A study on the micelle formation of surfactants in aqueous solutions under high pressure by laser light-scattering technique. I

Multistep Micellization of Standard Surfactants Evidenced by Second Harmonic Scattering

Processes involving in solution a reduced number of molecules are difficult to identify and characterize. Here, we show that micellization of standard surfactants, namely sodium dodecyl sulfate and trimethyl tetradecyl ammonium bromide, two nonefficient compounds for quadratic nonlinear optics, can be investigated by second harmonic scattering (SHS). In particular, the formation of aggregates at concentrations smaller than the critical micellar concentration is evidenced through a nonmonotonic behavior of the SHS intensity as a function of the surfactant concentration. A simple model based on chemical equilibria between monomers and micelles is proposed to account for the experimental observations. Signature of long-range molecular orientation correlation is revealed by polarization resolved experiments and is discussed regarding micellization and charge-induced effects.

Micelle stability in water under a range of pressures and temperatures; do both have a common mechanism?

Using molecular dynamics simulations, we present a description compatible with experimental data of the self-assembly aggregation of SDS molecules in H₂O and D₂O for a wide range of pressures and temperatures.

Role of water structure on the high pressure micellization and phase transformations of sodium dodecanoate aqueous solutions

The aim of this work is to study the micelle formation and possible subsequent transformations of sodium dodecanoate aggregates in aqueous solutions at pressures up to 700 MPa. This pressure range is much larger than in most available studies on surfactant solutions and allows for evaluating the possible effect of the low-density to high-density water transformation on the aggregative behavior of the surfactant. The speed-of-sound and attenuation coefficient were determined at 298.15 K as a function of pressure at concentrations up to 0.13 mol kg(-1) in water. The speed-of-sound behavior with concentration is maintained up to pressures around 350 MPa. The attenuation coefficient, initially insensitive to pressure, exhibits a sudden increase around 250 MPa, reaching a maximum around 350 MPa and a plateau above 500 MPa in the case of the highest studied surfactant concentrations. From the analysis of the changes observed in these properties, it was possible to extend the concentration-pressure phase diagram of sodium dodecanoate at constant temperature. Some peculiarities found were: (1) the critical micellar concentration reaches a maximum around 170 MPa, (2) the micellar phase disappears above 400 MPa, (3) a phase transformation starts around 250 MPa, setting the solubility limit of the surfactant at concentrations around 0.06 mol kg(-1) in this pressure region, and (4) further transformations occur between 350 and 500 MPa. We discuss in length the possibility that such transformations might be driven by structural changes linked to the so-called low-density-water to high-density-water transition.

Langmuir nanoarchitectonics: one-touch fabrication of regularly sized nanodisks at the air-water interface

In this article, we propose a novel methodology for the formation of monodisperse regularly sized disks of several nanometer thickness and with diameters of less than 100 nm using Langmuir monolayers as fabrication media. An amphiphilic triimide, tri-n-dodecylmellitic triimide (1), was spread as a monolayer at the air-water interface with a water-soluble macrocyclic oligoamine, 1,4,7,10-tetraazacyclododecane (cyclen), in the subphase. The imide moieties of 1 act as hydrogen bond acceptors and can interact weakly with the secondary amine moieties of cyclen as hydrogen bond donors. The monolayer behavior of 1 was investigated through π-A isotherm measurements and Brewster angle microscopy (BAM). The presence of cyclen in the subphase significantly shifted isotherms and induced the formation of starfish-like microstructures. Transferred monolayers on solid supports were analyzed by reflection absorption FT-IR (FT-IR-RAS) spectroscopy and atomic force microscopy (AFM). The Langmuir monolayer transferred onto freshly cleaved mica by a surface touching (i.e., Langmuir-Schaefer) method contained disk-shaped objects with a defined height of ca. 3 nm and tunable diameter in the tens of nanometers range. Several structural parameters such as the disk height, molecular aggregation numbers in disk units, and 2D disk density per unit surface area are further discussed on the basis of AFM observations together with aggregate structure estimation and thermodynamic calculations. It should be emphasized that these well-defined structures are produced through simple routine procedures such as solution spreading, mechanical compression, and touching a substrate at the surface. The controlled formation of defined nanostructures through easy macroscopic processes should lead to unique approaches for economical, energy-efficient nanofabrication.

Interactions between chitosan and SDS at a low-charged silica substrate compared to interactions in the bulk--the effect of ionic strength

The effect of ionic strength on association between the cationic polysaccharide chitosan and the anionic surfactant sodium dodecyl sulfate, SDS, has been studied in bulk solution and at the solid/liquid interface. Bulk association was probed by turbidity, electrophoretic mobility, and surface tension measurements. The critical aggregation concentration, cac, and the saturation binding of surfactants were estimated from surface tension data. The number of associated SDS molecules per chitosan segment exceeded one at both salt concentrations. As a result, a net charge reversal of the polymer-surfactant complexes was observed, between 1.0 and 1.5 mM SDS, independent of ionic strength. Phase separation occurs in the SDS concentration region where low charge density complexes form, whereas at high surfactant concentrations (up to several multiples of cmc SDS) soluble aggregates are formed. Ellipsometry and QCM-D were employed to follow adsorption of chitosan onto low-charged silica substrates, and the interactions between SDS and preadsorbed chitosan layers. A thin (0.5 nm) and rigid chitosan layer was formed when adsorbed from a 0.1 mM NaNO3 solution, whereas thicker (2 nm) chitosan layers with higher dissipation/unit mass were formed from solutions at and above 30 mM NaNO3. The fraction of solvent in the chitosan layers was high independent of the layer thickness and rigidity and ionic strength. In 30 mM NaNO3 solution, addition of SDS induced a collapse at low concentrations, while at higher SDS concentrations the viscoelastic character of the layer was recovered. Maximum adsorbed mass (chitosan + SDS) was reached at 0.8 times the cmc of SDS, after which surfactant-induced polymer desorption occurred. In 0.1 mM NaNO3, the initial collapse was negligible and further addition of surfactant lead to the formation of a nonrigid, viscoelastic polymer layer until desorption began above a surfactant concentration of 0.4 times the cmc of SDS.

Disruption and reassociation of casein micelles under high pressure: influence of milk serum composition and casein micelle concentration

In this study, factors influencing the disruption and aggregation of casein micelles during high-pressure (HP) treatment at 250 MPa for 40 min were studied in situ in serum protein-free casein micelle suspensions. In control milk, light transmission increased with treatment time for approximately 15 min, after which a progressive partial reversal of the HP-induced increase in light transmission occurred, indicating initial HP-induced disruption of casein micelles, followed by reformation of casein aggregates from micellar fragments. The extent of HP-induced micellar disruption was negatively correlated with the concentration of casein micelles, milk pH, and levels of added ethanol, calcium chloride, or sodium chloride and positively correlated with the level of added sodium phosphate. The reformation of casein aggregates during prolonged HP treatment did not occur when HP-induced disruption of casein micelles was limited (<60%) or very extensive (>95%) and was promoted by a low initial milk pH or added sodium phosphate, sodium chloride, or ethanol. On the basis of these findings, a mechanism for HP-induced disruption of casein micelles and subsequent aggregation of micellar fragments is proposed, in which the main element appears to be HP-induced solubilization of micellar calcium phosphate.

Mucin-chitosan complexes at the solid-liquid interface: multilayer formation and stability in surfactant solutions

The adsorption of a biologically important glycoprotein, mucin, and mucin-chitosan complex layer formation on negatively charged surfaces, silica and mica, have been investigated employing ellipsometry, the interferometric surface apparatus, and atomic force microscopy techniques. Particular attention has been paid to the effect of an anionic surfactant sodium, dodecyl sulfate (SDS), with respect to the stability of the adsorption layers. It has been shown that mucin adsorbs on negatively charged surfaces to form highly hydrated layers. Such mucin layers readily associate with surfactants and are easily removed from the surfaces by rinsing with solutions of SDS at concentrations > or =0.2 cmc (1 cmc SDS in 30 mM NaCl is equal to 3.3 mM). The mucin adsorption layer is negatively charged, and we show how a positively charged polyelectrolyte, chitosan, associates with the preadsorbed mucin to form mucin-chitosan complexes that resist desorption by SDS even at SDS concentrations as high as 1 cmc. Thus, a method of mucin layer protection against removal by surfactants is offered. Further, we show how mucin-chitosan multilayers can be formed.

Effect of Pressure and Temperature on the Phase Behavior of Microemulsions

Summary.

The optimal salinity of three different anionic microemulsions was found to increase as a function of increased hydrostatic pressure. This is equivalent to a phase transition from an upper [Winsor H(+) (VM +) microemulsion to a lower [Winsor II(-) (WII -)] microemulsion. Increased pressure induces a compressibility effect that is consistent with the observed phase transition. Increasing temperature also leads to increasing optimal salinity. Prediction of temperature effects is complicated by temperature-dependent interactions and entropic contributions caused by dispersion. Fluid models that account for temperature effects are needed; therefore, no attempt was made to develop a theoretical interpretation of this effect. The temperature range is 0 to 100deg.C, and the pressure was varied from 0.1 to 50 MPa.

Introduction

An evaluation of a chemical flooding process for deep reservoirs like those in the North Sea must consider the effect of high temperature and pressure on microemulsion properties. For practical reasons, laboratory screening tests use either stock-tank oil or a model oil at reservoir temperature and atmospheric pressure. Moving to Drill reservoir conditions is expected to change the chemical phase equilibrium significantly and thereby most likely to change the optimum surfactant composition. The reason for these changes is obvious if the oil has a large GOR, but we also believe that pressure has a direct influence on the microemulsion phase behavior.

The phase behavior of surfactant/oil/water mixtures is the single most critical factor determining the success of a chemical flood. Many papers describe the sensitivity of microemulsion phase behavior to such compositional parameters as surfactant concentration, cosolvent concentration, WOR, ionic strength, and the ratio of divalent to monovalent ions. Also, the structures of the surfactant, cosolvent, and oil are important to determine the actual phase behavior. The effect of temperature has been reported for both anionic and nonionic surfactants. The changes in phase behavior are more pronounced for nonionic surfactants that exhibit a sharp phase-inversion temperature.

Of the vast number of papers on surfactants, only a few consider pressure effects, and most of these cover studies of aqueous surfactant solutions. Micellar solutions of both anionic and nonionic surfactants show an increase in critical micelle concentration (CMC) and a decrease in aggregation number as a function of increased pressure.

The effect of pressure on the phase behavior of microemulsions has been the subject of several reports. The results so far dispute that pressure has any effect on phase behavior. In most of these studies, the pressure was increased by the addition of gas, and it is difficult to separate the pressure effect from changes in composition.

The interfacial tension (IFT) between oil and water is affected very little by pressure. In the presence of surfactants, however, low MT's can be achieved, and the phase behavior is critically dependent on the different intensive parameters, as described above. At these conditions, it is likely that pressure will affect both the IFT's and the phase behavior.

O'Connell and Walker found microemulsion phase behavior to change markedly under pressure, in direct disagreement with Nelson, who detected no phase volume change. Because of the difference in compressibility between oil and water, however, the water/oil volume ratio changes during pressure measurements. If no changes in phase volumes are observed, there still is an increase in water solubilization and a corresponding decrease in oil solubilization owing to changes in WOR. The phase behavior is thus shifted toward a lower-phase microemulsion. It is not clear whether these corrections in WOR were included in the reported phase-volume data.

The reports describing phase-behavior studies of both alkanes and crude oils pressurized with methane or other light alkanes concluded that with crude oils, the phase behavior shifts toward VM+. The shift is smaller than what would be predicted from the changes in alkane carbon number (ACN). Introducing methane lowers the average molecular weight and is expected to decrease the equivalent ACN. But because the density of the oil phase is increased under pressure, the presence of methane might reduce the effective shift in ACN. For alkanes pressurized with methane, the optimal salinity is found to increase with increasing gas concentrations, which correspond to a shift in phase behavior toward Wu. The above two results seem to contradict each other. Keep in mind, however, that crude oil contains aromatics and asphaltenes; therefore, comparison of alkanes with crude must be done with caution.

In this paper, we focus on the effect of pressure on the phase behavior of microemulsions at constant oil composition. The experiments were restricted to three surfactant systems in n-alkane/ n-butanol/NaCl/water. The surfactants were selected from groups of surfactants that are interesting for chemical EOR at high temperature and moderately high salinities. The effect of pressure on these systems was compared with changes in phase behavior with temperature and ACN. The IFT was also measured as a function of pressure and compared with changes in solubilization of oil and water in the microemulsion middle phase.

Materials

For the phase-behavior studies, we used three systems containing different types of commercial surfactants: alkyl-benzene sulfonate, secondary alkane sulfonate (SAS), and alkylaryl ethoxylated sulfonate.

The alkylbenzene sulfonate was a sodium n-dodecyl-benzene sulfonate (SDBS) from Akzo Chemie. The product was supplied as a concentrate at 35 wt % active matter. The organic impurities were less than 1 wt%, and the sodium sulfate residue was a maximum of 0.6 wt %.

The SAS sulfonate, from Hoechst AG, is made from a continuous process by sulf-oxidation of C 13 through C 18 n-paraffins. The dis-tribution in molecular weight is reported to be 1 % less than C 13–58 % C 13 through C 15, - 39 % C 16 through C 17, and a maximum of 3 % C 17 1. The surfactant was delivered as a paste at 60 wt% active matter. The oil content was less than 0.5 wt%, and the sodium sulfate concentration was a maximum of 4 wt%. The molecular weight is 328 g/mol.

The alkylaryl ethoxylated sulfonate is also a Hoechst product. This surfactant is a tributyl-phenolether-4-ethoxy-sulfonate (TBPE). The anionic active matter is 26 wt%, and the solid content is 40 wt%. The salt content is less than 5 wt%, and the rest of the solids (9 wt%) is the nonionic product.

A cosolvent, n-butanol from Merck (purity 99.5 % +), was used for all three surfactant systems.

SPERE

P. 601^

Interfacial adsorption of an inhalation anesthetic onto ionic surfactant micelles and its desorption by high pressure

The effects of pressure and temperature on the critical micelle concentration (CMC) of sodium dodecylsulfate (SDS) wer measured in the presence of various concentrations of an inhalation anesthetic, methoxyflurane. The change in the partial molal volume of SDS on micellization delta Vm, increased with the increase in the concentration of methoxyflurane. The CMC-decreasing power, which is defined as the slope of the linear plot between In(CMC) vs. mole fraction of anesthetic, was determined as a function of pressure and temperature. Since the CMC-decreasing power is correlated to the micelle/water partition coefficient of anesthetic, the volume change of the transfer (delta Vop) of methoxyflurane from water to the micelle can be determined from the pressure dependence of the CMC-decreasing power. The value of delta Vop amounts 6.5 +/- 1.8 cm3.mol-1, which is in reasonable agreement with the volume change determined directly from the density data, 5.5+/-0.6 cm3.mol-1. Under the convention of thermodynamics, this indicates that the application of pressure squeezes out anesthetic molecules from the micelle. The transfer enthalpy of anesthetic from water to the micelle is slightly endothermic. The partial molal volume of methoxyflurane in the micelle (112.0 cm3.mol-1) is smaller than that in decane (120.5 cm3.mol-1) and is larger than that in water (108.0 cm3. mol-1. This indicates that the anesthetic molecules are incorporated into the micellar surfaces region, i.e., the palisade layer of the micelle in contact with water molecules, rather than into the micelle core.