Surfactant Head Groups Research Articles

Microscopic Origin of Surfactant Irritation: An Experimental and Computational Study.

Surfactants, which are widely used in skin care products and cleansers, can cause skin irritation. The skin irritation potential of surfactants is fundamentally determined by their molecular structure and is directly related to their microscopic aggregation structure and specific interactions with the skin. The microscopic origin of the irritation of the surfactants remains unknown. In this work, irritation properties of four surfactant solutions were measured, and their microscopic aggregation behavior was systematically analyzed. The results indicate that the surfactants self-assembled in aqueous solution to form aggregates with different morphologies, where the head groups of surfactants were closer to each other. Furthermore, surfactants that can form larger and more stable aggregate structures in aqueous solutions will exhibit less irritation. These findings hold significant implications for the design and expanded applications of mild surfactants.

Molecular-Level Understanding of Phase Stability in Phase-Change Nanoemulsions for Thermal Energy Storage by NMR Spectroscopy.

Phase change materials (PCMs) are latent heat storage materials that can store or release thermal energy while undergoing thermodynamic phase transitions. Organic PCMs can be emulsified in water in the presence of surfactants to enhance thermal conductivity and enable applications as heat transfer fluids. However, PCM nanoemulsions often become unstable during thermal cycling. To better understand the molecular origins of phase stability in PCM nanoemulsions, we designed a model PCM nanoemulsion system and studied how the molecular-level environments and dynamics of the surfactants and oil phase changed upon thermal cycling using liquid-state nuclear magnetic resonance (NMR) spectroscopy. The model system used octadecane as the oil phase, stearic acid as the surfactant, and aqueous NaOH as the continuous phase. The liquid fraction of octadecane within the nanoemulsions was quantified noninvasively during thermal cycling by liquid-state 1H single-pulse NMR measurements, revealing the extent of octadecane supercooling as a function of temperature. The mean droplet size of the PCM nanoemulsions, measured by dynamic light scattering (DLS), was correlated with the liquid content of octadecane to explain phase instability in the solid-liquid coexistence region. Quantitative 13C single-pulse NMR experiments established that the carbonyl surfactant head groups were present in multiple distinct environments during thermal cycling. After repeated thermal cycling, the 13C signal intensity of the carbonyl surfactant head groups decreased, indicating that the surfactant head groups lost molecular mobility. The results explain, in part, the origin of phase instability of PCM nanoemulsions upon thermal cycling.

Diamine-surfactant adsorption at the air–water interface: The impact of the diamine structure, surfactant structure and solution pH

The biogenic amines, polyamines, such as putrescine and cadavarine, are small flexible polycations. They interact with charged biological species and have a range of important biological functions. Their strong interaction with anionic surfactants results in enhanced adsorption at interfaces. Aspects of the impact of the polyamine molecular weight and structure on the pattern of surfactant adsorption at the air–water interface have been explored previously using neutron reflectivity, NR, and surface tension, ST. However there is limited evidence for the extent to which surfactant adsorption can be manipulated and the impact of the diamine structure and surfactant headgroup structure on surfactant adsorption at the air–water interface is reported here.The addition of putrescine [1,4 diaminobutane or butanediamine] over the pH range 3 to 10 enhances significantly the adsorption of sodium dodecyl sulfate, SDS, sodium dodecyl diethylene sulfate, SLES, and sodium tetradecyl methyl ester sulfonate, MES. For SDS and to a lesser extent MES there are regions where surface multilayer adsorption occurs at both pH 3 and 10, and this extends significantly the range of polyamine structures for which this is observed. Modifying the diamine spacer with ethylene oxide groups of varying length still results in enhanced adsorption for SDS and MES, but the enhancement is less pronounced and depends upon the size of the ethylene oxide group. However the incorporation of the ethylene oxide group does suppresses the formation of the more complex surface multilayered structures.

Experimental Investigations and MD Simulation on Nanoparticle-Enhanced CO2-Responsive Foam (NECRF): Implications on CO2-EOR.

CO2-responsive foam (CRF) is a highly promising candidate for CO2-enhanced oil recovery (CO2-EOR) because it displays higher stability than the surfactant-stabilized foam owing to the formation of robust wormlike micelles (WLMs) upon exposure to CO2. In this work, the nanoparticle-enhanced CO2-responsive foam (NECRF) was properly prepared using lauryl ether sulfate sodium (LES)/diethylenetriamine/nano-SiO2, and its interfacial properties and EOR potential were experimentally and numerically assessed, aiming to explore the feasibility and effectiveness of NECRF as a novel CO2-EOR technique. It was found that the interfacial expansion elastic modulus increased 6-fold after CO2 stimulation. The modulus continued to increase with the introduction of nano-SiO2 owing to the pronounced synergistic effect of WLMs and nanoparticles. In addition to increasing the viscosity of the foaming liquid, WLMs and nano-SiO2 enhanced the shearing resistance of the NECRF as well. Calculations demonstrated that both the coarsening rate and the size distribution uniformity coefficient of NECRF were markedly lower than that of the LES foam, which subsequently inhibited NECRF decay and greatly improved its dynamic stability. Besides, molecular dynamics simulation revealed that adding inorganic salts to NECRF could notably enhance the foaming performance due to the intensified hydration of surfactant head groups and reduced binding energy of neighboring molecules. Nuclear magnetic resonance-assisted core flooding experiments validated the exceptional capacity of NECRF to sweep the low-permeability region and improve the conformance profile. Overall, these findings may provide valuable insights into the development and application of novel materials and strategies for the CO2-EOR.

Plasmid DNA Complexes in Powder Form Studied by Spectroscopic and Diffraction Methods.

Currently, new functional materials are being created with a strong emphasis on their ecological aspect. Materials and devices based on DNA biopolymers, being environmentally friendly, are therefore very interesting from the point of view of applications. In this paper, we present the results of research on complexes in the powder form based on plasmid DNA (pDNA) and three surfactants with aliphatic chains containing 16 carbon atoms (cetyltrimethylammonium chloride, benzyldimethylhexadecylammonium chloride and hexadecylpyridinium chloride). The X-ray diffraction results indicate a local hexagonal packing of DNA helices in plasmid DNA complexes, resembling the packing for corresponding complexes based on linear DNA. Based on the Fourier-transform infrared spectroscopy results, the DNA conformation in all three complexes was determined as predominantly of A-type. The two relaxation processes revealed by dielectric spectroscopy for all the studied complexes are connected with two different contributions to total conductivity (crystallite part and grain boundaries). The crystallite part (grain interior) was interpreted as an oscillation of the polar surfactant head groups and is dependent on the conformation of the surfactant chain. The influence of the DNA type on the properties of the complexes is discussed, taking into account our previous results for complexes based on linear DNA. We showed that the type of DNA has an impact on the properties of the complexes, which has not been demonstrated so far. It was also found that the layer of pDNA-surfactant complexes can be used as a layer with variable specific electric conductivity by selecting the frequency, which is interesting from an application point of view.

The Role of Surfactant Head Group Charge on Binding of Hoechst 33258 with Micelles

AbstractOwing to the diverse biological application, photophysical behaviour of Hoechst 33258 (H33258) is an important topic in biophysical research. Here, we have reported the photophysical behavior of H33258 in model biomembrane mimicking micellar microenvironment. In order to understand the molecular mechanism of the interaction of H33258 with micelles, we have used cationic (Hexadecetyltrimethyl ammonium bromide (CTAB)), anionic (sodium dodecyl sulphate (SDS)), and non‐ionic (t‐octylphenoxypolyoxyethanol, Triton‐X100 (TX 100)) micelles. From the steady‐state emission study the evaluated apparent binding constant (Kb) values for the complex formation between H33258 and CTAB, SDS and TX 100 micelle were (1.45±0.42)×105, (31.95±4.89)×105 and (3.10±0.73)×105 M−1, respectively. Thus, it was found that the Kb value was greater in the case of anionic micelle than non‐ionic and cationic micelles. Such differences in Kb values were due to the electrostatic interaction and hydrogen bond formation between the probe molecule and micelles. In addition, time‐resolved fluorescence decay and time‐resolved fluorescence anisotropy studies were performed to understand the dynamical behavior of H33258 in presence of micelles. The interaction of H33258 with micelles might be useful to study the structure and dynamics of different bio‐membranes and to formulate new drug delivery system.

Effect of ethylene oxide groups on calcite wettability reversal by nonionic surfactants: An experimental and molecular dynamics simulation investigation

HypothesisEthoxylated nonionic surfactants are promising candidates for enhanced oil recovery (EOR) from oil-wet carbonate reservoirs due to their ability to reverse the mineral wettability. The wettability-reversal efficiency increases with the number of the ethoxy (EO) groups in the surfactant molecule. MethodologyContact angle measurements, scanning electron microscopy (SEM) and molecular dynamics (MD) simulations were combined to investigate the wettability reversal of an oil-wet calcite by three ethoxylated nonionic surfactants with 1, 4 and 8 EO groups, respectively, to directly probe the role of the EO groups and to uncover the molecular mechanism responsible for the wettability reversal. FindingsBoth experiments and simulations consistently show a clear correlation between the number of EO groups and the wettability reversal efficiency of the surfactants, whereby the higher number of EO groups results in greater degree of wettability reversal. This is due to 1) the more hydrophilic surfactant headgroup weakening the carboxylate interactions with the surface by expanding the surface-adjacent water layer, and 2) the physically larger surfactant molecule attracting the carboxylates more strongly, thus aiding in their removal from the surface.

An Investigation of the Effect of pH on Micelle Formation by a Glutamic Acid-Based Biosurfactant

NMR spectroscopy, molecular modeling, and conductivity experiments were used to investigate micelle formation by the amino acid-based surfactant tridecanoic L-glutamic acid. Amino acid-based biosurfactants are green alternatives to surfactants derived from petroleum. NMR titrations were used to measure the monomeric surfactant’s primary and gamma (γ) carboxylic acid pKa values. Intramolecular hydrogen bonding within the surfactant’s headgroup caused the primary carboxylic acid to be less acidic than the corresponding functional group in free L-glutamic acid. Likewise, intermolecular hydrogen bonding caused the micellar surfactant’s γ carboxylic functional group to be less acidic than the corresponding monomer value. The binding of four positive counterions to the anionic micelles was also investigated. At pH levels below 7.0 when the surfactant headgroup charge was −1, the micelle hydrodynamic radii were larger (~30 Å) and the mole fraction of micelle-bound counterions was in the 0.4–0.7 range. In the pH range of 7.0–10.5, the micelle radii decreased with increasing pH and the mole fraction of micelle bound counterions increased. These observations were attributed to changes in the surfactant headgroup charge with pH. Above pH 10.5, the counterions deprotonated and the mole fraction of micelle-bound counterions decreased further. Finally, critical micelle concentration measurements showed that the micelles formed at lower concentrations at pH 6 when the headgroup charge was predominately −1 and at higher concentrations at pH 7 where headgroups had a mixture of −1 and −2 charges in solution.

Effects of surfactant properties on pore wetting of membrane distillation

Pore wetting is a major constraint to the performance of membrane distillation (MD) for hypersaline brine treatment. Despite the existence of surfactants with diverse properties, an explicit relationship between the properties of surfactants and their capabilities of inducing pore wetting has yet to be established. In this study, we perform a comparative analysis of the wetting behaviors of various surfactants with different charges and molecular weights in MD desalination. The induction time of surfactants to initiate pore wetting was correlated to the apparent contact angle and surface tension of the feedwater. Our results show that different surfactants resulting in similar feedwater surface tensions can lead to drastically different wetting potential, suggesting that both charge of the head group and molecular weight of surfactants have a significant influence on membrane pore wetting. Further, we demonstrate that parameters that have been commonly used to indicate wetting potential, including apparent contact angle and solution surface tension, are not reliable in predicting the wetting behavior of MD membranes, which is intricately linked with surfactant properties such as charge and molecular size. We envision that our results not only improve our fundamental understanding of surfactant-induced wetting but also provide valuable insights that necessitate thorough consideration of surfactant properties in evaluating wetting potential and membrane wetting resistance for MD desalination.

Interaction mechanisms between fibers and bubbles during foam forming

It is critical to understand the interaction mechanism of fiber-bubble before the foam-forming technique is used to disperse the long fiber and tailor the material microporous structure. Herein, the adhesion behavior of ionic and nonionic surfactants with different concentrations on five fibers was investigated, and the effect of electrostatic force and adhesion driving force on the interaction between fibers and bubbles was analyzed. In addition, the fiber dispersion mechanisms and the response mechanism of the fiber orientation and tensile strength of foam-formed paper on the interaction strength between fiber and bubble were clarified. The results show that the adhesion ability between hydrophobic fibers with weak surface polarity and cationic and amphoteric surfactants is more obvious. Though electrostatic repulsion weakens the interaction strength of fibers with anionic surfactants, the adhesion ability can exist when the adhesion driving force is larger than 6.6 mN/m. Furthermore, the bridge effect between ethoxylated surfactant headgroups with the acylamino on the aramid fiber surface also propels the adhesion behavior of bubble-fiber. The fiber dispersion mechanisms were proposed based on the adhesion effect of the fiber-bubble, electrostatic elimination, and high-viscosity (≥95 mPa∙s) properties of the “foam-fiber” slurry. Eventually, under the gradually decreased interaction strength between fibers and bubbles, the fiber orientation in the Z direction of foam-formed NBSKP fiber paper turned from −20°-20° to −60°-40°, and the tensile strength decreased from 693 KPa to 537 KPa. This work has implications for improving the long fiber dispersion and regulating the structure of foam-formed fiber composites.

Colloidal interactions between nanochitin and surfactants: Connecting micro- and macroscopic properties by isothermal titration calorimetry and rheology

This study elucidates the intricate interactions between chitin nanocrystals (ChNC) and surfactants of same hydrophobic tail (C12) but different head groups types (anionic, cationic, nonionic): sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), and polyoxyethylene(23)lauryl ether (Brij-35). Isothermal Titration Calorimetry (ITC) and rheology are used to study the complex ChNC-surfactant interactions in aqueous media, affected by adsorption, self-assembly and micellization. The ITC results demonstrate that the surfactant head group significantly influences the dynamics and nature of the involved phenomena. Cationic DTAB's reveal minimal interaction with ChNC, non-ionic Brij-25's interact moderately at low concentrations driven by hydrophobic effects while SDS's interacts strongly and show complex interaction patterns that fall across four distinct regimes with SDS addition. We attribute such behavior to initiate through electrostatic attraction and terminate in surfactant micelle formation on ChNC surfaces. ITC also elucidates the impact of ChNC concentration on key parameters including critical aggregation concentration (CAC) and saturation concentration (C2). Dynamic rheological analysis indicates the molecular interactions translate to non-linear variations in the elastic modulus (G') upon SDS addition mirroring that observed in ITC experiments. Such a direct correlation between molecular interactions and macroscopic rheological properties provides insights to aid in the creation of nanocomposites with tailored properties.

Interfacial characteristics and foam stability: A microscopic perspective from molecular dynamics simulation

Foams play a pivotal role in diverse industrial and academic applications, yet their stability, particularly in relation to surfactants, remains inadequately understood at the microscopic level. This study undertakes a comprehensive modeling of water films stabilized by 15 surfactants, spanning 4 ion types of hydrophilic groups. We conducted a systematic comparison and correlation of key interfacial characteristics at different stages of rupture. Our findings reveal that the critical film thickness and interface tension are integral in characterizing foam stabilities near equilibrium (metastable state) and near rupture (critical state), respectively. Furthermore, we observed a positive correlation between the hydrophilicity of surfactant head groups and both the thickness of Gibbs dividing surface and the orderliness of interfacial surfactant molecules. These factors are crucial in maintaining film stability in both metastable and critical states. This work presents a quantitative framework to generalize various interfacial characteristics commonly employed in describing foam stability, which is poised to enhance the data-driven methodology for surfactant selection and design.

Interaction, Surface Activity, and Application of Mixed Systems of Alcohol Ether Sulfate Anionic Surfactants with Multiple Ethylene Oxide Groups and Gemini Quaternary Ammonium Surfactant.

The surface activities and application properties for the mixtures of cationic surfactants tetramethylene-1,4-bis[N,N-bis(hydroxypropyl)-hexa/decyloxypropylammonium] bromide (GC10-P) and tetramethylene-1,4-bis[N,N-bis(hydroxyethyl)-hexa/decyloxypropylammonium] bromide (GC10-E) and anionic surfactant isomeric sodium fatty alcohol ether sulfates (iso-AE9S) were investigated using both the tensiometry and the conductometry. The interaction parameters and thermodynamic micellization parameters of GC10-P/iso-AE9S and GC10-E/iso-AE9S mixtures were evaluated by Clint-Rubingh and Motomura theoretical models. When the mole fraction of α1 for GC10-P/iso-AE9S mixed system was 0.2, the critical micelle concentration (CMC) reached a minimum of 1.61 × 10-4 mol/L, and the minimum critical micelle concentration of the GC10-E/iso-AE9S mixed system is 2.67 × 10-5 mol/L at α1 = 0.6. The CMC value of the mixed system is 1-2 orders of magnitude lower than that of any single component. The results indicate that the synergistic effects of the investigated mixed systems (evaluated by βm) are in order of GC10-P/iso-AE9S < GC10-E/iso-AE9S, with maximum βm values of -17.98 and -9.78, respectively. The change in zeta potential indicates that the poly(ethylene oxide) chain has weakened the charge density of the hydrophilic headgroup of the anionic surfactant. The interfacial tension at the oil-water interface in the mixed system of anionic/cationic surfactants is lower than that of any single component, exhibiting a higher interfacial activity. The mixed system exhibits a decreased contact angle and superior wetting ability over any single component, and it also enhances foam performance, emulsification performance, and degreasing performance.

Molecular dynamics simulations of adsorption behavior of mixtures of surfactant and polymer at C3A/water and Ettringite/water interfaces

In cementitious system, surfactant is used as air-entraining agents, which is often incompatible with other chemical admixtures, such as superplasticizers. The adsorption of surfactants at solid–liquid interface is critical for the bubble stability. The measurement of adsorption properties of surfactant in fresh cement or concrete is very difficult. In this study, classical molecular dynamics simulations is used to analysis the influence of two polymers (call as HPMC and PCA, respectively) on the adsorption behaviors of the two anionic surfactants (call as HSO4 and HPO4, respectively) at the C3A/water and ettringite/water interfaces. At solid/water interface, both of HSO4 and HPO4 molecules form a monolayer with hydrophilic head groups adsorbed on the solid surfaces. After adding polymers, the adsorption conformations are changed to spherical micelle or multilayer. The electrostatic interaction between oxygen atoms of hydrophilic head groups and calcium atoms of solid surface plays a decisive role. Therefore, the value of adsorption energy of HPO4 with more negatively charged hydrophilic head groups is greater than that of HSO4. The adsorption energies of the two surfactants are weakened by the two polymers, but the adsorption energy value of HPO4 is always greater. The influence mechanism of HPMC and PCA is different. The negatively charged PCA has a strongly competitive adsorption with the surfactants. The neutral HPMC has a weak adsorption capacity but it carries many hydroxyl groups to interact with the hydrophilic head groups of surfactants.

Formulation factors affecting foam properties during vacuum foam-drying

This paper explores how vacuum foam-drying of a protein is influenced by formulation parameters by investigating the foam structure, physical properties of the foam, and the stability of the protein. Recombinant human bile salt-stimulated lipase was used as a model of a protein drug. The stability of the lipase was evaluated through activity measurements. Two disaccharides (sucrose and trehalose), strongly tending to an amorphous form, were used as matrix formers, and the physical properties were assessed through residual water content, glass transition temperature, and crystalline state. Moreover, some formulations included surfactants with different sizes and structures of the head group. The alkyl chain length was kept constant to only investigate the impact of the surfactant head group, in the presence of the lipase, on the foamability and surface coverage of the lipase. The study demonstrated that the lipase allowed for a dry, solid foam with a foam overrun of up to 2600 %. The wall thickness of the dry, solid foam was estimated to be 20–50 µm. Clear differences between sucrose and trehalose as matrix former were identified. The lipase showed no tendency to lose activity because of the drying and rehydration, despite a proportion of the lipase covering the surfaces of the dry material.

Effect of counterions on the structure and dynamics of water near a negatively charged surfactant: a theoretical vibrational sum frequency generation study.

Charged aqueous interfaces are of paramount importance in electrochemical, biological and environmental sciences. The properties of aqueous interfaces with ionic surfactants can be influenced by the presence of counterions. Earlier experiments involving vibrational sum frequency generation (VSFG) spectroscopy of aqueous interfaces with negatively charged sodium dodecyl sulfate (Na+DS- or SDS) surfactants revealed that the hydrogen bonding strength of the interfacial water molecules follows a certain order when salts of monovalent and divalent cations are added. It is known that cations do not directly participate in hydrogen bonding with water molecules, rather they only influence the hydrogen bonded network through their electrostatic fields. In the current work, we have simulated the aqueous interfacial systems of sodium dodecyl sulfate in the presence of chloride salts of mono and divalent countercations. The electronic polarization effects on the ions are considered at a mean-field level within the electronic continuum correction model. Our calculations of the VSFG spectra show a blue shift in the presence of added countercations whose origin is traced to different relative contributions of water molecules from the solvation shells of the surfactant headgroups and the remaining water molecules in the presence of countercations. Furthermore, the cations shield the electric fields of the surfactant headgroups, which in turn influences the contributions of water molecules to the total VSFG spectrum. This shielding effect becomes more significant when divalent countercations are present. The dynamics of water molecules is found to be slower at the interface in comparison to the bulk. The interfacial depth dependence of various dynamical quantities shows that the interface is structurally and dynamically more heterogeneous at the microscopic level.

Molecular dynamics study on BS12/SDS stabilized CO2 foam: Evaluation of surfactant monolayer structure and interfacial properties

CO2 foam flooding holds great potential for enhanced oil recovery and geological storage. However, ensuring the stability of supercritical CO2 foam systems under high temperature and pressure reservoir conditions remains a challenge. Note that the term "supercritical CO2 foam" specifically denotes a foam system created by combining an aqueous solution containing chemical additives with CO2 in a supercritical state under high-temperature and high-pressure conditions. Surfactants can reduce the surface Gibbs free energy by adsorbing onto the CO2-water interface to form a monolayer structure, which hinders CO2 diffusion and improves foam stability. In this study, molecular dynamics simulations were employed to analyze the adsorption behavior and interfacial performance of betaine surfactant BS12 and the anionic surfactant SDS at the CO2-water interface. The results indicate that BS12 can reduce interfacial tension more effectively at near-saturated concentrations. Moreover, the monolayer formed by BS12 has a lower voidage and wider interfacial thickness. The carboxyl-nitrogen interaction among BS12 molecules impedes their further aggregation, thereby maintaining the mutually perpendicular "L" orientation of the headgroup and hydrophobic tail thus forming a rigid and stable monolayer structure. Additionally, the spatial distribution function, radial distribution function, and hydrogen bond statistics indicate that although BS12 exhibits slightly weaker hydrophobicity than SDS, its hydrophobicity is less affected by concentration. Specifically, the first hydration shell of the carboxyl group remains stable even at high concentrations. Besides, the steered simulations show that the rate-limiting step in the CO2 permeation process is the dense region of the surfactant headgroups. Besides, the permeation of CO2 molecules in the BS12 monolayer is characterized by a lower diffusion coefficient and higher energy barrier to overcome. In summary, BS12 forms a thicker and more uniform monolayer structure, effectively hindering the contact between CO2 and water molecules at the interface and further enhancing foam stability. Our findings further reveal the relationship between surfactant molecular architecture and interfacial properties, providing references for the screening and structural design of surfactants used in CO2 foam flooding.

Hidden Beneath the Layers: Extending the Core/Shell Model of Reverse Micelles.

Reverse micelles (RMs) provide a unique and highly tunable model system to study water in confined environments. The complex properties of water within RMs arise from the disruption of extended hydrogen bond (H-bond) networks that mediate local and long-range dynamics in bulk aqueous systems. Modulating the water pool size influences its H-bond dynamics, with smaller RMs increasingly restricting the H-bond network rearrangements leading to slower dynamics; however, within small confined systems, the dynamics of the surfactants also influence the water dynamics. Using ultrafast two-dimensional infrared spectroscopy, we investigate the effects of RM size on the surfactant headgroup rotamer populations and picosecond interfacial H-bond dynamics of aerosol-OT surfactants. We find that the increased water penetration accelerates H-bond dynamics, with larger RMs showing faster dynamics. These results imply that the changes in the RM structure alter the physical structure of the RM interface and thus alter the solvation dynamics. The findings in this study can be used for developing models for structure-specific solvation dynamics that account for the surfactant packing and hydration at the interface.

Counterion optimization for hydrophobic ion pairing (HIP): Unraveling the key factors

In the present study, various surfactants were combined with insulin (INS), bovine serum albumin (BSA) and horseradish peroxidase (HRP) via hydrophobic ion pairing to increase lipophilicity and facilitate incorporation into self-emulsifying drug delivery systems (SEDDS). Lipophilicity of model proteins was successfully increased, achieving log Dn-butanol/water values up to 3.5 (INS), 3.2 (BSA) and 1.2 (HRP). Hereby, key factors responsible for complex formation were identified. In particular, surfactants with branched alkyl chains or chain lengths greater than C12 showed favorable properties for hydrophobic ion pairs (HIP). Furthermore, flexibility of the carbon chain resulted in higher lipophilicity and suitability of polar head groups of surfactants for HIP decreased in the rank order sulfonate > sulfosuccinate > phosphate = sulfate > carbonate > phosphonic acids = sulfobetaines. Stability studies of formed HIP complexes were performed in various gastrointestinal fluids and their solubility was determined in commonly used SEDDS excipients. Formed complexes were stable in simulated gastrointestinal fluids and could be incorporated into SEDDS formulations (C1: 10% caprylocaproyl polyoxyl-8 glycerides, 20% PEG-40 hydrogenated castor oil, 20% medium-chain triglycerides, 50% n-butanol; C2: 10% caprylocaproyl polyoxyl-8 glycerides, 20% PEG-40 hydrogenated castor oil, 20% medium-chain triglycerides, 40% n-butanol, 10% 1,2-butanediol), resulting in suitable payloads of up to 11.9 mg/ml for INS, 1.0 mg/ml for BSA and 1.6 mg/ml for HRP.

Can wormlike micelle stiffness be estimated from zero‐shear viscosity? Experimental investigation with a model system and specific salt interactions

AbstractThe microstructure of wormlike micelles (WLM) directly affects the rheological properties of their solutions. Investigating the structure–property relationships of WLM has long been a popular topic for researchers who have developed theoretical and empirical models to describe their viscoelastic behavior. All these models rely on a collection of characteristic “length” parameters that are often difficult to estimate using relatively simple rheological tests. In particular, the micelle stiffness, as described by the persistence length, can be difficult to measure experimentally, while being very impactful on the bulk rheology of WLM solutions. Here, an array of inorganic salts (NaCl, LiCl, MgCl2, NaBr, NaI, and Na2SO4) have been used to induce wormlike micelle formation in an aqueous solution of the surfactant sodium lauryl ether sulfate. Ion dissociation/association with the surfactant head groups and the hydrogen bond network of the water alters the effective stiffness of the micelles, allowing for an estimation of the stiffness using three different methods: (1) small angle neutron scattering (SANS) measurements, (2) oscillatory rheological measurements, and (3) steady rheological measurements with a thermodynamic packing parameter model. Each of these methods are then compared and shown to be consistent with each other for the micelle solutions tested. The consistency of the results across all the measurements suggests that the approach used in this study, which estimates micelle parameters using steady shear rheology and a thermodynamic model, could provide a simpler and more accessible method for estimating micelle parameters in a wide range of surfactant systems.