M.C. van der Leeden

Effect of additives on nucleation rate, crystal growth rate and induction time in precipitation

Abstract A theory is proposed and general expressions are derived for the nucleation rate, the rate of crystal growth by two-dimensional nucleation and the introduction time in unseeded precipitation in the presence of additives. According to the theory, the additives behave as active centres for nucleation and do not alter the surface free energy and edge free energy of the nuclei by adsorption on them, because of the short lifetime of the nuclei and/or their small surface area. The surface free energy and edge free energy of the nuclei formed on the additives, however, can be different from the surface free energy and edge free energy of the nuclei on the original active centres in heterogeneous nucleation. Induction times in unseeded barium sulphate precipitation are measured as a function of the concentration of an additive, a co-polymer of maleic acid and vinyl sulphonic acid, and interpreted by the proposed theory. It is concluded that this theory describes fairly well the obtained experimental data, which show a stimulation of the precipitation process at very low additive concentrations and a change-over into retardation at an additive concentration of ≈ 0.005 ppm.

Precipitation of barium sulfate : induction time and the effect of an additive on nucleation and growth

Abstract On the basis of recently derived theoretical expressions for the induction time in seeded and unseeded precipitation, nucleation and growth rates of BaSO 4 are separately determined in the absence and presence of a copolymer of maleic acid and vinyl sulfonic acid (PMA-PVS). From the seeded precipitation experiments at different supersaturations, BaSO 4 is found to grow by the mechanism of two-dimensional nucleation and its growth rate is determined. Complete determination of the BaSO 4 nucleation rate is also done via a combined analysis of the induction times in both seeded and unseeded precipitation. The presence of PMA-PVS in the solution appears to retard the growth and to stimulate the nucleation of BaSO 4 .

How to develop globular proteins into adhesives.

To make globular proteins suitable for application in adhesives, the specific bonds and interactions which shape their structure have to broken. Only then, a layer of relatively large, flexible and interwoven polymer chains, which are firmly attached to the solid surface by adsorption, can be created. Such a network layer is essential to save the adhesive bond under an applied force, because it can distribute the concentration of stresses generated at the interface into the bulk. Unfolding and swelling of a protein can be achieved by changing the solvent quality. For the globular whey protein beta-lactoglobulin, the optimal conditions for unfolding and swelling is found with 98% formic acid as a solvent. In formic acid, beta-lactoglobulin looses its amphoteric character (it is protonated, probably for approximately 20%). In addition, formic acid is less polar than water and thus a better solvent for the apolar parts of the protein. The swelling and unfolding behaviour of beta-lactoglobulin is studied by viscosity and CD-spectroscopy measurements. For the interpretation of the results we apply the Kuhn formalism that the conformation of a protein can be described in terms of a statistical chain which consists of segments of an average persistence length P. The statistical segment length P, which varies with the experimental conditions, is directly related to the adsorption energy required for a strong adhesion between coil and surface. It determines the depletion energy kT P(-2) m(-2) which must be overcome by specific attraction between side groups of the protein chain and the surface. For beta-lactoglobulin in 98% formic acid, we find a P value of approximately 2.2 nm, pointing at a relatively flexible chain. The minimum net adsorption energy kT P(-2) is then approximately 1 mJ m(-2), a relatively small value to be exceeded. Preliminary results of destructive adhesion tests on beech wood lap-shear joints reveal promising tensile strengths of approximately 2.9+/-1.1 N mm(-2), indeed.

Surface properties of plastic materials in relation to their adhering performance

Adhesion between polymeric phases like adhesives and plastic surfaces is critical in many technological and industrial applications. In the last decades much progress has been made to understand the impact of the surface properties of both phases on the adhesive strength between them. These surface properties influence processes like wetting, molecular adsorption and inter-diffusion which determine how an interface develops into an interphase after the two materials have been brought into contact. Ultimately, the properties of this interphase determine the overall adhesion strength of an assembly. In this paper important parameters in the adhesion process will be reviewed, including methods to engineer these parameters in order to attain adhesion strengths ranging from complete release to irreversible bonding.

Aspects of additives in precipitation processes: Performance of polycarboxylates in gypsum growth prevention

Abstract A compendious survey of various types of additives in relation to their application is presented. Also some new measuring techniques to determine precipitation kinetics, agglomeration and other crystal features, which are subjected to additive adsorption, are briefly discussed. In the field of water treatment and desalination additives are widely used either alone as inhibitors or in combination with dispersants and corrosion inhibitors. To quantify the influence of polycarboxylate inhibitors with variations in their molecular structures on the crystallization of gypsum, suspension growth experiments were performed using the “constant composition” method. Additionally the synergistic effect of either two inhibitors or an inhibitor and a dispersant was determined. The most effective growth retarders at pH 5 appeared to be unsubstituted low-molecular weight polyacrylates. At pH 8, where the performance of polyacrylates is highly reduced owing to their far advanced ionization, introduction of itaconic acid segments into the polymer resulted into an enhanced effectiveness. The combination of polyacrylic acid and polystyrene sulfonic acid gave rise to a synergistic effect.

The degree of aggregation in solution controls the adsorbed amount of mussel adhesive proteins on a hydrophilic surface

Abstract The adsorption behaviour of the mussel adhesive protein Mefp-1 on a hydrophilic surface was studied by Surface Plasmon Resonance (SPR) at pH values of 4.5 and 6.5 under aerobic conditions and at an ion strength of 0.1 M NaCl. In this environment Mefp-1 molecules aggregate by crosslinking, likely via Dopa–Dopa quinone charge transfer interactions. The initial rate of aggregation increases with increasing pH, as could be derived from Photon Correlation Spectroscopy measurements. The degree of aggregation determines the adsorption plateau value of Mefp-1. Step-like adsorption curves have been found at pH 6.5, as well as at pH 4.5, which can be interpreted as the adsorption of an ad-layer of Mefp-1 aggregates onto the initially adsorbed Mefp-1 layer on the surface. The rate of formation of this second layer increases with increasing pH and Mefp-1 concentration. The affinity of the ad-layer for the first adsorbed layer appears to be much smaller than the affinity of the first layer for the surface (Poly Vinyl Alcohol). Probably, also the ad-layer formation proceeds by the establishment of specific crosslinks with the first layer of adsorbed Mefp-1.

Effect of the molecular weight of polyphosphinoacrylates on their performance in BaSO4 growth retardation

Abstract The retarding influence of three polyphosphinoacrylates and a polyacrylate with a phosphonate endgroup on the growth of barium sulfate (1000–5000 D), is related to their structure, their molecular weight distribution and to their adsorption behaviour. For the polyphosphinoacrylates the inhibiting effectiveness enhances with an increasing average molecular weight and with an increasing adsorption level. Although the highest adsorption plateau is reached by the polyacrylate with the phosphonate endgroup, this compound appears to be only a moderate inhibitor. This can be explained by a loopier confirmation at the BaSO 4 surface compared to an extended flat adsorption of the polyphosphinoacrylates under the prevailing conditions. Gel permeation chromatography (GPC) measurements of the inhibitor solutions before and after adsorption, reveal a preferential adsorption of the highest molecular weight fraction for all tested compounds. This is in agreement with a theoretical model developed for uncharged polymers.

Inhibition of Barium Sulfate Deposition by Polycarboxylates of Various Molecular Structures

To establish a relationship between the molecular structure of polycarboxylates and their growth-retarding influence on barium sulfate, seeded-suspension-growth experiments were performed at various inhibitor concentrations and pH values. Two types of polycarboxylates with a molecular structure based on their polyacrylic or maleic acid were studied. The molecular structure of these compounds were varied by particle substitution with monomers containing hydroxyl, amide, and sulfonic acid, as well as hydrophobic groups. Hydrophobic groups are detrimental to good inhibitor performance, whereas the introduction of OH, NH {sub 2}, or SO {sub 3} H groups presents opportunities to enhance the inhibitor effectiveness. The sequence in performance of the compounds on barium sulfate was compared with the sequence formerly obtained for calcium sulfate dihydrate.

β-Lactoglobulin as an ideal random polymer coil

Abstract Unfolding of globular proteins by breaking the bonds and interactions that shape their structure, results in more flexible macromolecular chains with a relatively high percentage of apolar side group. Such properties may be beneficial for application in coatings and adhesives. It is then essential to define the flexibility of the chains at very high protein concentrations. As a measure for the stiffness of an unfolded protein chain the average persistence length P , which varies with the experimental conditions, is introduced. We demonstrate that P at very high protein concentrations can be predicted by using measuring data of dilute solutions, as is required by the available analytical techniques. In this study the globular protein β-lactoglobulin is used as a model. Its secondary structure can be disrupted by applying 98% formic acid as a solvent and by additional chemical modification of the thiol groups to break the two remaining covalent SS bonds. The conformation of both unfolded structures, with and without chemical modification, can be described according to the Kuhn formalism, leading to a P value of ±2 nm This value is determined by viscosimetry and by SANS and can be held representative for very concentrated protein layers where θ-conditions are assumed.

Adsorption Behaviour of Polyelectrolytes in Relation to the Crystal Growth Kinetics of Barium Sulfate

For two polyphosphinoacrylates PPAA-I and PPAA-II and a copolymer of maleic acid and vinylsulfonic acid PMA-PVS, the adsorption levels on BaSO4 were determined as a function of time and compared with the growth inhibiting effectiveness of these compounds in BaSO4 crystallization. Only slightly different values are found for the equilibrium adsorption plateau of the best growth retarder PMA-PVS and the moderate inhibitor PPAA-II, but the time needed to establish the plateau coverage appears to be considerably shorter in the case of PMA-PVS. An indication is found that the adsorption rate of PPAA-II interferes with the growth rate of BaSO4 in the absence of an inhibitor under the given experimental conditions, which has a negative influence on its growth inhibiting performance.