Ionisation polyelectrolytes

A major drawback of synthetic thickeners when used with dyes is their sensitivity to electrolytes. Most soluble dyes behave as highly ionised electrolytes and disperse dyes contain anionic polyelectrolyte dispersing agents unless they have been formulated with nonionic systems specifically for use with acrylic thickeners. Consequently there is a loss of viscosity this can be quite pronounced although it depends on circumstances, particularly on the dye concentration. As already mentioned, this can be alleviated to some extent by copolymerisation with acrylamide during manufacture. Otherwise it is necessary to try to eliminate all electrolytes from the system or to increase the concentration of thickener. Such measures have their limitations in practice, however. Alternative synthetic thickening... 

Polymers having ionisable groups along the chain are known as polyelectrolytes. They generally exhibit properties in solution which are quite different from those with non-ionisable structures. Examples of polyelectrolytes include polyacids like poly (acrylic acid) and hydrolysed copolymers of maleic anhydride, polybases like poly (vinyl amine) and poly (4-vinyl pyridine), polyphosphates, nucleic acids, and proteins. 

Of the preponderance of small ions, the colligative properties of polyelectrolytes in ionising solvents measure counterion activities rather than Molecular weight. In the presence of added salt, however, correct Molecular weights of polyelectrolytes can be measured by membrane osmometry, since the small ions can move across the membrane. The second virial coefficient differs from that previously defined, since it is determined by both ionic and non-ionic polymer-solvent interactions. 

Polyelectrolytes are long chain molecules bearing ionisable sites. It is not always possible to predict with confidence the extent to which polyelectrolytes behaviour is exhibited. Thus, polyacrylic acid in water is only weakly ionised and in dioxan it behaves as a typical non-electrolyte. It is usual to overcome the complications imposed by ionic interactions by the inclusion of simple salts and LS studies in salt-free solutions are rather rare. The problems have been discussed recently by Kratochvil137), whilst the review of Nagasawa and Takahashi138 constitutes one of the few devoted exclusively to LS from polyelectrolyte solutions. LS from many biopolymers such as proteins is, of course, extremely relevant in this context. 

Polyelectrolytic nature arises from the high-charge density due to ionisation of coordination sites on medium to large complexants. 

Some synthetic polyelectrolytes with a high degree of ionisation in dilute aqueous solutions... 

A polyelectrolyte is defined as a polymer in which the monomeric units of its constituent macromolecules possess ionisable groups. In non-aqueous solvents a polyelectrolyte shows the same behaviour as a normal polymer. In aqueous solutions, however, the charged groups of the polyelectrolytes may be surrounded by small, oppositely charged counter-ions. The conformational properties of polyelectrolytes in aqueous solutions are highly dependent on the nature and concentration of the ions present. 

One consequence of dealing with polyelectrolytes and naturally occurring ionisable macromolcules is that it is necessary to know the extent of ionisation before osmotic data can be interpreted. For example, suppose the sodium salt of a polyelectrolyte dissociates into u sodium ions and the single polyanion Pv, then the van t Hoff equation read... 

Polydectrolytes Polyacrylic acid, -(CH2-CH-COOH) -. At pH > 5, ionisation of the carboxylic acid groups occurs forming an anionic polyelectrolyte. Polyacrylic acid/polymethacrylic acid, (CH2-CH-COO ) - (CH2-C (CH3)-COO ). Naphthalene formaldehyde sulphonated condensates, Hgnosulphonates. 

Nucleic acids are polyelectrolytes. They are strongly acidic pK -1.0 for the ionisable H) and in their physiological environment are found as negative ions with cations such as Na", Mg +, spermine, spermidine and so forth. The term nucleic acid in common use, nearly always refers to the nucleate anion , and these compounds are usually dealt with as their salts. 

Since in 1948, W. Kuhn [70], J.W. Breitenbach [71], and A. Katchalsky [72] independently found that water-swollen macromolecules can convert chemical energy directly into mechanical work under isothermal conditions. They exploited the principle of reversible contraction and dilatation that is based on the reversible ionisation of suitable groups (for example carboxylic acid groups) of a polyelectrolyte, by alternating addition of alkali and acid, respectively. 

E. Cussler proposed an interesting application that uses the difference of adsorption parameters between the ionised and unionised forms of polyelectrolyte gels refers to extraction of pigments particles from a large volume of aqueous dispersion [134, 135]. Thus, a diluted aqueous solution, containing the dispersed... 

In this system, the components are weak polyelectrolytes and have a charge that varies with pH. The degree of ionisation, a, of these groups can be expressed [63] as ... 

Under definite conditions of hydration and of ionisation, this hydrophobic polyelectrolyte may present several phases lamellar, cylindric or micellar [8]. They correspond to different specific areas [8] and conductivities [9]. 

In the present paper we hope on the one hand to present an original conception of interpreting the optical activity of synthetic polymers, and on the other one to show that optical activity is not only a means to study conformational phenomena but can also prove an excellent method of detection, and even of study, of the ionisation of polyelectrolytes and of the complexation of macromolecules with ions or small molecules. In order to show both the advantages as well as the disadvantages of such a technique, we shall first of all give a brief reminder of the origins of optical rotation and circular dichroism and of their sensibility to secondary structures and chemical modifications. 

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