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Scalable Synthesis of Salt-Free Quaternary Ammonium Carboxylate

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Scalable Synthesis of Salt-Free Quaternary Ammonium Carboxylate 270 DOI: 10.17344/acsi.2019.5413 Acta Chim. Slov. 2020, 67, 270–275 Scientific paper Scalable Synthesis of Salt-free Quaternary Ammonium Carboxylate Catanionic Surfactants Žiga Medoš, Miha Virant, Urša Štanfel, Boštjan Žener, Janez Košmrlj* and Marija Bešter-Rogač* Faculty of Chemistry and Chemical Technology, Večna pot 113, University of Ljubljana, SI-1000 Ljubljana, Slovenia * Corresponding author: E-mail: [email protected] Tel: +386 1 479 8558 E-mail: [email protected] Tel: +386 1 479 8537 Received: 07-16-2019 Abstract Surfactants in commercial products commonly contain catanionic mixtures thus many studies of aqueous surfactant mixtures have been carried out. However, hardly any studies have been dedicated to pure catanionic surfactants often termed salt-free catanionic surfactants. One of the difficulties is in acquirement of samples with required purity due to difficult separation of these compounds from inorganic salts. In this work we present an alternative method of synthesis using dimethyl carbonate as the alkylating agent in order to obtain alkyl trimethylammonium alkanecarboxylates with medium alkyl chain lengths (6–10). Keywords: Synthesis; surfactants; salt-free; catanionics; quaternization 1. Introduction tion techniques to ion-exchange columns or the precipi- tation of silver halides from water solutions. Unfortu- Commercial surfactants are commonly a mixture nately, these two approaches are not economically viable of cationic, anionic and non-ionic surfactants due to and some contamination with inorganic salt can still oc- their enhanced performance as mixtures.1,2 Therefore, cur. This can have significant impact on some of the sur- along the studies of non-ionic and ionic surfactants in factant’s properties and can crucially affect studied phys- their pure form as well as in binary aqueous solutions, ical and aggregation properties of aqueous solutions.4–6 significant focus has been dedicated to aqueous mixtures Thus, the development of novel and more effective meth- of cationic and anionic surfactants – catanionic mix- ods of synthesis is required. tures. An interesting group of catanionic surfactants are In this work we present two synthetic procedures to salt-free catanionics, where often the challenge of pre- prepare alkyltrimethylammonium alkanecarboxylate paring these surfactants in their pure form is the remov- catanionic surfactants (Figure 1). Generally, the synthesis al of all inorganic salts upon mixing the parent cation of quaternary ammonium surfactants starts with ammo- and anion salts. There are three main methods to remove nia or substituted amine precursor. It is alkylated in two inorganic salts from equimolar catanionic mixtures: (1) or more steps with the last step being quaternization. ion-exchange columns are used to prepare acids and hy- Most studied and produced quaternary ammonium sur- droxides and subsequently mixed; (2) liquid-liquid ex- factants are alkyltrimethylammonium halides followed by traction in organic phase and (3) precipitation method.3 dialkyldimethylammonium halides (gemeni surfac- When surfactants are poorly soluble in organic solvents tants).7 Their aqueous solutions are generally more stable precipitation is generally easily achieved. On the con- than quaternary ammonium surfactants with remaining trary, if solubility is limited in polar solvents liquid-liq- hydrogen(s) on quaternary ammonium. Quaternization uid extraction is possible. However, many surfactants, can be achieved by the Menshutkin reaction where tertia- especially those interesting for application, are soluble in ry amine is reacted with haloalkane. However, halide free both, organic and aqueous media. This limits the separa- methods are preferred for industrial application. Medoš et al.: Scalable Synthesis of Salt-free Quaternary ... Acta Chim. Slov. 2020, 67, 270–275 271 nal electrospray source at atmospheric pressure ionization (ESI) coupled to an HPLC instrument. 1H, 13C and 15N NMR spectra were recorded with a Bruker Avance III 500 Figure 1. N-alkyl-N,N,N-trimethylammonium alkanecarboxylates. MHz NMR (at 500 MHz, 126 MHz and 51 MHz, respective- ly) instrument at 296 K in DMSO-d6 using TMS as an inter- nal standard. Proton and carbon spectra were referenced to Recently, Jiang et. al. presented a synthetic procedure the residual chloroform shifts of 7.26 ppm and 77.16 ppm, for the preparation of salt-free quaternary ammonium car- respectively.13 Assignments of proton, carbon and nitrogen boxylates using dimethyl carbonate (DMC) as alkylating resonances were performed by 2D NMR techniques (1H−1H agent of appropriate tertiary amines, with one or two alkyl gs-COSY, 1H−13C gs-HSQC, 1H−13C gs-HMBC and 1H−15N chains exceeding 12 carbon atoms and a short-chain car- gs-HMBC). Carbon resonances without labels belong to the boxylate anion (acetate, propionate, lactate).8 They expand- chain carbons and could not have been differentiated. Ther- ed the scope to long-chain catanionic surfactants in the mogravimetric (TG) measurements were performed on a follow-up papers, namely tetradecyltrimethylammonium Mettler Toledo TGA/DSC1 Instrument in the temperature alkanecarboxylates (hexanoate, octanoate, decanoate, do- range from 25 to 400 °C under dynamic air flow (100 cm3 decanoate, tetradecanoate)9 and alkyl (decyl, dodecyl, tet- min−1) with a heating rate of 5 K min−1. Approximately 3–5 radecyl, hexadecyl, octadecyl) trimethylammonium deca- mg of sample was weighed into a 150 μL platinum crucible noates.10 DMC represents an attractive eco-friendly and the baseline was subtracted. Differential scanning calo- alternative to methyl halides.11 rimetry (DSC) measurements were performed separately However, additional challenges emerge when both on a Mettler Toledo DSC 1 Instrument in 40 μL aluminium alkyl chains are shorter than 10 carbon atoms. Mainly, due crucibles under the same conditions. to increased solubility in majority of polar and non-polar solvents the usual purification methods of precipitation, 2. 2. Synthesis recrystallization and liquid-liquid extraction become less efficient or impossible. Additionally, the commercial price Alkyltrimethylammonium Chlorides 3a–c of tertiary amines, required in the most common synthesis In a 200 mL autoclave chloroalkanes 2a–c (0.21 mol, approach, increases with shorter chains. Therefore, in this 1 eq) were mixed with 33% trimethylamine solution in work we present low cost synthetic alternative starting ethanol (1, 100 mL, 2 eq). The mixture was stirred in an oil with a secondary amine to produce quaternary ammoni- bath at 80 °C for 3 days. Excess 1 and solvents were re- um carboxylate surfactants with alkyl chains between 6 moved under reduced pressure. Products were dissolved and 10 carbon atoms long. in minimal amount of ethanol required (approx. 5 mL) Synthesis of decyltrimethylammonium alkanecar- and recrystallized by addition of ethyl acetate (100 mL). boxylates (acetate, butanoate, hexanoate, octanoate, nona- The precipitates were filtered and washed with diethyl noate, decanoate, undecanoate) has also been reported by ether (50 mL) and subsequently dried under reduced pres- reacting quaternary ammonium halides with silver hy- sure to yield products in 50% average yield. Products 3 are droxide. After precipitation of the silver halide salts, the very hygroscopic which reduces the yield of recrystalliza- resulting hydroxide solutions were reacted with carboxylic tion in open air (3a 26%, 3b 53%, 3c 55%). acid.12 For large scale application silver salts are not eco- nomically viable, therefore we present a competitive ap- Alkyltrimethylammonium Carboxylates 5a–e According to proach through precipitation of KCl from ethanol. We Procedure A compare the thermal properties of products obtained by In the next step, alkyltrimethylammonium chlorides both procedures. 3a–c as prepared above (0.07 mol) were dissolved in etha- nol (20 mL) and weighed precisely. The exact amount of chloride ions was determined by AgNO3 titration of small 2. Experimental samples and the required equimolar amounts of carboxylic acids 4a–c were weighted and added. 95% of KOH, re- 2. 1. General Information quired for neutralization, was weighed as solid pellets. Af- The reagents and solvents in general procedures were ter all of the added solid KOH dissolved and KCl precipitat- used as obtained from the commercial sources (Merck), un- ed (1 hour), the remaining KOH was titrated as an less noted otherwise. Octanoic acid (4b) and decanoic acid approximate 0.25 M solution of KOH in ethanol until po- (4c) were recrystallized from ethanol by addition of acetone. tential of glass electrode dropped below –250 mV which IR spectra were obtained with a Perkin–Elmer Spectrum was previously determined as the potential of the inflection 100, equipped with a Specac Golden Gate Diamond ATR as point. Solutions were filtered to separate the filtrate con- a solid sample support. High resolution mass spectra taining the products from the precipitated KCl. Filtrates (HRMS) were recorded on Agilent 6224 time-of-flight were concentrated under reduced pressure subsequently (TOF) mass spectrometer equipped with a double orthogo- precipitating more KCl which was filtered to obtain filtrate Medoš et al.: Scalable Synthesis of Salt-free Quaternary ... 272 Acta Chim. Slov. 2020, 67, 270–275 containing desired alkyltrimethylammonium carboxylates on cation and anion the better the yield due to the poorer 5a–e in quantitative yields. Solutions were dried first under solubility in acetonitrile and lower
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