Captive Bubble Contact Angle Measurements Research Articles

Stiffening Polymer Brush Membranes for Enhanced Organic Solvent Nanofiltration Selectivity.

Membrane-based separations allow energy-efficient purification of organic solvents which are typically carried out by energy-intensive distillation. Polymer membranes are inexpensive and have obtained widespread industrial acceptance for water and biotech applications but not organic solvent nanofiltration due to relatively low selectivities. In this work, a new class of polymer brush membranes was prepared with high selectivities for methanol-toluene separation. Stiffening the brush structure by cross-linking with aromatic trimesic acid and aliphatic itaconic acid resulted in an increase in selectivity from 1.4 to 6.5-11.5. This was achieved by graft polymerization of a primary amine monomer (aminoethyl methacrylate) using single electron transfer-living radical polymerization (SET-LRP) followed by cross-linking. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and captive bubble contact angle measurements were used to characterize these membranes. The stiffness of the brush membranes was measured using a quartz crystal microbalance-dissipation (QCM-D) and correlated positively with selectivity for separating organic feed mixtures. This new class of membranes offers a tunable and scalable method for purification of organics.

Characterizing surface porosity of porous membranes via contact angle measurements

This investigation attempts to establish and verify a novel method for quantifying surface porosity of porous polymeric membranes via contact angle measurements. Herein, we fabricate a series of porous membranes via nonsolvent induced phase separation (NIPS) comprising different concentrations of polyvinylidene fluoride (PVDF) and PVDF-poly (methyl methacrylate) block co-polymer (PVDF-PMMA) with different concentrations of water and isopropyl alcohol (IPA) in the coagulation bath. Both sessile drop and captive bubble contact angle measurements are used to determine contact angles (and porosity) for both dry and wet membranes, respectively. The former method is probably applicable for membrane distillation, aeration and de-aeration where liquid water does not saturate the membrane, whereas the latter may be more indicative of pressure-driven aqueous membrane separations where the membrane is saturated through its cross-section. Image analysis of scanning electron microscope (SEM) images quantified dry membrane surface porosity. We propose a simple analytical model to obtain wet and dry membrane surface porosity from contact angle measurements. Our results suggest that the surface porosity calculated from both wet and dry contact angle data correlates strongly with the surface porosity calculated from SEM values. However, wet contact angles of the membranes with high porosities produce significantly higher porosity values, which also establishes the importance of porous membrane swelling in determining membrane porosity for aqueous membrane separations.

Surface characterisation of pyrite exposed to A. brierleyi

The surface of pyrite exposed to the thermophilic archaeon Acidianus brierleyi was investigated to elucidate the impact of extracellular polymeric substances (EPS) and/or cells on mineral surface hydrophobicity. The interaction of cells with the surface of pyrite and the physicochemical properties of the mineral surface were studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), captive bubble contact angle measurements and time of flight – secondary ion mass spectrometry (ToF-SIMS). The ToF-SIMS results show no clear evidence of EPS on the pyrite surface after exposure to A. brierleyi and further suggests significant contamination of the surface by the yeast extract included in the recommended A. brierleyi growth medium. It was found that the presence of yeast extract made it impossible to accurately determine the action of EPS on the surface, and potentially impeded chemical and biological reactions and interactions on the surface. Investigation of the impact of A. brierleyi on pyrite in the absence of yeast extract found up to 24° decrease in contact angle compared to the control after 2 h. It was found that A. brierleyi produced very little EPS and as such is unlikely to alter the surface through a contact mechanism, with little to no cell attachment observed on the pyrite surface. Thus removal of yeast extract may improve the application of this archaea to biomining applications.

Surface Chemical Characterisation of Pyrite Exposed to Acidithiobacillus ferrooxidans and Associated Extracellular Polymeric Substances

A. ferrooxidans and their metabolic products have previously been explored as a viable alternative depressant of pyrite for froth flotation; however, the mechanism by which separation is achieved is not completely understood. Scanning electron microscopy (SEM), photoemission electron microscopy (PEEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and captive bubble contact angle measurements have been used to examine the surface physicochemical properties of pyrite upon exposure to A. ferrooxidans grown in HH medium at pH 1.8. C K-edge near edge X-ray absorption fine structure (NEXAFS) spectra collected from PEEM images indicate hydrophilic lipids, fatty acids and biopolymers are formed at the mineral surface during early exposure. After 168 h, the spectra indicate a shift towards protein and DNA, corresponding to an increase in cell population and biofilm formation on the surface, as observed by SEM. The Fe L-edge NEXAFS show gradual oxidation of the mineral surface from Fe(II) sulfide to Fe(III) oxyhydroxides. The oxidation of the iron species at the pyrite surface is accelerated in the presence of A. ferrooxidans and extracellular polymeric substances (EPS) as compared to HH medium controls. The surface chemical changes induced by the interaction with A. ferrooxidans show a significant decrease in surface hydrophobicity within the first 2 h of exposure. The implications of these findings are the potential use of EPS produced during early attachment of A. ferrooxidans, as a depressant for bioflotation.

Correlation of Surface Adsorption and Oxidation with a Floatability Difference of Galena and Pyrite in High-Alkaline Lime Systems.

When it comes to Pb-Zn ores with high amounts of pyrite, the major problem encountered is the low separation efficiency between galena and pyrite. By virtue of high dosage of lime and collector sodium diethyl dithiocarbamate (DDTC), pyrite and zinc minerals are depressed, allowing the galena to be floated. However, there have been significant conflicting reports on the flotation behavior of galena at high pH. In this context, correlation of the surface adsorption and oxidation with the floatability difference of galena and pyrite in high-alkaline lime systems would be a key issue for process optimization. Captive bubble contact angle measurements were performed on freshly polished mineral surfaces in situ exposed to lime solutions of varying pH as a function of immersion time. Furthermore, single mineral microflotation tests were conducted. Both tests indicated that the degree of hydrophobicity on the surfaces of galena and pyrite increased in the presence of DDTC at natural or mild pulp pH. While in a saturated lime solution, at pH 12.5, DDTC only worked for galena, but not for pyrite. Surface chemistry analysis by time-of-flight secondary ion mass spectrometry (Tof-SIMS) confirmed the preference of DDTC on the galena surface at pH 12.5, which contributed to a merit recovery. Further important evidence through measurements of Tof-SIMS, ion chromatography, and high-performance liquid chromatography indicated that in high-alkaline lime systems, the merit floatability of galena could exclude the insignificant contribution of elemental sulfur (S8) and was dominantly attributed by the strong adsorption of DDTC. In contrast, the poor flotation response of pyrite at high pH was due to the prevailing adsorption of CaOH+ species. This study provides an important surface chemistry evidence for a better understanding of the mechanism on the better selectivity in the galena-pyrite separation adopting high-alkaline lime systems.

Physical chemistry considerations in the selective flotation of bastnaesite with lauryl phosphate

Bastnaesite is one of the important mineral resources for the production of rare earth materials, and it is typically associated with calcite and quartz as gangue minerals. Fatty acid or hydroxamic acid are recommended collectors for bastnaesite flotation. In this research, potassium lauryl phosphate was expected to provide a stronger flotation response for bastnaesite than for the calcite and quartz gangue minerals. Using potassium lauryl phosphate as collector, the results of captive bubble contact angle measurements, zeta potential determinations and microflotation experiments for bastnaesite, calcite and quartz are reported. Greater selectivity for bastnaesite was achieved using potassium lauryl phosphate than using octyl hydroxamate as collector. These results indicate that potassium lauryl phosphate might be a promising collector in the flotation of bastnaesite ores. The computation results of universal force feld (UFF) interaction energies for the mineral-reagent interactions are also reported and were found to correlate well with the experimental microflotation results.

Hydration and chain entanglement determines the optimum thickness of poly(HEMA-co-PEG₁₀MA) brushes for effective resistance to settlement and adhesion of marine fouling organisms.

Understanding how surface physicochemical properties influence the settlement and adhesion of marine fouling organisms is important for the development of effective and environmentally benign marine antifouling coatings. We demonstrate that the thickness of random poly(HEMA-co-PEG10MA) copolymer brushes affect antifouling behavior. Films of thicknesses ranging from 50 to 1000 Å were prepared via surface-initiated atom-transfer radical polymerization and characterized using infrared spectroscopy, ellipsometry, atomic force microscopy and contact angle measurements. The fouling resistance of these films was investigated by protein adsorption, attachment of the marine bacterium Cobetia marina, settlement and strength of attachment tests of zoospores of the marine alga Ulva linza and static immersion field tests. These assays show that the polymer film thickness influenced the antifouling performance, in that there is an optimum thickness range, 200-400 Å (dry thickness), where fouling of all types, as well as algal spore adhesion, was lower. Field test results also showed lower fouling within the same thickness range after 2 weeks of immersion. Studies by quartz crystal microbalance with dissipation and underwater captive bubble contact angle measurements show a strong correlation between lower fouling and higher hydration, viscosity and surface energy of the poly(HEMA-co-PEG10MA) brushes at thicknesses around 200-400 Å. We hypothesize that the reduced antifouling performance is caused by a lower hydration capacity of the polymer for thinner films, and that entanglement and crowding in the film reduces the conformational freedom, hydration capacity and fouling resistance for thicker films.

The role of mineral surface chemistry in modified dextrin adsorption

The adsorption of two modified dextrins (phenyl succinate dextrin – PS Dextrin; styrene oxide dextrin – SO Dextrin) on four different mineral surfaces has been studied using X-ray photoelectron spectroscopy (XPS), in situ atomic force microscopy (AFM) imaging, and captive bubble contact angle measurements. The four surfaces include highly orientated pyrolytic graphite (HOPG), freshly cleaved synthetic sphalerite (ZnS), and two surfaces produced through surface reactions of sphalerite: one oxidized in alkaline solution (pH 9, 1h immersion); and one subjected to metal ion exchange between copper and zinc (i.e. copper activation: exposed to 1×10−3M CuSO4 solution for 1h). XPS measurements indicate that the different sphalerite surfaces contain varying amounts of sulfur, zinc, oxygen, and copper, producing substrates for polymer adsorption with a range of possible binding sites. AFM imaging has shown that the two polymers adsorb to a similar extent on HOPG, and that the two polymers display very different propensities for adsorption on the three sphalerite surface types, with freshly cleaved sphalerite encouraging the least adsorption, and copper activated and oxidized sphalerite encouraging significantly more adsorption. Contact angle measurements of the four surfaces indicate that synthetic sphalerite has a low contact angle upon fracture, and that oxidation on the timescale of one hour substantially alters the hydrophobicity. HOPG and copper-activated sphalerite were the most hydrophobic, as expected due to the carbon and di/poly-sulfide rich surfaces of the two samples, respectively. SO Dextrin is seen to have a significant impact on the wettability of HOPG and the surface reacted sphalerite samples, highlighting the difficulty in selectively separating sphalerite from carbonaceous unwanted minerals in flotation. PS Dextrin has the least effect on the hydrophobicity of the reacted sphalerite surfaces, whilst still significantly increasing the wettability of graphite, and thus has more potential for use as a polymer depressant in this separation.

Reduction of Surface Hydrophobicity Using a Stimulus-Responsive Polysaccharide

The adsorption of carboxymethyl cellulose (CMC) onto a hydrophobic self-assembled monolayer has been characterized using the quartz crystal microbalance (with dissipation monitoring, QCM-D). Adsorption was studied as a function of initial solution conditions. CMC adsorbs to a greater extent at high ionic strength (10(-1) M KCl as opposed to 10(-2) M KCl) or low pH (3 as opposed to 9). The solution conditions that yielded the lowest initial adsorbed amount (10(-2) M KCl, pH 9) were used as a reference to investigate the response of the adsorbed layer to a switch in solution conditions after adsorption (i.e., to higher ionic strength (10(-1) M KCl) or lower pH (pH 3)). The adsorbed layer released significant amounts of hydration water after each solution switch, as determined by the QCM-D measurements. This expulsion of hydration water was fully reversible. For the two solution switches, reducing the solution pH resulted in a more pronounced change in the amount of hydration water within the adsorbed CMC, accompanied by a distinct conformational change, as determined from a QCM D-f plot. In addition to studying adsorption using QCM-D, the effect of adsorbed CMC on surface hydrophobicity has been investigated using captive bubble contact angle measurements. The effect of the polymer on the contact angle of the surface was seen to be greatest when adsorbed at low pH or at higher ionic strength. CMC was also seen to have a significantly enhanced ability to reduce the surface hydrophobicity after both the ionic strength and pH switches, lowering the advancing water contact angle by 6 and 23° and the receding water contact angle by 10 and 40° for the ionic strength and pH switches, respectively. As with the change in hydration water content, the change in the contact angle of the polymer-coated surface following the solution switches was reversible.

Adsorption of tailored carboxymethyl cellulose polymers on talc and chalcopyrite: Correlation between coverage, wettability, and flotation

A carefully chosen selection of CMC polymers was used to probe the depression of talc and chalcopyrite and to highlight the effect of specific alterations of the bulk polymer chemistry on the power and selectivity of the depressants. In situ tapping mode atomic force microscopy (AFM), captive bubble contact angle measurements, and flotation tests were used to study the adsorption of the CMC polymers on chalcopyrite and talc. The degree of substitution and distribution of substitution of carboxymethyl groups on CMC was seen to have a strong influence on the ability of the polymers to adsorb onto the surface of talc and chalcopyrite. High substitution results in low coverage on both minerals whereas low substitution results in higher coverage. The influence of distribution of substitution (random or clustered) was also seen to affect the adsorption, with more random/even substitution giving rise to greater adsorption on talc and chalcopyrite. The contact angle of talc and chalcopyrite is significantly altered in the presence of the three polymers, with increased surface coverage (determined by AFM) correlating with the ability of the polymers to decrease the wettability of talc and chalcopyrite. This trend was also followed in single and mixed mineral talc and chalcopyrite flotation.

Adsorption of modified dextrins to a hydrophobic surface: QCM-D studies, AFM imaging, and dynamic contact angle measurements

The adsorption of three dextrin-based polymers, regular wheat dextrin (Dextrin TY), phenyl succinate dextrin (PS Dextrin), and styrene oxide dextrin (SO Dextrin) on a model hydrophobic surface, consisting of a mixed alkanethiol layer on gold, has been characterized using the quartz crystal microbalance with dissipation monitoring (QCM-D). The three polymers exhibited varying affinities and capacity for adsorption on the hydrophobic substrate. Atomic force microscope (AFM) imaging of the polymer layers indicates that all three polymers fully cover the surface. The effect of the three polymers on the static contact angle of the surface was studied using captive bubble contact angle measurements. The three polymers were seen to reduce the receding contact angle by similar amounts (approximately 14°) in spite of having varying adsorbed amounts and differences in adsorbed layer water content. Although no differences were observed in the ability of the polymers to reduce the static contact angle, measurements of the dynamic contact angle between a rising air bubble and the polymer covered substrate yielded stark differences between the polymers, with one polymer (SO Dextrin) slowing the dewetting by an order of magnitude more than the other two polymers. The differences in dewetting behavior correlate with the adsorbed layer characteristics determined by QCM-D and AFM. The role of the dynamic and static contact angle in the performance of a polymer as depressant is discussed.

In situ atomic force microscopy of modified dextrin adsorption on hydrophobic and hydrophilic layered silicate minerals

We have used in situ atomic force microscopy (AFM), captive bubble contact angle measurements, and colloid-probe AFM to investigate the adsorption of two modified dextrins (a phenyl succinate substituted dextrin, PS Dextrin, and a styrene oxide substituted dextrin, SO Dextrin) on the basal plane surfaces of talc and clinochlore. The experiments have probed the effect that the polymers have on the mineral hydrophobicity and on the expected particle–particle interactions in single mineral aggregation. Distinct correlations were seen between the adsorbed polymer layer morphology (coverage, thickness) and the ability of the polymers to reduce the contact angle of the talc basal plane surfaces (SO Dextrin > PS Dextrin). Both polymers were seen to lower the contact angle of clinochlore to an apparent limiting value of approximately 25°. The effect of the polymers on the forces between hydrophobic surfaces (talc basal plane and a hydrophobised titania sphere) and between hydrophilic surfaces (clinochlore cleaved surface and a silica sphere) was seen to be a complicated relationship between the adsorbed layer morphology and bulk polymer properties, with polymers exhibiting a clear tendency to bridge between hydrophobic surfaces. Neither of the dextrins was found to encourage attraction between hydrophilic surfaces, in spite of adsorbing to a significant extent on the clinochlore cleaved surface.

Surface chemistry features in the flotation of KCl

Despite the successful industrial practice to recover KCl by flotation, the adsorption state of collectors at KCl salt surfaces is still not fully understood. In this paper, the flotation behavior of KCl and NaCl using different collectors and captive bubble contact angle measurements at KCl and NaCl crystal surfaces in their saturated solutions are reported. The influence of collector concentration and collector structure is considered. The results show that both cationic and anionic collectors produce a hydrophobic state at the structure breaking KCl salt surface, with finite contact angle values of between 30 degrees and 60 degrees. In contrast, the contact angle at the structure making NaCl salt surface is zero at all collector concentrations, suggesting that no collector adsorption occurs, a supposition that is supported by results from molecular dynamics simulation (MDS). Further, it appears that the presence of surface defects such as edges and corners promote the adsorption of collectors at the KCl surface, and that the flotation is achieved by bubble attachment at the edges/corners of KCl particles with the formation of aeroflocs. (C) 2009 Elsevier Ltd. All rights reserved.

The Influence of Surface Hydrophobicity on Polyacrylamide Adsorption

The adsorption of a modified polyacrylamide on gold surfaces coated with varying proportions of -CH3- and -OH-terminated alkanethiols (producing substrates of varying hydrophobicity: thetac=75, 98, and 119 degrees), was investigated using quartz crystal microbalance with dissipation (QCM-D), tapping-mode atomic force microscopy (TM-AFM), and captive bubble contact angle measurements. The QCM-D data for the polymer adsorbing on the different substrates indicates that the polymer adsorbs faster and to a greater extent on surfaces with higher hydrophobicity. Dissipation data from the QCM-D suggests that the adsorbed polymer undergoes a conformational change when adsorbing onto the substrates of higher hydrophobicity, forming a less rigid extended layer as the adsorption progresses toward the maximum adsorbed amount. AFM imaging of the adsorbed layer illustrates that the polymer layer is incomplete on all three substrates, and that the underlying substrate hydrophobicity has a role in determining the morphology (distribution, coverage, and thickness) of the adsorbed layer. Contact angle measurements of the polymer-coated substrates show variation in the ability of the polymer to reduce the hydrophobicity of the substrates. The role of coverage and distribution of adsorbed polymer on the surface hydrophobicity reduction is discussed.

Physico-chemical and thermodynamic aspects of fibroblastic attachment on RGDS-modified chitosan membranes

In the present study, the cell attachment/spreading behaviour of L929 mouse fibroblasts on chitosan membranes was evaluated by using physico-chemical properties. For this purpose chitosan membranes were prepared and then photochemically modified with the cell adhesive peptide RGDS (Arg-Gly-Asp-Ser). The physico-chemical properties of unmodified (CHI) and RGDS-modified chitosan (CHI-RGDS) membranes were evaluated by calculating surface free energy ( γ sv) and interfacial free energy ( γ sw) values using captive bubble contact angle measurements and harmonic mean equation. The cell attachment experiments were performed both in 10% FBS containing and serum-free media with CHI and CHI-RGDS membranes. Eventually, it was not possible to predict a direct relationship between the change in physico-chemical properties and L929 cell attachment behaviour. The experimental results obtained from cell attachment agree with the theoretical prediction for the free energy of adhesion except for the cell attachment on CHI membrane in serum-free medium. Although a negative interfacial free energy of adhesion was calculated for CHI membrane in serum-free medium (Δ F adh = −2.19 ergs/cm 2), the cell attachment was poor (∼70%) compared to CHI-RGDS (∼90%) and none of the cells were spread on CHI surface to gain a fibroblastic morphology. Negative energy of adhesion was calculated for CHI and CHI-RGDS in 10% FBS medium, in which ∼100% of cells were attached on the membranes correlating with the thermodynamic approach. It can be suggested that, adsorption of serum proteins strongly affected the cell attachment meanwhile the presence of biosignal RGDS molecules triggered the cell spreading in serum medium.

A review of pyrrhotite flotation chemistry in the processing of PGM ores

The chemistry of pyrrhotite flotation using xanthate collectors is reviewed with respect to the processing of PGM ores and the recent results from captive bubble contact angle measurements at the University of Utah are presented. In some cases a low flotation recovery of PGM may be due to the surface state of pyrrhotite particles under conventional flotation conditions (open to air and pH 9.0). Thermodynamically pyrrhotite is not stable and reacts relatively quickly with its environment. Natural/collectorless flotation of pyrrhotite is observed only under a low oxidation potential in acidic solution. Its surface is easily oxidized to ferric hydroxide/oxide under conventional flotation conditions, creating a hydrophilic state at the pyrrhotite surface and low flotation recovery even though xanthate collectors can be adsorbed. Under these conditions, activation by copper is not easily achieved. These observations reported in the literature have been confirmed by captive bubble contact angle measurements. Based on the analysis of previous research, conditions for improved pyrrhotite flotation and increased PGM recovery are suggested.

Nanometer-Scale Roughness Having Little Effect on the Amount or Structure of Adsorbed Protein

Neither the adsorbed amount per unit actual surface area nor the structural stability of hen egg lysozyme is influenced by increasing the nanometer-scale roughness (5 < Rave < 60 nm) of a series of model substrates. Seven poly(ether sulfone) (PES) ultrafiltration membranes of increasing mean pore size with the same surface chemistry were chosen as model rough surface substrates. Topographical images, using atomic force microscopy, combined with attenuated total reflection Fourier transform infrared spectroscopy (ATR/FTIR) and sessile captive bubble contact angle measurements were used to characterize the surface properties of the substrates. ATR/FTIR spectroscopy together with a newly developed optimization algorithm for predicting the content of secondary structure motifs is used to correlate the secondary structure and amount of adsorbed lysozyme with the substrate surface roughness. From the adsorption measurements, the net adsorbed amount (total minus nonspecific adsorbed amount) of lysozyme correspon...

Bubble contact angle variation of sulphide minerals in relation to their self-induced flotation

Standard method for captive bubble contact angle measurements in distilled water usually yield no finite contact angles on common sulphide minerals such as chalcopyrite, galena and chalcocite although samples from such minerals have been shown to float in the absence of a collecting agent. The current study documents a case, where small but stable contact angles have been obtained on these minerals. Measured angles ranged from 0 to 36° for chalcopyrite, 0–21° for galena and 0–24° for chalcocite. During the initial period, volumes of attached air bubbles have been noted to decrease. A pronounced pH dependence has been notable from the results. High contact angles have been distinctly associated with low pH and relatively short induction periods. These effects are attributable to increased stability of elemental sulphur. Development of hydrophobicity on these minerals is in general agreement with the results of small-scale tests on their self-induced flotation. The correlation for chalcopyrite has been most notable for the acidic pH range. No correlation has been observed between the contact angle variation and flotation behavior of galena.

Photochemical modification of 10 kDa polyethersulfone ultrafiltration membranes for reduction of biofouling

Poly(ether sulfone) 10 kDa ultrafiltration membranes were modified by photolysis using ultraviolet light and graft polymerization of hydrophilic monomers onto the membrane surface to create more hydrophilic and lower fouling membrane surfaces. The modified membrane surfaces were characterized by FTIR/ATR and captive bubble contact angle measurements to determine chemical and hydrophilicity changes during modification. The modified membranes were compared with an unmodified poly(ether sulfone) (control) membrane as well as a commercial regenerated cellulose and a low protein adsorbing poly(ether sulfone) membrane using a newly developed standardized filtration protocol with 1 wt% bovine serum albumin. The best performing modified membrane was with N-vinyl-2-pyrrolidinone and showed a 25% increase in hydrophilicity, a 49% decrease in bovine serum albumin fouling, and a 4% increase in bovine serum albumin retention compared to the unmodified poly(ether sulfone) membrane. While the regenerated cellulose membrane had the lowest fouling and the low protein adsorbing membrane had the highest flux of all tested membranes, the N-vinyl-2-pyrrolidinone-modified membranes had the best combination of low fouling and high flux.