“…As a result, even if a sigmoidal shape of the titration curve is obtained, the slope deviates from the Henderson-Hasselbalch behavior (equation 4). Several models can be used to describe this deviation: a smeared out charge model [40], a Poisson-Boltzmann description [41], a "neighbour effect" model [44], or Monte-Carlo simulations either with implicit or explicit counterions [45][46][47]. One even simpler approach is to consider the pH value for which half of the sites are ionized (α = 0.5, horizontal line in Figure 1), the midpoint of the ionization process, as an estimate of an effective pK, noted p α hereafter.…”
The ionization of short weak polyelectrolytes in dilute solutions is investigated by 1 H Nuclear Magnetic Resonance (NMR). The increase of the pH of the medium causes both the ionization of the polyacids and, due to the intramolecular repulsions, the swelling of the chains. As a result, a noticeable effect of the pH change on the NMR chemical shift δ , and on the self-diffusion coefficient D is observed. The ionization of the polyelectrolyte occurs at higher pH as the length of the chain increases. The variation of the selfdiffusion coefficient with pH exhibits the opposite dependence on the length of the chain. However, no detectable effect of the length of the polyelectrolyte chain on the evolution of the chemical shift with pH is observed. These apparent contradictions are examined to clarify the impact of the chain length on the polyelectrolyte properties, and the counterion role in the ionization process.
“…As a result, even if a sigmoidal shape of the titration curve is obtained, the slope deviates from the Henderson-Hasselbalch behavior (equation 4). Several models can be used to describe this deviation: a smeared out charge model [40], a Poisson-Boltzmann description [41], a "neighbour effect" model [44], or Monte-Carlo simulations either with implicit or explicit counterions [45][46][47]. One even simpler approach is to consider the pH value for which half of the sites are ionized (α = 0.5, horizontal line in Figure 1), the midpoint of the ionization process, as an estimate of an effective pK, noted p α hereafter.…”
The ionization of short weak polyelectrolytes in dilute solutions is investigated by 1 H Nuclear Magnetic Resonance (NMR). The increase of the pH of the medium causes both the ionization of the polyacids and, due to the intramolecular repulsions, the swelling of the chains. As a result, a noticeable effect of the pH change on the NMR chemical shift δ , and on the self-diffusion coefficient D is observed. The ionization of the polyelectrolyte occurs at higher pH as the length of the chain increases. The variation of the selfdiffusion coefficient with pH exhibits the opposite dependence on the length of the chain. However, no detectable effect of the length of the polyelectrolyte chain on the evolution of the chemical shift with pH is observed. These apparent contradictions are examined to clarify the impact of the chain length on the polyelectrolyte properties, and the counterion role in the ionization process.
“…Indeed, while the p K A value is unique and well-defined for a monomeric acid, weak polyelectrolytes such as poly(acrylic acid), usually display an apparent broad p K A distribution with a mean apparent ⟨p K A ⟩ value larger than that of its monomeric constituent. The phenomenon is due to the polymer architecture and originates from the interactions between neighbor sites along the polymer chain: the presence of an already ionized site diminishes the ionizability of the neighbors, resulting in an apparent larger p K A . , 13 Derivatives ∂α B/A (1) /∂ p H (1) and ∂α B/A (2) /∂ p H (2) of the titration curves of Figure 5 of, respectively, (S 53 A 39 ) and A 111 both at 2.5 wt % in salt-free water. The lines are guide for the eye. On the other hand, the dispersion of (S 53 A 39 ) in the absence of NaOH (i.e., α B/A (1) = 0) is a stable, milky dispersion, of starting pH: pH 0,0 (1) = 4.9.…”
We consider a symmetrical poly(styrene- stat-(acrylic acid))- block-poly(acrylic acid), i.e., PSAA- b-PAA, diblock copolymer, with a molar fraction phi AA = 0.42 of acrylic acid, in the more hydrophobic PSAA statistical first block. We investigate its structural behavior at constant concentration in water using small-angle neutron scattering (SANS) by varying (i) the ionization of its acrylic acid motives via the pH by adding NaOH and (ii) the ionic strength of the solution by increasing the NaCl salt concentration c S. We present the resulting morphological phase diagram {pH, c S}, in which we identified two different lamellar phases presenting a smectic long-range order at small-to-intermediate ionizations and a spherical phase with a liquid-like short-range order at larger ionization. In the low-ionization regime, the first lamellar phase comprises a water-free PSAA lamellar core surrounded by a dense poly(acrylic acid) brush swollen with water. Its mostly hydrophobic core still being glassy, this phase is unable to reorganize and is frozen in. A detailed analysis of the SANS data shows the osmotic nature of the polyelectrolyte brush, in which the Na+ counterions are confined so that local electroneutrality is satisfied. Above the pH at which the PSAA statistical block starts ionizing, the PSAA lamellar core melts. The second lamellar phase identified then comprises a PSAA core thinner than that of the frozen-in previous phase, implying a significant increase of the core/water interface and a decrease of the brush surface density. The transition from the first lamellar phase to the second one can be quantitatively shown to result from the balance between the two contributions: (i) the extra interfacial cost between the thinner core and water and (ii) the associated gain in entropy of mixing for the counterions confined inside the brush. At even higher ionization, the diblocks finally form spherical objects with a very small, pH-dependent aggregation number and reach an apparent onset of self-association. When the highest ionization investigated is reached, the cores of these final spherical core-shell objects are found to contain a significant amount of water. We thereby demonstrate that at constant concentration, pH, and ionic strength both trigger a transition from frozen to molten hydrophobic phases as well as unexpected morphological transitions.
The self-assembly in water of an amphiphilic P(nBMA(50%) -stat-DMAEMA(50%) )(100)-b-PDMAEMA(235) diblock copolymer based on hydrophilic dimethylaminoethylmethacrylate (DMAEMA) units and hydrophobic n-butylmethacrylate (nBMA) ones is reported. DMAEMA units have been incorporated into the hydrophobic block of this copolymer to moderate its hydrophobic character. Light scattering experiments revealed the formation of micelles whose apparent aggregation number varied reversibly with the ionization degree of the DMAEMA units. Incorporating hydrophilic units into the hydrophobic block of an amphiphilic block copolymer is thus a way to generate dynamic aggregates in aqueous medium. As this strategy was also successful using other types of hydrophilic units, we believe it to be universal.
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