Copolymers at a liquid-liquid interface and their retarding effect on mass transfer between both phases

Abstract

In this study we have investigated the interfacial properties of several polyvinyl alcohol-acetate (PVA-Ac) copolymers, which only differ in the content and the intra-molecular distribution of their vinyl acetate monomers (VAc).In chapter I the relevant theoretical and experimental aspects of studies on polymer adsorption are reviewed. Because of experimental difficulties, studies on polymer adsorption at liquid-liquid interfaces have, up to this moment, only resulted in qualitative information on the adsorption mechanism and the structure of polymer adsorption layers. One of the objectives of the present study was to further elaborate a recently proposed method to determine the degree of coverage of a liquid-liquid interface from the retarding effect of polymers adsorbed at that interface, on mass transfer through it. To that end, KCl transfer measurements between water saturated with 1-butanol (wabu) and 1-butanol (buOH) saturated with water (buwa) have been performed, as described in the chapters 4 and 6.With the exception of the PVA(-Ac) (co)polymers, all materials and their relevant physical properties are described in chapter 2.In chapter 3, a short review is given on the properties of PVA-Ac, with special emphasis on the influence of the VAc monomers on the (dis)solution and interfacial properties of the copolymers. Since it was supposed that it is the intra- molecular VAc distribution that determines the interfacial properties of PVA-Ac to a large extent, this aspect has been investigated systematically. To that end, five blocky (B1-B5) and two random (R1 and R2) PVA-Ac copolymers have been prepared, the intra-molecular VAc distributions of which have been analysed in several ways (with complexometry, IR spectroscopy and thermal analysis).The solution properties of these copolymers in water have been studied by viscosimetry. Particular attention has been paid to the methods of processing the experimental data. For the random copolymers, the linear expansion factor in water steadily decreases with increasing VAc content, whereas it passes through a maximum for the blocky copolymers. These differences can not be explained by the often assumed inhibition of inter- and intra-molecular H-bonding between VA segments due to the size of the acetate groups. It, is suggested that the incompatibility of VA and VAc sequences causes the expansion of the blocky copolymers. The decrease in expansion with (a further) increase in VAc content is related to the more hydrophobic character of the VAc monomers.BuOH has a stabilizing influence on PVA in aqueous solution, probably due to preferential adsorption of buOH molecules with their hydrophobic part onto the C-C backbone of the polymers. This influence decreases with increasing VAc content of the copolymers, which is ascribed to the prevention of this adsorption on those parts of the chain to which the acetate groups are attached. An attempt is made to estimate the peripheral solvent quality for the copolymers from their Huggins coefficients. Probably the mean peripheral VAc content is less than the mean overall content of the copolymers.The interfacial activities of the copolymers have been studied by measuring the interfacial tension between a copolymer solution of wabu and buwa, both with the static drop-shape and the dynamic drop-volume method. It is shown experimentally that the more accurate and much simpler drop-volume method is also applicable to these solutions, provided the copolymer concentration is not too low. In this system, the interfacial activity of PVA-Ac increases with the VAc content and, in particular, with the average VAc sequence length of the copolymers. It is concluded that these VAc sequences are the anchors that adsorb at the wabu-buwa interface. The observed time effects are ascribed to the unfolding of the adsorbed copolymers, so that the longer VAc sequences of the inner part of the coil can adsorb.In chapter 4, the KCl transport from wabu to buwa is investigated. With a semi- empirical method it is attempted to separate the hydrodynamic from the physico-chemical contributions to the overall mass transfer coefficient, K b . Although no unambiguous quantitative results are obtained, it can yet be concluded that the partial buwa mass transfer coefficient, k b , is the rate determining step in the transfer process. This implies that the explanation of the retarding effect of PVA-Ac can not be found in a simple reduction of the interfacial area available for transfer. The effect is assumed to be mainly of a hydrodynamic nature. The experimental set-up used does not enable more quantitative results to be obtained, but the hypothesis that adsorption is the cause of the retardation is confirmed.Since the effect of PVA-Ac: can only be properly explained when the interphase mass transfer process itself is understood in all details, the fundamentals of those transport phenomena that are relevant to this process are treated in chapter 5. In addition, attention is paid to the experimental results found in the literature.In chapter 6, a transport vessel is described that has more systematic and better defined flow patterns at both sides of the interface. By assuming two simple but realistic models for the laminar boundary layer flow, k b is estimated theoretically for both a 'clean' (mobile) and a completely covered (stationary) interface. This makes it possible to draw more definite conclusions on the presence of any interfacial resistance.The agreement between the theoretical and experimental mass transfer coefficients for a 'clean' interface allows the conclusion to be made that the interfacial resistance is negligible for the system studied. This must mean that kb is changed drastically by the adsorption of PVA-Ac, which is confirmed by the agreement between the theoretical and experimental mass transfer coefficients for a completely covered interface. However, this method can not provide any direct information on the degree of coverage of a liquid-liquid interface.Yet it appears a sensitive method to distinguish between the interfacial activities of the blocky and random copolymers: the effects are diverse, depending on the total amount of PVA-Ac spread at the interface, on the VAc content and in particular on the average VAc sequence length. These differences are interpreted qualitatively in terms of the irreversibility of the desorption processes and the compressibility of the adsorption layers. Finally, a method is suggested to investigate more quantitatively the behaviour of adsorption layers in a variable, stationary shear-stress field.<p/