I read your explanation regarding the emulisification of biofuels and think you have mixed up the two major fuel types: Bioethanol (ethanol from a biological source rather than crude oil distillation) is blended with petrol not diesel. Ethanol is indeed hygroscopic and adsorbs water over time, especially in the higher blend ratios. In general, the compounds in petrol including those supplements with ethanol are more volatile than diesel and they evaporate at a faster rate than diesel.

Fatty acid methyl esters (FAMEs) are blended with diesel not petrol. These are relatively long chain compounds (typically C18 due to the prevalence of 18 carbon fatty acid in our vegetable oils) and are not hygroscopic. Diesel contains a less volatile suite of compounds compared to petrol and these bigger molecules are more waxy in nature. Any loss of the shorter chain, more volatile components through storage will leave the waxy compounds behind making the resulting fuel more viscous.