Abstract:The self-consistent field Poisson–Boltzmann framework
is
applied to analyze equilibrium partitioning of ampholytic nanoparticles
(NPs) between buffer solution and polyelectrolyte (PE) polyanionic
brush. We demonstrate that depending on pH and salt concentration
in the buffer solution, interactions between ionizable (acidic and
basic) groups on the NP surface and electrostatic field created by
PE brush may either lead to the spontaneous uptake of NPs or create
an electrostatic potential barrier, preventing the … Show more
“…Insertion of the ampholytic nanocolloidal particle from the bulk of the solution into the PE brush leads to the change in the ionization degree of both basic and acidic monomer groups. The respective free energy change can be presented 68,69 as…”
Section: ■ Results and Discussionmentioning
confidence: 99%
“…Insertion of the ampholytic nanocolloidal particle from the bulk of the solution into the PE brush leads to the change in the ionization degree of both basic and acidic monomer groups. The respective free energy change can be presented , asnormalΔFion(z)/NΣkBT=f+ln(1−α+false(zfalse)1−αb+)goodbreak+(1−f+).25emln(1−α−false(zfalse)1−αb−)where α + ( z ), α b + and α – ( z ), α b – are the respective degrees of ionization of basic and acidic groups on the surface of the NPs placed at distance z from the grafting surface and in the bulk of the solution (at z = ∞), respectively. We remark that eq is a particular case of the more general equation derived in ref for the case when the particle surface is decorated by multiple types of cationic and anionic groups.…”
Section: Resultsmentioning
confidence: 99%
“…The position z * can be found from the equationψ(z*)=prefix−δnormalpHblog(e)which implies that pH( z *) = pH IEP . By taking the derivative of the free energy Δ F ion ( z ), eq , with respect to z, it is straightforward to demonstrate that Δ F ion ( z ) exhibits a maximum in the point of the charge inversion, z = z *true(∂normalΔFion(z)∂ztrue)z=z*=0;.25emtrue(∂2normalΔFion(z)∂z2true)z=z*<0;goodbreak0em2emnormalΔFion(z*)>0(see ref for detailed discussion).…”
Section: Resultsmentioning
confidence: 99%
“…Insertion of the ampholytic nanocolloidal particle from the bulk of the solution into the PE brush leads to the change in the ionization degree of both basic and acidic monomer groups. The respective free energy change can be presented , aswhere α + ( z ), α b + and α – ( z ), α b – are the respective degrees of ionization of basic and acidic groups on the surface of the NPs placed at distance z from the grafting surface and in the bulk of the solution (at z = ∞), respectively. We remark that eq is a particular case of the more general equation derived in ref for the case when the particle surface is decorated by multiple types of cationic and anionic groups.…”
Section: Resultsmentioning
confidence: 99%
“…13 It is anticipated that in general cases, both mechanisms driving spontaneous protein absorption by the PE brush both below and above the IEP may be involved. 64 In our previous studies, 68,69 we have systematically studied the effects of the buffer pH b and salt concentration on the uptake of nanoparticles (NPs) by negatively charged PE brushes using a simplified model of NPs comprising equal numbers of ionizable cationic and anionic groups on its surface with equal acidic ionization constants, K + = K − . The position-dependent differential free energy of the NP insertion into the brush and net particle charge were calculated within the nonlinear Poisson−Boltzmann approximation.…”
Electrostatic interaction of ampholytic nanocolloidal particles (NPs), which mimic globular proteins, with polyelectrolyte brushes is analyzed within mean-field Poisson−Boltzmann approximation. In accordance with experimental findings, the theory predicts that an electrostatic driving force for the particle uptake by the brush may emerge when the net charge of the particle in the buffer and the charge of the brush are of the same sign. The origin of this driving force is change in the ionization state of weak cationic and anionic groups on the NP surface provoked by interaction with the brush. In experimental systems, the ionic interactions are complemented by excluded-volume, hydrophobic, and other types of interactions that all together control NP uptake by or expulsion from the brush. Here, we focus on the NP−brush ionic interactions. It is demonstrated that deviation between the buffer pH and the NP isoelectric point, considered usually as the key control parameter, does not uniquely determine the insertion free energy patterns. The latter depends also on the proportion of cationic and anionic groups in the NPs and their specific ionization constants as well as on salt concentration in the buffer. The analysis of the free energy landscape proves that a local minimum in the free energy inside the brush appears, provided the NP charge reversal occurs upon insertion into the brush. This minimum corresponds either to a thermodynamically stable or to a metastable state, depending on the pH offset from the IEP and salt concentration, and is separated from the bulk of the solution by a free energy barrier. The latter, being fairly independent of salt concentration in height, may strongly impede the NP absorption kinetically even when it is thermodynamically favorable. Hence, change reversal is a necessary but insufficient condition for the uptake of the NPs by similarly charged polyelectrolyte brushes.
“…Insertion of the ampholytic nanocolloidal particle from the bulk of the solution into the PE brush leads to the change in the ionization degree of both basic and acidic monomer groups. The respective free energy change can be presented 68,69 as…”
Section: ■ Results and Discussionmentioning
confidence: 99%
“…Insertion of the ampholytic nanocolloidal particle from the bulk of the solution into the PE brush leads to the change in the ionization degree of both basic and acidic monomer groups. The respective free energy change can be presented , asnormalΔFion(z)/NΣkBT=f+ln(1−α+false(zfalse)1−αb+)goodbreak+(1−f+).25emln(1−α−false(zfalse)1−αb−)where α + ( z ), α b + and α – ( z ), α b – are the respective degrees of ionization of basic and acidic groups on the surface of the NPs placed at distance z from the grafting surface and in the bulk of the solution (at z = ∞), respectively. We remark that eq is a particular case of the more general equation derived in ref for the case when the particle surface is decorated by multiple types of cationic and anionic groups.…”
Section: Resultsmentioning
confidence: 99%
“…The position z * can be found from the equationψ(z*)=prefix−δnormalpHblog(e)which implies that pH( z *) = pH IEP . By taking the derivative of the free energy Δ F ion ( z ), eq , with respect to z, it is straightforward to demonstrate that Δ F ion ( z ) exhibits a maximum in the point of the charge inversion, z = z *true(∂normalΔFion(z)∂ztrue)z=z*=0;.25emtrue(∂2normalΔFion(z)∂z2true)z=z*<0;goodbreak0em2emnormalΔFion(z*)>0(see ref for detailed discussion).…”
Section: Resultsmentioning
confidence: 99%
“…Insertion of the ampholytic nanocolloidal particle from the bulk of the solution into the PE brush leads to the change in the ionization degree of both basic and acidic monomer groups. The respective free energy change can be presented , aswhere α + ( z ), α b + and α – ( z ), α b – are the respective degrees of ionization of basic and acidic groups on the surface of the NPs placed at distance z from the grafting surface and in the bulk of the solution (at z = ∞), respectively. We remark that eq is a particular case of the more general equation derived in ref for the case when the particle surface is decorated by multiple types of cationic and anionic groups.…”
Section: Resultsmentioning
confidence: 99%
“…13 It is anticipated that in general cases, both mechanisms driving spontaneous protein absorption by the PE brush both below and above the IEP may be involved. 64 In our previous studies, 68,69 we have systematically studied the effects of the buffer pH b and salt concentration on the uptake of nanoparticles (NPs) by negatively charged PE brushes using a simplified model of NPs comprising equal numbers of ionizable cationic and anionic groups on its surface with equal acidic ionization constants, K + = K − . The position-dependent differential free energy of the NP insertion into the brush and net particle charge were calculated within the nonlinear Poisson−Boltzmann approximation.…”
Electrostatic interaction of ampholytic nanocolloidal particles (NPs), which mimic globular proteins, with polyelectrolyte brushes is analyzed within mean-field Poisson−Boltzmann approximation. In accordance with experimental findings, the theory predicts that an electrostatic driving force for the particle uptake by the brush may emerge when the net charge of the particle in the buffer and the charge of the brush are of the same sign. The origin of this driving force is change in the ionization state of weak cationic and anionic groups on the NP surface provoked by interaction with the brush. In experimental systems, the ionic interactions are complemented by excluded-volume, hydrophobic, and other types of interactions that all together control NP uptake by or expulsion from the brush. Here, we focus on the NP−brush ionic interactions. It is demonstrated that deviation between the buffer pH and the NP isoelectric point, considered usually as the key control parameter, does not uniquely determine the insertion free energy patterns. The latter depends also on the proportion of cationic and anionic groups in the NPs and their specific ionization constants as well as on salt concentration in the buffer. The analysis of the free energy landscape proves that a local minimum in the free energy inside the brush appears, provided the NP charge reversal occurs upon insertion into the brush. This minimum corresponds either to a thermodynamically stable or to a metastable state, depending on the pH offset from the IEP and salt concentration, and is separated from the bulk of the solution by a free energy barrier. The latter, being fairly independent of salt concentration in height, may strongly impede the NP absorption kinetically even when it is thermodynamically favorable. Hence, change reversal is a necessary but insufficient condition for the uptake of the NPs by similarly charged polyelectrolyte brushes.
A large number of experimental studies have demonstrated that globular proteins can be absorbed from the solution by both polycationic and polyanionic brushes when the net charge of protein globules is of the same or of the opposite sign with respect to that of brush-forming polyelectrolyte chains. Here, we overview the results of experimental studies on interactions between globular proteins and polycationic or polyanionic brushes, and present a self-consistent field theoretical model that allows us to account for the asymmetry of interactions of protein-like nanocolloid particles comprising weak (pH-sensitive) cationic and anionic groups with a positively or negatively charged polyelectrolyte brush. The position-dependent insertion free energy and the net charge of the particle are calculated. The theoretical model predicts that if the numbers of cationic and anionic ionizable groups of the protein are approximately equal, then the interaction patterns for both cationic and anionic brushes at equal offset on the “wrong side” from the isoelectric point (IEP), i.e., when the particle and the brush charge are of the same sign, are similar. An essential asymmetry in interactions of particles with polycationic and polyanionic brushes is predicted when fractions of cationic and anionic groups differ significantly. That is, at a pH above IEP, the anionic brush better absorbs negatively charged particles with a larger fraction of ionizable cationic groups and vice versa.
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