Aqueous Boundary Layer Research Articles

A mixing length model for the aqueous boundary layer including the effect of wave breaking on enhancing gas transfer

A mixing length model for air-water gas transfer is developed to include the effects of wave breaking. The model requires both the shear velocity induced by the wind and the integrated wave dissipation. Both of these can be calculated for tanks and oceans by a full spectrum wave model. The gas transfer model is calibrated, with laboratory tank measurements of carbon dioxide flux, and transported to oceanic conditions to yield air-sea transfer velocity versus wind speed.

Polar interactions drug/phospholipids estimated by IAM-HPLC vs cultured cell line passage data: Their relationships and comparison of their effectiveness in predicting drug human intestinal absorption

The relationships between data of passage through Caco-2 cultured cell lines (logPapp), taken from the literature, for 38 structurally unrelated compounds and both n-octanol lipophilicity parameters (logPN and logD7.4) and phospholipid affinity indexes were investigated. Phospholipid affinity(logkWIAM) was experimentally determined by HPLC on two different phospholipid stationary phases and the polar/electrostatic interaction component drug/phospholipids (Δlog kWIAM) was calculated according to a method we previously proposed. LogPapp moderately related to lipophilicity values measured at pH 7.4 (logD7.4), according to a parabolic pattern, but poorly related with log kWIAM. Furthermore, a significant inverse linear relationship with Δlog kWIAM values was only observed for the analytes with m.w. >300Da, for which paracellular diffusion can be considered a minor transport route in vivo. Indeed, it has been reported that Caco-2 passage data also encode secondary passage mechanisms, which participate in a different extent to the jejunal absorption in vivo and cannot be directly equated to the corresponding human in situ logPeff values, unless a normalization is performed.In an attempt to elucidate this issue, 47 structurally unrelated compounds whose cultured cell line passage data were corrected for the effects of the aqueous boundary layer and paracellular permeability, so as to express transcellular intrinsic permeability, logP0Caco-2/MDCK, were also considered. Highly significant inverse linear relationships were observed between logP0Caco-2/MDCK and ΔlogkWIAM values from both IAM.PC.MG (r2=0.765) and IAM.PC.DD2 (r2=0.806) stationary phases whereas the relationships with either lipophilicity in n-octanol or logkWIAM values were very poor. The results of the present study, in complete agreement with those of our recent study on the relationships between jejunal absorption data measured in situ and ΔlogkWIAM values, confirm the soundness of ΔlogkWIAM parameters in the prediction of the intestinal absorption of drugs. From a mechanistic point of view, they suggest that the polar/electrostatic forces between drugs and phospholipids play a major role in the passage through biomembranes.

Modeling the transport of organic chemicals between polyethylene passive samplers and water in finite and infinite bath conditions.

Understanding the transfer of chemicals between passive samplers and water is essential for their use as monitoring devices of organic contaminants in surface waters. By applying Fick's second law to diffusion through the polymer and an aqueous boundary layer, the authors derived a mathematical model for the uptake of chemicals into a passive sampler from water, in finite and infinite bath conditions. The finite bath model performed well when applied to laboratory observations of sorption into polyethylene (PE) sheets for various chemicals (polycyclic aromatic hydrocarbons, polychlorinated biphenyls [PCBs], and dichlorodiphenyltrichloroethane [DDT]) and at varying turbulence levels. The authors used the infinite bath model to infer fractional equilibration of PCB and DDT analytes in field-deployed PE, and the results were nearly identical to those obtained using the sampling rate model. However, further comparison of the model and the sampling rate model revealed that the exchange of chemicals was inconsistent with the sampling rate model for partially or fully membrane-controlled transfer, which would be expected in turbulent conditions or when targeting compounds with small polymer diffusivities and small partition coefficients (e.g., phenols, some pesticides, and others). The model can be applied to other polymers besides PE as well as other chemicals and in any transfer regime (membrane, mixed, or water boundary layer-controlled). Lastly, the authors illustrate practical applications of this model such as improving passive sampler design and understanding the kinetics of passive dosing experiments.

Supersaturation induced by Itraconazole/Soluplus® micelles provided high GI absorption in vivo

To investigate the effect of supersaturation induced by micelle formation during dissolution on the bioavailability of itraconazole (ITZ)/Soluplus® solid dispersion. Solid dispersions prepared by hot melt extrusion (HME) were compressed into tablets directly with other excipients. Dissolution behavior of ITZ tablets was studied by dissolution testing and the morphology of micelles in dissolution media was studied using transmission electron microscopy (TEM). Drug transferring from stomach into intestine was simulated to obtain a supersaturated drug solution. Bioavailability studies were performed on the ITZ tablets and Sporanox® in beagle dogs. The morphology of micelles in the dissolution media was observed to be spherical in shape, with an average size smaller than 100 nm. The supersaturated solutions formed by Soluplus® micelles were stable and no precipitation took place over a period of 180 min. Compared with Sporanox®, ITZ tablets exhibited a 2.50-fold increase in the AUC(0–96) of ITZ and a 1.95-fold increase in its active metabolite hydroxyitraconazole (OH-ITZ) in the plasma of beagle dogs. The results obtained provided clear evidence that not only the increase in the dissolution rate in the stomach, but also the supersaturation produced by micelles in the small intestine may be of great assistance in the successful development of poorly water-soluble drugs. The micelles formed by Soluplus® enwrapped the molecular ITZ inside the core which promoted the amount of free drug in the intestinal cavity and carried ITZ through the aqueous boundary layer (ABL), resulting in high absorption by passive transportation across biological membranes. The uptake of intact micelles through pinocytosis together with the inhibition of P-glycoprotein-mediated drug efflux in intestinal epithelia contributed to the absorption of ITZ in the gastrointestinal tract. These results indicate that HME with Soluplus®, which can induce supersaturation by micelle formation, may be of great assistance to the successful development of poorly water-soluble drugs.

Capric Acid Absorption in the Presence of Hydroxypropyl-β-Cyclodextrin in the Rat Ileum using the In Situ Single-Pass Perfusion Technique

The purpose of the present study was to gain quantitative mechanistic insight into the role cyclodextrin carriers may play in the intestinal absorption of highly lipophilic molecules. The physical model approach was employed to investigate capric acid absorption in the rat ileum using the in situ single-pass method with 2-hydroxypropyl-β-cyclodextrin (HPB) present in the perfusate. Two physical models were examined: the flat surface model in which the intestinal wall was treated as a hollow, smooth, circular cylinder, and the villus model in which the intestinal surface allowed for the presence of villi. Capric acid absorption was found to be essentially 100% aqueous boundary layer controlled at low HPB concentrations and increasingly membrane controlled at the higher HPB concentrations. Theoretical calculations based on the experimental data and model parameters were found to be consistent with: at low HPB concentrations, capric acid was mainly absorbed at the villus tips and there was very little capric acid penetration into the intervillus space; in contrast, at 50 mM HPB, there was considerable capric acid penetration into the intervillus space, this corresponding to around a 4.5-fold increase in the accessible area for absorption when compared with 0 mM HPB.

Phenolic wastewater treatment through extractive recovery coupled with biodegradation in a two-phase partitioning membrane bioreactor

A two-phase partitioning membrane bioreactor (TPPMB) was designed and operated for treatment of high strength phenolic wastewater through extraction/stripping and concomitant biodegradation. Tributyl phosphate dissolved in kerosene was used as the organic phase, sodium hydroxide as the stripping phase and Pseudomonas putida for biodegradation. In a semi-dispersive approach, organic phase dispersed in the stripping solution was contacted with wastewater through semi-permeable membranes for removal of phenol from wastewater, while the microorganisms were inoculated directly into the wastewater for biodegradation. The TPPMB exhibited high phenol removal rates, and phenol concentrations of 1000–3000mg/L were reduced to undetected amounts within 2–4h. Up to 80% phenol was recovered through extraction, while the remaining was metabolized by the microorganisms. Phenol recovery in the TPPMB was enhanced by increasing the mass transfer rate of phenol through the membranes, and it was also estimated that phenol diffusion through the aqueous boundary layer on the tube side was the rate limiting step. The flexibility in adjusting inoculation time in the TPPMB prevented microorganisms from adverse effects of substrate inhibition, which facilitated complete removal of phenol from the wastewater. TPPMB retained the advantages of both solvent extraction and biodegradation, and it can be highly promising for the treatment of toxic industrial wastewater.

Reactive mass transfer in a membrane-based microcontactor

The extraction kinetics of Zn(II) from sulfate solutions with di-(2-ethylhexyl) phosphoric acid (D2EHPA) in isododecane was studied in a flat-sheet, membrane-based microcontactor. The potential of process intensification due to microstructured length scales, a dispersion-free contacting, laminar flow and the constant interfacial area allow a detailed view in reactive extraction kinetics, where the rate-determining step can be diffusion- or reaction-limited (or both in a mixed regime). The individual mass transfer coefficients can be manipulated by the experimental conditions, consequently, the chemical reaction resistance is negligible at high pH and high extractant concentrations and the extraction process is mainly governed by aqueous boundary layer resistance (>95%). Therefore, the developed diffusion model accounts for the overall mass transfer coefficient. In the next step, the reaction resistance is quantified at lower pH. The relative reaction resistance reaches up to 78% at pH 1.33.

Phase Coexistence in Lipid Membranes Induced by Buffering Agents and Charged Lipid Headgroups

Recent literature has shown that buffers affect the interaction between lipid bilayers through a mechanism that involves van der Waals forces, electrostatics, hydration forces and membrane bending rigidity. We endeavour to show phase coexistence as an effect of charges from the aqueous boundary layer on the mixed chain 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers. We will present data that suggests one phase dehydrates below the value in pure water while the other phase swells as the concentration of buffer is increased. However, since the two phases must be in osmotic equilibrium with one another, this behavior challenges theoretical models of lipid interactions and introduces new variables to consider for the Gibbs phase rule. This model of lipid charging was then applied to explain the mechanisms behind phase separation in lipid mixtures containing phosphatidylinositols.This work was supported by NIH NCI K01-CA169078-01 and Indiana University Collaborative Research Grants.

In vitro porcine blood–brain barrier model for permeability studies: pCEL-X software pKaFLUX method for aqueous boundary layer correction and detailed data analysis

In vitro blood–brain barrier (BBB) models from primary brain endothelial cells can closely resemble the in vivo BBB, offering valuable models to assay BBB functions and to screen potential central nervous system drugs. We have recently developed an in vitro BBB model using primary porcine brain endothelial cells. The model shows expression of tight junction proteins and high transendothelial electrical resistance, evidence for a restrictive paracellular pathway. Validation studies using small drug-like compounds demonstrated functional uptake and efflux transporters, showing the suitability of the model to assay drug permeability. However, one limitation of in vitro model permeability measurement is the presence of the aqueous boundary layer (ABL) resulting from inefficient stirring during the permeability assay. The ABL can be a rate-limiting step in permeation, particularly for lipophilic compounds, causing underestimation of the permeability. If the ABL effect is ignored, the permeability measured in vitro will not reflect the permeability in vivo. To address the issue, we explored the combination of in vitro permeability measurement using our porcine model with the pKaFLUX method in pCEL-X software to correct for the ABL effect and allow a detailed analysis of in vitro (transendothelial) permeability data, Papp. Published Papp using porcine models generated by our group and other groups are also analyzed. From the Papp, intrinsic transcellular permeability (P0) is derived by simultaneous refinement using a weighted nonlinear regression, taking into account permeability through the ABL, paracellular permeability and filter restrictions on permeation. The in vitro P0 derived for 22 compounds (35 measurements) showed good correlation with P0 derived from in situ brain perfusion data (r2=0.61). The analysis also gave evidence for carrier-mediated uptake of naloxone, propranolol and vinblastine. The combination of the in vitro porcine model and the software analysis provides a useful tool to better predict BBB permeability in vivo and gain better mechanistic information about BBB permeation.

Desorption kinetics of hydrophobic organic contaminants from marine plastic pellets

This study investigated the desorption behavior of polychlorinated biphenyls (PCBs) from marine plastic pellets. Long-term desorption experiments were conducted using field-collected polyethylene (PE) pellets. The results indicate that the desorption kinetics highly depends on the PE-water partition coefficients of PCB congeners. After 128d of the experiment, the smallest congener considered (CB 8) had desorbed nearly completely (98%), whereas major fractions (90–99%) of highly chlorinated congeners remained in the pellets. An intraparticle diffusion model mostly failed to reproduce the desorption kinetics, whereas an aqueous boundary layer (ABL) diffusion model well approximated the data. The desorption half-lives are estimated to 14d to 210years for CB 8 to CB 209 in an actively stirred solution (ABL thickness: 30μm). Addition of methanol to water enhanced the desorption to a large extent. A need for further work to explore roles of organic matter in facilitating solute transfer is suggested.

In silico predictions of gastrointestinal drug absorption in pharmaceutical product development: Application of the mechanistic absorption model GI-Sim

Oral drug delivery is the predominant administration route for a major part of the pharmaceutical products used worldwide. Further understanding and improvement of gastrointestinal drug absorption predictions is currently a highly prioritized area of research within the pharmaceutical industry. The fraction absorbed (fabs) of an oral dose after administration of a solid dosage form is a key parameter in the estimation of the in vivo performance of an orally administrated drug formulation. This study discloses an evaluation of the predictive performance of the mechanistic physiologically based absorption model GI-Sim. GI-Sim deploys a compartmental gastrointestinal absorption and transit model as well as algorithms describing permeability, dissolution rate, salt effects, partitioning into micelles, particle and micelle drifting in the aqueous boundary layer, particle growth and amorphous or crystalline precipitation. Twelve APIs with reported or expected absorption limitations in humans, due to permeability, dissolution and/or solubility, were investigated. Predictions of the intestinal absorption for different doses and formulations were performed based on physicochemical and biopharmaceutical properties, such as solubility in buffer and simulated intestinal fluid, molecular weight, pKa, diffusivity and molecule density, measured or estimated human effective permeability and particle size distribution. The performance of GI-Sim was evaluated by comparing predicted plasma concentration–time profiles along with oral pharmacokinetic parameters originating from clinical studies in healthy individuals. The capability of GI-Sim to correctly predict impact of dose and particle size as well as the in vivo performance of nanoformulations was also investigated. The overall predictive performance of GI-Sim was good as >95% of the predicted pharmacokinetic parameters (Cmax and AUC) were within a 2-fold deviation from the clinical observations and the predicted plasma AUC was within one standard deviation of the observed mean plasma AUC in 74% of the simulations. GI-Sim was also able to correctly capture the trends in dose- and particle size dependent absorption for the study drugs with solubility and dissolution limited absorption, respectively. In addition, GI-Sim was also shown to be able to predict the increase in absorption and plasma exposure achieved with nanoformulations. Based on the results, the performance of GI-Sim was shown to be suitable for early risk assessment as well as to guide decision making in pharmaceutical formulation development.

Metal Ions: Driving the Orderly Assembly of Polyelectrolytes at a Hydrophobic Surface

The accumulation of polyelectrolytes at the interface between water and nonpolar fluids is an important process in both environmental and biological systems. For instance, polyelectrolytes such as humic acids are highly charged molecules that play a role in the remediation of water contaminated by oil, and the adsorption of other polyelectrolytes such as proteins and DNA to cellular surfaces is essential in biological processes. The properties of these naturally occurring polyelectrolytes are highly tunable and depend strongly on the binding of metal ions commonly found in environmental and biological systems. While the metal complexation behaviors of many polyelectrolytes and biomolecules are well characterized in bulk solution, this work shows in molecular detail that the behavior of a common polyelectrolyte in the presence of metal ions can be quite different when it adsorbs to a hydrophobic-aqueous liquid interface. In these studies, vibrational sum frequency spectroscopy and interfacial tension measurements conducted on poly(acrylic acid) (PAA) at a model oil-water interface show how small amounts of monovalent and divalent cations significantly alter the interfacial conformation of PAA at the interface and act to enhance its interfacial adsorption. The results provide important new insights that have direct relevance for understanding the effect of metal ions on the adsorption of charged macromolecules to a hydrophobic-aqueous boundary layer, specifically in biological and environmental systems.

Transport of a Lipophilic Ionizable Permeant (Capric Acid) Across a Lipophilic Membrane (Silicone Polymer Membrane) from Aqueous Buffered Solutions in the Presence of Hydroxypropyl-β-Cyclodextrin

The present study describes a physical model approach applicable to understanding the transport of highly lipophilic, ionizable drugs across a lipophilic membrane between two aqueous compartments in the presence of a cyclodextrin in the aqueous phase. Model predictions were compared with experimental results of capric acid (HA) transport across a silicone polymer membrane in the presence and in the absence of 2-hydroxypropyl-β-cyclodextrin (HPB) in the aqueous phase over wide ranges of conditions. Key parameters entering into the physical model calculations were the HA-HPB and the A(-)-HPB binding constants, the unionized and ionized free and the complexed HA species diffusion coefficients, the HA pKa, the HA intrinsic silicone polymer membrane permeability coefficient, and the aqueous boundary layer thickness. All of these key parameters were determined from independent or essentially independent experiments. The agreement between the model predictions and the experiments were generally quite good over the entire ranges of the studied independent variables. The results of this study provide an approach that is useful in the mechanistic understanding of how cyclodextrins may enhance the passive absorption of highly lipophilic, low solubility drug molecules in the intestinal tract.

Sorption-Induced Effects of Humic Substances on Mass Transfer of Organic Pollutants through Aqueous Diffusion Boundary Layers: the Example of Water/Air Exchange

This study examines the effect of dissolved humic substances (DHS) on the rate of water-gas exchange of organic compounds under conditions where diffusion through the aqueous boundary layer is rate-determining. A synthetic surfactant was applied for comparison. Mass-transfer coefficients were determined from the rate of depletion of the model compounds by means of an apparatus containing a stirred aqueous solution with continuous purging of the headspace above the solution. In addition, experiments with continuous passive dosing of analytes into the water phase were conducted to simulate a system where thermodynamic activity of the chemical in the aqueous phase is identical in the presence and absence of DHS. The experimental results show that DHS and surfactants can affect water-gas exchange rates by the superposition of two mechanisms: (1) hydrodynamic effects due to surface film formation ("surface smoothing"), and (2) sorption-induced effects. Whether sorption accelerates or retards mass transfer depends on its effect on the thermodynamic activity of the pollutant in the aqueous phase. Mass transfer will be retarded if the activity (or freely dissolved concentration) of the pollutant is decreased due to sorption. If it remains unchanged (e.g., due to fast equilibration with a sediment acting as a large source phase), then DHS and surfactant micelles can act as an additional shuttle for the pollutants, enhancing the flux through the boundary layer.

Separation of zinc from lean sphalerite leached liquor in nitrate medium with D2EHPA using supported liquid membrane

Separation of zinc and lead from an aqueous nitrate medium using supported liquid membrane with di(2-ethylhexyl)phosphoric acid (D2EHPA) as mobile carrier was studied. Celgard-2500, a microporous polypropylene film, was used as the solid support for the liquid membrane with a plate and frame type of cell used for the experiments. The effects of different parameters such as pH of feed solution (1–3·5), D2EHPA concentration in the membrane phase (10–100 mol m−3), acid concentration in strip solution and flowrate of the aqueous solution on separation of lead and zinc were studied. It was observed that zinc flux across the membrane increased with pH of the feed solution up to 3·0 and thereafter decreased while zinc flux increased with the D2EHPA concentration in the membrane phase up to 75 mol m−3 and thereafter decreased. The flowrate of 100 mL min−1 was sufficient to reduce the resistances due to the aqueous boundary layer while 950 mol m−3 H2SO4 was found sufficient to strip the metals from the membrane phase in the strip phase. Under the study conditions, the copermeation of lead with zinc was observed to be negligible.

Extraction of phenol from aqueous effluent using triglycerides in supported liquid membrane

This research work focuses on the removal of phenol using a flat sheet supported liquid membrane impregnated with triglycerides (vegetable oils). The mass transfer of phenol was found to be dependent on various factors such as type of vegetable oil, support material, feed phase pH and concentration, stripping phase concentration and stirring speed. The phenol transport was found to be higher when PTFE membrane (pore size 0.45 μm) impregnated with coconut oil was used and mass transfer coefficient was found to be 3.50 × 10-6 m/s. The corresponding stirring speed, pH of feed solution, concentration of stripping (NaOH) solution were 350 rpm, 4.0 and 0.2 M, respectively. The 92.5% of phenol was extracted from feed phase to stripping phase in 20 h. The aqueous boundary layer thickness reduced with an increase in stirring speed up to a certain extent (350 rpm), beyond which, it resulted in turbulence leading to displacement of impregnated oil from membrane pores. Phenol can be transported through the membrane, ...

Non-dispersive solvent extraction with strip dispersion (NDSXSD) pertraction of Cd(II) in HCl medium using ionic liquid CYPHOS IL101

The transport of Cd(II) from hydrochloric acid medium in NDSXSD technology is investigated using the ionic liquid CYPHOS IL101 (tetraalkyl)phosphonium salt) dissolved in cumene as carrier phase. The permeation of the metal is investigated as a function of various experimental variables: hydrodynamic conditions, concentration of cadmium(II) and HCl in the feed phase, carrier concentration, support characteristics, etc. In NDSXSD, a pseudo-emulsion is temporarily formed between the organic and stripping solutions. The overall mass transfer coefficient ( K = 4.1 × 10 −3 cm s −1) and the thickness of the aqueous boundary layer ( d aq = 1.8 × 10 −3 cm) were calculated from experimental data. Of the different reagents used, 1 M ammonium hydroxide served most efficiently as the stripping agent. Furthermore, the selectivity of CYPHOS IL101-based membrane towards different metals is investigated, whereas the system is compared with other carriers in the liquid membrane phase.

Kinetic limitations on tracer partitioning in ganglia dominated source zones

Quantification of the relationship between dense nonaqueous phase liquid (DNAPL) source strength, source longevity and spatial distribution is increasingly recognized as important for effective remedial design. Partitioning tracers are one tool that may permit interrogation of DNAPL architecture. Tracer data are commonly analyzed under the assumption of linear, equilibrium partitioning, although the appropriateness of these assumptions has not been fully explored. Here we focus on elucidating the nonlinear and nonequilibrium partitioning behavior of three selected alcohol tracers — 1-pentanol, 1-hexanol and 2-octanol in a series of batch and column experiments. Liquid–liquid equilibria for systems comprising water, TCE and the selected alcohol illustrate the nonlinear distribution of alcohol between the aqueous and organic phases. Complete quantification of these equilibria facilitates delineation of the limits of applicability of the linear partitioning assumption, and assessment of potential inaccuracies associated with measurement of partition coefficients at a single concentration. Column experiments were conducted under conditions of non-equilibrium to evaluate the kinetics of the reversible absorption of the selected tracers in a sandy medium containing a uniform entrapped saturation of TCE-DNAPL. Experimental tracer breakthrough data were used, in conjunction with mathematical models and batch measurements, to evaluate alternative hypotheses for observed deviations from linear equilibrium partitioning behavior. Analyses suggest that, although all tracers accumulate at the TCE-DNAPL/aqueous interface, surface accumulation does not influence transport at concentrations typically employed for tracer tests. Moreover, results reveal that the kinetics of the reversible absorption process are well described using existing mass transfer correlations originally developed to model aqueous boundary layer resistance for pure-component NAPL dissolution.

A measurement of the unstirred aqueous boundary layer in a Franz diffusion cell

The unstirred aqueous boundary layer (ABL) thickness was determined for modified Franz diffusion cells using three test compounds—parathion, 4-hexylresorcinol, and 2,4-dinitrochlorobenzene (DNCB). Steady-state fluxes through one, two, and three dialysis membranes in series were achieved in donor-saturated, infinite-dose permeation experiments. These results were then used to calculate the total diffusive resistance for each case, as well as an extrapolated zero membrane value which was used in conjunction with an estimate of the aqueous diffusivities to calculate the ABL thickness. The resulting value (mean ± SE) was 0.070 ± 0.004 cm.

Real-time UV imaging of drug diffusion and release from Pluronic F127 hydrogels

The objective of this study was to introduce and evaluate UV imaging technology for real-time characterization of drug diffusion in and release from hydrogels. Piroxicam and human serum albumin diffusion in Pluronic F127 hydrogel was monitored by measuring the absorbance of light passing through the diffusion cell at 26 °C, thus providing real-time concentration maps (7 × 3 mm imaging area) within the gel as a function of time. Apparent diffusion coefficients were obtained on the basis of Fick’s second law. Piroxicam and human serum albumin diffusivities in 20% (w/w) F127 gel were 8 and 24 times lower than those determined in the phosphate buffer (pH 7.4). The effect of increasing polymer concentration (20%, 25% and 30% (w/w)) on piroxicam diffusion was further investigated. The decreasing diffusion rate with increasing F127 concentration agreed well with small-angle X-ray scattering (SAXS) measurements. UV imaging was also successfully applied to monitor piroxicam release from 30% (w/w) F127 gel into a stirred aqueous buffer solution, providing simultaneous information on gel dissolution rate, change in thickness of gel–aqueous boundary layer as well as the release of piroxicam into bulk aqueous phase. The current study indicates that UV imaging has great potential for measuring drug diffusion in and release from gel matrices. Compared to the currently used conventional techniques, this technology has several advantages including high information content, non-intrusive measurements without the need for labeling, flexibility with respect to experimental design and simplicity of operation.