x-ray reflectivity was conduted at beamline x19c at NSLS (Brookhaven National Laboratory,USA) with a liquid surface spectrometer and measurement techniques described elsewhere. the kinematics of reflectivity is illustrated in fig 1. for spercular reflection, the wave vector transfer Q=K(scat)-K(in) in only in the Z direction, normal to the interface:Q(z)=Q(y)=0 where x and y are in the plane of the interface,and Q(z)=(4P/a)sin ํC