Model-based design of gradient elution in liquid-liquid chromatography: Application to the separation of cannabinoids.

Liquid-liquid chromatography (LLC) is a separation technique that utilizes a biphasic solvent system as the mobile and stationary phases. The components are separated solely due to their different distributions between the two liquid phases. Gradient change in the mobile phase composition during the chromatographic process is a powerful method for improving the resolution of separation or shortening the process time. Gradient elution readily applies to LLC with biphasic solvent systems in which the stationary phase composition remains nearly constant when the mobile phase composition changes. This work proposes a model-based approach to optimize gradients in LLC and circumvent tedious trial-and-error experiments. The solutes' distribution constant depends on the mobile phase composition. Thus, the distribution constants were described as a function of the content of one of the solvents (= modifier) in the mobile phase. The dispersive and mass-transfer effects in the tubing and the column are modeled with a stage model. Only a few experiments are required to determine the model parameters. After the validation of the model and its parameters, the model can be used for LLC gradient optimization. The proposed approach was demonstrated for a gradient LLC separation of a mixture of four cannabinoids. Two different gradient shapes, one-step and linear gradient, were considered. For a pre-selected minimal purity requirement, the gradient was optimized for maximum process efficiency, defined as the product of productivity and yield. An experiment conducted with the optimized gradient conditions was in good agreement with the simulation, showing the potential of the proposed method.

Nonlinear liquid-liquid chromatography: Modeling a binary mixture separation.

In liquid-liquid chromatography (LLC), mixture components are separated due to their different distribution between the phases of a biphasic liquid system composed of three or four solvents. LLC separations are typically modeled assuming that only the solutes distribute between the two liquid phases and their distribution can be described with a concentration-independent distribution constant. With increasing solute concentration, the physicochemical properties of the biphasic system change, and the distribution of the solutes becomes a function of their concentration. However, the experimental determination of liquid-liquid equilibria in multicomponent systems is time-intensive, and its prediction using thermodynamic models is often not sufficiently accurate for process design purposes. Thus, in this work, we propose a simple approach to model and simulate LLC separations in the nonlinear (concentration-dependent) range of the solutes' distribution equilibria, namely cannabidiol (CBD) and cannabigerol (CBG). Using the inverse method, the distribution equilibrium equation parameters were estimated from pulse injection experiments of single solutes at concentrations ranging from 1 to 100 mg/mL and 1-50 mg/mL for CBD and CBG, respectively. The obtained parameters were then successfully used to predict the elution profiles of binary mixtures of different compositions at 40 mg/mL total cannabinoid concentration. The approach was demonstrated and validated for CBD and CBG as model compounds and n-hexane/methanol/water 10/7.5/2.5 (v/v/v) as the biphasic solvent system. It should be noted that the applicability of the proposed approach is system-dependent, and hence, it should be evaluated for each separation task individually.

Nonlinear liquid-liquid chromatography: Beyond a constant distribution coefficient.

Liquid-liquid chromatography (LLC) is a technique in which the separation of mixture components is achieved due to their different distribution between the two phases of a pre-equilibrated biphasic solvent system. In this work, the LLC operation in the nonlinear range of the distribution isotherm was systematically examined for the first time. The influence of the feed concentration on the elution profiles of a model component (cannabidiol, CBD) was studied in three LLC units of different types and sizes ranging from ∼20 mL to ∼2 L. A series of pulse injections with CBD concentrations varying from 1 to 300 mg/mL was performed with n-hexane/methanol/water 5/4/1 (v/v/v) in descending mode (lower phase as the mobile phase). The elution profiles were simulated using the equilibrium-cell model and an anti-Langmuir-like equation for describing the CBD distribution equilibria. The distribution equilibria equation parameters were fitted to the CBD elution profiles using the peak fitting method. The model was validated and provided good predictions of the CBD elution profiles in the entire concentration range for all three LLC units.

Separation of minor cannabinoids from hemp extract with trapping multiple dual mode liquid-liquid chromatography.

Aside from Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD), other less common cannabinoids have recently gained an increasing popularity, mostly due to their promising biological potential. However, time-saving and cost-effective methods for their preparative purification are missing. In this study, trapping multiple dual mode (MDM), a flow-reversal liquid-liquid chromatography (LLC) operating mode, was used for the separation of different minor cannabinoids from a hemp extract. Separation task specific biphasic solvent systems were selected for the purification of the target constituents, as follows: n-hexane/methanol/water 10/6.5/3.5 for cannabielsoin (CBE); n-hexane/methanol/water 10/7/3 for cannabidivarin (CBDV) and cannabigerol (CBG); n-hexane/methanol/water 10/8/2 for cannabinol (CBN) and n-hexane/methanol/water 10/9/1 for cannabichromene (CBC) and cannabicylol (CBL). For each separation task, the concentration of the hemp extract in the feed stream and mobile phase flow rate were selected by shake-flask and stationary phase retention experiments, respectively. For the determination of the trapping MDM operating parameters, the short-cut method was implemented and followed by equilibrium-cell model-based simulations. The trapping MDM allowed the separation of the targeted cannabinoids with purities of 93-99%, yields of 73-95%, solvent consumption 2-4-fold lower and productivities almost double than those obtained using batch separation.

Magneto-Adaptive Surfactants Showing Anti-Curie Behavior and Tunable Surface Tension as Porogens for Mesoporous Particles with 12-Fold Symmetry.

Gaining external control over self-organization is of vital importance for future smart materials. Surfactants are extremely valuable for the synthesis of diverse nanomaterials. Their self-assembly is dictated by microphase separation, the hydrophobic effect, and head-group repulsion. It is desirable to supplement surfactants with an added mode of long-range and directional interaction. Magnetic forces are ideal, as they are not shielded in water. We report on surfactants with heads containing tightly bound transition-metal centers. The magnetic moment of the head was varied systematically while keeping shape and charge constant. Changes in the magnetic moment of the head led to notable differences in surface tension, aggregate size, and contact angle, which could also be altered by an external magnetic field. The most astonishing result was that the use of magnetic surfactants as structure-directing agents enabled the formation of porous solids with 12-fold rotational symmetry.

Nanoparticle shape anisotropy and photoluminescence properties: Europium containing ZnO as a Model Case.

The precise control over electronic and optical properties of semiconductor (SC) materials is pivotal for a number of important applications like in optoelectronics, photocatalysis or in medicine. It is well known that the incorporation of heteroelements (doping as a classical case) is a powerful method for adjusting and enhancing the functionality of semiconductors. Independent from that, there already has been a tremendous progress regarding the synthesis of differently sized and shaped SC nanoparticles, and quantum-size effects are well documented experimentally and theoretically. Whereas size and shape control of nanoparticles work fairly well for the pure compounds, the presence of a heteroelement is problematic because the impurities interfere strongly with bottom up approaches applied for the synthesis of such particles, and effects are even stronger, when the heteroelement is aimed to be incorporated into the target lattice for chemical doping. Therefore, realizing coincident shape control of nanoparticle colloids and their doping still pose major difficulties. Due to a special mechanism of the emulsion based synthesis method presented here, involving a gelation of emulsion droplets prior to crystallization of shape-anisotropic ZnO nanoparticles, heteroelements can be effectively entrapped inside the lattice. Different nanocrystal shapes such as nanorods, -prisms, -plates, and -spheres can be obtained, determined by the use of certain emulsification agents. The degree of morphologic alterations depends on the type of incorporated heteroelement M(n+), concentration, and it seems that some shapes are more tolerant against doping than others. Focus was then set on the incorporation of Eu(3+) inside the ZnO particles, and it was shown that nanocrystal shape and aspect ratios could be adjusted while maintaining a fixed dopant level. Special PL properties could be observed implying energy transfer from ZnO excited near its band-gap (3.3 eV) to the Eu(3+) states mediated by defect luminescence of the nanoparticles. Indications for an influence of shape on photoluminescence (PL) properties were found. Finally, rod-like Eu@ZnO colloids were used as tracers to investigate their uptake into biological samples like HeLa cells. The PL was sufficient for identifying green and red emission under visible light excitation.