Mounting TV on a weird wall that has some material between the drywall and stud
Trying to mount an articulating arm that is designed to mount to a single stud will not stand up mounted to that type of wall construction. Even with long lag bolts there is just too much chance that mount will move around and cut into the drywall, become loose and make a mess of things. 

What you should be doing is to mount a panel of good quality 3/4" plywood that is large enough to span across at least two studs with three preferred. Make the height of this panel at least three times greater than the arm's mounting bracket. Use multiple large screws to mount the plywood into place into each stud. Now you can mount the articulating arm to the plywood surface and be assured of a secure attachment that will stay tight and firm against the wall.

You can sand and paint the plywood to match the wall color and it will hardly be noticable behind the TV. If you use flat head screws to mount the plywood they can be countersunk and then spackled over to make a nice flat surface.