## 4 Discussion  
The ethoxylation of sodium lauryl sulphate (i.e., the addition of ethylene oxide (EO) groups) has been proven to reduce its hazards to humans regarding the skin irritation effects of this surfactant while maintaining its functionalities [28], although a parallel assessment of the influence of such ethoxylation on the toxicity of SLE\({}_{\rm n}\)S to the biota has rarely been conducted, even though this chemical is commonly discharged in freshwater ecosystems. Thus, the present work aimed to elucidate the effects of different degrees of ethoxylation (number of EO groups) of SLE\({}_{\rm n}\)S on the viability of the _X. laevis_ kidney cell line A6 through MTT and resazurin assays. Although both assays estimate cellular viability due to the reduction of the active ingredient by oxidoreductaseness present in the cell, there is a preference for the enzymes that catalyze this reaction towards MTT or resazurin [29]. While MTT is mainly reduced by the oxidoreductaseness, dehydrogenases, oxidases, and peroxididases using NADH, NADPH, succinate, or pyruvate as electron donors that are present in the mitochondria and plasma membrane, resazurin is mainly reduced by reductases present in the cytosol, mitochondrial, or microsomal enzymes; by enzymes in the respiratory chain; or by electron transfer agents, preferably N-methylphenazinium methosulfate [27; 30]. The results obtained varied according to the assay used. The resazurin assay indicated that SLE\({}_{\rm n}\)S variants with a fewer number of EO units were the most toxic to A6 cell lines, exhibiting lower values of LC\({}_{50}\). Though the results from the MTT assay showed a similar pattern of fewer EO units associated with higher toxicity, no significant association was observed between the computed LC\({}_{50}\) values and the number of EO groups. This pattern of toxicity is in line with previously published works. [PERSON] et al. [1] assessed the effects of SLE\({}_{\rm n}\)S on the bioluminescence of the marine bacterium _Vibrio fischeri_ and reported that SLE\({}_{\rm n}\)S with a greater number of EO units were comparatively less toxic to the bacterium than those with a fewer number of EO units. These authors reported acute toxicity ranges (LC\({}_{50}\)) from 0.56 to 2.81 mg/L\({}^{-1}\) for SLE\({}_{0}\)S, SLE\({}_{1}\)S, SLE\({}_{2}\)S, and SLE\({}_{4}\)S, while SLE\({}_{11}\)S, SLE\({}_{30}\)S, and SLE\({}_{50}\)S showed an acute toxicity from 8.09 to 8.60 mg/L\({}^{-1}\)[1]. Possible mechanisms for SLE\({}_{\rm n}\)S with a fewer number of EO units being more toxic were suggested by [PERSON] et al. [1]. First, a higher number of EO units is linked with a high stabilization of micelles due to the screening of headgroup repulsion, which contributes to the presence of a lower quantity of monomers in the suspension. Therefore, this mechanism lowers the amount of surfactant (i.e., monomers) able to penetrate the cell membrane, resulting in lower toxicity [1]. The second mechanism is represented by the fact that surfactants with a fewer number of EO units are more hydrophobic and, consequently, more efficient at associating with cellular membranes, thus being more bioavailable [1]. Similarly, [PERSON] et al. [11] determined the acute toxicity of nine variants of alcohol ethoxylate surfactants with different alkyl chain lengths and different numbers of EO groups on _P. promelas_ and on _D. magna_. The authors reported increasing toxicity with decreasing number of EO units; for example, for _D. magna_ the EC\({}_{50,48\rm{ns}}\) ranged from 0.46 mg/L (for the variant with 5 EOs) to 1.2 mg/L (for the variant with 13 EOs) [11]. The exposure duration had no significant effect on the cell viability for any variant of SLE\({}_{\rm n}\)S in the resazurin assay, but a significant influence was registered for SLE\({}_{1}\)S and SLE\({}_{11}\)S when the MTT assay was performed. This latter assay also showed a higher sensitivity to the variants of SLE\({}_{\rm n}\)S compared to the resazurin assay.