Search

Antimicrobial peptides are one of the most promising alternatives to antibiotics for targeting pathogens without developing resistance. In this study, pore formation in 1,2-Dimyristoyl-snglycero-3-phosphocholine (DMPC) / cholesterol liposome induced by native melittin, its two mutant variants (G1I and I17 K), and cecropin P1 was investigated by monitoring the dynamics of fluorescence dye leakage. A critical peptide concentration was required for dye leakage with the rate of leakage being dependent on peptide concentration above a critical value. A lag time was required for dye leakage for low peptide concentrations that are above the critical value, which decreased at higher peptide concentrations eventually approaching zero. Lag time was found to be in the order I17 K mutant with lower hydrophobicity and higher net charge > G1I with higher hydrophobicity > melittin > cecropin P1. Cecropin P1 exhibited the highest rate of dye leakage followed by melittin, G1I, and I17 K. Size distribution and transmission electron microscopy (TEM) of liposomes exposed to peptides of different concentrations indicated pore formation with accompanied stretching of liposomes at low peptide concentrations for both melittin and cecropin P1. At much higher concentrations, however, size distribution indicated three peaks for both peptides. In both cases, TEM images show that the middle and small peaks are shown to be due to stretched liposome and broken stretched liposome respectively. For melittin, the large peak is due to peptide aggregates as well as aggregates of liposome. For cecropin P1, however, the large peak indicates cecropin P1 aggregates with solubilized lipids thus suggesting carpet mechanism.

In our previous investigation (Zhang et. al., J. Functional Foods 40 (2018) 700-706), we have proposed a method of complexation of alamethicin (ALM) with γ-cyclodextrin (γ-CD) to increase the solubility and demonstrated an enhancement in its antimicrobial activity against Listeria monocytogenes. In this study, transmission electron microscopy of L. monocytogenes treated with γ-CD/ALM complex indicated cell lysis due to pore formation. This was further corroborated by fluorescent dye leakage from DMPC/cholesterol liposomes when exposed to γ-CD/ALM complex for molar ratios of 1, 5 and 10. The extent of dye leakage increased with ALM-lipid ratio in the range of 0.00015 to 0.16. Dye leakage of γ-CD/ALM complex was found to be the highest for molar ratio of 5, consistent with our earlier results of antimicrobial activity of the complex against L. monocytogenes. All atom molecular dynamics (MD) simulation showed that γ-CD/ALM complex can effectively bind to the 3:1 POPE/POPG bilayer, a mimic of bacterial cell membrane. In addition, circular dichroism spectrum indicated that ALM in the complex has a helical conformation in solution as well as in the presence of liposome. Transmembrane MD simulation of six γ-CD/ALM complex aggregates in α helical conformation showed water channel with barrel stave like pore structure.

Antimicrobial peptides (AMPs) have the ability to penetrate the cell membrane, form pores which eventually lead to cell death. Immobilization of AMP on nanoparticles can play a major role in antimicrobial materials, biosensors for pathogen detection and in food safety. The minimum inhibitory concentration (MIC) of free Cecropin P1 (CP1, sequence SWLSTAKKLENSAKKRLSEGIAIAIQGGPR) and adsorbed on silica nanoparticle against E. coli O157:H7 EDL933 were 0.78μg/ml. This was found to be consistent with preservation of α-helical secondary structure of CP1 upon adsorption as indicated by circular dichroism (CD). Cysteine-terminus modified Cecropin P1 (CP1C, sequence SWLSTAKKLENSAKKRLSEGIAIAIQGGPRC) was chemically immobilized onto silica nanoparticles with maleimide-PEG-NHS ester cross-linkers of different PEG chain lengths. The antimicrobial activity of CP1C in solution and adsorbed on silica nanoparticles against E. coli O157:H7 EDL933 were found to be the same as those for CP1. However, tethered CP1C exhibited much higher MIC of 24.38, 37.55 and 109.82μg/ml for (PEG)20, (PEG)6 and (PEG)2 linkers respectively. The antimicrobial activity of CP1C tethered to silica nanoparticles with (PEG)20 linker was found to be lower for lower surface coverage with MIC values being 86.06, 36.89, 24.38 and 17.84μg/ml for surface coverage of 12.3%, 24.4%, 52.8% and 83.8% respectively. All atom MD simulation of 1:3 DOPG/DOPC mixed membrane interacting with free and PEGlyated CP1C indicated that presence of PEG linker prevented CP1C from interacting with the bilayer which may explain the loss of antimicrobial activity of tethered CP1C.

A model for drainage of aqueous foam film with a partial coverage of particles accounts for an inner clean air-liquid film surrounded by a film stabilized by close packed particles at the outer periphery. The drainage of the inner film occurs due to capillary pressure gradient. The film thickness is found to decrease fastest at the center with the inner film being thinner than the outer film eventually leading to its rupture as a result of film thickness becoming zero at the center. The velocity of film drainage is found to be higher for lower particle concentration and higher surface tension. The lifetime (time of rupture) of the film is found to be larger for larger bubble size and liquid holdups (higher initial film thickness). For a film stabilized by close packed monolayer or multilayer of particles, the disjoining pressure is larger for larger particle number concentration and particle size, increases with surface tension only for film stabilized by single or two layers of particles and is a function of bubble size only for film stabilized by multilayers of particles. The maximum disjoining pressure decreases with contact angle approaching zero for a critical contact angle of 129° and increases linearly with surface tension. At mechanical equilibrium, the particle stabilized foam gets drier for larger bubble sizes, contact angles and particle number concentrations.

A new approach to the theory of nucleation, formulated relatively recently by Ruckenstein, Narsimhan, and Nowakowski (see Refs. [7–16]) and developed further by Ruckenstein and other colleagues, is presented. In contrast to the classical nucleation theory, which is based on calculating the free energy of formation of a cluster of the new phase as a function of its size on the basis of macroscopic thermodynamics, the proposed theory uses the kinetic theory of fluids to calculate the condensation (W+) and dissociation (W−) rates on and from the surface of the cluster, respectively. The dissociation rate of a monomer from a cluster is evaluated from the average time spent by a surface monomer in the potential well as obtained from the solution of the Fokker–Planck equation in the phase space of position and momentum for liquid-to-solid transition and the phase space of energy for vapor-to-liquid transition. The condensation rates are calculated using traditional expressions. The knowledge of those two rates allows one to calculate the size of the critical cluster from the equality W+ = W− as well as the rate of nucleation. The developed microscopic approach allows one to avoid the controversial application of classical thermodynamics to the description of nuclei which contain a few molecules. The new theory was applied to a number of cases, such as the liquid-to-solid and vapor-to-liquid phase transitions, binary nucleation, heterogeneous nucleation, nucleation on soluble particles and protein folding. The theory predicts higher nucleation rates at high saturation ratios (small critical clusters) than the classical nucleation theory for both solid-to-liquid as well as vapor-to-liquid transitions. As expected, at low saturation ratios for which the size of the critical cluster is large, the results of the new theory are consistent with those of the classical one. The present approach was combined with the density functional theory to account for the density profile in the cluster. This approach was also applied to protein folding, viewed as the evolution of a cluster of native residues of spherical shape within a protein molecule, which could explain protein folding/unfolding and their dependence on temperature.

A model for creaming and formation of a foam layer in an aerated system consisting of Newtonian liquid is proposed. The variation of air volume fraction in the dispersion layer is described by hindered creaming which is coupled to syneresis in the top foam layer that is described by flow of liquid through a network of Plateau borders due to gravitational and capillary forces. The present analysis accounts for the compressibility of foam layer by coupling creaming analysis with syneresis in the foam layer. The behavior of the system is described by three parameters: (a) characteristic time scale of creaming of an isolated bubble, (b) hydrodynamic interaction factor, and (c) capillary number, ratio of capillary and gravitational forces in the foam layer. System behavior is shown to be different for four different regions of initial air volume fractions for which the phase diagram and evolution of the profile of air volume fraction for batch dispersion are presented.

Antimicrobial peptides (AMPs) are relatively short peptides that have the ability to penetrate the cell membrane, form pores leading to cell death. This study compares both antimicrobial activity and cytotoxicity of native melittin and its two mutants, namely, melittin I17K (GIGAVLKVLTTGLPALKSWIKRKRQQ) with a higher charge and lower hydrophobicity and mutant G1I (IIGAVLKVLTTGLPALISWIKRKRQQ) of higher hydrophobicity. The antimicrobial activity against different strains of Listeria was investigated by bioassay, viability studies, fluorescence and transmission electron microscopy. Cytotoxicity was examined by lactate dehydrogenase (LDH) assay on mammalian Caco-2 cells. The minimum inhibitory concentration of native, mutant I17K, mutant G1I against Listeria monocytogenes F4244 was 0.315±0.008, 0.814±0.006 and 0.494±0.037μg/ml respectively, whereas the minimum bactericidal concentration values were 3.263±0.0034, 7.412±0.017 and 5.366±0.019μg/ml respectively. Lag time for inactivation of L. monocytogenes F4244 was observed at concentrations below 0.20 and 0.78μg/ml for native and mutant melittin I17K respectively. The antimicrobial activity against L. monocytogenes F4244 was in the order native>G1I>I17K. Native melittin was cytotoxic to mammalian Caco-2 cells above concentration of 2μg/ml, whereas the two mutants exhibited negligible cytotoxicity up to a concentration of 8μg/ml. Pore formation in cell wall/membrane was observed by transmission electron microscopy. Molecular dynamics (MD) simulation of native and its mutants indicated that (i) surface native melittin and G1I exhibited higher tendency to penetrate a mimic of bacterial cell membrane and (ii) transmembrane native and I17K formed water channel in mimics of bacterial and mammalian cell membranes.

A model for drainage of a power-law fluid through a Plateau border is proposed which accounts for the actual Plateau border geometry and interfacial mobility. The non-dimensionalized Navier-Stokes equations have been solved using finite element method to obtain the contours of velocity within the Plateau border cross section and average Plateau border velocity in terms of dimensionless inverse surface viscosity and power-law rheological parameters. The velocity coefficient, the correction for the average velocity through a Plateau border of actual geometry compared to that for a simplified circular geometry of the same area of cross section, was expressed as a function of dimensionless inverse surface viscosity and flow behavior index of the power-law fluid. The results of this improved model for Plateau border drainage were then incorporated in a previously developed foam drainage model [G. Narsimhan, J. Food Eng. 14 (1991) 139] to predict the evolution of liquid holdup profiles in a standing foam. Foam drainage was found to be slower for actual Plateau border cross section compared to circular geometry and faster for higher interfacial mobility and larger bubble size. Evolution of liquid holdup profiles in a standing foam formed by whipping and stabilized by 0.1% beta-lactoglobulin in the presence of xanthan gum when subjected to 16g and 45g centrifugal force fields was measured using magnetic resonance imaging for different xanthan gum concentrations. Drainage resulted in the formation of a separate liquid layer at the bottom at longer times. Measured bubble size, surface shear viscosity of beta-lactoglobulin solutions and literature values of power-law parameters of xanthan gum solution were employed in the current model to predict the evolution of liquid holdup profile which compared well with the experimental data. Newtonian model for foam drainage for zero shear viscosity underpredicted drainage rates and did not agree with the experimental data.

A generalized formalism for the rupture of an equilibrium foam film due to imposed random thermal and mechanical perturbations, modeled as a Gaussian white noise, is presented. The evolution of amplitude of perturbation is described by a stochastic differential equation. The average film rupture time is the average time for the amplitude of perturbation to equal to one half the film thickness and is calculated by employing a first passage time analysis for different amplitudes of imposed perturbations, wavenumbers, film thickness, van der Waals and electrostatic interactions and interfacial mobilities. A critical film thickness is identified based on the sign of the disjoining pressure gradient, below which the film is unstable in that the rupture time is very small. There exists an optimum wavenumber for minimum rupture time only for an unstable film. The calculated values of rupture time as well as the optimum wavenumber in the present analysis agree well with the results of linear stability analysis for unstable film with immobile film interfaces. For stable films, the rupture time is found to increase dramatically with film thickness near the critical film thickness. As expected, the average rupture time was found to be higher for smaller amplitudes of imposed perturbations, larger surface potentials and smaller Hamaker constants. The interfacial mobility is accounted for through surface viscosity as well as Marangoni effect. The mean rupture time for partially mobile gas–liquid interface, as characterized by two dimensionless groups, dimensionless surface viscosity and dimensionless Marangoni number, increased with the two dimensionless groups and approached the limiting value for immobile interfaces. The second moment of the rupture time distribution was also evaluated and a beta distribution was fitted to the first two moments. The fitted rupture time distribution was found to be broader for larger film thickness, smaller pressure amplitude, lower Hamaker constant and smaller surface potential.

Evolution of liquid holdup profile in a standing foam formed by whipping and stabilized by sodium caseinate in the presence of xanthan gum when subjected to 16 and 29g centrifugal force fields was measured using magnetic resonance imaging for different pH, ionic strength, protein and xanthan gum concentrations. Drainage resulted in the formation of a separate liquid layer at the bottom at longer times. Foam drainage was slowest at pH 7, lower ionic strength, higher protein and gum concentrations. Foam was found to be most stable at pH 5.1 near the isoelectric point of protein, lower ionic strength and higher protein and xanthan gum concentrations. A predicted equilibrium liquid holdup profile based on a previous model (G. Narsimhan, J. Food Eng. 14 (1991) 139) agreed well with experimental values at sufficiently long times. A proposed model for velocity of drainage of a power law fluid in a Plateau border for two different simplified geometries was incorporated in a previously developed model for foam drainage (G. Narsimhan, J. Food Eng. 14 (1991) 139) to predict the evolution of liquid holdup profiles. The model predictions for simplified circular geometry of Plateau border compared well with the experimental data of liquid holdup profiles at small times. At longer times, however, the predicted liquid holdup profile was larger than the observed, this discrepancy being due to coarsening of bubble size and decrease in foam height not accounted for in the model. A Newtonian model for foam drainage under predicted drainage rates did not agree with the experimental data.

The proposed model views drop coalescence in a turbulent flow field as a two-step process consisting of formation of a doublet due to drop collisions followed by coalescence of the individual droplets in a doublet due to the drainage of the intervening film of continuous phase under the action of colloidal (van der Waals and electrostatic) and random turbulent forces. The turbulent flow field was assumed to be locally isotropic. A first-passage-time analysis was employed for the random process of intervening continuous-phase film thickness between the two drops of a doublet in order to evaluate the first two moments of coalescence-time distribution of the doublet. The average drop coalescence time of the doublet was dependent on the barrier for coalescence due to the net repulsive force (net effect of colloidal repulsive and turbulent attractive forces). The predicted average drop coalescence time was found to be smaller for larger turbulent energy dissipation rates, smaller surface potentials, larger drop sizes, larger ionic strengths, and larger drop size ratios of unequal-sized drop pairs. The predicted average drop coalescence time was found to decrease whenever the ratio of average turbulent force to repulsive force barrier became larger. The calculated coalescence-time distribution was broader, with a higher standard deviation, at lower energy dissipation rates, higher surface potentials, smaller drop sizes, and smaller size ratios of unequal drop pairs. The model predictions of average coalescence-rate constants for tetradecane-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) in a high-pressure homogenizer agreed fairly well with the inferred experimental values as reported by Narsimhan and Goel (J. Colloid Interface Sci. 238 (2001) 420-432) at different homogenizer pressures and SDS concentrations.

Antimicrobial peptides (AMP) represent a class of compounds to combat antibiotic resistance to microorganisms, neutralize biological warfare agents, and as topical antimicrobial agents. AMP kills microbial cells through insertion and permeabilization of the cytoplasmic membranes. It is important to predict the efficacy of AMP at low concentration to circumvent their toxicity. Leakage of fluorescent dyes (calcein, FD4 and FD20) of different molecular weights entrapped within 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/cholesterol mixed liposomes by AMP melittin indicated (i) a critical melittin concentration for pore formation and (ii) a lag time for pore formation above this critical concentration. The lag time decreased with an increase in melittin concentration and was in the order FD20>FD4>calcein. % α helix of melittin increased when exposed to liposome with this increase being more pronounced at higher concentrations eventually reaching an asymptotic value. The rate of dye leakage following the lag time was found to be larger at higher melittin concentration. A simplified mathematical model for nucleation and growth of pores formed by an aggregate of melittin in lipid bilayer is proposed to predict the variation of rate of dye leakage with melittin concentration which agreed fairly well with the data for calcein and seems to suggest a toroidal mechanism of pore formation with participation of large number of phospholipid heads.

The present study investigates the effects of homogenizer pressure, surfactant concentration, ionic strength, and dispersed phase fraction on the coalescence rate of tetradecane-in-water emulsions during their formation in a high-pressure homogenizer. Experiments were conducted in a recirculating system consisting of a Rannie laboratory-scale single-stage homogenizer and a stirred vessel for tetradecane-in-water emulsions stabilized by sodium dodecyl sulfate (SDS). The initial evolution of the number concentration of droplets in the stirred tank was measured when subjected to a negative stepchange in the homogenizer pressure. The average drop coalescence rate constant in the homogenizer was inferred by fitting the experimental evolution of the number concentration of drops to a simple model accounting for the coalescence in the homogenizer under the assumption of a quasi steady state in the homogenizer. The residence time of the emulsion in the homogenizer was evaluated from the analysis of radial turbulent flow between disks. The step down homogenizer pressure was varied in the range 20.7-48.3 MPa, the drop size in the range 174-209 nm, the dispersed phase fraction in the range 5%-15%, SDS concentration in the range 0.0033-0.25 wt%, and ionic strength in the range 0.01-0.1 M. The coalescence rate constants were found to be in the range from 3.34x10(-17) to 2.43x10(-16) m(3) s(-1). The coalescence rate constant was found to be higher for higher homogenizer pressures, smaller drop sizes, lower dispersed phase fractions, and lower SDS concentrations and was insensitive to variations in ionic strength. Copyright 2001 Academic Press.

Coalescence rate constants in a high-pressure homogenizer were inferred for pure emulsions of a neutrally buoyant mixture of carbon tetrachloride and benzene dispersed in aqueous medium, as well as for emulsions stabilized by sodium caseinate or whey protein, from the temporal evolution of number of drops per unit volume in the exit stream of the homogenizer in a recirculating system consisting of the homogenizer and a stirred tank when subjected to a negative step change in applied homogenizer pressure (Ph). The homogenization pressure was varied in the range 6.9 to 41.4 MPa, pH in the range 5 to 7, dispersed phase fraction in the range 0.05 to 0.15, ionic strength in the range 0.01 to 0.05 M, and protein concentration in the range 0.01 to 0.05 wt%. Functional dependence of drop coalescence rate on the homogenizer pressure was derived for pure emulsions for mechanisms of drop collisions due both to turbulence and to shear. The inferred collision rate constant for pure emulsions was found to be proportional to Ph0.722, which was closer to the functional dependence for drop collisions due to turbulence. The coalescence rate constant was found to be higher for higher homogenizer pressures, larger drop sizes, and higher dispersed phase fractions. For emulsions stabilized by proteins, drop coalescence rate constant was found to be higher at lower protein concentrations, near the isoelectric point of the protein, and higher ionic strengths because of the smaller interdroplet colloidal repulsive forces, thus clearly demonstrating the effect of colloidal forces on drop coalescence. Sodium caseinate was found to result in a lower coalescence rate constant than whey protein.

Destabilization of protein-stabilized food foams by some ingredients has been shown to be due to disruption of protein-protein interactions in the adsorbed protein layer. The structure of adsorbed protein layers at air/water interfaces has been elucidated by neutron reflectance. A new mechanism of thin film drainage at high protein concentration has been demonstrated. Drainage models have been developed to predict.syneresis and collapse of equilibrium foams.

Monolayers of bovine serum albumin (BSA) were prepared with Trurnit's method; deposition of thin aqueous layers on an aqueous substrate followed by adsorption on the surface. The experiments demonstrated that adsorption was irreversible, initially diffusion controlled as determined before, but also that diffusional effects toward the bulk were important. Three two-layer-diffusion and adsorption models were solved numerically to gain insight into these effects and determine the conditions at which Trurnit's method is reliable. The method is most reliable for irreversible diffusion-controlled adsorption. For reversible diffusion-controlled adsorption the method can have significant error mostly at very long adsorption times. For slower adsorption kinetics and reversible adsorption, the method is unreliable even at small adsorption times.

The dynamics of adsorption of bovine serum albumin (BSA) and its derivatives at the air–water interface were investigated through the measurement of dynamic surface pressure using a Langmuir minitrough. BSA was chemically modified through reductive alkylation to yield methylated and pentylated BSA with different surface hydrophobicities. Adsorption isotherms (Π–A) of native and modified BSAs at pH 7 were measured, and the surface density (Γ) was calculated directly from the area of the spread monolayer produced with Trurnit's method. The areas per molecule for BSA and modified BSAs at sufficiently high surface pressures were found to be much larger than those corresponding to the molecular dimensions, thus indicating unfolding of the molecules at the interface. The modified BSAs were found to occupy much larger areas per molecule than the native BSA, possibly due to more unfolding. The rate of surface pressure increase was higher for higher bulk concentrations, especially at smaller times. The deviation from diffusion-controlled adsorption was found to occur at smaller times for smaller ionic strengths, and for pH 7 rather than for pH 5.5 (at the isoelectric point). The steady-state surface pressures for native BSA ranged from 10 to 17 mN/m for bulk concentrations of 0.5 to 10 ppm and were higher for higher bulk concentrations, higher ionic strengths, and at the isoelectric point. The initial rate of surface pressure increase and the steady-state value were highest for pentylated BSA and lowest for native BSA. A previously developed model (Narsimhan and Uraizee,Biotech. Prog.8,187, 1992) was able to predict well the effects of bulk concentration, pH, ionic strength, and surface hydrophobicity on the dynamics of adsorption. The model predictions of the dynamics of surface pressure agreed fairly well with the experimental data at short times and at longer times. However, the model was found to overpredict the surface pressure at intermediate times, probably because of the use of the simplifying assumption that the surface equation of state during adsorption is given by the steady-state Π–Γ isotherm.

The viscoelastic properties of adsorbed protein layer in food emulsions and foams are important in providing stability to such systems. Linear stability analysis for a protein stabilized aqueous film sandwiched between two semi-infinite oil phases with a viscoelastic liquid-liquid interface is presented. The interfacial dilatational and shear viscoelastic properties are described by Maxwell models. The aqueous film is found to be more stable for smaller values of dilatational (shear) relaxation times and larger values of interfacial dilatational (shear) viscosities. The asymptotic values of maximum growth coefficient for very large and very small values of interfacial dilatational (shear) viscosities were found to be independent of relaxation times and correspond to those for immobile and fully mobile liquid-liquid interfaces respectively. The aqueous film is shown to be more stable for larger viscosities of the oil phase with the maximum growth coefficient approaching zero as the ratio of viscosities of oil and aqueous phases approach very large values and an asymptotic value corresponding to that for a foam film for very small viscosity ratios.

Previous analysis of Narsimhan [G. Narsimhan, J. Colloid Interface Sci. 287 (2005) 624-633] for the evaluation of rupture of a nondraining thin film on a solid support due to imposed random mechanical perturbations modeled as a Gaussian white noise has been extended for partially mobile gas-liquid interfaces. The average rupture time of film is evaluated by first passage time analysis (as the mean time for the amplitude of perturbation to become equal to film thickness). The interfacial mobility is accounted for through surface viscosity as well as Marangoni effect. The mean rupture time for partially mobile gas-liquid interface, as characterized by two dimensionless groups, dimensionless surface viscosity and Marangoni number, lies between the two extreme limits for fully mobile and immobile films. The critical wavenumber for minimum rupture time is shown to be insensitive to interfacial mobility. However, the critical dimensionless surface viscosity and critical Marangoni number at which the behavior of thin film deviates from that of fully mobile film and the behavior approaches that of fully immobile film are smaller for higher Hamaker constants, smaller film thickness and smaller surface potentials.

Linear stability analysis for a film on a solid surface with a viscoelastic air–liquid interface is presented. The interfacial dilatational and shear viscoelastic properties were described by Maxwell models. Dilatational and shear interfacial elasticity and viscosity were shown to improve film stability. When the interfacial rheological properties are extremely large or small, the maximum perturbation growth coefficient is shown to reduce to those for immobile and mobile interfaces respectively. Calculated values of maximum growth coefficient for thin film stabilized by 0.5% β-lactoglobulin approached those of mobile films for thick ( > 2000 nm ) and those for immobile films for thin ( 100 nm ) films respectively with the values lying between the two limits for intermediate film thicknesses.

A generalized formalism for the rupture of a nondraining thin film on a solid support due to imposed random thermal and mechanical perturbations, modeled as a Gaussian white noise, is presented. The evolution of amplitude of perturbation is described by a stochastic differential equation. The average film rupture time is the average time for the amplitude of perturbation to equal to the film thickness and is calculated by employing a first passage time analysis for different amplitudes of imposed perturbations, wavenumbers, film thickness, van der Waals and electrostatic interactions and surface tensions. The results indicate the existence of an optimum wavenumber at which the rupture time is minimum. A critical film thickness is identified based on the sign of the disjoining pressure gradient, below which the film is unstable in that the rupture time is very small. The calculated values of rupture time as well as the optimum wavenumber in the present analysis agree well with the results of linear stability analysis for immobile as well as completely mobile gas-liquid film interfaces. For stable films, the rupture time is found to increase dramatically with film thickness near the critical film thickness. As expected, the average rupture time was found to be higher for smaller amplitudes of imposed perturbations, larger surface potentials, larger surface tensions and smaller Hamaker constants.

The changes in the secondary conformation and surface hydrophobicity of beta-lactoglobulin subjected to different thermal treatments were characterized at pH values of 7, 5.5 and 4 using circular dichroism (CD) and hydrophobic dye binding. Heating resulted in a decrease in alpha-helix content with a corresponding increase in random coil at all pH values, this change being more pronounced for small heating times. Heating also resulted in an increase in surface hydrophobicity as a result of partial denaturation, this increase being more pronounced at pH 4. Thermal treatment resulted in a shift of the spread monolayer isotherm at air-water interface to smaller area per molecule due to increased flexibility and more loop formation. Thermal treatment led to an increase in interfacial shear elasticity and viscosity of adsorbed beta-lactoglobulin layer at pH 5.5 and 7. Interfacial shear elasticity, shear viscosity, stability of beta-lactoglobulin stabilized emulsion and average coalescence time of a single droplet at a planar oil-water interface with adsorbed protein layer exhibited a maximum for protein subjected to 15 min heat treatment at pH 7. At pH 5.5, the interfacial shear rheological properties and average single drop coalescence time were maximum for 15 min heat treatment whereas emulsion stability was maximum for 5 min heat treatment. At pH 7, thermal treatment was found to enhance foam stability. Analysis of thin film drainage indicated that interfacial shear rheological properties do not influence thin film drainage.

Dynamics of adsorption of 14C radiolabeled beta-lactoglobulin and alpha-lactalbumin at the air-water interface was investigated through the measurement of surface pressure (pi) and surface concentration (Gamma) via a radiotracer technique. Adsorption was diffusion controlled at short times, the rates of increase of pi and Gamma being lower at longer times because of an energy barrier. At low concentrations, an apparent time lag was observed in the evolution of pi for beta-lactoglobulin but not for alpha-lactalbumin which was shown to be due to the nonlinear nature of the pi-Gamma relationship for the former. The area per molecule of an adsorbed beta-lactoglobulin during the dynamics of adsorption was smaller than that for spread monolayer since beta-lactoglobulin was not fully unfolded during adsorption. For alpha-lactalbumin, however, no such difference in the molecular areas for adsorbed and spread monolayer was observed indicating thereby that alpha-lactalbumin unfolded much more rapidly than beta-lactoglobulin. Evolution of Gamma for alpha-lactalbumin was found to occur in two steps possibly due to the change in the orientation of the adsorbed protein from a side-on to an end-on orientation. A previously developed mechanistic model (G. Narsimhan and F. Uraizee, Biotechnology Prog. 8, 187 (1992)) was improved to account for the presence of hydrophobic patches on the surface of the protein molecule as well as an adsorbed protein layer at the air-water interface. The model predictions agreed quite well with the experimental evolution of Gamma for beta-lactoglobulin and alpha-lactalbumin. The model calculations seem to indicate that alpha-lactalbumin changes its orientation at the air-water interface from side-on to other orientations at higher surface concentrations. Copyright 1999 Academic Press.

A formalism is developed for the prediction of the rate of nucleation in liquids which as opposed to the usual quasi-thermodynamic treatment does not employ (i) the macroscopic concept of interfacial tension for small clusters and (ii) the principle of detailed balance. The rate of dissociation of a surface monomer from a cluster is calculated by using a first passage time analysis. An expression for the critical cluster size is obtained in terms of the characteristics of the interaction potential between the monomers of a molecular cluster which, for illustrative purposes, is taken here as the square well potential. A generalized Kelvin equation is derived to obtain the dependence of the solubility on the cluster size. Finally, an expression for the rate of nucleation is obtained on the basis of the kinetics of dissociation. The predicted nucleation rates for spherical clusters with uniform distribution of molecules are found to be much larger than those predicted by the classical theory. In contrast to the classical theory, the nucleation rates exhibit a gradual increase at very low supersaturations, increase catastrophically at a critical supersaturation, and taper off gradually at high supersaturations. The predicted nucleation rates for approximately spherical clusters with fee packing are found to be much smaller than those for spherical amorphous clusters and exhibit behavior qualitatively different from that of the latter. The classical theory is shown to overpredict the interfacial tension and hence underpredict the nucleation rates. Since the spherical amorphous cluster and a cluster with a definite structure such as fee are probably appropriate models for large and small clusters, respectively, the actual nucleation rates should lie between the two, coinciding with the former at low and the latter at high supersaturations. The classical theory for the rate of nucleation can be recovered from the present formalism when the critical cluster size is sufficiently large, i.e., larger than 10 6 monomers. Such a condition, however, is satisfied only for extremely small supersaturations.

The collisions between small aerosol particles do not necessarily result in coagulation, since the dispersion forces responsible for coagulation are relatively weak; i.e., the interaction potential well due to the van der Waals attraction and Born repulsion is not deep. In addition, because of weak dispersion forces, the constituents of a doublet of small particles can derive sufficient energy from the collisions with the molecules of the suspending medium to overcome the potential well and dissociate. The rate of formation of a doublet, i.e., the rate of coagulation per particle, is calculated by accounting for the sticking probability in terms of the depth of the interaction potential well. The average dissociation time of a doublet has been determined as a first passage time in the energy of the relative motion of the two constituent particles. For sufficiently small particles, the rate of dissociation of a doublet is much larger than its rate of formation. This unstable doublet reaches, therefore, an equilibrium with the single particles over times much shorter than the time scale of coagulation. Because of the dramatic decrease in the rate of dissociation of a doublet with an increase in the size of its constituents, there exists a critical particle radius above which the doublet is stable; i.e., its rate of formation becomes greater than its rate of dissociation. When the doublet is unstable, aerosol growth occurs via a nucleation mechanism, i.e., via successive equilibria of unstable agglomerates leading to a critical size aggregate, which constitutes a nucleus for growth. On the other hand, when the doublet is stable, the singlets themselves act as nuclei for aerosol growth.

If the dispersion forces, responsible for coagulation, are not too strong (i.e., if the aerosol particles are sufficiently small), then the coagulated aerosol doublets can derive enough energy from the collisions with the molecules of the suspending medium to overcome the interaction potential well, thus leading to their dissociation. Because of the great disparity between the time scales of oscillation in the potential well and Brownian motion of the coagulated particle pairs, the Fokker-Planck equation can be averaged with respect to the relative position of the constituents of the doublet. One thus obtains a one-dimensional Fokker-Planck equation in terms of the energy of the relative motion of the two constituents. As a result, the average lifetime of a doublet can be calculated as a first passage time in the energy of the relative motion of its constituents. The average dissociation time of the doublets, in air at 1 atm and 298 K, for a Hamaker constant of 10 −12 erg, has been thus calculated for different radii of the constituent particles. The average dissociation time is found to increase dramatically, from 10 −7 to 10 −1 s, as the radius changes from 15 to 50 A. This is a result of the rapid increase in the depth of the interaction potential well with increasing radii. The doublets consisting of particles whose radii are greater than 50 A are found to be extremely stable.

The existing models for Brownian coagulation of aerosols account for the effect of interparticle forces through a phenomenological sticking probability, i.e., the probability of coagulation upon collision. This probability is usually assumed to be equal to unity. A model for Brownian coagulation of equal-sized electrically neutral aerosol particles is proposed, which takes into account explicitly the van der Waals attraction and Born repulsion, instead of the phenomenological sticking probability. In this model, the relative motion between two particles in the vicinity of the sphere of influence is considered to be free molecular, the thickness of this region is taken to be equal to the average correlation length for the relative Brownian motion. The relative motion of the particles outside this region is described by the Fokker-Plank equation. The sticking probability predicted by the model, in terms of the interaction potential between two particles, becomes vanishingly small for very small particles. The collisions with the medium that a particle experiences during its escape from the potential well and the corresponding energy dissipation, neglected in this model, will increase the coagulation coefficient, in particular for the large particles because they stay longer in the region in which the interaction potential is acting. For this reason the model is likely to be valid for sufficiently small particles and to provide a lower bound for the coagulation coefficient of the larger particles. However, for large particles the interaction potential is deep and the decay length of the potential is small compared to the particle sizes. As a result, the region of the interaction potential can be replaced by a sink. This is equivalent to considering the sticking probability as equal to unity. In contrast to the earlier models, the expression derived for the coagulation coefficient in the latter case displays the proper continuum and free molecular limits and agrees well with the Fuchs empirical formula. It yields an upper bound for sufficiently small particles but can be exceeded for particles of intermediate sizes. Comparison with the experimental data seem to indicate the validity of this upper bound for particles as small as 0.01 μm.

The effect of interparticle forces on Brownian coagulation of aerosols is usually accounted for through a phenomenological sticking probability, i.e., the probability of coagulation upon collision. This probability is customarily assumed to be equal to unity even though it is recognized that it is smaller than unity for very small particles. A Monte Carlo method for the simulation of Brownian coagulation of equal-sized electrically neutral spherical aerosol particles is presented, which takes into account the interparticle forces due to van der Waals attraction and Born repulsion instead of the phenomenological sticking probability. The particle motion is described by the generalized Brownian dynamics. The Brownian coagulation coefficient for particles of unit density, for a Hamaker constant of 10−12 erg, is calculated over the entire range of Knudsen numbers. For large particles, the results of the simulation agree very well with those provided by the model proposed previously by the authors (1) and also with the Fuchs interpolation formula. In this case the results indicate that the sticking probability is close to unity. For sufficiently small particles, the coagulation coefficient agrees with the lower bound provided by the model proposed by the authors (1). The extrapolation to sufficiently small particles of the model (1) proposed for large particles provides an upper bound with much larger values. For particles of intermediate sizes, the coagulation coefficient is found to be even higher than the values predicted by the model valid for large particles (1). The coagulation coefficients of the particles of intermediary size are greater than those corresponding to a sticking probability of unity whereas those of the sufficiently small particles are much smaller because of the following two opposite effects: (i) As the particle size decreases, the ratio of the decay length of the interaction potential (based on the shortest distance between the particles) and the particle size increases; this relative increase in the range of the van der Waals attraction increases the rate of collisions between particles. (ii) As the particle size decreases, the interaction potential becomes less deep and, as a result, the probability of escape of the particles from the potential well increases. In the intermediary size range, the former effect, which was not accounted for in the model for large particles (being negligible in that case), is more important, whereas for very small particle sizes, the latter effect predominates. The calculated values of the collision efficiency are small for sufficiently small particles, increase with particle size and eventually reach the value of unity for large particles. Extrapolating to particles of molecular dimensions, one can understand why in the cases in which only physical interactions are involved the collision efficiency is negligibly small.