1. Here it means something like "to come into contact with" 2. This doesn't make sense as a full sentence. It would be helpful if you could provide more context as there are some articles and verbs missing. Let me know if you have any questions

Yeah the second one isn't complete or is otherwise lacking something. Colloidal particles are dispersed in a matrix, usually a liquid. The surface area of the colloidal particles is exposed to the molecules of the surrounding matrix. A small particle size leads to a larger surface-area-to-volume ratio; stabilization of the colloidal system is accomplished with amphiphilic substances like surfactants. More or less, exposed, refers to the 'interface' between the material in the dispersed colloidal particles and the surrounding matrix. Exposed just mean two different 'materials' are in open contact with one another, as this is a science-y thing, usually this is called an interface. This is analogous to Number 1, but in Number 1 the particles is a solid and the surrounding matrix is a gas...in this example, gas molecules from the surrounding gas 'matrix' will absorb onto the surface of the solid or be adsorbed at the interface.

Like so many English words, it derives from Latin. To best understand what it means, go back to the Latin roots. (There are websites for this. The key word is "etymology".) All of the modern meanings come directly from the roots: "Ex" = from, forth "ponere" = put, place So it means to "put forth", or in current lingo to "reveal", "explain", and so forth. For a long list of such words, use a thesaurus (also available online) to find a long list of synonyms for "expose". All of them mean something close to "put forth". I agree with what Sunny said.