Influence of the solution pH and the macromolecule structure on the polymer adsorption on the Cr2O3 surface
In the case of the polymers containing the functional groups capable of electrolytic dissociation process, the solution pH is the key factor determining the adsorption mechanism. Changes involving the hydrogen ion concentration modification have influence on both the solid surface charge and the polymer coil conformation. Moreover, the copolymer block affinity for the metal oxide surface can be easily affected by fixing the proper pH value.
As one can see in Fig. 2, both the solid surface structure and the analyzed polymer macromolecule conformation strongly depend on the solution pH. At the acidic pH conditions, the Cr2O3 surface is positively charged whereas the carboxyl groups belonging to the ASP chains remain undissociated. With the hydroxyl ion concentration growth, a larger number of the negative charges appear on the metal oxide surface. Simultaneously, the ASP segment dissociation degree increase leads to the formation of the more extended spatial conformation of the polymer chains. With regard to the nonionic poly(ethylene glycol) (PEG) fragment, the solution pH adjustments do not affect the polymer coil structure, but they can contribute to the changes of the macromolecule affinity for the solid particles. This is related to the PEG segment hydrogen bond formation ability; such connections can be created only between the positively charged surface groups or the amphoteric ones. Therefore, the adsorbent and adsorbate property changes described above have significantly influence on the polymer adsorption mechanism.
The adsorbed amounts of the tested macromolecular compounds (both homo- and copolymers) are placed in Table 1. At pH 3, the Cr2O3 particle surface is positively charged (σ
0 = 11.23 μC cm−2). Under these conditions, the ASP segments can either interact electrostatically with Cr2O3 or form hydrogen bridges (just as PEG). As one can see, the adsorption of all of the compounds gradually increases with the growth of the polymer initial concentration. The maximum of the polymeric chains linked to the adsorbent surface is reached in the case of the AP diblock copolymer, whereas the ASP homopolymer exhibits the lowest adsorption. The differences in the quantities of the polymer macromolecules bound at the metal oxide–aqueous solution interface can be explained by the analyzed substance structure. The AP copolymer chains consist of a long nonionic block and the relatively short polyamino acid ones. Under the acidic pH conditions, both copolymer structural units can undergo the adsorption on the Cr2O3 surface. The low ASP segment dissociation degree favors the more closely packed adsorption layer. Moreover, the presence of long PEG tail contributes to the screening of charges originating from the adjacent adsorbed ASP coils which contribute to further adsorption amount increase (related to the homopolymer). This effect is considerably less visible regarding the APA triblock copolymer. In this case, the PEG fragment is too short (in comparison with the ASP units length) in order to improve the proper screening of charges. A higher APA adsorption in relation to the homopolymer containing only the poly(aspartic acid) monomers comes from a larger number of the carboxylic groups capable of interacting with the solid particle surface.
At pH 7.6, the number of the positively and negatively charged groups present on the Cr2O3 particle surface is nearly equal (pHpzc—point of zero charge). The solution pH growth leads to reduction of the adsorption of all studied polymers by half in relation to the values obtained at pH 3 (except for APA—in this case, the decrease is significantly larger). The reasons for such a behavior are the electrostatic repulsion forces acting between the ASP segments in the macromolecules and the solid surface as well as between the adjacent adsorbed ASP chains. The presence of the strongly extended macromolecules on the metal oxide surface results that the lower number of polymer chains can be bound to the Cr2O3 surface. As regards the block copolymers, the PEG fragment has less capabilities of hydrogen bond formation due to a smaller content of the positively charged surface groups. The adsorption maximum is reached for the AP diblock copolymer, similar to pH 3. The APA adsorbed amount reduction is associated with the presence of the two large ASP units which can block the surface active sites making them inaccessible for the successive macromolecules.
The solution pH increase to 10 results in the further polymer adsorption amount reduction. Under these conditions, the PEG structural unit loses the affinity for the Cr2O3 surface. At pH 10, the negatively charged surface groups predominate at the Cr2O3–aqueous solution interface. Under these conditions, the repulsion forces between the ether groups belonging to the PEG chains and the solid surface significantly impede the nonionic polymer binding. As a consequence, the external part of copolymer adsorption layer consists of the nonionic structural units directed toward the bulk solution. In the case of the APA triblock copolymer, the loop formed by the PEG segments located between the two spatially extended ASP blocks provides another steric hindrance precluding the adsorption of polymer chains.
Solid particle surface charge density changes in the presence of the poly(l-aspartic acid) and the block copolymers
The potentiometric data obtained for the Cr2O3 particle surface charge density (σ
0) changes in the absence and presence of the analyzed homo- and copolymers as a function of the solution pH are shown in Figs. 3 and 4. The analysis of the results at lower polymer concentration (C
p = 10 mg L−1; Fig. 3) reveals that the curves obtained for the Cr2O3 particles in the background electrolyte and the Cr2O3/ASP homopolymer system overlap. The addition of the other two compounds causes the surface charge density reduction up to pH 10. At the initial pH values, the highest σ
0 drop is observed for the solid particles covered with numerous adsorbed AP copolymer macromolecules. A considerable surface charge decrease in the presence of APA triblock copolymer (despite lower adsorption) can be explained by a larger number of the dissociated carboxyl groups in the polymer chains. Another conclusion which may be drawn is that the lower the adsorption is, the closer and closer to the results obtained for the Cr2O3/NaCl system the potentiometric curves are (Fig. 3).
For the systems containing the studied polymers at a concentration of 100 mg L−1, the noticeable surface charge density drop is observed (compared with the solid suspensions with a lower polymer content). A greater number of the macromolecules adsorbed on the solid surface is responsible for the significant σ
0 value decrease. In addition, the change of curves order can be explained by the impact of the macromolecule structure on the polymer adsorption layer conformation. In the samples containing a lower concentration, the polymer chains adopt more parallel conformation, while at higher concentration the adsorption layer is rich in the loops and tails directed toward the bulk solution. Increase in the number of polymer macromolecules linked to the Cr2O3 surface results in higher amount of the functional groups able to interact with the adsorbent active sites.
Despite the fact that the highest adsorption is reached for the samples containing the AP copolymer, the lowest surface charge values are measured in the presence of both the ASP homopolymer and the APA triblock copolymer. The reason for such a behavior is a larger number of the negatively charged functional groups located in the mentioned macromolecular compound chains in relation to the AP copolymer. In the case of the AP diblock copolymer, a slight changes in the surface charge density values (compared with the lower concentration) is related to the adsorption mechanism. Due to the presence of the long PEG chain in the macromolecule structure, AP can be bonded to the Cr2O3 surface on account of the hydrogen bond formation between the nonionic block and the adsorbent surface active sites (especially in the acidic solution). The tests carried out previously indicate that the presence of PEG presence in the by surface by-surface layer of the metal oxide does not change the surface charge density . Therefore, the inconsiderable drop of the surface charge density values can be associated with the marked contribution of the PEG block in the copolymer adsorption process (Fig. 5). Amount of the polymer charged functional groups occurring near the adsorbent particle surface plays a crucial role in the suspension stabilization or destabilization process.
The polymer adsorption impact on the aqueous Cr2O3 suspension stability
In order to prepare a comprehensive analysis of the examined systems, the TSI parameter values in the absence and presence of the ionic polyamino acid and their copolymers were calculated. The analysis of the data collected in Fig. 6 indicates that in the absence of the studied polymers, the Cr2O3 suspensions are stable only at pH 3, and under other pH conditions, the solid samples exhibit a low durability. At pH 3, the Cr2O3 particles contain mainly positively charged surface groups contributing to the electrostatic sample stabilization. As one can see from the TSI analysis, at pH 7.6, the Cr2O3 suspensions without the polymer exhibit the lowest stability (TSI = 62.91). Under these conditions, the overall surface charge is equal to zero which means that there is the same number of positively and negatively charged groups. The attraction forces between them are responsible for the stability reduction. A lower TSI parameter value was reached at pH 10 (TSI = 49.82). It can be driven by electrostatic repulsion between the negative surface groups.
The analysis of the turbidimetric data (Fig. 6; Table 2) exhibited that the lowest durability of the sample containing the ASP homopolymer at pH 3 (in comparison with the other studied polymers) is related to the absence of PEG fragment in the macromolecule structure. The nonionic structural unit performs two functions. First of all, it ensures the screening of charges originating from the ASP blocks. Secondly, under these conditions, the PEG segments can undergo adsorption process at the Cr2O3 surface providing to the more densely packed polymer adsorption layer formation. As a result, the sample aggregation is partially impeded by the impact of steric effects. In the presence of ASP homopolymer, the solid particle charge is more neutralized due to the electrostatic interactions with the dissociated carboxyl groups leading to the formation of flocs characterized by a small diameter. In the case of the AP or APA block copolymer, much more larger aggregates are the results of the PEG ability of the formation polymer bridges between different solid particles.
At higher pH values (7.6 or 10), the Cr2O3 suspension stability improvement in the presence of all tested polymers is observed (in comparison with pH 3 and the solid in the background electrolyte under the same conditions). Another conclusion that can be drawn on the basis of data analysis in Fig. 6 is that the polymer chain structure has a considerable impact on the suspension stability. At pHpzc, the sample containing the ASP homopolymer exhibits the highest durability among other analyzed compounds, whereas the samples with the APA triblock copolymer are unstable. In the basic environment, this tendency is maintained. Such a behavior can be explained by the two effects. First of all, the adsorption of all tested compounds decreases with the solution pH growth. Therefore, the solid surface is not fully covered. Secondly, with the pH increase, the PEG block present in the AP and APA copolymers chain loses the affinity for the Cr2O3 surface. The nonionic tails directed toward the bulk solution can form the hydrogen bonds leading to the suspension destabilization. Such a behavior is not observed for the ASP homopolymer. In this case, the extended negatively charged chains impede the sample aggregation. The proposed mechanism is confirmed by the average floc size and sedimentation velocity analysis (Table 2). The smallest values at both pH are noted for ASP, whereas the large aggregates are formed in the presence of the copolymers. Slight differences between AP and APA follow from the higher adsorption of the diblock copolymer causing decrease in the solid active sites available for the polymer macromolecules.
Thermogravimetric analysis of the poly(l-aspartic acid) homopolymer and copolymers impact on the Cr2O3 particles
The interactions between various polymers and the solid particles using the thermogravimetric methods are subject of numerous research [38–50]. Figures 7–10 present the TG, DTA and DTG curves for the chromium (III) oxide samples, both unmodified and modified with the adsorption layer formed by the analyzed homo- and copolymers. Changes in the solid particle thermal stability in the presence and absence of the macromolecular compounds were measured at two extreme pH values in order to investigate the polymer influence on the Cr2O3 properties.
Analyzing the DTG curve obtained for Cr2O3 without the adsorbed polymer one can distinguish two regions [51–53]. The first region ranges from 30 to 360 °C with the minimum at 96.9 °C, and the mass loss of 0.16 % corresponds to the endothermic desorption of physically (the BET measurements indicate that the solid is nonporous and possess only numerous spaces between crystallites). In the second stage (360–930 °C, minimum at 432.6 °C), the mass loss of 0.21 % corresponds to dehydroxylation of the solid particles. The addition of the polymers containing the poly(l-aspartic acid) segments significantly changes the course of thermogravimetric curves. First of all, in the initial temperature range (up to 150 °C), the endothermic process of the physically adsorbed water desorption is observed. Above 150 °C, curved-down peaks indicate the exothermic decomposition of polymeric substances adsorbed on the Cr2O3 surface. The shift of the peak maximum (DTG curves) involves the changes in the type of interactions between the polymer macromolecules and the solid particles. It is worth noting that the course of thermogravimetric curves course as well as the peak position depends on the solution pH value which is associated with the polymer adsorption mechanism.
Under the acidic pH conditions (Figs. 7, 8), at the temperature up to 200 °C, the polymer adsorption on the solid surface is responsible for the lower mass losses compared with the Cr2O3 sample. This is related to the replacement of water molecules by the polymer chain bound to the adsorbent surface. The DTA peaks obtained between 250 and 420 °C depend on the macromolecule structure. The analysis of the dependencies shows that the system containing the APA triblock copolymer is characterized by the greatest mass decrement, whereas the fewest polymer chains undergo decomposition in the case of the ASP homopolymer addition. Simultaneously, the adsorption amount of this macromolecular compound at pH 3 is the lowest. This indicates that the ASP macromolecules linked to the Cr2O3 particles demonstrate the most flat conformation (Fig. 5). As a result, the polymer functional groups are able to interact with numerous solid surface active groups giving a less space accessible to the successive macromolecules present in the solution. Such a polymer chain conformation is responsible for the low ASP adsorption, but it ensures stronger binding to the metal oxide surface. The AP and APA copolymers contain in their structure a larger number of the functional groups able to interact with the solid. This contributes to the formation of more densely packed polymer adsorption layer in which the number of “train” segments (the macromolecule fragments bound directly to the surface) is smaller than in the ASP case. As a result, the copolymer can be more readily removed from the solid particles. The shift of the decomposition peak maximum toward a higher temperature is related to the carboxyl group number interacting with the surface active groups in the electrostatic manner. The opposite situation takes place for the dehydroxylation peak of Cr2O3 surface groups (420–480 °C). Under these conditions, the polymer segment binding density plays a key factor.
The analysis of the thermogravimetric curves measured for the samples at pH 10 (Figs. 9, 10) indicates that the samples mass decrement are larger in comparison with those at pH 3. This confirms the polymer adsorption mechanism which assumes that the hydrogen bond formation is a driving force for macromolecule binding. The obtained values are in good agreement with the adsorbed amounts of the analyzed polymeric compounds. In the polymer decomposition temperature range (150–400 °C), the mass increment increases in the following order: APA triblock copolymer, ASP homopolymer, AP diblock copolymer. This phenomenon can be explained on the basis of the macromolecule conformation. In the basic environment, the carboxyl groups belonging to the poly(l-aspartic acid) chains are totally dissociated. As a result, the polymer macromolecules adsorbed on the Cr2O3 surface due to the van der Waals interactions adopt an extended conformation. Additionally, under these conditions, the PEG unit present in the block copolymers cannot form hydrogen bridges. Hence, the AP macromolecules containing a long nonionic chain can be readily removed from the solid surface (two intensive peaks on DTA curve with minimum at 179.4 and 296 °C). The lowest mass loss measured for the sample with APA is related to the low polymer adsorption.
Interesting observations can be done comparing the DTA data obtained for samples prepared in acidic and basic solutions. As one can see, at pH 3 (Fig. 7), only one peak on DTA curve for each polymeric substance is detected. Moreover, the signals are shifted toward the higher temperatures in the following order: AP, ASP and APA (the curves obtained for the last two substance are nearly overlapping). On the basis of these results, the interactions between the particular suspension constituents can be drawn. As it was mentioned before, the AP diblock copolymer forms a densely packed polymer layer in which the adjacent macromolecules can interact on account of the hydrogen bond formation. Such connections can be easily destroyed. The greater the number of the polymer segments linked with the solid surface, the higher the temperature required to remove the macromolecules is.
At pH 10, two peaks observed for AP copolymer on DTA curves (Fig. 9) can be related to the two-step process of the polymer chain removal. Such a behavior can be connected with the amount of the adsorbed polymer macromolecules and the strength of the polymer–solid surface interactions. As one can see, at pH 10 those two peaks are observed for AP, one signal (with the minimum shifted toward higher temperatures) is detected for ASP homopolymer. In the case of the APA triblock copolymer, there are no significant changes in the curve course. In addition, a higher quantity of the adsorbed polymer chains is noted for the AP, while the lowest value is reached in the system containing APA. Therefore, the explanation of this fact is the peaks is related to the polymer chain removal phenomena from the Cr2O3 particles. First signal detected for the AP copolymer can be associated with the breaking of the hydrogen bonds formed between the fewer PEG segments adsorbed on the solid surface or the destruction of the hydrogen bridges between the adjacent blocks. The second one (with the maximum at 300 °C) corresponds to the ASP block removal. Since these fragments are strongly linked to the adsorbent compared with the nonionic PEG ones (and the number of the adsorbed polyamino acid segments is also greater), the higher temperature is required to destroy the polymer–surface active group interactions. Such a behavior is not observed at pH 3. The main reason is fact that the adjacent adsorbed macromolecules in the polymer layer formed in the acidic solution are connected by the numerous hydrogen bonds. As the pH value increases, the number of the hydrogen bridges is considerably reduced. As a result, the adsorbed layer is less densely packed with a higher distance between the successive chains. In the case of the ASP homopolymer, temperature necessary to the polymer decomposition increases which can be related to the ASP chain conformation. Under these conditions, the anionic polyamino acid macromolecules adopt more flat arrangement with respect to the AP diblock copolymer. A lack of the considerable effect in the probe containing APA is a consequence of the low polymer adsorption. The spatially extended copolymer macromolecules are easily removed from the Cr2O3 surface. These results are in a good agreement with the data obtained from the mass loss analysis.
MS profile of CO2 and H2O released in the decomposition stage versus temperature obtained for the analyzed systems at pH 3 and 10 are presented in Figs. 11 and 12. As one can see, under acidic conditions in 250–420 °C range the highest amount of carbon dioxide (m/z = 44) originates from the APA containing sample while the lowest value is emitted in the case of the AP copolymer. The reason for such a behavior is a lower content of carboxyl groups which undergo decomposition with the CO2 evolution. Considering water (m/z = 18) originating from oxidation of hydrogen atoms located in the polymeric chain, the largest number of the solvent molecules was adsorbed to the Cr2O3 surface in the case of the ASP homopolymer on account of low polymer adsorption level. A more densely packed adsorption film formed in the presence of the block copolymers effectively impedes the water molecule sorption.
Different situation is observed analyzing the MS profiles of CO2 and H2O emitted in the decomposition stage versus temperature obtained for the analyzed systems at pH 10 (Figs. 11, 12). In the case of both released gases, the highest emission is observed for the particles covered by the AP diblock copolymer. This is associated with a larger number of the polymer functional groups undergoing the oxidation process on account of high adsorption.