Adsorption Isotherms

Adsorption isotherms show the variation of the quantity of macromolecules adsorbed per surface unit (or by mass) of the adsorbent as a function of the concentration in solution at the adsorption equilibrium state. Langmuir's model

FIG. 8 Normalized adsorbed amount of adsorbed oligonucleotide (dT35) as a function of time (at 20°C, 0.1 mg mL-1 of dT35, 0.1 mg mL-1 of latex particles, 10-2 M ionic strength, and pH 5). (From Ref. 11.)

[12], first defined to describe the adsorption of gas on solid supports, is often adapted to macromolecule adsorption onto colloidal particles. It is based on the following hypotheses: (1) the surface is homogeneous; (2) a single molecule is adsorbed per adsorption site (i.e., monolayer of interfacial molecules); (3) there are no lateral interactions between the adsorbed macromolecules; (4) there is no competitive adsorption; (5) the adsorption is reversible (equilibrium between adsorption and desorption).

By considering the adsorption equilibrium and the Langmuir adsorption hypothesis, the adsorbed amount (As) and the macromolecule concentration equilibrium (Ceq) are related to the adsorption affinity constant (K) via the following equation:

As K

where Asmax is the maximal quantity of the macromolecule adsorbed onto solid surface. The determination of K constant provides information on the affinity of the macromolecule for the adsorbent surface, whereas the plateau of the isotherm corresponds to the saturation of the surface under the adsorption conditions (expressed as Asmax). Although this model fits such isotherms quite well, interpretations are restricted as neither the equilibrium adsorption nor the conformation of the macromolecules at the interface is taken into account.

After kinetics, adsorption isotherms of nucleic acids onto latexes as a function of various parameters, such as pH, salinity, and temperature, are the starting point of any adsorption study. Both cationic and anionic polystyrene latexes were examined by Elaissari et al. [10]. The adsorption isotherms using anionic latexes particles were found to exhibit low affinity compared to the cationic latexes [9,10], using oligonucleotides of 27 and 30 nucleotides, respectively (Fig. 9a). The adsorption isotherms exhibit two marked domains: (1) a rapid increase in the adsorbed amount for low bulk concentration of oligonucleotide and then (2) a slight increase of the amount adsorbed before reaching a plateau value.

The isotherms of oligonucleotides adsorption onto cationic polystyrene particles bearing noncharged surfactant (Triton X-405, 0.1 mg/m2) have been also investigated (Fig. 9b). The presence of the interfacial surfactant on the colloidal particles affects the adsorption affinity and the maximal amount of adsorbed ODNs. In fact, low affinities and adsorbed amounts were observed principally at basic pH rather than at acidic pH, whereas high affinities are observed in the case of bare cationic latexes.

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