Interfacial Free Energy Changes Research Articles

Heterogeneous Nucleation of Colloidal Crystals on a Glass Substrate with Depletion Attraction.

The heterogeneous nucleation of colloidal crystals with attractive interactions has been investigated via in situ observations. We have found two types of nucleation processes: a cluster that overcomes the critical size for nucleation with a monolayer, and a method that occurs with two layers. The Gibbs free energy changes (ΔG) for these two types of nucleation processes are evaluated by taking into account the effect of various interfacial energies. In contrast to homogeneous nucleation, the change in interfacial free energy, Δσ, is generated for colloidal nucleation on a foreign substrate such as a cover glass in the present study. The Δσ and step free energy of the first layer, γ1, are obtained experimentally based on the equation deduced from classical nucleation theory (CNT). It is concluded that the ΔG of q-2D nuclei is smaller than of monolayer nuclei, provided that the same number of particles are used, which explains the experimental result that the critical size in q-2D nuclei is smaller than that in monolayer nuclei.

Determination of the Point of Zero Charge pH of Borosilicate Glass Surface Using Capillary Imbibition Method

The point of zero charge pH of an oxide surface is a fundamental surface chemistry property or solids or metal oxides that determine the nature of interaction at the solid-aqueous solution interface. In colloid chemistry this physical parameter controls the evolution of the electric double layer as well as adsorption and desorption processes.In colloid chemistry a number of methods have been used for the determination of the point of zero charge pH of an oxide surface. This ranges from titrimetric to radiation chemistry approach that deals with scanning electron microscopy.In this study, the direct effect of aqueous solution acidity on the solid-liquid interfacial free energy and the consequence of this effect on spontaneous imbibition of aqueous solution into borosilicate glass have been exploited for the determination of the point of zero charge pH of this type of glass. What is new in this method is that while the traditional titration method relies on neutralization of surface charges, the present method relies on interfacial free energy changes due to aqueous solution pH changes and the effect of this on the wettability of borosilcate glass surface. Result of point of zero charge pH obtained from the present method has been compared with those obtained using traditional methods. The comparison shows close agreements and this proves the technique used in the present work as a novel method for the determination of the point of zero charge pH of oxide surfaces.

Phase field modeling of facet formation during directional solidification of silicon film

Adaptive phase field modeling is conducted to study the facet formation during directional solidification of silicon film. The necessary condition for morphological instability is examined first based on the classic theory for the reported experiments. With a proper thermal gradient, the simulated onset velocity is found consistent with the experimental observations and the classic theory. The effects of anisotropy of kinetic coefficients and interfacial energy are also considered, and the simulated morphologies are significantly affected. The range of the kinetic cusp function affects the facet tips, while the anisotropy of interfacial free energy changes the wavelength of facets. Nevertheless, the simulation of kinetic undercooling on the facets is not successful, and the computational difficulty is discussed.

Preparation of hemispherical polyimide particles by inverse emulsion technique from poly(amic acid) ammonium salts

Nonspherical polymer particles have attracted increasing attention recently. In this paper, micron-scale hemispherical polyimide (PI) particles were fabricated using water-soluble poly(amic acid) ammonium salts (PAAS) by a novel inverse emulsion technique. In the process, liquid paraffin was used as a continuous phase, the mixed solution of PAAS and water as a dispersed phase and sorbitan monooleate (Span80) as a surfactant. The research suggested that water as a stabilizing agent played an important role in forming stable emulsion. As the amount of water increased, stability of the emulsion increased gradually and morphology of PI particles transformed from sphere to ellipsoid, and finally to hemisphere. The concentration of PAAS solution and Span80 both affected the shape of particles, which changed from spherical to hemispherical by increasing the PAAS/Span80 concentration. The mechanism of forming hemispherical PI particles was discussed based on interfacial tension and interfacial free energy changes. Via adjusting the composition of the system to change the corresponding interfacial tension, we could get the particles with different morphologies. Furthermore, the change in structure characterized by FT-IR spectroscopy demonstrated that PAAS had been converted to PI after adding the dehydrating agent to the emulsion. And TGA results showed that the obtained PI particles had excellent thermal stability.

Analysis of heterogeneous nucleation on rough surfacesbased on Wenzel model

Heterogeneous nucleation plays an important part in the solidification process. In order to describe the effects of the substrate surface on the heterogeneous nucleation, an idealized planar interface between the nucleus and the substrate is assumed in the consideration of the classical nucleation theory, but this may lead to the deviation of the experimental results from the theoretical predictions, since there is no idealized planar surface on any actual substrate. The heterogeneous nucleation procedure of solid phase on rough substrate surface is investigated, and the effect of the roughness factor of the substrate on nucleation energy is analyzed based on the Wenzel wetting model. The results show that when the intrinsic contact angle between the substrate and the nucleus in the base phase is less than 90°, higher roughness of the substrate surface makes it easier to induce heterogeneous nucleation; when intrinsic contact angle is greater than 90°, higher roughness would restrict the nucleation on the substrate surface. Meanwhile, attaching and wetting drift embryos on the substrate play an important part in the formation of effective nucleus on the substrate, while the change of interfacial free energy in the wetting process of the embryos on the rough substrate surface has an important effect on heterogeneous nucleation behaviors.

Solvent Effect on the Unfolding Force of a Single Hydrophobic Polymer

The hydrophobic collapse of a homopolymer is a much simplified model for studying the hydrophobic collapse of a protein. It is widely believed and theorized that the driving force for the hydrophobic collapse is the interfacial free energy between the polymer and the solvent. Therefore, changes in interfacial free energy should be directly proportional to the force to unfold the polymer in bad solvents. To test this hypothesis, we used single molecule force spectroscopy to unfold a single polystyrene chain in water-ethanol mixtures. Different percentage of binary mixture is used to create solvents with different interfacial energy with polystyrene. However, we do not see a linear correlation between the interfacial tensions with the unfolding forces. This is an indication that macroscopic properties such as the interfacial free energy cannot be directly applied to study certain systems in microscopic scale. In this study we also hypothesized a mechanism for the cause of this inconsistency.

EDGE emulsification for food-grade dispersions

In this paper, we use the Edge-based Droplet GEneration (EDGE) emulsification method to produce food-grade emulsions (including double emulsions) and foams. This newly developed mild technology proved to be very stable and robust in the production of all these products. The products are made with food-grade components in an up-scaled micro device, which does not show any changes in time in wettability and fouling. The size of the droplets and bubbles is as needed for food stuffs. Air bubbles generated with EDGE were much larger than emulsion droplets, which could be explained through the viscosity ratio of the phases and changes in interfacial free energy caused by dynamic interfacial tension effects. In the outlook section of this paper, the obtained results are related to the dimensions of the devices, which are in a practically feasible range, also due to the simplicity of the EDGE structure and its operation. Preliminary estimations show that a 300L system can produce 1 m/h 4% (v/v) emulsion.

Attachment of Shiga toxigenic Escherichia coli to stainless steel

Shiga toxigenic Escherichia coli (STEC) are important foodborne pathogens causing gastrointestinal disease worldwide. Bacterial attachment to food surfaces, such as stainless steel may lead to cross contamination of foods and subsequent foodborne disease. A variety of STEC isolates, including E. coli O157:H7/H− strains, were grown in planktonic (broth) and sessile (agar) culture, following which initial attachment to stainless steel was determined using epifluorescence microscopy. Experiments were performed to determine whether the number of bacteria attached to stainless steel differed between STEC strains and between the two modes of growth. No relationship was found between STEC strains and the number of bacteria attached to stainless steel. Five STEC strains, including one non-toxigenic O157 isolate, attached in significantly greater ( p < 0.05) numbers to stainless steel following growth in planktonic culture compared to sessile culture. In contrast, two clinical strains of O157:H7 attached in significantly greater ( p < 0.05) numbers following growth in sessile culture compared to planktonic culture. Thirteen out of twenty E. coli strains showed no significant difference ( p > 0.05) in attachment when grown in planktonic or sessile culture. The change of interfacial free energy between the bacterial strains and stainless steel was calculated and the influence of free energy in attachment was determined. Although a significant variation ( p < 0.05) in free energy values was found between STEC strains, no correlation was found between free energy values and bacterial counts on stainless steel. In addition, no correlation was also found between bacterial hydrophobicity and surface charge values or production of surface structures (type I fimbriae or flagella) (previously determined) with the number of bacteria attached to stainless steel. The results of this study suggest that different growth conditions (planktonic and sessile) can influence the attachment of STEC to stainless steel. Factors other than bacterial physicochemical properties and these surface structures may also influence STEC attachment to stainless steel.

Enthalpy and interfacial free energy changes of water capillary condensed in mesoporous silica, MCM-41 and SBA-15

The effect of confinement on the solid-liquid phase transitions of water was studied by using DSC and FT-IR measurements. Enthalpy changes upon melting of frozen water in MCM-41 and SBA-15 were determined as a function of pore size and found to decrease with decreasing pore size. The melting point also decreased almost monotonically with a decrease in pore size. Analysis of the Gibbs-Thomson relation on the basis of the thermodynamic data showed that there were two stages of interfacial free energy change after the constant region, i.e., below a pore size of 6.0 nm: a gradual decrease down to 3.4 nm and another decrease after a small jump upward. This fact demonstrates that the simple Gibbs-Thomson relation, i.e., a linear relation between the melting point change and the inverse pore size, is limited to the range not far from the melting point of bulk water. FT-IR measurements suggest that the decrease in enthalpy change and interfacial free energy change with decreasing pore size reflect the similarity of the structures of both liquid and solid phases of water in smaller pores at lower temperatures.

Electrocapillarity behavior of Au111 in SO4(2-) and F-.

We present the first set of results measuring the change in interfacial free energy and surface stress for Au(111) electrodes in an electrolyte containing a nonspecifically adsorbing anion and compare this behavior to that in an electrolyte containing an anion known to undergo specific adsorption. Generally, we find that the surface stress is more sensitive to changes in electrode potential and adsorption then the interfacial free energy. The results obtained in fluoride electrolytes are compared to the predictions of a thermodynamic analysis.

The formation mechanism of poly(vinyl acetate)/poly(butyl acrylate) core/shell latex in two-stage seeded semi-continuous starved emulsion polymerization process

In this paper, the morphological evolution of latex particles on poly(vinyl acetate) (PVAc)/poly(butyl acrylate) (PBA) in two-stage seeded semi-continuous starved emulsion polymerization is studied, and the thermodynamic analysis and a mathematic model are derived to describe the interfacial free energy changes corresponding to various possible thermodynamically unstable morphologies during the polymerization process. The comparisons between the morphological evolution and the related thermodynamic analysis show that the morphological evolution of latex particles is oriented to the thermodynamic preferred morphology which is an inverted core/shell structure with PBA as core and PVAc as shell. Based on the comparisons, the possible morphological developmental pathway of PVAc/PBA core/shell latex is described. The possible morphological developmental pathway suggests that the observed multi-particle morphology in the polymerization process is due to restricted chain mobility related to high internal viscosity. The formation mechanism of PVAc/PBA core/shell latex, based on the experiments as well as the thermodynamic and dynamic analyses, is proposed, which signifies that PBA is first formed outside the PVAc seed and then migrate to the inside of the PVAc seed.

Stabilization of the monoolein Pn 3 m cubic structure on trehalose glasses

Trehalose is known to protect some organisms from various stresses due to drought and high temperature. To explore the molecular mechanism of the protective function, the mesomorphic properties of the monoolein-water system, dried in the presence of trehalose, were studied by X-ray diffraction. While, in pure water, two bicontinuous inverse cubic structures (the Pn3m and Ia3d phases) and a lamellar Lα phase exist as a function of concentration, only the Pn3m cubic phase has been detected in concentrated trehalose solutions or in trehalose glasses, even under extremely dry conditions. Depending on the sugar concentration, or after glass dehydration, the Pn3m cubic unit cell decreases to very low values, much below the smaller one observed in pure water. However, as no phase transitions occur, a simple osmotic mechanism can be excluded. An additional stabilization of the lipid phase, arising from interfacial free energy changes due to trehalose-water-lipid direct interactions, and large enough to affect the energetic balance between the Pn3m and the Ia3d cubic phases, evidently occurs. Moreover, no differences in the Pn3m cubic structure were observed when the sugar platelets convert to the glassy state; no apparent structural modifications that can be related to mechanical pressure exerted on the lipid phase have been detected.

On the interfacial energy of extended-chain crystals from polyethylene melt by revised Flory equations of fusion

To analyze extended-chain crystalline systems composed of linear polyethylene, Flory's conventional theory of fusion is reconsidered by introducing a new concept of crystallinity. When this new treatment is applied to a melting case of a low molecular weight polyethylene fraction (Mn = 5600) isothermally bulk crystallized, a certain result that very large lamellar thickness was caused by a very small increase in crystallization temperature can satisfactorily be explained by a significant change in interfacial free energy of the crystallite end. Further, it shows 14–17 kJ/mol as a nonequilibrium value range of interfacial free energy for highly crystalline polyethylene fractions of low molecular weight Mn ≦ 5600 by using the previous data presented by other workers. A similar result is also obtained on the Mn = 5600 fraction by analyzing from a standpoint of equilibrium crystallinity. In either case, the estimated range of interfacial free energy is consistent with the conventional range. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1293–1303, 1998

Correlation between reduction of surface hydrophobicity of S. aureus and the decrease in its adhesiveness induced by subinhibitory concentrations of brodimoprim

Hydrophobic interactions are involved in the mechanism of adhesion of a variety of bacteria to host tissues. Bacterial attachment to human cells is modulated by a change in interfacial free energy and this is correlated with surface hydrophobicity of bacterial cells. In S. aureus (one ATCC25923+four clinical isolates) hydrophobicity before and after incubation with subinhibitory concentrations (sub-MICs) of brodimoprim (BMP), a dimethyoxypyrimidine recently entered clinical practice, was measured by sessile drop technique as the contact angle. BMP is a new molecule derived from trimethoprim by substitution of the OCH3 group in position 4 of the benzyl-ring with a bromine atom. Bacterial adhesiveness of the same S. aureus strains was measured under the same experimental conditions. BMP significantly decreased the surface hydrophobicity of S. aureus strains at one-half MIC and one-quarter MIC. At sub-MICs concentrations BMP also reduced the adhesiveness to human epithelial buccal cells but this effect was significant down to one-sixteenth MIC. The two phenomena are correlated and hydrophobicity is involved in bacterial adhesiveness but the molecular mechanisms for the two phenomena do not completely overlap, with adhesiveness the more complex and based on a system involving both the bacteria and the epithelial cells with their specific surface characteristics.

Interfacial Free Energies in Protein Solution Droplets on FEP-Teflon by Axisymmetric Drop Shape Analysis by Profile—IgG versus BSA

In this paper, interfacial free energy changes occurring during IgG adsorption to FEP-Teflon from protein solution droplets are measured and compared with those occurring during BSA adsorption. Protein solution droplets (100 μl) were put on FEP-Teflon and the contact angle Θ and surface tension γ lv were determined by ADSA-P. Subsequently, these results were combined with the Young equation while assuming a constant surface free energy γ sv to yield the interfacial free energy γ sl. For both proteins, γ sl decreased in time up to at least 1 h in a concentration-dependent way despite the fact that similar experiments with radiolabeled proteins indicated stationary state adsorption within 30 min. IgG showed a greater decrease in interfacial free energy γ sl than did BSA, particularly at the higher concentrations. Hypothetically, this was suggested to be due to a lower internal stability of IgG as compared to BSA at the experimental pH, enabling more extensive conformational changes. Analysis of the results using the Gibbs equation yielded unrealistic amounts of proteins adsorbed to the various interfaces, most likely due to the irreversible nature of protein adsorption. Analysis of the results by the Lucassen-Reynders equation gave possibly more realistic results, since this approach is based on the ratio between surface excess quantities and thus possible errors due to the irreversible nature of protein adsorption processes might cancel each other.

Interfacial free energy changes occurring during BSA adsorption in solution droplets on FEP-Teflon surfaces as measured by ADSA-P

Axisymmetric drop shape analysis by profile (ADSA-P) was employed to determine the interfacial free energy changes occurring during bovine serum albumin (BSA) adsorption from solution droplets on fluoroethylenepropylene-Teflon (FEP-Teflon). 100 μl droplets of BSA solutions on FEP-Teflon were followed by ADSA-P as a function of time, for up to 60 min, yielding the contact angle and the surface tension, γ lv. By combining these data with Young's equation, while assuming the surface free energy, γ lv of FEP-Teflon to be 20 mj m −2, the interfacial free energy γ sl could be calculated. Both γ lv and γ sl were found to decrease as a function of time, since after positioning of a droplet, BSA adsorption occurs both at the liqui—vapor as well as the solid—liquid interface. Furthermore, linear relations were obtained when plotting γ lv ( t) · cosθ( t), vs γ lv, ( t) with BSA concentration dependent slopes. By employing the Lucassen-Reynders equation and radio-activity data obtained for BSA adsorption on FEP-Teflon, it was possible to estimate BSA adsorption at the liquid—vapor interface.

Membrane glycoprotein and surface free energy changes in hypoxic fibroblast cells

Hypoxia affects the biochemistry of mammalian cells and thus alters their sensitivity to subsequent chemo- and radiotherapy. When V79 Chinese hamster lung fibroblasts were grown under conditions of extreme hypoxia ( <10 ppm O 2) there was a significant shift in the membrane glycoprotein composition. Scanning electron microscopy revealed altered cell surface morphology including loss of pseudopodial projections. Experiments to determine changes in interfacial free energy of these cells using equilibrium two phase systems of poly(ethylene glycol) (PEG) and dextran were carried out. Test fluid droplets of the denser dextran-rich phase were formed on layers of cells in the PEG-rich phase as the bathing medium, and the contact angles the droplets made with the cell layers were measured from photomicrographs. The contact angles on cells in the plateau phase increased significantly with time of exposure to hypoxia, from 25° (zero time) to 35° (6 h) to 60° (9 h). Contact angles on cells in the exponential phase increased from 80° (zero time) to 150° after 20 h of hypoxia. It appears that the altered contact angles reflect changes in cell surface hydrophobicity that may, in part, reflect alterations in the membrane glycoprotein composition.

Rabbit arterial endothelium and subendothelium. A change in interfacial free energy that may promote initial platelet adhesion.

Endothelial damage and exposure of the subendothelium is accompanied by platelet adhesion and aggregation. Thermodynamically, adhesion will occur if this process decreases the total free energy of the system. In this investigation, we have developed a contact angle technique to measure relative changes in the surface free energy of the arterial wall, before and after the endothelium has been removed. Sections of descending thoracic aorta from seven rabbits were tested with an equilibrium two-phase system of 4% polyethylene glycol (mol wt: 20,000)/4% dextran (mol wt: 2,000,000) in Hepes-buffered physiological saline. The tissue was split longitudinally, immersed in the polyethylene glycol phase, and test droplets of the denser dextran phase were placed on its surface. Advancing contact angles of the drop were measured from photomicrographs taken with polarizing optics. After testing, the endothelium was removed with a saline jet, which we had calibrated previously with scanning electron microscopy, and the tissue was retested. We measured angles of 86.0 +/- 1.1 degree (SEM), n = 64 for the intact endothelium and 20.0 +/- 0.8 degree (SEM), n = 61 on the subendothelial surface. Since the physical behavior of blood is probably similar to our polyethylene glycol bathing medium the subendothelium may also represent a surface of high interfacial free energy in vivo. Assuming that the surface energy of platelets is low, platelet adhesion to the subendothelium could reduce the total free energy of the system, by covering this high interfacial free energy surface.

Crystallization of low molecular weight fractions of poly(octamethylene oxide)

Poly(octamethylene oxide) fractions ranging in molecular weight from 3400 to 9000 have been isothermally crystallized in the interval 61–70°C. From the dilatometric isotherms, the Avrami exponent is an integral number, 4, and is independent of temperature and molecular weight; the total crystallinity ranges from 78% for M = 3400 to 51% for M = 9000. The crystallization temperature coefficient was studied using the three-dimensional nucleation theory and it was found that the basal interfacial free energy changes from 3700 cal/mol to 2200 cal/mol, decreasing with molecular weight. When the change of the interfacial free energy is considered, the crystallization is described by a unique function of the free energy for nucleation.

Epitaxy and heterogeneous nucleation theory

On the basis of heterogeneous nucleation theory a model is developed which successfully explains a number of observations about epitaxy. The relative nucleation rates are calculated for two types of cap-shaped nuclei, “epitaxial” and “random”, differing only in the magnitude of the nucleus-substrate interfacial free energy. There is a sharp rise in the ratio of the epitaxial/random nucleation rates over a relatively small temperature range, thus predicting an epitaxial temperature. The epitaxial temperature increases significantly when the impingement flux is increased. The combined effects of impingement flux and temperature are shown to produce conditions for which either no nucleation, epitaxial nucleation, or random nucleation will occur. The potentially large effect of impurities is illustrated by the variation of the relative nucleation rates with changes in interfacial free energy. The results are in qualitative agreement with the limited amount of experimental information available.