J Surfact Deterg (2010) 13:399–407 DOI 10.1007/s11743-010-1185-8
ORIGINAL ARTICLE
Synthesis and Characterization of Novel Surfactants: Combination Products of Fatty Acids, Hydroxycarboxylic Acids and Alcohols Hans-Josef Altenbach • Rachid Ihizane • Bernd Jakob • Karsten Lange • Manfred Schneider • Zeynep Yilmaz • Sukhendu Nandi
Received: 5 August 2009 / Accepted: 22 January 2010 / Published online: 5 March 2010 Ó AOCS 2010
Abstract The reaction of hydroxycarboxylic acids, such as citric, malic and tartaric species with an excess of fatty acid chlorides produces the corresponding O-acylated hydroxycarboxylic anhydrides in one step and in a near quantitative yield. These molecules are excellent electrophiles which react readily with a variety of nucleophiles including alcohols, diols and polyols. Their reaction with triethylene glycol and triethylene glycol monomethyl ether leads to two series of novel anionic surfactants, which are unsymmetrical gemini surfactants. The determination of their properties (CMC, foaming, HLB) revealed that these molecules are—depending on the chain length of the fatty acid—excellent emulsifiers, and that they also display interesting antimicrobial activity. These novel functional surfactants are of interest for applications in food and personal care products and for the formulation of pharmaceuticals. Keywords Gemini surfactants Anionic surfactants Foaming properties Surface activity Synthesis
Introduction Nature, and in this context agriculture, provides a wealth of renewable and highly useful raw materials like fats and oils, plant proteins and carbohydrates. By selective combination of their molecular constituents (e.g., fatty acids, glycerol, amino acids, mono- and disaccharides) a wide
H.-J. Altenbach R. Ihizane B. Jakob (&) K. Lange M. Schneider Z. Yilmaz S. Nandi Organic Chemistry Department, Bergische Universita¨t Wuppertal, 42097 Wuppertal, Germany e-mail:
[email protected]
variety of surface active materials can be prepared, all of them—due to their molecular structure—being potentially highly biodegradable. Numerous research groups, including ours, as well as industrial companies, have prepared and studied a wide variety of such molecules, e.g., partial glycerides (monoand diglycerides of various structures) [1–5], N-acylated amino acids, protein hydrolysates [6–14] and sugar esters [15–19]. In an extension of this general concept, we felt that bi-functional molecules such as natural hydroxycarboxylic acids would be highly attractive linkers (spacers) between hydrophobic (lipophilic) fatty acids and a wide variety of hydrophilic building blocks such as alcohols, diols and polyols, amino acids and a variety of saccharides. This would, in fact, lead to an extensive series of novel surfactants with possibly interesting and useful properties. Although described mainly in the patent literature, O-acylated hydroxy carboxylic acids such as O-acylated fatty acid derivatives of citric, malic and tartaric acid [20–23] seem to have made—to the best of our knowledge—so far little or no impact in the detergent field. In an attempt to acylate citric acid with fatty acids for the production of oil soluble derivatives, we discovered that with an excess of fatty acid chlorides the corresponding O-acylated anhydrides were obtained in near quantitative yield. Further studies revealed that this reaction could also be extended to other hydroxy carboxylic acids such as malic acid and tartaric acid [24]. All of these hydroxy carboxylic acids can now be converted in a one pot reaction and nearly quantitatively into the corresponding O-acylated anhydrides indicated as 1a–f and 2a–f, respectively, in Fig. 1. These molecules are, of course, excellent electrophiles for ring opening reactions and can react with nucleophiles
123
400
J Surfact Deterg (2010) 13:399–407 OH
O
O O
HO
70 - 100 °C
+ 2 RCOCl
O
no solvent - 2 HCl; - RCO 2H
O
low CMC and thus make up highly efficient surfactants [13, 14, 26–30]. In the present paper we describe the synthesis of such a novel class of gemini surfactants based on the above concept.
O
HO
O
R
(R,S) - Malic acid
Experimental Section
1a- f O
OH
O
HO O
O
90 - 120 °C
+ 3 RCOCl
O
HO
R
O
no solvent - 3 HCl; - RCO 2H
O O
OH (R,R) - Tartaric acid
O
R
2a- f R= C7H 15 (a); C9H19 (b); C11H 23 (c); C13H 27 (d) ; C15H31 (e); C17H 35 (f)
Fig. 1 Syntheses of O-acylmalic—and tartaric acid anhydrides 1a–f and 2a–f
from renewable resources, such as glycerol, sugar alcohols, amino acids and various carbohydrates [25]. Based on the chemical structures of the resulting compounds, one could expect that they would display properties similar to the socalled gemini surfactants. In this first paper of a series, we describe here reaction products with alcohols and diols based on triethylene glycol, namely triethylene glycol monomethyl ether as a mono-functionalized nucleophile (Fig. 2) and triethylene glycol for bis-functionalizations (Fig. 3). These molecules were chosen in order to obtain surface active compounds with a reasonable solubility in water and a low critical micelle concentration (CMC) (see below). In contrast to linear surfactants, gemini surfactants— molecules with two hydrophilic head groups linked by a short to medium spacer—are characterized by an unusually O O R
O O
O HO
O
OH
3
CHCl3; molecular sieve
R
O
O
O O
R
O
O
HO
O
3
CHCl3; molecular sieve
O
R
O O
2a- f
2,5-Dioxo-Tetrahydrofuran-3-yl-Dodecanoate (1c) Lauroylchloride (106.4 mL, 0.45 mol) was added to R,Smalic acid (30 g, 0.22 mol) in a dry round-bottom flask under stirring and the resulting mixture was heated to 70 °C for 4 h, and then cooled to room temperature. A total of 400 mL n-hexane was added and the precipitate filtered, washed thoroughly with 200 mL n-hexane, and dried under a vacuum. 1c was obtained as a white powder (m.p. 43– 44 °C; 60.0 g, 90% Yield). 1H NMR (400 MHz, CDCl3): d = 5.41 (dd, 1H, CH2–CH), 3.36 (dd, 1H, CH2–CH), 2.99 (dd, 1H, CH2–CH), 2.42 (t, 2H, CO–CH2), 1.65 (m, 2H, CO–CH2–CH2), 1.26 (m, 16H, CH2, aliphatic), 0.88 (t, 3H, CH2–CH3) ppm. 13C NMR (100 MHz, CDCl3): d = 172.55, 167.74, 166.31, 67.40, 35.16, 33.32, 31.86, 29.54, 29.53, 29.34, 29.27, 29.11, 28.90, 24.57, 22.64, 14.05 ppm. [a]D ?33.2° (c = 0.6, Acetone). (3R,4R)-2,5-Dioxo-Tetrahydrofuran-3,4-DiylDidodecanoate (2c)
OH O
O
3
O 4a- f
Fig. 2 Reactions of 1a–f and 2a–f with triethylene glycol monomethyl ether
123
Syntheses
O
R
O O
3
3a- f
O O
O
O
1a- f
R
O
O
All chemicals were purchased from the Fluka Company. All solvents used were HPLC grade. Structure determinations were carried out using a Bruker 400 MHz NMR and a Bruker microTOF-MS. Melting points were determined on a Bu¨chi melting point apparatus and are uncorrected. The described synthetic procedures were identical for all fatty acids employed. For the preparation of the triethanolammonium salts, the samples were dissolved in chloroform, then, a triethanolamine equivalent amount was added at room temperature. After 30 min of stirring, the solvent was removed and the salts obtained were dried under a high vacuum.
Lauroyl-chloride (142.7 mL, 0.6 mol) was added to L-(?) tartaric acid (30 g, 0.2 mol) in a dry round-bottom flask under stirring and the resulting mixture was heated to 90 °C for 24 h, and then cooled to room temperature. A total of 400 mL n-hexane was added, the precipitate filtered, washed thoroughly with 200 mL n-hexane, and dried under a vacuum. 2c was obtained as a white powder (m.p. 65–66 °C; 89 g 90% Yield). 1H NMR (400 MHz, CDCl3):
J Surfact Deterg (2010) 13:399–407
401
Fig. 3 Reactions of 1a–f and 2a–f with triethylene glycol
O
O
HO O
2 O
O
O
O OH
3OH
O
O
CHCl3; molecular sieve
O R
R
O
HO
O
O
O
O
O O
O OH
3OH
CHCl3; molecular sieve
R
O
O
O O
O R
5a- f
R O
O O
1a- f
2
HO O
O
R
O
O
O
O
O
O
O
2a- f
d = 5.41 (s, 2H, CH), 2.46 (t, 4H, CO–CH2), 1.66 (m, 4H, CO–CH2–CH2), 1.26 (m, 32H, CH2, aliphatic), 0.88 (t, 6H, CH2–CH3) ppm. 13C NMR (100 MHz, CDCl3): d = 172.59, 163.45, 72.07, 33.32, 31.88, 29.56, 29.54, 29.36, 29.30, 29.11, 28.86, 24.52, 22.65, 14.06 ppm. [a]D -25.8° (c = 0.6, Acetone). 13-(Dodecanoyloxy)-12-Oxo-2,5,8,11Tetraoxapentadecan-15-Oic Acid (3c) A solution of 1c (5 g, 16.76 mmol) in dry chloroform (60 mL) with added molecular sieves (Type 3A) (3 g) was stirred for 30 min at room temperature, after which triethylene glycol monomethyl ether (2.44 mL, 16.76 mmol) was added under argon. The reaction mixture was stirred at room temperature for 48 h. After removal of the molecular sieves by filtration and evaporation of the solvent under reduced pressure, the residue was dried under a vacuum. 3c was obtained as colorless oil (6.9 g 89% yield). 1H NMR (400 MHz, CDCl3): d = 10.20 (m, 1H, –COOH), 5.41 (t, 1H, CH2–CH), 4.21–4.31 (m, 2H, CO2–CH2–CH2–O), 3.52–3.65 (m, 10H, O–CH2CH2–O), 3.33 (s, 3H, O–CH3), 2.84 (d, 2H, CH2–CH), 2.33 (m, 2H, CO–CH2), 1.58 (m, 2H, CO–CH2–CH2), 1.20 (m, 16H, CH2, aliphatic), 0.82 (t, 3H, CH2–CH3) ppm. 13C NMR (100 MHz, CDCl3): d = 172.52, 172.38, 168.21, 165.92, 71.55–70.00, 68.39, 64.94, 58.75, 33.61, 31.81, 29.51, 29.23, 29.14, 28.91, 24.65, 22.57, 13.98 ppm. MS (ES?): calcd. for C23H42O9 [M ? H]? 480.3173; found 480.3090. [a]D -21.2° (c = 0.6, Acetone). (13R,14R)-13,14-Bis(Dodecanoyloxy)-12-Oxo2,5,8,11-Tetraoxa-Penta-Decan-15-Oic Acid (4c) A solution of 2c (5 g, 10 mmol) in dry chloroform (60 mL) with added molecular sieves (3 g) was stirred for 30 min at room temperature, after which triethylene glycol
O O
O
R
R
O O
HO
R
6a- f
monomethyl ether (1.47 mL, 10 mmol) was added under argon. The reaction mixture was heated to reflux and stirred for 48 h. After removal of the molecular sieves by filtration and evaporation of the solvent under reduced pressure, the residue was dried under a vacuum. The final product was obtained as a colorless oil (5.66 g, 85% yield). 1 H NMR (400 MHz, CDCl3): d = 10.12 (m, 1H, –COOH), 5.51 (s, 2H, CH), 4.19–4.37 (m, 2H, CO2–CH2–CH2–O), 3.55–3.65 (m, 10H, O–CH2CH2–O), 2.98 (s, 3H, O–CH3), 2.38 (m, 4H, CO–CH2), 1.59 (m, 4H, CO–CH2–CH2), 1.23 (m, 32H, CH2, aliphatic), 0.85, (t, 6H, CH2–CH3) ppm. 13C NMR (100 MHz, CDCl3): d = 172.52, 172.38, 168.21, 165.92, 71.55–70.00, 68.39, 64.94, 58.75, 33.61, 31.81, 29.51, 29.23, 29.14, 28.91, 24.65, 22.57, 13.98 ppm. MS (ES?): calcd. for C35H64O11 [M ? NH4]? 678.4793; found 678.4544. [a]D -7.3° (c = 0.6, Acetone). 3,16-Bis(Dodecanoyloxy)-4,15-Dioxo-5,8,11,14Tetraoxaoctadecane-1,18-Dioic Acid (5c) A solution of 1c (5 g, 16.76 mmol) in dry chloroform (60 mL) with added molecular sieves (3 g) was stirred for 30 min at room temperature, after which (1.26 mL, 8.38 mmol) triethylene glycol was added under argon. The reaction mixture was stirred at room temperature for 48 h. After removal of the molecular sieves by filtration, the solvent was evaporated under reduced pressure, and the residue was dried under a vacuum. 5c was obtained as a colorless oil (5.67 g 91% yield). 1H NMR (400 MHz, CDCl3): d = 9.93 (m, 1H, –COOH), 5.45 (t, 2H, CH2– CH), 4.37–4.22 (m, 4H, CO2–CH2–CH2–O), 3.68–3.60 (m, 8H, O–CH2CH2–O), 2.95–2.90 (m, 4H, CH2–CH), 2.37 (m, 4H, CO–CH2), 1.63 (m, 4H, CO–CH2–CH2), 1.24 (m, 32H, CH2, aliphatic), 0.86 (t, 6H, CH2–CH3) ppm. 13C NMR (100 MHz, CDCl3): d = 174.08, 172.73, 168.58, 70.26, 68.72, 67.58, 64.47, 35.97, 33.76, 31.84, 29.53, 29.25, 28.96, 24.67, 22.59, 14.04 ppm. MS (ES?): calcd. for
123
402
C23H42O9 [M-H]- 745.4309; found 745.4371. [a]D -20.7° (c = 0.6, Acetone).
J Surfact Deterg (2010) 13:399–407 Table 1 Synthesis of the title compounds—reaction conditions, yields and properties Product Reactant 1 Reactant 2 T [°C] Yields (%) Properties
(2R,3S,16S,17R)-2,3,16,17-Tetrakis(Dodecanoyloxy)4,15-Dioxo-5,8,11,14-Tetraoxaoctadecane-1,18-Dioic Acid (6c) A solution of 2c (3 g, 6.04 mmol) in dry chloroform (60 mL) with added molecular sieves (3 g) was stirred for 30 min at room temperature, after which triethylene glycol (0.4 mL, 3.02 mmol) was added under argon. The reaction mixture was stirred at room temperature for 48 h. After removal of the molecular sieves by filtration the solvent was evaporated under reduced pressure and the residue was dried under a vacuum. 6c was obtained as a colorless oil (3.5 g 87% yield). 1H NMR (400 MHz, CDCl3): d = 10.12 (m, 1H, –COOH), 5.51 (s, 2H, CH), 4.19–4.37 (m, 2H, CO2–CH2–CH2–O), 3.55–3.65 (m, 10H, O–CH2CH2–O), 2.98 (s, 3H, O–CH3), 2.38 (m, 4H, CO–CH2), 1.59 (m, 4H, CO–CH2–CH2), 1.23 (m, 32H, CH2, aliphatic), 0.85, (t, 6H, CH2–CH3) ppm. 13C NMR (100 MHz, CDCl3): d = 172.52, 172.38, 168.21, 165.92, 71.55–70.00, 68.39, 64.94, 58.75, 33.61, 31.81, 29.51, 29.23, 29.14, 28.91, 24.65, 22.57, 13.98 ppm. MS (ES?): calcd. for C35H64O11 [M H]- 1,141.7687; found 1,141.7591. [a]D -9.7° (c = 0.6, Acetone). All obtained compounds were characterized regarding purity ([97%) by 400 MHz 1H- and 100 MHz 13CNMR. Molecular weights were determined by high resolution MS. No trace analysis was carried out. Reaction conditions, yields and properties are summarized in Table 1.
3a
1a
I
RT
55
Oil
3b
1b
I
RT
86
Oil
3c
1c
I
RT
89
Oil
3d
1d
I
RT
88
Oil
3e
1e
I
RT
95
Oil (yellow)
3f
1f
I
60
91
High viscosity
4a 4b
2a 2b
I I
60 60
59 84
Oil Oil
4c
2c
I
60
85
Oil
4d
2d
I
60
83
Oil
4e
2e
I
60
96
Oil (yellow)
4f
2f
I
60
92
High viscosity
5a
1a
II
RT
48
Oil
5b
1b
II
RT
88
Oil
5c
1c
II
RT
91
Oil
5d
1d
II
RT
81
Oil
5e
1e
II
RT
92
m.p. 37.4 °C
5f
1f
II
RT
89
m.p. 50.6 °C
6a
2a
II
60
51
Oil (yellow)
6b
1b
II
60
90
Oil (yellow)
6c
1c
II
60
87
Oil (yellow)
6d 6e
1d 1e
II II
60 60
83 90
Oil (yellow) m.p. 46.5 °C
6f
1f
II
60
86
m.p. 60.0 °C
All reactions were carried out in chloroform for 2 days I = triethylene glycol monomethyl ether; II = triethylene glycol
Hydrophilic-Lipophilic-Balance (HLB) Values Surface Tension and CMC The surface tension was measured using the ring method [31]. For this, sample solutions within a suitable range of concentrations (normally 10-5, 10-4, 10-3, 10-2, 10-1, 1, 10 mg/mL) were prepared by dissolving the corresponding compound in bi-distilled water. The solutions were equilibrated at least 20 min before taking measurements. As a control, the surface tension of water was measured regularly in order to check the accuracy of the method. The thus determined values for the surface tension (in mN/m) were plotted against the surfactant concentration (logarithmic scale). The CMC was determined as the concentration where the surface tension remains practically constant, i.e., the intersection of the two—more or less—straight lines in the diagrams, exemplified in Fig. 4. All measurements were carried out by a commercial provider. The results are summarized in Table 2.
123
HLB values were determined using an emulsion comparative method for oil/water emulsions 20:80 (O/W) based on a published procedure [32] as follows: To 2 g of the corresponding oil 200 mg of a mixture of two emulsifiers were added, one with a known HLB-value and one for which the HLB-value is unknown. The mixture was heated to 70–80 °C until the emulsifiers were fully dispersed. Using this approach, a series of mixtures with different weight proportions were prepared (normally 6–10). To these mixtures 8 mL portions of hot (70–80 °C) water were added and the mixtures were shaken intensively for 10 s. or treated with an Ultra Turrax high performance disperser for 10 s. The thus prepared samples were subsequently stored at R.T. and inspected visually after 24 and 48 h, respectively. As the oil phases, paraffin oil (rHLB*1 = 10), canola oil (rHLB = 7) and toluene (rHLB = 15) were employed. 1
*rHLB: required HLB value.
J Surfact Deterg (2010) 13:399–407
403 70
70
60
60
[mN/m]
[mN/m]
Fig. 4 Surface tension versus concentration for 3c and 4c
3c
50 40 30 20 1E-5
4c
50 40 30
1E-4
1E-3
0,01
0,1
1
10
20 1E-5
1E-4
Table 2 Surfactant properties of the title compounds cCMC HLB-value Foaming Foaming Compound CMC ability stability (mmol/l)a (mN/m)a (mL)a (mL)a 3a
–
–
20
0
0
3b
5
27
19
180
152
3c
0.5
27
19
293
150
3d
0.03
28
18
680
586
3e
0.03
34
17
395
332
3f
0.02
33
17
40
25
4a
–
–
14
60
45
4b
0.3
26
14
773
644
4c
0.004
29
13
310
280
4d
0.09
30
10
0
0
4e 4f
0.003 0.0009
60 35
9 8
45 30
5 25
5a
0.75
56
10
0
0
5b
0.15
28
17
210
195
5c
0.024
31
12
290
240
5d
0.023
29
11
180
153
5e
0.017
46
12
120
102
5f
0.021
27
12
0
0
6a
0.001
46
10
45
24
6b
0.038
28
12
240
192
6c
2.4
36
9
40
35
6d
0.45
35
9
80
60
6e
0.012
45
9
0
0
6f
0.028
43
9
0
0
SDS
8.0
22
40
710
580
a
Triethanolammonium-salts
Foaming Properties There are various ways determining both foaming behavior and foaming stabilities. Standardized methods are described in ASTM D1173-07 (US) and DIN 53902—part1 (Germany) [33]. The foam production was measured using 0.1% solutions of the corresponding surfactants in
1E-3
0,01
0,1
1
10
C[g/l]
C[g/l]
bi-distilled water. For the measurement 200 mL of the solution in question was poured into a graduated 1 L graduated cylinder. The solution was then manually beaten with a perforated disc with a frequency of 60 beats per minute. The volume of the foam produced was measured after 30 s (foaming ability) and after 300 s (foam stability).
Antimicrobial Properties All of the synthesized gemini surfactants were tested with regard to their activities against a series of bacteria (Gram negative: Pseudomonas putida mt2 (DSM3931), Escherichia coli (DSM498), Enterobacter aeruginosa (DSM30053); Gram positive: Staphylococcus aureus (DSM346); Micrococcus luteus (DSM20030)) and eukaryotic species such as fungi (Aspergillus niger (DSM63263), Candida albicans (DSM1386)). For the antimicrobial tests, standard agar plates were employed which were prepared in the following way: a.
For bacteria by suspending 37 g LB-agar (Bertani-agar after Miller) containing 5 g of yeast extract, 10 g of peptone, 10 g of NaCl, and 12 g agar in 800 mL of bi-distilled water. b. For fungi by suspending 4 g of peptone, 24 g of malt extract, and 25 g agar in 800 mL of bi-distilled water. The mixtures were heated until all components were dissolved and afterwards sterilized for 15 min at 121 °C. Afterwards ca. 20 mL of the resulting mixture was poured onto a standard agar plate which was allowed to cool for 2 days. In every agar plate 100 lL of the corresponding cell suspension (105–106 cells/mL) was plated out with a glass spatula and allowed to dry for ca. 30 min. Then, 10 lL of a 1% solution of the corresponding test samples were applied to defined areas of the agar plates which were then incubated (a) for bacteria at 30 °C for 24 and 48 h and (b) for fungi and yeasts at room temperature for 48 and 72 h, respectively.
123
404
J Surfact Deterg (2010) 13:399–407
Table 3 Antimicrobial properties (inhibition zones) Substance
Ps. putida mt2
E. coli
Enterobacter aer.
S. aureus
3a
???
???
???
???
3b
???
???
???
?
3c
???
???
???
??
3d
???
(?)
?
3e
???
(?)
??
M. luteus
Asp. niger
Ca. albicans
???
??
???
???
???
???
???
???
??
???
???
???
???
???
???
-
???
3f
???
(?)
(?)
???
-
-
??
4a
???
???
???
???
???
(?)
???
4b
???
??
???
???
???
-
???
4c
???
???
???
-
???
-
???
4d
??
-
?
-
-
??
-
4e 4f
? -
-
(?) -
-
-
? ???
-
5a
??
???
???
???
???
-
???
5b
???
???
???
???
???
-
?
5c
???
???
???
???
???
-
?
5d
???
???
??
?
-
-
(?)
5e
???
-
(?)
-
-
-
??
5f
-
-
-
-
-
-
??
6a
??
??
???
???
-
-
??
6b
???
???
??
???
-
-
(?)
6c
??
(?)
?
-
-
-
??
6d
(?)
-
(?)
-
(?)
(?)
?
6e
-
-
-
-
-
-
??
6f
?
-
-
-
-
-
-
- = No spot; (?) = overgrown spot; ? = spot with colonies; ?? = spot with only a few colonies; ??? = clear spot without colonies
The agar plates were inspected visually at regular intervals and the inhibition zones were estimated. The results are summarized in Table 3.
Results and Discussion Syntheses As already described above, the transformation of hydroxycarboxylic acids with fatty acid chlorides leads to the corresponding O-acylated hydroxycarboxylic acid anhydrides in near quantitative yield (Fig. 1) [15–19]. These molecules are accessible in multi kilogram quantities, even in the laboratory. The method allows the synthesis of these materials to be carried out on a scale large enough to be suitable for industrial production. We are presently actively engaged in developing alternative methods for the syntheses of these central building blocks. The reaction of (R/S)-malic acid with 2 equivalents of fatty acid chlorides at 70 °C for 4 h leads quantitatively (isolated yields ca. 90%) to the corresponding O-acylated
123
malic acid anhydrides 1a–f. The reaction of (L)-tartaric acid with 3 equivalents of the same fatty acid chlorides produces the corresponding O,O0 -diacyl tartaric acid anhydrides 2a–f again in quantitative yield (isolated yields ca. 90%). As already pointed out, the molecules obtained in this way (1a–f and 2a–f) are excellent electrophiles and prone to ring opening reactions with a wide variety of nucleophiles. In a series of reactions, products 1a–f were combined with one equivalent of triethylene glycol monomethyl ether in chloroform and in the presence of molecular sieves at room temperature for 48 h, leading to the corresponding monoesters 3a–f in near quantitative yield. It should also be noted that the ring opening reactions of the malic acid anhydride derivatives are highly regioselective, thus confirming corresponding literature reports [34]. The monoester in proximity to the O-acylated function is always obtained (Fig. 2). In a similar way, 2a–f were reacted with one equivalent of triethylene glycol monomethyl ether under the same conditions. However, this transformation requires a
J Surfact Deterg (2010) 13:399–407
somewhat higher temperature and 60 °C was found to be optimal for this process. Since (L)-tartaric acid is of C2-symmetry, 4a–f are obtained as single isomers with yields of around 90% (Fig. 2). Clearly, the availability of 1a–f and 2a–f also allows the synthesis of combination products with triethylene glycol itself for a second series of compounds. Two equivalents of the anhydrides were combined with one equivalent of triethylene glycol to the corresponding doubly acylated products 5a–f and 6a–f, which can be regarded as gemini surfactants in near quantitative yield (Fig. 3).
Surfactant Properties All the synthesized compounds were characterized with respect to their surface active properties such as surface tension and critical micellar concentration (CMC), ccmc, the HLB value, foaming properties and antimicrobial activity.
Surface Tension and Critical Micellar Concentration (CMC) The CMC and the related surface tension provide a basic characterization of any surfactant. Representative results are shown in Fig. 4. In order to compare the obtained values for the CMC regarding their dependence on structure and chain length of the acyl groups, all results are listed together with sodium dodecyl sulfate (SDS) as the universal standard for surfactants (Table 2). The results in Table 2 show that the tartaric acid based surfactants 4a–f display considerably lower CMCs than the corresponding malic acid derivatives 3a–f. In the case of 4a–f, the CMC decreases strongly with increasing chain length, a known definite trend only interrupted by the behavior of 4d where the CMC slightly increases again. This observation is so far unclear (but reproducible) and will be studied in detail in the near future. Nevertheless, all the CMCs of 4a–f are typical values for gemini surfactants. In the case of 3a–f we observed a similar trend, albeit on a much higher level of ca. 2 orders of magnitude. Only 3c–f show CMCs considerably lower than the standard SDS. In the series of malic acid derivatives 5a–f with increasing chain length, the CMC values decrease gradually and reach a minimum with 5e. In the case of the tartaric acid based surfactants, the trend is entirely different; the CMC values increase with increasing chain length and reach a maximum in 6c and decrease again in systems with
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longer chain lengths.These results also need further investigation in order to be understood. HLB Values Numerous compounds with surface-active properties are useful for the preparation of emulsions such as oil in water (O/W) or water in oil (W/O). Emulsifier properties are closely related to the so-called HLB value, which describes the Hydrophilic-Lipophilic Balance of a surface active molecule [35–37]. The HLB values were determined as described in the Experimental Section. The results are summarized in Table 2. An inspection of Table 2 reveals, that only the surfactants based on tartaric acid 4a–f are interesting emulsifiers with 4c–e showing the most desirable HLB values. Again the HLB values decrease with increasing chain length. The corresponding malic acid based derivatives display HLB values considerably higher and are thus not very useful for the formation of emulsions. It also turned out that the stability of these emulsions is less than 2 h. Foaming Properties and Foam Stability Another important property of surfactants is their ability to form foams, essential for certain industrial applications and also used in consumer products and cosmetics. In general, surfactants with HLB [ 10 exhibit good foaminess and foam stability, whereas compounds with HLB \ 3 are referred to as antifoaming agents. There are various ways to determine both foaming behavior and foaming stabilities. Standardized methods are described in ASTM D117307 (US) and DIN 53902—part 1 (Germany) [33]. In Table 2, the foaming ability (foam volume after 30 s) and foam stability (foam volume after 300 s) are summarized for the above series of surfactants. Clearly, the malic acid derivatives 3a–f display better foaming properties than the tartaric acid derivatives 4a–f. The best foaming agents are 3d and 4b with values similar to those of SDS; the other surfactants display low foaming ability (3b, c, e and 4c) while (3a and 4a, d, e, f) do not foam at all. Antimicrobial Properties Frequently, for numerous applications, it is highly desirable to develop so-called functional surfactants, i.e., surface-active compounds which show evidence of additional benefits. In this context the antimicrobial properties of surface active compounds are highly useful for applications in the cosmetic field, in consumer products, in cleaning liquids for medical instruments, as well as for the
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stabilization of industrial emulsions against deterioration by bacteria and fungi [38, 39]. Hence, the above synthesized gemini surfactants were tested regarding their activities against a series of bacteria (Gram negative: Pseudomonas putida mt2, Escherichia coli, Enterobacter aeruginosa; Gram positive: Staphylococcus aureus; Micrococcus luteus, and eukaryotic species such as fungi (Aspergillus niger, Candida albicans). The agar plates (compare the experimental, Sect. 2.4.) were inspected visually at regular intervals and the inhibition zones were estimated. The results are summarized in Table 3. Most of the synthesized surfactants showed activity against a variety of the above microorganisms, particularly against Pseudomonas putida, E. coli, Enterobacter aeruginosa, and Candida albicans with somewhat lesser activity. Malic acid derivatives 3 and 5 exhibited high antimicrobial activity against all the investigated microorganisms. With increasing chain length the microbial activity gradually decreases.
Conclusions As outlined above, the synthesis of combination products from fatty acids, with hydroxycarboxylic acids as linkers and triethylene glycol monomethyl ether or triethylene glycol leads to a series of novel gemini-type surfactants. Several of the resulting molecules also show additional benefits such as antibacterial properties and are thus potentially useful as multifunctional emulsifiers in cosmetics and food technology. Low CMCs, an interesting range of HLB-values, foaming and antifoaming properties and also anti-microbial properties of certain derivatives have provided a series of very attractive molecules, the practical applications of which are presently being studied in detail in our laboratory and in collaboration with industry. Although a number of interesting compounds have been obtained, first tendencies for a structure–activity relationship cannot be deduced yet, and further experiments are required. Acknowledgments We wish to thank Prof. W. Reineke, Department of Microbiology, Bergische Universita¨t Wuppertal, for support in determining antimicrobial activities and Boris Ihmenkamp for technical assistance. We are grateful to the German Department of Agriculture (Bundesministerium fu¨r Erna¨hrung, Landwirtschaft und Verbraucherschutz BMELV administered by the Fachagentur Nachwachsende Rohstoffe e.V., Gu¨lzow, Germany) for the financial support of this research within their program of establishing young scientist groups at several universities.
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Author Biographies Hans-J. Altenbach received his diploma and Ph.D. from the University of Cologne with Prof. E. Vogel and worked as a Postdoctoral Fellow with Prof. R. E. Ireland at Caltech, Pasadena. Since 1992 he has been Full Professor for Organic Chemistry at the University of Wuppertal. His research interests include the development of flexible, stereoselective methods for the synthesis of biologically active compounds, especially polyhydroxylated carboand heterocyclic systems, f.i. inositols and carba- as well as azasugars. Rachid Ihizane was born in Morocco in 1977, studied at the University Mohamed V in Rabat (Department of Science) and at the University of Wuppertal (Department of Mathematics and Natural Science). After concluding his Masters study in 2006, he started Ph.D. studies at the University of Wuppertal. His areas of research include synthesis und characterization of novel surfactants and emulsifiers based on renewable resources. Bernd Jakob was born in 1966 and earned a Ph.D. in Chemical Sciences at the University of Wuppertal in 1999. After working in a Company for custom synthesis he came back to the University of Wuppertal in 2006 as leader of a research group. His research interests are novel compounds synthesized from renewable resources to make surfactants, detergents, emulsifiers and thickeners. Karsten Lange was born in 1967 in Germany and studied chemistry at the University of Wuppertal. After obtaining his Ph.D. in organic chemistry in 2004 he joined a company for custom synthesis for a short time. Then he went back to the University of Wuppertal for a research assignment. At the moment, he is working on new surface active compounds based on renewable resources. Manfred Schneider was born in Germany in 1940. He received his masters degree in inorganic chemistry and a doctoral degree in organic chemistry in 1965 and 1969, respectively, from the University of Stuttgart, Germany. He worked as a professor between 1972 and 1980 at the University of Hohenheim. He joined the University of Wuppertal in 1980. He has a lot of experience in academic research, especially related to synthetic organic chemistry, photochemistry and physical-organic (mechanistic) chemistry. He retired in 2006. Zeyneb Yilmaz was born in Turkey in 1975. She completed her M.Sc. in 2007 at the University of Wuppertal, where she is currently working as a Ph.D. student on the synthesis of hydroxycarboxylic acid-based novel polymers and their interesting applications. Sukhendu Nandi was born in India in 1985. He finished his M.Sc. in 2007 at the Indian Institute of Technology, Madras. Currently he is a Ph.D. student at the University of Wuppertal. His research focuses on the synthesis of carbohydrate based novel amphiphiles and the measurement of their surface properties.
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