Surfactants Of Different Chain Lengths Research Articles

Assembly nature and physicochemical variables of a mixture of cetyltrimethylammonium bromide + tetradecyltrimethylammonium bromide in aq. levofloxacin hemihydrate drug medium: A conductivity and theoretical approach

This study describes the development of mixed micelles between two cationic surfactants of different chain lengths, hexadecyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB), in an aqueous solution of levofloxacin hemihydrate (LFH). The LFH is, an antibiotic drug, used to treat many bacterial infections. The conductivity method was used in this investigation at varying temperatures and concentrations of CTAB. From the conductivity versus [surfactant] plots, the critical micelle concentration (CMC) and degree of ionization (g) were determined. The differences between the experimental CMC and its ideal value (CMCid) revealed the presence of interactions amid two employed amphiphiles in creating the mixed micelles. The CMC of the TTAB + CTAB mixed system decreased as the mole fraction (α1) of CTAB was increased at a specific temperature. The values of X1Rub and X1id were larger than the corresponding stoichiometric mole fraction of CTAB (α1), which indicated that CTAB substituted the TTAB molecules from the mixed micelles, and the contribution of CTAB increased in the mixed micelles with increase of α1. The activity coefficients (f1Rub and f2Rub) also showed the contribution of CTAB in the mixed micellization. Thermodynamic variables including excess free energy (ΔGexRub) were computed. The enthalpy (ΔHm0) and entropy (ΔSm0) of the association process indicated the presence of electrostatic (ion–dipole, ion–ion) interactions as well as hydrophobic interactions between the constituents of the study system.

Redox Control of Particle Deposition from Drying Drops.

The coffee-ring effect (CRE), which denotes the accumulation of nonvolatile compounds at the periphery of a pinned sessile drying drop, is a universal and ubiquitous yet complex phenomenon. It is crucial to better understand and control it, either to avoid its various deleterious consequences in many processes requiring homogeneous deposition or to exploit it for applications ranging from controlled particle patterning to low cost diagnostics. Here, we report for the first time the use of a reduction-oxidation (redox) stimulus to cancel the CRE or harness it, leading to a robust and tunable control of particle deposition in drying sessile drops. This is achieved by implementing redox-sensitive ferrocenyl cationic surfactants of different chain lengths in drying drops containing anionic colloids. Varying surfactant hydrophobicity, concentration, and redox state allows us not only to control the overall distribution of deposited particles, including the possibility to fully cancel the CRE, but also to modify the microscopic organization of particles inside the deposit. Notably, with all other parameters being fixed, this method allows the adjustment of the deposited particle patterns, from polycrystalline rings to uniform disks, as a function of the oxidation rate. We show that the redox control can be achieved either chemically by the addition of oxidants or electrochemically by applying a potential for additive-free and reversible actuation in a closed system. This correlation between the redox state and the particle pattern opens a perspective for both redox-programmable particle patterning and original diagnostic applications based on the visual determination of a redox state. It also contributes to clarify the role of surfactant charge and its amphiphilic character in directing particle deposition from drying suspensions.

Hydrogen storage performance of methyl-substituted mesoporous silica with tailored textural characteristics

The present study reports a systematic analysis of morphology and hydrogen sorption capacity of mesoporous organic-inorganic silica prepared by varying the silica synthesis parameters. Cationic surfactants of different chain lengths and different proportions of functionalised precursors were introduced in the synthesis mixture. Improved hydrogen storage properties have been obtained, already below 20 bar, reaching a maximum storage capacity around 4.75 wt% at 80 bar. The textural characteristics were analysed by nitrogen sorption and small angle X-ray scattering, and the thermal stability has been characterized by thermogravimetric analysis. The materials show specific surface area between 400 and 700 m2/g and a weakly ordered mesoporosity induced by the short chain surfactant template.

Synthesis and Properties of Amide Gemini Surfactants

Abstract In the present study, three amide Gemini surfactants of different chain lengths were synthesized, namely N,N-bis-dodecyl-2,6-pyridinium dimethylamide propionate sodium (DLP-12), N,N-bis-tetradecyl-2,6-pyridinium dimethylamide propionate sodium (DLP-14) and N,N-bis-hexadecyl-2,6-pyridinium dimethylamide propionate sodium (DLP-16). Their chemical compositions and structures were characterized by FTIR, 1H NMR and mass spectrometry. Moreover, tensiometry, Langmuir monolayer and conductivity measurements were performed to investigate the micellization and chemical surface properties of the surfactants. The results of the characterization proved that the synthesis of DLP-12, DLP-14 and DLP-16 was successful. Among the investigated surfactants DLP-16 had optimum properties such as a higher emulsification ability as well as a lower surface tension. Furthermore, by calculating the critical packing parameter (CPP), the aggregation morphology of the DLP-12, DLP-14 and DLP-16 was also predicated in this study.

Fluorocarbon versus hydrocarbon organosilicon surfactants for wettability alteration: A molecular dynamics approach

This paper presents a theoretical model based on molecular dynamics to represent and explain the experimental measurements of water droplet contact angles on fluorocarbon and hydrocarbon organosilicon surfactants of different chain lengths. In addition to the contact angles, we evaluated the distribution of the surfactants on the surface (surface density, packing degree, density profile, and coating thickness) and the interaction energies (surfactant–surfactant, surfactant–water, and water–water) to better understand the effect generated by the differences in the molecular structure of the surfactants. We found that the surface densities of the fluorocarbon surfactants were lower than those of hydrocarbon surfactants. This is because fluorocarbon chains are more rigid and because the attractive interaction energy between CF chains is less than that between CH chains because the fluorine–fluorine repulsive electrostatic interactions are stronger than the corresponding hydrogen–hydrogen interactions. The wettability alteration resulted from the modified surface–water interactions. The initial state was hydrophilic (water angle contact of 24°), produced by the high interaction energy between water and the silica surface. Upon application of surfactant, the surface–water interaction energies were considerably lowered, generating hydrophobic surfaces, with water angles contact between 87° and 99°.

The oversolubility of methane gas in nano-confined water in nanoporous silica materials

Abstract The oversolubility of the non-polar methane (CH4) gas in nano-confined liquid in nanoporous silica materials is investigated. A series of mesoporous silica materials with different pore sizes, pore volumes, and different amounts of nano-confined water (H2O) are prepared using a pore-expanding reagent and surfactants of different chain-lengths, and the CH4 absorption by the silica nano-materials is studied at 298 K under low CH4 gas pressures. Analysis of the CH4 absorption data reveals unequivocal evidence for oversolubility of CH4 in the nano-confined H2O of all the hydrated nano-materials. The solubility enhancements are several hundred fold relative to the CH4 solubility in bulk H2O. Interestingly, the enhancements are 25–30% higher when a tricopper cluster complex capable of efficient selective CH4 oxidation under ambient conditions is immobilized into the nano-channels of the silica materials. This dramatic enhancement of the CH4 solubility is attributed to specific CH4 binding to the tricopper cluster complexes embedded within the mesopores of the nanoporous materials.

Use of organo-montmorillonite for the nitrate retention in water: influence of alkyl length of loaded surfactants

The removal of NO3− from water was performed and compared among four organo-montmorillonites (OMt) with quaternary amine surfactants of different chain lengths [octadecyl trimethyl-(ODTMA), hexadecyl trimethyl-(HDTMA), tetradecyl trimethyl-(TDTMA) and dodecyl trimethyl-ammonium (DDTMA)] and loadings [100%, 200%, 400% of the Mt cation exchange capacity (CEC)]. The maximum adsorption of NO3− was attained by the long chain length surfactants (64 and 100 mg NO3− ions per gram of OMt, for HDTMA and ODTMA, respectively) with initial loading of 400% with respect to the CEC of Mt. The short chain length surfactants (DDTMA and TDTMA) did not show adsorption of NO3− except for TDTMA 400 sample. The NO3− adsorption produced a slight expansion of the interlayer thickness of the OMt samples loaded with long-chain surfactants that was assigned to the entrance of NO3− by a synergic effect with these surfactants. The decrease of the negative zeta potential, found for OMt samples relative to that of Mt sample, was attributed to the surfactant chain length rather than to the actual surfactant loaded amounts, which allowed attaining positive zeta potential values for the OMt loaded with long-chain surfactants. The NO3− adsorption on OMt samples caused a slight decrease in the positive zeta potential values, reflecting the external surface coating by NO3−. For long-chain molecules, this behaviour also indicated the formation of greater interactions such as ion pairs between the positive polar group of the surfactant and NO3− ions.

Analytical modeling of micelle growth. 1. Chain-conformation free energy of binary mixed spherical, wormlike and lamellar micelles

HypothesesA quantitative molecular-thermodynamic theory of the growth of giant wormlike micelles of nonionic surfactants can be developed on the basis of a generalized model, which includes the classical “phase separation” and “mass action” models as special cases. The generalized model describes spherocylindrical micelles, which are simultaneously multicomponent and polydisperse in size. TheoryBy analytical minimization of the free-energy functional we derived explicit expressions for the chain-extension and chain-end distribution functions in the hydrocarbon core of mixed micelles from two surfactants of different chainlengths. FindingsThe hydrocarbon core of a two-component micelle is divided in two regions, outer and inner, where the ends of the shorter and longer chains are located. The derived analytical expression for the chain-conformation free energy implies that the mixing of surfactants with different chainlengths is always nonideal and synergistic, i.e. it leads to decrease of the micellar free energy and to enhancement of micellization and micelle growth. The derived expressions are applicable to surfactants with different headgroups (nonionic, ionic, zwitterionic) and to micelles of different shapes (spherical, wormlike, lamellar). The results can be incorporated in a quantitative theory of the growth of giant mixed micelles in formulations with practical applications in detergency.

Organic intercalation of structure modified vermiculite

The experiment used cationic surfactants of different chain lengths to intercalate structure modified vermiculites. The influences of structure modification, chain length and dosage of surfactants on the intercalation behavior of vermiculites were studied, and intercalation mechanism and features of interlayer chemical reactions were discussed. Results indicate that structure modified vermiculites with different layer charge have different intercalation behavior. The basal spacing of the organic intercalated modified vermiculite is the largest when acid concentration used in structure modification is 0.003mol/L, and increases with increasing the chain length and dosage of the organics. Molecular dynamics simulation verifies that interlayer organics align almost parallel to structure layer of vermiculite, with alkyl chain stretching to the middle of interlayer space. –N+ groups of the three surfactants locate above the leached [SiO4], which has stronger interaction with interlayer organic cations. Electrostatic force is the main interaction force between interlayer organics and structure layer of vermiculite, and then is Van der Waals force, no chemical bond formed.

High-Resolution Ultrasonic Spectroscopy Study of Interactions between Hyaluronan and Cationic Surfactants

Interactions in a cationic surfactant-hyaluronan system in water and in sodium chloride solution were investigated by high-resolution ultrasonic spectroscopy at 25 °C. Two alkyltrimethylammonium bromide surfactants of different chain lengths (tetradecyl and hexadecyl) were used; hyaluronan molecular weight ranged from 10 to 1750 kDa. Two main parameters-ultrasonic velocity and attenuation-were measured in the titration regime. Up to six different regions could be identified in the velocity titration profiles in water in a narrow interval of surfactant concentration. These regions differed primarily in the compressibility of structures formed in the system. The number of detected transitions was higher for the tetradecyl surfactant; therefore, the increased length of the hydrophobic chain simplified the details of the structure-forming behavior. The measurement of attenuation was much less sensitive and detected only the formation of microheterogeneous structures or visible phase separates. The richness of the titration profiles was depressed in salt solution, where essentially only two principal regions were observed. On the other hand, the effect of hyaluronan molecular weight on the positions of boundaries between regions was more significant in the presence of salt. Besides electrostatic interactions, hydrophobic interactions are also relevant for determining the behavior of hyaluronan-surfactant systems and the properties of formed complexes (aggregates).

Impact of cationic surfactant chain length during SAPO-11 molecular sieve synthesis on structure, acidity, and n-octane isomerization to di-methyl hexanes

The structure, acidity, and n-octane di-branched isomerization of SAPO-11 synthesized with surfactants of different chain lengths in water–propanol system were investigated. The results showed that with the increasing chain length of surfactants, the crystallite size of SAPO-11 firstly decreased and then increased, but the external surface area and the active B acid sites of SAPO-11 firstly increased and then decreased. The smallest crystallite, the highest external surface area, and the most B acid sites were obtained simultaneously with the surfactant of 12 carbon atoms. The isomerization results of n-octane indicated that among the series Pt/SAPO-11 catalysts, the catalyst with DoTAB-directed SAPO-11 as support had the highest selectivity to di-branched isomers and the lowest cracking selectivity due to the largest external surface area that created more active sites to promote the formation of di-branched isomers by the key-lock mechanism and improve the diffusion of intermediates and products for reducing cracking reactions.

Evidence of Higher Complexes Between Cucurbit[7]uril and Cationic Surfactants

The host-guest assembly of CB7 with a series of alkyl(trimethyl)ammonium (C(n)TA(+)) surfactants of different chain lengths (n=6-18) has been studied. The complexation behaviour was investigated by NMR spectroscopy, isothermal titration calorimetry and kinetics measurements. The combined results of these techniques provided evidence for the formation of 1:1 inclusion and 2:1 external complexes in the cases of C(n)TA(+) with n=12-18. The binding constants for the 1:1 complexes are independent of the alkyl chain length of the surfactant, whereas a relationship between K(2:1) and the chain length of the surfactant was found for the 2:1 complexes.

Correlation between Electrical Conductivity, Relative Humidity, and Pore Connectivity in Mesoporous Silica Monoliths

Incipient wetness was used to load cobalt nitrate into the pores of a set of silica monoliths with pores arranged in a hierarchal fashion, in the form of either a bimodal pore structure containing only macropores and textural mesopores, or trimodal pore structures with additional mesopores as a result of surfactant templating. The volume and diameter of the surfactant templated mesopores were controlled by varying concentration and using surfactants of different chain length, tetradecyl-, hexadecyl-, and octadecyltrimethylammonium bromide (C14TAB, C16TAB, and C18TAB, respectively). Platinum replication of samples made with C14TAB and C18TAB was performed. The native and the cobalt nitrate impregnated monoliths were characterized with nitrogen physisorption, water sorption, and electron microscopy. The electrical conductivity of the cobalt nitrate impregnated monoliths was measured as a function of relative humidity. The presence of the surfactant templated mesopores resulted in significantly higher conductivity at all relative humidities, and generally the conductivity increased as a function of surfactant chain length (going from C14TAB to C18TAB surfactants) and with increasing surfactant concentration in the precursor sols. The findings are suggested to originate from differences in the macroscopic mesopore connectivity throughout the monoliths, a property which is otherwise difficult to assess by other characterization techniques.

How Chain Length and Charge Affect Surfactant Denaturation of Acyl Coenzyme A Binding Protein (ACBP)

Using intrinsic tryptophan fluorescence, equilibria and kinetics of unfolding of acyl coenzyme A binding protein (ACBP) have been investigated in sodium alkyl sulfate surfactants of different chain length (8-16 carbon atoms) and with different proportions of the nonionic surfactant dodecyl maltoside (DDM). The aim has been to determine how surfactant chain length and micellar charge affect the denaturation mechanism. ACBP denatures in two steps irrespective of surfactant chain length, but with increasing chain length, the potency of the denaturant rises more rapidly than the critical micelle concentration (cmc) declines. Increasing proportions of DDM, which significantly reduce the amount of monomeric sodium dodecyl sulfate (SDS), make the first denaturation step occur at lower concentrations but weaken and eventually remove the second denaturation step. The logarithm of the unfolding rate constants increases linearly with denaturant concentration below the cmc but declines at higher concentrations. Both shortening chain length and decreasing micellar charge reduce the overall kinetics of unfolding and makes the dependence of unfolding rate constants on surfactant concentration more complex. This behavior contrasts with the simplicity of unfolding in chemical denaturants and highlights the changing properties of surfactant micelles. We suggest that the transition from spherical to more elongated micelles leads to inhibition of unfolding kinetics, while weaker binding sites may cause a subsequent rise in unfolding rate constants at higher surfactant concentrations. We propose that shifting micellar binding sites on globular proteins such as ACBP, as opposed to the predefined binding sites on membrane protein surfaces, may lead to nonlinear correlations between activation unfolding energies and SDS mole fraction.

Control of Phase Behavior and Properties of Vesicle Gels by Admixing Ionic Surfactants to the Nonionic Surfactant Brij 30

The effect of the addition of two cationic surfactants of different chain length (decyl and dodecyl trimethylammonium bromide, DeTMABr and DTMABr, respectively) and one anionic surfactant of identical chain length (sodium dodecyl sulfate, SDS) on phase behavior, structure, and macroscopic properties of a bilayer forming nonionic surfactant (Brij 30) has been investigated by means of phase studies, rheology, turbidity measurements, dynamic light scattering, and freeze-fracture transmission electron microscopy. We concentrated on DTMABr because of the generically similar behavior for the other ionic surfactants. It is found that already very small amounts of added ionic surfactant have a very pronounced effect on the phase behavior of these systems. The pure nonionic surfactant forms bilayers and has a tendency for the formation of vesicles which becomes enhanced by charging the bilayer through the incorporation of the ionic surfactant. The presence of the ionic surfactant leads to much more viscous systems, which already at a total surfactant concentration of 150 mM become gel-like. For a given surfactant concentration, the elastic properties of the gels increase largely upon the addition of ionic surfactant. This effect is strongly synergistic, requiring only very small amounts of added ionic surfactant, and the elastic properties pass through a maximum for a content of ionic surfactant of about 3-5 mol %. This behavior can be explained in a self-consistent way by a simple rheological model and by combining it with light scattering data. For the addition of larger amounts, the elastic properties decrease again and the formed vesicles become structurally less defined as one is leaving the range of conditions for forming well-defined vesicles, which are required for forming elastic vesicle gels.

Quantitatively Modeling the Equilibrium Properties of Thiol-Decorated Gold Nanoparticles

We employ a molecular mean-field theory to quantitatively understand the sizes, surfactant surface coverage, and size fluctuations of gold nanocrystals decorated with thiol surfactants of different chain lengths. Our model assumes that surfactant-coated nanoparticles are equilibrium structures. We find that packing constraints experienced by the surfactant tails are less significant for more curved (smaller) particles. This effect enables us to rationalize the experimental observations/deductions that the thiol coverage per unit area increases with decreasing particle size. The reduction of surface coverage with increasing size also explains the fact that size polydispersity increases with increasing nanoparticle size. We find that increasing the length of the surfactants results in larger nanoparticles.

Premicellar and micelle formation behavior of dye surfactant ion pairs in aqueous solutions: Deprotonation of dye in ion pair micelles

The premicellar and micelle formation behavior of dye surfactant ion pairs in aqueous solutions monitored by surface tension and spectroscopic measurements has been described. The measurements have been made for three anionic sulfonephthalein dyes and cationic surfactants of different chain lengths, head groups, and counterions. The observations have been attributed to the formation of closely packed dye surfactant ion pairs which is similar to nonionic surfactants in very dilute concentrations of the surfactant. These ion pairs dominate in the monolayer at the air–water interface of the aqueous dye surfactant solutions below the CMC of the pure surfactant. It has been shown that the dye in the ion pair deprotonates on micelle formation by the ion pair surfactants at near CMC but submicellar surfactant concentrations. The results of an equilibrium study at varying pH agree with the model of deprotonated 1:1 dye–surfactant ion pair formation in the near CMC submicellar solutions. At concentrations above the CMC of the cationic surfactant the dye is solubilized in normal micelles and the monolayer at the air–water interface consists of the cationic surfactant alone even in the presence of the dyes.

Modulating the Emission Intensity of Poly-(9,9-bis(6‘-N,N,N-trimethylammonium)hexyl)-Fluorene Phenylene) Bromide Through Interaction with Sodium Alkylsulfonate Surfactants

The interaction between the cationic HTMA-PFP (Poly-(9,9-bis(6'-N,N,N-trimethylammonium)hexyl-fluorene phenylene) bromide) and oppositely charged sodium n-alkyl sulfonate surfactants of different chain lengths has been studied in DMSO-water solutions (4% v/v) by UV-visible absorption, fluorescence spectroscopy, fluorescence lifetimes, electrical conductivity, and (1)H NMR spectroscopy. Polymer-surfactant interactions lead to complex spectroscopic behaviors which depends on surfactant concentration. At low surfactant concentrations, the observed strong static fluorescence quenching of fluorescence seems to be associated with formation of aggregates between polymer chains neutralized through interaction with surfactants. This is supported by conductivity and by analysis of absorption spectra deconvoluted at each surfactant concentration using an adapted iterative method. In contrast, above the surfactant critical micelle concentration, there is a strong fluorescence enhancement, leading in some cases to higher intensities than in the absence of surfactants. This is attributed to the transformation of the initially formed aggregates into some new aggregate species involving surfactant and polymer. These changes in HTMA-PFP fluorescence as a function of n-alkyl sulfonate concentration are important for the general understanding of polymer-surfactant interactions, and the aggregates formed may be important as novel systems for applications of these conjugated polyelectrolytes.

X‐ray Diffraction Study of the Structure of Carboxymethylcellulose–Cationic Surfactant Complexes

We have investigated dilute aqueous solutions of an anionic polymer (carboxymethylcellulose) mixed with cationic surfactants of different chain lengths (dodecyl to octadecyl trimethylammonium bromides: DTAB, TTAB, CTAB and OTAB). The structures of the concentrated phases formed above the precipitation threshold were studied by X-ray diffraction. Different body-centred cubic structures with space groups Pm3n were observed in the presence of surfactant with a short aliphatic chain (DTAB), despite the fact that the polymer persistence length is comparable to the repeat distance of the structure (5 nm). For larger surfactant chain lengths (TTAB and CTAB), the structure of the precipitates can be either cubic (Pm3n) or 2D hexagonal depending on the initial surfactant and polymer concentrations. For still larger chain length (OTAB), the structure becomes lamellar. This structural evolution from micellar cubic towards 2D hexagonal and lamellar is attributed to the decrease of the local curvature of the surfactant aggregates, as observed for flexible synthetic polymers and short DNA fragments under similar conditions. Furthermore, the structure of the bulk complexes formed just below the precipitation threshold anticipates the structure seen in the precipitated phases.

Theoretical estimation of stability ratios for hexadecane-in-water (H/W) emulsions stabilized with nonylphenol ethoxylated surfactants

The effect of steric interactions on the stability of oil-in-water emulsions is studied here by means of emulsion stability simulations (ESS). For this purpose, a new steric potential based on a modification of the one formerly proposed by Vincent et. al. [Colloids Surf. 18 (1986) 261] is employed. The parameters of the calculation correspond to hexadecane in water emulsions stabilized with nonylphenol ethoxylated surfactants of different chain lengths (NPE m). Stability ratios ( W) were calculated using the half life time of the number of drops per unit volume of these systems. A functional relationship between W and the repulsive potential barrier, (Δ V), similar to the one previously found by Prieve and Ruckenstein for electrostatically stabilized suspensions [J. Colloid Interface Sci. 73 (1980) 539] was obtained. However, according to our simulations there exists a threshold for the stability of emulsions with respect to coalescence which is approximately located around 12.7 k B T .