as far as I know, yes, that's almost always true. sedimentation in your case might be due to the large particle size, I think.

A higher zeta only means that you probably have a stronger repulsion between your particles. There are no absolute truths regarding colloid stability. Yet, apart from the considerable amount of variables that may influence such behaviour, an increase of particle size from smaller nano leaning to the micro range will most likely be the cause of sedimentation.

That is the Surface Potential, which is responsible for the Colloidal Stability, not Zeta potential.

The Colloidal Stability in Electrolyte Solutions is described within DLVO Theory. It is impossible to describe major theory within answer on RG.

As far as zeta potential and colloidal stability concerns, almost higher the zeta potential (irrespective to sign) MORE the stable collide and vice-versa.

My dear Amir hossein Aref; here are few links which will be very useful to you. thanks

http://www.nature.com/nnano/journal/v3/n2/abs/nnano.2007.451.html

http://customers.hbci.com/~wenonah/riddick/ref.htm

http://www.sciencedirect.com/science/article/pii/S0925963504004078

http://www.sciencedirect.com/science/article/pii/S0021979702001765

Paulo Roque Lino @ I agree with your explanation. 

If particles size become  larger gravitational force come to the picture. 

Nature of solvent and also may have effect on stability as stated already by  Dr. Iva Ninova Kuznetsova. 

Colloidal dispersions are subject to several kinds of instability.  The particles can stick to each other (aggregation, coagulation, flocculation), they can stick to surrounding surfaces (deposition) and they can separate under gravity (sedimentation or creaming).  These mechanisms all can be counteracted by strong electrostatic repulsion, and the strength of that repulsion can be parameterized by the zeta potential.  In some cases, however, electrostatic interactions can destabilize an dispersion.  Details matter.  

Strong electrostatic repulsion can prevent aggregation by keeping colloidal particles well separated from each other and from surfaces.  To be effective, the electrostatic repulsion should be substantially larger than the thermal energy scale at length scales where van der Waals attraction and other molecular-scale attractions could become dominant.  This means that the range of the electrostatic interaction must be long enough, and typically means that the ionic strength must be low.  Adding salt can destabilize a dispersion that otherwise would be electrostatically stabilized.

In your case, it sounds like the particles' interaction with gravity may be the most important consideration.  Electrostatic interactions can help to keep particles dispersed, but only if the concentration is high enough that the buoyant force acting on each particle is balanced by electrostatic interactions with neighbors.  For particles much denser (or much less dense) than the surrounding medium, the relevant scale for separation can be small, so that the particles still will separate.

Finally, it is possible for electrostatic interactions to destabilize a dispersion.  This happens if some other element of your system is strongly polarizable or carries an opposite charge.

To learn more, follow Alexander Babchin's advice an look up the DLVO theory.

Stability of colloids is controlled by forces between them, but electrostatic interaction (zeta potential) is just one of the forces between colloids. Here are other foreces mediating stability such as van der Waals attraction, acid-base interations, or even molecular-scale attractions. These forces are described in the DLVO and extended DLVO theories.

http://www.sciencedirect.com/science/article/pii/S0021979783712051

Separation stability and colloidal stability may go in the same direct but not necessarily. You refer to separation stability: 'colloid which settles after a while'. If individual particles are large enough and/or density difference is high enough / viscosity low enough settling is observed.

Colloidal dispersions are all thermodynamically unstable. What is meant by colloidal stability is stability against agglomeration. Agglomeration will lead to lower separation stability as long as agglomerates can settle freely, but to stabilization when a space filling network is formed. In polar media high surface potential might be helpful in colloidal stabilization. But it is a sum of several stabilization mechanisms counteracting van der Waals attraction which determines colloidal stability. Results of zeta potential measurements are therefore misleading in a number of cases. In case of nanoparticles situation is often more complicated and correlation to zeta potential readings of instruments gets less good compared to larger submicron particles. Another example is silica which exhibit exceptional stability at its isoelectric point.

Hi. zeta potential of my microemulsion is too big. it reached +-10-5. I used Tween 40, which is nonionic surfactant for my microemulsion. what causes zeta potential too big ?

Pure error . microemulsions have no particles droplets in common sense, so you do not have an interface of particles/droplets were you can measure electrophoretic mobility.

zeta potential is directly proportional to the stability of nano particles higher the value of zeta potential increase the stability of nano particles   

The higher value of zeta potential increase the particles stability. we can say that when the zeta potential lays in the range 30/-30 mV (irrespective to sign) the stability of colloid is low and the probability to generate aggregates increases. but when the zeta potential value increase in the range from ±40 to ±60 we can obtain a good stability. in particular, the aggregation phenomenon seemed to occur when the zeta potential below 20 mV.

Zeta potential isn't enough to infer stability. You need a reasonable understanding of the attraction between particles and you need to know the ionic composition of the dispersion medium.

thank you