Self-assembly structures of gemini surfactants are characterized, among others, for their low CMC. This characteristic could be due to great hydrophobic parts in their molecular structures. That availability could imply great stability of self-assembly structures or monolayers absorbed in an interface. The micellization behavior of two cationic gemini surfactants, α,ω-bis(S-alkyl dimethylammonium) alkane bromides, were studied by a modelization of dynamic surface tension (DST) experimental data and isothermal titration calorimetry (ITC) measurements. The adsorption data at the air/water interface was taken through the analysis of the profile changes of a pendant drop. The thermodynamic characterization of the micellization process of the gemini surfactants was carried out using ITC. A model based on the Frumkin adsorption isotherm and the Ward-Tordai diffusion equation was developed to obtain the characteristic parameters of the adsorption without the need of using the Gibbs adsorption equation. Positive values of lateral interaction show good stability of the adsorbed monolayer. The ITC data were analyzed following a novel protocol based on the identification of the different energetic contributions and regimens observed in the titration enthalpograms from demicellization processes. The presence of exothermic peaks would explain the low values of CMC.
Following up a recent paper on grafted sliding polymer layers (Macromolecules 2005, 38, 1434-1441), we investigated the influence of the sliding degree of freedom on the self-assembly of sliding polymeric surfactants that can be obtained by complexation of polymers with cyclodextrins. In contrast to the micelles of quenched block copolymer surfactants, the free energy of micelles of sliding surfactants can have two minima: the first corresponding to small micelles with symmetric arm lengths, and the second corresponding to large micelles with asymmetric arm lengths. The relative sizes and conc…
We develop a simulation method which allows us to calculate the critical micelle concentrations for ionic surfactants in the presence of different salts. The results are in good agreement with the experimental data. The simulations are performed on a simple cubic lattice. The anionic interactions with the alkyl chains are taken into account based on the previously developed theory of the interfacial tensions of hydrophobic interfaces: the kosmotropic anions do not interact with the hydrocarbon tails of ionic surfactants, while chaotropic anions interact with the alkyl chains through a dispersi…
Flow-induced configuration dynamics and scission of rodlike micelles are studied for the first time using molecular dynamics simulations in presence of explicit solvent and salt. Predicted dependence of tumbling frequency and orientation distribution on shear rate S agrees with mesoscopic theories. However, micelle stretching increases the distance between the cationic head groups and adsorbed counter ions, which reduces electrostatic screening and increases the overall energy Phi linearly with micelle length. Micelle scission occurs when Phi exceeds a threshold value, independent of S.
We present a new theoretical approach to the kinetics of micelle formation in surfactant solutions, in which the various stages of aggregation are treated as constrained paths on a single free-energy landscape. Three stages of well-separated time scales are distinguished. The first and longest stage involves homogeneous nucleation of micelles, for which we derive the size of the critical nuclei, their concentration, and the nucleation rate. Subsequently, a much faster growth stage takes place, which is found to be diffusion-limited for surfactant concentrations slightly above the critical mice…
We study a simple model of DNA divalent cationic surfactant complexation. We find that the combination of electrostatic and hydrophobic effects leads to a cooperative phenomenon in which as the amphiphile is added to the solution containing DNA, a large fraction of the DNA's charge is neutralized by the condensed divalent cationic surfactants, forming the surfoplex. This binding transition occurs for concentrations that are lower for divalent than for monovalent surfactants. Since the electrostatic strength is larger in the first case and the amount of surfactant lower, we suggest that multiva…
We report the synthesis of complex salts made from the cationic surfactant dodecyltrimethyl-ammonium and diblock copolymers poly(acrylic acid)-block-poly(acrylamide) of different molecular weights. In water, the complex salts self-assemble into stable hierarchical aggregates with a dense core and a diffuse shell. In contrast to earlier reports, the surfactant/polymer aggregates exhibit a liquid crystalline structure of Pm3n cubic symmetry. The crystal structure is analogous to that obtained with homopolymer. Size and aggregation numbers were estimated from a combination of light and small-angl…
The $L_α$ phase of lipid bilayers is a fluid self-assembled state, key to the formulation of cosmetics, detergents and pharmaceutics. Despite having been extensively scrutinized in self-assembled phospholipid or surfactant bilayers, the formation of a fluid $L_α$ state has defied understanding in mixtures of fatty alcohols, surfactants and water, where is viewed as the essential step for the preparation of creamy dispersions. Here, atomistic molecular dynamics simulations show the existence of a fluid bilayer in aqueous mixtures of cetyl (C$_{16}$OH) and stearyl (C$_{18}$OH) alcohols, and cety…
We report on an investigation of the structural and dynamical properties of n-nonyltrimethylammonium chloride (C9TAC) and erucyl bis [2-hydroxyethyl] methylammonium chloride (EMAC) micelles in aqueous solution. A fully atomistic description was used, and the time evolution was computed using molecular dynamics.
We report the formation of colloidal complexes resulting from the electrostatic co-assembly between anionic surfactants and cationic polyelectrolytes or block copolymers. Combining light and x-ray scattering experiments with cryogenic transmission and optical microscopy, we emphasize a feature rarely addressed in the formation of the electrostatic complexes, namely the role of the mixing concentration on the microstructure. At low mixing concentration, electrostatic complexes made from cationic-neutral copolymers and alkyl sulfate surfactants exhibit spherical core-shell microstructures. With …
Concentrated fabric conditioners are water-based formulations containing 10 wt. % of cationic surfactants that are deposited on textile fibers during the rinse cycle in a washing machine to make them smoother and softer to touch. In topical formulations, the concentration of cationic surfactants is reduced by half, this reduction being compensated by the addition of environment-friendly polysaccharides. Using atomic force microscopy (AFM), quartz crystal microbalance with dissipation (QCM-D) and ellipsometry, the deposition of formulations with or without polysaccharides on model cellulose sub…
We investigate the morphology and energetics of a self-associating model cationic surfactant in water using coarse-grained molecular dynamics simulations. We develop an algorithm to track micelles contours and quantify various microstructural features, such as contour length, persistence length, mesh size, and entanglement-length. We demonstrate that branched and multiconnected structures govern the anomalous dependence of zero-shear viscosity on salt concentration. We predict reliably the end-cap energy of micelles, for the first time, from simulations.
Modification of calcium carbonate particles with surfactant significantly improves the properties of the calcium carbonate coating on paper. Unmodified and CTAB and oleate-modified calcium carbonate nanoparticles were prepared using the wet carbonation technique for paper coating. CTAB (cationic surfactant) and sodium oleate (anionic surfactant) were used to modify the size, morphology, and surface properties of the precipitated nanoparticles. The obtained particles were characterized by XRD, FT-IR spectroscopy, zeta potential measurements, TGA and TEM. Coating colors were formulated from the …
Hypotheses: The aggregation number and length of spherocylindrical (rodlike, wormlike) micelles in solutions of an ionic surfactant and salt can be predicted knowing the molecular parameters and the input concentrations of the species. This can be achieved by upgrading the quantitative molecular thermodynamic model from the previous parts of this series with an expression for the electrostatic component of micelle scission energy that is the excess free energy of the spherical endcaps with respect to the cylindrical part of the micelle. Theory: The thermodynamics of micellization is extended t…
We present the first molecular dynamics study to probe the mechanisms of anomalous diffusion in cationic surfactant micelles in the presence of explicit salt and solvent-mediated interactions. Simulations show that when the counter ion density increases, saddle-shaped interfaces manifest leading to the formation of branched structures. In experiments, branched structures exhibit lower viscosity as compared to linear and wormlike micelles, presumably due to stress relaxation arising from the sliding motion of branches along the main chain. Our simulations provide conclusive evidence and a mecha…
Concentrated fabric softeners are water-based formulations containing around 10 - 15 wt. % of double tailed esterquat surfactants primarily synthesized from palm oil. In recent patents, it was shown that a significant part of the surfactant contained in today formulations can be reduced by circa 50 % and replaced by natural guar polymers without detrimental effects on the deposition and softening performances. We presently study the structure and rheology of these softener formulations and identify the mechanisms at the origin of these effects. The polymer additives used are guar gum polysacch…
The micellization process of the aqueous mixed system Triton X-100 (TX100)-Dodecyltrimethylammonium Bromide (DTAB) was studied with a battery of procedures: surface tension, static and dynamic light scattering and ion-selective electrodes. Results were also analysed with two thermodynamic procedures. The system shows some changes in its behaviour with changing the mole fraction of DTAB, $α_{DTAB}$, in the whole surfactant mixture. For $ α_{DTAB} < 0.40$ micelles are predominantly TX100 with scarce solubilized DTA+ ions, and TX100 acts as a nearly ideal solvent. In the range $0.50 < α_{DTAB} < …
The encapsulation of fragile biomacromolecules is crucial for many biotechnological applications but remains challenging. Interfacial complexation (IC) in water-in-oil emulsions turned out to be an efficient process for the formation of protective polymer layers at the surface of capsule-precursor water droplets. We propose to further enhance this IC process by introducing thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) strands in the interfacial polymer layer. In this work, we implement surfactant-polymer IC in water-in-fluorocarbon oil emulsions between a water-soluble poly(L-lysine)-g…
Fabric conditioners are household products used to impart softness and fragrance to tex-tiles. They are colloidal dispersions of cationic double chain surfactants that self-assemble in vesicles. These surfactants are primarily derived from palm oil chemical modification. Reducing the content of these surfactants allows to obtain products with lower environmental impact. Such a reduction, without adverse effects on the characteristics of the softener and its performance, can be achieved by adding hydrophilic biopolymers. Here, we review the role of guar biopolymers modified with cationic or hyd…
The binding of a cationic surfactant (hexadecyltrimethylammonium bromide, CTAB) to a negatively charged natural polysaccharide (pectin) at air-solution interfaces, was investigated on single interfaces and in foams, versus the linear charge densities of the polysaccharide. Beside classical methods to investigate polymer/surfactant systems, we applied, for the first time concerning these systems, the analogy between the small angle neutron scattering by foams and the neutron reflectivity of films to measure in situ film thicknesses of foams. CTAB/pectin foam films are much thicker than that of …
A theory is presented which quantitatively accounts for the cooperative adsorption of cationic surfactants to anionic polyelectrolytes. For high salt concentration we find that the critical adsorption concentration (CAC) is a bilinear function of the polyion monomer and salt concentrations, with the coefficients dependent only on the type of surfactant used. The results presented in the paper might be useful for designing more efficient gene delivery systems.
Hypotheses: A 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. Theory: By 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 t…
The adsorption kinetics of the cationic surfactant dodecyltrimethylammonium bromide at the air-water interface has been studied by the maximum bubble pressure method at concentrations below the critical micellar concentration. At short times, the adsorption is diffusion-limited. At longer times, the surface tension shows an intermediate plateau and can no longer be accounted for by a diffusion limited process. Instead, adsorption appears kinetically controlled and slowed down by an adsorption barrier. A Poisson-Boltzmann theory for the electrostatic repulsion from the surface does not fully ac…
A microemulsion of decane droplets stabilized by a non-ionic surfactant film is progressively charged by substitution of a non-ionic surfactant molecule by a cationic surfactant. We check that the microemulsion droplets remain identical within the explored range of volume fraction (0.02 to 0.18) and of the number of charge per droplets (0 to 40) . We probe the dynamics of these microemulsions by dynamic light scattering. Despite the similar structure of the uncharged and charged microemulsions the dynamics are very different . In the neutral microemulsion the fluctuations of polarization relax…
In this paper we study the main surface characteristics which control the foamability of solutions of various surfactants. Experiments with anionic, cationic and nonionic surfactants with different head groups and chain lengths are performed in a wide concentration range, from 0.001 mM to 100 mM. The electrolyte concentration is also varied from 0 up to 100 mM. For all surfactants studied, three regions in the dependence of the foamability, VA, on the logarithm of surfactant concentration, lgCS, are observed. In Region 1, VA is very low and depends weakly on CS. In Region 2, VA increases steep…
We report the presence of a correlation between the bulk and interfacial properties of electrostatic coacervate complexes. Complexes were obtained by co-assembly between cationic-neutral diblocks and oppositely charged surfactant micelles or 7 nm cerium oxide nanoparticles. Light scattering and reflectometry measurements revealed that the hybrid nanoparticle aggregates were more stable both through dilution and rinsing (from either a polystyrene or a silica surfaces) than their surfactant counterparts. These findings were attributed to a marked difference in critical association concentration …
There is currently a renewed interest for improving household and personal care formulations to provide more environment friendly products. Fabric conditioners used as softeners have to fulfill a number of stability and biodegradability requirements. They should also display significant adsorption on cotton in the conditions of use. The quantification of surfactant adsorption remains however difficult because the fabric woven structure is complex and deposited amounts are generally small. Here we propose a method to evaluate cellulose/sur-factant interactions with increased detection sensitivi…
Surfactant choice is key in starting the phenomena of artificial morphogenesis, the bottom-up growth of geometric particles from cooled emulsion droplets, as well as the bottom-up self-assembly of rechargeable microswimmer robots from similar droplets. The choice of surfactant is crucial for the formation of a plastic phase at the oil-water interface, for the kinetics, and for the onset temperature of these processes. But further details are needed to control these processes for bottom-up manufacturing and understand their molecular mechanisms. Still unknown are the minimum concentration of th…
Binding and phase behavior of oppositely charged polyelectrolytes and surfactants were studied in aqueous bulk solutions and at air-water interfaces. In particular, we have investigated the polyanion poly(sodium 4-styrenesulfonate) (NaPSS) and the cationic surfactants dodecyltrimethylammonium bromide (C$_{12}$TAB), tetradecyltrimethylammonium bromide (C$_{14}$TAB) and cetyltrimethylammonium bromide (C$_{16}$TAB). In order to reveal surfactant/polyelectrolyte binding, aggregation and phase separation of the mixtures, we have varied the NaPSS concentration systematically and have kept the surfac…
Hypothesis: A broad range of phenomena, such as emulsification and emulsion stability, foam formation or liquid evaporation, are closely related to the dynamics of adsorbing colloidal particles. Elucidation of the mechanisms implied is key to a correct design of many different types of materials. Experiments: Microspheres forced to rotate near a fluid interface exhibit a roto-translational hydrodynamic mechanism that is hindered by capillary torques as soon as the particles protrude the interface. Under these conditions, the time evolution in the ratio of moving spheres provides a direct descr…
