Synthesis and surface properties of chemodegradable anionic ...

Synthesis and Surface Properties of Chemodegradable Anionic Surfactants: Sodium Carboxylates1 of cis-1,3-Dioxane Derivatives Andrzej Piasecki*, Bogdan Burczyk*, and Piotr Ruchala Institute of Organic and Polymer Technology, Wroclaw University of Technology, 50-370 Wroclaw, Poland

ABSTRACT: The 2-n-alkyl-5-carboxy-5-methyl-1,3-dioxanes were obtained in good yield from the reaction of aliphatic aldehydes with 2,2-bis(hydroxymethyl)propionic acid in dichloromethane solution, catalyzed by p-toluenesulfonic acid monohydrate. Nuclear magnetic resonance analysis indicated that they were pure cisisomers with the axial configuration of the carboxylic group at the C-5 carbon atom of the 1,3-dioxane ring. The acids were converted, with retention of the configuration, to their sodium salts by reaction with sodium methoxide or sodium hydroxide in methanol. The physicochemical properties of the acids and sodium salts, as well as their surface properties at the aqueous solution-air interface, were determined. Critical micelle concentration, surface excess concentration, surface area demand per molecule of sodium salts at the monomolecular surface layer, and standard free energy of micellization were determined based on surface tension measurements. JSD 1, 29–35 (1998). KEY WORDS: Aliphatic aldehyde, 2,2-bis(hydroxymethyl)propionic acid, carboxylic acid soap, cis-2-n-alkyl-5-carboxy-5methyl-1,3-dioxane, cis-trans isomerism, critical micelle concentration, 1,3-dioxane derivative, sodium cis-(2-n-alkyl-5methyl-1,3-dioxan-5-yl)carboxylate, surface activity.

From an ecological standpoint, there are two reasons for carboxylic acid soaps to still be regarded as valuable components of detergents: their good performance, and biodegradability usually exceeding the biodegradability of synthetic surfactants obtained from petrochemicals. However, the carboxylic acid soaps exhibit relatively low surface activity compared to other surfactants having a hydrophobic alkyl chain of the same length. They are poorly soluble in cold water and form insoluble calcium and magnesium soaps in hard water (1). Introduction of an additional inter1

Part XXXIV in the series: Chemical Structure and Surface Activity. Part XXXIII: A. Piasecki, A. Sokolowski, B. Burczyk, R. Gancarz, and U. Kotlewska, Synthesis, Surface Properties and Hydrolysis of Chemodegradable Anionic Surfactants: Diastereomerically Pure cis- and trans2,5-Disubstituted-1,3-dioxanes, J. Colloid Interface Sci. 192:74–82 (1997). *To whom correspondence should be addressed at Institute of Organic and Polymer Technology, Wroc/ law University of Technology, Wybrzeze Stanislawa Wyspian´skiego 27, 50-370 Wroc/ law, Poland. E-mail: [email protected]. Copyright © 1998 by AOCS Press

mediate fragment, e.g., an amide grouping (2) or an oligooxyethylene chain (3–7), between the hydrophobic alkyl chain and the carboxylate group in a soap molecule causes an increase of its resistance to hard water and the dispersibility of its calcium soap, with the preservation of detergency performance and good biodegradability. Investigations dealing with the synthesis and surface properties of anionic surfactants containing chemodegradable i.e., susceptible to hydrolysis, 1,3-dioxacyclane groups (8–18) and hydrophilic sodium carboxylate fragment(s) (8,10–12,15,17) have recently been described. Hydrophobic fragments of the latest group of surfactants (chemodegradable soaps) were constructed from longchain aliphatic aldehydes (8,11,15), alkyl-methyl ketones (8), 1-O-alkylglycerols (10), or alkyl oxiranes (12). Ethyl 2,3-epoxybutyrate (8), ethyl esters of ketoacids (10,12), diethyl tartrate (11), or diethyl 2,2-bis(hydroxymethyl)malonate (17) has been used as intermediates to introduce one or two carboxylic acid groups into soap-type chemodegradable surfactant molecules. Surface properties of these compounds, expressed by critical micelle concentration (CMC) value, exceed surface properties of standard alkyl soaps with the same hydrophobic alkyl chains. Moreover, the 1,3-dioxacyclane ring is prone to chemical degradation in aqueous acid solutions (8,10–12). Chemodegradable soaps bearing a 1,3-dioxolane ring have recently been shown to be biodegradable. In many cases, their biodegradability exceeds the biodegradability of a standard alkyl soap such as sodium dodecanoate (19). This work focuses on the synthesis of a new group of chemodegradable anionic soap-type surfactants, containing a 1,3-dioxane ring, prepared from easily accessible aliphatic aldehydes 1 and 2,2-bis(hydroxymethyl)propionic acid (DMPA) 2. The acetalization reaction products, i.e., the 2-n-alkyl-5-carboxy-5-methyl-1,3-dioxanes 3, were converted to sodium salts by simple neutralization with sodium hydroxide or methoxide. If the acetalization reaction is carried out in a solution of boiling dichloromethane, used as an azeotropic agent, the reaction product consists of the cis-1,3-dioxane derivative. The physicochemical properties and structures of the acids 3a–e and their

Journal of Surfactants and Detergents, Vol. 1, No. 1 (January 1998)

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A. PIASECKI ET AL.

sodium salts 4a–e, were determined as well as the surface properties at the aqueous solution–air interface (sodium salts).

EXPERIMENTAL PROCEDURES Materials. n-Octanal, n-nonanal, n-decanal, n-undecanal, and n-dodecanal (Merck, Darmstadt, Germany) were used after purification by fractional distillation and stabilization with 0.1% wt of hydroquinone. DMPA (Merck) was used as received. All other auxiliary reagents were of analytical grade and used as received. Synthesis of cis-2-n-alkyl-5-carboxy-5-methyl-1,3-dioxanes 3a–e. A mixture of 0.15 mol of an aliphatic aldehyde 1 (from n-octanal 1a to n-dodecanal 1e), 20.1 g (0.15 mol) of DMPA 2, 0.1–0.15 g of p-toluenesulfonic acid monohydrate, and 200 mL of dichloromethane was refluxed with the continuous separation of water with a Dean-Stark trap. After hot filtration, solvent and other low-boiling components were removed by evaporation under reduced pressure. The residue was crystallized from methanol to obtain cis-2-n-alkyl-5-carboxy-5-methyl-1,3-dioxanes 3a–e in 75–83% yield. Synthesis of sodium cis-(2-n-alkyl-5-methyl-1,3-dioxan-5yl)carboxylates 4a–e. 0.05 Mol of acid 3a–e was introduced in small portions at room temperature to a stirred solution of 0.051 mol of sodium hydroxide or sodium methoxide in 150 mL of methanol. The reaction mixture solvent was then evaporated yielding crude sodium carboxylate 4a–e, which was then crystallized from ethyl acetate (95–97.5% yield). Surface tension measurements. The equilibrium surface tensions of aqueous solutions were measured at 25°C by the Wilhelmy plate method. We used a 3.70-cm long, microroughened platinum plate vertically withdrawn from solution with a torsional balance. Solutions of 4a–e were prepared in quadruply distilled water containing 7.5 × 10−4 M NaOH. Accuracy of the surface tension measurements was about ± 0.2 mN m−1.

Analytical methods. 1H nuclear magnetic resonance (NMR) (300 MHz) spectra were recorded at 27°C for solutions of 3a–e in CDCl3 and 4a–e in D2O, with a Bruker Avance DRX300 spectrometer (Bruker, Karlsruhe, Germany). The chemical shifts δ (ppm) were downfield from tetramethylsilane (TMS) as an internal standard in CDCl3, or introduced as an external standard in a sealed-glass capillary tube.

RESULTS AND DISCUSSION Preparation, physicochemical properties, and cis, trans structure of the hydrophobic intermediates. The acetalization reactions of aliphatic aldehydes 1a–e with DMPA 2 were carried out in a standard manner, with the use of equimolar amounts of reactants and a low-boiling azeotropic agent (dichloromethane) to avoid the possible decomposition of DMPA. Our further investigations have shown that DMPA is stable at considerably higher temperatures. Acetalization reaction products 3a–e (Scheme 1) were obtained in about 80% yields and were easily crystallizable from methanol to give white solids with sharp melting points in the range of 133–140°C (Table 1). Elemental analyses of acids 3a–e fully confirmed their molecular compositions (Table 1). 1H NMR spectra of the acids showed, surprisingly, that all were pure cis-isomers, and not the expected mixtures of cis- and transisomers usually obtained in the synthesis of 2,5-di- or 2,5,5trisubstituted 1,3-dioxane derivatives (20–23). It has been reported that in thermodynamically equilibrated mixtures of isomers of 2,5,5-trisubstituted 1,3-dioxane derivatives with an equatorial “holding” substituent at C-2, and a methyl group geminally located with such substituents as -OH (20), -CH2OH (20,21,23), -COCH3 (22,24), -NO2 (20), or -COOCH3 (20) at the C-5 carbon atom, the axial configuration is preferred. The rate of preference is dependent both on the kind of substituent and on the conditions of equilibration process—mostly on the polarity of the solvent used (20). Probably, the formation of only cis-isomers of 3 from 1 and 2 in the presence of a polar azeotropic agent is an extreme ex-

SCHEME 1


SODIUM CARBOXYLATES OF CIS-1,3-DIOXANE DERIVATIVES

31

TABLE 1 Physicochemical Constants of the cis-2-n-Alkyl-5-carboxy-5-methyl-1,3-dioxanes 3a–e and Their Sodium Salts 4a–e

Compound 3a 3b 3c 3d 3e 4a 4b 4c 4d 4e

Alkyl substituent

Yielda (mol%)

m.p.b (°C)

Molecular formula

n-C7H15 n-C8H17 n-C9H19 n-C10H21 n-C11H23 n-C7H15 n-C8H17 n-C9H19 n-C10H21 n-C11H23

78 80 83 80 75 95 95 95 97 97.5

140–140.5 137.5 133.5–134.5 136.5–138 137.5–138 241–241.5 230.5–231 222.5–223 209–211 195–196

C13H24O4 C14H26O4 C15H28O4 C16H30O4 C17H32O4 C13H23O4Na C14H25O4Na C15H27O4Na C16H29O4Na C17H31O4Na

Elemental analysis measured (calculated), wt% C H 63.96 (63.91) 65.18 (65.08) 66.06 (66.14) 66.80 (67.09) 67.94 (67.96) 58.51 (58.63) 59.98 (59.98) 61.16 (61.20) 62.15 (62.32) 63.42 (63.33)

9.80 (9.90) 10.08 (10.14) 10.27 (10.36) 10.58 (10.56) 10.65 (10.73) 8.65 (8.71) 8.89 (8.99) 9.18 (9.25) 9.40 (9.48) 9.58 (9.69)

a

Crude products. For the products crystallized from methanol (3a–e) or ethyl acetate (4a–e).

b

ample of such preference. The 1H NMR signals of axial and equatorial protons of 3a–e at the C-4 and C-6 carbon atoms of the 1,3-dioxane ring were observed as two doublets with large coupling constants J4gem = J6gem ≅ 11.6 Hz. The axial protons resonate at higher field than their equatorial counterparts (22,25). The axial methine proton H(a)-2 at C-2 appears as a triplet at 4.47 ppm (Table 2). Protons of the equatorial methyl group at C-5 absorb as a singlet at 1.02 ppm (0.78 ppm when TMS is used as an external standard) (Table 2, Fig. 1A). The singlet for an axial -CH3 group would be observed at a much lower field [1.45 ppm in trans-2-isopropyl5-carbomethoxy-5-methyl-1,3-dioxane (20) or 1.44 ppm in trans-2-isopropyl-5-acetyl-5-methyl-1,3-dioxane (22)]. This analysis showed that acids 3a–e were diastereomerically pure cis-2-n-alkyl-5-carboxy-5-methyl-1,3-dioxanes with an equatorial alkyl group at C-2 and an axial carboxyl group at C-5 of the 1,3-dioxane ring. We are convinced that the direct acetalization reaction of DMPA with the aliphatic aldehydes

is a much more convenient method for the synthesis of 3 than the procedures presented earlier, where 1,3-dioxane derivatives with methyl and carboxyl groups at C-5 were obtained with poor or moderate yields by oxidation of the appropriate 5-hydroxy-methyl-5-methyl-substituted 1,3-dioxanes with KMnO4 (26) or with lead tetraacetate followed by silver nitrate (20). Physicochemical and surface properties of sodium cis-(2-nalkyl-5-methyl-1,3-dioxan-5-yl)carboxylates 4a–e. Sodium salts 4a–e were obtained in almost theoretical yields by a simple neutralization reaction of acids 3a–e with sodium hydroxide or sodium methoxide in methanolic solution. Their physicochemical properties are presented in Table 1. Sodium salts 4a–e are easily crystallizable from ethyl acetate to give white solids with high melting points (without decomposition). Elemental analysis confirmed molecular composition (Table 1), while analysis of 1H NMR spectra (Table 2 and Fig. 1B) confirmed that the neutralization reaction

TABLE 2 Analysis of 1H Nuclear Magnetic Resonance Spectra of the cis-2-n-Alkyl-5-carboxy-5-methyl-1,3-dioxanes 3a–e and Their Sodium Salts 4a–e

Compound 3a 3b 3c 3d 3e 4a 4b 4c 4d 4e

Alkyl substituent

H(a)-2 (t)

n-C7H15 n-C8H17 n-C9H19 n-C10H21 n-C11H23 n-C7H15 n-C8H17 n-C9H19 n-C10H21 n-C11H23

4.47 4.47 4.47 4.47 4.47 4.55 4.49 4.41 4.37 4.37

Chemical shifts δ (ppm)a,b H(e)-4,6 H(a)-4,6 CH3(e)-5 (d) (d) (s) 4.47 4.47 4.47 4.46 4.45 4.34 4.38 4.39 4.41 4.41

3.44 3.45 3.44 3.44 3.45 3.40 3.34 3.31 3.27 3.27

1.02 (0.78)c 1.02 (0.78)c 1.02 (0.78)c 1.02 (0.78)c 1.02 (0.78)c 0.81 (0.64)c 0.81 (0.64)c 0.81 (0.64)c 0.81 (0.64)c 0.81 (0.64)c

JH(a)-2-CH 5.2 5.2 5.3 5.2 5.2 5.2 5.2 5.2 5.3 5.2

a

2(α)

J4gem = J6gem 11.6 11.6 11.6 11.7 11.6 11.3 11.2 11.3 11.2 11.3

Downfield from tetramethylsilane (TMS) as internal standard for the solutions of 3a–e in CDCl3. In the spectra for 4a–e in D2O the signal of H2O protons (as impurity in D2O) was used as an internal standard at δ = 4.700 ppm. b Multiplets of equatorial alkyl group protons (CH3(CH2)nCH2-) at C-2: 0.87 (t, 3H, CH3); 1.20–1.40 (m, 2 × nH, CH3(CH2)nCH2); 1.58–1.66 (m, 2H, CH3(CH2)nCH2). c Downfield from TMS as external standard in sealed-glass capillary.


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FIG. 1. 1H nuclear magnetic resonance spectra of (A) cis-2-n-heptyl-5-carboxy-5-methyl-1,3-dioxane 3a (solution in CDCl3) and (B) sodium cis-(2n-heptyl-5-methyl-1,3-dioxan-5-yl)carboxylate 4a (solution in D2O). Tetramethylsilane as external standard.

proceeded with retention of configuration to give pure cisisomers. Surface tension measurements of aqueous solutions of sodium salts 4a–e were performed at a standard temperature of 25.0 ± 0.1°C because the Krafft point temperatures are lower than 15°C. Aqueous solutions were prepared in quadruply distilled water containing 7.5 × 10−4M NaOH. Plots of equilibrium surface tension vs. the logarithm of molar concentration [logarithm (log)c] are presented in Figure 2. The surface activity of 4a–e gradually increases with an increase of the hydrophobic alkyl chain length. The log CMC is a linear function of alkyl chain length (Fig. 3), which may be expressed by Kleven’s equation (Eq. 1) (27): log CMC = A − Bn

[1]

where A and B are constants and n is the number of carbon atoms in the alkyl chain. The value of the slope B for the sodium salts under study equals 0.43. This is a significantly higher value than that determined for standard alkyl soaps CnH2n+1COOX [B = 0.30 when X = Na and B = 0.29 when X = K (28)]. The values of B for soap-type surfactants depend on their chemical structures. Among the chemodegradable soaps containing a 1,3-dioxacyclane moiety, B changes from 0.22 for bis-functional derivatives of malonic

acid (calculated from the data presented in Ref. 17), through 0.36, 0.38, and 0.40 for sodium salts of 2-oxopropionic, 3-oxobutyric, and 4-oxovaleric acid derivatives, respectively (calculated from the data presented in Ref. 10). Considerably higher values of B were determined for nonacetal potassium soaps of 2-S-alkylthioacetic acids (B = 0.48) and 3-S-alkylthiopropionic acids (B = 0.45) (29). Sodium salts 4a–e are more surface active than the typical sodium alkyl soaps CnH2n+1COONa with the same alkyl chain lengths. For example, the surface activity of sodium dodecanoate, as expressed by its CMC value [CMC = 2 × 10−2 mol dm−3 (10)], is equal to that of sodium cis-(2-n-nonyl-5methyl-1,3-dioxan-5-yl)carboxylate 4c. This agrees with our earlier finding that the insertion of a 5-methyl-1,3-dioxane moiety between the hydrophobic alkyl chain and a hydrophilic sodium sulfate group leads to a CMC decrease that can be expressed in terms of about two methylene groups of an alkyl chain (16). Surface properties of sodium salts 4a–e are presented in Table 3. Surface excess concentration, ΓCMC, and surface area demand per molecule at the adsorption layer, ACMC, were calculated from surface tension isotherms γ = f (log c), based on the Gibb’s isotherm equation for 1:1 electrolytes (30) and isotherm points near the CMC. Pre-CMC data were fit to a quadratic line. The standard free energy of micellization (∆Gomic) was calculated from Equation 2:



33

FIG. 2. Equilibrium surface tension vs. logarithm of aqueous molar concentration (log C) of sodium cis-(2-n-alkyl)-5-methyl-1,3-dioxan-5-yl)carboxylates 4a–e at 25°C.

o ∆Gmic = (2 − α)RT lnx±CMC,

[2]

where x±CMC = x±CMC × y± is the standard mean activity in mole fraction units at the CMC (y± is the mean activity coefficient, log y± = −0.509J0.5, J is ionic strength), α is the mi-

cellar degree of dissociation [we assume α = 0.35 as determined for sodium dodecanoate (31)], R is the gas constant, and T is temperature. It was assumed that NaOH concentration in the aqueous solution was low enough not to influence the Gibb’s

FIG. 3. Relationship between the logarithm of critical micelle concentration (log CMC) and alkyl chain length (Cn) of sodium cis-(2-n-alkyl-5methyl-1,3-dioxan-5-yl)carboxylates 4a–e.


34

A. PIASECKI ET AL. TABLE 3 Surface Properties of Sodium cis-(2-n-Alkyl-5-methyl-1,3-dioxan-5-yl)carboxylates 4a–e Compound 4a 4b 4c 4d 4e Sodium dodecanoatea

Alkyl

CMC × 103 (mol m−3)

γCMC (mN m−1)

ΓCMC × 106 (mol m−2)

ACMC × 1020 (m2)

−∆Gomic (kJ mol−1)

n-C7H15 n-C8H17 n-C9H19 n-C10H21 n-C11H23

77.3 38.5 15.0 4.2 1.6

38.1 38.0 38.2 36.1 36.5

2.23 2.11 2.14 2.12 2.07

75 ± 2 79 ± 2 78 ± 2 78 ± 2 80 ± 2

27.0 30.6 34.2 39.1 42.9

n-C11H23

20.0

37.5

—

69

—

a

At 20°C, pH 12 (data from Ref. 10). CMC, critical micelle concentration.

equation coefficient. The surface areas, ACMC, of sodium salts 4a–e are higher compared to sodium dodecanoate [69 × 10−20m2 (10)], because they contain a bulky 1,3-dioxane ring. However, they are much lower than those determined for the sodium salts of the 2-(4-alkoxymethyl-2methyl-1,3-dioxolan-2-yl)acetic acids (10). The sodium salts 4a–e effectively decrease the surface tension of water up to 36–38 mN m−1.

12.

13.

ACKNOWLEDGMENTS The authors are grateful to Urszula Walkowiak for performing the 1 H NMR analyses. Support of this work by the State Committee for Scientific Research, Grant No. 341305 is gratefully acknowledged.

14.

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canoate Micellar Aggregation Number, Colloid Polym. Sci. 272:1259–1263 (1994). [Received December 11, 1996; accepted August 13, 1997]

Dr. Andrzej Piasecki studied chemistry at the Wroclaw University of Technology, Poland, and received his Ph.D. (1977) and Habilitation (1989) degrees from the Wrocl/aw University of Technology to become a professor of technical sciences in 1993 at the same university. Research activity constitutes the search for new groups of chemodegradable amkphiphiles and relations between their geometric structure and chemical reactivity and surface activity. Dr. Bogdan Burczyk studied chemistry at the Wrocl/aw University of Technology, Poland, and received his Ph.D. (1962) and Habilitation (1970) degrees from the Wroclaw University of Technology to become a professor of chemistry in 1976 at the same university. His research interests are in the field of surfactant sciences: synthesis of chemodegradable, acetal-type surfactants; chemical structure–surface activity relationships of amkphiphilic substances; application of surfactants; and selected problems of industrial organic chemistry, including the fate of chemicals in the environment.
