I’m not sure that “alone” and solitude mean the same thing. Technically, they are synonyms but the concept of “alone” implies loneliness while solitude implies a choice to remain without company. Solitude speaks of private time while alone implies the absence of people.
ALONE – adjective & adverb
1. Having no one else present; on one’s own.
“she was alone that evening”
Synonyms: by oneself, on one’s own, all alone, solitary, single, singly, solo, solus.
2. Indicating that something is confined to the specified subject or recipient.
“we agreed to set up such a test for him alone”
Synonyms: only, solely, just.
1. The state or situation of being alone. “She savored her few hours of freedom and solitude.”
Synonyms: loneliness, solitariness, isolation, seclusion, sequestration, withdrawal, privacy, peace. “She savored her solitude.”
A lonely or uninhabited place.
Plural noun: solitudes
Synonyms: wilderness, rural area, wilds, backwoods.