I cannot really add to the above in terms of a basic explanation. I will however make a side point. Chaotropic agents and in the context of these kits we are talking about guanidium salts are designed to inhibit DNAses and RNAses and thus provide a stable extraction environment.

However their effect is generic and thus if guanidium salts bleed through into your final DNA/RNA they can potentially inhibit downstream reactions:  I have seen this. Salts in general - and of relevance to extraction guanidium in particular- absorb at 230nM and 260/230 ratios of 1.0 and increases the likely hood of a functional sample 

Hi there,

From Qiagen QIAquick® Spin Handbook

"The binding buffers in QIAquick Kits provide the correct salt concentration and pH for adsorption of DNA to the QIAquick membrane. The adsorption of nucleic acids to  silica surfaces occurs only in the presence of a high concentration of chaotropic salts (1), which modify the structure of water (2)."

Excellent explanation David. I remember gene clean !!

These explanations are so clear. Thanks.