Surface Tension

Discussion

The objective of the experiment is to use the capillary rise method to determine the surface tension of the working solution – in this case, the increasing concentrations of n-butanol solution.

In a solution, molecules experience intermolecular forces with each other. However, the molecules in the surface of the solution experience less intermolecular force because part of it is exposed to a different phase. Therefore, there is a tendency for the “bulk” solution to pull the molecules from the surface towards them. This concept is applied in a rain droplet, where because of the pull by the bulk part of the solution, it shapes into a form of a sphere – a shape with the least surface area. The rise of the solution in the capillary tube is the result of cohesion and adhesion.

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Cohesion is the attraction of molecules within the same phase while adhesion refers to the attraction of molecules of different phases; say the n-butanol solution and the walls of the capillary tube. If the adhesion force is stronger than the cohesion force, the walls of the capillary tube will be wet, which in turn attracts molecules from the bulk of the solution upward until the pressure exerted from outside (environment) of the capillary tube is equal to the forces that lifts the solution upwards.

This equilibrium point will be used to determine the height of the rise of the solution, which is a factor in determining the surface tension of the solution. Different concentrations of n-butanol solution were prepared in volumetric flasks. Then the radius of the capillary tube was identified by performing the capillary rise method using deionized water. With the given surface tension of water at 30 degrees Celsius, the radius was calculated: r=2? pgh Surfactants are molecules that have a nonpolar tail and a polar end.

It lowers the surface tension from two different phases because of its ability to “pull” the molecules toward the molecules in the surface area. N-butanol is a surfactant therefore, it is hypothesized that the surface tension will decrease as the concentration of this surfactant in the solution increases. However, in the experiment, the results stated otherwise, that the more concentrated the n-butanol in the solution is, the higher the surface tension it manifests.

The equation used to compute for the surface tension is: ? = pghr2 where p is the density, g is the acceleration due to gravity, h is the height of the solution and r is the radius of the capillary. Sources of error can come from the deviating temperatures of the balance room and the laboratory. The capillary rise method should have been performed immediately right after identifying the density of the solution since a little change in temperature could greatly affect the behavior of the solution.

Moreover, the long duration of time in performing the capillary rise method could possibly turn the solution back again to two layered phases, instead of a homogenous mixture therefore, what is measured is the height of the crude deionized wated rather than the solution. Also, another source of error could be the prolonged “stagnant” state of the other solutions in the volumetric flask where the alcohols present in the solution could possibly be turned into vapor state inside the flasks.

The behavior of the n-butanol is to converge to each other and replace the water molecules at the surface. The concentration of this surfactant becomes bigger than the molecules in the bulk which gives the excess of concentration denoted by: ? = -sRT where s is the slope of the best-fit line by plotting the surface tension against ln C (concentration in mol m-3), R is the ideal gas constant, and the T is the temperature in Kelvin. The value of ? , is used to calculate the value of the cross-sectional molecule of n-butanol, as well as the molecular radius of the chemical.

Conclusion

Although the result stated otherwise, the concept of surface tension and the relationship of the concentration of the surfactant were understood with further research of other related experiments. Despite this, the use of capillary rise method gave way to compute for the radius without directly measuring it, but instead by having a given surface tension and determining the other sufficient factors in the Laplace equation. Sample Calculations: radius of capillary= 2(0. 07118Nm)995. 67kgm39. 8ms2(0. 0320m)=0. 00046m urface tension= 910kgm39. 8ms20. 022m(0. 00046m)2=0. 045Nm excess concentration= -0. 0028. 314JKmol(303K)= 0. 00000079Jmol cross-sectional area= 10. 00000079Jmol10101m216. 022X1023molecules=2. 09X10^48A molecular radius= 2. 09X10^48Avalue of pi=8. 16X10^23A

Literature Cited

1. Chang, Raymond. Physical Chemistry for the Chemical and Biological Sciences. 3rd ed. Sausalito, CA: University Science Books, 2000. Print. Page 840. csustan.
2. Surface Tension and Soap Bubbles . 03 February 1999. 25 June 2012. <http://www. chem. csustan. du/chem2000/Exp5/Bkg. htm>.
3. Prpich, A. , et. al. Tension at the Surface: Which Phase Is More Important, Liquid or Vapor?. 2009 Value of density @ 30 degrees Celsius taken from: <http://van. physics. illinois. edu/qa/listing. php? id=2170>
4. Value of ideal gas constant taken from: < Mohr, Peter J. ; Taylor, Barry N. ; Newell, David B. (2008). "CODATA
5. Recommended Values of the Fundamental Physical Constants: 2006". Rev. Mod. Phys. 80 (2): 633–730. >. Value of surface tension of water @ 30 taken from: < Lange, p. 1663>

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