Atypical behaviour of special urea-formaldehyde pre-condensate
A phenomenon where hydration forces play the main role is the so-called critical degree of dilution (CDD), an untypical case of a limited dilution ability characteristic of some concentrated hydrophilic water systems, e.g. special prepared urea-formaldehyde pre-condensate. The volume of the original pre-condensate (approximately 50% w/w concentration), Vo, being gradually diluted with water or water solutions does not undergo any changes until a moment when the volume of added water or its solution exceeds a critical value, Vk. Now, the originally transparent water system becomes turbid due to precipitated microgel particles. The value CDD is (Vo + Vk)/Vo. The turbidity is cause by a coacervation of the originally homogeneous system by addition of water or water solutions.
Experimental modelling system of hydration forces. Evaluation of the effect of hydration forces by means of modelling water system, e.g. urea-formaldehyde pre-condensate.
The super-molecular phenomenon of this behaviour can be simply explained by concept of hydration forces functioning among interacting hydrophilic molecules or their fragments in a water environment. The condition of such behaviour is a presence of hydrated hydrophilic anisometric oligomer molecules. The parts or ends of these molecules influence each other by the attractive hydration forces, while other parts of molecules by the repulsive hydration forces. However, the effect of attractive forces must prevail to that repulsive hydration forces. The uniform distribution of oligomer molecules is prerequisite of following behaviour. When this system is diluted, molecules, their parts and ends are drawn away from each to other and their mutual force action is thus weakened. After exceeding the critical concentration or critical dilution state characterized by critical volume of added water, Vk , the coacervation takes place, owing to fluctuation of the affecting mutual forces and the kinetic energy of interacting oligomer molecules. In this case, the composition and properties of the separated coacervates correspond to the original concentrated system. The influence rate of the added substances on hydration forces can be evaluated by means of so called hydration factor
g = CDD/CDDo, where CDDo means the CDD value in distilled water. The value g > 1 means that the added substances in form of water solution decrease the attractive forces influence or increase action the repulsive hydration forces and vice versa.
By use of this modelling system it is possible to make a sophisticated characterization the role of hydration in papermaking suspension , predominantly a new concept of chemistry refining processes.
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