoxylated (forms the metal alkoxyethoxylate). After the catalyst picks up a mole of EO, it then transesterifies with the ester to form methyl ester ethoxylate, alkyl ester ethoxylate, and metal-coordinated methoxide. These steps occur continuously until the available EO is exhausted and a distribution of methyl ester ethoxylate homologues (ethoxymers) is obtained. 

With the exception of some small volume 1- and 2-mol products, all ethoxylates consist of a distribution of ethoxymers. These distributions are not statistically determined because there exist finite differences in the relative reactivities of various homologues to... 

Figure 13.13. Distribution of ethoxymers for the preparation of 60% EO ethoxylates by using sodium hydroxide (NaOH) and a proprietary peaked distribution catalyst...

Oxyethylated alcohols are prepared at 130°C-180°C in the presence of a catalyst whose concentration is on the order of a few tenths of a percent. Various kinds of catalysts are used. Earlier, basic catalysts (sodium hydroxide) were commonly used, which yielded a wide distribution of ethoxymers. Currently, acidic catalysts with a considerably narrower distribution of ethoxymers and a lower content of unreacted alcohol are used. It follows from this comparison that products obtained by means of different syntheses may have different compositions due to the presence of both different ethoxymers and residual free alcohol, which is often hard to vaporize. Additionally, alcohol as a reactant may be a mixture of compounds with various alkyl chain lengths. Therefore, particularly in the case of ethoxylates prepared from natural sources on an industrial scale, one may have to deal with a mixture of compounds with various ethylene oxides (m) and alkyl (n) chain lengths. 

Ethoxylation of methyl esters using conventional hydroxide catalysts (NaOH, KOH, etc,) does not proceed efficiently because of the absence of an active hydrogen. As shown in the chromatograrm of the resulting ethoxylate (Fig. 3A), conversion is poor, and the product contains a broad distribution, of ethoxymers as well as significant concentration of unethoxyl-ated methyl esters. 

In contrast, ethoxylation of methyl esters with a more complex catalyst system (activated calcium and aluminum alkolide [27]) is significantly more successful in achieving efficient ethoxylation, reducing the level of unethoxyl-ated methyl ester, and yielding a more peaked distribution of ethoxymers (Fig. 3B). 

The distribution of methyl ester ethoxymers (the relative concentrations of unethoxylated feedstock, of 1-mol ethoxylate, of 2-mol ethoxylate, etc.)... 

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