pH-responsive diblock copolymer micelles at the silica/aqueous solution interface: Adsorption kinetics and equilibrium studies

The adsorption behavior of two examples of a weakly basic diblock copolymer, poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA), at the silica/aqueous solution interface has been investigated using a quartz crystal microbalance with dissipation monitoring and an optical reflectometer. Dynamic and static light scattering measurements have also been carried out to assess aqueous solution properties of such pH-responsive copolymers. In alkaline solution, core-shell micelles are formed above the critical micelle concentration (cmc) by both copolymers, whereas the chains are molecularly dissolved (as unimers) at all concentrations in acidic solution. As a result, the adsorption behavior of PDMA-PDEA diblock copolymers on silica is strongly dependent on both the copolymer concentration and the solution pH. Below the cmc at pH 9, the cationic PDMA-PDEA copolymers adsorb as unimers and the conformation of the adsorbed polymer is essentially flat. At concentrations just above the cmc, the initial adsorption of copolymer onto the silica is dominated by the unimers due to their faster diffusion compared to the much larger micelles. Rearrangement of the adsorbed unimers and/or their subsequent displacement by micelles from solution is then observed during an equilibration period, and the final adsorbed mass is greater than that observed below the cmc. At concentrations well above the cmc, the much higher proportion of micelles in solution facilitates more effective competition for the surface at all stages of the adsorption process and no replacement of initially adsorbed unimers by micelles is evident. However, the adsorbed layer undergoes gradual rearrangement after initial adsorption. This relaxation is believed to result from a combination of further copolymer adsorption and swelling of the adsorbed layer.     

Adsorption kinetics of laponite and ludox silica nanoparticles onto a deposited poly(diallyldimethylammonium chloride) layer measured by a quartz crystal microbalance and optical reflectometry

A quartz crystal microbalance with dissipation (QCM-D) and an optical reflectometer (OR) have been used to investigate the adsorption behavior of Laponite and Ludox silica nanoparticles at the solid-liquid interface. The adsorption of both Laponite and Ludox silica onto poly(diallyldimethylammonium chloride) (PDADMAC)-coated surfaces over the first few seconds were studied by OR. Both types of nanoparticles adsorbed rapidly and obtained a stable adsorbed amount after only a few minutes. The rate of adsorption for both nanoparticle types was concentration dependent. The maximum adsorption rate of Ludox nanoparticles was found to be approximately five times faster than that for Laponite nanoparticles. The QCM data for the Laponite remained stable after the initial adsorption period at each concentration tested. The observed plateau values for the frequency shifts increased with increasing Laponite particle concentration. The QCM data for the Ludox nanoparticles had a more complex long-time behavior. In particular, the dissipation data at 3 ppm and 10 ppm Ludox increased slowly with time, never obtaining a stable value within the duration of the experiment. We postulate here that this is caused by slow structural rearrangements of the particles and the PDADMAC within the surface adsorbed layer. Furthermore, the QCM dissipation values were significantly smaller for Laponite when compared with those for Ludox for all nanoparticle concentrations, suggesting that the Laponite adsorbed layer is more compact and more rigidly bound than the Ludox adsorbed layer.     

A quartz crystal microbalance study of the adsorption of asphaltenes and resins onto a hydrophilic surface

The adsorption of extracted and purified samples of asphaltenes and resins onto gold surfaces has been studied as a function of bulk concentration using a quartz crystal microbalance with dissipation measurements (QCM-D). With this device, which works equally well in transparent, opaque, and nontransparent samples, the adsorbed amount is measured through a change in resonant frequency of the quartz oscillator. The measured change in dissipation reports on changes in layer viscoelasticity and slip of the solvent at the surface. The results show that the adsorbed amount for resins from heptane corresponds to a rigidly attached monolayer. The adsorbed amount decreases with increasing amount of toluene in the solvent and is virtually zero in pure toluene. Asphaltenes, on the other hand, adsorb in large quantities and the mass and dissipation data demonstrate the presence of aggregates on the surface. The aggregates are firmly attached and cannot be removed by addition of resins. On the other hand, resins and asphaltenes associate in bulk liquid and the adsorption from mixtures containing both resins and asphaltenes is markedly different from that obtained from the pure components. Hence, we conclude that preformed resin aggregates adsorb to the surface. These results are compared and discussed in relation to adsorption from crude oil diluted in heptane/toluene mixtures.     

Cationic ester-containing gemini surfactants: adsorption at tailor-made surfaces monitored by SPR and QCM

Adsorption of a series of ester-containing cationic surfactants at a surface containing 90% methyl groups and 10% carboxyl groups was studied by two surface analysis techniques, surface plasmon resonance (SPR) and quartz crystal microbalance (QCM). Such a surface, which is at the same time hydrophobic and negatively charged, is of interest as a model for many polymeric surfaces. Two different types of ester gemini surfactants and their monomeric counterparts were included together with nonester containing surfactants of similar structure. The results show that the gemini surfactants give the same adsorbed amount at the surface as the monomeric surfactants when compared at the same bulk concentration normalized to the critical micelle concentration (cmc) in bulk. Since the cmc of the geminis is around 20 times lower than the cmc of the corresponding monomeric surfactants, the gemini surfactants are much more effective in covering the surface. The two techniques gave similar relative values but the QCM values were always higher than those from SPR, which is due to the former method taking also adsorbed water into account. The adsorption, as measured by both methods, was found to follow closely the Langmuir adsorption model.     

Bovine serum albumin adsorption on nano-rough platinum surfaces studied by QCM-D

The adsorption of bovine serum albumin (BSA) on platinum surfaces with a root-mean-square roughness ranging from 1.49nm to 4.62nm was investigated using quartz crystal microbalance with dissipation (QCM-D). Two different BSA concentrations, 50microg/ml and 1mg/ml, were used, and the adsorption studies were complemented by monitoring the antibody interaction with the adsorbed BSA layer. The adsorption process was significantly influenced by the surface nano-roughness, and it was observed that the surface mass density of the adsorbed BSA layer is enhanced in a non-trivial way with the surface roughness. From a close examination of the energy dissipation vs. frequency shift plot obtained by the QCM-D technique, it was additionally observed that the BSA adsorption on the roughest surface is subject to several distinct adsorption phases revealing the presence of structural changes facilitated by the nano-rough surface morphology during the adsorption process. These changes were in particular noticeable for the adsorption at the low (50microg/ml) BSA concentration. The results confirm that the nano-rough surface morphology has a significant influence on both the BSA mass uptake and the functionality of the resulting protein layer.     

Surface Changes of Ochrobactrum Anthropi in Cr（Ⅵ） Treatment for 1 Hour

Bioremediation has been a considerable method for treating Cr（VI） contamination. Bacterial surface changes of Ochrobactrum anthropi during Cr biosorption was investigated in this study. We found that Cr adsorption capacity increased with the increase of initial Cr（Ⅵ） concentration. Atomic force microscope （AFM） morphologic analysis combined with surface roughness analysis indicated that the bacterial surfaces became rougher during Cr uptake process. X-ray photoelectron spectroscopy （XPS） showed that Cr（Ⅲ） was adsorbed on the bacterial surfaces. Fourier transform infrared （FT-IR） analysis showed that surface functional groups including C-O and C-N might be involved in the Cr biosorption process.     

Experimental studies on the adsorption of two surfactants on solid-aqueous interfaces: adsorption isotherms and kinetics

A quartz crystal microbalance with dissipation (QCM-D) was used to measure the adsorption from aqueous solutions of CTAB (cationic) and C(12)E(6) (nonionic) surfactants on gold and silica surfaces. QCM-D allows for the determination of adsorption isotherms and also the monitoring of the dynamics of adsorption in real time. By considering the atomic-scale roughness of the solid surfaces and the surface area per head group at the air/water interface, our experiments indicate that at bulk concentrations above the critical micelle concentration adsorbed C(12)E(6) forms a monolayer-like structure on both surfaces and CTAB yields a bilayer-like structure. Although our measurements do not allow us to discriminate between the morphology of the aggregates (i.e., between flat monolayers, hemicylinders, or hemispheres in the case of C(12)E(6) and between flat bilayers, cylinders, or spheres in the case of CTAB), these results are particularly significant when compared to recent QCM-D data reported by Macakova et al. (Macakova, L.; Blomberg, E.; Claesson, P. M. Langmuir 2007, 23, 12436). These authors reported that QCM-D overestimates the amount of CTAB adsorbed on silica by as much as 30-40% as a result of entrapped water. Our analysis suggests that the effect of entrapped solvent is not as important as previously assumed and, in fact, QCM-D may not overestimate the amount of CTAB adsorbed when roughness is considered. Results for the kinetics of adsorption suggest that the aggregate structure as well as whether micelles are present may influence the adsorption mechanism. We discuss our results in the perspective of molecular theories for both the equilibrium and kinetics of surfactant adsorption.     

Adsorption of alkyl polyglucosides on the solid/water interface: equilibrium effects of alkyl chain length and head group polymerization

The equilibrium adsorption behavior of two n-alkyl-beta-D-glucosides (octyl (C8G1) and decyl (C10G1)) and four n-alkyl-beta-D-maltosides (octyl (C8G2), decyl (C10G2), dodecyl (C12G2), and tetradecyl (C14G2)) from aqueous solution on a titania surface, as measured by ellipsometry, has been investigated. The main focus has been on the effect of changes in the alkyl chain length and headgroup polymerization, but a comparison with their adsorption on the silica/water and air/water interfaces is also presented. Some comparison with the corresponding adsorption of ethylene oxide surfactants, in particular C10E6 and C12E6, is given as well. For all alkyl polyglucosides, the maximum adsorbed amount on titania is reached slightly below the critical micelle concentration (cmc), where it levels off to a plateau and the amount adsorbed corresponds roughly to a bilayer. However, there is no evidence that this is the actual conformation of the surfactant assemblies on the surface, but the surfactants could also be arranged in a micellar network. On hydrophilic silica, the adsorbed amount is a magnitude lower than on titania, corresponding roughly to a layer of surfactants lying flat on the surface. A change in the alkyl chain length does not result in any change in the plateau molar adsorbed amount at equilibrium; however, the isotherm slope for the alkyl maltosides increases with increasing chain length. Headgroup polymerization on the other hand affects the adsorbed amount. The alkyl glucosides start adsorbing at lower bulk concentrations than the maltosides and equilibrate at higher adsorbed amounts above the cmc. When compared with the ethylene oxide (EO) surfactants, it is confirmed that the EO surfactants hardly adsorb on titania, since the measured changes in the ellipsometric angles are within the noise level. They do, however, adsorb strongly on silica.     

Spreading of proteins and its effect on adsorption and desorption kinetics

The kinetics of adsorption of lysozyme and alpha-lactalbumin from aqueous solution on silica and hydrophobized silica has been studied. The initial rate of adsorption of lysozyme at the hydrophilic surface is comparable with the limiting flux. For lysozyme at the hydrophobic surface and alpha-lactalbumin on both surfaces, the rate of adsorption is lower than the limiting flux, but the adsorption proceeds cooperatively, as manifested by an increase in the adsorption rate after the first protein molecules are adsorbed. At the hydrophilic surface, adsorption saturation (reflected in a steady-state value of the adsorbed amount) of both proteins strongly depends on the rate of adsorption, but for the hydrophobic surface no such dependency is observed. It points to structural relaxation ("spreading") of the adsorbed protein molecules, which occurs at the hydrophobic surface faster than at the hydrophilic one. For lysozyme, desorption has been studied as well. It is found that the desorbable fraction decreases after longer residence time of the protein at the interface.     

Fibronectin and bovine serum albumin adsorption and conformational dynamics on inherently conducting polymers: a QCM-D study

Quartz crystal microbalance with dissipation monitoring (QCM-D) was employed to characterize the adsorption of the model proteins, bovine serum albumin (BSA) and fibronectin (FN), to polypyrrole doped with dextran sulfate (PPy-DS) as a function of DS loading and surface roughness. BSA adsorption was greater on surfaces of increased roughness and was above what could be explained by the increase in surface area alone. Furthermore, the additional mass adsorbed on the rough films was concomitant with an increase in the rigidity of the protein layer. Analysis of the dynamic viscoelastic properties of the protein adlayer reveal BSA adsorption on the rough films occurs in two phases: (1) arrival and initial adsorption of protein to the polymer surface and (2) postadsorption molecular rearrangement to a more dehydrated and compact conformation that facilitates further recruitment of protein to the polymer interface, likely forming a multilayer. In contrast, FN adsorption was independent of surface roughness. However, films prepared from solutions containing the highest concentration of DS (20 mg/mL) demonstrated both an increase in adsorbed mass and adlayer viscoelasticity. This is attributed to the higher DS loading in the conducting polymer film resulting in presentation of a more hydrated molecular structure indicative of a more unfolded and bioactive conformation. Modulating the redox state of the PPy-DS polymers was shown to modify both the adsorbed mass and viscoelastic nature of FN adlayers. An oxidizing potential increased both the total adsorbed mass and the adlayer viscoelasticity. Our findings demonstrate that modification of polymer physicochemical and redox condition alters the nature of protein-polymer interaction, a process that may be exploited to tailor the bioactivity of protein through which interactions with cells and tissues may be controlled.     

Adsorption isoterms and capillary condensation in a nanoslit with rough walls: a density functional theory

Adsorption isoterms and capillary condensation in an open slit with walls decorated with arrays of pillars are examined using the density functional theory. Compared with the main substrate, the pillars can have the same or different parameters in the Lennard-Jones interaction potential between them and the fluid in the slit. The roughness of the solid surface, defined as the ratio between the area of the actual surface and the area of the surface free of pillars, is controlled by the height of the pillars. It is shown that the capillary condensation pressure first increases with increasing roughness, passes through a maximum, and then decreases. The amount of adsorbed fluid at constant volume of the slit has, in general, a nonmonotonic dependence on roughness. These features of adsorption and capillary condensation are results of increased surface area and changes in the fluid-solid potential energy due to changes in roughness.     

Forces between glass surfaces in mixed cationic-zwitterionic surfactant systems

We report atomic force microscopy (AFM) measurements of the forces between borosilicate glass solids in aqueous mixtures of cationic and zwitterionic surfactants. These forces are used to determine the adsorption of the surfactant as a function of the separation between the interfaces (proximal adsorption) through the application of a Maxwell relation. In the absence of cationic surfactant, the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS) undergoes little adsorption to glass at concentrations up to about 2/3 critical micelle concentration (cmc). In addition, DDAPS does not have much effect on the forces over the same concentration range. In contrast, the cationic surfactant dodecylpyridinium chloride (DPC) does adsorb to glass and does affect the force between glass surfaces at concentrations much lower than the cmc. In the presence of a small amount of DPC (0.05 mM = cmc/300), the net force between the glass surfaces is quite sensitive to the solution concentration of DDAPS. A model-independent thermodynamic argument is used to show that the surface excess of DDAPS depends on the separation between the glass interfaces when the cationic surfactant is present and that the surface excess of the cationic surfactant is more sensitive to interfacial separation in the presence of the zwitterionic surfactant. The change in adsorption of the zwitterionic surfactant is explained in terms of an intermolecular coupling between the long-range electrostatic force acting on the cationic surfactant and the short-range hydrophobic interaction between the alkyl chains on the cationic and zwitterionic surfactants. The adsorptions of cationic and zwitterionic surfactants in mixtures were measured independently and simultaneously by attenuated total internal reflection infrared spectroscopy (ATR-IR). The adsorption of the zwitterionic surfactant is enhanced by the presence of a small amount of cationic surfactant.     

Fibrinogen adsorption on three silica-based surfaces: conformation and kinetics

Using AFM (atomic force microscopy) to probe protein conformation and arrangement, and TIRF (total internal reflectance fluorescence) to monitor kinetics, fibrinogen adsorption on three different silica-based surfaces was studied: the native oxide on silicon, acid-etched microscope slides, and acid-etched polished glass. The three are chemically similar, but the microscope slide is rougher and induces AFM tip instabilities that appear as high spots on the bare surface. Fibrinogen's conformation and transport-limited adsorption kinetics are found to be quantitatively similar on all three surfaces. Further, the number of adsorbed proteins in progressive AFM micrographs quantitatively match the coverages measured by TIRF during early adsorption. Surfaces appear full, via AFM, when adsorbed amounts are about an order of magnitude below their true saturation levels (via TIRF) because, above about 0.26 mg/m(2), individual proteins cannot be discerned. The results demonstrate how the appearance of AFM micrographs can be misleading regarding surface saturation. On all three surfaces, fibrinogen is, at most, slightly aggregated, showing limited, if any, surface mobility. The complexities of the microscope slide's surface landscape minimally impact adsorption.     

Adsorption and activity of Thermomyces lanuginosus lipase on hydrophobic and hydrophilic surfaces measured with dual polarization interferometry (DPI) and confocal microscopy

The adsorption and activity of Thermomyces lanuginosus lipase (TLL) was measured with dual polarization interferometry (DPI) and confocal microscopy at a hydrophilic and hydrophobic surface. In the adsorption isotherms, it was evident that TLL both had higher affinity for the hydrophobic surface and adsorbed to a higher adsorbed amount (1.90 mg/m²) compared to the hydrophilic surface (1.40–1.50 mg/m²). The thickness of the adsorbed layer was constant (3.5 nm) on both surfaces at an adsorbed amount >1.0 mg/m², but decreased on the hydrophilic surface at lower surface coverage, which might be explained by partially unfolding of the TLL structure. However, a linear dependence of the refractive index of the adsorbed layer on adsorbed amount of TLL on C18 surfaces indicated that the structure of TLL was similar at low and high surface coverage. The activity of adsorbed TLL was measured towards carboxyfluorescein diacetate (CFDA) in solution, which upon lipase activity formed a fluorescent product. The surface fluorescence intensity increase was measured in a confocal microscope as a function of time after lipase adsorption. It was evident that TLL was more active on the hydrophilic surface, which suggested that a larger fraction of adsorbed TLL molecules were oriented with the active site facing the solution compared to the hydrophobic surface. Moreover, most of the activity remained when the TLL surface coverage decreased. Earlier reports on TLL surface mobility on the same surfaces have found that the lateral diffusion was highest on hydrophilic surfaces and at low surface coverage of TLL. Hence, a high lateral mobility might lead to a longer exposure time of the active site towards solution, thereby increasing the activity against a water-soluble substrate.     

Adsorption of cationic gemini surfactants at solid surfaces studied by QCM-D and SPR: effect of the rigidity of the spacer

Two small series of cationic gemini surfactants with dodecyl tails have been synthesized and evaluated with respect to self-assembly in bulk water and at different solid surfaces. The first series contained a flexible alkane spacer and is denoted 12-n-12, with n = 2, 4, and 6. The second series had a phenylene group connected to the quaternary nitrogens in either the meta or para position and the surfactants are referred to as 12-m-Φ-12 and 12-p-Φ-12, respectively. The phenylene group is a rigid linker unit. The critical micelle concentration (cmc) was determined both by tensiometry and by conductometry, and the packing density of the surfactants at the air-water interface was calculated from the Gibbs equation. The cmc values for the geminis with a rigid spacer, 12-m-Φ-12 and 12-p-Φ-12, were of the same order of magnitude as for 12-4-12, which is the flexible surfactant that most closely matches the phenylene-based surfactants with respect to hydrophobicity, measured as log P, and distance between the positively charged nitrogen atoms. The adsorption of flexible and rigid surfactants was investigated on gold, silicon dioxide (silica), gold made hydrophobic by the self-assembly of hexadecanethiol, and gold made hydrophilic by the self-assembly of 16-hydroxyhexadecanethiol. On all of the surfaces, there was a reverse relationship between the adsorbed amount at the cmc and the length of the spacer (i.e., 12-2-12 gave the highest and 12-6-12 gave the lowest amount of adsorbed material). The adsorption pattern was similar for all of the surfactants when recorded at 25 °C. Thus, one can conclude that a rigid spacer does not render the self-assembly of a gemini surfactant difficult, neither in bulk water nor at solid surfaces. However, on one of the surfaces-untreated gold-the adsorbed amount of the geminis with a rigid spacer at 40 °C was approximately twice the values obtained at 25 °C. This is interpreted as the formation of an interdigitated bilayer at 25 °C and a regular bilayer without interpenetration of the alkyl chains at 40 °C.     

Surface activity of solid particles with extremely rough surfaces

The solid particles are adsorbed at liquid-liquid interfaces and form self-assembled structures when the particles have suitable wettability to both liquids. Here, we show theoretically how the extreme roughness on the particle surface affects their adsorption properties. In our previous work, we discussed the adsorption behavior of the solid particles with microstructured surfaces using the so-called Wenzel model [Y. Nonomura et al., J. Phys. Chem. B 110 (2006) 13124]. In the present study, the wettability and the adsorbed position of the particles with extremely rough surfaces are studied based on the Cassie-Baxter model. We predict that the adsorbed position and the interfacial energy depend on the interfacial tensions between the solid and liquid phases, the radius of the particle, and the fraction of the particle surface area that is in contact with the external liquid phase. Interestingly, the initial state of the system governs whether the particle is adsorbed at the interface or not. The shape of the particle is also an important factor which governs the adsorbed position. The disk-shaped particle and the spherical particle which is partially covered with the extremely rough surface, i.e. Janus particle, are adsorbed at the liquid-liquid interface in an oriented state. We should consider not only the interfacial tensions, but also the surface structure and the particle shape to control the adsorption behavior of the particle.     

Nano-scale superhydrophobicity: suppression of protein adsorption and promotion of flow-induced detachment

Wall adsorption is a common problem in microfluidic devices, particularly when proteins are used. Here we show how superhydrophobic surfaces can be used to reduce protein adsorption and to promote desorption. Hydrophobic surfaces, both smooth and having high surface roughness of varying length scales (to generate superhydrophobicity), were incubated in protein solution. The samples were then exposed to flow shear in a device designed to simulate a microfluidic environment. Results show that a similar amount of protein adsorbed onto smooth and nanometer-scale rough surfaces, although a greater amount was found to adsorb onto superhydrophobic surfaces with micrometer scale roughness. Exposure to flow shear removed a considerably larger proportion of adsorbed protein from the superhydrophobic surfaces than from the smooth ones, with almost all of the protein being removed from some nanoscale surfaces. This type of surface may therefore be useful in environments, such as microfluidics, where protein sticking is a problem and fluid flow is present. Possible mechanisms that explain the behaviour are discussed, including decreased contact between protein and surface and greater shear stress due to interfacial slip between the superhydrophobic surface and the liquid.     

Water adsorption and desorption pretreatment of surfaces increases the maximum amount of adsorbed water molecules by a multiple

The amount of adsorbed water on surfaces in an atmosphere with 100 % relative humidity can be increased by a multiple, if the surfaces are pretreated by cycles of adsorption and desorption of water. This was observed on surfaces of diamond, titanium dioxide and silicon dioxide at temperatures around 22 °C. With a sufficient number of such cycles a faster and stronger adsorption of water molecules was obtained, if compared with untreated surfaces. This also means an increased energy transfer from the atmosphere to the surface. Due to the pretreatment the amount of adsorbed water was more than three times increased. The observed effect is explained by small amounts of specially arranged water molecules, which remain on the surface after the desorption process and which support the adsorption of water. The observed effect can be used to moisten surfaces of small particles very efficiently from the gas phase.     

Adsolubilization of toluene and acetophenone as a function of surfactant adsorption

Adsolubilization, solubilization of organic compounds into adsorbed surfactant aggregates, has attracted much attention in the past few years. It is being explored for a variety of new commercial applications including the formation of engineered surfaces, pharmaceutical applications, and nanotechnology. Adsolubilization is strongly influenced by the amount of adsorbed surfactant, which in turn depends upon pH, ionic strength, and surfactant type and concentration. In this study, the adsorption of a cationic surfactant, cetyltrimethylammonium bromide (CTAB) on precipitated silica and the adsolubilization of organic solutes (toluene and acetophenone) into the adsorbed surfactant aggregates were examined as a function of surfactant adsorption at two pH values (5 and 8). Three levels of surfactant adsorption, corresponding to equilibrium concentration low in region II, high in region II, and low in region III of the adsorption isotherm, and the adsolubilization of toluene and acetophenone were investigated both in single- and mixed-solute systems. The results showed that the adsorption of CTAB depended strongly on pH and for each pH the adsolubilization increased with increasing surface adsorption. However, the adsolubilization behaviors of the adsorbed CTAB aggregates in different adsorption regions were quite distinct, suggesting that the structural arrangement of the surfactant aggregates may play an important part in addition to the amount of adsorbed surfactant. This phenomenon was more noticeable at low surfactant adsorption than at higher surface adsorption. In mixed-solute systems, the presence of acetophenone had little effect on the toluene adsolubilization. In contrast, a synergetic effect was observed in the adsolubilization of acetophenone in the presence of toluene.     

Mobility of Thermomyces lanuginosus lipase on a trimyristin substrate surface

We have studied the mobility of active and inactive Thermomyces lanuginosus lipase (TLL) on a spin-coated trimyristin substrate surface using fluorescence recovery after photobleaching (FRAP) in a confocal microscopy setup. By photobleaching a circular spot of fluorescently labeled TLL adsorbed on a smooth trimyristin surface, both the diffusion coefficient D and the mobile fraction f could be quantified. FRAP was performed on surfaces with different surface density of lipase and as a function of time after adsorption. The data showed that the mobility of TLL was significantly higher on the trimyristin substrate surfaces compared to our previous studies on hydrophobic model surfaces. For both lipase variants, the diffusion decreased to similar rates at high relative surface density of lipase, suggesting that crowding effects are dominant with higher adsorbed amount of lipase. However, the diffusion coefficient at extrapolated infinite surface dilution, D0, was higher for the active TLL compared to the inactive (D0 = 17.9 x 10(-11) cm2/s vs D0 = 4.1 x 10(-11) cm2/s, data for the first time interval after adsorption). Moreover, the diffusion decreased with time after adsorption, most evident for the active TLL. We explain the results by product inhibition, i.e., that the accumulation of negatively charged fatty acid products decreased the diffusion rate of active lipases with time. This was supported by sequential adsorption experiments, where the adsorbed amount under flow conditions was studied as a function of time after adsorption. A second injection of lipase led to a significantly lower increase in adsorbed amount when the trimyristin surface was pretreated with active TLL compared to pretreatment of inactive TLL.