Pre 1923: The First National Pictures movies were made by Chaplin under a complicated arrangement in which he leased the movies to the company for 5 years and then gained the copyright afterwards - so they are arguably not all public domain under the 'everything is public domain if it was first published before 1923 in the USA rule'. Chaplin wasn't a US citizen so it adds a complicating factor here (as a UK citizen the copyright will be death +70 years on those films on which he has a copyright claim - he died 25th December 1977 so they won't fall into the public domain until 2048). I have found this copyright claim for a number of the Chaplin films from 1918 onwards here, pdcomedy has none of these films on the site.
I have put everything Charlie Chaplin has done that is in the public domain, apart from the following:
The only copy of A Jitney Elopement I could find was the restored copyright version.
Her Friend The Bandit, 1914: No copy of this exists, apparently.
The Bond was brought out by Essany in 1918 and is a compilation of scenes deleted from other movies. It was a very poor quality print.