Alkyl Tails Of Surfactants Research Articles

Influence of water saturation on interlayer properties of HDTMA-, HDTMP-, and HDPy-modified montmorillonite organoclays

Surfactant-modified clay minerals, organoclays, are well-studied and used commercially for removing contaminants from water. Nevertheless, dry organoclay properties are typically used to understand and predict contaminant sorption. In this work, the effects of water saturation on two properties important to contaminant sorption were examined: organoclay interlayer space and surfactant alkyl tail conformational ordering. Montmorillonite was modified to 0.25–3.0 CEC using three surfactants: hexadecyltrimethylammonium (HDTMA), hexadecylpyridinium (HDPy), and hexadecyltrimethylphosphonium (HDTMP). At low surfactant-loadings (< 1 CEC and < 15% fOC), water saturation expanded the maximum organoclay interlayer by 4–8 Å, depending on the surfactant and its loading but had little effect on surfactant tail conformational ordering. At low surfactant loadings, water saturation likely promotes contaminant sorption relative to expectations based on dry organoclay properties, particularly for larger contaminants, due to a relaxation of the physical constraints of the interlayer. At higher surfactant loadings, HDTMA and HDTMP organoclays showed relatively small effects of water saturation, but effects were significant for HDPy organoclays. The maximum interlayer space of the HDPy organoclay at 20.7% fOC expanded 3-fold (+22.3 Å) upon hydration, which is attributed to a substantial rearrangement of the pyridinium headgroup. All HDPy organoclays with fOC > 20% experienced a notable increase in the alkyl tail conformational ordering upon hydration, becoming fully solid-like, even as negligible interlayer expansion occurred at the highest loadings. At high HDPy-loading water saturation would likely restrict sorption of contaminants relative to what might be expected based on dry properties, due to the energetic barrier of cavity formation in the more solid-like interlayer phase. This work provides evidence from three types of organoclays that water-saturated organoclay properties can differ substantially from those of dry organoclays. Characterization of organoclays under water-saturated conditions that are relevant to contaminant sorption will facilitate our understanding and improvement of organoclay performance.

Self-aggregation of cationic gemini surfactants with amide groups in the spacer and variable alkyl chain length

AbstractSynthesis, aggregation parameters and antimicrobial activity of novel cationic gemini surfactants with two amide groups in gemini spacer structure and a variable number of carbon atoms in alkyl tails ranging from 12 to 15 are reported. The critical micelle concentration of gemini surfactants was determined using surface tension and electrical conductivity methods. The cmc values were found in the range 0.83 to 0.06 mM. The interfacial area, micelle ionisation degree and the Gibbs free energy per molecule and alkyl chain were calculated from the surface tension and conductivity curves. Particle size analysis using the dynamic light scattering method confirmed the formation of small spherical micelles 6–7 nm large in size for gemini surfactants with 12 and 13 carbon atoms in the alkyl chain. A large size above 50 nm was found for the aggregates composed of long-chain gemini molecules with 14 and 15 carbon atoms. The zeta potential of gemini surfactants shows a continuous increase with the increasing alkyl chain length. Micelle aggregation number of gemini surfactants correlates well with the hydrodynamic size data. Small aggregation number values were found for short-chain gemini molecules with 12 and 13 carbon atoms in the alkyl chain. Long-chain gemini molecules with 14 and 15 carbon atoms exhibit aggregate growth represented by an increase in the aggregation number values while maintaining the spherical or spheroidal shape of micelles. The investigations of antimicrobial activity against Gram-positive bacteria, Gram-negative bacteria and yeast indicate the increasing antimicrobial efficiency towards the short-chain surfactant with 12 carbon atoms in the alkyl chain. A possible cut-off effect presence is proposed to explain the dependence of antimicrobial activity on the surfactant alkyl tail length.

Model of the charged mosaic surface of the cement particle based on the adsorption behavior of surfactants using ATR-FTIR spectroscopy

A charged mosaic structure is often introduced to explain the adsorption behavior of ionic additives in cementitious materials, no matter whether the apparent zeta potential of the solid phase is positive or negative. However, an in-depth model of this mosaic structure has not been developed. In this study, the difference spectra of selected cationic, anionic and nonionic surfactants adsorbing onto cement particles were investigated using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. For concentration-resolved ATR-FTIR spectra by adding cement particles to surfactant solutions, and for time-resolved ATR-FTIR spectra by adding cement particles to the anionic surfactant/Ca(OH)2 mixtures, specific features appear at ~2920 and ~2850 cm−1. If these bands are positive, it indicates that an ordered structure of the surfactant alkyl tail is formed, and if these bands are negative, a disordered structure is formed. Therefore, to explain the adsorption process, a charged mosaic surface model of the cement particle is developed. The results indicate that the heterogeneously charged mosaic surface consists of negatively charged parts which have higher charge density and are relatively isolated, and positively charged parts which have lower charge density and are relatively continuous. This structure arises from the distribution of hydrate phases with different electrical properties.

Direct Observation of Adsorption Morphologies of Cationic Surfactants at the Gold Metal-Liquid Interface.

Understanding interfacial phenomena is important in processes like corrosion, catalysis, and electrochemical reactions. Specifically, in corrosion inhibition, the assembly of adsorbed surfactants at metal-water interfaces in well-packed, ordered layers is desired. We provide direct evidence of the role of alkyl tails of surfactants in the formation of ordered adsorbed layers at metal-water interfaces. We have employed surface-specific sum frequency generation (SFG) spectroscopy to probe the in situ adsorption and self-assembly of cationic surfactants, alkyldimethylbenzyl ammonium bromides of tail lengths n = 4 (C4) and 12 (C12), without any applied potential or stimulus, at the gold-water interface. Our SFG measurements show that C12 Quat adsorbs as an ordered monolayer, whereas C4 Quat adsorbs in a disordered monolayer. All-atom molecular dynamics (MD) simulations of these surfactants corroborate with SFG results. These findings provide new insights on how hydrophobic interactions between alkyl tails of surfactants affect their self-assembly at metal-water interfaces.

Nanoencapsulation of food bioactives in supramolecular assemblies based on cyclodextrins and surfactant

The bioavailability of poorly soluble food ingredients can be achieved by their encapsulation into the 3D macrocyclic structure or self-assembled surfactant micelles. In this study, the mixed self-assembly of cyclodextrins (CDs) and cationic surfactant, 1-hexadecyl-4-(2-(1-methylpyridin-1-ium-4-yl)vinyl)pyridin-1-ium chloride, was studied by a set of physicochemical methods. It was shown using tensiometry and conductometry that the presence of CDs in the micellar medium of this surfactant increases the critical aggregation concentration. Moreover, NMR spectroscopy and spectrophotometry revealed the interaction between the alkyl tail of surfactant and the internal cavity of CDs. It is interesting to note that in the case of γ-CD and HP-β-CD there was additional binding by the vinyl group of surfactant. The observed patterns of aggregation behavior were confirmed by the method of solubilization of hydrophobic substances. Spectrophotometric monitoring of the solubilization of hydrophobic flavonoids (rutin and quercetin) demonstrated that mixed assemblies enriched with surfactant are capable of binding the flavonoid, while they show poor binding ability towards flavonol compound in the presence of a small amount of surfactant.. Variation of the component ratio in the supramolecular CD–surfactant system also allows adjusting the biological properties. Systems with an excess of CD are less toxic to the cells of test microorganisms, herewith, an increase of the surfactant content in a mixture with CD leads to an increase in both antimicrobial and hemolytic activities. Therefore, when using CD complex-based supramolecular approach for food safety, it is necessary to take into account the toxic properties of the base components of the supramolecular mixture.

Counterion-Dependent Access to Low-Symmetry Lyotropic Sphere Packings of Ionic Surfactant Micelles.

The water-driven self-assembly of homologous dianionic surfactants into lyotropic liquid crystals (LLCs) is investigated, with a focus on understanding how surfactant headgroup and counterion identities guide supramolecular spherical mesophase selection. Using temperature-dependent small-angle X-ray scattering (SAXS), we demonstrate that 2-alkylmalonate surfactants (CnMal-M2) with n = 8 (octyl) or 10 (decyl) and M = K+, Cs+, or (CH3)4N+ form both simple and complex micelle packings. Observed spherical morphologies include body-centered cubic (BCC), hexagonally closest-packed (HCP), and tetrahedrally closest-packed Frank-Kasper (FK) A15 and σ phases (Pm3(-)n and P42/mnm symmetries, respectively). Previously observed in only one other minimally hydrated surfactant, the σ phase is a rare LLC morphology comprising a low-symmetry unit cell containing 30 sub-2-nm quasispherical micelles, each of which belongs to one of five symmetry-equivalent classes with discrete aggregation numbers. Temperature versus water concentration phase maps for CnMal-M2 LLCs reveal that σ-phase formation depends sensitively on the size and polarizability of the surfactant counterion and the length of the surfactant alkyl tail. These observations are rationalized in terms of a delicate interplay between global packing symmetry and local particle symmetry, and the extent to which counterion-headgroup correlations enforce the latter structures in these LLC phases.

Tuning the position of head groups by surfactant design in mixed micelles of cationic and carbohydrate surfactants

HypothesisEmerging applications of carbohydrate/cationic surfactant mixtures require not only synergistic mixing, but also accessible sugar headgroups at the exterior of micelles. A previous study showed that the glucoside headgroups of octyl-β-d-glucopyranoside aggregate at the interior of mixed micelles with equimolar cetyltrimethylammonium bromide rather than mixing with trimethylammonium groups at the corona. The current study tests the hypothesis that structural characteristics of the surfactants (the relative lengths of the alkyl tails and the type of linker) can be tuned to shift the carbohydrate groups to micelle surfaces. ExperimentsThe structural arrangement of 30 mM equimolar mixed micelle solutions in D2O is investigated using NMR. The dynamics in different regions are probed using 1H spin-lattice (T1) and spin-spin (T2) relaxation measurements, and relative positioning by nuclear Overhauser effect spectroscopy (NOESY). Additional micellar properties are determined using solvatochromic fluorescent probes. FindingsMatching surfactant alkyl tail lengths is found ineffective at “pushing out” the carbohydrate headgroups due to a large mismatch in interactions between the headgroups and D2O. However, inserting a novel polar triazole group between the carbohydrate head group and the hydrophobic tail (e.g. in n-octyl-β-d-xylopyranoside) using click chemistry is able to “pull out” the carbohydrate, thus giving accessible sugar moieties at the surface of mixed micelles.

Experimental and MD simulation study on the physical and mechanical properties of organically modified montmorillonite clay and compatibilized linear low density polyethylene nanocomposites

ABSTRACTUnderstanding the interfacial interactions plays a key role in controlling mechanical and physical properties of polymer/clay nanocomposites (PCNs). In this work, the surface interactions between constituents of experimentally prepared PCNs which are the pristine linear low density polyethylene (PE) chains, PE compatibilizers, montmorillonite clay surface layer, and surfactants were studied quantitatively by employing molecular dynamics simulation technique. The interaction energy between the polymer and the clay was found to be inversely proportional with the volume of the surfactant which decreases the electrostatic interactions between the compatibilizer and the hydrophilic clay surface. However, the van der Waals (vdW) interactions between alkyl tails of surfactants and the PE chains increase with the tail length of the surfactants. The most attractive interaction was between the surfactant's head group and the clay surface. We showed that there existed fine balance between the electrostatic and vdW type forces on the stability and the enhanced properties of the PE–organoclay nanocomposites. Calculated interaction energies were then correlated to the experimentally measured mechanical properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45817.

Interaction between DNA and cationic diester-bonded Gemini surfactants

The formation of the polyion-complex between three cationic diester-bonded Gemini surfactants and DNA has been demonstrated systematically. This was studied through the electrostatic attraction between ammonium head groups of Gemini surfactants and the phosphate groups of DNA. Ethidium bromide exclusion assay indicates the interaction between DNA and diester-bonded Gemini surfactants. DNA binding abilities with the Gemini surfactant depends on tail length which has been demonstrated by agarose gel electrophoresis and circular dichroism (CD) measurements. Dynamic light scattering measurements reveal that the ester-bonded Gemini surfactants can induce the collapse of DNA into densely packed bead-like structures with smaller size. Molecular docking technique was also utilized to understand the mode and mechanism of interaction between DNA and the Gemini surfactants (pre-micellar form). In addition to electrostatic interactions between the negatively charged phosphate backbone of DNA and positively charged head groups of Gemini surfactants, self-association due to hydrophobic interactions between the alkyl tails of surfactant and the hydrogen bonding between the ester group of surfactant and nucleotide bases, result in the compaction of nucleotides.

Mixed aggregate formation in gemini surfactant/1,2-dialkyl-sn-glycero-3-phosphoethanolamine systems

An evaluation of the physical interactions between gemini surfactants, DNA, and 1,2-dialkyl-sn-glycero-3-phosphoethanolamine helper lipid is presented in this work. Complexation between gemini surfactants and DNA was first investigated using surface tensiometry where the surface tension profiles obtained were found to be consistent with those typically observed for mixed surfactant–polymer systems; that is, there is a synergistic lowering of the surface tension, followed by a first (CAC) and second (CMC) break point in the plot. The surfactant alkyl tail length was observed to exhibit a significant effect on the CAC, thus demonstrating the importance of hydrophobic interactions during complexation between gemini surfactants and DNA. The second study presented is an investigation of the mixing interactions between gemini surfactants and DOPE using Clint’s, Rubingh’s, and Motomura’s theories for mixed micellar formation. The mixing interactions between the 16–3–16/16–7–16/16–12–16/16–7NH–16 gemini surfactants and DOPE were observed to be antagonistic, where the strength of antagonism was found to be dependent upon the gemini surfactant spacer group and the solution composition.

NaBr induced strong adsorption of bis-(quaternary ammonium) gemini surfactants with long polymethylene spacer chain at air/water interface

The adsorption of alkanediyl-α,ω-bis(dimethyldodecylammonium bromide) (referred to as 12- s-12, s = 12, 14 and 16) at the air/water interface in the absence and the presence of 10 mmol L −1 NaBr has been investigated using surface tension measurements. The addition of salt strongly promoted the adsorption of 12- s-12, resulting in the formation of the double layers in the subphase below the main adsorption layer. In the double layers, the alkyl tails of surfactants were wrapped in the two layers of the heads and the long and flexible spacer chains were bent to insert into the alkyl tails. The π– A isotherm and surface dilational elasticity measurements supported this hypothesis. The reason may be due to the high hydrophobicity of the 12- s-12 with a long and flexible spacer chain. With addition of salt, the polarity of the solution environment was increased, which expelled the 12- s-12 from the bulk solution. The 12- s-12 molecules concentrated in the domain close to the interface formed the core–shell structure in order that the hydrocarbon chains could escape from the contact with water.

Propylammonium Nitrate as a Solvent for Amphiphile Self-Assembly into Micelles, Lyotropic Liquid Crystals, and Microemulsions

The phase behavior and self-assembled microstructures of a range of oligo(oxyethylene)-n-alkyl ether (C(i)E(j)) surfactants has been investigated in propylammonium nitrate (PAN), a room temperature ionic liquid. Micelles and single-phase microemulsions were all found to form at alkyl chain lengths from dodecyl to octadecyl, and lyotropic liquid crystals formed with hexadecyl chains or longer. Small-angle neutron scattering (SANS) shows that self-assembly occurs by solvophobic interactions driving the aggregation of the alkyl chains, but several results indicate that these are weaker in PAN than in water or ethylammonium nitrate, due chiefly to the hydrophobicity of PAN. Longer alkyl chains are needed for lyotropic liquid crystals to form, and higher surfactant concentrations are needed to form a single phase microemulsion. Conductivity shows these microemulsions to be weakly structured, and relatively insensitive to oil or surfactant molecular structure, unlike water-based systems. However, SANS contrast variation reveals a nanosegregation of oil from the alkyl tails of surfactants within the microemulsion, and may suggest a cosurfactant-like role for the propylammonium cation. Molecular areas within microemulsions and lamellar phases are larger than corresponding water- or ethylammonium nitrate-based systems due to the large molecular volume of the solvating PANs.