When the surface is protected by {HEAD ~ an SiO2 film}, there exist {TAIL ~ a large number} of hydroxyl groups 
When the surface is protected by { HEAD ~ an SiO2 film } , there exist { TAIL ~ a large number } of hydroxyl groups
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH