I assume you've measured electrophoretic mobility and not zeta potential.  One word of caution - you can't quote zeta potential values to 2 decimal places - even the fist place before the decimal point is probably suspect. Zeta potential is the determined charge at the slipping plane and you do not say what your systems are.  With steric stabilization you could get a charge of close to 0 mV at the slipping plane but the system would be stable.  You could have - 30 mV for a 300 nm TiO2 and this would be unstable until more ionic additive (usually phosphate for inorganic oxides) is adsorbed on the surface.

Typo: 'first' and not 'fist' obviously...

We measured for three different nano clays those are showed increased negative values what that means? 

What do you mean 'we measured'?  How?  At what pH?  Was the pH the same for all the materials? More negative values mean more negative charge on the slipping plane.

Thanks. All are measured at neutral pH 7.

Sir can you give a reference and I detail principle involved in this measurements. 

I is in error in typos 

How did you stabilize the pH?  In buffer?

And on what instrument?

Zeta PALS Instrument was used for the measurements. 

Millipore water used as solvent for neutral pH 7. 

What is the difference between layer charge and zeta potential. 

Please provide me a reference and working principle and method. Electrode used in instrument.  Which method follow for measure . Among the electro kinetic effects.  Like electrophoresis, electro osmosis, streaming potential or sedimentation potential. 

You need background conductivity in the continuous phase for zeta potential measurement.  There's no conductivity in DI water and you'll get electrode polarization.  You can use Google to find out the difference between surface charge and that on the slipping plane.  Read Bob Hunter's book on Colloid Science and zeta potential.