Whats the difference?
The most apparent difference between the two ties into the presence of groups. When someone is alienated, they are kicked out of leave a group they were previously in or had knowledge of. This leaves the person missing something they once had. When someone is isolated, they were never part of a group. This could be due to something keeping them from joining, or from them having no knowledge of unity anywhere; they are lost.
Where do we see these themes?
In the modernist era, it was common for writers to write about self consciousness. This includes what people feel when they are alone. Modernists played with the idea of diving deep into a persons mind to reveal their inner thoughts, which often included ideas of being alone. In postmodernist literature, authors were more fascinated with the idea of disintegration, or the idea that all pre existing groups dissolve into nothing. This has an effect in some stories that show the main character feel lost in a world where there is nothing present.