- Email: [email protected]
Enzymatically Prepared n-Alkyl Esters of Glucuronic Acid: The Effect of Hydrophobic Chain Length on Surface Properties
Journal of Colloid and Interface Science 247, 424–428 (2002) doi:10.1006/jcis.2001.8154, available online at http://www.idealibrary.com on
Enzymatically Prepared n-Alkyl Esters of Glucuronic Acid: The Effect of Hydrophobic Chain Length on Surface Properties Christophe Blecker,∗,1 Salvator Piccicuto,∗, † Georges Lognay,‡ Claude Deroanne,∗ Michel Marlier,‡ and Michel Paquot† ∗ Unit´e de Technologie des Industries Agro-alimentaires; †Unit´e de Chimie Biologique Industrielle; and ‡Unit´e de Chimie G´en´erale et Organique, Facult´e des Sciences Agronomiques de Gembloux, Gembloux B-5030, Belgium Received February 20, 2001; accepted December 10, 2001
The effect of hydrophobic chain length on surface properties of enzymatically prepared n-alkyl esters of glucuronic acid are examined. Dynamic parameters from Hua and Rosen’s mathematical model and equilibrium surface tension are presented for esters with octyl, decyl, dodecyl, and tetradecyl alkyl segments. Increasing the alkyl chain length has a significant influence on the surface activity. Decyl and dodecyl glucuronate exhibit an interesting adsorption speed associated with foaming capacity. Octyl glucuronate exhibits a micellar organization as its bulk concentration is over 10.68 mM. C 2002 Elsevier Science (USA) Key Words: Nonionic surfactants; glucuronic acid; biosurfactants; hydrophobic; surface tension; critical micellar concentration; adsorption kinetics; hydrophilic–lipophilic balance; enzymatic synthesis; lipase.
INTRODUCTION
For several years, there has been great interest in the use of sugar fatty esters as nonionic surfactants. Surfactant and emulsifying properties result from the presence of both hydrophilic and hydrophobic regions on the same molecule (1). Sucroesters show this characteristic: the hydrophilic moiety is here provided by a sugar. The use of a linear alkyl chain as hydrophobic moiety allows us to modulate its importance. So an investigation of the relative hydrophobicity influence can be envisaged. The hydrophilic–lipophilic balance is the main parameter that determines the interfacial activity of an amphiphilic molecule. This value is controlled by the nature of the chain length and by the importance of the hydrophilic head. The number of esterified hydroxyl groups has also a great influence. Sugar esters have a very wide arbitrary hydrophilicity and lipophilicity balance (HLB) (2). Indeed their properties can range from those of water soluble surfactants (HLB higher than 10) to oil soluble emulsifiers (HLB lower than 4). As stated above, both occurring moieties exhibit relevant properties: they are made from renewable and inexpensive
1 To whom correspondence should be addressed. E-mail: blecker.c@fsagx. ac.be.
0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
424
feedstocks. In addition, they are completely biodegradable, nontoxic, nonskin-irritant, odourless, and tasteless (3). Despite all these advantages and since a long time, the synthesis of sugar esters has been limited. The reason is that the classical chemical way of synthesis shows some major drawbacks. This conventional approach requires a high temperature, which causes coloration, high energy consumption, and the recovery of considerable amounts of sideproducts (3, 4). The use of biological catalysts under mild conditions can help to overcome these problems. The enzymatic synthesis has given new potentialities to sugar esters production (5, 6). Most studies about sugar esters present the results obtained with sucrose, glucose, and fructose esters. The particularity of the surfactants used within the present study is linked to the nature of the hydrophilic moiety: glucuronic acid. Otto et al. (1) reported for the first time the lipase-catalyzed esterification of this sugar acid with an alcohol (n-butanol). To our knowledge, no other glucuronic acid esters (longer hydrophobic chain) were synthesized and no data are available on the surface properties of such compounds. So, our main objective was to investigate the effect of the hydrophobic chain length on the surface properties of enzymatically prepared glucuronic acid esters. The surface properties of a surfactant provide the basis of various applications and are usually characterized by the equilibrium surface tension, the critical micellar concentration (CMC), the surface concentration, and the surface area per molecule. However, in many interfacial process, such as wetting or foaming, equilibrium conditions are not attained and dynamic processes play a major role. So adsorption kinetics of the surfactant must be characterized. MATERIALS AND METHODS
Esters Synthesis and Purification Sugar esters were prepared using immobilized Candida antarctica lipase B (Novozym SP 435, Novo Nordisk) in an organic medium. Glucuronic acid (0.025 M) and fatty alcohol (0.025 M) were dissolved in 10 ml of t-butanol. After an addition of 100 mg of lipase and 500 mg of activated molecular sieve (8–12 mesh, Sigma), an enzymatic reaction was carried out
425
SURFACE PROPERTIES OF GLUCURONIC ACID ESTERS
FIG. 1.
Ester of glucuronic acid and fatty alcohol.
in a shaking (150 rpm) water bath at 60◦ C for 48 h. Lipase and molecular sieve were then removed by filtration and ester was extracted by dichloromethane (20 ml) and purified using silica gel chromatography (chloroform : methanol 70 : 30 vol/vol as eluant). The purity of all synthesized products was proved to be more than 99% by HPLC-ELSD. Enzymatic synthesis allowed us to obtain esters of glucuronic acid and aliphatic alcohols from 8 to 14 carbon atoms. Their structure is illustrated in Fig. 1. Adsorption Kinetics Recording The automatic drop tensiometer TRACKER (IT Concept, France) was used for measuring the surface tension–time dependence of surfactant solutions by a static procedure. It consisted in forming a rising air bubble at the tip of a curved capillary. The volume of the bubble was set to 1 µl. The surface tension was measured and recorded every second. The length of the measurement was sufficient to reach the equilibrium. The cell in which the bubble was formed contained 7 ml of solution. All measurements were performed at 25 ± 0.5◦ C. Dynamic Drop Volume Method The automated drop volume tensiometer TVT 1 (Lauda, Germany) was employed to perform dynamic measurements. Drops of solution were formed with a growing formation speed. The lifetime of the drops was measured as a function of their volume, which made it possible to calculate the surface tension. All measurements were performed at 25 ± 0.5◦ C. Each measurement was repeated twice. For high concentrations of surfactant, equilibrium surface tension (σe ) was taken as the mean of the values obtained with the last 4 drops. For low concentrations, σe was deduced by extrapolating the surface tension to time t → ∞ in the σ − t −1/2 (7).
Surface tension (mN/m)
80 70 60 50 40 30 20 10 0 0
500
1000
1500
2500
3000
FIG. 2. Adsorption kinetics of decyl glucuronate (), dodecyl glucuronate (), and tetradecyl glucuronate () (results of two repetitions).
was realized by the software SigmaPlot 5.0 (Jandel Scientifics, USA), γ t − γm =
γ0 − γm , 1 + (t/t ∗ )n
[1]
where σ0 is the surface tension of the pure solvent, γm the surface tension at mesoequilibrium, and σt the surface tension at time t; t ∗ and n are estimated by the fitting. RESULTS AND DISCUSSION
Adsorption Kinetics Adsorption kinetics were established at 6 mg/l. This concentration is the limit of solubility for tetradecyl glucuronate, the most hydrophobic compound. Curves are shown in Fig. 2 and remarkable results are presented in Table 1. At this concentration, dodecyl glucuronate acted quickly: it provided an important decrease of surface tension during the first 100 s.
TABLE 1 Surface Tension (γ) after 1, 500, and 2000 s for a Solution at 6 and 83 mg/l γ
Application of the Hua and Rosen Mathematical Model Data obtained with the automated drop volume tensiometer in dynamic mode were treated according to the mathematical model proposed by Hua and Rosen (8–10). Fitting was performed using Eq. [1]. In a first attempt, the parameters n and t ∗ were estimated by a logarithmic linear transformation of Eq. [1]. The estimated values were then employed for the fitting of Eq. [1] to the dynamic surface tension data. This fitting
2000
Time (s)
1s Surfactant Octyl glucuronate Decyl glucuronate Dodecyl glucuronate Tetradecyl glucuronate
500 s
2000 s
6 mg/l 83 mg/l 6 mg/l 83 mg/l 6 mg/l 83 mg/l 72 70 69 68
66 45 — —
71 59 52 59
65 37 — —
71 58 39 48
— — — —
426
BLECKER ET AL.
TABLE 2 Hua and Rosen Parameters for a Solution at 6 and 83 mg/l t ∗ (s)
FIG. 3. Adsorption kinetics of octyl glucuronate () and decyl glucuronate() (results of two repetitions).
Then dodecyl glucuronate adsorbed to the surface and so led to a continuous decrease of surface tension. Decyl glucuronate led to a 10 mN/m decrease over the first 200 s, and after this first step, it did not provoke an important decrease over the following 3000 s. Tetradecyl glucuronate showed a behavior similar to that of decyl glucuronate during 400 s. However, after this delay, its adsorption did not stop and caused a significant continuous decrease in surface tension. So adsorption kinetics depended on the hydrophobic chain length, and an acyl moiety with 12 carbon atoms seems to be an optimum in terms of adsorption speed. Adsorption kinetics were also recorded at 83 mg/l for octyl glucuronate and decyl glucuronate, the two more soluble esters. This concentration correspond to the solubility limit of decyl glucuronate. Curves are shown in Fig. 3, and the main results are presented in Table 1. Observation of the adsorption kinetics of octyl and decyl glucuronate indicated the ability of the second compounds to adsorb quickly to the surface at this concentration: after 500 s, decyl glucuronate led to a decrease of surface tension equal to 35 mN/m (Table 1). Octyl glucuronate induced a lower decrease of the surface tension (7 mN/m). Actually, kinetics recorded for both esters are quite similar. Octyl glucuronate caused a 5 mN/m decrease during the first seconds and then showed a linear and slow evolution. Decyl glucuronate caused a 33 mN/m decrease during the first seconds and then showed the same linear behavior. So, even for higher concentrations, a hydrophobic chain length seemed to play an important role in the adsorption speed of this kind of surfactant and the low solubility limit of dodecyl glucuronate caused the decrease of the optimal hydrophobic chain length. Application of the Hua and Rosen Mathematical Model The previous study of adsorption kinetics had a qualitative aspect. The use of the Hua and Rosen mathematical model would help us to determine quantitative parameters. These authors have suggested a method of treatment that elucidated the factors that
γm (mN/m)
Surfactant
6 mg/l
83 mg/l
6 mg/l
83 mg/l
Octyl glucuronate Decyl glucuronate Dodecyl glucuronate Tetradecyl glucuronate
— 20.05 9.92 37.2
9.15 0.40 — —
— 62.0 49.5 48.1
60.1 32.9 — —
determine the rate at which the surface tension was reduced. Results obtained with the automated drop tensiometer in dynamic mode were treated according to the Hua and Rosen mathematical model and gave information about the surfactant behavior when the surface area was growing. Two parameters could be compared: —t ∗ : the time at which the rate of change of surface tension in function of the logarithm of time reaches a maximum. So this parameter decreases as the adsorption speed of the surfactant increases. —γm : the surface tension at mesoequilibrium. This is the surface tension reached after the first step of quick adsorption of the surfactant to the newly created surface. Table 2 illustrates the values obtained for decyl glucuronate, dodecyl glucuronate, and tetradecyl glucuronate at both concentrations. For this concentration, the adsorption of octyl glucuronate was very slow and did not fit to the Hua and Rosen model. Values obtained for t ∗ confirmed the quick adsorption of dodecyl glucuronate at this concentration. The same mathematical treatment was applied to the plots of surface tension against the logarithm of time for octyl glucuronate and decyl glucuronate with a surfactant concentration equal to 83 mg/l. Values of t ∗ indicated the rapidity of decyl glucuronate under those conditions. So this quantitative characterization corroborated the results of the previous qualitative study. Equilibrium Surface Tension The equilibrium surface tension reached for a concentration also depended on the hydrophobic chain length. If a concentration of 6 mg/l was considered, dodecyl glucuronate provided the greatest decrease of surface tension (Table 3). If a concentration TABLE 3 Equilibrium Surface Tension (σe ) for a Solution at 6 and 83 mg/l σe (mN/m) Surfactant
6 mg/l
83 mg/l
Octyl glucuronate Decyl glucuronate Dodecyl glucuronate Tetradecyl glucuronate
71 57 37 44
57 29 — —
427
SURFACE PROPERTIES OF GLUCURONIC ACID ESTERS
of 83 mg/l was considered, decyl glucuronate induced a greater drop in surface tension than octyl glucuronate did (Table 3). This last observation could be confirmed by the calculation of the efficiency (10). The efficiency of a surfactant (pC20 ) is defined by the value of the negative logarithm of the bulk concentration necessary to reduce the surface tension by 20 mN/m. The calculated efficiency for octyl glucuronate was 0.276 and for decyl glucuronate 2.369. These values confirmed that decyl glucuronate was a very efficient compound for this bulk concentration. Zhang and Marchant (11) have shown that efficiency increases as the hydrophobic chain length increases for another group of sugar-based surfactants. The optimum chain length is different according to the effect that is suited: if a significant decrease of surface tension is needed with a very low surfactant concentration, dodecyl glucuronate can be used. However, if the lowest surface tension is needed, decyl glucuronate is the most efficient compound. Critical Micellar Concentration
max
Surfactant SDS CTAB CHAPS Octyl glucoside Octyl glucuronate
(7) (7) (7) (12)
σCMC (mN/m)
CMC (mM)
max (mol · cm−2 ) × 10−10
ACMC (m2 /molec) × 10−20
38.5 37.6 45.2 32 28.0
8.19 0.98 6.66 18 10.68
2.51 2.85 1.73 — 3.85
66.0 58.3 96.0 — 43.1
Note. Numbers in parentheses indicate references.
( dσ/d ln C)CMC was calculated by the derivative of the polynomial function σe = f (ln C) at the CMC. The molecular area (ACMC ) was given by ACMC =
From all esters, octyl glucuronate is the only one that caused the formation of micelles. Critical micellar concentration was determined from a plot of equilibrium surface tension against logarithm of the concentration. Curves giving the best fit of the equilibrium surface tension–concentration data were obtained and are shown in Fig. 4. Both curves were plotted using polynomial and straight-line equations, corresponding respectively to the descending and the horizontal parts. Those equations were used to calculate the CMC and the surface tension at the CMC (σCMC ) defined at the intersection point of the fitted curves. The maximum surface excess (max ) was calculated with the Gibbs adsorption equation applied to a nonionic surfactant: 1 = RT
TABLE 4 Surface Properties of Various Surfactants
dσ d ln C
.
[2]
CMC
1 , max N
[3]
where N is Avogadro’s number. The results for octyl glucuronate and those for various surfactants found in the literature are presented in Table 4. The molecular area determined for octyl glucuronate at the CMC (43.1 × 10−20 m2 /molecule) suggested that molecules are rather closely packed. Such a value is typical for nonionic surfactants with a bulky hydrophilic part, as esters of monosaccharides, derivatives of cycloalkanes or dioxacyclanes, and similar compounds. Comparatively, some kinds of ionic (SDS, CTAB) or zwitterionic (CHAPS) surfactants exhibit a higher molecular area at the CMC. In conclusion, it is possible to obtain various surface properties by adapting the hydrophobic chain length of glucuronic acid esters. For example, the important adsorption speed of decyl glucuronate and dodecyl glucuronate allowed them to cause a significant decrease of surface tension in a few seconds. This characteristic can be associated to foaming properties (8). Micellar structures could be obtained using a relatively small bulk concentration of octyl glucuronate. So the utilization of this molecule can be envisaged in detergents or generally speaking, in systems where an hydrophobic compound must be dispersed in an aqueous phase. Such systems are found in drugs and cosmetics formulations. Interfacial and emulsifying properties should also be investigated as sugar esters are employed as emulsifying agents (13, 14). ACKNOWLEDGMENT This work received financial support from the “Minist`ere de la Communaut´e Fran¸caise de Belgique” (Project ARC).
REFERENCES FIG. 4. Plot of surface tension of solutions against logarithm of octyl glucuronate concentration (results of two repetitions).
1. Otto, R. T., Bornscheuer, U. T., Syldatk, C., and Schmidt, R. D., J. Biotechnol. 64, 231 (1998).
428
BLECKER ET AL.
2. Liu, X., Gong, L., and Liu, J., J. Mol. Catal. A: Chem. 147, 37 (1999). 3. Ducret, A., Giroux, A., Trani, M., and Lortie, R., Biotech. Bioeng. 48, 3 (1995). 4. Fregapane, G., Sarney, D. B., Greenberg, S. G., Knight, D. J., and Vulfson, E. N., J. Am. Oil Chem. Soc. 71, 1 (1994). 5. Vulfson, E., in “Novel Surfactants” (K. Holmberg, Ed.), Surfactant Science Series, Vol. 74, p. 279. Marcel Dekker, New York, 1998. 6. Holmberg, K., in “Novel Surfactants” (K. Holmberg, Ed.), Surfactant Science Series, Vol. 74, p. 333. Marcel Dekker, New York, 1998.
7. Razafindralambo, H., Blecker, C., Delhaye, S., and Paquot, M., J. Colloid Interface Sci. 174, 373 (1995). 8. Hua, X. Y., and Rosen, M. J., J. Colloid Interface Sci. 124, 2 (1988). 9. Rosen, M. J., and Hua, X. Y., J. Colloid Interface Sci. 139, 2 (1990). 10. Hua, X. Y., and Rosen, M. J., J. Colloid Interface Sci. 141, 1 (1991). 11. Zhang, T., and Marchant, R. E., J. Colloid Interface Sci. 177, 419 (1990). 12. Otto, R., Bornscheuer, U., Syldatk, C., and Schmid R., J. Biotechnol. 64, 231 (1998). 13. Nakamura, S., Information 8, 8 (1997). 14. Arcos, J. A., Bernab´e, M., and Otero, C., Biotechnol. Bioeng. 60, 1 (1998).
Enzymatically Prepared n-Alkyl Esters of Glucuronic Acid: The Effect of Hydrophobic Chain Length on Surface Properties Christophe Blecker,∗,1 Salvator Piccicuto,∗, † Georges Lognay,‡ Claude Deroanne,∗ Michel Marlier,‡ and Michel Paquot† ∗ Unit´e de Technologie des Industries Agro-alimentaires; †Unit´e de Chimie Biologique Industrielle; and ‡Unit´e de Chimie G´en´erale et Organique, Facult´e des Sciences Agronomiques de Gembloux, Gembloux B-5030, Belgium Received February 20, 2001; accepted December 10, 2001
The effect of hydrophobic chain length on surface properties of enzymatically prepared n-alkyl esters of glucuronic acid are examined. Dynamic parameters from Hua and Rosen’s mathematical model and equilibrium surface tension are presented for esters with octyl, decyl, dodecyl, and tetradecyl alkyl segments. Increasing the alkyl chain length has a significant influence on the surface activity. Decyl and dodecyl glucuronate exhibit an interesting adsorption speed associated with foaming capacity. Octyl glucuronate exhibits a micellar organization as its bulk concentration is over 10.68 mM. C 2002 Elsevier Science (USA) Key Words: Nonionic surfactants; glucuronic acid; biosurfactants; hydrophobic; surface tension; critical micellar concentration; adsorption kinetics; hydrophilic–lipophilic balance; enzymatic synthesis; lipase.
INTRODUCTION
For several years, there has been great interest in the use of sugar fatty esters as nonionic surfactants. Surfactant and emulsifying properties result from the presence of both hydrophilic and hydrophobic regions on the same molecule (1). Sucroesters show this characteristic: the hydrophilic moiety is here provided by a sugar. The use of a linear alkyl chain as hydrophobic moiety allows us to modulate its importance. So an investigation of the relative hydrophobicity influence can be envisaged. The hydrophilic–lipophilic balance is the main parameter that determines the interfacial activity of an amphiphilic molecule. This value is controlled by the nature of the chain length and by the importance of the hydrophilic head. The number of esterified hydroxyl groups has also a great influence. Sugar esters have a very wide arbitrary hydrophilicity and lipophilicity balance (HLB) (2). Indeed their properties can range from those of water soluble surfactants (HLB higher than 10) to oil soluble emulsifiers (HLB lower than 4). As stated above, both occurring moieties exhibit relevant properties: they are made from renewable and inexpensive
1 To whom correspondence should be addressed. E-mail: blecker.c@fsagx. ac.be.
0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
424
feedstocks. In addition, they are completely biodegradable, nontoxic, nonskin-irritant, odourless, and tasteless (3). Despite all these advantages and since a long time, the synthesis of sugar esters has been limited. The reason is that the classical chemical way of synthesis shows some major drawbacks. This conventional approach requires a high temperature, which causes coloration, high energy consumption, and the recovery of considerable amounts of sideproducts (3, 4). The use of biological catalysts under mild conditions can help to overcome these problems. The enzymatic synthesis has given new potentialities to sugar esters production (5, 6). Most studies about sugar esters present the results obtained with sucrose, glucose, and fructose esters. The particularity of the surfactants used within the present study is linked to the nature of the hydrophilic moiety: glucuronic acid. Otto et al. (1) reported for the first time the lipase-catalyzed esterification of this sugar acid with an alcohol (n-butanol). To our knowledge, no other glucuronic acid esters (longer hydrophobic chain) were synthesized and no data are available on the surface properties of such compounds. So, our main objective was to investigate the effect of the hydrophobic chain length on the surface properties of enzymatically prepared glucuronic acid esters. The surface properties of a surfactant provide the basis of various applications and are usually characterized by the equilibrium surface tension, the critical micellar concentration (CMC), the surface concentration, and the surface area per molecule. However, in many interfacial process, such as wetting or foaming, equilibrium conditions are not attained and dynamic processes play a major role. So adsorption kinetics of the surfactant must be characterized. MATERIALS AND METHODS
Esters Synthesis and Purification Sugar esters were prepared using immobilized Candida antarctica lipase B (Novozym SP 435, Novo Nordisk) in an organic medium. Glucuronic acid (0.025 M) and fatty alcohol (0.025 M) were dissolved in 10 ml of t-butanol. After an addition of 100 mg of lipase and 500 mg of activated molecular sieve (8–12 mesh, Sigma), an enzymatic reaction was carried out
425
SURFACE PROPERTIES OF GLUCURONIC ACID ESTERS
FIG. 1.
Ester of glucuronic acid and fatty alcohol.
in a shaking (150 rpm) water bath at 60◦ C for 48 h. Lipase and molecular sieve were then removed by filtration and ester was extracted by dichloromethane (20 ml) and purified using silica gel chromatography (chloroform : methanol 70 : 30 vol/vol as eluant). The purity of all synthesized products was proved to be more than 99% by HPLC-ELSD. Enzymatic synthesis allowed us to obtain esters of glucuronic acid and aliphatic alcohols from 8 to 14 carbon atoms. Their structure is illustrated in Fig. 1. Adsorption Kinetics Recording The automatic drop tensiometer TRACKER (IT Concept, France) was used for measuring the surface tension–time dependence of surfactant solutions by a static procedure. It consisted in forming a rising air bubble at the tip of a curved capillary. The volume of the bubble was set to 1 µl. The surface tension was measured and recorded every second. The length of the measurement was sufficient to reach the equilibrium. The cell in which the bubble was formed contained 7 ml of solution. All measurements were performed at 25 ± 0.5◦ C. Dynamic Drop Volume Method The automated drop volume tensiometer TVT 1 (Lauda, Germany) was employed to perform dynamic measurements. Drops of solution were formed with a growing formation speed. The lifetime of the drops was measured as a function of their volume, which made it possible to calculate the surface tension. All measurements were performed at 25 ± 0.5◦ C. Each measurement was repeated twice. For high concentrations of surfactant, equilibrium surface tension (σe ) was taken as the mean of the values obtained with the last 4 drops. For low concentrations, σe was deduced by extrapolating the surface tension to time t → ∞ in the σ − t −1/2 (7).
Surface tension (mN/m)
80 70 60 50 40 30 20 10 0 0
500
1000
1500
2500
3000
FIG. 2. Adsorption kinetics of decyl glucuronate (), dodecyl glucuronate (), and tetradecyl glucuronate () (results of two repetitions).
was realized by the software SigmaPlot 5.0 (Jandel Scientifics, USA), γ t − γm =
γ0 − γm , 1 + (t/t ∗ )n
[1]
where σ0 is the surface tension of the pure solvent, γm the surface tension at mesoequilibrium, and σt the surface tension at time t; t ∗ and n are estimated by the fitting. RESULTS AND DISCUSSION
Adsorption Kinetics Adsorption kinetics were established at 6 mg/l. This concentration is the limit of solubility for tetradecyl glucuronate, the most hydrophobic compound. Curves are shown in Fig. 2 and remarkable results are presented in Table 1. At this concentration, dodecyl glucuronate acted quickly: it provided an important decrease of surface tension during the first 100 s.
TABLE 1 Surface Tension (γ) after 1, 500, and 2000 s for a Solution at 6 and 83 mg/l γ
Application of the Hua and Rosen Mathematical Model Data obtained with the automated drop volume tensiometer in dynamic mode were treated according to the mathematical model proposed by Hua and Rosen (8–10). Fitting was performed using Eq. [1]. In a first attempt, the parameters n and t ∗ were estimated by a logarithmic linear transformation of Eq. [1]. The estimated values were then employed for the fitting of Eq. [1] to the dynamic surface tension data. This fitting
2000
Time (s)
1s Surfactant Octyl glucuronate Decyl glucuronate Dodecyl glucuronate Tetradecyl glucuronate
500 s
2000 s
6 mg/l 83 mg/l 6 mg/l 83 mg/l 6 mg/l 83 mg/l 72 70 69 68
66 45 — —
71 59 52 59
65 37 — —
71 58 39 48
— — — —
426
BLECKER ET AL.
TABLE 2 Hua and Rosen Parameters for a Solution at 6 and 83 mg/l t ∗ (s)
FIG. 3. Adsorption kinetics of octyl glucuronate () and decyl glucuronate() (results of two repetitions).
Then dodecyl glucuronate adsorbed to the surface and so led to a continuous decrease of surface tension. Decyl glucuronate led to a 10 mN/m decrease over the first 200 s, and after this first step, it did not provoke an important decrease over the following 3000 s. Tetradecyl glucuronate showed a behavior similar to that of decyl glucuronate during 400 s. However, after this delay, its adsorption did not stop and caused a significant continuous decrease in surface tension. So adsorption kinetics depended on the hydrophobic chain length, and an acyl moiety with 12 carbon atoms seems to be an optimum in terms of adsorption speed. Adsorption kinetics were also recorded at 83 mg/l for octyl glucuronate and decyl glucuronate, the two more soluble esters. This concentration correspond to the solubility limit of decyl glucuronate. Curves are shown in Fig. 3, and the main results are presented in Table 1. Observation of the adsorption kinetics of octyl and decyl glucuronate indicated the ability of the second compounds to adsorb quickly to the surface at this concentration: after 500 s, decyl glucuronate led to a decrease of surface tension equal to 35 mN/m (Table 1). Octyl glucuronate induced a lower decrease of the surface tension (7 mN/m). Actually, kinetics recorded for both esters are quite similar. Octyl glucuronate caused a 5 mN/m decrease during the first seconds and then showed a linear and slow evolution. Decyl glucuronate caused a 33 mN/m decrease during the first seconds and then showed the same linear behavior. So, even for higher concentrations, a hydrophobic chain length seemed to play an important role in the adsorption speed of this kind of surfactant and the low solubility limit of dodecyl glucuronate caused the decrease of the optimal hydrophobic chain length. Application of the Hua and Rosen Mathematical Model The previous study of adsorption kinetics had a qualitative aspect. The use of the Hua and Rosen mathematical model would help us to determine quantitative parameters. These authors have suggested a method of treatment that elucidated the factors that
γm (mN/m)
Surfactant
6 mg/l
83 mg/l
6 mg/l
83 mg/l
Octyl glucuronate Decyl glucuronate Dodecyl glucuronate Tetradecyl glucuronate
— 20.05 9.92 37.2
9.15 0.40 — —
— 62.0 49.5 48.1
60.1 32.9 — —
determine the rate at which the surface tension was reduced. Results obtained with the automated drop tensiometer in dynamic mode were treated according to the Hua and Rosen mathematical model and gave information about the surfactant behavior when the surface area was growing. Two parameters could be compared: —t ∗ : the time at which the rate of change of surface tension in function of the logarithm of time reaches a maximum. So this parameter decreases as the adsorption speed of the surfactant increases. —γm : the surface tension at mesoequilibrium. This is the surface tension reached after the first step of quick adsorption of the surfactant to the newly created surface. Table 2 illustrates the values obtained for decyl glucuronate, dodecyl glucuronate, and tetradecyl glucuronate at both concentrations. For this concentration, the adsorption of octyl glucuronate was very slow and did not fit to the Hua and Rosen model. Values obtained for t ∗ confirmed the quick adsorption of dodecyl glucuronate at this concentration. The same mathematical treatment was applied to the plots of surface tension against the logarithm of time for octyl glucuronate and decyl glucuronate with a surfactant concentration equal to 83 mg/l. Values of t ∗ indicated the rapidity of decyl glucuronate under those conditions. So this quantitative characterization corroborated the results of the previous qualitative study. Equilibrium Surface Tension The equilibrium surface tension reached for a concentration also depended on the hydrophobic chain length. If a concentration of 6 mg/l was considered, dodecyl glucuronate provided the greatest decrease of surface tension (Table 3). If a concentration TABLE 3 Equilibrium Surface Tension (σe ) for a Solution at 6 and 83 mg/l σe (mN/m) Surfactant
6 mg/l
83 mg/l
Octyl glucuronate Decyl glucuronate Dodecyl glucuronate Tetradecyl glucuronate
71 57 37 44
57 29 — —
427
SURFACE PROPERTIES OF GLUCURONIC ACID ESTERS
of 83 mg/l was considered, decyl glucuronate induced a greater drop in surface tension than octyl glucuronate did (Table 3). This last observation could be confirmed by the calculation of the efficiency (10). The efficiency of a surfactant (pC20 ) is defined by the value of the negative logarithm of the bulk concentration necessary to reduce the surface tension by 20 mN/m. The calculated efficiency for octyl glucuronate was 0.276 and for decyl glucuronate 2.369. These values confirmed that decyl glucuronate was a very efficient compound for this bulk concentration. Zhang and Marchant (11) have shown that efficiency increases as the hydrophobic chain length increases for another group of sugar-based surfactants. The optimum chain length is different according to the effect that is suited: if a significant decrease of surface tension is needed with a very low surfactant concentration, dodecyl glucuronate can be used. However, if the lowest surface tension is needed, decyl glucuronate is the most efficient compound. Critical Micellar Concentration
max
Surfactant SDS CTAB CHAPS Octyl glucoside Octyl glucuronate
(7) (7) (7) (12)
σCMC (mN/m)
CMC (mM)
max (mol · cm−2 ) × 10−10
ACMC (m2 /molec) × 10−20
38.5 37.6 45.2 32 28.0
8.19 0.98 6.66 18 10.68
2.51 2.85 1.73 — 3.85
66.0 58.3 96.0 — 43.1
Note. Numbers in parentheses indicate references.
( dσ/d ln C)CMC was calculated by the derivative of the polynomial function σe = f (ln C) at the CMC. The molecular area (ACMC ) was given by ACMC =
From all esters, octyl glucuronate is the only one that caused the formation of micelles. Critical micellar concentration was determined from a plot of equilibrium surface tension against logarithm of the concentration. Curves giving the best fit of the equilibrium surface tension–concentration data were obtained and are shown in Fig. 4. Both curves were plotted using polynomial and straight-line equations, corresponding respectively to the descending and the horizontal parts. Those equations were used to calculate the CMC and the surface tension at the CMC (σCMC ) defined at the intersection point of the fitted curves. The maximum surface excess (max ) was calculated with the Gibbs adsorption equation applied to a nonionic surfactant: 1 = RT
TABLE 4 Surface Properties of Various Surfactants
dσ d ln C
.
[2]
CMC
1 , max N
[3]
where N is Avogadro’s number. The results for octyl glucuronate and those for various surfactants found in the literature are presented in Table 4. The molecular area determined for octyl glucuronate at the CMC (43.1 × 10−20 m2 /molecule) suggested that molecules are rather closely packed. Such a value is typical for nonionic surfactants with a bulky hydrophilic part, as esters of monosaccharides, derivatives of cycloalkanes or dioxacyclanes, and similar compounds. Comparatively, some kinds of ionic (SDS, CTAB) or zwitterionic (CHAPS) surfactants exhibit a higher molecular area at the CMC. In conclusion, it is possible to obtain various surface properties by adapting the hydrophobic chain length of glucuronic acid esters. For example, the important adsorption speed of decyl glucuronate and dodecyl glucuronate allowed them to cause a significant decrease of surface tension in a few seconds. This characteristic can be associated to foaming properties (8). Micellar structures could be obtained using a relatively small bulk concentration of octyl glucuronate. So the utilization of this molecule can be envisaged in detergents or generally speaking, in systems where an hydrophobic compound must be dispersed in an aqueous phase. Such systems are found in drugs and cosmetics formulations. Interfacial and emulsifying properties should also be investigated as sugar esters are employed as emulsifying agents (13, 14). ACKNOWLEDGMENT This work received financial support from the “Minist`ere de la Communaut´e Fran¸caise de Belgique” (Project ARC).
REFERENCES FIG. 4. Plot of surface tension of solutions against logarithm of octyl glucuronate concentration (results of two repetitions).
1. Otto, R. T., Bornscheuer, U. T., Syldatk, C., and Schmidt, R. D., J. Biotechnol. 64, 231 (1998).
428
BLECKER ET AL.
2. Liu, X., Gong, L., and Liu, J., J. Mol. Catal. A: Chem. 147, 37 (1999). 3. Ducret, A., Giroux, A., Trani, M., and Lortie, R., Biotech. Bioeng. 48, 3 (1995). 4. Fregapane, G., Sarney, D. B., Greenberg, S. G., Knight, D. J., and Vulfson, E. N., J. Am. Oil Chem. Soc. 71, 1 (1994). 5. Vulfson, E., in “Novel Surfactants” (K. Holmberg, Ed.), Surfactant Science Series, Vol. 74, p. 279. Marcel Dekker, New York, 1998. 6. Holmberg, K., in “Novel Surfactants” (K. Holmberg, Ed.), Surfactant Science Series, Vol. 74, p. 333. Marcel Dekker, New York, 1998.
7. Razafindralambo, H., Blecker, C., Delhaye, S., and Paquot, M., J. Colloid Interface Sci. 174, 373 (1995). 8. Hua, X. Y., and Rosen, M. J., J. Colloid Interface Sci. 124, 2 (1988). 9. Rosen, M. J., and Hua, X. Y., J. Colloid Interface Sci. 139, 2 (1990). 10. Hua, X. Y., and Rosen, M. J., J. Colloid Interface Sci. 141, 1 (1991). 11. Zhang, T., and Marchant, R. E., J. Colloid Interface Sci. 177, 419 (1990). 12. Otto, R., Bornscheuer, U., Syldatk, C., and Schmid R., J. Biotechnol. 64, 231 (1998). 13. Nakamura, S., Information 8, 8 (1997). 14. Arcos, J. A., Bernab´e, M., and Otero, C., Biotechnol. Bioeng. 60, 1 (1998).