On the Sinking of Dispersed Oil
A very common misconception is that dispersants cause oil to sink to the sea bottom, thus polluting it. I always found it hard to believe, because oils generally are less dense than water and they will naturally tend to resurface rather than sink. Also, the effect of surfactants alone cannot increase the density of oil droplets above that of water.
So, does oil sink in the first place, and what can be the cause? To answer this question, the book "Understanding Oil Spill Dispersants: Efficacy and Effects - Committee on Understanding Oil Spill Dispersants: Efficacy and Effects, National Research Council - ISBN: 0-309-54793-8, 396 pages, 6 x 9, (2005)" comes handy.
First thing: dispersants disperse oil into minute droplets, that can be transported vertically down the water column. In simple terms, this means that while bulk oil remains on the surface, dispersed oil will reach a certain depth. As usual, many factors determine the depth that oil can reach but an indicative value is around 20 meters. The main driver here is waves.
So, at superficial look the effect of dispersants may appear as sinking the oil. But it is not.
But oil droplets have been observed to sink. That happens when denser than water aggregates with mineral particles are formed. In seawater there always is a certain amount of suspended mineral particles, and if they encounter oil droplets they can form aggregates; the buoyancy of these aggregates depends on the type and amount of oil and type and amount of minerals.
Oil droplet size is also important, and it appears that chemically dispersed oil is actually less prone to sinking than naturally (by wave action) dispersed oil. Also the type and amount of surfactants used to disperse oil has an influence.
In fact there are oil-siking agents designed for the purpose; they are made from sand treated with surfactants to make it oleophilic and thus adhere to the oil; the resulting mixture is stable and denser than water. In any case, intentionally sinking oil is a procedure used seldom if ever these days.
There is another plausible mechanisms for oil sinking I would say. Weathering: after light components evaporate and/or dissolve in water, a very dense residual tar may be left behind and sink to the bottom. Mineral particles can also be incorporated in the tar making it even more dense. I am not sure of what effect dispersants may have on this phenomenon. There are reports of accelerated loss of oil light fractions when using dispersants but no comprehensive studies.
So the conclusion is... not that much is known. But from what it is known. it appears that dispersants do not generally cause sinking of oil although they may have a more important role in some cases.