Startseite Poly(N-octyl-4-vinylpyridinium bromide) copolymers in aqueous solutions: potentiometric and thermodynamic studies
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Poly(N-octyl-4-vinylpyridinium bromide) copolymers in aqueous solutions: potentiometric and thermodynamic studies

  • Lahcene Tennouga EMAIL logo , Wassila Bensalah und Ali Mansri
Veröffentlicht/Copyright: 12. September 2018
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e-Polymers
Aus der Zeitschrift e-Polymers Band 18 Heft 6

Abstract

In this work, the P4VP was synthesized by radical polymerization. The quaternization of this polymer by octyl bromide leads to the two copolymers [poly(N-octyl-4-vinylpyridinium bromide] named P4VPC8Br 48.8% and P4VPC8Br 72%. The thermodynamic behavior associated with the potentiometric titration of the copolymers, was reported in the temperature range (293.16–333.16 K) and as a function of the concentrations (0.25×10−4 mmol/dm3 12.3×10−4 mmol/dm3). The free energy of dissociation ΔGdiss variation versus the neutralization degree shows the negative value due to the steric and electrostatic effect of the alkyl chains. The positive values of ΔH and ΔS confirmed the spontaneity and disorder of the reaction. The critical concentration C* of the two copolymers was determined from the enthalpy ΔH0 and entropy ΔS0 changes. The transition in conformation of the copolymer chains was influenced by the presence of hydrophobic-hydrophilic and hydrophobic-hydrophobic interactions.

Keywords: conformation; interactions; poly(N-octyl-4-vinylpyridinium bromide); potentiometry; thermodynamic

1 Introduction

Polyelectrolyte solutions present multiple points of interest in physical chemistry and biochemists. The polymeric and electrolytic solutions are being investigated in terms of electrolytic dissociation and polymeric behavior (1).

The thermodynamic parameters for the protonation of polymeric bases generally depend on the degree of protonation of the macromolecule on its total charge (2). Kogej (3) studied the conformation transition of polyelectrolytes in water and a complete thermodynamic analysis of the transition of polyelectrolyte chains, this study is conducted by determining the free energy, enthalpy and entropy changes.

Polyelectrolytes, which are hydrosolubles have been labeled either as hydrophylics, or as polysoaps that are similar to surfactant as regards certain properties (4). In a solvating system, the steric and electrostatic phenomena appear after the basic polymer is quaternized (5). The weak neutralization degree effect on the surfactant properties of (P4VPC8-50) has been studied (6). The partially protonated potentiometric titration of P4VP was carried out by Fuoss and Strauss (7). Potentiometric titration has been used to investigate various aspects of the interactions of polyelectrolyte counter-ions (8). The associated thermodynamics with the potentiometric titration behavior of polyelectrolytes in aqueous solution were studied in literature, for being either polybasic or polyacid (9). A natural explanation of thermodynamic signatures of hydrophobic hydration, particularly entropy convergence, emerges from these theoretical advances. How those temperature behaviors are involved in cold denaturation or the stability of thermophilic proteins will be a topic for future research (10).

The calculus of thermodynamic parameters, the Gibbs free energy, ΔG, the enthalpy, ΔH0, and the entropy, ΔS0 are of major importance, because they quantify the relative importance of hydrophobic interactions of polymers in aqueous solutions (11).

The aim of this research was to study the poly(N-octyl-4-vinylpyridinium bromide) copolymers in aqueous solutions using potentiometric and thermodynamic analysis. The effect of concentration, temperature and neutralization degree was examined.

2 Materials and methods

2.1 Materials

2.1.1 Synthesis of poly(N-octyl-4-vinylpyridinium bromide) copolymers (P4VP-C8Br)

The poly(4-vinylpyridine) P4VP (12) was prepared by radical polymerization. The benzoyl peroxide 1% was used as an initiator in 70% volume of toluene at 90°C, then precipitated in the ethanol/ether solvent.

The quaternization of P4VP (Scheme 1) was prepared by refluxing poly(4-vinylpyridine) in ethanol with octyl-bromide. The reactions were carried out in thermostated water both under nitrogen. The solution was then poured into diethyl ether to obtain a solid which was also washed with diethyl ether, filtered and dried under vacuum at room temperature (13).

Scheme 1: Ground of copolymer poly(N-octyl-4-vinylpyridinium bromide) [P4VPC8-X]; X rate of quaternization.
Scheme 1:

Ground of copolymer poly(N-octyl-4-vinylpyridinium bromide) [P4VPC8-X]; X rate of quaternization.

2.2 Methods

Molecular weights were determined using a Ubbelhode-Schott Gerat AVS400 viscosimeter (Mainz, Germany). Measurements were conducted at 25±0.1°C in a thermostat bath (14). Then we measured the M¯V=6.1⋅104 g/mol of each sample in absolute ethanol using the equation [1].

[1][η]=2.35⋅10−5⋅MV0.68

We measured the rate of quaternization using the conductivity titration of the bromide ions with AgNO3 using a Mettler DL 40 RC titrator (Schwerzenbach, Switzerland) with a silver electrode; we confirmed the results by [1H-nuclear magnetic resonance (NMR)] with a Bruker 200 MHz (Rheinstetten, Germany) at room temperature (solvent methanol deterred), and thermogravimetric analysis by using a SDT Q600V20.9 Build 20 (New Castle, DE, USA). Dynamic measurements were done at a heating rate of 5 K/min. The values of quaternization rates were 72% and 48.8%.

2.3 Potentiometric titration

Potentiometric titrations were collected with a Denver Instrument model 225 star (Arvada, CO, USA) equipped with a Schott combined electrode calibrated at 25°C using Gran plots created by the software Glee (15), the system was standardized with two aqueous buffers solutions having pH values of 4.01 and 7.02 at this temperature.

All the titrations were performed in a thermostatic bath regulated to 0.05°C at the desired temperatures of 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C and 60°C, the results were discarded.

Twenty milliliters of the copolymer solution poly(N-octy-4-vinylpyridinium bromide), (P4VPC8Br 48.8% and P4VPC8Br 72%) soluble in water and the addition of hydrochloric acid HCl solution already prepared in water (CHCl=10×CP4VP) were used as the titrant. Ten minutes of lag time was allowed between the two dosages to ensure that the reaction had reached equilibrium.

2.4 Study of poly(N-octy-4-vinylpyridinium bromide) and proton interactions (P4VP-C8Br)-HCl by the potentiometric and thermodynamic techniques

We used the potentiometric titration with HCl to study the two copolymers, poly(N-Octyl bromide-4 vinyl pyridinium) which have different rates of charges, P4VC8Br 72% and P4VPC8Br 48.8%.

Taken at an interval of concentration between 0.25×10−4 mmol/dm3 and 12.3×10−4 mmol/dm3, that were already chosen from the phase diagram (Figure 1) such that both copolymers are soluble in water.

Figure 1: Phase diagram of the P4VPC8Br 72% and P4VPC8Br 48.8% on the solubility at 25°C in the mixture of water-ethanol.
Figure 1:

Phase diagram of the P4VPC8Br 72% and P4VPC8Br 48.8% on the solubility at 25°C in the mixture of water-ethanol.

The diagram of solubility (Figure 1) was carried out in order to estimate the capacity of P4VPC8Br solubility in water. A solution of P4VPC8Br in absolute ethanol with an original stock concentration of 20 g/l was prepared. Other solutions of various concentrations were obtained by dilution of the stock solution. Dilutions were made starting from a mixture of water/ethanol.

The limits of solubility were determined by visual observations; we could delimit two fields of a single phase and two phases (precipitate-solution) (16).

In the concentration field of the P4VPC8Br solutions the single phase aspect disappears with the profit from a turbid solution. When the copolymer concentration increases the domain with two-phase (precipitate-solution) was observed for both copolymers.

We calculate the neutralization degree α using the following equation:

[2]α=[H3O+ ][p]

where [p] represent the polymer concentration.

The potentiometric titration of the polyelectrolyte solutions is generally treated in terms of a negative logarithm of the acid dissociation constant (pKa) which is defined by the following equation:

[3]pKa=pH+log1−αα

where pKa is the sum of both terms:

[4]pka=pka0+0.434dGdissRTdα

pk0 is the intrinsic acid dissociation constant that is independent of the α parameter, R is the universal constant of perfect gas, T is the absolute temperature, and G is the electrostatic Gibbs energy which corresponds to the required energy to overcome the electrostatic force that is necessary to extract a proton from a charged cationic polyelectrolyte (3, 17).

The values of pk0 were obtained by extrapolating curves of titration at the neutralization degree α=0, ΔGdiss ranging from α=0 to 1 can be obtained using graphical integration based on equation [5]:

[5]ΔGdiss=2.303∫01[pk(α)−pk0 ]dα
[6]ΔGdiss=2.303RT(pka−pk0 )α

3 Results and discussion

The curves of potentiometric titration show the Gibbs enthalpy ΔGdiss as a function of P4VPC8Br 48.8% neutralization degree α for multiple studied temperatures.

Both figures (Figures 2 and 3) show an enthalpy change as a function of neutralization degree α for different concentrations in an interval of temperature varying between 293.16 K and 333.16 K.

Figure 2: This figure shows an enthalpy change as a function of neutralization degree for different concentrations in large intervals of temperature of P4VPC8Br 48.8% by potentiometric test.Free energy change ΔGdiss vs. neutralization degree α of P4VPC8Br 72%, (a) CP4VPC8Br 72%=0.25×10−4 mmol/dm3; (b) CP4VPC8Br 72%=6.1× 10−4 mmol/dm3; (c) CP4VPC8Br 72%=8.1×10−4 mmol/dm3; (d) CP4VPC8Br 72%=12.3×10−4 mmol/dm3.
Figure 2:

This figure shows an enthalpy change as a function of neutralization degree for different concentrations in large intervals of temperature of P4VPC8Br 48.8% by potentiometric test.

Free energy change ΔGdiss vs. neutralization degree α of P4VPC8Br 72%, (a) CP4VPC8Br 72%=0.25×10−4 mmol/dm3; (b) CP4VPC8Br 72%=6.1× 10−4 mmol/dm3; (c) CP4VPC8Br 72%=8.1×10−4 mmol/dm3; (d) CP4VPC8Br 72%=12.3×10−4 mmol/dm3.

Figure 3: An enthalpy change as a function of neutralization degree for different concentrations in large intervals of temperature of P4VPC8Br 72% by potentiometric test.Free energy change ΔGdiss vs. neutralization degree α of P4VPC8Br 48.8%, (A) CP4VPC8Br 48.8%=0.25×10−4 mmol/dm3; (B) CP4VPC8Br 48.8%= 6.1×10−4 mmol/dm3; (C) CP4VPC8Br 48.8%=8.1×10−4 mmol/dm3; (D) CP4VPC8Br 48.8%=12.3×10−4 mmol/dm3.
Figure 3:

An enthalpy change as a function of neutralization degree for different concentrations in large intervals of temperature of P4VPC8Br 72% by potentiometric test.

Free energy change ΔGdiss vs. neutralization degree α of P4VPC8Br 48.8%, (A) CP4VPC8Br 48.8%=0.25×10−4 mmol/dm3; (B) CP4VPC8Br 48.8%= 6.1×10−4 mmol/dm3; (C) CP4VPC8Br 48.8%=8.1×10−4 mmol/dm3; (D) CP4VPC8Br 48.8%=12.3×10−4 mmol/dm3.

We observe that the plots share the same behavior in all aspects which can be explained in two phases:

  1. Phase I: (α ranging from 0 to 0.1): We observe a considerable semi-linear decrease of enthalpy. This explains a partial dissociation of copolymer in water. The decrease of ΔGdiss affirms the copolymer’s dissociation.

    The partial dissociation of the copolymer is smaller or equal to zero: (ΔG≤0).

  2. Phase II: (α ranging from 0.1 to 1): We observe stability accompanied by small change (an increase then a decrease) in enthalpy ΔGdiss<0 for all the concentrations in an interval of enthalpy ranging from [ΔGdiss=−20 kJ/mol to ΔGdiss=−14 kJ/mol].

Partition of the Gibbs energy of the relation has shown a large non-electrostatic contribution to the solubility (11). Negative heat capacity changes in all the systems are correlated to the involvement of the significant hydrophobic forces (18).

The partial dissociation of the copolymer in water is due to the lack of numbers of hydrophobic chains in the copolymers P4VPC8Br 72% and P4VPC8Br 48.8%, this implies that the electrostatic interactions and the hydrophobic-hydropholic characteristics are stronger than the one of the hydrophibic-hydrophibic interactions (19).

The uncharged pyridinium group becomes charged by H+, this one penetrates inside this structure.

This phenomenon provokes a conformation of the transition of the copolymers. The steric effect occurring between alkyl chains reduces the nitrogen atom N. This reduction phenomenon provokes copolymer’s neutralization along with the counter-ion effect. This phenomenon reduces the electrostatic interactions occurring between the charged patterns of the polyelectrolyte in water.

Based on this experience, we have confirmed that the neutralization energy ΔGdiss reaches an interval of static values (Scheme 2); this interval does not get affected by the quaternization rate, nor the concentration, or the temperature in a diluted environment.

Scheme 2: Transitional conformation of poly(N-octyl-4-vinylpyridinium bromide) in aqueous solution with HCl and thermodynamic effect (20).
Scheme 2:

Transitional conformation of poly(N-octyl-4-vinylpyridinium bromide) in aqueous solution with HCl and thermodynamic effect (20).

This cooperation occurring between the precedent phenomena has caused a conformation transition on the structure of these copolymers (3).

The extrapolation procedures suggested by Leyte and Mandel were used (21). Equation [6] explaines the conformational transition interference of the P4VPC8Br intermolecular association and HCl which has been determined experimentally.

Based on integration, we can plot ΔG0; the intrinsic extrapolation is determined by the thermodynamic equation [7].

The value of ΔG0 is governed by two factors, enthalpy and entropy, for an isothermal system, the change in Gibbs energy is the sole measure to determine these factors (Figure 4).

Figure 4: Free intrinsic energy ΔG0 vs. temperature.(1): P4VPC8Br 72%; (2): P4VPC8Br 48.8%.
Figure 4:

Free intrinsic energy ΔG0 vs. temperature.

(1): P4VPC8Br 72%; (2): P4VPC8Br 48.8%.

[7]ΔG0=ΔHdiss0−TΔSdiss0

ΔHdiss0 is defined as the enthalpy of protonation, ΔSdiss0 are values of the change regarding the entropy of dissociation.

The linear extrapolation is the most used method among the developed methods of thermodynamics analysis. It was originally based upon experimental observation.

As in (22), the potentiometric titration was used for its linear dependency of observation regarding deployment of free energy changes (23).

According to the theoretical development of thermodynamics (24) which has been proved, the utilization of methods, linear extrapolation or free energy interception has allowed the use of equation (7).

The values of ΔH0 and ΔS0 for both copolymers are shown in Table 1.

Table 1:

Resume the variation of thermodynamics parameters as a function of copolymers concentration.

Copolymers concentration (mmol/dm3)P4VPC8Br 48.8%P4VPC8Br 72%
∆H0 (kJ/mol)∆S0 (kJ/mol)∆H0 (kJ/mol)∆S0 (kJ/mol)
0.25·10−42.2200.0650.0460.052
3.5·10−42.3400.06670.7430.0534
5.2·10−42.5700.06861.06650.0576
6.1·10−42.7480.0671.1930.0593
7.1·10−411.560.0993.450.065
8.1·10−418.2380.1244.8940.072
10.2·10−418.7240.1274.8970.0807
12.3·10−418.9240.1285.0360.086

ΔH° and ΔS° were plotted as a function of concentration which is already calculated using equation [7] for the potentiometric titration in Figures 2 and 3, nine temperatures for two copolymers were studied with a variation in the neutralization degree for all concentrations.

We notice that the values of ΔH° and ΔS° remain positive in the previous table despite the change in concentration and quaternization rate for both copolymers.

We note that the spontaneity is clearly visible in the some figures.

In an endothermic system, ΔH0>0 and ΔS0>0, therefore we conclude that the spontaneity occurs for all temperatures in which exothermicity becomes less important. The average potential energy of molecules’ interaction measured the enthalpy part, and the order or intermolecular correlations measured the entropic part (25).

In an endothermic reaction where ΔH0>0, we observe that the plots behave differently, this can be explained in three phases (Figure 5).

Figure 5: Enthalpies of dissociation ΔH0 and the entropy change of dissociation ΔS0 vs. concentration of copolymer P4VPC8Br 72% and P4VPC8Br 48.8%.
Figure 5:

Enthalpies of dissociation ΔH0 and the entropy change of dissociation ΔS0 vs. concentration of copolymer P4VPC8Br 72% and P4VPC8Br 48.8%.

  1. Phase 1: In the concentration area 0–6.475×10−4 mmol/dm3, we observe a linear stability regarding the entropies values for both copolymers, a gradual, yet an insignificant increase (semi-stable), this stability affirms the change in transitional conformation in the presence of H+ which causes an intra polymer interaction due to the dissociation of the polymer in water.

  2. Phase 2: In the concentration area 6.475×10−4 mmol/dm3 to 8.1×10−4 mmol/dm3, we observe that the value of entropies for both copolymers remains null during the phase whereas the enthalpy for two copolymers increase differently in unequal tangential amounts.

    We explain the enthalpy increase by a partial formation of micro domains that are caused by a solubility limit or a transition of phases. In this state, there exists intrapolymer and polymer-solvent interactions.

  3. The interaction “polymer-polymer” occurs when the associated concentration of recovery is exceeded.

  4. Phase 3: (Concentration from 8.1×10−4 mmol/dm3 to 12.3×10−3 mmol/dm3): The values of entropies for both copolymers remain null. The enthalpy of both copolymers increases insignificantly with a tangential coefficient almost equal to zero. The latter observation shows that the net enthalpy has an implication in conformational changes, besides the protonation with deprotonation (26).

We explain this behavior by the intermolecular interactions. The intermolecular interactions are introduced by the value of critical concentrations recovery of the polymer in the solvent, which is equal to 6.475·10−4 mmol/dm3. This value was determined from the slop. These occurring interactions have caused a decrease of “polymer-solvent” interactions. Consequently, there is a formation of micro-domains along with a possibility of the formation of microscopic precipitates. In this state, the “hydrophobic-hydrophobic” interactions are more numerous compared to “hydrophilic-hydrophobic” interactions. The dissociation degree is characteristic of polymers basic. On the other hand, it was seen, that the net enthalpies increased with concentration of polymers. Thus, we may finish, that the overpowering part of the experimental endothermic originates not from the deprotonation, but rather from desolvation and the conformational changes. Copolymer desolvation and conformational changes are bound to each other, as conformational changes necessarily involve desolvation and this latter induces conformational changes (26).

The study of thermodynamic behavior accurately measures the enthalpy and the entropic contributions to the Gibbs energy.

Entropy and stoichiometry of binding method with affinity ranging from: [0.052 to 0.086 kJ/mol] of P4VPC8Br 72% and [0.065 to 0.128 kJ/mol] of P4VPC8Br 48.8%, which are the values of the binding constants usually found for interactions commanding the nanoparticle formation and association (27).

The stability of positive enthalpy ΔS0>0 of both copolymers indicates the reaction’s disorder. This is due to the existence of the hydrophylic-hydrophobic balance; this balance has been caused by the steric and electrostatic phenomena of the alkylic chains along with charged N+ and non-charged N of the pyridineum group that is charged with H+.

4 Conclusion

A thorough thermodynamic and potentiometric titration analysis of the conformation transition in P4VPC8Br 48.8% and P4VPC8Br 72% has confirmed the presence of the hydrophobic-hydrophilic effect and hydrophobic-hydrophobic of this conformation.

The plots of free energy ΔG vs. neutralization degree have confirmed that the conformation of transition became stable when α reaches the value of 0.1; this is due to the steric and electrostatic effect of the alkyl chains.

ΔH0>0 along with ΔS0>0 the positive values of enthalpy and entropy have confirmed the spontaneity and disorder of the reaction.

The plots of enthalpy ΔH0 in function of concentration have emphasized the critical concentration where the intermolecular interaction begins; this is due to the hydrophobic-hydrophilic balance.

Acknowledgments

The authors thank the National Directorate General of Scientific Research and Technological Development (DGRSDT) in Algeria for financial support.

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Received: 2018-04-11
Accepted: 2018-07-17
Published Online: 2018-09-12
Published in Print: 2018-10-25

©2018 Walter de Gruyter GmbH, Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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https://doi.org/10.1515/epoly-2018-0079
Schlagwörter für diesen Artikel
conformation; interactions; poly(N-octyl-4-vinylpyridinium bromide); potentiometry; thermodynamic
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Artikel in diesem Heft

  1. Frontmatter
  2. In this Issue
  3. Full length articles
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Heruntergeladen am 18.9.2025 von https://www.degruyterbrill.com/document/doi/10.1515/epoly-2018-0079/html
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