Unified concept of solubilization in water by hydrotropes and cosolvents

A Review of Hydrotropic Solubilization Techniques for Enhancing the Bioavailability of Poorly Soluble Drugs

Hydrotropic solubilization is a technique that can be used to improve the solubility of drugs that are poorly soluble. This technique involves adding a large amount of a second solute, known as a hydrotrope, which increases the aqueous solubility of the poorly soluble drug. Hydrotropes such as sodium citrate, sodium benzoate, and urea have been shown to be effective in enhancing the solubility of poorly soluble drugs. This technique has several advantages over other solubility enhancement techniques, including its cost-effectiveness, eco-friendliness, and the fact that it does not require chemical modification of hydrophobic drugs or the use of organic solvents. Hydrotropic agents are now being used to develop various dosage forms, including solid dispersions, mouth-dissolving tablets, and injections, to improve poorly water-soluble drugs' therapeutic effectiveness and bioavailability. This review paper will provide an overview of hydrotropic solubilization techniques.

Synthesis and Performance of Biobased Surfactants Prepared by the One-Pot Reductive Amination of l-Arabinose and d-Galacturonic Acid

Herein, we report a method for the synthesis of biobased surfactants derived from sugar beet pulp (SBP) monosaccharides, l-Ara and d-GalA. The surfactants were prepared via one-pot reductive amination, allowing the introduction of different alkyl chain lengths and methyl modifications. Optimal reaction conditions were established to achieve high yields and easy purification. The synthesized surfactants including the tertiary amines exhibited desirable properties, including solubility, foamability, and reduction of surface tension. Notably, the anionic surfactants derived from d-GalA demonstrated better solubility and foam performance compared to those derived from l-Ara. In addition, these surfactants exhibited surface tension and critical micelle concentration (CMC) comparable to those of the commercial surfactant sodium lauryl ether sulfate (SLES). Furthermore, the biodegradable surfactant GalA1.8 displayed excellent emulsifying properties and low skin irritation potential. On the l-Ara surfactant with a short chain, Ara1.6 has potential as a hydrotrope. These findings suggest that biobased surfactants derived from SBP monosaccharides have promising applications in various industries, including pharmaceuticals, cosmetics, detergents, and chemicals.

Exploring the impact of sodium salts on hydrotropic solubilization

Some ionic liquids (ILs) were shown to display a strong ability to enhance the solubility of phenolic compounds through hydrotropy. However, evidence shows that salt ions in hydrotropic aqueous solutions may change the behavior of molecules by promoting possible interactions between the components of the system, thus causing changes in solubility. Herein, we study the impact of sodium salt anions on the hydrotropic dissolution of syringic acid using 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) as a hydrotrope, with a focus on dicyanamide Na[N(CN)2] and thiocyanate Na[SCN] salts. Dynamic light scattering, Raman spectroscopy, and nuclear magnetic resonance spectroscopy were used to investigate how the mixture of IL-salts affects the solvation. The results obtained show that [C4mim]Cl is able to increase the solubility of syringic acid 80-fold. Despite their structural similarities, the presence of Na[N(CN)2] or Na[SCN] in an aqueous solution of [C4mim]Cl induced opposite solubility trends. The addition of Na[N(CN)2] promotes a higher ability to solubilize syringic acid than in the corresponding IL system due to a pH buffering effect, resulting in the deprotonation of the solute. The addition of Na[SCN], on the other hand, induces a relative decrease in syringic acid solubilization at higher concentrations of ILs due to the negative contribution of the NaCl formed by anion-exchange. These results emphasise the often overlooked pH contribution provided by ILs for biomolecule solubilisation whilst providing experimental insights into the structure of aqueous solutions of ionic liquids and the role it plays in the formation of IL-salt aggregates.

Hydrophobic dye solubilization via hybrid imogolite nanotubes probed using second harmonic scattering

This article explores the organization and interactions of Disperse Orange 3 (DO3) hydrophobic dye molecules within hybrid organic-inorganic imogolite nanotubes. In pure water, the DO3 dye molecules self assemble into large insoluble 2D nanosheets whose structure is also explored by molecular dynamics simulations. The dye molecules are however efficiently solubilized in the presence of hybrid imogolite nanotubes. The filling of the internal hydrophobic cavity of the nanotubes is quantified. The organization of the molecules inside the nanotube is probed using the polarization resolved second harmonic scattering (SHS) technique coupled with simulation. At the highest loading, the dyes fill the nanotube with their principal axis parallel to the nanotube walls showing a strong SHS signal due to this encapsulation.

Aqueous solubilization of hydrophobic compounds by inorganic nano-ions: An unconventional mechanism

HYPOTHESIS

Solubilization of hydrophobic compounds in water is commonly performed by using organic solubilizers such as hydrotropes, surfactants, co-solvents, and macrocycles to form host-guest complexes. 3,3'-commo-bis[closo-1,2-dicarba-3-cobaltadodecaborane] derivatives (COSANs) are fully inorganic and non-amphiphilic ionic boron clusters with nanometric size (nano-ions) showing superchaotropic properties as they strongly bind to neutral organic molecules. Therefore, we expect COSANs to act as solubilizers of sparingly water-soluble molecules, but with a mechanism different from all other organic solubilizers known so far.

EXPERIMENTS

The aqueous solubilization efficiency of COSANs towards butanol was evaluated by determining phase diagrams and comparing them to classical solubilizers. Nanostructuration of the mixture was studied using UV spectroscopy, small-angle X-ray, and neutron scattering with contrast variation.

FINDINGS

COSANs act as efficient aqueous solubilizers of medium-chain alcohols (0.6 < log P < 1.5). Unlike surfactants, COSAN is an efficient solubilizer in its monomeric state, at concentrations well below its critical aggregation concentration. Solubilization by COSAN takes place with a bi-dimensional anisotropic growth of COSAN/butanol co-assemblies, whereas solubilization by surfactant occurs via an isotropic swelling of micelles. Appealingly, COSANs/2-butanol co-assemblies efficiently solubilize more hydrophobic compounds with log P values up to around 6, offering new opportunities in many applied fields.

Elucidating the Hydrotropic Mechanism of the Antagonistic Salt PPh4Cl

PPh₄Cl is an antagonistic salt that recently showed promise as a hydrotropic agent. Here, we give mechanistic insights into the PPh₄Cl-assisted solubility of a dye molecule using molecular dynamics simulations. Our findings reveal that dye molecules aggregate into a cluster which leads to an accumulation of PPh₄⁺ ions in its vicinity and subsequent exclusion of water molecules from the region. The structural organization is attributed to the preferential interaction of dye molecules and PPh₄Cl. The origin of such preference arises from the difference in π-π and CH-π interaction among the pairs. The hydrodynamic radius of PPh₄Cl indicates a low propensity for cluster formation, which enhances its hydrotropic behavior. The process of dye dissolution is thermodynamically favored and occurs through a cooperative mechanism. Our studies provide molecular insight into experimental observations crucial for the design of novel hydrotropes with enhanced solubilizing properties.

Betaine- and L-Carnitine-Based Ionic Liquids as Solubilising and Stabilising Agents for the Formulation of Antimicrobial Eye Drops Containing Diacerein

The therapeutic efficacy of topically administered drugs, however powerful, is largely affected by their bioavailability and, thus, ultimately, on their aqueous solubility and stability. The aim of this study was to evaluate the use of ionic liquids (ILs) as functional excipients to solubilise, stabilise, and prolong the ocular residence time of diacerein (DIA) in eye drop formulations. DIA is a poorly soluble and unstable anthraquinone prodrug, rapidly hydrolysed to rhein (Rhe), for the treatment of osteoarthritis. DIA has recently been evaluated as an antimicrobial agent for bacterial keratitis. Two ILs based on natural zwitterionic compounds were investigated: L-carnitine C6 alkyl ester bromide (Carn6), and betaine C6 alkyl ester bromide (Bet6). The stabilising, solubilising, and mucoadhesive properties of ILs were investigated, as well as their cytotoxicity to the murine fibroblast BALB/3T3 clone A31 cell line. Two IL-DIA-based eye drop formulations were prepared, and their efficacy against both Staphylococcus aureus and Pseudomonas aeruginosa was determined. Finally, the eye drops were administered in vivo on New Zealand albino rabbits, testing their tolerability as well as their elimination and degradation kinetics. Both Bet6 and Carn6 have good potential as functional excipients, showing solubilising, stabilising, mucoadhesive, and antimicrobial properties; their in vitro cytotoxicity and in vivo ocular tolerability pave the way for their future use in ophthalmic applications.

Molecular Mechanism of Hydrotropic Properties of GTP and ATP

Hydrotropes are small amphiphilic compounds that increase the aqueous solubility of hydrophobic molecules. Recent evidence suggests that adenosine triphosphate (ATP), which is the primary energy carrier in cells, also assumes hydrotropic properties to prevent the aggregation of hydrophobic proteins, but the mechanism of hydrotropy is unknown. Here, we compare the hydrotropic behavior of all four biological nucleoside triphosphates (NTPs) using molecular dynamics (MD) simulations. We launch all atom MD simulations of aqueous solutions of NTPs [ATP, guanosine triphosphate (GTP), cytidine triphosphate (CTP), and uridine triphosphate (UTP)] with pyrene, which acts both as a model hydrophobic compound and as a spectroscopic reporter for aggregation. GTP prevents pyrene aggregation effectively. Dissolution is not achieved in the presence of CTP and UTP. The higher stability of the base stacking in guanine is responsible for the higher hydrotropic efficiency of GTP. Consistent with the simulations, spectroscopic measurements also suggest that the hydrotropic activity of GTP is higher than ATP. Stacking of aromatic pyrene with the aromatic base of NTPs is a characteristic feature of this hydrotropic property. Both ATP and GTP also dissolve clusters of di- and tripeptides containing tryptophan but with equal potency. Importantly, the presence of aromatic amino acids is a necessary condition for the hydrotropic potency of ATP and GTP. Our results can have broad implications for hydrotrope design in the pharmaceutical industry, as well as the possibility of cells employing GTP as a hydrotrope to regulate the hydrophobic protein aggregation in membrane-less biological condensates.

Progress in the field of hydrotropy: mechanism, applications and green concepts

Sustainability and greenness are the concepts of growing interest in the area of research as well as industries. One of the frequently encountered challenges faced in research and industrial fields is the solubility of the hydrophobic compound. Conventionally organic solvents are used in various applications; however, their contribution to environmental pollution, the huge energy requirement for separation and higher consumption lead to unsustainable practice. We require solvents that curtail the usage of hazardous material, increase the competency of mass and energy and embrace the concept of recyclability or renewability. Hydrotropy is one of the approaches for fulfilling these requirements. The phenomenon of solubilizing hydrophobic compound using hydrotrope is termed hydrotropy. Researchers of various fields are attracted to hydrotropy due to its unique physicochemical properties. In this review article, fundamentals about hydrotropes and various mechanisms involved in hydrotropy have been discussed. Hydrotropes are widely used in separation, heterogeneous chemical reactions, natural product extraction and pharmaceuticals. Applications of hydrotropes in these fields are discussed at length. We have examined the significant outcomes and correlated them with green engineering and green chemistry principles, which could give an overall picture of hydrotropy as a green and sustainable approach for the above applications.

Phosphorylated resveratrol as a protein aggregation suppressor in vitro and in vivo

The stability of proteins in solution poses a great challenge for both technical applications and molecular biology, including neurodegenerative diseases. In this work, a phosphorylated resveratrol material was examined for its anti-aggregation properties in vitro and in vivo. Here, an anti-fibrillation effect could be measured for amyloid beta and human insulin in vitro and general anti-aggregation properties for crude chicken egg white in solution. Using a drosophila fly model for the overexpression of amyloid beta protein, changes in physiological protein aggregation and improved locomotor abilities could be observed in the presence of dietary phosphorylated resveratrol.

Phosphorylated resveratrol can prevent the aggregation of globular and intrinsically disordered proteins in vitro and in vivo.

Influence of Hydrotropes on the Solubilities and Diffusivities of Redox-Active Organic Compounds for Aqueous Flow Batteries

In this study, we explored the extent to which hydrotropes can be used to increase the aqueous solubilities of redox-active compounds previously used in flow batteries. We measured how five hydrotropes influenced the solubilities of five redox-active compounds already soluble in aqueous electrolytes (≥0.5 M). The solubilities of the compounds varied as a function of hydrotrope type and concentration, with larger solubility changes observed at higher hydrotrope concentrations. 4-OH-TEMPO underwent the largest solubility increase (1.18 ± 0.04 to 1.99 ± 0.12 M) in 20 weight percent sodium xylene sulfonate. The presence of a hydrotrope in solution decreased the diffusion coefficients of 4-OH-TEMPO and 4,5-dihydroxy-1,3-benzenedisulfonate, which was likely due to the increased solution viscosity as opposed to a specific hydrotrope-solute interaction because the hydrotropes did not alter their molecules' hydraulic radii. The standard rate constants and formal potentials of both 4-OH-TEMPO and 4,5-dihydroxy-1,3-benzenedisulfonate remained largely unchanged in the presence of a hydrotrope. The results suggest that using hydrotropes may be a feasible strategy for increasing the solubilities of redox-active compounds in aqueous flow batteries without substantially altering their electrochemical properties.

Role of Hydrotropes in Sparingly Soluble Drug Solubilization: Insight from a Molecular Dynamics Simulation and Experimental Perspectives

Drug molecules' therapeutic efficacy depends on their bioavailability and solubility. But more than 70% of the formulated drug molecules show limited effectiveness due to low water solubility. Thus, the water solubility enhancement technique of drug molecules becomes the need of time. One such way is hydrotropy. The solubilizing agent of a hydrophobic molecule is generally referred to as a hydrotrope, and this phenomenon is termed hydrotropy. This method has high industrial demand, as hydrotropes are noninflammable, readily available, environmentally friendly, quickly recovered, cost-effective, and not involved in solid emulsification. The endless importance of hydrotropes in industry (especially in the pharmaceutical industry) motivated us to prepare a feature article with a clear introduction, detailed mechanistic insights into the hydrotropic solubilization of drug molecules, applications in pharma industries, and some future directions of this technique. Thus, we believe that this feature article will become an adequate manual for the pharmaceutical researchers who want to explore all of the past perspectives of the hydrotropic action of hydrotropes in pharmaceutics.

Cholinium-based ionic liquids as bioinspired hydrotropes to tackle solubility challenges in drug formulation

Hydrotropy is a well-established strategy to enhance the aqueous solubility of hydrophobic drugs, facilitating their formulation for oral and dermal delivery. However, most hydrotropes studied so far possess toxicity issues and are inefficient, with large amounts being needed to achieve significant solubility increases. Inspired by recent developments in the understanding of the mechanism of hydrotropy that reveal ionic liquids as powerful hydrotropes, in the present work the use of cholinium vanillate, cholinium gallate, and cholinium salicylate to enhance the aqueous solubility of two model drugs, ibuprofen and naproxen, is investigated. It is shown that cholinium vanillate and cholinium gallate are able to increase the solubility of ibuprofen up to 500-fold, while all three ionic liquids revealed solubility enhancements up to 600-fold in the case of naproxen. Remarkably, cholinium salicylate increases the solubility of ibuprofen up to 6000-fold. The results obtained reveal the exceptional hydrotropic ability of cholinium-based ionic liquids to increase the solubility of hydrophobic drugs, even at diluted concentrations (below 1 mol·kg^-1), when compared with conventional hydrotropes. These results are especially relevant in the field of drug formulation due to the bio-based nature of these ionic liquids and their low toxicity profiles. Finally, the solubility mechanism in these novel hydrotropes is shown to depend on synergism between both amphiphilic ions.

Cooperativity in micellar solubilization

Sudden onset of solubilization is observed widely around or below the critical micelle concentration (CMC) of surfactants. It has also been reported that micellization is induced by the solutes even below CMC and the solubilized solute increases the aggregation number of the surfactant. These observations suggest enhanced cooperativity in micellization upon solubilization. Recently, we have developed a rigorous statistical thermodynamic theory of cooperative solubilization. Its application to hydrotropy revealed the mechanism of cooperative hydrotropy: hydrotrope self-association enhanced by solutes. Here we generalize our previous cooperative solubilization theory to surfactants. We have shown that the well-known experimental observations, such as the reduction of CMC in the presence of the solutes and the increase of aggregation number, are the manifestations of cooperative solubilization. Thus, the surfactant self-association enhanced by a solute is the driving force of cooperativity and a part of a universal cooperative solubilization mechanism common to hydrotropes and surfactants at low concentrations.

Salt-tolerance of alkyl-glyceryl ether carboxylates hydrotropes and surfactants. Dramatic effect of the methylation of the glyceryl spacer

HYPOTHESIS

The insertion of polyether spacers between the anionic head and the alkyl chain of ionic surfactants significantly improves their salt-tolerance. The aim of this work is to study whether the petro-based polyethoxy spacer can be replaced by a glyceryl ether group for high salinity applications.

EXPERIMENTS

A series of amphiphilic sodium salts of alkyl glyceryl ether carboxylates are synthesized with different alkyl chain lengths from 4 to 12 and various spacers between the glyceryl and the carboxylate groups. Their aggregation behavior is studied by tensiometry and their amphiphilicities are assessed by the PIT-slope method. The dramatic effect of the methylation of the glyceryl spacer on the salt-tolerance is highlighted, and rationalized by DFT calculations and molecular dynamics.

FINDINGS

In contrast to the corresponding sodium soap, n-C₆H₁₃-CO₂Na, and to the non-methylated counterpart, the sodium salt of 1-pentyl-3-methyl glyceryl ether methylene carboxylate ([5.0.1]-CH₂CO₂Na) exhibits an excellent salt-tolerance since it remains water-soluble with NaCl or CaCl₂ concentrations greater than 20 wt% at 25 °C. Amphiphiles with short alkyl chains (<C₈) act as hydrotropes whereas longer compounds behave as surfactants whose CMC are lower than their corresponding isomers with ethoxy spacers n-C_i(EO)₂CH₂CO₂Na.

Understanding the Effect of the Phase Modifier n-Octanol on Extraction, Aggregation, and Third-Phase Appearance in Solvent Extraction

Phase modifiers are often added to solvent extraction processes to avoid the third-phase formation. While this important issue was attributed to sticky interactions between reverse aggregates, structural effects of phase modifiers remain ambiguous. As they are similar to reverse hydrotropes, phase modifiers may act as cosurfactants or cosolvents in the organic phase in a solvent extraction system. We therefore applied an innovative small-angle scattering approach coupled with surface tension measurements on the industrially applied AMEX process to evaluate how phase modifiers repel the third phase and affect the extraction properties. We first confirmed that adding 1-octanol has a small influence on the extraction performance. By varying the scattering contrast of the solution with deuterated 1-octanol, we found that 1-octanol is located both in the solvent, acting as a cosolvent and diluting the aggregates, and in an outer shell of the aggregates. Further surface tension measurements demonstrated that instead of penetrating till the core of the aggregates as a cosurfactant, 1-octanol only penetrates their shell and forms a shielding barrier avoiding the coalescence of aggregates.

Enhancement of Simvastatin ex vivo Permeation from Mucoadhesive Buccal Films Loaded with Dual Drug Release Carriers

BACKGROUND

Simvastatin (SMV), a hypocholesterolemic agent, suffers from very low bioavailability due to its poor aqueous solubility and extensive first-pass metabolism.

METHODS

Two SMV carrier systems, namely, polymeric drug inclusion complex (IC) and mixed micelles (MM) nanoparticles, were developed and loaded into mucoadhesive buccal films to enhance SMV bioavailability. The two carrier systems were characterized and their permeation across human oral epithelial cells (OEC) was studied. The effect of IC to MM ratio (X1) and the mucoadhesive polymer concentration (X2) on the cumulative percent of drug released, elongation percent and the mucoadhesive strength, from the prepared mucoadhesive films, were optimized. Ex vivo permeation across bovine mucosal tissue was investigated. The permeation parameters for the in vitro and ex vivo release data were calculated.

RESULTS

Complexation of SMV with hydroxypropyl beta-cyclodextrin (HP β-CD) was superior to all other polymers as revealed by the equilibrium saturation solubility, stability constant, complexation efficiency and thermodynamic potential. SMV-HP β-CD IC was utilized to develop a saturated polymeric drug solution. Both carrier systems showed enhanced permeation across OEC when compared to pure drug. X1 and X2 were significantly affecting the characteristics of the prepared films. The optimized mucoadhesive buccal film formulation loaded with SMV IC and drug MM nanoparticles demonstrated superior ex vivo permeation when compared to the corresponding pure drug buccal film, and the calculated permeation parameters confirmed this finding.

CONCLUSION

Mucoadhesive buccal films containing SMV IC and drug MM can be used to improve drug bioavailability; however, additional pharmacokinetic and pharmacodynamic studies are required.

Unveiling the mechanism of hydrotropy: evidence for water-mediated aggregation of hydrotropes around the solute

The mechanism of hydrotropy is experimentally proven in this work. Apolarity is shown to be the driving force of hydrotropy.

Quantifying non-specific interactions via liquid chromatography

Determinations of solute-cosolute interactions from chromatography have often resulted in problems, such as the "antibinding" (or a negative binding constant) between the solute and micelle in micellar liquid chromatography (MLC) or indeterminacy of salt-ligand binding strength in high-performance affinity chromatography (HPAC). This shows that the stoichiometric binding models adopted in many chromatographic analyses cannot capture the non-specific nature of solvation interactions. In contrast, an approach using statistical thermodynamics handles these complexities without such problems and directly links chromatographic data to, for example, solubility data via a universal framework based on Kirkwood-Buff integrals (KBI) of the radial distribution functions. The chromatographic measurements can now be interpreted within this universal theoretical framework that has been used to rationalize small solute solubility, biomolecular stability, binding, aggregation and gelation. In particular, KBI analysis identifies key solute-cosolute interactions, including excluded volume effects. We present (i) how KBI can be obtained directly from the cosolute concentration dependence of the distribution coefficient, (ii) how the classical binding model, when used solely as a fitting model, can yield the KBIs directly from the literature data, and (iii) how chromatography and solubility measurements can be compared in the unified theoretical framework provided via KBIs without any arbitrary assumptions about the stationary phase. To perform our own analyses on multiple datasets we have used an "app". To aid readers' understanding and to allow analyses of their own datasets, the app is provided with many datasets and is freely available on-line as an open-source resource.

Ab initio prediction of structuring/mesoscale inhomogeneities in surfactant-free microemulsions and hydrogen-bonding-free microemulsions

Prediction of microemulsion-like structuring in surfactant-free and hydrogen-bonding-free systems.

Formulation and clinical investigation of optimized vinpocetine lyoplant-tabs: new strategy in development of buccal solid dosage form

BACKGROUND

This work aimed to develop a new solid dosage formulation of vinpocetine (VPN) in the form of buccal freeze-dried pullulan-based tablets (lyoplant-tabs) loaded with physically modified drug binary system.

METHODS

Different polyvinyl pyrrolidone (PVP) grades were studied to prepare an efficient VPN binary system characterized by enhanced equilibrium saturation solubility, solubilization efficiency, thermodynamic stability, and permeation through oral mucosal cell lines. The concentrations of pullulan and swelling-aid polymer that affect the quality attributes of lyoplant-tabs were optimized. Clinical pharmacokinetics study on human volunteers for the optimized lyoplant-tabs compared to marketed product was accomplished.

RESULTS

A promising drug binary system with polyvinyl pyrrolidone vinyl acetate (PVP-VA64) utilizing the lyophilization technique was developed. Solid-state characterization confirmed transformation of VPN completely into the amorphous form. The concentrations of pullulan and swelling-aid polymer were significantly affecting the characteristics of the tablets. Compared to the commercial VPN tablets, pullulan-based buccal tablets demonstrated enhancement in the studied pharmacokinetic parameters with positive impact on the drug bioavailability.

CONCLUSION

These VPN lyoplant-tabs containing lyophilized PVP-VA64-VPN binary system can be considered as an alternative to currently available marketed tablets; however, further preclinical investigations using large number of volunteers are required.

Enhanced dissolution of ibuprofen using ionic liquids as catanionic hydrotropes

Ionic liquids as powerful hydrotropes for ibuprofen, where both cation and anion may contribute to the hydrotropic mechanism in a synergistic manner.

Strategies for improving the payload of small molecular drugs in polymeric micelles

In the past few years, substantial efforts have been made in the design and preparation of polymeric micelles as novel drug delivery vehicles. Typically, polymeric micelles possess a spherical core-shell structure, with a hydrophobic core and a hydrophilic shell. Consequently, poorly water-soluble drugs can be effectively solubilized within the hydrophobic core, which can significantly boost their drug loading in aqueous media. This leads to new opportunities for some bioactive compounds that have previously been abandoned due to their low aqueous solubility. Even so, the payload of small molecular drugs is still not often satisfactory due to low drug loading and premature release, which makes it difficult to meet the requirements of in vivo studies. This problem has been a major focus in recent years. Following an analysis of the published literature in this field, several strategies towards achieving polymeric micelles with high drug loading and stability are presented in this review, in order to ensure adequate drug levels reach target sites.

Unifying hydrotropy under Gibbs phase rule

A rational approach, aiming at constructing a unified theory of hydrotropy, will be presented based upon the first principles of statistical thermodynamics.

PPh4Cl in aqueous solution – the aggregation behavior of an antagonistic salt

The aggregation behavior of the antagonistic salt PPh₄Cl was investigated. This salt was found to be an excellent hydrotrope which does not aggregate in binary aqueous solutions or in a ternary one, upon the addition of the exemplary hydrophobic molecule DR-13.

The impact of the structuring of hydrotropes in water on the mesoscale solubilisation of a third hydrophobic component

The complex structuring of alcohols in water influences the mesoscale solubilisation of a third component depending on its nature.

Hydrotropy and scattering: pre-ouzo as an extended near-spinodal region

The relationship between hydrotropic solubilization and mesoscale structuring has been clarified from the first principles of chemical and statistical thermodynamics.

Hydrotropic Solubilization of Lipophilic Drugs for Oral Delivery: The Effects of Urea and Nicotinamide on Carbamazepine Solubility-Permeability Interplay

Hydrotropy refers to increasing the water solubility of otherwise poorly soluble compound by the presence of small organic molecules. While it can certainly increase the apparent solubility of a lipophilic drug, the effect of hydrotropy on the drugs’ permeation through the intestinal membrane has not been studied. The purpose of this work was to investigate the solubility–permeability interplay when using hydrotropic drug solubilization. The concentration-dependent effects of the commonly used hydrotropes urea and nicotinamide, on the solubility and the permeability of the lipophilic antiepileptic drug carbamazepine were studied. Then, the solubility–permeability interplay was mathematically modeled, and was compared to the experimental data. Both hydrotropes allowed significant concentration-dependent carbamazepine solubility increase (up to ∼30-fold). A concomitant permeability decrease was evident both in vitro and in vivo (∼17-fold for nicotinamide and ∼9-fold for urea), revealing a solubility–permeability tradeoff when using hydrotropic drug solubilization. A relatively simplified simulation approach based on proportional opposite correlation between the solubility increase and the permeability decrease at a given hydrotrope concentration allowed excellent prediction of the overall solubility–permeability tradeoff. In conclusion, when using hydrotropic drug solubilization it is prudent to not focus solely on solubility, but to account for the permeability as well; achieving optimal solubility–permeability balance may promote the overall goal of the formulation to maximize oral drug exposure.

Molten fatty acid based microemulsions

We show that ternary mixtures of water (polar phase), myristic acid (MA, apolar phase) and cetyltrimethylammonium bromide (CTAB, cationic surfactant) studied above the melting point of myristic acid allow the preparation of microemulsions without adding a salt or a co-surfactant. The combination of SANS, SAXS/WAXS, DSC, and phase diagram determination allows a complete characterization of the structures and interactions between components in the molten fatty acid based microemulsions. For the different structures characterized (microemulsion, lamellar or hexagonal phases), a similar thermal behaviour is observed for all ternary MA/CTAB/water monophasic samples and for binary MA/CTAB mixtures without water: crystalline myristic acid melts at 52 °C, and a thermal transition at 70 °C is assigned to the breaking of hydrogen bounds inside the mixed myristic acid/CTAB complex (being the surfactant film in the ternary system). Water determines the film curvature, hence the structures observed at high temperature, but does not influence the thermal behaviour of the ternary system. Myristic acid is partitioned in two "species" that behave independently: pure myristic acid and myristic acid associated with CTAB to form an equimolar complex that plays the role of the surfactant film. We therefore show that myristic acid plays the role of a solvent (oil) and a co-surfactant allowing the fine tuning of the structure of oil and water mixtures. This solvosurfactant behaviour of long chain fatty acid opens the way for new formulations with a complex structure without the addition of any extra compound.

Mechanism of Hydrotropic Action of Hydrotrope Sodium Cumene Sulfonate on the Solubility of Di-t-Butyl-Methane: A Molecular Dynamics Simulation Study

Hydrotropes are special class of amphiphilic molecules that have an ability to solubilize the insoluble or sparingly soluble molecules in water. To find out the mechanism of hydrotropic action of hydrotropes on hydrophobic molecules, we have carried out classical molecular dynamics simulation of hydrophobic solute di-t-butyl-methane (DTBM) and hydrotrope sodium cumene sulfonate (SCS) in water with a regime of SCS concentrations. Our study demonstrates that, above the minimum hydrotrope concentration (MHC), the self-aggregation of SCS starts, and it creates a micellar-like environment in which the hydrophobic tail part of SCS points inward while its hydrophilic sulfonate group points outward to make favorable contact with water molecules. The formation of the hydrophobic core of SCS cluster creates a hydrophobic environment where the hydrophobic DTBM molecules are encapsulated. Interestingly, the determination of average water-SCS hydrogen bonds further suggests that the aggregate formation of SCS molecules has a negligible influence on it. Moreover, the calculations of Flory-Huggins interaction parameters also reveal favorable interactions between hydrotrope SCS and solute DTBM molecules. The implications of these findings on the mechanism of hydrotrope assisted enhanced solubility of hydrophobic molecules are discussed.

Revealing microheterogeneities and second order phase transitions in aqueous mixtures of 1-propoxypropan-2-ol at 298 K

Second-order phase transitions, identified by Kirkwood–Buff integrals, assigned to solution microheterogeneities are corroborated by ESI-MS, surface tension and solvatochromic parameters.