Impact of Surface Area on Dissolution
Yesterday, I started working through the Noyes-Whitney equation with the intrinsic dissolution rate (k2). Today, let’s discuss the impact of surface area on dissolution. The equation shows us that as the surface area of a formulation increases, so does the dissolution rate. Using this, a formulator can greatly increase or decrease the dissolution rate of a formulation by changing the surface area. This change in surface area can be achieved with the product being presented in a different form or altering its dissolution behavior.
If you want to achieve a quicker dissolution, then increase the surface area can be a good place to start. In solid oral dosage forms, the most common approach is to have a disintegrating dosage form or a capsule containing powder or beads. The cumulative surface area of all of this powder, beads, etc. is much higher than if a dosage form is non-disintegrating. In this way, you can have rates exponentially higher.
Not only can the product be made disintegrating, but we can also decrease the particle sizes as well to further increase these surface areas. For some products which need rapid dissolution, products may be presented in a powder or effervescent tablet format. Suspensions in liquid – or just having the product already dissolved in liquid – are common choices as well.
When we have disintegrating dosage forms, especially with smaller particle sizes, filtration becomes incredibly important. We always need to make sure that our filter is able to efficiently remove undissolved drug particles. This is not only a decision of filter pore size and chemistry, but also making sure that we filter as soon as possible after sampling (if not during) to prevent artificial dissolution. If you have filtration challenges or questions – I’m happy to answer those or point you to some other posts I’ve made.
If we want slower dissolution rates, we tend to want a non-disintegrating dosage forms. These products have a much lower surface area which can slow dissolution rates. The formulator can have these products erode slowly or choose other options such as polymer matrixes or osmotic tablets to control release.
We have a lot of leverage to increase or decrease the rate of our dissolution through the type and behavior of our formulation!

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Resource Person: Ken Boda