Is there a connection between frequency of use of tender words and close relationships? Maybe something about perception of importance of use of tender words and quality of close relationships?

I'm not sure how you define 'tender words', but the work of Hans IJzerman on Social Thermoregulation provides interesting ideas on how and why we use words like 'warm' or 'cold' on social relationships. 

I agree with the IJzerman reference. The use of the "warm" words - and physical warmth --with warm social meaning has also been tested in neuroscience. I think Naomi Eisenberger just put something out on this. Bargh work of course too.

In clinical relationship literature I found:

PERSONAL IDIOM USE AND AFFECT REGULATION

IN ROMANTIC RELATIONSHIPS

by CATHERINE NICHOLE MORELOCK, B.S., M.S.

Might have some information in there to help find more researchers on the topic.

Good luck!

Thanks Oriana - this is exactly what I was looking for.