I.
Introduction:
At a given temperature and pressure, a body of liquid does work against the cohesive forces in the interior in order for the molecules to make a transfer into the surface and increase the surface area. As a result, the free energy tends to become greater in the surface than in the bulk of the liquid. The measure of free energy of the surface per unit area is known as surface tension. It is also the work required to expand the surface per unit area. It has been known that the surface tensions of pure solvents are different from those of solutions. Substances such as non-electrolytes lower the surface tension of the solvent, such that when they are mixed together, the solute acquires a high free energy, making them more unstable in the interior of the solution. Thus, only a small amount of work is required for them to come into the surface . They are said to be “positively absorbed” into the interface. On the other hand, when electrolytes are added to the solvent, surface tension is increased as the solvent molecules do more work for them to reach the interior of the solution. The solute is then is said
to be “negatively absorbed.” (Moore, 1962) A common way for determining determining surface tensions is through the use of the capillary rise method. The basis for this method is to measure the height, h, of the meniscus in a capillary tube of known inner radius, r. The rise or depression of h depends on the cohesive forces acting between the liquid molecules and on the forces of adhesion acting between the liquid and the walls of the tube. Two different liquids of known surface tension are used in the apparatus. Determination of the interfacial tension can be done using the simplified equation
wherein
() ( )
The surface excess concentration, calculated using the equation:
(7-1)
derived from the Gibbs isothermal equation can be
(7-2)
The temperature dependence of the surface tension of a liquid may be represented by the Katayama equation:
) ( ) (
(7-3)
where M is the molecular weight of the solvent, Tc the critical temperature and K the Katayama constant. This experiment aims to: (1) determine the surface tension of aqueous solutions (1propanol,2-propanol, sodium chloride detergent) at various concentrations; (b) determine effect of temperature; (c) obtain Katayama constant and the critical temperature of 1-propanol and 2propanol; and (d)calculate the surface excess concentration μ, and effective cross -sectional area per molecule (ECAPM).
II.
Materials and Methods
Figure 7.1 shows a schematic illustration of the apparatus used. The relative humidity, atmospheric temperature, and pressure were recorded. Water, acetone and ethyl acetate were used as calibration liquids and 1-propanol, 2-propanol and methanol as test liquids. 100mL of the solutions were placed in the solution containers and were equilibriated in a thermostatted bath at about 30°C. The apparatus was calibrated by measuring the differential capillary rise (∆h) and densities of the three calibration liquids. Using serial dilution, 0.05, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70 and 0.80 M aqueous solutions of the test liquids were prepared from a 1.0M stock solution and ∆h were again measured using 100mL of each concentration. For the electrolyte solutions, NaCl was used and 2.5M stock solution was prepared from the pure solid. From this, 0.50, 0.75, 1.00, 1.50 and 2.00M solutions of the electrolyte was prepared using
serial dilution. Again, ∆h was measured using solutions prepared. The same was done for 1, 2, 3, 4 and 5% (w/v) detergent solutions. The density of each solution was measured using the pycnometer:
(7-4)
The surface tension of the solutions were calculated using the working equation, (7-1) and the surface excess concentration using equation (7-2)
The differential capillary rise was then determined using pure test liquids from 30°C to 70°C at 5 degree interval.
Fig 7.1 Schematic diagram of the apparatus used for measuring surface tension using capillary rise method.
III.
Results and Discussion
The surface tensions of pure liquids and solutions at varying concentration and temperature were determined using the capillary rise method. The apparatus was first calibrated by measuring ∆h of water, acetone and ethyl acetate. The density of the atmosphere was determined at the given conditions of the experiment and using the following equation:
(7-5)
The densities as well as the surface tensions of the calibrating liquids were obtained from handbooks (Table 7.1). The constants of apparatus A and B were then obtained by plotting
vs. ∆h and getting the values of the slope and y-intercept, respectively. () Table 7.1 Data obtained in the calibration of the apparatus.
Calibrating liquids
∆h, cm
density, , g/mL
Water Acetone Ethyl Acetate
1.5 1 0.8
0.99565 0.771468242 0.8884
Surface tension, dyne/cm 71.1833 22.7910775 22.69494
, g/mL
density,
0.001151
80 y = 69.225x - 33.9 R² = 0.9596
70 60 o ρ ρ
/ y
50 40 30 20 10 0 0
0.5
1
1.5
2
∆h,cm
Fig. 7.2 Plot of
vs ∆h obtained from the calibration of the apparatus. ()
From the plot, the values of A and B were determined to be 69.224854 and 33.889775, respectively. It is also observed from the data that water has the highest magnitude of surface tension among all the calibrating liquids used. Because surface tension can be defined as the work needed to increase the surface per unit area, liquids having strong intermolecular forces will be expected to have higher surface tensions since greater work will be required to break these forces in order for the molecules to be able to come into the surface. Due to extensive hydrogen bonding, the cohesive forces in water are relatively large resulting to a high measure
of surface tension (Rosenbaum, 1970). Acetone and ethyl acetate, on the other hand, exhibit only dipole-dipole interaction and thus have lower surface tensions. The effect of concentration on surface tension was investigated using the aqueous solutions prepared from a 1.0M stock solution of 1-propanol and 2-propanol. The obtained values of A and B were substituted in equation (7-1) in order to calculate for experimental surface tensions of the test liquids. Tables 7.2 and 7.3 shows the data used for the calculation of surface tension. Table 7.2 Experimental surface tension of 1-Propanol at different concentrations. Concentration, M
ln C
mpyc+soln, g
Density, g/mL
∆h, cm
0.05 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-2.99573 -1.60944 -1.20397 -0.91629 -0.69315 -0.51083 -0.35667 -0.22314
26.1327 26.1379 26.1029 26.0918 26.1019 26.0866 26.0733 26.0628
0.995468 0.995996 0.992447 0.991322 0.992346 0.990795 0.989446 0.988382
0.55 1 1.65 1.6 1.9 1.8 1.9 2.1
Surface tension, dyne/cm 4.150175 35.14296 79.62211 76.1045 96.76779 89.76556 96.48471 110.049
Table 7.3 Experimental surface tension of 2-Propanol at different concentrations. Concentration, M
ln C
mpyc+soln, g
Density, g/mL
∆h, cm
0.05 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-2.99573 -1.60944 -1.20397 -0.91629 -0.69315 -0.51083 -0.35667 -0.22314
27.3167 27.3103 27.3046 27.2933 27.27 27.2056 27.2412 27.2421
0.994113 0.993494 0.992942 0.991849 0.989594 0.983362 0.986807 0.986894
4.5 4.1 3.9 3.8 3.7 1.6 1.9 2.3
Surface tension, dyne/cm 80.8413 73.01751 69.0924 67.07611 64.98775 24.18453 30.06009 37.78441
Generally for 1-propanol, we see that the magnitude of the surface tension increases with concentration while for 2-propanol, the surface tension generally decreases with increasing concentration. The effect of concentration on surface tension was further studied as well as the effect of the nature of solute by analyzing electrolytic and detergent solutions. Results are shown in Table 7.4 and 7.5.
Table 7.4 Experimental surface tension of NaCl solutions at different concentrations Concentration, M
ln C
mpyc+soln, g
Density, g/mL
∆h, cm
0.5 0.75 1 1.5 2 0.5
-0.69315 -0.28768 0 0.405465 0.693147 -0.69315
26.3185 26.4193 26.528 26.7075 26.8985 26.3185
1.014306 1.024526 1.035546 1.053745 1.07311 1.014306
2.4 2.6 2.6 2.5 2.7 2.4
Surface tension, dyne/cm 133.9795 149.4995 151.1094 146.4815 164.0175 133.9795
Table 7.5 Experimental surface tension of detergent solutions at different concentrations Concentration, %(w/v)
ln C
mpyc+soln, g
Density, g/mL
∆h, cm
1 2 3 4 5
0 0.693147 1.098612 1.386294 1.609438
27.3163 27.4291 27.4831 27.5565 27.5966
0.994075 1.00499 1.010216 1.017319 1.021199
1.9 0.9 0.7 0.6 0.4
Surface tension, dyne/cm 30.28173 10.95609 7.060945 5.12065 1.145012
Notice that large values of surface tension were obtained for the electrolytic solutions while relatively low values were calculated for the detergent solutions. It was found that the surface tension of solutions generally differ from those of pure liquids. Ionic salts, such as NaCl, for instance, increase the surface tension of aqueous solutions above the value for pure water due to electrostatic interactions. Greater work is done as the water molecules are pulled into the bulk of the liquid by the dissociated ions and a new surface is created. On the other hand, polar solutes and solutes having both polar and non-polar groups, upon its addition to water, tend to decrease the surface tension of the liquid as they concentrate more likely in the surface of the liquid than the interior due to opposing forces (Moore, 1962). Examples of such solutes are 1propanol and 2-propanol. While the –OH group on these compounds make them soluble to water, their alkyl groups, being non-polar, make the whole compound easier to be pushed out of the interior and into the surface, which reduces the surface tension of water by disrupting the Hbonds. The same is true for detergents, only the decrease brought about by these substances are generally much larger. This concentration dependence of surface tension may also be explained using the equation:
) (
(7-6)
where c is the concentration of the solute. This relation is known as Gibbs isotherm and is known as the surface excess concentration. A positive indicates that the solute added concentrates at the surface of the liquid and thus decreases the surface tension of the solvent
while a negative value indicates otherwise. This value was calculated in order to verify our observations on the effect of the added solutes. Values were also obtained for 1-propanol and 2-propanol solutions. Equation (7-2), which was derived from equation (7-6), was used to solve for . Plotting vs ln c, the value of is obtained from the slope, . The ECAPM, or the effective cross-sectional area per molecule, was also calculated by simply getting the reciprocal of . Table 7.6 shows the calculated values.
Table 7.6 Calculated surface excess concentration of different solutions. Surface excess concentration, molecule/ Ǻ2 -9.14395 1.62577013 -3.89479 4.132792
Solution 1-propanol 2-propanol NaCl Detergent
ECAPM, Ǻ2/molecule -0.10936 0.61509311 -0.25675 0.241967
The effect of temperature on surface tension was also studied using pure methanol and 2-propanol. The differential capillary rise was measured for every 5 degree interval from 30 to 70 C. Table 7.7 Experimental surface tension of methanol and 2-propanol at varying temperature. Surface Tension, dyne/cm
Temperature, K
Methanol
2-Propanol
303.25 308.55 313.15 318.15 323.15 333.25 338.15 343.15
67.67781 81.1028 75.73281 73.04781 70.36281 27.40282 16.66282 16.66282
43.4068357 52.52170773 54.04085307 55.55999841 57.07914374 58.59828908 58.59828908 31.25367299
m c / e n y d , n o i s n e t e c a f r u s
2000 1500 1000 500 0 300
310
320
330
340
Temperature, K
Fig. 7.3. Plot of surface tension vs. T for methanol
350
1500
, n o i s m1000 n c e t / e e n c y 500 a d f r u s
0
300
310
320
330
340
350
Temperature, K
Fig. 7.3. Plot of surface tension vs. T for 2-propanol Generally for methanol, the surface tension decreases with increasing temperature while this trend is not observed with 2-propanol. Supposedly, a rise in temperature results in a decrease in the surface tension of most liquids. The reason for this may simply be because of the expansion of the liquid or the disruption of the interactions between the liquid molecules as the temperature is increased. The temperature dependence of surface tension has been adequately represented by several known equations, which include the equation proposed by Katayama:
) ( ) (
(7-7)
where M is the molecular weight of the solvent, T c the critical temperature and K the Katayama constant. The relationship of and T is clearly seen in the equation. It is also observed that as T approaches T c, the value of approaches zero. This is because at T c the sample exists in the gas phase only and cannot be condensed back into its liquid form. The value for K and T c is calculated for both liquid samples using equation (7-7) and distributing K to T c and T. Plotting
) vs T, K is obtained from the slope, -K and T (
c from
the value of the y-intercept, KT c.
Table 7.8. Calculated Katayama constant and T c for methanol and 2-propanol Parameter Katayama Constant Experimental Critical T, K Theoretical Critical T, K % error
Methanol 31.86277 353.1594 513 -31.16%
2-propanol 1.198790047 1102.0782 536.78 105.31%
The errors encountered in this experiment which can be accounted for the percent errors calculated and inaccurate trends may be personal errors, such as improper dilution, incorrect readings of ∆h, and not letting the apparatus come into equilibrium with the thermostatted bath and instrumental errors such as incorrect measurement of the temperature by the thermocouple probe.
There are several methods used in determining some of which include the capillary rise method, the ring method, drop-weight method and the bubble drop method. The method
used in this exercise, known as the capillary rise method, is known to provide high accuracy in the determination of In this method, a glass tube of small internal diameter, r , is immersed in the test liquid and the liquid rise due to its surface tension is measured. If r is small enough, the meniscus may be taken to be spherical in shape, and the surface tension of the liquid can be calculated using the equation:
(7-8)
where is the contact angle. The rise or depression of h depends on the cohesive forces acting between the liquid molecules and on the forces of adhesion acting between the liquid and the walls of the tube. These forces determine the contract angle :
Fig. 7.4 Capillary rise of a liquid that wets the walls of the tube In the ring method, the surface tension is readily determined from the force required to pull a platinum ring of known radius R from the surface of a liquid (Masutani, 1984). For an idealized system, wherein the surface tension acts over the circumference of the ring, the force necessary to break the liquid film is equal to 4πRɣ . The surface tension, then, can be calculated using:
(7-9)
where P is the force necessary to lift the ring from the solution surface and F is the correction factor due to the shape of the liquid held up and ring dimensions (Daniels, et al, 1970).
The drop-weight method allows the calculation of from the physical properties of a drop of mass formed at the end of a tube with known external radius r. In this method, is determined by equating the downward force on the drop as it falls off the tube tip, mg, to the force acting upward, (Daniels et al, 1970).
(7-10)
Another method which can be used is the bubble drop method. Here, the calculation of depends on the maximum pressure that can exist in a liquid bubble with radius of curvature r. A tube is immersed into a liquid to a depth h and a bubble of an inert gas is formed at the end of the tube. The pressure of the gas then is balanced by the sum of the hydrostatic pressure at the tip and the pressure caused by the surface tension.
(7-11)
The surface tension is measured by increasing the gas pressure until the bubble detaches from the tip of the tube. From the measurement of maximum pressure is calculated using the equation given above (Rosenbaum, 1970).
Surface tension has several known practical applications which include its use in the study of macromolecular chemistry. Surface-tension measurements are also applied in biological sciences, such as bacteriology. Many of the agricultural phenomena which can be observed also involve surface tension such as the movement of soil moisture and passage of plant saps, to name just a few (Daniels et al, 1970).
IV.
Summary and Conclusion
The surface tension of pure liquids and solutions were determined using the capillary rise method. The apparatus was first calibrated using water, acetone, and ethyl acetate and determining their respective differential capillary rise, ∆h. It was known that depends on the strength of IMFA, such that the stronger the intermolecular force exhibited in a liquid, the greater will be . The concentration dependence of was also observed. Electrolytes, such as NaCl, upon addition to water, increases its as the water molecules are pulled into the interior and thus increasing the work expended. The opposite is true for non- or weak electrolytes such as 1propanol, 2-propanol, and detergent. The surface excess concentration and ECAPM were also calculated and negative values for 1-propanol and NaCl were obtained and positive values for 2-propanol and detergent. The effect of temperature on was also studied using methanol and 2-propanol. It was observed that for methanol is inversely related to T while this was not observed for 2-propanol. Generally, is supposed to decrease with increasing T until at T c where becomes equal to zero. T c was calculated for methanol and 2-propanol and the resulting values were 353.1594K and 1102.0782K, respectively. High percent errors were calculated for both liquids due to personal and instrumental errors encountered during the course of the experiment.
In addition to the capillary rise method used in this exercise, there are several other methods used for the determination of including the ring method, drop-weight method, bubble drop method and many others.
The measurement of surface tension has been found to be of many practical uses including its use in the study of some aspects of chemical and biological sciences.
V.
Sample Calculations From Table 7.2 and 7.3:
() (For 1-propanol) () ()
4.150174456 dyne/cm From Table 7.4 and 7.5:
(For 1-propanol) () () ()
From Table 7.6:
(for NaCl solution) Using LR analysis: slope= 16.30628; y-int= 148.6 slope= 16.30628=
()
()() -3.89479 molecule/Ǻ From Table 7.8:
) (For methanol) ( Using LR analysis: slope= -31.8628; y-int.= 11252.64 Slope=-31.8628=-K
K=31.8628
y-int.= 11252.64= KT c
Tc= 353.1594K
VI.
References: Daniels F. et al. 1970. Experimental Physical Chemistry . 7th ed. USA: McGraw-Hill, Inc. Moore, WJ. 1962. Physical Chemistry. 4th ed. Englewood Cliffs, NJ: Prentice-Hall. Rosenbaum EJ. 1970. Physical Chemistry. USA: Meredith Corporation Shoemaker, DP, CW Garland and JW Nibler. 2009. Experiments in Physical Chemistry. 8th ed. NY: McGraw-Hill Book Co., Inc. Masutani G. (July 1, 1984). “ A Review of Surface Tension Measuring Techniques, Surfactants, And Their Implications For Oxygen Transfer In Wastewater Treatment Plants. ” Retrieved Feb. 8, 2014 from
EXERCISE 7 SURFACE TENSION OF PURE LIQUIDS AND SOLUTIONS
Carmina Angelica E. Garcia
Groupmates: Aldwin Ralph Briones Kristine Joy Belgica Roxanne Del Rosario Hannah Angelie Olivarez
Date Performed: February 20, 2014 Date Submitted: February 28, 2014 CHEM 111.1 6L Leeza DC. Servidad
