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C J Radke

Publications and source records attributed to C J Radke.

At least 19 recordsLinked to original sources
PubMed2006

Diblock copolymer surfactant transport across the interface between two homopolymers.

Dynamics of adsorption and desorption of a diblock copolymer to an interface between two homopolymers was measured using dynamic secondary-ion mass spectrometry (SIMS). Thin films were constructed consisting of a layer of saturated polybutadiene with 90% 1,2-addition (sPB90), followed by a layer of saturated polybutadiene with 63% 1,2-addition (sPB63), and finally by another layer of the sPB90 homopolymer. A sPB90-sPB63 diblock copolymer was initially included only in the top sPB90 layer of the film at a volume fraction of 0.05. The thin films were annealed at ambient temperature for times ranging between 0.2 and 108 h, and the concentration profiles of the diblock copolymer through the films were measured using SIMS. The dynamics of adsorption and desorption of the diblock copolymer at the two sPB90-sPB63 interfaces was gauged by comparing the different transient concentration profiles. The sorption process was modeled as diffusion in an external field, generated from self-consistent field theory (SCFT). All parameters for the model were determined independently. Although the model neglects the dynamics of conformational change, experimental results matched theory very well.

Polymers↗
PubMed2006

Relationship between macroscopic and microscopic models of surfactant adsorption dynamics at fluid interfaces.

In a companion preceding paper, we presented an experimental investigation into the adsorption dynamics of a diblock copolymer surfactant to a polymer/polymer interface and found them to be well-described by a microscopic model of diffusion in a potential generated using self-consistent field theory. We compare the predictions of the microscopic approach with a macroscopic (adsorption-diffusion) model and demonstrate the equivalence of the two models when the free-energy well underlying surfactant adsorption is flanked by barriers that are significantly larger than thermal energy (kT). However, when the energy barriers are nonexistent, as is the case for the experimental system of interest, a finite interfacial width must be introduced into the classical model to obtain physically meaningful results (i.e., nonnegative desorption rates). Surprisingly, we find that the predictions of the macroscopic finite interfacial width model with no adjustable parameters are in excellent agreement with experimental data presented in the companion paper even though the latter was obtained with molecular resolution. This agreement provides insight into aspects of the free-energy landscape that determine surfactant transport.

Adsorption↗
PubMed2006

Three-dimensional lattice Monte Carlo simulations of model proteins. IV. Proteins at an oil-water interface.

Lattice Monte Carlo simulations describe the adsorption of protein-like heteropolymer chains at an oil/water interface. The heteropolymers are designed sequences of 27 and 64 amino acid-type lattice sites taken from a 20-letter alphabet. We use our recently suggested energy scale to model oil and water. We investigate the effect of the oil parameters on adsorption properties of a single chain and on the aggregation of adsorbed chains while keeping the water parameters fixed to their optimum values found previously. By varying the oil parameters, we can cause a large range of adsorption behavior: from no adsorption to reversible adsorption to irreversible adsorption. We compare adsorption at a liquid/solid interface to that at a liquid/liquid interface. A liquid interface leads to stronger adsorption and denaturation than a solid interface with the same water and oil interaction parameters. We propose "optimal" oil parameters and use them to study multichain adsorption at a liquid interface.

Adsorption↗
PubMed2006

Sorption kinetics and equilibrium uptake for water vapor in soft-contact-lens hydrogels.

A gravimetric-sorption technique was used to obtain kinetic and equilibrium adsorption/desorption data for water vapor in four different soft-contact-lens (SCL) polymers at 35 degrees C. The SCL materials are a conventional hydrogel (polymacon) with a low water content at saturation (<50 wt %); two conventional hydrogels (hilafilcon A and alphafilcon A) with a high water content at saturation (>50 wt %); and a siloxane hydrogel (balafilcon A). Absorption and desorption equilibrium isotherms (water activity versus water weight fraction) overlap at high water contents, whereas significant hysteresis is observed at low water contents. The hysteresis loop is likely due to trapping of water in the polymer during the desorption process because of a rubber-to-glass transition of the SCL-film surfaces. Sorption data were interpreted using Flory-Rehner theory. The positive Zimm and Lundberg cluster function suggests that water tends to cluster in these SCL materials, except at very low water content. For polymacon and hilafilcon A, Fickian diffusion is observed for all activities for both water sorption and desorption. However, for alphafilcon A and balafilcon A, non-Fickian features appear at intermediate/low activities, in particular during water desorption, suggesting coupling of the diffusion process with polymer-matrix relaxation. The diffusion coefficient increases significantly with water concentration for polymacon and hilafilcon A (from approximately 0.3 x 10(-8) to 4.0 x 10(-8) cm2/s) because of augmented mixture free volume induced by water sorption, whereas a more complex composition dependence is observed for alphafilcon A and balafilcon A probably as consequence of a combined effect of polymer relaxation, plasticization, and water clustering.

Adsorption↗
PubMed2005

A combined streaming-potential optical reflectometer for studying adsorption at the water/solid surface.

A novel in-situ streaming-potential optical reflectometry apparatus (SPOR) was constructed and utilized to probe the molecular architecture of aqueous adsorbates on a negatively charged silica surface. By combining optical reflectometry and electrokinetic streaming potentials, we measure simultaneously the adsorption density, gamma, and zeta potential, zeta, in a rectangular flow cell constructed with one transparent wall. Both dynamic and equilibrium measurements are possible, allowing the study of sorption kinetics and reversibility. Using SPOR, we investigate the adsorption of a classic nonionic surfactant (pentaethylene glycol monododecyl ether, C12E5), a simple cationic surfactant (hexadecyl trimethylammonium bromide, CTAB) of opposite charge to that of the substrate surface, and two cationic polyelectrolytes (poly(2-(dimethylamino)ethyl methacrylate), PDAEMA; (poly(propyl methacrylate) trimethylammonium chloride, MAPTAC). For the polyethylene oxide nonionic surfactant, bilayer adsorption is established above the critical micelle concentration (cmc) both from the adsorption amounts and from the interpretation of the observed zeta potentials. Near adsorption saturation, CTAB also forms bilayer structures on silica. Here, however, we observe a strong charge reversal of the surface. The SPOR data, along with Gouy-Chapman theory, permit assessment of the net ionization fraction of the CTAB bilayer at 10% so that most of the adsorbed CTAB molecules are counterion complexed. The adsorption of both C12E5 and CTAB is reversible. The adsorption of the cationic polymers, however, is completely irreversible to a solvent wash. As with CTAB, both PDAEMA and MAPTAC demonstrate strong charge reversal. For the polyelectrolyte molecules, however, the adsorbed layer is thin and flat. Here also, a Gouy-Chapman analysis shows that less than 20% of the adsorbed layer is ionized. Furthermore, the amount of charge reversal is inversely proportional to the Debye length in agreement with available theory. SPOR provides a new tool for elucidating aqueous adsorbate molecular structure at solid surfaces.

Journal Article↗
PubMed2005

Oscillating drop/bubble tensiometry: effect of viscous forces on the measurement of interfacial tension.

The oscillating drop/bubble technique is increasingly popular for measuring the interfacial dilatational properties of surfactant/polymer-laden fluid/fluid interfaces. A caveat of this technique, however, is that viscous forces are important at higher oscillation frequencies or fluid viscosities; these can affect determination of the interfacial tension. Here, we experimentally quantify the effect of viscous forces on the interfacial-tension measurement by oscillating 100 and 200 cSt poly(dimethylsiloxane) (PDMS) droplets in water at small amplitudes and frequencies ranging between 0.01 and 1 Hz. Due to viscous forces, the measured interfacial tension oscillates sinusoidally with the same frequency as the oscillation of the drop volume. The tension oscillation precedes that of the drop volume, and the amplitude varies linearly with Capillary number, Ca=DeltamuomegaDeltaV/gammaa(2), where Deltamu=mu(D)-mu is the difference between the bulk Newtonian viscosities of the drop and surrounding continuous fluid, omega is the oscillation frequency of the drop, DeltaV is the amplitude of volume oscillation, gamma is the equilibrium interfacial tension between the PDMS drop and water, and a is the radius of the capillary. A simplified model of a freely suspended spherical oscillating-drop well explains these observations. Viscous forces distort the drop shape at Ca>0.002, although this criterion is apparatus dependent.

Models, Theoretical↗
PubMed2005

The effect of water hydraulic permeability on the settling of a soft contact lens on the eye.

PURPOSE: Silicone-elastomer soft contact lenses (SCLs) adhere to the cornea during wear, whereas silicone-hydrogel soft contact lenses exhibit adequate on-eye movement. One explanation for the observed immunity to binding of silicone-hydrogel lenses is that some interstitial water is expelled during blinking, therefore maintaining a more stable post-lens tear film (PoLTF). We examine quantitatively whether or not water can be squeezed by hydrodynamic flow through a silicone-hydrogel membrane driven by the applied lid force during a blink. METHODS: A rigid, porous-disk model of a contact lens was devised to calculate the relative settling rates of a permeable versus a completely impermeable SCL. The settling rate depended strongly on the value of the hydraulic permeability for pressure-driven water flow through the lens. Because the hydraulic permeability of water through silicone-hydrogel materials is not well-known, we measured this value. At steady state, water was forced through flat membranes of representative lens materials under known pressure drops. The resulting volumetric flows were measured by following the transient rise height of water in a vertical, precision-bore glass capillary. Darcy's law permitted calculation of the hydrodynamic permeability. RESULTS: The settling-rate model indicated that tear can be squeezed through a SCL only when the Darcy-law hydrodynamic permeability is greater than about 10 microm2 (i.e., greater than 10 Darcy). Our measurements for silicone and HEMA hydrogel membranes reveal hydrodynamic permeabilities of the order 10(-8) microm2, almost 9 orders of magnitude smaller than that necessary to initiate hydrodynamic flow through a SCL. CONCLUSIONS: We conclude that the squeeze-through mechanism cannot quantitatively account for the observed on-eye movement of silicone-hydrogel lenses. Also, we find that the lid-applied pressure cannot squeeze enough water out of a SCL during a blink to stabilize the PoLTF. Neither a squeeze-through nor a squeeze-out mechanism can maintain a stable PoLTF and prevent adherence.

Blinking↗
PubMed2003

3D-Lattice Monte Carlo simulations of model proteins. Size effects on folding thermodynamics and kinetics.

Recently, we devised an energy scale to vary systematically amino-acid residue-solvent interactions for Monte Carlo simulations of lattice-model proteins in water. For 27-mer proteins, the folding behavior varies appreciably with the choice of interaction parameters. We now perform similar simulations with 64-mers to study the size dependence of the optimal energy parameter set for representing realistic behavior typical of many real proteins (i.e. fast folding and high cooperativity for single chains). We find that 64-mers are considerably more stable and more cooperative compared to 27-mers. The optimal interfacial-interaction-energy parameter set, however, is relatively size independent.

Amino Acid Sequence↗
PubMed2003

Evaluation of DLVO theory with disjoining-pressure and film-conductance measurements of common-black films stabilized with sodium dodecyl sulfate.

We develop a unique film holder combining a thin-film balance with AC impedance spectroscopy to measure disjoining pressure, film conductance, and film thickness simultaneously. Foam films stabilized by sodium dodecyl sulfate (SDS) are investigated with and without added sodium chloride (NaCl) electrolyte. Classical colloidal theory, Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, is tested rigorously over a wide range of solution conditions by comparing the surface charge densities fit to disjoining-pressure isotherms with those estimated independently from film-conductance and surface-tension data. Film-conductance measurements strongly suggest that the adsorbed anionic surfactant is partially complexed with counterions. Therefore, to reconcile the different values of charge densities calculated from surface tension and film conductance with those from disjoining pressure, we propose a simple ion-binding electrostatic model. The ion-complexation framework predicts increased ion complexing with increasing solution ionic strength, in agreement with surface-tension and film-conductance data. Unfortunately, it is not possible to describe similarly the trends of the measured disjoining-pressure isotherms because the diffuse-layer charge density increases, or equivalently, the ion complexation decreases with increasing ionic strength. Accordingly, the ion-binding extension of classical DLVO theory does not permit agreement between theory and independent experimental data from surface tension, disjoining pressure, and film conductance.

Journal Article↗
PubMed2003

Dynamics of surfactant sorption at the air/water interface: continuous-flow tensiometry.

Dynamic interfacial tensiometry, gauged by axisymmetric drop shape analysis of static drops or bubbles, provides useful information on surfactant adsorption kinetics. However, the traditional pendant-drop methodology is not readily amenable to the study of desorption kinetics. Thus, the question of sorption reversibility is difficult to assess by this technique. We extend classical pendant/sessile drop dynamic tensiometry by immersing a sessile bubble in a continuously mixed optical cell. Ideal-mixed conditions are established by stirring and by constant flow through the cell. Aqueous surface-active-agent solutions are either supplied to the cell (loading) or removed from the cell by flushing with water (washout), thereby allowing study of both adsorption and desorption kinetics. Well-mixed conditions and elimination of any mass transfer resistance permit direct identification of sorption kinetic barriers to and from the external aqueous phase with time constants longer than the optical-cell residence time. The monodisperse nonionic surfactant ethoxy dodecyl alcohol (C(12)E(5)), along with cationic cetyltrimethyl ammonium bromide (CTAB) in the presence of added salt, adsorbs and desorbs instantaneously at the air/water interface. In these cases, the experimentally observed dynamic-tension curves follow the local-equilibrium model precisely for both loading and washout. Accordingly, these surfactants below their critical micelle concentrations (CMC) exhibit no detectable sorption-activation barriers on time scales of order a min. However, the sorption dynamics of dilute CTAB in the absence of electrolyte is markedly different from that in the presence of KBr. Here CTAB desorption occurs at local equilibrium, but the adsorption rate is kinetically limited, most likely due to an electrostatic barrier arising as the charged surfactant accumulates at the interface. The commercial, polydisperse nonionic surfactant ethoxy nonylphenol (NP9) loads in good agreement with local-equilibrium theory but shows deviation from the theoretical washout curve, presumably due to slow desorption of solubilized but otherwise water insoluble components. The polymeric nonionic triblock copolymer Pluronic exhibits almost complete irreversible adsorption at the air/water interface over a molecular-weight range from 3 to 14 kDa. Similar irreversible dynamic behavior is observed for adsorption/desorption of the protein bovine serum albumin (BSA) from dilute aqueous solutions at the air/water interface. The new continuous-flow tensiometer (CFT) is a simple, yet powerful, tool to investigate sorption dynamics at fluid/fluid interfaces, especially for larger molecular weight surface-active agents that exhibit significant hindrance to desorption.

Adsorption↗
PubMed2002

Disjoining pressures, zeta potentials and surface tensions of aqueous non-ionic surfactant/electrolyte solutions: theory and comparison to experiment.

A self-consistent electrostatic theory is presented to predict disjoining pressure isotherms of aqueous thin-liquid films stabilized by non-ionic surfactants and air/water surface tensions and zeta potentials of electrolyte solutions with and without non-ionic surfactant. The proposed model combines specific adsorption of hydroxide ions at the interface with image charge and dispersion forces on ions in the diffuse double layer. The result is a quantitative description of aqueous solution interfaces as a function of surfactant concentration, ionic strength and pH. Disjoining pressure isotherms of thin-liquid films stabilized by non-ionic surfactants and electrophoresis experiments on air bubbles and oil droplets in aqueous solutions demonstrate that hydroxide ions specifically adsorb at air/water and oil/water interfaces. The surface charge increases with pH, decreases with increasing surfactant concentration, increases slightly with ionic strength, and for n-alkyl polyethylene oxide non-ionic surfactants is not significantly affected by surfactant molecular structure. Concentrated electrolyte-solution surface tensions, however, indicate that ions are repelled from the air/water interface by an 'image charge' force, that is a parameterized by the ion valence and the ionic strength of the aqueous solution. Additionally, differences in induced-induced dipole forces on an ion near an interface lead to a van der Waals dispersion interaction force that depends on the ion polarizabilites and the molecular properties of the two surrounding bulk phases. Incorporation of these two additional ion free energies into the Poisson-Boltzmann equation along with a simple model for hydroxide-ion specific adsorption at the air/water interface results in a non-linear second-order ordinary differential equation containing two adjustable parameters. The proposed modified Poisson-Boltzmann (MPB) theory accurately predicts newly measured disjoining pressures of thin-liquid foam films stabilized by polyethylene oxide n-alkyl ether surfactants. With no additional adjustable parameters, zeta potentials of nascent air bubbles in water and surface tensions of aqueous electrolyte solutions are successfully predicted. The new electrostatic model also explains the fascinating existence of a surface tension minimum in dilute electrolyte solutions, known as the Jones-Ray effect.

Journal Article↗
PubMed2002

Settling and deformation of a thin elastic shell on a thin fluid layer lying on a solid surface.

placement of a soft contact lens onto the cornea, the upper eyelid deforms and settles the lens by squeezing fluid out of the post-lens tear film or POLTF (i.e., the tear fluid layer sandwiched between the lens and the cornea). This paper studies the physical mechanisms that control the dynamic state of the contact lens during blinking, i.e., its shape and its distance from the cornea, especially a long time after insertion. We model the lens as a deformable elastic shell and the cornea as a flat nondeformable body. The tear fluid is assumed to be Newtonian, and the lens is characterized by an elastic modulus and a Poisson ratio. Lubrication equations under creeping flow are used to solve the fluid problem, while the thin-shell approximation is applied to the solid lens. The solid and fluid mechanics problems are coupled by maintaining continuity of stress and velocity at the solid/liquid interface. Lid applied pressure causes the lens to approach the cornea by squeezing tear fluid out and also leads to the deformation of the lens. Subsequently, in the interblink period, since there is no applied force, the elastic energy stored in the lens due to its deformation is released causing it to move away from the cornea by imbibing tear fluid into the POLTF. If the POLTF thickness is large, the inward motion of the lens in the blink is more than the outward motion during interblink, and this causes the lens to settle closer to the cornea. Eventually, there may be a balance of the inward motion during the blink and the outward motion during the interblink. If so, the lens subsequently exhibits periodic steady-state motion. However, it is also possible that a balance of inward and outward motion is never achieved, and the lens continues to settle endlessly. If this happens, then the thinfilm interactions between the mucin-covered corneal surface and the lens material determine whether the lens actually touches the cornea and possibly adheres. Our elastohydrodynamic analysis serves as a useful tool to elucidate the effects of various lens parameters on the final settled state of the lens. In particular, we are concerned about eventual adherence and/or mechanical abrasion to the cornea, which is very important to the ocular health of soft contact lens wearers.

Journal Article↗
PubMed2001

Role of organic solvents on Pa-hydroxynitrile lyase interfacial activity and stability.

Catalytic activity and adsorption of Pa-hydroxynitrile lyase (Pa-Hnl) was investigated at various organic solvent/water interfaces. We focused on the role of solvent polarity in promoting activity and stability in two-phase systems, specifically for the solvents heptane, dibutyl ether (DBE), diisopropyl ether (DIPE), butylmethyl ether (BME), and methyl tert-butyl ether (MTBE). Enzyme activity towards mandelonitrile cleavage was determined in a recycle reactor with a well-defined interfacial area as described by Hickel, et al. 1999. Here the recycle reactor was modified to permit exchange of the aqueous phase. With this modification, irreversibility of enzyme adsorption was determined as a function of the adsorption time at the interface. Irreversibility of enzyme adsorption was also investigated by measuring the surface pressure of a sessile-drop upon washout. We find that Pa-Hnl exhibits the highest stability but the lowest initial catalytic activity at the aqueous/organic solvent interface with the most polar organic solvents. Thus, DIPE and MTBE display no loss in enzyme activity over a period of several hours. However, the more apolar the solvent is the higher the initial Pa-Hnl activity, but the faster the loss of activity. Dynamic tensiometry reveals that Pa-Hnl adsorbs more strongly at the interface of the more apolar solvents. Surprisingly, Pa-Hnl develops some irreversible adsorption after 30 min at the DIPE/water interface, but does not lose catalytic activity.

Adsorption↗
PubMed2001

Modeling the vertical motion of a soft contact lens.

PURPOSE: The motion of a soft contact lens in the up-down or vertical (inferior-superior) and in-out (anterior-posterior) directions drives mixing in the post lens tear film. Thus, it is important to obtain an accurate assessment of lens motion. The commonly used experimental technique to measure the vertical motion, video microscopy, only gauges motion during the interblink period. Since most of the eyelid force, which drives the motion, is exerted during the blink, it is reasonable to assume that the majority of the lens motion in the up-down direction takes place during the blink and is, therefore, hidden from view. Thus, experimentally measured values of vertical lens travel are currently underpredicted. METHODS: In this paper, we use a simple mechanical force balance on the lens to predict its motion in the vertical direction. The forces included in our model are due to the upper and the lower eyelids, gravity, elasticity, and viscous stresses. We input the lens physical parameters, the various tear film thicknesses, and the upper eyelid velocity. Then we integrate a macroscopic force balance to obtain the lens vertical position as a function of time. RESULTS: The proposed model predicts that the downward lens motion during a blink is about 2--3 times the downward motion during the interblink (centration). CONCLUSIONS: The up-down motion observable during the interblink period is only a small fraction of the total lens vertical travel.

Biomechanical Phenomena↗
PubMed2001

The role of fenestrations and channels on the transverse motion of a soft contact lens.

PURPOSE: Ineffectual removal of potentially harmful species from the postlens tear film (POLTF) may lead to adverse responses among extended wearers of soft contact lenses. It is apparently important to remove bacteria, cell debris, and metabolic products from the postlens tear film to the outer tear lake; the flushing or dispersion rate of these species is enhanced by increasing fluid movement in the tear film driven by periodic lens motion. The contact lens moves laterally (up-down) and transversally (in-out) due to the action of the eyelid forces during blinking. Viscous drag in the POLTF resists lens motion. Consequently, any design change in the lens that reduces viscous drag increases motion and improves flushing of unwanted species from the POLTF. We investigate quantitatively the effect of channels cut on the back surface of the lens and fenestrations (holes) drilled through the lens on transverse lens motion. METHODS: We model the lens as a curved solid body with a periodic arrangement of channels/holes. The cornea is treated as a flat surface, and the hydrodynamic equations of motion are solved for Newtonian fluid transport in the POLTF assuming lubrication and creeping flow. POLTF pressure profiles, obtained by solving these equations, are integrated to determine the lens settling velocity in the transverse direction for a given amount of applied lid force. Lens settling velocity is then compared with the same velocity in the absence of channels/fenestrations. Further, we calculate the total transverse motion in a blink for lenses with and without channels/fenestrations to estimate the possible enhancement in transverse motion due to the channels/fenestrations. RESULTS: Variables that affect the fluid mixing in the POLTF are the postlens tear film thickness, lens thickness, channel length, depth and spacing, and the hole diameter, location, and spacing. We study the effect of each of these variables on the enhancement of transverse motion for channels and holes of diameters varying from 0.1 to 2 mm with spacing varying from 1 to 5 mm. CONCLUSIONS: We demonstrate that incorporation of channels and holes reduces viscous resistance and increases transverse lens motion, and thus increases fluid mixing and dispersive flushing from the POLTF. The increase in transverse motion depends strongly on the postlens tear film thickness. Enhancement of the transverse motion varies from a factor of about 2 to 20 depending on the particular lens design and the postlens tear film thickness. Because fluid mixing increases up to the square of the transverse motion, channels/holes are expected to render flushing of the POLTF considerably more effective. We find that channels and/or fenestrations, when appropriately designed, can provide significant improvement in flushing from the POLTF. This work provides a new quantitative tool for the efficient design of channels/holes in soft contact lenses.

Biomechanical Phenomena↗
PubMed2000

Peptide interfacial adsorption is kinetically limited by the thermodynamic stability of self association.

We present a study of the adsorption of two peptides at the octane-water interface. The first peptide, Lac21, exists in mixed monomer-tetramer equilibrium in bulk solution with an appreciable monomer concentration. The second peptide, Lac28, exists as a tetramer in solution, with minimal exposed hydrophobic surface. A kinetic limitation to interfacial adsorption exists for Lac28 at moderate to high surface coverage that is not observed for Lac21. We estimate the potential energy barrier for Lac28 adsorption to be 42 kJ/mol and show that this is comparable to the expected free energy barrier for tetramer dissociation. This finding suggests that, at moderate to high surface coverage, adsorption is kinetically limited by the availability of interfacially active monomeric "domains" in the subinterfacial region. We also show how the commonly used empirical equation for protein adsorption dynamics can be used to estimate the potential energy barrier for adsorption. Such an approach is shown to be consistent with a formal description of diffusion-adsorption, provided a large potential energy barrier exists. This work demonstrates that the dynamics of interfacial adsorption depend on protein thermodynamic stability, and hence structure, in a quantifiable way.

Adsorption↗
PubMed1999

Hydroxynitrile lyase at the diisopropyl ether/water interface: evidence for interfacial enzyme activity.

A novel recycle reactor has been designed to determine the interfacial activity of hydroxynitrile lyase in a diisopropyl ether (DIPE)/water two-phase system. The reactor provides a known interfacial area. Enzyme activity toward mandelonitrile cleavage is continuously measured in the reactor by following benzaldehyde product formation in the DIPE organic phase with an optical flow cell. For the first time, we establish that this enzymatic reaction is carried out by the hydroxynitrile lyase residing at the organic solvent/water interface and not in the aqueous bulk phase. Hydroxynitrile lyase adsorbs at the interface and exhibits extraordinary stability. Denaturation does not occur over several hours, although the surface pressure increases under the same conditions over this time span. Increases in surface pressure indicate enzyme penetration through the interface although no loss of enzyme activity is observed. Adsorption of p-Hnl at the interface is fit by the Langmuir equilibrium adsorption model with an adsorption equilibrium constant of 0.032 L mg(-1). For the mandelonitrile-cleavage reaction at ambient temperature, p-Hnl follows Michaelis-Menten kinetics at the interface with a Michaelis constant of 14.4 mM and a specific activity close that for the bulk aqueous phase.

Acetonitriles↗