Surfactant Aggregation Number Research Articles

Influence of Hydrophobic and Hydrophilic Chain Length of CiEj Surfactants on the Solubilization of Active Pharmaceutical Ingredients.

Up to 90% of all newly developed active pharmaceutical ingredients (APIs) are poorly water soluble, most likely also showing a low oral bioavailability. In order to increase the aqueous solubility of these APIs, surfactants are promising excipients to increase both solubility and consequently bioavailability (e.g., in lipid- and surfactant-based drug delivery systems). In this work, we investigated the influence of hydrophobic and hydrophilic chain lengths of CiEj surfactants (C8E6, C10E6, and C10E8) toward the solubilization of fenofibrate, naproxen, and lidocaine. Furthermore, we investigated the partitioning of these APIs between the surfactant aggregates and the surrounding aqueous bulk phase. For all APIs considered, we determined the locus of API solubilization as well as the individual aggregation numbers (Nagg) of surfactants and API molecules in an API/surfactant aggregate. We further determined the hydrodynamic radius (Rh) of the API/surfactant aggregates in the absence and presence of the APIs. The size of the API/surfactant aggregates (Nagg, Rh) passes through a minimum upon lidocaine solubilization; it gradually increases upon naproxen solubilization and is almost constant upon fenofibrate solubilization. The results give valuable insights into the solubilization mechanisms of APIs in the CiEj surfactant aggregates. Our results reveal that fenofibrate is solely solubilized in the hydrophobic core of the CiEj surfactant aggregates, as only the hydrophobic chain length of the surfactant influences its solubilization. Naproxen is solubilized in the palisade layer of the surfactant aggregates, as both the hydrophobic and hydrophilic chain lengths are decisive for its solubilization. Lidocaine is mainly solubilized in the rather hydrophilic corona region of the surfactant aggregates, as the hydrophilic chain length of the surfactant governs its solubilization. The results further reveal that the hydrophilic/lipophilic balance is not an appropriate measure to estimate the solubilization capacity of surfactant aggregates.

Transport Properties of Surfactants in Micellar Systems

A methodological supplement has been proposed to author previous paper on the correlation between the mobility and diffusion coefficient (Colloid J., 2016, vol. 78, p. 88). Now, the self-diffusion coefficient has been added to the aforementioned parameters. It has been found when it coincides with the diffusion coefficient and when not. The correlation between the three transport properties has been established, and appropriate schemes of their experimental study have been considered. The computational relations based on the traditional assumption of constant aggregation numbers have been generalized taking into account recent results of studying the effect of variability of surfactant aggregation numbers.

Micellization theory based on the law of mass action with a variable aggregation number

Variations in the aggregation number of spherical micelles are considered within the micellization theory based on the law of mass action. The mechanism of micellization in a polydisperse aggregated system and the transition to a monodisperse model are explained. A relation between aggregation numbers and chemical potentials of molecules or ions is determined using the curve for equilibrium distribution of aggregates over the aggregation numbers. It is shown that the aggregation numbers of nonionic surfactants unambiguously grow with concentration; however, such a conclusion cannot be drawn for ionic surfactants. For the explicit concentration dependence of the aggregation number, two versions of an analog of the Langmuir equation are proposed to be used, i.e., versions comprising the total surfactant concentration and the concentration of monomers. Comparison with experimental data is carried out by the example of conventional surfactants, namely, sodium dodecyl sulfate and hexadecyltrimethylammonium bromide.

On the calculation of diffusion coefficients and aggregation numbers of nonionic surfactants in micellar solutions

The calculation of the diffusion coefficients of nonionic surfactants as functions of their concentrations in micellar solutions has been analyzed within the framework of the quasi-chemical version of the law of mass action. The methods of the introduction of initial calculation parameters, calculation scheme for an ideal mixture of monomeric molecules and micelles, and corrections for varying solution viscosity have been considered. Numerical estimations have been performed using aqueous tetraoxyethylene octyl ether, pentaoxyethylene hexyl ether, and octyl-β-D-glucopyranoside solutions as examples.

Acquisition of Water Solubility Diagrams in Ternary Systems (AOT/Organic Solvent/Alcohol) and Extraction of α‐Lactalbumin Using Reverse Micellar Systems

AbstractIn this study water solubility curves were constructed and calorimetric measurements obtained for reverse micellar systems consisting of an alcohol (isopropanol or butanol), surfactant (AOT) and organic solvent (isooctane or hexane). Also evaluated were the effects of alcohol and solvent type and surfactant concentration on the extraction of the α‐lactalbumin (α–la). From the obtained solubility diagrams for ternary systems, it was concluded that isooctane presented the highest water solubility capacity in the center of the micelle systems with hexane, since isooctane has greater molecular volume and greater effect of the surfactant aggregation number. With respect to the alcohols, it was observed that isopropanol and butanol act in the system as a co‐surfactant, since they prefer to adsorb at the water/solvent interface. It was also verified that butanol improved water solubility inside the reverse micellar due to its contribution to increase the critical packing parameter. The amount of α‐la extracted increased proportionally with the AOT concentration for systems with isooctane and hexane. However, for systems with the latter solvent, the concentration of extracted protein first increases and then decreases. The extraction power of reverse micellar systems with isooctane was influenced by the type of alcohol with butanol showing better results. For systems containing hexane there was no effect of the alcohol added to the system on extraction power of α‐lactalbumin.

Structural Features of Micelles of Zwitterionic Dodecyl-phosphocholine (C₁₂PC) Surfactants Studied by Small-Angle Neutron Scattering.

Small-angle neutron scattering (SANS) was used to investigate the size and shape of zwitterionic dodecyl phosphocholine (C12PC) micelles formed at various concentrations above its critical micelle concentration (CMC = 0.91 mM). The predominant spherical shape of micelles is revealed by SANS while the average micellar size was found to be broadly consistent with the hydrodynamic diameters determined by dynamic light scattering (DLS). Cryogenic tunneling electron microscopy (cryo-TEM) shows a uniform distribution of structures, proposing micelle monodispersity ( Supporting Information ). H/D substitution was utilized to selectively label the chain, head, or entire surfactant so that structural distributions within the micellar assembly could be investigated using fully protonated, head-deuterated, and tail-deuterated PC surfactants in D2O and fully deuterated surfactants in H2O. Using the analysis software we have developed, the four C12PC contrasts at a given concentration were simultaneously analyzed using various core-shell models consisting of a hydrophobic core and a shell representing hydrated polar headgroups. Results show that at 10 mM, C12PC micelles can be well represented by a spherical core-shell model with a core radius and shell thicknesses of 16.9 ± 0.5 and 10.2 ± 2.0 Å (total radius 27.1 ± 2.0 Å), respectively, with a surfactant aggregation number of 57 ± 5. As the concentration was increased, the SANS data revealed an increase in core-shell mixing, characterized by the emergence of an intermediate mixing region at the spherical core-shell interface. C12PC micelles at 100 mM were found to have a core radius and shell thicknesses of 19.6 ± 0.5 and 7.8 ± 2.0 Å, with an intermediate mixing region of 3.0 ± 0.5 Å. Further reduction in the shell thickness with concentration was also observed, coupled with an increased mixing of the core and shell regions and a reduction in miceller hydration, suggesting that concentration has a significant influence on surfactant packing and aggregation within micelles.

Self-assembly of sodium bis(2-ethylhexyl) sulfosuccinate in aqueous solutions: Modulation of micelle structure and interactions by cyclodextrins investigated by small-angle neutron scattering

The modulating effects of α-cyclodextrin (α-CD) and hydroxypropyl β-cyclodextrin (HP-β-CD) on micelles formed in water by the double tail surfactant sodium bis(2-ethylhexyl) sulfosuccinate (also known as dioctyl sulfosuccinate (DOSS) or Aerosol-OT (AOT)) have been investigated using small angle neutron scattering (SANS). SANS data were fitted to the core–shell oblate ellipsoid form factor and Hansen–Hayter-based structure factor in order to quantify the effect of cyclodextrins on the micelle structure and inter-micelle interactions. The results suggested that addition of cyclodextrins caused a sharp decrease in surfactant aggregation numbers, micelle dimensions and micelle charge. The micelle shape changed from elongated oblate to sphere upon addition of cyclodextrin at stoichiometric ratio of 1:1 CD:AOT for HP-β-CD and 2:1 for α-CD. HP-β-CD was found more effective than α-CD in influencing the micelle properties. For example, the decrease in surfactant aggregation number upon addition of 1mM CD at constant total surfactant concentration was about double for HP-β-CD compared to α-CD. The results are consistent with the localization of small amount of cyclodextrins in the micelle, while the remainder stays in the solution pseudophase. The present study offers an improved understanding on the modulating effects of CDs on micelle structure and enables better design of emulsifiers or dispersants in formulations suitable for controlled release of actives.

Surfactant-polymer interactions: molecular architecture does matter.

Polymer-surfactant mixtures are found in many industrial formulations, and hence there is a significant interest in understanding, at a molecular level, how the self-assembly of surfactant is affected by oppositely-charged polyelectrolytes (PEs). We use self-consistent field modeling and show that the modes of interaction of PEs strongly depend on the architecture of the PE on the segmental level. Hydrophilic cationic PEs with their charge proximal to the linear backbone are expected to bind electrostatically to the outsides of the coronas of the spherical micelles of anionic surfactants, such as sodium laureth sulphate (SLES). As a result, the surfactant aggregation number increases, but at the same time the colloidal stability deteriorates, due to bridging of the PEs between micelles. PEs with their charge somewhat displaced from the backbone by way of short hydrophobic spacers, are expected to be present inside a micelle at the core-corona boundary. In this case the aggregation number decreases, yet the colloidal stability is retained. Hence, SLES tends to remove hydrophilic PEs from an aqueous solution, whereas it solubilizes more hydrophobic ones. The binding isotherm shows that the uptake of PEs remains typically below charge compensation and in this case the spherical micelle topology remains the preferred state.

A Simple Approach to Calculate the Micelle Aggregation Numbers of Ionic Liquid-Based Surfactants: Electrochemical Behavior of Aggregate-Solubilized Ferrocene Studied by Microelectrode Voltammetry

Micellar diffusion coefficient (DMDC), micellar hydrodynamic radius (RH) and average aggregation numbers (Nagg) of 1-alquil-3- methylimidazolium chloride (CxMeImCl x = 12, 14, 16) surfactants in aqueous solution have been determined by using voltammetric information obtained with disk microelectrodes. Ferrocene was used as redox-active electrochemical probe. DMDC values determined by extrapolation of the plot of diffusion coefficient versus surfactant concentration to the critical micelle concentration were found to be in good agreement with the literature values, based on static light scattering and theoretical calculations.

Studies on the Hydrodynamic Diameter and Aggregation Number of Nonionic Switchable Surfactant

An Investigation of the micelle aggregation number (N) and hydrodynamic diameter (Dh) is conducted on nonionic switchable surfactant 11-ferrocenylundecyl polyoxyethylene ether (FPEG), the micelle aggregation number of FPEG solution is determined by steady-state fluorescence probe, the results show that the micelle aggregation number (N) increase linearly with the concentration range from 5 to 12 times CMC of FPEG solutions. The critical micelle aggregation number Nmwhich means that the aggregation number of surfactant at the concentration of CMC is calculated to be 66. The hydrodynamic diameter (Dh) of FPEG micelle is determined by dynamic light scattering (DLS), the measurement angle is fixed at 90°and the wavelength is 532nm, the distribution data analyses by NNLS (non-negatively constrained least square) algorithm, the results reveal that the mean value of Dhat the concentration of 0.5 mmolL-1is 7.8nm. the POLY value, which used to explain the micelle polydispersity in the aqueous solution, is measured and the results show that the POLY values are slightly different, this indicates that the micelle aggregation morphology of FPEG is polydispersity in the aqueous solution.

Interfacial composition, thermodynamic properties, and structural parameters of water-in-oil microemulsions stabilized by 1-pentanol and mixed surfactants

The present study is focused on the evaluation of the interfacial composition, thermodynamic properties, and structural parameters of water-in-oil mixed surfactant microemulsions [(cetylpyridinium chloride, CPC+polyoxyethylene (20) cetyl ether, Brij-58 or polyoxyethylene (20) stearyl ether, Brij-78)/1-pentanol/n-heptane, or n-decane] under various physicochemical environments by the Schulman method of cosurfactant titration of the oil/water interface. The estimation of the number of moles of 1-pentanol at the interface (nai) and bulk oil (nao) and its distribution between these two domains at the threshold level of stability have been emphasized. The thermodynamics of transfer of 1-pentanol from the continuous oil phase to the interface have been evaluated. nai,nai, standard Gibbs free energy (ΔGt0), standard enthalpy (ΔHt0), and standard entropy (ΔGt0) of transfer process have been found to be dependent on the molar ratio of water to surfactant (ω), type of nonionic surfactant and its content (XBrij-58 or Brij-78), oil and temperature. A correlation between (ΔHt0) and (ΔSt0) is examined at different experimental temperatures. Bulk surfactant composition dependent temperature insensitive microemulsions have been reported. Associated structural parameters, such as droplet dimensions and aggregation number of surfactant and cosurfactant at the droplet interface have been evaluated using a mathematical model after suitable modifications for mixed surfactant systems. In light of these parameters, the prospect of using these microemulsion systems for the synthesis of nanoparticles and the modulation of enzyme activity has been discussed. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.

Gas phase charged aggregates of bis(2-ethylhexyl)sulfosuccinate (AOT) and divalent metal ions: first evidence of AOT solvated aggregates

Assembling and chelating properties of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) towards divalent metal ions have been investigated in the gas phase by electrospray ionization mass spectrometry. A variety of positively charged monometallated and mixed metal aggregates are formed. Interestingly, several ions contain solvent (MeOH, H(2)O) molecules and constitute the most abundant AOT cationic aggregates not containing sodium. These species are the first example of solvated AOT-metal ion aggregates in the gas phase. By increasing the surfactant aggregation number, the abundance of solvated species becomes lower than that of unsolvated ones. Decompositions of ionic species have been studied by tandem mass spectrometry, and their stability has been determined through energy resolved mass spectrometry. In contrast with positively charged AOT-alkaline metal ion aggregates, whose decompositions are dominated by the loss of individual surfactant molecules, AOTNa-divalent ion aggregates mainly dissociate through the cleavage of the AOT H(2)C-O bond followed by further intramolecular fragmentations. This finding, that is consistent with an enhanced chelation of divalent ions with AOT(-) head groups, has been taken as an indication that such aggregates are characterized by a reverse micelle-like organization with a ionic core formed by the metal cations interacting with the negatively charged surfactant polar heads, whereas the surfactant alkyl chains point outside.

Self-Assembly of Hydrophobin and Hydrophobin/Surfactant Mixtures in Aqueous Solution

The self-assembly of the protein hydrophobin, HFBII, and its self-assembly with cationic, anionic, and nonionic surfactants hexadecylterimethyl ammonium bromide, CTAB, sodium dodecyl sulfate, SDS, and hexaethylene monododecyl ether, C(12)E(6), in aqueous solution have been studied by small-angle neutron scattering, SANS. HFBII self-assembles in solution as small globular aggregates, consistent with the formation of trimers or tetramers. Its self-assembly is not substantially affected by the pH or electrolytes. In the presence of CTAB, SDS, or C(12)E(6), HFBII/surfactant complexes are formed. The structure of the HFBII/surfactant complexes has been identified using contrast variation and is in the form of HFBII molecules bound to the outer surface of globular surfactant micelles. The binding of HFBII decreases the surfactant micelle aggregation number for increasing HFBII concentration in solution, and the number of hydrophobin molecules bound/micelle increases.

A Mechanistic Study of the Selective Retention of SDS-Suspended Single-Wall Carbon Nanotubes on Agarose Gels

Elution chromatography through columns packed with agarose beads has been used to separate metallic from semiconducting single-wall carbon nanotubes (SWCNTs). Prior studies have attributed the separation to either selective adsorption or size-exclusion (due to selective aggregation) of semiconducting SWCNTs. Initial SWCNT suspensions with different aggregation states were prepared to test these compet- ing theories. Retention characteristics of the SWCNT suspen- sions were not affected by changes to aggregation state, except when the centrifugation time was short and aggregation ex- cessive. On the other hand, selective adsorption of nanotubes on the agarose matrix is confirmed by modifying the surfactant structure around the SWCNTs without changing the aggregation state of the suspension. In addition, salt-modifiers and solvent- modifiers allow systematic changes to the surfactant aggregation number, orientation, and sidewall coverage. The retention characteristics from these modified SWCNT suspensions suggest that surfactant orientation rather than the exposed regions on the surface of the nanotubes is the dominant factor in the adsorption process.

Micellar Properties and Related Thermodynamic Parameters of the 14-6-14, 2Br− Gemini Surfactant in Water + Organic Solvent Mixed Media

Micellar parameters such as critical micelle concentration (cmc), degree of counterion dissociation (α), and aggregation number (Nagg) of the cationic gemini surfactant hexamethylene-1,6-bis(tetrad...

Interfacial Behavior, Structure, and Thermodynamics of Water in Oil Microemulsion Formation in Relation to the Variation of Surfactant Head Group and Cosurfactant

Interfacial behavior, structural, and thermodynamic parameters in relation to the formation of water-in-oil (w/o) microemulsion (μE) with varied surfactant head groups and cosurfactants have been evaluated through dilution technique at different temperature and [water]/[surfactant] mole ratio. Dodecyltrimethylammonium bromide (DTAB), sodium dodecylsulphate (SDS), and polyoxyethylene sorbitan monolaurate (Tween-20) were used as surfactants and n-butanol and n-pentanol were used as cosurfactants. Distribution of cosurfactants between bulk oil and the interface using fixed amount of surfactant at varied [water]/[surfactant] mole ratio and temperature has been studied to evaluate thermodynamic parameters. Associated structural parameters, such as droplet dimension and aggregation number of surfactant and cosurfactant at the droplet interface, have also been evaluated. Spontaneity of the μE formation followed the order DTAB>SDS>Tween-20 for both butanol and pentanol in the studied range of temperature. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.

Effect of Graft Density on the Nonionic Bottle Brush Polymer/Surfactant Interaction

The effect of graft density on the interaction of nonionic bottle brush polymers with an anionic surfactant (sodium dodecyl sulfate) was investigated. The graft density of 45 units long poly(ethylene oxide) (PEO) side chains was varied in a wide range (30, 50, 75, 90, and 100%) on a methacrylate type polymer backbone. The surfactant binding isotherms were determined by the potentiometric method in the presence of 0.1 M sodium bromide. It was found that due to the grafting of the PEO chains to a polymer backbone the surfactant binding becomes significantly suppressed. The amount of bound surfactant at the critical micelle concentration (cmc) decreases almost 2 orders of magnitude compared to the binding on a linear PEO having a similar molecular weight. The binding of the surfactant was found to occur in cooperative fashion, though the critical aggregation concentration (cac) of the binding was found surprisingly small. This result was interpreted in terms of the surfactant aggregation numbers that were found much smaller in the case of the bottle brush polymers than in the case of linear PEOs due to the steric crowding of the grafted PEO chains. To confirm the results of the binding isotherm measurements, steady-state fluorescence probe (pyrene) measurements as well as static and dynamic light scattering measurements were performed.

Surfactant Self-Assembly in Oppositely Charged Polymer Networks. Theory

The interaction of ionic surfactants with polyion networks of opposite charge in an aqueous environment is analyzed theoretically by applying a recent theory of surfactant ion-polyion complex salts (J. Colloid. Int. Sci. 2009, 332, 183). The theory takes into account attractive and repulsive polyion-mediated interactions between the micelles, the deformation of the polymer network, the mixing of micelles, polyion chains, and simple ions with water, and the hydrophobic free energy at the micelle surface. The theory is used to calculate binding isotherms, swelling isotherms, surfactant aggregation numbers, compositions of complexes,and phase structure under various conditions. Factors controlling the gel volume transition and conditions for core/shell phase coexistence are investigated in detail, as well as the influence of salt. In particular, the interplay between electrostatic and elastic interactions is highlighted. Results from theory are compared with experimental data reported in the literature. The agreement is found to be semiquantitative or qualitative. The theory explains both the discrete volume transition observed in systems where the surfactant is in excess over the polyion and the core/shell phase coexistence in systems where the polyion is in excess.

Dilution method study on the interfacial composition, thermodynamic properties and structural parameters of W/O microemulsions stabilized by 1-pentanol and surfactants in absence and presence of sodium chloride

The phase behaviors, interfacial composition, thermodynamic properties and structural characteristics of water-in-oil microemulsions under varied molar ratio of water to surfactant ( ω) at 303 K and also by varying temperatures at a fixed ω ( = 40 ) by mixing with 1-pentanol and decane or dodecane in absence and presence of sodium chloride have been studied by the method of dilution. The surfactants used were cetyl pyridinium chloride (CPC), sodium dodecyl sulfate (SDS) and polyoxyethylene (23) lauryl ether (Brij-35). The compositions of 1-pentanol and the surfactant at the interfacial region, the distribution of 1-pentanol between the interfacial region and the continuous oil phase, and the effective packing parameter ( P eff ) at the threshold level of stability have been estimated. The thermodynamics of transfer of 1-pentanol from the continuous oil phase to the interface have been evaluated. The structural parameters viz. radii of the droplet ( R e ) and the waterpool ( R w ), effective thickness of the interfacial layer ( d I ), average aggregation numbers of surfactants ( N ¯ s ) and the cosurfactant (1-pentanol) ( N ¯ a ) and the number of droplets ( N d ) have also been estimated. The prospect of using these w/o microemulsions for the synthesis of nanoparticles with small size, have been discussed in the light of the radii of the droplet, and waterpool, the extent of variation of effective thickness of the droplet under varied molar ratio of water to surfactant and temperature. An attempt has been made to rationalize the results in a comprehensive manner.

Determination of the physico-chemical parameters and aggregation number of surfactant in micelles in binary alcohol–water mixtures

Abstract The effect of ethanol on the micellization of the anionic surfactant, sodium dodecyl sulfate (SDS), was investigated using potentiometric and pulsed field gradient–NMR spectroscopic techniques. Potentiometry studies showed that the critical micelle concentration (CMC) of surfactant decreases to a minimum value at around 10% ethanol (v/v) because of the co-surfactant effect. The mean values of the hydrodynamic radius, Rh, and aggregation number, NA, of micelles were determined by a combination of viscosity and self-diffusion coefficient measurements. The viscosity of the micellar solutions was approximately independent of ethanol concentration, indicating that the presence of the alcohol does not induce substantial changes in the micellar structure. The number of attachment of ethanol in the micellar core and the degree of dissociation of the counterion were evaluated using a potentiometric technique for alcohol concentrations between 5% and 40% v/v. In addition, an explicit expression is developed for the average size of micelles formed in aqueous solvents containing alcohol.