Interesting question, I have never thought about that. I am not sure that the reason is due to the size of the units. Or course, the atoms of a crystal are smaller than molecules, but you don't need many units to build a nanocrystal, so the reason should be somewhere else.

I rather think that big organic crystals are observed because of the way they are usually prepared, i.e. from a solution which is slowly evaporated. In these conditions, big single crystals can be obtained, just like big crystals of various salts can be obtained in similar conditions. So in my opinion, there is no real difference, except the way organics are usually separated and purified, not often used for inorganic materials.

Alain

Some INorganic crystals think that they are normal, but people are being nano )

http://j-times.ru/wp-content/uploads/2010/06/z6a.jpg

Many organic compounds have a molecular lattice - and perfect crystal structure takes time, so if you reach supersaturation too fast, get oil instead of crystals. Over time, the oil can turn into a crystal.

Inorganics may form an ionic lattice, a much more stronger with less stringent requirements on the orientation of the fragments. Therefore, in the case of supersaturation, many centers of crystallization is produced . Without the addition of surfactants, fine crystals eventually becoming larger.

However, in the case of organic crystals one can form very small crystals. You can read for yourself how process chemists have a lot of trouble with irbesartan. The molecule consists essentially of rigid and polar cyclic moieties. http://books.google.ru/books?id=I_nwhvhp1BAC&pg=PA358&dq=irbesartan+crystals&hl=ru&sa=X&ei=UbyxUoHxLKSQ5ASmpoDoDw#v=onepage&q=irbesartan%20crystals&f=false

The reason is not the size of the unit, rather its polarity and stereochemical flexibility.

i agree with prof Andrie Rogoza

Thank you very much for answers. These are really helpful for me !

The conditions to prepare organic crystals mostly are different than in case of inorganic crystals. The growth conditions generally slow which is advantageous to prepare large crystals. The diffusion of larger organic moelcuels are generalyl slower, etc.

This question is scientifically incorrect , since from the observation it comes out that one can find SiO2 (alfa-quartz) single crystals (in nature) weighing 20 kilos, along with mica crystals (in natural pegmatites ) several meters large... Calcite (CaCO3) crystals can easily reach cm - size in geological or lab environment, while they always are nano or microcrystals in all biomineralization phenomena.

From crystal growth theoretical considerations, it follows that the parameters controlling the size of crystals are:

a) the distance of the system from the equilibrium (supersaturation)

b) the temperature

c) the interfacial tension between the crystal phase and the surrounding growth medium

a), b) and c) control the frequency of crystal nucleation ( number of nuclei per unit volume and unit time). Then, the final size of the growing crystals will depend on the "availability" of crystallizing material... ( Gypsum crystals from Naica mine are eleven meters long, since they grew at very low supersaturation, at moderate T close to 50°C, in natural caves containing thousands of cubic meters of aqueous solution)

In general, the size of inoranic crystals is typically much more larger, than these of organic crystals.

@Dino Aquilano

The art of professorship mean the ability to give correct answers to the wrong questions)))

Can you explain influence of "interfacial tension between the crystal phase" on size of crystals as you mentioned in c) ? For example, it is close to zero? and at maximal value? What results will obtained?

Andrei is right :). The championship between the size of inorganic and organic crystals will never be finished, but I think, the question related to the laboratory crystallization techniques and not natural minerals or some special artificially-grown single crystals (e.g. elemmental Si).