The phrase "treacle of sentimentality" can refer to an overly sugary or cloying sentimentality in language or entertainment. Synonyms for this phrase might include "mawkishness," "sappiness," or "schmaltz." All of these words describe a kind of exaggerated emotionality that can seem insincere or manipulative, particularly in media that is designed to be emotionally affecting, like romance movies or greeting cards. Other related words might include "melodrama" or "corniness," which similarly describe over-the-top emotional themes. None of these words necessarily imply that sentimentality is bad in all cases -- after all, sometimes it can be nice to indulge in a little sweetness -- but they do suggest a certain kind of cheesy or saccharine style that can become overwhelming.