M. A. Acquavia
1,2, A. Onzo
1, T.R.I Cataldi
3, M. Ligonzo
1, D. Coviello
1, R. Pascale
1, G.
Martelli
1, M. Bondoni
2, L. Scrano
4and G. Bianco
11Università degli Studi della Basilicata, Dipartimento di Scienze, Via dell’Ateneo Lucano 10, 85100, Potenza,
Italy, 2ALMAGISI s.r.l, Corso Italia, 27, 39100, Bolzano, Italy, 3Università degli Studi di Bari Aldo Moro,
Dipartimento di Chimica, via E. Orabona 4, 70126, Bari, Italy, 4Università degli Studi della Basilicata,
Dipartimento delle Culture Europee e del Mediterraneo: Arch., Ambiente, Patrimoni Culturali, Via Lanera, 20, 75100 Matera, Italy.
Keywords: Green surfactants, sodium coceth sulfate, tandem mass spectrometry
Introduction
Alkyl ether sulfates (AESs) represent one of the major class of anionic surfactants used in soaps and detergent formulations [1]. Due to their favorable physicochemical characteristics, they are currently employed also in many fields of technology and research, such as for the enhancement of active ingredients’ efficacy in pharmaceutical and agricultural formulations, or in biotechnological and industrial processes. Like all the surfactants, after their use AESs as well as their products, are mainly discharged into sewage treatment plants and then dispersed into the environment, through effluent discharge into surface waters and sludge disposal on lands [2]. As such, they are among the most relevant organic pollutants of anthropogenic origin which cause eutrophication and acidification of rivers and lakes [3]. Besides, the use of detergent-contaminated water for cultivation reduces the photosynthetic rate and chlorophyll content in plants [4]. Due to the growing awareness of environmental issues related to the extensive use of tensioactives and to the increasing consumer demand for products that are both more biocompatible and efficient, milder and biodegradable, the use of “green surfactants” has recently being proposed [5,6]. Green surfactants are defined as bio-based amphiphilic molecules obtained from renewable raw materials and whose synthesis results in a less CO2 liberation
compared to the production of conventional petrolchemicals-based compounds [7]. Several renewable raw materials could serve as starting compounds for surfactant synthesis, such as fatty acids, carbohydrate sources and organic acids; clearly fatty acids or sterols contribute to the hydrophobic part of green tensioactives while sugars and amino acids contribute to the hydrophilic moiety [7]. Fatty acids could be recovered from a wide range of vegetable oils; as example, fatty acids deriving from coconut, palm, and palm kernel oils are used to obtain substantial yields of hydrophobic surfactants. Anyway, regardless of their derivation source, different oleochemical transformations, hydrogenation, hydrolysis, trans-esterification as well as certain specific modifications of fatty acids to yield various surfactants and surfactant precursors are needed.
Coconut oil is the base ingredient of the surfactant sodium coceth sulfate (CES), i.e. an alkyl ether sulfates mixture added in a lot of cleaning products and detergents defined as non-aggressive and biodegradable, and it is obtained as the sodium salt of the sulfate esters of the polyethylene glycol ether of coconut alcohols [8].
To date, no structural investigations on green alkyl ether sulfates constituting CES have been made. The length of their alkyl chains and the degree of ethoxylation in their chemical formula (CxHy(OCH2CH2)nOSO3Na) remain still unknown, therefore making difficult their identification in
products that are marketed with the “eco-frindly” label. Thus, in this work a structural characterization of coceth sulfate was performed, for the first time, by direct-injection ESI(-)-LTQ-MS and MS/MS technique.
Materials and Methods
A methanolic solution of commercial coceth sulfate raw material was analysed by direct-injection into the ESI-LTQ-MS instrument, in order to investigate the structure of the green alkyl ether sulfates constituting it. Full-scan MS experiments were performed in the range m/z 100–1500 and negative ion ESI-MS was used for the detection of the intact AESs of CES. To identify the alkyl chain length “x” and the degree of ethoxylation “n” of all the species corresponding to the m/z signals occurring in the
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Virtual International Pre-Congress 62 MS spectrum of CES, an in-house database of putative molecular formulae, written in RStudio software (https://www.r-project.org/), was used. For confirming the proposed structures, tandem mass spectrometry (MS/MS) analyses were performed on single members of CES MS spectrum. Tandem mass spectrometry experiments were conducted by using Collision Induced Dissociation (CID) as fragmentation technique and the collision energy was optimized for each precursor ion, between 0.9-1.5 eV of applied electrical potential.
Results
ESI(-)-LTQ-MS analysisof commercial CES revealed a well-defined and asymmetric molecular weight distribution. There were apparently two ion series with 44 Da apart within the series and 28 Da apart between the series. Thus, the presence of two alkyl homologues, whose the alkyl chain differed for a ethylene (C2H4) moiety, with a variable number of monomeric (-CH2CH2O-) units was suggested.
Starting from the existing data reported in literature about alkyl ether sulfates and by using an in-house dedicated database of putative molecular formulas, alongside with MS/MS experiments, it was possible to characterize the alkyl chain composition and the number of ethylene oxide units of the anionic species occurring in CES raw material. Two series of oligomeric species characterized by a C12 and C14 alkyl chain, i.e. [C12H25(OCH2CH2)nOSO3]- and [C14H29(OCH2CH2)nOSO3]- with n comprised between 0 to
7, were successfully ascertained.
Discussion/Conclusions
Direct-injection mass spectrometric and tandem mass spectrometric analysis were used to conduct, for the first time, a detailed investigation on the structure of biodegradable alkyl ether sulfates constituting sodium coceth sulfate. Due to its desirable ecological profile and to its good compatibility with eyes, skin and mucous membranes, CES is used in a wide variety of consumer products in which viscosity building and foam characteristics are of importance. Therefore, the definition of its characteristic m/z signals is remarkably useful for its detection in green labelled detergents. Direct-injection mass spectrometry turned out to be a simple and low-cost technology to that purpose, as it allowed to quickly define the m/z signals pattern of the CES, ensuring its identification in commercial products.
References
1. N.J. Turro, M. Grätzel, A.M. Braun; Angewandte Chemie International Edition in English, 19 (1980), pp 675-696.
2. G. G. Ying; Environment international, 32 (2006), pp 417-431.
3. S. González, D. Barceló, M. Petrovic; TrAC Trends in Analytical Chemistry, 26 (2007), pp 116-124.
4. B. R. Jovanic, S. Bojovic, B. Panic, B. Radenkovic, M. Despotovic; health, 5 (2010), pp 395-399. 5. K. Holmberg; Current Opinion in Colloid & Interface Science, 6 (2001), pp 148-159
6. T. Benvegnu, J. F. Sassi; Carbohydrates in Sustainable Development I, (2010), pp 143-164. 7. S. Rebello, A. K. Asok, S. Mundayoor, M. S. Jisha; Environmental chemistry letters, 12 (2014), pp
275-287.
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Forensic
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