Adsorption of polyelectrolytes at liquid-liquid interfaces and its effect on emulsification
In this study we have investigated the adsorption behaviour of a number of synthetic polyelectrolytes at the paraffin oil-water interface and the properties of paraffin oil-in-water emulsions stabilized by these polyelectrolytes.Polyacrylic acid (PAA), polymethacrylic acid (PMA) and the copolymers of the monomeric acids with their methyl esters (resp. PAA-pe and PMA-pe) are used. Most of the experiments have been performed with PMA-pe, whereas the other polyelectrolytes are mainly used for the sake of comparison. In chapter 2 the synthesis and some relevant bulk properties are described. These properties are determined by potentiometric titrations and by viscosimetry. In agreement with the literature it is found that PMA and PMA-pe undergo a reversible conformational transition as a function of pH. At low charge density these polyelectrolytes are characterized by a compact hypercoiled conformation (aconformation) and by increasing the charge density they unfold, resulting finally in an expanded conformation (b-conformation). This conformational transition is not observed for PAA and PAA-pe.Several techniques are used to obtain information about the mode of adsorption of these polyelectrolytes at the paraffin oil-water interface and about the properties of emulsions stabilized by them. At undisturbed interfaces the interfacial tension as a function of time γ( t ) has been determined from interfacial tension measurements with the Wilhelmy-plate technique. Adsorbed (section 3.2.) as well as spread (section 3.3.) monolayers are investigated. It is found that the attainment of the steady state of the interfacial tension γ(∞) is faster the more compact the molecules are (i.e. lower charge density, higher ionic strength, Ca 2+counterions) and the higher the polyelectrolyte concentration. These findings agree with the fact that the rate of reduction of the interfacial tension dγ/d t is determined by three processes, viz. diffusion of the polymer molecule to the interface with adsorption in its bulk conformation, reconformation and spreading of the arrived molecules. As all three processes usually occur simultaneously, the adsorption behaviour of polymers is very complex. For compact molecules diffusion leads to a rapid accumulation of segments at the interface and reconformation and spreading hardly take place. For more expanded molecules diffusion is slow and the rate of reduction of γtakes place mainly through reconformation. As at higher c p the supply by diffusion is faster, it is understandable that the steady state is attained faster according as c p is higher and reconformation and spreading are reduced.If spreading can take place to a high degree, the fraction of segments per molecule adsorbed in the first layer p , becomes relatively high. From a semiquantitative interpretation of the interfacial tension measurements it is indeed found that p increases the less compact the molecules are (section 3.5.). To be able to do these calculations also adsorption experiments at an undisturbed liquid-solid (L/S) interface have been performed (section 3.4.). The amount of adsorbed PMA-pe at the polystyrene latex of low surface charge has been determined and it is assumed that Γ at the oil-water interface and the latex surface does not differ.Additional information about the mode of adsorption has been obtained from experiments in which the conditions in the water phase were changed after adsorption (chapter 4). Especially changes in pH produced interesting results (so-called γ-pH cycles). It was found that the mode of adsorption determines the degree of (ir)reversibility of the adsorbed layer by increasing pH. The longer the average train length of the adsorbed molecule, the higher the degree of irreversibility. These findings agree with the mode of adsorption as described already in chapter 3. If reconformation and spreading can take place to a high degree (i.e. at high pH) the average train length is large and the molecules are irreversibly adsorbed. If diffusion is the main factor (i.e. at low pH) the average train length is small and desorption is possible.The properties of emulsions stabilized by these polyelectrolytes have been investigated by the specific interfacial area S (expressed in m 2per ml paraffin oil), the amount of adsorbed polyelectrolyte at the emulsified interface Γ e (expressed in mg per m 2) and the viscosity of the emulsions. Γ e has been obtained by a depletion method (section 5.3.) and S by turbidity measurements (section 5.4.). As these properties did not change with time after finishing emulsification, they enable us to characterize the adsorbed layer at an emulsified interface and they reflect the interesting but complex dynamic processes occurring during emulsification with polyelectrolytes as stabilizers. The results of these investigations are summarized in chapter 5 ( S , Γ e and the mechanism of emulsification) and in chapter 6 (rheology of emulsions and characterization of the adsorbed layer at an emulsified interface).The interfacial tension at undisturbed interfaces and the emulsion properties have been investigated as a function of a number of parameters. These parameters are polyelectrolyte concentration c p or polyelectrolyte supply c p ', degree of neutralization α(charge density) or pH, ionic strength, nature of the counterion (Na +, Ca 2+) and chemical constitution of the polyelectrolytes. Moreover, it was investigated whether the a- and b-conformation in bulk was reflected in the adsorbed layer, the emulsion properties and the behaviour during emulsification. We will here summarize the effects of these parameters on the interfacial tension (undisturbed interface) and on the emulsion properties (disturbed interface). The comparison between both conditions of the interface is not meant to predict emulsion properties from the adsorption behaviour at an undisturbed interface. We intend to find out which of the investigated parameters play a major role in the interfacial activity at undisturbed interfaces, the emulsification process and the properties of the emulsions once they are formed. To predict the emulsifying behaviour of a polyelectrolyte it is necessary to investigate its interfacial behaviour under dynamic conditions. Such measurements are available (section 3.6.). However, it appears that the theory for the interpretation of these results is not yet applicable to polymers.In the course of this study the parameters mentioned before have been investigated seperately for each emulsion property or for each technique to determine the interfacial properties at undisturbed interfaces. Therefore it seems worth while to summarize the results here systematically for each parameter.Polyelectrolyte concentration c P or polyelectrolyte supply c P 'The steady-state value of the interfacial pressure for adsorbed layers always increases with c p , d Π(∞)/d c p >0 (see figs. 3.4. and 3.5.). For spread monolayers it is always found that d Π(∞)/d Γ sp >0 (see figs. 3.11.-3.14.). From the literature it is known that at L/S interfaces d Γ/d c p ≥0. Hence it is acceptable to assume dΓ ad /d c p ≥0 for the adsorption of polyelectrolytes onto undisturbed L/L interfaces. However, for adsorbed polymer layers an unambiguous relation between c p , Πand Γ ad cannot be given. In this study it is argued that Πis primarily related to the occupation of the first layer θ ad (see section 3.5.).At emulsified interfaces it is always found that dΓ e /d c P ' ≥0 (see section 5.5.2.). S as a function of c P ' is not univocal and depends on both c P ' and the flexibility of the polyelectrolyte (see below: degree of neutralization). In the region of low c P ' it is always found that S increases with c P ' and that the flexibility only affects the value of S . Above a given c P ' the relation S ( c P ') depends on the flexibility of the polyelectrolyte molecule (see section 5.6.). As the flexibility depends strongly on the degree of neutralization the relation between S and c P ' is a function of α.The viscosity of emulsions - restricted to the highly viscous emulsions with PMA-pe at α ≤0.30 as the stabilizer - increases with c P '. It is found that the viscosity is a function of the amount of PMA-pe adsorbed in loops in the a-conformation (see chapter 6).Degree of neutralization α(or pH)As shown in chapter 2 an increase of α gives an expansion of the coil in bulk. At an oil-water interface a higher charge density will reduce the interfacial activity and if adsorption takes place it will result in a thin layer. Moreover, a higher charge density of the adsorbed molecules will give and increased stability of the emulsion droplets by electrostatic repulsion.To summarize our findings for this parameter it is necessary to distinguish between esterified (PAA-pe and PMA-pe) and non-esterified (PAA and PMA) polyelectrolytes.In general the steady-state interfacial pressure is reduced at higher pH or α(see figs. 3.6.-3.9.). PAA and PMA are even not interfacially active at pH>6. From the Π(∞) values of PMA-pe it is deduced that the adsorption mechanism at high pH differs from that at low pH. The γ-pH cycles underline this conclusion (chapter 4). It appears that adsorption of PMA-pe at pH = 4 enables desorption upon pH increase. However, adsorption at pH = 9, followed by a reduction of the pH to 4, gives an adsorbed layer that is not desorbable upon pH increase. Adsorption at pH 4 takes place with short sequences of segments in trains, whereas at pH 9 these sequences are longer. Desorption of the long sequences is unlikely.Spread monolayers of PMA-pe do not confirm the important differences in the interfacial properties as a function of pH. The Π- A isotherms of PMA-pe are hardly dependent on the pH (figs. 3.12-3.14.).The differences between esterified and non-esterified polyelectrolytes are also reflected in the emulsifying and stabilizing capacity of these polyelectrolytes as a function of α. At α< 0.5 emulsions stabilized by any of the polyelectrolytes are stable to coalescence, whereas at α>0.5 emulsions prepared with PMA and PAA coalesce directly but emulsions prepared with PMA-pe and PAA-pe remain stable to coalescence (see fig. 5.3.). The obtained area of emulsions stabilized by PMA-pe and PAA-pe as a function of αreflects again the difference in adsorption mechanism between low and high pH.As the emulsions have been prepared under standard conditions (time of agitation, intensity of agitation and volume fraction of dispersed phase constant) the differences in S as a function of αand c P ' reflect the mechanism of emulsification. The main factor is the way in which the gradients of γalong the interfaces of newly created droplets are reduced. The supply by diffusion to the interface and the rate of reconformation and spreading reduce these gradients and it is the ratio of their contributions that determine the mechanism of emulsification. From earlier experiments with PVA as emulsifying agent it was found that for polymers with a relatively low flexibility S passes through a maximum as a function of c P '. For polymers with a relatively high flexibility S levels off with increasing c P '. Analogous results are now found for PMA-pe as a function of α. Especially at α ≥0.70 the flexibility is low and a maximum in S is observed. However, the higher αthe less pronounced the maximum is, because of additional factors such as the increased electrostatic repulsion and the reduced diffusion (see section 5.6.). At 0.30 ≤ α ≤0.50 S levels off with increasing c P '. At α ≤0.10 an enhanced coalescence without desorption results in an increasing S with c P ' and the leveling-off or the maximum is not reached in the region of c P ' investigated by us.Γ e decreases with increasing αfor all types of polyelectrolytes (see fig. 5.5.). From a comparison with the adsorption at the polystyrene latex surface it is concluded that emulsions stabilized by Na +PMA-pe at α< 0.30 and emulsions stabilized by Ca 2+PMA-pe over the whole region of αare characterized by a coalescence without desorption during emulsification (see section 5.5.4.).The viscosity of emulsions stabilized by PMA-pe decreases considerably with higher α. For the explanation see below the section on the conformational transition.Ionic strength (NaCI)A high ionic strength reduces the electrostatic repulsion on the polyelectrolyte chain and hence increases its flexibility. Moreover, the diffusion to the interface increases with increasing ionic strength. These effects will be more pronounced the higher the degree of neutralization. It is anticipated that Γand Π(∞) will increase with increasing ionic strength. The effect on the emulsion interface is not easy to predict. It depends on the sum effect of the increased diffusion and flexibility and the reduced electrostatic repulsion.The effect of ionic strength is only investigated for PMA-pe. In general the interfacial pressure increases with higher ionic strength (see fig. 3.4). It is notable that the ionic strength has no effect on the interfacial pressure at pH ~ 9, whereas it is still measurable at pH = 4. This again indicates the differences in adsorption mechanism as a function of pH.At the polystyrene latex surface it is indeed found that Γ increases with increasing ionic strength.S decreases and Γ e increases slightly with increasing ionic strength. The viscosity of PMA-pe stabilized emulsions at α< 0.30 does not depend on ionic strength.In general the effects of ionic strength are small compared with the more drastic effects for αand the nature of the counterion.Nature of the counterion (Na+, Ca 2+)It is anticipated that Ca 2+counterions considerably affect all investigated properties. The bivalent bonding of Ca 2+ions with dissociated carboxylic groups reduces the flexibility of the polyelectrolyte molecule.The interfacial pressure is increased and becomes much less a function of αand c p (see figs. 3.5. and 3.8).S is decreased and Γ e considerably increased in the presence of Ca 2+ ions. Moreover, S and Γ e are also hardly dependent on α. It is concluded that bivalent bonding is indeed responsible for this behaviour. From a comparison with the adsorption on polystyrene latex it is deduced that emulsions stabilized by PMA-pe in the presence of Ca 2+ ions undergo coalescence without desorption during emulsification (see section 5.5.4). The effect of Ca 2+ions on the viscosity of emulsions is not investigated.Chemical constitution of the polyelectrolytesA number of the results are already mentioned in the summary of the effect of the degree of neutralization. On account of bulk properties a distinction between PMA(-pe) and PAA(-pe) is obvious (see chapter 2). However, it appears that in the interfacial pressure, S and Γ e the difference between esterified and non-esterified polyelectrolytes is more pronounced. The interfacial activity of the esterified polyelectrolytes is higher than that of the non-esterified ones (see fig. 3.9). In general S is higher for the esterified polyelectrolytes, although at α< 0.30 the presence of the ester groups is responsible for an additional coalescence during emulsification, resulting in a lower S for emulsions stabilized by esterified polyelectrolytes (see fig. 5.3). Γ e is higher for the esterified polyelectrolytes than for the non-esterified ones. This effect is very obvious at α< 0.30.It is found that the viscosity of emulsions is strongly affected by the presence of the methyl groups in the main chain but not by the presence of esterified groups (see fig. 6.1.). It appears that the high viscosity of emulsions stabilized by PMA-pe at α< 0.30 is related to the occurrence of a hypercoiled conformation in the molecules.Conformational transitionThe occurrence of two different conformations in bulk is partly reflected in the interfacial and emulsion properties. S and Γ e are not affected by the presence of the hypercoiled conformation, but only by the presence of the ester groups. Moreover, the interfacial pressure as such does not depend on the conformation. However, the γ-pH cycles (chapter 4) allow us to locate a transition region in the conformation of the adsorbed layer, which is related to the occurrence of the a- and b-conformation in bulk. Adsorption of the PMA-pe molecules onto the interface from the solution in which the a-conformation exists, leads to reversibility of the adsorbed molecules upon increasing pH after adsorption. However, the molecules are irreversibly adsorbed if the b-conformation exists in solution. This difference in the conformation is only detectable for the esterified polyelectrolytes, since for PMA and PAA desorption always occurs upon increasing pH.The reversible adsorption of PMA-pe at low pH as compared to the irreversibility at high pH is interpreted by proposing an adsorption model in which the length of the trains plays a dominant role. Adsorption of PMA-pe at pH = 4 gives relatively short trains which are desorbable by increasing pH. However, adsorption from solutions in which the b-conformation exists gives relatively long trains and desorption is less probable by increasing pH.The occurrence of two different conformations in the adsorbed layer is indicated in the viscosity of emulsions stabilized by PMA-pe and/or PAA-pe. The viscosity is only very high for emulsions stabilized by PMA-pe at α ≤0.30. It appears to be caused by intermolecular foces of attraction between molecules adsorbed at different droplets and not by a bridging of the droplets by adsorption of one molecule at two or more droplets. These forces are of the same origin as those responsible for the stabilization of the hypercoiled a-conformation. From the viscosity of emulsions stabilized by PMA-pe at α= 0.10 in a mixture of water and methanol as the continuous phase it is concluded that the main stabilizing factor for the hypercoiled structure is hydrophobic bonding. This conclusion disagrees with some literature references in which v.d. Waals forces of attraction are held responsible.It appears that the prediction of emulsion properties from the interfacial tension measurements at undisturbed interfaces and from the bulk properties of the polyelectrolyte is a difficult problem, because of the very complex dynamic processes occurring during emulsification. However, this study demonstrates that a number of parameters exert an influence on one or more of the investigated emulsion properties and on the interfacial properties at undisturbed interfaces. It provides more insight into the factors which influence the behaviour of polyelectrolytes at disturbed and undistrubed liquid-liquid interfaces. It contributes to a better conception of the behaviour of proteins at interfaces in more practical systems.
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| Format: | Doctoral thesis biblioteca |
| Language: | English |
| Published: |
Veenman
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| Subjects: | acrylic acid, adsorption, carboxylic acids, chemistry, colloids, emulsions, industry, macromolecular materials, oils, oleic acid, plastics, sorption, surface chemistry, surfaces, unsaturated fatty acids, water, acrylzuur, adsorptie, carbonzuren, chemie, colloïden, emulsies, industrie, kunststoffen, macromoleculaire stoffen, oleïnezuur, oliën, onverzadigde vetzuren, oppervlaktechemie, oppervlakten, sorptie, |
| Online Access: | https://research.wur.nl/en/publications/adsorption-of-polyelectrolytes-at-liquid-liquid-interfaces-and-it |
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