Surface-active and associative properties of ionicp polymeric surfactants based on carboxyrnethylcellulose.
The increasing interest for biocompatible and biodegradable materials makes natural polymers like polysaccharides attractive raw materials for the preparation of biopolymeric surfactants, which have potential application in food and non-food sectors (1). Modification of highly hydrophilic water-soluble polysaccharides by grafting a low amount of hydrophobic groups like alkyl chains leads to water-soluble amphiphilic derivatives, which usually exhibit associative properties in water, viscosity-increasing effects and surface-active properties enabling to control foaming or emulsion stability (2), (3). A considerable number of studies reported on polymeric surfactants prepared by partial hydrophobization of commercial and non-commercial polysaccharides such as hydroxyethyl cellulose (HEC) (4), (5), puliulan (6), dextran (7), (8), carboxymethylstarch (CMS) (9), and beech wood xylan (10).
The commercially available anionic cellulose derivative carboxymethylcellulose (CMC) was used for targeted hydrophobic modification. Water-soluble associative polymers exhibiting tensio-active properties were obtained by amidation of the carboxyl group of CMC using various alkylamines (11), (12). Etherification of the free hydroxyl groups of CMC with [C.sub.12]-[C.sub.18] alkyl halides in DMF/H20 using NaOH as catalyst yielded partially hydrophobized CMC ether derivatives, with a very low degree of substitution. They showed excellent emulsifying properties for emulsions of the oil in water type and effective antiredeposition efficiency, but a poor washing power (13). Surface active amphiphilic CMC esters prepared by non-conventional esterification with mixed anhydrides and vinyl laurate exhibited acceptable detergent performance properties (14). Recently, a substantial decrease of surface tension of water (from 72.0 up to about 30 mN/m) and the oil/water interfacial tension (from 30 up to 1.7 mN/m) was reported for CMC containing dodecylpolyoxyethylene aery late groups (15).
The reaction temperatures used in esterification reactions ranged from 40 up to 190C. Nowadays, microwave radiation has become an increasingly popular heating source, which replaces the classical one in many diverse areas of chemistry, including chemical modifications of carbohydrates (16), (17). In comparison with the conventional heating technique, the application of microwave heating extensively shortened the reaction time in organic syntheses. There are few reports concerning the application of microwave heating in the esterification of polysaccharides such as cellulose (18), (19), starch (20), CMS (21), and HEC (22) whereby some of the reactions were performed under solvent-free conditions.
In our recent article (23) CMC was partially hydrophobized by transesterification using the complex of fatty acid methyl esters of rapeseed oil (MERO) and microwave radiation with controlled power as heating source. The transesterification reaction of higher fatty acid methyl esters was successfully applied already on starch under solvent-free conditions (24). Since fatty acid esters are found in nature as triacyl glycerides, the authors used them as nonexpensive acylation reagents in a later study on fatty acid starch esters (25).
In this study the microwave-assisted transesterification between CMC and the triacyl glycerides of rapeseed oil was investigated and the chemical, surface-active and associative properties of the obtained water-soluble amphophilic CMC derivatives were characterized.
Materials and Methods
CMC ([Na.sup.Plus] salt; [DS.sub.CM] = 1; [M.sub.w] = 741 kDa, Walsrode, Germany) was a gift from Prof Thomas Heinze (Centre of Excellence of Polysaccharide Research, Friedrich Schiller University of Jena, Germany). The rapeseed oil (RO) was a commercial product from PALMA-TUMYS, a. s. (Bratislava, Slovakia). It contained ~60% oleic acid and lower amounts of linoleyl and linolenyl acids (~30%) and saturated [C.sub.14-20] fatty acids (~8%) as triacyl glycerides (Gly-(OCOR)3). All chemicals were used without further purification. Tween 20 was from Aldrich Chemical Co. (Steinheim, Germany). Coomassie Brilliant Blue G-250 (CBB) was from Serva Electrophoresis (Heidenberg, Germany).
Fourier-transform infrared (FT-IR) spectra were obtained on the NICOLET Magna 750 spectrometer with DTGS detector and OMNIC 3.2 software using 128 scans at a resolution of 4 [cm.sup.-1]. The samples (2.0 mg) were pressed into pellets of KBr (200 mg). The spectra were baseline corrected and normalized to the intensity of the absorption band at 1602 [cm.sup.-1]. The absorbance of the bands at 2922 [cm.sup.-1] (baseline 3000-2600 [cm.sup.-1]) and at 1065 [cm.sup.-1] (base line 1935-870 [cm.sup.-1]) were used to express the extent of esterification (EE) as the ratio [A.sub.2922]/[A.sub.1065]
[sup.1.H] and [sup.13.C] nuclear magnetic resonance (NMR) spectra (in [D.sub.2]O) in the inverse gated decoupling mode were recorded at 40 [degrees] C on a VNMRS 400 MHz Varian spectrometer operating at 399.90 MHz for [sup.1.H] and 100.55 MHz for [sup.13.C], and equipped with 1H-19F/15N-31P 5 mm PFG AutoX DB NB Probe. The sample for NMR measurements were dissolved in water (0.1 g in 50 mL) and lyophilized, and the procedure was twice repeated.
Synthesis of RO-Hydrophohized CMC Derivatives (RO-CMC)
In a typical experiment, CMC (1.0 g) was dissolved in 25 mL distilled water under stirring at room temperature for 1 h. Subsequently, RO (0.5-2.0 g) in 20 mL DMF was added to the CMC solution. The reaction mixture with addition of potassium laurate (17.9 mg in 1 mL water) as catalyst, was under permanent stirring exposed to microwave heating for 1 to 5 min, using a modified domestic microwave oven SENCOR SMW 2220 (2.45 GHz) at microwave power 140 W or 550 W, whereby the temperature increased from room to reaction temperatures (70-110 [degrees] C). After reaction, the product was poured into four to six volumes of acetone and the precipitated derivative was separated by filtration, thoroughly washed with acetone, and extracted in a Soxhlet apparatus with acetone for 8 h to remove the unreacted acylation agent and degradation products.
Surface-Active Properties of RO-CMC Derivatives
Surface Tension. The surface tension was measured at 25 [degrees] C using the Du Nouy ring apparatus. The surface tension data were plotted against the logarithm of the polysaccharide concentration in water or 0.5 M aqueous NaCl in the concentration range 0.019 to 5.0 g/L. From the inflection point of the plot, the critical micelle concentration (c.m.c) and minimum surface tension (ymin) were derived (23).
Emulsifying Efficiency. The emulsifying properties were tested on emulsion of the "oil in water" (O/W) type (23). The emulsion was prepared by mixing 9 mL water containing 0.05 g of the tested derivative and 1 mL of paraffinic oil dyed with SUDAN IV using the laboratory minimixer (Heidolph DIAX 600) at 20,500 rpm for 1 min. The emulsifying efficiency was expressed as the height (mm) of the oil and cream layers formed on the surface of the emulsion at three time intervals after the emulsions had been prepared, i.e. 5 min ([h.sub.1]), 1 ([h.sub.2]), and 24 h ([h.sub.3]). The stability of the emulsions was measured in dependence on the time of rest in course of 24 h and expressed by the onset of creaming and the final height of the formed cream layer.
Associative Properties of RO-CMC Derivatives
Viscometry. The dilute solution properties of the CMC derivatives were examined by viscometry of RO-CMC solutions in aqueous 0.1 M NaCl using the Ubbelohdetype capillary viscometer (0.64 mm, diameter). The concentration of RO-CMC solutions ranged between 0.32 and 0.09 g/dL. The flow times were measured at 25 [degrees] C maintained by a circulating water bath. Before the measurements, the polymer solutions were filtered through paper filter (Whatman Schleicher & Schuell 589/1). The ratio of the flow times of the solution and solvent (t/[t.sub.0]) representing the relative viscosity ([[eta].sub.rel]) was used to calculate the specific viscosity as [[eta].sub.sp] = [[eta].sub.rel] - 1. The concentration dependence of the reduced viscosity [[eta].sub.red] of the solutions was described by the Huggins equation:
[[eta].sub.red] = [[eta].sub.sp]/c = [[eta]] + [k.sub.H] * [[eta].sup.2] * c (1)
The intrinsic viscosity ([[eta]]) and Huggins constant ([k.sub.H]) reflecting the nature of polymer interactions in solution was obtained by plotting the reduced viscosity [[eta].sub.red] as a function of polymer concentration (c).
Detection of Hydrophobic Aggregates (Clusters). Hydrophobic microdomains in RO-CMC solutions were evidenced with the Coomassie Brilliant Blue (CBB) dye, as described by Samsonoff et al. (26). An aqueous solution of RO-CMC derivative (15.0 g/L) was diluted to a set of solutions with polymer concentration between 15.0 and 0.4 g/L. To 2 mL of these solutions, 2 mL 0.03 g/L of aqueous CBB solution was added. Final solutions were homogenized by stirring and incubated 10 min at room temperature and the absorption of CBB was monitored between 500 and 700 nm using the SPEKORD UV-VIS spectrometer. The critical aggregation concentration (c.a.c.) corresponding to the onset of detection of hydro phobic clusters, was determined by plotting the absorption change at 618 nm ([DELTA][A.sub.618]) versus concentration (c) of the RO-CMC derivative.
[FIGURE 1 OMITTED]
RESULTS AND DISCUSSION
Based on the results from the previous study of CMC hydrophobization by transesterification of MERO (23), the reaction was performed in H20/DMF, catalyzed with potassium laurate at various reaction conditions and controlled low and high microwave power (Table 1). As shown in Table 1, the temperature at the end of the reaction increased with reaction time and microwave power, similarly as described by Antova et al. , but no marked effects on the yields of the RO-CMC derivatives were observed. They ranged from 0.88 to 0.99 g per g CMC and were similar to those of the MERO-CMC derivatives prepared at 300 W (23). Esterification technique is schematically presented in Fig. 1.
TABLE 1. Effect of reaction conditions on the yield and FT-IR characteristics of RO-CMC derivatives prepared at different controlled microwave power. RO-CMC CMC:RO Time Temperature [Yield.sup.a] sample (mass ratio) (min) ([degrees]C) (g/g) I (d) 1:1 1 70 0.97 II (d) 1:2 3 74 0.88 III (d) 1:2 5 96 0.99 V (e) 1:2 3 110 0.89 V (e) 1:1 2 104 0.96 VI (d) 2:1 2 101 0.96 V[(0H).sup.c] RO-CMC [EE.sup.b] (a + b) a/b sample ([A.sub.2922]/[A.sub.1065]) I (d) 0.218 68 0.58 II (d) 0.157 62 0.59 III (d) 0.152 55 0.57 V (e) 0.178 53 0.61 V (e) 0.360 51 0.65 VI (d) 0.230 75 0.56 (a) Expressed as g of the recovered derivative per g CMC (on dry mass basis). (b) Extent of esterification expressed as the intensity of the v[sub.as] CH[sub.2] absorption band. (c) Half width of the v(OH) absorption maximum (a + b) and its asymmetry, expressed as the ratio of the left (a) and right (b) sides; The corresponding values for CMC are 74 and 0.55, respectively. (d) Controlled power: 140W (e) Controlled power: 550W
As the obtained RO-CMC derivatives showed nonsignificant differences in elemental analysis data compared to the starting CMC (not shown), the FT-IR spectra were used to estimate the extent of esterification. As illustrated in Fig. 2, the spectral patterns of the derivatives were similar to those of the starting CMC and showed strong asymmetric stretching carbonyl vibration [[v.sub.as]([COO.sup.-])] of the carboxymethyl substitutents at 1602 [cm.sup.-1]. The symmetric carbonyl stretching vibration [[v.sub.s]([COO.sup.-])] is overlapped by the [CH.sub.2] symmetric bending mode of cellulose at ~421 [cm.sup.-1]. The absorption band at Tilde1730 [cm.sup.-1] attributed to the v(C=0) vibration of the ester groups appeared as a shoulder only in the spectrum of RO-CMC-V indicating a very low degree of esterification. However, the presence of ester groups is demonstrated also by the intensity increase of the absorption bands at ~2922 [cm.sup.-1] and 2852 [cm.sup.-1], respectively, attributed to the [v.sub.as]([CH.sub.2]) and [v.sub.s] ([CH.sub.2]) vibrations of the fatty acyl substituents, and is again the most intense in RO-CMC-V. The esterification of CMC caused further changes in the region at 1200 to 1000 c[m.sup.-1] dominated by ring vibrations, stretching vibrations of v(C--OH) side groups and glycosidic bond vibrations, which is characteristic for particular polysaccharides (27). The extent of esterification (EE) was expressed by the absorbance ratio the v[sub.as](CH[sub.2]) vibration and the skeletal v(C--O--C) vibration at 1065 c[m.sup.-1] typical of cellulose, i.e. [A.sub.2922]/[A.sub.1065].
[FIGURE 2 OMITTED]
As shown in Table 1, the EE values of derivatives, except of RO-CMC-V, were low and slightly decreased with increasing radiation time. The results indicated that the microwave power played a significant role in the esterilication reaction. When compared with the previous report (23), one can suggest that the applied MW power of 140 W was too low to achieve a noticeable esterification degree. In accord with Satge et al. (18) the extension of the microwave radiation time from 1 to 3 and 5 min probably induced the de-esterification reaction, similarly as reported for sulfated chitosan (28). These suggestions were confirmed also by changes in the region of v(OH) vibrations reflecting the character of the cellulose supramolecular structure (29). This was demonstrated by the asymmetry of the broad v(OH) absorption band at about 3414 cm"1 showing a shoulder shifted to lower wave-numbers (around 3300 c[m.sup.-1]), what is indicative of OH groups involved in strong hydrogen bond systems. The changes were characterized by the half width of the v(OH) absorption maximum (a + b) and the asymmetry by the ratio (a/h) of the left (a) and right (b) sides. The decrease of the (a + h) and increase of the alb values indicated disruption of the supramolecular structure of CMC. As seen in Table 1, the course of both values followed the same trend as the estimated EE values. However, the observed structural changes can be, only in part, caused by esterification of accessible OH groups. A detrimental effect of microwave radiation might be suggested, similarly as was observed during the microwave-assisted depolymerization of highly ordered polysaccharides such as cellulose and hyaluronic acid (30), (31),
The structure of the derivative RO-CMC-V with the highest EE was characterized by NMR spectroscopy. The C-NMR spectrum displayed in Fig. 3a was poorly-resolved and showed the typical pattern of CMC with clusters of peaks in the range from 60 to 103 ppm, which are attributed to C-l-C-6 of the glucopyranosyl units free and carboxymethylated at positions 2, 3, and 6; the inserted region at Tilde178 ppm correspond to carbonyl signals of the carboxymelhyl groups (32), (33). Similarly as in the very low-esterified MERO-CMC derivative (23), only the carbon peak substituted by acyl groups (C6e) at 65.26 ppm was distinguished from the noise. The signals in the high field region of the *H-NMR spectrum at 2.8 to 1.0 ppm (Fig. 3b) confirmed the low EE indicated by the FT-IR analysis.
Regardless the low degree of esterification, the RO-CMC derivatives displayed surface-active properties, similarly as reported with the MERO-CMC derivatives (23). The critical micelle concentration (c.m.c.) and the corresponding minimum surface tension (ymin) of the derivatives in water and 0.5 M aqueous NaCl were assessed from the logarithmic concentration dependence of surface tension: y = f(log c). The obtained data are summarized in Table 2. As seen, the RO-CMC derivatives had no pronounced suppressing effect on the surface tension of water, because they lowered the } ymin from 72.8 only to 60.8-64.9 mN/m. The c.m.c. values ranged between 2.41 and 2.63 g/L, except of RO-CMC-IV (0.30 g/L). Based on the reported effect of the size of carbohydrate segments of polymeric surfactants (34), the particularly low c.m.c. of RO-CMC-III indicated a lower molar mass than had samples I and II. As its esterification extent was the lowest, a depolymerization effect of microwave heating cannot be excluded (30). The surface activity of all RO-CMC derivatives was strengthened when measured in 0.5 M aqueous NaCl, what corresponds with findings reported for other hydrophobically modified CMC (12) and ethyl(hydroxyethyl)cellulose (35). In general, the abilities of polymeric surfactants to decrease surface and interfacial tension are much lower than those of the classical, low molar-mass surfactants (36), (37). They depend on number of factors, such as the type and content of hydrophobic substituents, their hydrophobicity, the chemical structure, molecular size and flexibility of the polymer. The RO-CMC derivatives consist of a mixture of various acyl substituents and their proportions may vary due to the different reactivity of the RO fatty acid components during the transesterification reaction and/or the above supposed microwave-induced deesterification (28). This has to be taken in account and might explain the scattering of the data for the tensio-active properties in Table 2.
TABLE 2. Surface-active properties of RO-CMC derivatives and CMC. RO-CMC ymin (mN/m) c.m.c. (g/L) NaCl sample Water NaCl Water I 64.0 60.6 2.41 2.40 II 64.7 51.6 2.44 2.31 III 60.8 59.3 2.63 2.46 IV 66.3 63.7 0.30 0.61 V 61.2 62.0 2.46 0.15 VI 64.9 62.8 2.46 2.45 CMC 65.9 -- 0.81 -- ymin, Minimal surface tension. c.m.c. Critical micelle concentration.
[FIGURE 3 OMITTED]
The hydrophobization effect achieved in case of the RO-CMC derivatives is demonstrated by the emulsifying efficiency. It was evaluated by testing the stability of emulsions of the oil in water (O/W) type during 24 h by measuring the height of oil and cream layers formed on the top of the emulsion. The commercial emulsifier Tween 20 was used as control. A "delay time" is sometimes observed in emulsion systems before creaming. This is the time at which the height of cream layer exceeded the limit of 0.1 mm. The delay was quantified by extrapolation of the cream layer height to zero movement (see Fig. 4) according to Santiago et al. (38). As shown in Table 3, the unmodified CMC was able to form and stabilize an O/W emulsion like Tween 20. Evidently, the weak hydrophobization of CMC leads to emulsifying efficiency improvement and the emulsions of the RO-CMC derivatives are stabilized several hours with delay time of cream layer forming longer then unmodified CMC (0.57 h) and standard Tween 20 (0.97 h). The differences in emulsion stability are the best illustrated in Fig. 4. In comparison to Tween 20, the unmodified CMC slowed down the creaming process and a somewhat lower maximum cream layer was formed at about 20 h later. Significant differences in stability were observed between the emulsions from RO-CMC samples I to III and samples IV to VI. The first group exhibited an about 1.5 times higher stability than the commercial emulsifier Tween 20. They markedly decelerated the creaming process and increased the height of the cream layer formed after more than 100 h. The emulsions of samples V and VI followed the same course of creaming as the before mentioned samples, but the maximum heights of the cream layer was lower with sample V, and even below the level of the CMC emulsion in case of sample VI. As shown in Table 3, the both cream layers contained dispersed oil droplets. The emulsion from sample IV behaved as CMC, but formed after 24 h a much lower (Tilde60%) cream layer as well a separated oil layer (Table 3). Studies on various amphiphilic polysaccharide derivatives (39), (40) as well as the presented results on partially RO-hydrophibized CMC indicated that, although not obligatory for the active adsorption, the introduced hydrophobic moieties enhance the emulsifying activities.
TABLE 3. Emulsifying efficiency for oil in water (O/W) type emulsions of RO-CMC derivatives and controls. Oil/cream layer.(b) (mm/mm) RO-CMC sample Delay time (a) (h) h1 h2 h3 I 1.38 0/0 0/0 0/5 II 1.68 0/0 0/0 0/5 III 1.63 0/0 0/0 0/6 IV 0.77 0/0 0/0 2/4 V 1.55 0/0 0/0 0/6 (c) VI 0.90 0/0 o/1 0/5 (v) CMC 0.57 0/0 0/0 0/8 Tween 20 0.97 0/0 0/0 0/11 (a)a Time at which the height of cream layer exceeded the limit 0.1 mm. (b) Height of the oil and cream layers formed on the surface of the emulsion after 5 min ([h.sub.1]), 1 h ([h.sub.2]), and 24 h ([h.sub.3]). (c) Oil drops dispersed in the cream layer.
[FIGURE 4 OMITTED]
Moreover, amphiphilic polymers display associative properties (2), (3), (6) and form aggregates in aqueous solutions which result from hydrophobic interaction of the side chains investigated by rheology and viscometry (6), (8) and/or by spectroscopy (4), (8), (26), (36), (41). The associative properties of the RO-CMC derivatives were investigated by viscometry and spectroscopy using the Coomassie Brilliant Blue (CBB) technique.
The viscometric measurements of the unmodified CMC and RO-CMC derivatives were performed on dilute solutions in aqueous 0.1 M NaCl at 25 [degrees] C. The intrinsic viscosity ([[eta]]]) and Huggins constant ([k.sub.H]) derived from the Huggins Eq, 1 are displayed in Table 4. In general, these quantities provide insight to molecular structure and interactions of the polymers in solution. The Huggins constant is generally below 0.4 for noninteracting macromolecules in good solvent, but can reach values above 5 in case of highly interacting macromolecules (6), (8). However, these effects are strongly dependent on the type and amount of the hydrophobic substituents as well as on the hydrophilic backbone, i.e. polysaccharide type (neutral or ionic). As seen in Table 4, the Huggins constant for the unmodified CMC (DS = 1.0) was 0.2. A similar low value was reported for carboxymethylpullulan (0.29) (6). The intrinsic viscosities of all RO-CMC derivatives were markedly lower than for the unmodified CMC and the [k.sub.H] remained relatively small. Only for derivative RO-CMC-V, the [k.sub.H] value was close to 0.5. In accord with the low extent of esterification and presence of a mixture of acyl substituents, probably differing in the [C.sub.n] length and in the proportions, the relatively small increase of [k.sub.H] indicated the existence of associates formed through hydrophobic interactions of the substituents. The decrease of intrinsic viscosities of RO-CMC derivatives could be, in part, ascribed to a coil contraction of the molecular chains due to hydrophobic interactions (42) resulting in decrease of the hydrodynamic volume, which is directly related to viscosity. However, depolymerization of CMC during the esterification under microwave radiation (30) can not be excluded and a combination of both effects seems to be plausible.
TABLE 4. Intrinsic viscosity ([[eta]]), Huggins constant (kH), and critical aggregation concentration (c.a.c.) of CMC and RO-CMC derivatives. RO-CMC sample [[eta]] (dL/g) [K.sub.H] c.a.c. (g/L) I 6.43 0.34 1.40 II 8.16 0.21 2.29 III 7.67 0.33 1.80 IV 8.14 0.36 - V 6.49 0.48 1.89 VI 7.75 0.24 0.89 CMC(DS - 1.0) 10.26 0.20 None (-) Not determined.
The (CBB) method (26) has been used to test the RO-CMC derivatives for hydrophobic interactions. The absorption spectra of CBB in RO-CMC-III solutions of different concentrations (0.8-7.5 g/L) are illustrated in Fig. 5a. The shiftening of the maximum absorption peak towards higher wavelengths with increasing concentration of sample III and the RO-CMC derivatives indicated that the environment of CBB became apolar. The absorption change at 618 nm ([DELTA]A [sub.618]) as a function of RO-CMC concentration is illustrated in Fig. 5b. In the case of RO-CMC II, as for other RO-CMC derivatives, the absorb-ance at 618 nm increases significantly beyond a critical aggregate concentration. However, for unmodified CMC, absorbance remains constant or moderate increase. This confirmed that the increase of [DELTA] [A.sub.618] in the RO-CMC derivatives is truly attributable to the presence of hydrophobic clusters and not to other kinds of interactions, such as electrostatic ones, that could occur between CBB and the polysaccharide. A similar behavior was observed with the hydrophobically modified pullulans (36) and carboxy-methyl starch octenylsuccinate (43), In case of amphi-philic polymers the critical aggregation concentration (c.a.c.) corresponds to the onset of enhanced absorbance at 618 nm and indicates the formation of hydrophobic microdomains. For low molecular surfactants, c.a.c. corresponds to the critical micelle concentration (c.m.c.) (26). As seen in the Table 4 the c.a.c for RO-CMC-1I (2.29 g/ L) is similar to the c.m.c. in the Table 2 (2.44 g/L). However, as illustrated in Fie. 5b, the c.a.c. derived from the concentration dependence of [DELTA] [A.sub.618] for RO-CMC-III (1.80 g/L) was markedly lower than the corresponding c.m.c. value (2.63 g/L), similarly as for other RO-CMC derivatives (Tables 2 and 4). The different critical concentrations observed can be related to different type of organization of the macromolecules in solution, analogically as described in case of hydrophobically modified pullulans (36).
[FIGURE 5 OMITTED]
Like low molecular surfactants, amphiphilic polymers tend to minimize contacts between their hydrophobic segments and water molecules. This can be achieved either by adsorbing at the air/water interface or by self-association of hydrophobic segments in bulk solution leading to intra- or intermolecular aggregation. In general, the predominance of one of both phenomena or their simultaneous occurrence might be considered. In case of the RO-CMC-II derivative one can suggest that it preferentially adsorbs at the air/water interface and do not favor the formation of hydrophobic microdomains, and hence the c.m.c. and c.a.c. values are very close. Such behavior was reported previously for other hydrophobically modified anionic polysaccharides (36), (44), (45). As polymeric surfactants establishing exclusively intramolecular associations display no "c.m.c.-like" critical concentration (46), the different critical concentrations (c.m.c. and c.a.c.) observed in all the other RO-CMC derivatives (Tables 2 and 4) suggested the existence of intermolecular associations, however, intramolecular associations are likely to exist, too. These derivatives displayed low but distinct surface activity indicating that both phenomena occurred simultaneously, similarly as reported for others amphiphilic polysacharides (46), (47). It has to be noted that also the associative properties of the RO-CMC derivatives might be affected by the above mentioned composition and distribution of the acyl substituents. Further studies about the effect of type and proportion of fatty acyl groups on the emulsifying and associative properties of the CMC derivatives will be the subject of future investigations.
The results implied the rapeseed oil triacyl glycerides to be as efficient in the transesterification reaction with CMC as the complex of fatty acid methyl esters (MERO) produced from rapeseed oil (23). Similar yields of water-soluble RO-CMC derivatives were obtained under mild reaction conditions and the advantage of the microwave radiation used as heating source in shortening the reaction time up to 5 min was confirmed. Moreover, they point at the marked relation between the microwave power and the reaction time. At higher microwave power, the applied reaction time must be shortened in order to avoid loss of emulsifying efficiency. The water-soluble RO-CMC derivatives showed a very low esterification extent. Although their surface-tension lowering effect was low, some of the derivatives exhibited excellent emulsifying properties comparable to that of the control--the synthetic emulsifier Tween 20, They were shown to established hydrophobic associations in bulk solution while adsorbing on the air/water interface. Due to the mildly hydrophobized anionic polymeric backbone, the derivatives substantially contribute to the stabilization of the oil-water interface and, therefore, the creaming process started several hours later than in case of Tween 20. However, it has to be noted that, similarly as the tensio-active properties, the associative behavior of the RO-CMC derivatives are affected by the above mentioned structural diversity of the acyl substituents, their proportion and distribution. Studies about these supposed effects using defined CMC fatty acid esters will be the subject of future investigations.
The results suggested that suitable polymeric biosurfactants applicable as emulsifying agents can be prepared from CMC under mild reaction conditions using the microwave radiation. The presented microwave-assisted transesterification method might substitute the toxic, hazardous and time-consuming classical esterification processes in preparing polysaccharide-based surfactants. Of importance is also the proved applicability of rapeseed oil as acylation agent.
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Correspondence to: Vladimira Tomanova; e-mail: email@example.com Contract grant sponsor: Slovak grant agency Vega; contract grant number: 2/0062/09.
Vladimira Tomanova, (1) Iva Srokova, (1) Anna Ebringerova, (2) Vlasta Sasinkova (2)
(1) Faculty of Industrial Technologies, Trencin University of Alexander Dubcek, 020 01 Puchov, Slovakia
(2) Institute of Chemistry, Slovak Academy of Sciences, 845 38 Bratislava, Slovakia
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|Author:||Tomanova, Vladimira; Srokova, Iva; Ebringerova, Anna; Sasinkova, Vlasta|
|Publication:||Polymer Engineering and Science|
|Date:||Aug 1, 2011|
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