I am learning about (introductory level) biological psychology and the synapse structure came into mind. I do understand that neurotransmitters are transmitted between two neurons via the synapse structure.

What I found about neurotransmitters was that there are several types of them, including amino acid, and other stuff (I don't really know chemistry..). The problem is that, according to the following picture, neurotransmitters are transmitted across the gap. What are the chemicals in the gap at a synapse? There must be some medium that transmits neurotransmitters right? enter image description here

My guess is that there is a special solution in which neurotransmitters are soluble. Any help will be appreciated. Thanks in advance!