Closing Android Web Browser inside an App does not close CodeIgniter session

Hi, I am building a Mobile App using CodeIgniter and PhoneGap/Cordova.

My App opens a local static HTML document in the android web browser and then I use window.open to redirect to an external url to the server where my web application is runing. I log in the web application, close the mobile app and when I open it again the sesión on the web application did not close when it should have because in the codeigniter config.php i set the variable 

$config['sess_expire_on_close'] = TRUE; 

which should close sessions when closing all Windows of a browser, it works well using any web browser on a pc but doesnt using the android web browser within my mobile App built using phonegap and compiled using phonegap build.

Any help would be very welcome.

Thanks a lot in advance.