Dilational viscoelasticity and relaxation properties of interfacial electrostatic complexes between oppositely charged hydrophobic and hydrophilic polyelectrolytes.

Strongly adsorbing hydrophobic cationic polyelectrolyte, Eudragit RS, containing approximately 2.5 mol% of pendent hydrophilic trimethylammonium (TMA) groups irreversibly adsorbs from its methylene chloride (MCl) solution at the MCl/water interface and forms solid-like adsorption layers (ALs). Submitted to periodic dilational deformations with the standard radial frequency omega(0)=0.63 rad/s, these ALs exhibit relatively high dilational storage modulus E' approximately 20 mN/m and practically zero loss modulus E'' at the bulk concentration C(Eud)=4 x 10(-3)g/L. The frequency scanning of these ALs in the diapason omega=0.01-0.63rad/s and the approximation of the experimental dependences E'(omega) and E''(omega) by two relaxation times rheological model makes it possible to estimate the crossing frequency of these ALs determined from the condition E'(omega(c))=E''(omega(c)) as omega(c) approximately 5 x 10(-4)rad/s. Upon dissolving the hydrophilic anionic polyelectrolyte, chitosan sulfate (ChS), in the water phase (C(ChS)=3 x 10(-2)g/L) the electrostatic interpolyelectrolyte complexes form at the MCl/water interface. The elasticity moduli E' and E'' of these mixed AL did not undergo remarkable variations, but the crossing frequency is sharply increased by approximately 10 times becoming equal to omega(c) congruent with 3 x 10(-3)rad/s. The increase of omega(c) certifies for the liquefaction of mixed Eudragit RS/ChS adsorption layers. A remarkable decrease of the storage modulus down to E'=8 mN/m and simultaneous increase of the crossing frequency up to omega(c) congruent with 10(-2)rad/s occurs upon increasing the concentrations of both components, Eudragit RS and ChS, up to 0.1g/L. The liquefaction effect in the mixed ALs of oppositely charged polyelectrolytes was explained on the basis of the proposed relaxation mechanism. The effect of the liquefaction of adsorption layers of strongly adsorbing hydrophobic polyelectrolytes by formation of interpolyelectrolyte complexes with hydrophilic polyelectrolytes must be taken into account in the production of nano-capsules and nano-fibers.

Effect of electrolyte concentration on the dynamic surface tension and dilational viscoelasticity of adsorption layers of chitosan and dodecyl chitosan.

The effect of an external salt (AcONa) on the kinetics of adsorption and structure formation inside the adsorption layers (ALs) of chitosan (Ch) and dodecyl chitosan (C12Ch) as well as on the frequency dependence of the complex dilational elasticity modulus of these layers has been studied. The complex dilational elasticity modulus of adsorption layers of polymers has been measured on the drop tensiometer (Tracker, IT Concept, France) upon applying a small sinusoidal variation of the drop area with a given frequency, omega, in the range from 10(-2) to 0.63 rad/s and recording the variation of the surface pressure. It has been found that, in the absence of the salt, the dilational storage modulus, E'(omega), of ALs of both Ch and C12Ch is lower with regard to the loss modulus, E' '(omega), in the whole range of frequencies used, testifying for the liquidlike rheological behavior of these layers. With an increase of the salt concentration up to CAcONa > 0.1 M, the ALs become solidlike, as shown when E'(omega) > E' '(omega). Consequently, the characteristic frequency, omega c, corresponding to the intercept between the E'(omega) and E' '(omega) curves, gradually varies from omega c > 1 rad/s to omega c < 0.01 rad/s when the salt concentration is increased from zero to CAcONa = 1 M. Hydrophobically modified C12Ch, having long grafted alkyl chains, exhibited a higher sensitivity to the presence of salt than Ch: the former solidifies more readily and at lower salt concentrations than the latter. It has been found that the experimental E'(omega) and E' '(omega) curves exhibit two characteristic relaxation frequencies, omega 01 approximately 1 rad/s and omega 02 approximately 10(-3)-10(-2) rad/s, whose physical meaning and values were related to the structure of the ALs and to the competitive contribution of electrostatic and hydrophobic interactions between amino and nonpolar groups of Ch and C12Ch to the formation of a gel-like network inside the polymeric film at the interface.

Impact of bulk and surface properties of some biocompatible hydrophobic polymers on the stability of methylene chloride-in-water mini-emulsions used to prepare nanoparticles by emulsification-solvent evaporation.

The emulsifying and stabilizing ability of several hydrophobic (insoluble in water and soluble in volatile organic solvents) polymers, such as Eudragit RL, Eudragit RS, PLGA, PCL, and their mixtures, with regard to the methylene chloride (MC)-in-water mini-emulsions, has been compared to the viscosity of MC solutions and to the properties of adsorption and spread monolayers of these polymers. Eudragits RS and RL contain approximately 2.5 and approximately 5 mol% of pendent cationic trimethylammonium (TMA) groups per approximately 164 g/mol segments, whereas PLGA and PCL contain 1 and 2 polar carbonyl groups per 130 and 114 g/mol, respectively. The electrostatic attraction between the dipoles, formed by TMA groups and the condensed counter ions in the MC solutions, leads to the contraction of macromolecular coils of Eudragits, whereas the PLGA and PCL macromolecules, interacting by low polar carbonyl groups (with dipole moment mu = 2.7 D) retain more extended conformation in MC. This explains why the characteristic viscosities [eta] of MC solutions are much lower for the former polymers ( approximately 0.1 dL/g) with regard to PLGA and PCL solutions whose [eta] is equal to 0.3 and 0.6 dL/g, respectively. The ionization of TMA groups in contact with the water phase leads to the irreversible adsorption of Eudragits at the MC/water interface and to high decrease of the interfacial tension gamma (down to 4 mN/m for the 5% MC solutions). Whereas PLGA and PCL possessing low polar carbonyl groups adsorb poorly at the MC/water interface exhibiting gamma congruent with 28 mN/m. Higher stability of spread monolayers of Eudragits (pi* approximately 40 mN/m) with regard to PLGA and PCL (pi* < 20 mN/m) correlates well with higher interfacial activity of the former with regard to the later. The higher surface potential DeltaV of Eudragits (0.9 V) with regard to PLGA (0.3 V) and PCL (0.4V) is explained by the formation of electric double layer (DL) by the former, whereas the later contribute to the DeltaV only by cumulative dipole moments of carbonyl groups. The experimental values of surface potentials correlate well with the Gouy-Chapman model of the DL and the Helmholtz model of the monolayer. The ensemble of experimental results leads to the conclusion that higher emulsifying and stabilizing ability of Eudragits with regard to PLGA and PCL is due to higher adsorption activity of the former which form the corona of polymeric chains with ionized TMA groups around the droplets. It can be postulated that Eudragit polymers have good surface active properties which may allow manufacturing of biocompatible nanoparticles by emulsification-solvent evaporation method without surfactants.