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Why does boiling point increase start to diminish when alkanes get larger?

When dealing with unbranched alkanes, the general trend is for boiling points to increase as CH2 groups are added. However, the boiling point increase between smaller alkanes, such as from Methane to Ethane, is greater than the boiling point increase between bigger alkanes, such as from Nonane to Decane. Why does this happen?
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2 Answers

David's spot on in his responce, but also as alkanes, especially strait-chained, get larger, the ability for induced-induced dipole interactions also decreases as the number of carbon atoms increases, as there becomes a larger number of atoms for dispersion forces to act through increases.
With the larger alkanes, you adding a smaller percentage of extra mass and surface area, so the increases in intermolecular forces, and thus BP, are not as large, relatively.  From methane to ethane, you're nearly doubling everything.  From nonane to decane, the increase is less than 10%.