Although viscosity varies over a factor of more than two in the series from hexane to nonane, the surface tension at 25 degrees C increases by only about 20% in the same series. How would I account for this?
Surface tension is the result of intermolecular forces. Viscosity is also the result of intermolecular forces plus additional factors such as entanglement of long molecules. What intermolecular forces are involved from C6H14 to C9H20? How would the increasing length plus increasing London forces affect the viscosity? For more on this, check:
http://www.newton.dep.anl.gov/askasci/mats05/mats05069.htm
To account for the observed change in surface tension despite the significant variation in viscosity within the series from hexane to nonane, we need to consider the underlying molecular factors that influence both properties.
Viscosity is primarily determined by intermolecular forces and molecular size. In the present case, hexane and nonane have similar intermolecular forces (Van der Waals forces) and differ mainly in their molecular size. Larger molecules experience more steric hindrance and internal friction, resulting in higher viscosity.
Surface tension, on the other hand, is mainly influenced by intermolecular forces at the liquid-air interface. The molecules at the surface experience unbalanced forces, leading to a cohesive force that generates surface tension. Although the size of the molecules does affect surface tension, other molecular factors, such as polarizability and the presence of functional groups, can have a more significant impact.
In the series from hexane to nonane, the increase in surface tension can be attributed to changes in molecular interactions as the chain length increases. Longer alkane chains introduce more polarizable regions within the molecule, which increases the London dispersion forces, a type of Van der Waals force. These enhanced intermolecular forces contribute to higher surface tension.
To summarize, the difference in molecular size primarily affects viscosity, while changes in molecular interactions, such as increased polarizability with longer chain lengths, have a more pronounced effect on surface tension.