Do Polysaccharides Such as Dextran and Their Monomers Really

Dec 3, 2005 - 5 King's College Road, Toronto, Ontario, Canada M5S 3G8. ReceiVed May 13, 2005. In Final Form: October 31, 2005. It has been reported in...
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Langmuir 2006, 22, 52-56

Do Polysaccharides Such as Dextran and Their Monomers Really Increase the Surface Tension of Water? Mina Hoorfar, Michael A. Kurz, Zdenka Policova, Michael L. Hair, and A. Wilhelm Neumann* Department of Mechanical and Industrial Engineering, UniVersity of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8 ReceiVed May 13, 2005. In Final Form: October 31, 2005 It has been reported in the literature that sugars such as dextrose and sucrose increase the surface tension of water. The effect was interpreted as a depletion of the solute molecules from the water-air interface. This paper presents accurate measurements of the surface tension of different concentrations of dextrose solution as well as its polymer (i.e., dextran). An automated drop shape technique called axisymmetric drop shape analysis (ADSA) was used for the surface tension determination. The surface tension measurement is presented as a function of a shape parameter, Ps, which has been used to quantify the range of the applicability of ADSA. The results of the above study show that dextrose solutions decrease the surface tension of water in contradiction to the results obtained from the weight drop method in the literature. The surface tension decreases continuously with increasing concentration. A similar effect was observed for the dextran solutions. To verify that the setup and the methodology are capable of accurately measuring increases in surface tension, a similar experiment was conducted with a sodium chloride solution with a concentration of 1 M. It is well-known that electrolyte solutions, e.g., sodium chloride, increase the surface tension of water. The results obtained from ADSA verify that the sodium chloride increases the surface tension of water by 1.6 mJ/m2. It is concluded that dextrose and dextran decrease the surface tension of water. Thus, there is no evidence of depletion. To identify the sources of discrepancy between the results of ADSA and those reported in the literature, the experiments were repeated for different concentrations and the rate of drop formation using the drop weight method. It was found that the rate of drop formation is most likely the source of error in the results reported in the literature.

1. Introduction It has been reported in the literature that polysaccharides and their monomers increase the surface tension of water.1,2 The elevation in surface tension was interpreted as a depletion of the sugar molecules from the water-air interface.2 The above conclusion is based on results obtained for sugar solutions from the early 1920s3,4 using the drop weight method. The experiments were repeated5 for sucrose solutions in buffered spring water (Volvic, pH 7.0) using the Wilhelmy plate technique. The results of ref 5 show that sucrose increases the surface tension of the buffered spring water, but the excess in surface tension is considerably smaller than that reported earlier. A few theoretical studies1,6 have been carried out on the determination of the surface tension of sugar solutions, but surprisingly little experimental work has been done. The accuracy of the results reported in the literature is also questionable since there are a number of experimental complications in the use of the above methods (i.e., the Wilhelmy plate technique and the drop weight method). With respect to the Wilhelmy plate technique, vapor adsorption at parts of the hangdown mechanism is a typical experimental concern which cannot be studied easily. In the use of the drop weight method possible complications were recognized for * To whom correspondence should be addressed. Phone: (416) 9781270. Fax: (416) 978-7753. E-mail: [email protected]. (1) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997; p 328. (2) Docoslis, A.; Giese, R. F.; van Oss, C. J. Colloids Surf., B 2000, 19, 147-162. (3) Washburn E. W., Ed. International Critical Tables; McGraw-Hill: New York, 1928; Vol. 4, p 469. (4) Clark G. L.; Mann, W. A. J. Biol. Chem. 1922, 52, 157-182. (5) Aroulmoji, V.; Aguie´-Be´ghin, V.; Mathlouthi, M.; Douillard, R. J. Colloid Interface Sci. 2004, 276, 269-276. (6) van Oss, C. J. Interfacial Forces in Aqueous Media; Marcel Dekker: New York, 1994.

experiments with solutions. It has been pointed out that the rate of drop formation must be slow enough to make sure that adsorption equilibrium is reached. However, questions of rate have not been quantified.7 This paper presents accurate surface tension measurements of different concentrations of dextrose solutions (ranging from 0.05 to 50 mg/mL) using a drop shape technique called axisymmetric drop shape analysis (ADSA). The ADSA technique and experimental procedure are explained in section 2. The surface tension measurements were obtained as the volume of the drop was changed by a stepper motor. To evaluate the results, surface tension is presented as a function of a quantitative parameter called the “shape parameter Ps” defined as

Ps )

|

∫02π∫0r r dr dθ - πR02| ∫02π∫0r r dr dθ θ

θ

(1)

where the numerator presents the absolute value of the difference between the projected area of the drop and an inscribed circle with radius R0 (i.e., the radius of curvature at the apex) and the denominator presents the projected area of the drop. The shape parameter Ps is used to quantify the range of the applicability of ADSA.10 It has been found10,11 that drop shape techniques, (7) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons Inc.: New York, 1990; p 23. (8) Lahooti, S.; del Rı´o, O. I.; Cheng, P.; Neumann, A. W. In Applied Surface Thermodynamics; Neumann, A. W., Spelt, J. K., Eds.; Marcel Dekker Inc.: New York, 1996; Chapter 10. (9) Spelt, J. K.; Vargha-Butler, E. I. In Applied Surface Thermodynamics; Neumann, A. W., Spelt, J. K., Eds.; Marcel Dekker Inc.: New York, 1996; Chapter 8. (10) Hoorfar, M.; Kurz, M. A.; Neumann, A. W. Colloids Surf., A 2005, 260, 277-285. (11) Hoorfar, M.; Neumann, A. W. J. Adhes. 2004, 80, 727-743.

10.1021/la0512805 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/03/2005

Sugar Solution Surface Tension Measurements by ADSA

Figure 1. Image of a pendant drop formed at the end of a 4 mm stainless steel holder.

which use numerical optimization to find the best fit between the Laplacian curve and an experimental profile, have limitations in the determination of the interfacial properties of the drops close to spherical shape. The limitation is probably due to the accumulation of round-off error that may be the ultimate limitation of all numerical schemes. In view of this limitation, the shape parameter was developed to quantify the range of the applicability of the methodology.10 In that study a “critical shape parameter” was defined on the basis of the minimum value of the shape parameter that guarantees an error of less than (0.1 mJ/m2 in the surface tension measurements of ADSA. It has been shown that the critical shape parameter is invariant from one type of liquid to another.10 In other words, the critical shape parameter is the same for liquids with different values of surface tension and density. However, the critical shape parameter depends on the size and the material of the holder used to form the drop. The more hydrophilic the material and the larger the size of the holder, the larger the range of the applicability of ADSA.10 In this paper, a 4 mm stainless steel holder (see Figure 1) was used in all ADSA experiments. The edge of the holder is sharp with an angle of approximately 45° to prevent spreading of the liquid onto the outer surface of the holder.12 The drop will form at the circular edge since the holder is hydrophilic. Thus, the contact diameter of the drop is definite. The sharp edge also facilitates the exact measurement of the drop volume. A previous study10 showed that the critical shape parameter for a 4 mm stainless steel holder is 0.16 regardless of the type of liquid. Therefore, only those drops whose shape parameters are larger than 0.16 should be considered for further analysis. To avoid any doubts about the applicability of ADSA, only drops whose shape parameters are larger than 0.4 are considered here. The results obtained from ADSA for dextrose solutions are presented in section 3.1. The surface tension of an aqueous solution of a polysaccharide (i.e., dextran) was also measured for different concentrations (ranging from 0.1 to 20 mg/mL). In the literature, a negative surface pressure has been reported for polymer solutions.13,14 This has been attributed to depletion that causes a measurable increase in the surface tension of liquid-vapor interfaces. (12) Yu, L. M. Y.; Lu, J. J.; Hoorfar, M.; Policova, Z.; Zhang, L.; Ng, A.; Grundke, K.; Neumann, A. W. J. Appl. Physiol. 2004, 97, 704-715. (13) Allain, C.; Ausserre´, D.; Rondelez, F. Phys. ReV. Lett. 1982, 49, 16941697. (14) Ausserre´, D.; Hervet, H.; Rondelez, F. Phys. ReV. Lett. 1985, 54, 19481951.

Langmuir, Vol. 22, No. 1, 2006 53

Specifically, depletion has been reported for a polysaccharide (polymer JR400) by Regismond et al.15 Because of the molecular weight influence on depletion, one could anticipate that if an increase in surface tension were found, it would become manifest at a lower concentration of the polymer compared to the monomer. The surface tension measurements of solutions of different concentrations of dextran are presented in section 3.2. To verify that the methodology and experimental setup are capable of measuring an increase in surface tension, the surface tension of a sodium chloride solution with a concentration of 1 M (i.e., 58.44 mg/mL) was also measured. It is well-known that water-soluble electrolytes, such as sodium chloride, raise the surface tension of water due to the electrostatic image repulsive interaction between electrolyte ions and air so that the electrolyte ions are repelled from the air-water interface (i.e., negative adsorption or surface depletion of the electrolyte ions).16 The surface tension measurements of the sodium chloride solution are presented in section 3.3. Finally, a number of experiments were performed for selected concentrations of dextrose solutions using the drop weight method, to clarify the discrepancy between the literature data and the main results of the present study, as described in section 3.4. 2. Materials and Method 2.1. Materials. Dextrose (catalog no. 49158 with a purity of 99.5%), dextran (catalog no. D-4751, Mw ) 68800), and sodium chloride (catalog no. 204439, with a purity of 99.999%) were supplied from Sigma-Aldrich. They were dissolved in distilled water, which was first demineralized and then glass distilled. 2.2. Axisymmetric Drop Shape Analysis. In the experimental setup of ADSA a spot white light source is used to illuminate the drop. A heavily frosted diffuser is used in front of the light source to provide a uniformly lit background and to minimize heat transfer to the drop during image acquisition. Also, a blue filter is positioned in front of the diffuser to reduce chromatic aberration of the white light.11 The drop is formed at the end of a stainless steel holder with a diameter of 4 mm (see Figure 1). A stepper motor is used to change the volume of the drop. To reduce the role of all possible dynamic effects, the drop volume is changed slowly with a rate of 0.04 µL/s. During the experiment, the images of the drop are acquired using a microscope and a CCD camera. The aperture of the microscope is fully open to minimize the depth of focus and to facilitate acquisition of a drop image at the meridian plane.11 The images are acquired with a frequency of one image per second and transferred to the host computer to run the software of ADSA. In the software of ADSA, the edge of the drop is detected using the Canny method,17 which has been found to be the most effective edge detection technique among those that were tested.11 Several profile correction procedures18,19 (including a correction for optical distortion associated with the microscope lens) are applied to the experimental profile to obtain the edge coordinates with high accuracy. The inputs to the numerical scheme of ADSA are the coordinates of the experimental profile, gravity, and the density of the liquid (measured at room temperature using the digital density meter Paar DMA 45 (Graz, Austria). In the numerical scheme, the experimental profile is fitted to a series of theoretical curves using an optimization method.8,20,21 The optimization method uses an objective function that specifies (15) Regismond, S. T. A.; Policova, Z.; Neumann, A. W.; Goddard, E. D.; Winnik, F. M. Colloids Surf., A 1999, 156, 157-162. (16) Ohshima, H.; Matsubara, H. Colloid Polym. Sci. 2004, 282, 1044-1048. (17) Canny, J. IEEE Trans. Pattern Anal. Mach. Intell. 1986, 8, 679-698. (18) Cheng, P. Automation of Axisymmetric Drop Shape Analysis Using Digital Image Processing. Ph.D. Thesis, University of Toronto, 1990. (19) Cheng, P.; Li, D.; Boruvka, L.; Rotenberg, Y.; Neumann, A. W. Colloids Surf. 1990, 43, 151-167. (20) del Rı´o, O. I.; Neumann, A. W. J. Colloid Interface Sci. 1997, 196, 136147. (21) Rotenberg, Y.; Boruvka, L.; Neumann, A. W. J. Colloid Interface Sci. 1983, 93, 169-183.

54 Langmuir, Vol. 22, No. 1, 2006

Figure 2. Surface tension as a function of the shape parameter for different pendant drops of water.

Hoorfar et al.

Figure 3. Surface tension measurements of water and two concentrations of dextrose.

the discrepancy between the theoretical curve and the actual profile as the sum of the squares of the normal distances between the measured points and the calculated curve. The objective function is minimized numerically to obtain the surface tension, the radius of the curvature at the apex, and other properties including the contact angle, drop surface area, and drop volume.8,20,21

3. Surface Tension Measurements and Evaluation The surface tension measurements of dextrose, dextran, and sodium chloride solutions are presented. All experiments were conducted at room temperature (i.e., 24 °C). A small drop was formed at the end of a 4 mm stainless steel holder. The volume of the drop was then increased to the maximum volume that the holder can support, i.e., just before the drop falls off. At this position the drop is large and hence well-deformed (with a noticeable neck area). The volume was then decreased to the initial size at the same rate. The shape parameter Ps was used to define the range of the acceptable drop shapes. In a previous study,10 a critical shape parameter of 0.16 was found; i.e., only drop shapes whose shape parameters are larger than 0.16 should be considered. To be well above this cutoff, only drop sizes whose shape parameters are larger than 0.4 (i.e., much larger than the critical value) are considered here. To compare the surface tension of the solutions with that of water, first the surface tension of water was measured. Figure 2 presents the surface tension values of water for four different runs. In each run, a new drop of water was formed. The average of the surface tension values of drops with Ps > 0.4 was calculated for each run (see Figure 2). The error limits of the surface tension values were obtained with a 95% confidence level. The average values for all four runs agree very well and are virtually identical with the literature value (i.e., 72.28 mJ/m2 at 24 °C).22 Therefore, it can be concluded that the results are accurate and reproducible. 3.1. Dextrose. The surface tensions of different concentrations of dextrose solution (ranging from 0.5 to 50 mg/mL) were measured. Figure 3 compares the surface tension values of two concentrations of the dextrose solution (i.e., 5 and 50 mg/mL) to that of water. The surface tension measurements obtained during increases and decreases of the volume are denoted by “open” and “solid” symbols, respectively. The surface tension remains quite constant as the volume is changed. It is observed that the surface tensions of the dextrose solutions are lower than the surface tension of water, in contradiction to the results in the literature.1-6 The experiments were performed four times for every concentration. The results of four runs are virtually consistent (similar to that of water shown in Figure 2). (22) Jasper, J. J. J. Phys. Chem. Ref. Data 1972, 1 (4), 948.

Figure 4. Surface tension measurements of water and two concentrations of dextran (Mw ) 68800). Table 1. Surface Tension Measurements of Different Concentrations of Dextrose at 24 °C concn density (mg/mL) (g/cm3) 0 (water) 0.5 1 2

0.9973 0.9973 0.9974 0.9977

γ (mJ/m2) 72.26 ( 0.01 72.16 ( 0.01 72.05 ( 0.02 71.93 ( 0.01

concn density (mg/mL) (g/cm3) 5 10 20 50

0.9991 1.0011 1.0046 1.0165

γ (mJ/m2) 71.79 ( 0.01 71.64 ( 0.02 71.39 ( 0.01 71.23 ( 0.02

Table 1 presents the density and surface tension values of dextrose solutions with different concentrations. The density values were measured using a digital density meter, Paar DMA 45 (Graz, Austria). The surface tension values are the average of the results of four runs. For each run, the surface tension value is also the average of surface tension values of drops with Ps > 0.4. The error limits of the surface tension values were determined at a 95% confidence level. Clearly, dextrose does not increase the surface tension of water. One would anticipate that a depletion effect would become manifest at least at higher concentrations. However, the surface tension decreases continuously with increasing concentration. There is no evidence of depletion. 3.2. Dextran. The surface tension values of different concentrations of dextran solutions (ranging from 0.1 to 20 mg/mL) were measured in a similar fashion. Figure 4 shows the surface tension as a function of the shape parameter for water and for solutions of two concentrations of dextran (0.5 and 20 mg/mL). Again, the experiments were performed four times for every concentration. Figure 4 shows the surface tension values of one run for each concentration. The results show that dextran decreases the surface tension of water similar to dextrose. Table 2 summarizes the density and surface tension values of solutions

Sugar Solution Surface Tension Measurements by ADSA

Figure 5. Surface tension measurements of water and a sodium chloride solution with a concentration of 1 M. Table 2. Surface Tension Measurements of Different Concentrations of Dextran at 24° Ca concn density (mg/mL) (g/cm3) 0 (water) 0.1 0.5 1 a

0.9973 0.9973 0.9974 0.9979

γ (mJ/m2) 72.26 ( 0.01 72.16 ( 0.02 71.92 ( 0.01 71.69 ( 0.01

concn density (mg/mL) (g/cm3) 2.5 10 20

γ (mJ/m2)

0.9990 71.55 ( 0.02 1.0010 71.39 ( 0.01 1.0044 71.26 ( 0.01

Dextran Mw ) 68800.

of different concentrations of dextran. The surface tension values are the average of the results of four runs. For each run, the surface tension value is also the average of surface values of drops with Ps > 0.4. Again, the surface tension decreases continuously with increasing concentration. 3.3. Sodium Chloride. The surface tension of a sodium chloride solution with a concentration of 1 M (58.44 mg/mL) was measured in a similar fashion. The density of the solution was found to be 1.0391 g/cm3. The surface tension values of water and the sodium chloride solution are presented in Figure 5. Again, the results of one of the four runs are presented. It is clear that the sodium chloride solution increases the surface tension of water. The average of the surface tension values of the sodium chloride solution is 73.85 ( 0.01 mJ/m2 (i.e., the average of the surface tension values of four runs). Thus, the increase in surface tension is 1.59 mJ/m2, in good agreement with results reported in the literature (i.e., 1.55 mJ/m2).23,24 It can be concluded that ADSA is capable of measuring increases in surface tension. 3.4. Comparison between the Results Obtained from the Drop Weight Method and ADSA. The results obtained from ADSA clearly show that dextrose decreases the surface tension of water (see Table 1). However, the results obtained from the drop weight method in refs 3 and 4 demonstrate an opposite effect. Despite the simplicity and success of the drop weight method, there has been a concern about the rate of drop formation and its impact on the results especially in the case of solutions.7 Therefore, the effect of the drop formation rate on the drop weight results for dextrose solutions was investigated. Drop weight experiments were performed at four different rates of volume increase (i.e., 0.009, 0.045, 0.09, and 0.45 µL/s) for a concentration of dextrose solution (i.e., 10 mg/mL). Also, to study the effect of concentration on the drop weight results, a series of experiments were performed for selected concentrations at a rate of 0.045 µL/s. (23) Weissenborn, P. K.; Pugh, R. J. J. Colloid Interface Sci. 1996, 184, 550563. (24) Matubayasi, N.; Matsuo, H.; Yamamoto, K.; Yamaguchi, S.; Matuzawa, A. J. Colloid Interface Sci. 1999, 209, 398-402.

Langmuir, Vol. 22, No. 1, 2006 55

Figure 6. A comparison between the results obtained in refs 3 and 4 using the drop weight method (DWM) and those obtained in this work using both ADSA and the DWM at four different rates of volume increase.

The surface tension values were calculated from

γ ) Mg/2πrf

(2)

where M is the mass of the falling drop of volume V, g is gravity, r is the radius of the holder (i.e., the capillary used to form the drop), and f is an empirical factor which is a function of r/V1/3. It has been reported in ref 25 that the results are most accurate when 0.3 e r/V1/3 e 1.2. Therefore, a holder with a diameter of 4 mm (i.e., the same size of holder used in ADSA experiments) was used so that the measurements fall in the above range. In these experiments, first a drop with a volume of 45 µL was formed manually (i.e., fast) at the end of the holder. Then the volume of the drop was increased at a given rate using a stepper motor. The maximum drop volume (i.e., the volume of the drop just before detachment) is approximately 72 µL. The volume of the falling drop is approximately 60 µL. The drop formation time (i.e., the time during which the drop volume is increased from the initial size (i.e., 45 µL) to the maximum size (i.e., 72 µL)) was approximately 50 min for the slowest rate (i.e., 0.009 µL/s) and 1 min for the fastest rate (0.45 µL/s). The masses of 10 drops (after falling from the holder) were measured at room temperature (i.e., 24 °C) using a microbalance (Mettler H20, Switzerland). Thus, the mass of a falling drop was obtained from the average of the mass of 10 drops. The volume of the falling drop was obtained from the measured mass and density. The values of the empirical factor were obtained from ref 25. Figure 6 presents the results of the drop weight experiments performed for different concentrations as well as different rates of volume increase, i.e., different drop formation times. To facilitate the comparison, the results of ADSA (see Table 1) and those reported in refs 3 and 4 were also incorporated into Figure 6. The results show that the drop formation rate has a significant effect on the drop weight output. Specifically, for a certain concentration (i.e., 10 mg/mL) significantly different surface tension values were obtained for different rates. As the rate decreases, the weight of the falling drops and hence the surface tension value obtained from eq 2 decrease. For instance, the surface tension value obtained at a rate of 0.45 µL/s is significantly higher than the surface tension of water. At the slowest rate, i.e., 0.009 µL/s, the surface tension value obtained from the drop weight method is lower than the surface tension of water, and it is in fair agreement with the value obtained from ADSA. At this rate, the drop formation takes about 1 h. Finally, the surface tension value (25) Lando, J. L.; Oakley, H. T. J. Colloid Interface Sci. 1967, 25, 526-530.

56 Langmuir, Vol. 22, No. 1, 2006

reported in the literature3,4 (i.e., 72.37 mJ/m2) can be readily reproduced at a rate of 0.07 µL/s. At this rate, the drop formation time is approximately 6 min. The effect of the rate of drop formation has been pointed out in ref 7, where a “slow rate of drop formation” is recommended. Intuitively, and in the absence of quantitative information, a drop formation time of 6 min might seem acceptable as a slow rate of drop formation. The present results show that it is not. In fact, an appropriate slow rate cannot be specified for a solution with unknown surface tension. For the dextrose solution of 10 mg/mL concentration, the comparison between the drop weight results and those of ADSA shows that acceptable measurements are obtained when the drop was formed extremely slowly (e.g., taking a full hour), which may not be practicable. Obviously, this minimum rate of drop formation may vary from one system to another. The concentration dependence of the drop weight results was also studied for selected concentrations at a rate of 0.045 µL/s. For water, the result reported in the literature3,4 and that obtained here using the drop weight method are close to the ADSA result, which is virtually identical with the known surface tension value of water (i.e., 72.28 mJ/m2 at 24 °C22). However, for dextrose solutions, there is a significant difference between the results reported in the literature and those obtained here. Unlike the results reported in the literature,3,4 the surface tension values obtained from both the drop weight method and ADSA decrease with increasing concentration, at the chosen rate of 0.045 µL/s. However, it is significant that the surface tension values obtained from the drop weight method are lower than the surface tension of water, although generally greater than those obtained from ADSA. As explained above, this difference is related to the rate dependence of the drop weight results.

4. Discussion The surface tension measurements of sugar solutions reported in the literature have been obtained from either the drop weight method3,4 or the Wilhelmy plate technique.5 The Wilhelmy method is deceptively straightforward. While the underlying analysis and the resulting equation are indeed rigorous and unambiguous, there can be a number of experimental complications. Vapor adsorption at parts of the hangdown mechanism and solute adsorption/precipitation on the plate or cylinder are typical examples which can cause an increase in the calculated surface tension value. There are difficulties in the use of the drop weight method for surface tension measurement of solutions, as shown in section 3.4. The rate of drop formation has a significant effect on the drop weight results. We have shown that the surface tension values of dextrose solutions increase to well above the surface tension of water as the rate increases. Clearly, such results are erroneous. The underlying reasons for this effect are not clear, and this elucidation is outside the scope of this study. However, possible parameters that one might want to consider are the (1) momentum of the liquid as pumped into the drop, (2) rate of adsorption, and (3) viscosity. The momentum is not likely to be the source of the above effect since the drop weight results obtained for water, both in this study and in the literature,4 are correct even at rates for which the results for dextrose solutions are not. The rate of adsorption also cannot be the reason because even if the adsorption equilibrium was not reached, the ultimate surface tension could not have been higher than the surface tension of water. The effect of viscosity is not clear, and it might be a starting point for future study. Drop shape methods on the other hand are free of these concerns. Specifically, ADSA has no adjustable parameters and

Hoorfar et al.

depends only on direct input of liquid density and drop profile coordinates, i.e., quantities that are well understood and readily measurable. The only remaining potential sources of errors can be (1) impurities in the water or solutes (i.e., dextrose and dextran) and (2) the inadequacies of the experimental technique (ADSA) in some specific situations (i.e., nearly spherical drop shapes). The measured surface tension of the distilled water used in these experiments agrees very well with the literature value (see Figure 2), indicating its purity. Highly purified dextrose and dextran were used (see section 2.1). Exploratory experiments were also conducted using dextrose obtained from various sources, and the results were virtually identical. Therefore, the purity cannot be the underlying reason for the patterns observed in this study. Finally, previous studies10,11,26 showed that ADSA is capable of producing surface tension values which are highly accurate and identical with the literature values. The only known limitation of ADSA is a lack of deviation of the experimental drop profile from spherical.10,11 The shape parameter Ps was developed to quantify the range of the applicability of ADSA.10,11 A critical shape parameter was defined on the basis of the minimum value of the shape parameter that guarantees an error of less than (0.1 mJ/m2 in the surface tension measurements. Thus, only drop shapes whose shape parameters are larger than the critical value should be considered. To avoid any doubts about the applicability of ADSA, the results presented in this paper are only for drop sizes whose shape parameters are much larger than the critical value.

5. Summary The surface tensions of solutions of dextrose and the corresponding polysaccharide (i.e., dextran) were determined using ADSA. The accuracy of the results was controlled using a quantitative criterion called the shape parameter Ps. The results of this study show that both dextrose and dextran solutions decrease the surface tension of water. The decrease in the surface tension increases with increasing concentrations. To verify that ADSA is capable of measuring increases in the surface tension of water, the surface tension of a sodium chloride solution (with a concentration of 1 M) was measured. The results show that sodium chloride increases the surface tension of water by a value of 1.6 mJ/m2, which agrees well with the results reported in the literature. The experiments were repeated using the drop weight method to find the source of the discrepancy between the results obtained from ADSA and those reported in the literature. The results show that the output of the drop weight method depends strongly on the rate of drop formation. It has been shown that for a drop formation time of approximately 1 h the drop weight result is in reasonable agreement with that obtained from ADSA. As the rate increases, the drop weight surface tension values increase. Therefore, the rate of drop formation can easily be the source of error in the results reported in the literature.3,4 It can be concluded that sugar solutions (unlike electrolyte solutions) do not increase the surface tension of water. There is no evidence of depletion. Acknowledgment. This work was supported by the Natural Science and Engineering Research Council (NSERC) of Canada under Grant No. 8278. Financial support through an Ontario Graduate Scholarship (OGS) (M.H.) is gratefully acknowledged. LA0512805 (26) Zuo, Y. Y.; Ding, M.; Bateni, A.; Hoorfar, M.; Neumann, A. W. Colloids Surf., A 2004, 250, 233-246.