Photon Density Waves Research Articles

Real-Time Monitoring of Particle Size in Emulsion Polymerization: Simultaneous Turbidity and Photon Density Wave Spectroscopy.

Particle size evolution in seeded semibatch emulsion polymerization is monitored by two real-time monitoring techniques: online turbidity spectroscopy (TUS) and inline photon density wave spectroscopy (PDWS). An automatic dilution system that withdraws a sample from the reactor and upon dilution transfers to the measurement cell is used for the online TUS analysis. A PDWS probe is immersed in the reactor and collects inline the scattered light directly from the reacting latex. The particle sizes retrieved from TUS and PDWS are compared to offline dynamic light scattering (DLS) values. The particle size obtained by TUS is close to the intensity-average particle size obtained offline by DLS, while the particle size obtained by PDWS lies closer to the number-average particle size from DLS.

Compression and self-compression of frequency modulated spherical photon density waves in anisotropic scattering media

We report on the results of a simulation of the photon density waves with pulse amplitude modulation by a complex frequency modulated signal. The problem is considered for the optical properties typical for sea water with anisotropy factor values varying from 0.75 to 0.93 at source-detector distances up to 120 m. It is shown that multiple scattering in a medium does not prevent effective compression of a signal registered using matched detection. Two competing phenomena affecting the detected pulse duration and depending on the central frequency of the modulation signal are discussed. The effect of faster attenuation of high harmonics in a complexly modulated signal leads to the detected signal duration increase as a consequence of multiple scattering. On the other hand, anomalous dispersion of photon density waves in media with scattering anisotropy leads to the pulse self-compression. The simulation results presented in the paper demonstrate the prevalence of different phenomena depending on the central frequency of the modulation signal resulting in a pulse duration decrease or increase in different frequency ranges covering the band from 107 to 2⋅109 Hz. The effect of the phase function shape on the observed effect is also discussed.

Effect of alkali metal salt addition on disintegration of titania particles precipitated from tetraethyl orthotitanate in ethanol

AbstractInline (or in situ) photon density wave spectroscopy was used to monitor the disintegration of secondary titania particles into their primary particles. Photon density wave spectroscopy can be applied to determine the reduced scattering coefficient of a dispersion without dilution or calibration, and thus enables process analysis in materials that are usually unsuitable for established particle characterization techniques. In this work, amorphous titania particles were precipitated from tetraethyl orthotitanate in ethanol by addition of water in presence of different alkali metal salts (NaCl, KCl, CsCl, K2SO4) with concentrations between 0 and 1.6 mM. The present results suggest that the synthesized titania secondary particles disintegrate into their primary particles if the electrostatic repulsion between the primary particles is promoted. This can be achieved by an increased alkali chloride concentration in the synthesis or by addition of larger alkali metal ions. In contrast, the particles are only weakly charged upon addition of sulfate ions, and the disintegration stops. The conclusions drawn from photon density wave spectroscopy results are supported by gravimetric determination of the particle yield, dynamic light scattering measurements, zeta‐potential measurements, and electron micrographs. Additionally, the disintegration was driven to completion by addition of hydrochloric acid to create a transparent suspension of titania primary particles as small as 4.7 nm.

Photon Density Wave spectroscopy to monitor the particle size in seeded semibatch emulsion copolymerization reactions

The performance of Photon Density Wave spectroscopy (PDW) is assessed to inline monitor the particle size during the seeded semibatch emulsion copolymerizations of methyl methacrylate, butyl acrylate, methacrylic acid (MMA/BA/MAA = 51/47/2 wt% composition) with particle size and solids content ranges relevant in industrial formulations. Moreover, the suitability of the technique to early detect some of the most probable deviations, such as particle aggregation and secondary nucleation, is studied. It is shown that the PDW can be successfully used to inline monitor the reduced scattering coefficient, and consequently the particle size evolution in emulsion polymerization reactions for a broad range of particle sizes and solids contents. Nevertheless, for particle sizes higher than 350 nm and solids content above 40 %, the low signal to noise ratio together with the characteristic multiple solutions regime of the theoretical reduced scattering versus the particle size curve, produces uncertainties in the determination of the particle size by PDW.

Evolution of temporal and frequency characteristics of spherical photon density waves in scattering media

The paper reports on the numerical study of temporal characteristics, frequency and phase responses of the photon density waves generated by a point-sized source in scattering media with different scattering anisotropy and absorption index. The study is performed with time-resolved Monte-Carlo simulations for a wide range of medium optical properties covering values typical for aerosols and sea water. We demonstrate that although the normalized pulse delay increases monotonically with distance in the course of propagation in the medium, for particular sets of optical properties its normalized width shows a non-monotonous trend with a maximum corresponding to distances where ballistic and multiply scattered photons give comparable contribution to the irradiance field. In the frequency responses of photon density waves for certain combinations of optical properties at particular distances a deep minimum is formed, also associated with the interference of partial waves. The dimensionless frequency corresponding to this minimum monotonically increases with the increase in anisotropy factor. Dependencies of local and averaged phase and group velocities on distance derived from phase responses revealed a formation of a minima in local group velocities dependencies with further increase to the average velocity values at higher distances, which is also associated with relative contribution of low- and multiply scattered photons. The revealed dependencies can be useful for verifying the analytical models of nonstationary radiation transfer and directly for assessing the achievable parameters of optical systems for various purposes operating in natural scattering media, such as the atmosphere and sea water.

Advanced Particle Size Analysis in High-Solid-Content Polymer Dispersions Using Photon Density Wave Spectroscopy.

High-solid-content polystyrene and polyvinyl acetate dispersions of polymer particles with a 50 nm to 500 nm mean particle diameter and 12-55% (w/w) solid content have been produced via emulsion polymerization and characterized regarding their optical and physical properties. Both systems have been analyzed with common particle-size-measuring techniques like dynamic light scattering (DLS) and static light scattering (SLS) and compared to inline particle size distribution (PSD) measurements via photon density wave (PDW) spectroscopy in undiluted samples. It is shown that particle size measurements of undiluted polystyrene dispersions are in good agreement between analysis methods. However, for polyvinyl acetate particles, size determination is challenging due to bound water in the produced polymer. For the first time, water-swelling factors were determined via an iterative approach of PDW spectroscopy error (Χ2) minimization. It is shown that water-swollen particles can be analyzed in high-solid-content solutions and their physical properties can be assumed to determine the refractive index, density, and volume fraction in dispersion. It was found that assumed water swelling improved the reduced scattering coefficient fit by PDW spectroscopy by up to ten times and particle size determination was refined and enabled. Particle size analysis of the water-swollen particles agreed well with offline-based state-of-the-art techniques.

Real-time monitoring of biomass during Escherichia coli high-cell-density cultivations by in-line photon density wave spectroscopy.

An efficient monitoring and control strategy is the basis for a reliable production process. Conventional optical density (OD) measurements involve superpositions of light absorption and scattering, and the results are only given in arbitrary units. In contrast, photon density wave (PDW) spectroscopy is a dilution-free method that allows independent quantification of both effects with defined units. For the first time, PDW spectroscopy was evaluated as a novel optical process analytical technology tool for real-time monitoring of biomass formation in Escherichia coli high-cell-density fed-batch cultivations. Inline PDW measurements were compared to a commercially available inline turbidity probe and with offline measurements of OD and cell dry weight (CDW). An accurate correlation of the reduced PDW scattering coefficient µs ' with CDW was observed in the range of 5-69 g L-1 (R2 = 0.98). The growth rates calculated based on µs ' were comparable to the rates determined with all reference methods. Furthermore, quantification of the reduced PDW scattering coefficient µs ' as a function of the absorption coefficient µa allowed direct detection of unintended process trends caused by overfeeding and subsequent acetate accumulation. Inline PDW spectroscopy can contribute to more robust bioprocess monitoring and consequently improved process performance.

Novel data types for frequency-domain diffuse optical spectroscopy and imaging of tissues: characterization of sensitivity and contrast-to-noise ratio for absorption perturbations.

In frequency-domain (FD) diffuse optics it is known that the phase of photon-density waves (ϕ) has a stronger deep-to-superficial sensitivity ratio to absorption perturbations than the alternate current (AC) amplitude, or the direct current intensity (DC). This work is an attempt to find FD data types that feature similar or even better sensitivity and/or contrast-to-noise for deeper absorption perturbations than phase. One way is to start from the definition of characteristic function (Xt(ω)) of the photon's arrival time (t) and combining the real () and imaginary parts () with phase to yield new data types. These new data types enhance the role of higher order moments of the probability distribution of the photon's arrival time t. We study the contrast-to-noise and sensitivity features of these new data types not only in the single-distance arrangement (traditionally used in diffuse optics), but we also consider the spatial gradients, which we named dual-slope arrangements. We have identified six data types that for typical values of the optical properties of tissues and depths of interest, have better sensitivity or contrast-to-noise features than phase data and that can be used to enhance the limits of imaging of tissue in FD near infrared spectroscopy (NIRS). For example, one promising data type is which shows, in the single-distance source-detector arrangement, an increase of deep-to-superficial sensitivity ratio with respect to phase by 41% and 27% at a source-detector separation of 25 and 35 mm, respectively. The same data type also shows an increase of contrast-to noise up to 35% with respect to phase when the spatial gradients of the data are considered.

Characteristics of the Image System based on Photon Density Waves in Observing Underwater Objects through a Waved Surface

The wavy surface effect on the resolution of optical imaging systems employing narrow modulated probing beams is studied. Relations are formulated connecting the complex amplitudes of the photon density waves (propagating through the wavy interface and the water layer) to the main parameters of the problem. These relations consider the finite waves height and the change in the photon trajectory length due to random refraction of rays upon entering the water. The optical transfer function of the wavy surface and the beam scattering function averaged over the ensemble of surface waving realizations are introduced. The dependences of these functions on the surface waves integral parameters, namely, the elevations and slopes dispersions, are examined. The contributions of surface waving and water layer to the generation of optical transfer functions and to the overall signal level from an underwater object when it is imaged using photon density waves are estimated. It is shown that in a certain range of depths, spatial frequencies, and illumination beam modulation frequencies, the systems employing photon density waves can demonstrate advantages over the systems with stationary illumination.

Exploitation of inline photon density wave spectroscopy for titania particle syntheses

AbstractIn this work, inline (or in‐situ) monitoring of titania particle formation with photon density wave spectroscopy is shown to provide helpful information for process optimization for higher particle yields. Titania particles have been synthesized by the controlled hydrolysis of tetraethyl orthotitanate. Without calibration or dilution, photon density wave spectroscopy provides access to dispersions' reduced scattering coefficient μs'. Hence, information can be obtained inline about processes that are otherwise challenging for established particle characterization methods. Photon density wave spectroscopy results suggest that the titania particles disintegrate over time after their formation. Consistent with that hypothesis, a decreasing particle yield with reaction time has been observed. The disintegration can be hindered by adding a sufficient amount of hexadecylamine, such that the particle yield after 24 h can be increased by 44% for the given syntheses.

Continuous feeding strategy for polyhydroxyalkanoate production from solid waste animal fat at laboratory- and pilot-scale.

Bioconversion of waste animal fat (WAF) to polyhydroxyalkanoates (PHAs) is an approach to lower the production costs of these plastic alternatives. However, the solid nature of WAF requires a tailor-made process development. In this study, a double-jacket feeding system was built to thermally liquefy the WAF to employ a continuous feeding strategy. During laboratory-scale cultivations with Ralstonia eutropha Re2058/pCB113, 70% more PHA (45 gPHA L-1 ) and a 75% higher space-time yield (0.63 gPHA L-1 h-1 ) were achieved compared to previously reported fermentations with solid WAF. During the development process, growth and PHA formation were monitored in real-time by in-line photon density wave spectroscopy. The process robustness was further evaluated during scale-down fermentations employing an oscillating aeration, which did not alter the PHA yield although cells encountered periods of oxygen limitation. Flow cytometry with propidium iodide staining showed that more than two-thirds of the cells were viable at the end of the cultivation and viability was even little higher in the scale-down cultivations. Application of this feeding system at 150-L pilot-scale cultivation yielded in 31.5 gPHA L-1 , which is a promising result for the further scale-up to industrial scale.

Autoquenching of spherical photon density waves during propagation in a turbid medium.

The effect of the formation of deep minima in frequency characteristics of photon density waves (PDWs) during their propagation in scattering media with different optical characteristics has been studied by statistical Monte Carlo modeling. The simulation was performed for the Henyey-Greenstein scattering phase function with the anisotropy factor value varying in the range of 0-0.93. The dependence of the position and magnitude of the minimum in the frequency response of PDWs on the combination of the parameters of the scattering medium and the distance to the radiation source is demonstrated.

Scale-up of Emulsion Polymerisation up to 100 L and with a Polymer Content of up to 67 wt%, Monitored by Photon Density Wave Spectroscopy

The scale-up process of the high solid content (up to 67 wt%) emulsion polymerisation of vinyl acetate and Versa®10 from 1 L over 10 L to 100 L was investigated. An emulsion copolymerisation of vinyl acetate and neodecanoic acid vinyl ester in a molar ratio of 9:1 was carried out in a starved-fed semi-batch operation. As a radical source, a redox initiator system consisting of L-ascorbic acid, tert-butyl hydroperoxide and ammonium iron (III) sulphate was used. The process parameters, such as the required stirring speed and heat dissipation, were determined and adjusted beforehand via reaction calorimetry to ensure a successful scale-up without safety issues. In addition, the emulsion polymerisation was monitored inline by Raman (qualitative monomer accumulation), as well as Photon Density Wave spectroscopy (particle size and scattering coefficient) and temperature measurements. The data provided by Raman spectroscopy and temperature measurements revealed mixing difficulties due to an insufficient stirring rate, while the inline measurement with Photon Density Wave spectroscopy offered an insight into the development of the product properties. It proved to be reliable and precise throughout the entire scale-up process, especially compared to conventional offline methods, such as dynamic light scattering or sedimentation analysis by means of a disc centrifuge, both of which encountered issues when using higher polymer contents.

Angular distribution of photon density waves radiance in media with different scattering anisotropy.

Statistical modeling of pulsed frequency responses of the light field radiance by an isotropic point source was performed by Monte Carlo technique. Scattering properties of the medium were simulated by the Henyey-Greenstein phase function with different anisotropy factor values. Angular distributions of the pulsed field and amplitudes of the photon density waves in a certain range of parameters were shown to have a qualitatively different character for media with quasi-isotropic and strongly anisotropic scattering. A comparison of the impulse and frequency characteristics was performed for media with strongly anisotropic scattering with different scattering phase functions yet the same anisotropy factor. The main difference in the angular distributions of the fields is observed in the rear hemisphere.

Process Characterization of Polyvinyl Acetate Emulsions Applying Inline Photon Density Wave Spectroscopy at High Solid Contents.

The high solids semicontinuous emulsion polymerization of polyvinyl acetate using poly (vinyl alcohol-co-vinyl acetate) as protective colloid is investigated by optical spectroscopy. The suitability of Photon Density Wave (PDW) spectroscopy as inline Process Analytical Technology (PAT) for emulsion polymerization processes at high solid contents (>40% (w/w)) is studied and evaluated. Inline data on absorption and scattering in the dispersion is obtained in real-time. The radical polymerization of vinyl acetate to polyvinyl acetate using ascorbic acid and sodium persulfate as redox initiator system and poly (vinyl alcohol-co-vinyl acetate) as protective colloid is investigated. Starved–feed radical emulsion polymerization yielded particle sizes in the nanometer size regime. PDW spectroscopy is used to monitor the progress of polymerization by studying the absorption and scattering properties during the synthesis of dispersions with increasing monomer amount and correspondingly decreasing feed rate of protective colloid. Results are compared to particle sizes determined with offline dynamic light scattering (DLS) and static light scattering (SLS) during the synthesis.

Effect of scattering anisotropy on the properties of photon density waves.

The frequency characteristics of spherical photon density waves excited in media with different degrees of scattering anisotropy are studied. Statistical modeling of the frequency and phase responses of the spatial irradiance of the light field emitted by a point-sized isotropic source were performed employing the Monte Carlo technique. The scattering anisotropy of the medium was determined by the Henyey-Greenstein phase function with different values of the mean scattering cosine. It is shown that the scattering anisotropy factor determines the frequency range, in which the effect of the photon path length distribution on the magnitude of the photon density wave dispersion is maximal. In media with quasi-isotropic scattering, dispersion effects are manifested at lower frequencies as compared to those for anisotropic media. The simulation results are compared with the analytical solution for the asymptotic regime of the light field in an isotropically scattering medium.

Depletion-Induced Flocculation of Concentrated Emulsions Probed by Photon Density Wave Spectroscopy.

Stable, creaming-free oil in water emulsions with high volume fractions of oil (ϕ = 0.05-0.40, density matched to water) and polysorbate 80 as an emulsifier were characterized without dilution by Photon Density Wave spectroscopy measuring light absorption and scattering behavior, the latter serving as the basis for droplet size distribution analysis. The emulsion with ϕ = 0.10 was used to investigate flocculation processes induced by xanthan as a semi-flexible linear nonabsorbing polymer. Different time regimes in the development of the reduced scattering coefficient μs' could be identified. First, a rapid, temperature-dependent change in μs' during the depletion process was observed. Second, the further decrease of μs' follows a power law in analogy to a spinodal demixing behavior, as described by the Cahn-Hilliard theory.

In-line monitoring of latex-particle size during emulsion polymerizations with a high polymer content of more than 60.

The photon density wave (PDW) spectroscopy is established in the fields of biochemistry and food chemistry as an online analytical method for the determination of mean particle sizes. This work examines PDW spectroscopy regarding its potential in high solid content emulsion polymerization. For this reason, emulsion copolymerization with a tendency for agglomeration of vinyl acetate and Versa® 10 in a molar ratio of 9 : 1, and with varying emulsifier content, was carried out in semi-batch operation mode with different target particle sizes from 50 to 325 nm. A redox initiator system, consisting of l-ascorbic acid, tert-butyl hydroperoxide and ammonium iron(iii) sulfate, was used as a radical source. The mean particle sizes of PDW spectroscopy were compared with those of conventional offline measurement methods, such as dynamic light scattering (DLS) and sedimentation analysis, by means of a disc centrifuge. The determined mean particle sizes show a very good reproducibility and agreement between DLS and sedimentation analysis up to a polymer content of 36%, after which measurements were rendered difficult due to agglomeration. Nevertheless, PDW spectroscopy was able to continue providing reproducible measurements until reaching a polymer content of 63%.

In-Line Monitoring of Polyhydroxyalkanoate (PHA) Production during High-Cell-Density Plant Oil Cultivations Using Photon Density Wave Spectroscopy

Polyhydroxyalkanoates (PHAs) are biodegradable plastic-like materials with versatile properties. Plant oils are excellent carbon sources for a cost-effective PHA production, due to their high carbon content, large availability, and comparatively low prices. Additionally, efficient process development and control is required for competitive PHA production, which can be facilitated by on-line or in-line monitoring devices. To this end, we have evaluated photon density wave (PDW) spectroscopy as a new process analytical technology for Ralstonia eutropha (Cupriavidus necator) H16 plant oil cultivations producing polyhydroxybutyrate (PHB) as an intracellular polymer. PDW spectroscopy was used for in-line recording of the reduced scattering coefficient µs’ and the absorption coefficient µa at 638 nm. A correlation of µs’ with the cell dry weight (CDW) and µa with the residual cell dry weight (RCDW) was observed during growth, PHB accumulation, and PHB degradation phases in batch and pulse feed cultivations. The correlation was used to predict CDW, RCDW, and PHB formation in a high-cell-density fed-batch cultivation with a productivity of 1.65 gPHB·L−1·h−1 and a final biomass of 106 g·L−1 containing 73 wt% PHB. The new method applied in this study allows in-line monitoring of CDW, RCDW, and PHA formation.

Real-time inline monitoring of zeolite synthesis by Photon Density Wave spectroscopy

The formation process of zeolite A (Linde Type A) was monitored inline at 1.5 L scale by Photon Density Wave (PDW) spectroscopy as novel process analytical technology for highly turbid liquid suspensions. As a result, the reduced scattering coefficient, being a measure for particle number, size, and morphology, provides distinct process information, including the formation of amorphous particles and their transfer into crystalline zeolite structures. The onset and end of the crystallization process can be detected inline and in real-time. Analyses by powder X-ray diffraction and electron microscopy, based on a sampling approach, support the interpretation of the results obtained by PDW spectroscopy. In addition, the influence of the molar water content was investigated, indicating a linear increase of the time needed to reach the end of the zeolite A crystallization with increasing molar water content. Further experiments indicate a strong influence of the silica source on the course of the crystallization. The applicability of PDW spectroscopy under even more demanding chemical and physical conditions was investigated by monitoring the synthesis of zeolite L (Linde Type L).