Binary Surfactant-water System Research Articles

Structure and dynamics of aqueous solutions containing poly-(acrylic acid) and non-ionic surfactant pentaethylene glycol n-octyl ether (C8E5): A molecular simulations study

Self-assembly of non-ionic surfactant pentaethylene glycol n-octyl ether (C8E5) in aqueous solution containing anionic polyelectrolyte poly (acrylic acid) PAA was investigated via atomistic molecular dynamics (MD) simulations. The concentration of surfactant (Cs) varied in the range (Cs = 9.6 × 10−4–0.3 M) of dilute to concentrated systems while polymer concentration was in dilute solution condition (i.e. Cp = 0.01 M). Binary and ternary systems show formation of nearly spherical micellar aggregate at low Cs and lamellar aggregates at high Cs. Strong interaction between PAA and C8E5 is observed at low Cs, in agreement with experimental data. PAA chain adsorbs onto the micellar aggregate and wraps around the aggregate. For lamellar phase aggregates PAA chain shows polar interactions with hydrophilic moieties of surfactant, originating from interactions between COOH groups of PAA and CO groups of C8E5. The concentration at which free-micelles form in ternary (polymer, surfactant and water) solution is greater than CMC of the binary surfactant-water system, in qualitative agreement with experimental data. At low Cs the interaction between surfactants and polymer chain is weak and does not perturb chain conformations. The radius-of-gyration Rg of PAA increases with Cs. Interaction of surfactant molecules with water is more cooperative at higher values of surfactant concentration, as evident from the values of solvation enthalpy. Significant hydrogen bonds were observed to occur between polar oxygens and hydroxyl oxygen (O2HA) of PAA. The number of h-bonds between polar oxygens and O2HA of PAA increase with Cs. Hydrogen bonds of PAA with surfactant molecules increases with Cs as the polymer expands at high Cs. Formation of surfactant aggregate is enthalpically favored across the entire range of concentration in binary and ternary solution. Binding of PAA to surfactant aggregate is favorable at higher surfactant concentration. The enthalpy of formation of ternary solution exhibits a transition from unfavorable to favorable which interestingly is driven by the structural transition from spherical micellar to the lamellar phase. This signature for this transition is also seen by conformation change of PAA upon binding to surfactant aggregate.

Phase diagram and surface adsorption behavior of benzyl dimethyl hexadecyl ammonium bromide in a binary surfactant-water system

Phase diagram provides valuable information to describe the phase and aggregation behavior of surfactants. In this study, surface adsorption behavior of benzyl dimethyl hexadecyl ammonium bromide (BDHAB) surfactant in water and their phase formations have been explored. The formation of lyotropic liquid crystalline (LLC) phases in the binary surfactant–water system was analyzed by polarized light microscopy (PLM). A comprehensive phase diagram was established presenting the existence of LLC phases as a function of surfactant concentration and temperature. The observed PLM images reveal that the BDHAB–H2O system exhibits isotropic (L1), hexagonal liquid crystalline (H1), cubic (V1), and lamellar (L∞) phases in a sequential pattern with the increase of surfactant concentration. The recorded LLC phases for bi-tail BDHAB–H2O system have also been compared with the phase diagram of single tail cetyl trimethyl ammonium bromide (CTAB)–H2O system. The present study evidence that the surfactant morphology (i.e., alkyl chain length and surfactant head-group area) has a significant influence on the formation of micellar aggregates. In addition, surface tension measurements were also performed to observe the surface adsorption behavior of BDHAB surfactant. This study will be beneficial in understanding the micelle formation and solubilization behavior of bi-tailed quaternary ammonium cationic surfactants to optimize its colloidal formulations.

Liquid Crystal Demonstration of Binary Phase Behavior for the Classroom

A demonstration showing binary phase formation and transformation, two concepts relevant to understanding binary phase diagrams, is presented using lyotropic liquid crystals. An optical microscope, in cross-polarized illumination, is used to observe phase formation as a function of composition at a surfactant–water interface. Variations in optical texture as a function of position with respect to the interface, and hence composition, reveal distinct lyotropic liquid crystal phases. This phenomenon is illustrated using two different binary surfactant–water systems: one exhibiting simple phase behavior (three distinct phases through the compositional space) and one exhibiting complex phase behavior (six distinct phases through the compositional space). The change in phase fraction during a temperature-induced phase transition and the effect of cooling rate on microstructure development are also demonstrated by examining the microstructure, and changes in the microstructure, of a fixed-composition lyotropic ...

A phase-rule study of multiple-phase formation in a model emulsion system containing water, n-octanol, n-dodecane and a non-ionic surface-active agent at 10 and 25 degrees.

Abstract A four-component emulsion system containing the non-ionic surfactant C8H17 (O·CH2·CH2)6OH has been investigated, particularly in regions in which liquid crystal or more than two condensed phases are formed. Liquid crystal does not exist in the binary surfactant-water system at 25° but extensive regions form when the organic liquids octanol and dodecane are present, although these are not joined to the main two-liquid regions of the emulsion system. The surfactant is almost completely dissolved in the aqueous phase in water-dodecane mixtures but when substantial amounts of octanol are also present the partition coefficient is reversed in favour of the organic phase. At certain intermediate compositions three liquid phases are present. The significance of these results to the system as an emulsion are briefly discussed.

Lamellar Miscibility Gap in a Binary Catanionic Surfactant−Water System

The coexistence of two lamellar liquid-crystalline phases in equilibrium for binary surfactant-water systems is a rare and still puzzling phenomenon. In the few binary systems where it has been demonstrated experimentally, the surfactant is invariably ionic and the miscibility gap is thought to stem from a subtle balance between attractive and repulsive interbilayer forces. In this paper, we report for the first time a miscibility gap for a catanionic lamellar phase formed by the surfactant hexadecyltrimethylammonium octylsulfonate (TASo) in water. Synchrotron small-angle X-ray scattering, polarizing light microscopy, and 2H NMR unequivocally show the coexistence of a dilute (or swollen) lamellar phase, Lalpha', and a concentrated (or collapsed) lamellar phase, Lalpha' '. Furthermore, linear swelling is observed for each of the phases, with the immiscibility region occurring for 15-54 wt % surfactant. In the dilute region, the swollen lamellar phase is in equilibrium with an isotropic micellar region. Vesicles can be observed in this two-phase region as a dispersion of Lalpha' in the solution phase. A theoretical cell model based on combined DLVO and short-range repulsive potentials is presented in order to provide physical insight into the miscibility gap. The surfactant TASo is net uncharged, but it undergoes partial dissociation owing to the higher aqueous solubility of the short octylsulfonate chain. Thus, a residual positive charge in the bilayer is originated and, consequently, an electrostatic repulsive force, whose magnitude is dependent on surfactant concentration. For physically reasonable values of the solubility of the octyl chain, assumed to be constant with surfactant volume fraction, a fairly good agreement is observed between the experimental miscibility gap and the theoretical one.

Polymerization in the cubic phase and studies of phase behavior in two nonionic surfactant-water-acrylamide systems

Polymerization of acrylamide in two bicontinuous cubic phases formed by two different surfactant didodecyldimethylaminooxide (DDAO) and hexaethylene glycol mono- n-dodecyl ether (C 12E 6)-water-acrylamide systems was studied. SAXS data from both polymerized and unpolymerized samples in both systems showed that the cubic crystallographic symmetry and the lattice parameter were unaffected by the polymerization. NMR self-diffusion of DDAO in cubic phases showed no difference between polymerized and unpolymerized samples. Also studied was the effect of acrylamide on the phase behavior of the binary surfactant-water systems. The effect of acrylamide was found to be different for the two systems. In the DDAO case, only minor effects were observed, while large effects were observed in the C 12E 6 case where the hexagonal phase was stabilized while the lamellar phase was destabilized by the addition of acrylamide.

Lyotropic liquid crystalline phase behavior and structure of cesium n-tetradecanoate-water mixtures

The compositional sequence and microstructure of liquid crystalline phases formed by binary mixtures of cesium n-tetradecanoate (CsTD) and water (deuterium oxide) were studied from 297 to 338 K and from 35 to 95 wt% surfactant. The experimental methods used were deuterium ( 2H) quadrupole nuclear magnetic resonance (NMR) spectroscopy and optical polarizing microscopy. The Krafft boundary in the CsTD-water system is about 2–5°C, more than 45° lower than that in the comparable sodium n-tetradecanoate-water system. The structured surfactant phases observed were micellar; normal hexagonal; ribbon, a viscous birefringent phase of biaxial symmetry; viscous isotropic; and lamellar. The ribbon phase structure was deduced from the biaxial deuterium NMR spectral lineshape and by the contiguity of hexagonal and ribbon phases. This phase has not been observed previously by quadrupole NMR spectroscopy in a simple binary surfactant-water system. The phase transition from hexagonal to ribbon (ca. 65 wt%) is apparently second order as indicated by the lack of an observable two-phase region. A higher order transition is consistent with the proposed continuous variation in microstructure from rodlike aggregates to those of ribbonlike symmetry. The viscous isotropic phase, which has been observed in mixtures of potassium dodecanoate or sodium octanoate with water, was stable at temperatures above 310 K, at a composition of about 70 wt% CsTD. The lamellar phase is stable to very high surfactant concentrations at modest temperatures, >95 wt% CsTD at 338 K. The change in counterion from sodium to cesium lowers the Krafft temperature, enabling the study of liquid crystalline phase behavior from 5 to 100°C.