Physicochemical investigations of anionic–nonionic mixed surfactant microemulsions in nonaqueous polar solvents: I. Phase behavior

Phase behaviors of AOT/heptane (Hp)/formamide (FA), ethylene glycol (EG), propylene glycol (PG), triethylene glycol (TEG) and glycerol (GLY) have been investigated in the absence and presence of a nonionic surfactant, polyoxyethylene(2) cetyl ether (Brij-52) at 303 K. The phase characteristics of (AOT+Brij-52)/Hp/(EG or PG or TEG) have been found to be different from that of AOT/Hp/FA systems in respect of both the area of monophasic domain and the appearance of other mesophases. The area of monophasic domain of (AOT+Brij-52)/Hp/EG depends on the content of Brij-52 (X _Brij-52) and shows a maximum at X _Brij-52=0.4. A negligible effect on the area of the monophasic domain has been shown by more hydrophobic surfactants, polyoxyethylene(2) stearyl ether (Brij-72) and polyoxyethylene(2) oleyl ether (Brij-92). The effect of oils (dodecane and hexadecane) on the mixed systems stabilized by (AOT+Brij-52) in EG has been investigated. The area of monophasic domain has been found to be dependent on the type of nonaqueous solvents and follows the order GLY>EG>PG>TG. A systematic investigation on the measurement of phase volumes of mixed surfactant systems [AOT+nonionic surfactant(s)] stabilized in oils of different chain lengths (heptane, dodecane and hexadecane) and polar solvent (EG) has been carried out at different compositions of the ingredients to identify the phase transitions of these systems as a function of X _Brij-52. The threshold point of phase transition (both W I→W IV and W IV→W II transitions) has been found to be a function of the configuration of added nonionic surfactant, nature of the polar solvent and oil. The conversion of the initial oil/EG droplets into EG/oil droplets with increasing X _nonionic has been facilitated for hydrophobic surfactants polyoxyethylene(4) lauryl ether (Brij-30), Brij-52, and Brij-72 in comparison to the hydrophilic surfactants polyoxyethylene(10) cetyl ether (Brij-56) and polyoxyethylene(20) cetyl ether (Brij-58).     

Phase behavior of mixtures of polyoxyethylene(10) stearyl ether (Brij-76), 1-butanol, isooctane, and mixed polar solvents I. Water and formamide (or N,N-dimethyl formamide)

The isothermal phase diagram of the quaternary system polyoxyethylene(10) stearyl ether (Brij-76)/1-butanol/isooctane/water has been constructed at 30 degrees C with equal amounts of oil and water. A regular fishtail diagram was obtained, confirming the establishment of hydrophile-lipophile balance (HLB) in the system. Mixing of formamide (FA) [or N,N-dimethyl formamide (DMF)] with water as a cosolvent altered the HLB and decreased the solubilization capacity of the quaternary system. No three-phase body appeared at high FA or DMF content. Similar observations were noted for temperature-induced phase diagrams. The effect of DMF was more pronounced than that of FA in reducing the maximum solubilization capacity. The results have been summarized on the basis of HLB and mutual solubility of the components.     

Phase behavior, interfacial composition and thermodynamic properties of mixed surfactant (CTAB and Brij-58) derived w/o microemulsions with 1-butanol and 1-pentanol as cosurfactants and n-heptane and n-decane as oils

Phase diagrams of pseudo-quaternary systems of cetyltrimethylammonium bromide (CTAB)/polyoxyethylene(20)cetyl ether (Brij-58)/water/1-butanol (or 1-pentanol)/n-heptane (or n-decane) at fixed omega (=[water]/[surfactant]) of 55.6 were constructed at different temperatures (293, 303, 313, and 323 K) and different mole fraction compositions of Brij-58 (X(Brij-58)=0, 0.5, and 1.0 in CTAB + Brij-58 mixture). Pure CTAB stabilized systems produced larger single-phase domains than pure Brij-58 stabilized systems. Increasing temperature increased the single-phase domain in the Brij-58 stabilized systems, whereas the domain decreased in the CTAB stabilized systems. For mixed surfactant systems (with X(Brij)=0.5) negligible influence of temperature in the studied range of 293 to 323 K on the phase behavior was observed. Interfacial compositions of the mixed microemulsion systems at different temperature and different compositions were evaluated by the dilution method. The n(a)(i) (number of moles of alcohol at the interface) and n(a)(o) (number of moles of alcohol in the oil phase) determined from dilution experiments were found to decrease and increase respectively for CTAB stabilized systems, whereas an opposite trend was witnessed for Brij-58 stabilized systems. The energetics of transfer of cosurfactants from oil to the interface were found to be exothermic and endothermic for CTAB and Brij-58 stabilized systems, respectively. At equimolar composition of CTAB and Brij-58, the phase diagrams were temperature insensitive, so that the enthalpy of the aforesaid transfer process was zero.     

Analyzing solubility of acid gas and light alkanes in triethylene glycol

Physical solvents such as ethylene glycol （EG）, diethylene glycol （DEG）, and triethylene glycol （TEG） are commonly used in wet gas dehydration processes with TEG being the most popular due to ease of regeneration and low solvent losses. Unfortunately, TEG absorbs significantly more hydrocarbons and acid gases than EG or DEG. Quantifying this amount of absorption is therefore critical in order to minimize hydrocarbon losses or to optimize hydrocarbon recovery depending on the objective of the process. In this article, a new correlation that fully covers the operating ranges of TEG dehydration units is developed in order to determine the solubility of light alkanes and acid gases in TEG solvent. The influence of several parameters on hydrocarbon and acid gas solubility including temperature, pressure, and solvent content is also examined.     

On the perturbation of the H-bonding interaction in ethylene glycol clusters upon hydration

Ab initio and density functional methods have been employed to study the structure, stability, and spectral properties of various ethylene glycol (EG(m)) and ethylene glycol-water (EG(m)W(n)) (m = 1-3, n = 1-4) clusters. The effective fragment potential (EFP) approach was used to explore various possible EG(m)W(n) clusters. Calculated interaction energies of EG(m)W(n) clusters confirm that the hydrogen-bonding interaction between EG molecules is perturbed by the presence of water molecules and vice versa. Further, energy decomposition analysis shows that both electrostatic and polarization interactions predominantly contribute to the stability of these clusters. It was found from the same analysis that ethylene glycol-water interaction is predominant over the ethylene glycol-ethylene glycol and water-water interactions. Overall, the results clearly illustrate that the presence of water disrupts the ethylene glycol-ethylene glycol hydrogen bonds.     

Fragments of the phase diagrams of ethylene glycol systems in the region of low second component concentrations

Fragments of the phase diagrams of the ethylene glycol (EG)-tert-butanol (t-BuOH) and EG-tet-ramethylurea (TMU) binary systems were studied by differential scanning calorimetry in the region rich in EG up to a ～40 mol % concentration of the second component. Liquid-phase stratification occurred at ～10–37 mol % t-BuOH and ～8–25 mol % TMU at ?37°C in the EG-t-BuOH system and at ?22°C in the EG-TMU system. The occurrence of liquid stratification was substantiated by visual observations of the solutions of the concentrations specified at a constant temperature of ?20°C. A comparison of the results obtained with the data on light scattering in the systems under study over the temperature range 25–50°C lends support to the hypothesis of the subcritical state of solutions of nonelectrolytes in solvents with a 3D network of hydrogen bonds at low solute concentrations.     

The use of acridine orange base (AOB) as molecular probe to characterize nonaqueous AOT reverse micelles

The behavior of acridine orange base (AOB) in nonaqueous reverse micelles composed of n-heptane/AOT/polar solvent has been performed. Ethylene glycol (EG), propylene glycol (PG), glycerol (GY), formamide (FA), dimethylformamide (DMF), and dimethylacetamide (DMA) were employed as water substitutes. The studies were performed by static and time-resolved emission spectroscopy. Thus, the distribution of AOB between the two pseudophases of the aggregates was quantified by measuring the partition constants from emission spectra at different surfactant concentration. Similar values to those obtained by means of absorption spectroscopy were obtained. This match is indicating that AOB is not experiencing partition during the lifetime of the excited state. Partitioning to the micelles is strongly favored in micelles containing hydrogen-bond donor (HBD) solvents rather than non-HBD solvents. Variations of fluorescence lifetimes with AOT concentration confirm these results. By the solvatochromic behavior of AOB in the different systems it is shown that the microenvironment at the interface is distinct from that of the bulk polar solvent, indicating that the probe senses no "free" solvent. The steady state anisotropy (r) was measured for EG/AOT/n-heptane and DMF/AOT/n-heptane systems as representatives for HBD and non-HBD polar solvents, respectively. The value of r is higher in the micelles containing EG than that obtained with DMF, and increases with AOT concentration. This is explained as due to highly structured polar solvents in the inner core. EG is interacting with the polar heads of AOT through hydrogen-bond interaction, while DMF can only interact with the Na+ counterions. This is confirmed by the time-resolved emission spectra (TRES) of the probe in the micellar systems, in comparison with the bulk solvents.     

Temperature-induced percolation behavior of AOT reverse micelles affected by poly(ethylene glycol)s

The influence of poly(ethylene glycol)s additives viz. mono- (EG), di- (DEG), tri- (TEG), tetra- (TeEG) and poly(ethylene glycol)-400 (PEG-400) on temperature-induced electrical percolation of water/AOT/isooctane microemulsion system has been investigated. The composition of microemulsion systems has been kept constant to omega=22 and [additive] = 0.1 M w.r.t. dispersion medium. The effect of increase in the non-polar continuum (S= [Oil]/[AOT]) is indicated by increase in the percolation threshold, theta(c). The findings have been elaborated in terms of validity of scaling laws in the light of the dynamic percolation theory. The activation energy of the process, DeltaEp, has been estimated from Arrhenius plots. Pseudophase concept of the micellar aggregation has been utilized to assess the thermodynamics of clustering of the nanodroplets. The state of trapped water in the micellar core and the corresponding interactions with the AOT head group has been visualized through 1H NMR and FTIR analysis. Results show that at higher omega (>16.0), encapsulated water behaves like free or the bulk water.     

Effect of temperature and salt on the phase behavior of nonionic and mixed nonionic-ionic microemulsions with fish-tail diagrams

The phase behavior of Brij-56/1-butanol/n-heptane/water is investigated at 30 degrees C with alpha [weight fraction of oil in (oil+water)]=0.5, wherein a 2-->3-->2 phase transition occurs with increasing W1 (weight fraction of 1-butanol in total amphiphile) at low X (weight fraction of both the amphiphiles in the mixture) and a 2-->1-->2 phase transition occurs at higher X. Addition of an ionic surfactant, sodium dodecylbenzene sulfonate, destroys the three-phase body and decreases the solubilization capacity of the system at different delta (weight fraction of ionic surfactant in total surfactant). A three-phase body appears at alpha=0.25, but not at alpha=0.75 for the single system. No three-phase body appears with the mixed system at either alpha value. Increased temperature increases the solubilization capacity of the Brij-56 system; on the other hand, a negligible effect of temperature on the Brij-56/SDBS mixed system has been observed. Addition of salt (NaCl) produces a three-phase body for both single and mixed systems and increases their solubilization capacities. The monomeric solubility of 1-butanol in oil (S1) and at the interface (S1s) has been calculated using the equation hydrophile-lipophile balance plane for both singles- and mixed-surfactant systems. These parameters have been utilized to explain the increase in solubilization capacity of these systems in the presence of NaCl.     

SANS studies of the effects of surfactant head group on aggregation properties in water/glycol and pure glycol systems

Aggregation in mixed water-glycol and pure glycol solvents has been investigated with four related surfactants, bearing common C12 tails: anionic, sodium dodecylsulfate (SDS); cationic, dodecyltrimethylammonium bromide (C12TAB); zwitterionic C12-amidopropyldimethylamine betaine (betaine) and nonionic, octaethyleneglycol monododecyl ether (C12E8). The solvent media were water, water/ethylene glycol, and water/propylene glycol mixtures, as well as pure ethylene glycol (EG) and propylene glycol (PG), spanning relative dielectrics epsilon(r) from 79 to 30. Results from small-angle neutron scattering (SANS) experiments, employing deuterated solvents, were consistent with the presence of ellipsoidal, or cylindrical micelles, depending on solvent and surfactant type. In pure EG and PG solvents the ionic and zwitterionic surfactants exhibit only weak aggregation, with much smaller micelles than normally found in water. However, interestingly, pure EG is identified as a solvent in which nonionic C12E8 aggregates strongly, mirroring the behavior in water. In contrast when the solvent is changed to PG (epsilonr=30) aggregation of C12E8 is only minimal. Hence, aggregation is shown to be strongly dependent on surfactant type and identity of the glycol solvent.     

Mixed micelles of polyethylene glycol (23) lauryl ether with ionic surfactants studied by proton 1D and 2D NMR

(1)H NMR chemical shift, spin-lattice relaxation time, spin-spin relaxation time, self-diffusion coefficient, and two-dimensional nuclear Overhauser enhancement (2D NOESY) measurements have been used to study the nonionic-ionic surfactant mixed micelles. Cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were used as the ionic surfactants and polyethylene glycol (23) lauryl ether (Brij-35) as the nonionic surfactant. The two systems are both with varying molar ratios of CTAB/Brij-35 (C/B) and SDS/Brij-35 (S/B) ranging from 0.5 to 2, respectively, at a constant concentration of 6 mM for Brij-35 in aqueous solutions. Results give information about the relative arrangement of the surfactant molecules in the mixed micelles. In the former system, the trimethyl groups attached to the polar heads of the CTAB molecules are located between the first oxy-ethylene groups next to the hydrophobic chains of Brij-35 molecules. These oxy-ethylene groups gradually move outward from the hydrophobic core of the mixed micelle with an increase in C/B in the mixed solution. In contrast to the case of the CTAB/Triton X-100 system, the long flexible hydrophilic poly oxy-ethylene chains, which are in the exterior part of the mixed micelles, remain coiled, but looser, surrounding the hydrophobic core. There is almost no variation in conformation of the hydrophilic chains of Brij-35 molecules in the mixed micelles of the SDS/Brij-35 system as the S/B increases. The hydrophobic chains of both CTAB and SDS are co-aggregated with Brij-35, respectively, in their mixed micellar cores.     

Excited state proton transfer in di-(2-hydroxy-3-formyl-5-tert butyl phenyl) methane and solvent effect

The proton transfer reaction and the spectroscopic properties of di-(2-hydroxy-3-formyl-5-tert butyl phenyl) methane (HFPM) have been examined in different nonpolar and polar solvents at room temperature and 77 K, by means of absorption, emission and time resolved fluorescence spectroscopy. In the ground state, the primary closed form has been identified in all the nonpolar and polar solvents and the anion is detected only in presence of base in some of the polar solvents. After photoexcitation, the excited state intramolecular proton transfer (ESIPT) is indicated by a large Stokes shifted emission (approximately 10,600 cm-1) in all the nonpolar and polar solvents used, except in water and ethylene glycol (EG). The ESIPT band is likely to be originated from the enol tautomer of the HFPM. Two types of anion and H-bonded complex have been detected in the excited state. In water and EG, only anion and H-bonded complex have been detected in the excited state. At 77 K, HFPM shows phosphorescence in pure ethanol, and in n-hexane in presence of triethylamine. It has been suggested that the appearance of phosphorescence is due to the rotation of the formyl group. The measured nonradiative decay rates have always been found to dominate in the decay processes of the excited state of HFPM. Some semiempirical calculations have been undertaken to rationalize the experimental findings.     

有序介孔材料H6P2W18O62/TiO2(Brij-76)的制备与微波增强光催化降解一氯苯

采用非离子表面活性剂C18H37(OCH2CH2)10OH(Brij-76)作为模板剂, 在以杂多酸H6P2W18O62对TiO2掺杂改性基础上, 通过模板-溶胶-凝胶-程序升温溶剂热一步法在较低温度下制备了有序复合介孔材料H6P2W18O62/TiO2(Brij-76). 通过傅立叶变换红外(FT-IR)光谱, X射线衍射(XRD), 扫描电子显微镜(SEM), 能量色散X射线(EDX), N2吸附-脱附测定和NH3程序升温脱附(NH3-TPD)等手段对其进行了表征. 结果表明, 以非离子表面活性剂Brij-76为模板剂制得的复合材料H6P2W18O62/TiO2(Brij-76)平均孔径约为3.31 nm, BET比表面积为99.78 m2·g-1. 与TiO2相比, 其孔径有序性大幅度提高, 粒子的聚集度降低, 表面酸性显著增加. 微波增强光催化性能研究结果显示, H6P2W18O62/TiO2(Brij-76)在微波作用下催化活性更高, 可有效地降解一氯苯溶液.     

Broadband dielectric study of dynamics of poly(vinyl pyrrolidone)-ethylene glycol oligomer blends

Broadband dielectric measurements for blends of poly(vinyl pyrrolidone) (PVP) and ethylene glycol oligomer (EGO) from 0 to 40 wt % PVP were carried out at 25 degrees C in the frequency range from 20 Hz to 20 GHz. The EGOs used in this study were ethylene glycol (EG), diethylene glycol (2EG), and PEG400 (MW = 400). For the PVP-EG, -2EG, and -PEG400 blends, relaxation processes caused by the motion of EGO in the GHz range and the micro-Brownian motion of the PVP chain at 10 kHz-1 MHz were observed. Although the PVP-EGO blend is miscible, relaxation processes caused by the molecular motion of EGO and the local chain motion of PVP were observed individually. The relaxation time of the local chain motion of PVP showed a strong PVP concentration dependence and a solvent viscosity dependence, which are similar to those reported so far for the solutions in nonpolar solvents.     

Interface of AOT/Brij mixed reverse micellar systems: conductometric and spectrophotometric investigations

Solubilization and conductivity studies are carried out with AOT/Brijs (Brij-30, Brij-35, Brij-52, Brij-56, Brij-58, Brij-72, Brij-76, Brij-78)/isooctane/water mixed reverse micellar systems. Replacement of AOT molecules with large head group Brij molecules (Brij-30, Brij-35, Brij-56, Brij-58, Brij-76, Brij-78) decreases the solubilization capacity, whereas those with smaller polar head groups (Brij-52 and Brij-72) increases it. The former blends assist the conductance percolation whereas the latter retard it. An attempt has been taken to obtain more insight on the interfacial composition of the mixed interface with the help of spectrophotometric studies using 7-hydroxycoumarin as the fluorophore. The results obtained from the solubilization and conductometric studies have been correlated with those obtained from the spectroscopic studies.     

Physicochemical investigations of microemulsification of eucalyptus oil and water using mixed surfactants (AOT+Brij-35) and butanol

Microemulsification of a vegetable oil (eucalyptus) with single and mixed surfactants (AOT and Brij-35), cosurfactant of different lipophilicities (isomers of butanol), and water were studied at different surfactant and cosurfactant mixing ratios. The phase diagrams of the quaternary systems were constructed using unfolded and folded tetrahedron, wherein the phase characteristics of different ternary systems can be underlined. The microemulsion zone was found to be dependent upon the mixing ratios of surfactant and cosurfactant; the largest microemulsion zone was formed with 1:1 (w/w) S:CS. The effects of temperature and additives (NaCl, urea, glucose, and bile salts of different concentrations) on the phase behavior were examined. The mixed microemulsion system showed temperature insensitivity, whereas the Brij-35 (single) stabilized system exhibited a smaller microemulsion zone at elevated temperature. NaCl and glucose increased the microemulsion zone up to a certain concentration, beyond which the microemulsion zones were decreased. These additives decreased the microemulsion zones as temperature was increased. The effect of urea on microemulsion zone was found to be insignificant even at the concentration 3.0 mol dm(-3). Little effect on microemulsion zone was shown by NaC (sodium cholate) at 0.25 and 0.5 mol dm(-3) at different temperatures. The conductance of the single (AOT) and mixed microemulsion system (AOT+Brij-35) depends upon the water content and mixing ratios of the surfactants, and a steep rise in conductance was observed at equal weight percentages of oil and water. Viscosities for both single (AOT) and mixed (AOT+Brij-35) surfactant systems passed through maxima at equal oil and water regions showing structural transition. The viscosities for microemulsion systems increased with increasing Brij-35 content in the AOT+Brij-35 blend. Conductances and viscosities of different monophasic compositions in the absence and presence of additives (NaCl and NaC) were measured at different temperatures. The activation energy of conduction (DeltaE(cond)( *)) and the activation enthalpy for viscous flow (DeltaH(vis)( *)) were evaluated. It was found that both DeltaE(cond)( *) and DeltaH(vis)( *) were a function of the nature of the dispersion medium. Considering the phase separation point of maximum solubility, the free energy of dissolution of water or oil (DeltaG(s)(0)) at the microdispersed state in amphiphile medium was estimated and found to be a function of surfactant composition.     

Interaction between poly(ethylene glycol) and water as studied by differential scanning calorimetry

The interaction between poly(ethylene glycol) (PEG) and water was studied by differential scanning calorimetry (DSC). The DSC curves of PEG–water systems were classified into three groups according to the difference in molecular weight. The melting peaks of eutectic mixture appeared for PEG with molecular weight higher than 1000. The eutectic point temperature shifted to higher temperatures and the eutectic point composition shifted to lower concentrations of PEG with increasing molecular weight. The maximum hydration number per ethylene glycol (EG) unit was estimated as 1.6, 2.4, and 3.3 for samples with molecular weights 400, 1540, and 70,000, respectively. No thermal change was found in PEG1540‐water system for a narrow weight fraction range of 0.585–0.605 for overall measuring temperatures due to perfect supercooling. The glass transition temperature shifted to higher temperatures with increasing molecular weight of PEG. A modified Flory–Huggins equation was used to fit curves for experimental liquidus data in phase diagrams. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 496–506, 2001     

Controlling aggregation of nonionic surfactants using mixed glycol media

The extent of aggregation of nonionic surfactants can be controlled by the composition of mixed solvents with two miscible glycols, ethylene glycol (EG)/propylene glycol (PG). Three nonionic surfactants bearing a common E8 ethoxylated headgroup, but with variations in the hydrocarbon chain, have been investigated: octaethylene monododecyl ether (C12E8), octaethylene monotetradecyl ether (C14E8), and octaethylene monohexadecyl ether (C16E8). The hydrogen-bonding solvents were EG/PG mixtures at different PG levels, defined in terms of the concentration (mol %) of PG. Aggregation was investigated using small-angle neutron scattering (SANS) with h-CiE8 surfactants, at 10 and 5 wt %, in deuterated glycol solvents to improve contrast. Increasing PG concentration (mol %) in the background EG/PG solvent leads to a consistent decrease in the SANS intensity, until in pure d-PG only very weak scattering is observed. These SANS data were analyzed using cylinder or ellipsoidal form factors for the EG-rich and PG-rich systems, respectively, hence demonstrating an aggregate shape change as a function of solvent composition. The results show that aggregation of nonionic surfactants occurs in glycol solvents and that the EG:PG ratio may be used as an effective means to switch aggregation "on" or "off", as required.     

Interaction of self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiols with water studied by vibrational sum-frequency generation

Vibrational sum-frequency generation (VSFG) was used to investigate the conformational changes in self-assembled monolayers (SAMs) of (1-mercaptoundec-11-yl) hexa(ethylene glycol) monomethylether (EG6-OMe) on gold when exposed to liquid water. VSFG spectra of the EG6-OMe SAMs were recorded before, during, and after exposure of the films to water and after a subsequent evacuation step. While in contact with water the entire ethylene glycol chains are found in a random, solvated state, after removal from the fluid water molecules remain absorbed only at the terminal groups of the film giving rise to distinct conformational changes. After evacuation, the structure of the EG6-OMe SAM reverts to its original state, indicating that water has been removed from the monolayer. Our findings support recent ab initio calculations and Monte Carlo simulations on the interaction of ethylene glycol-terminated monolayers with water.     

The Distribution of mixtures of dodecyl ether of poly(23)ethylene glycol with sodium dodecyl sulfate and dodecyltrimethylammonium bromide in the water/octane system

The scintillation phase and tensiometry methods were used to study the mutual influence of dodecyl ether of poly(23)ethylene glycol (Brij-35) with sodium dodecyl sulfate and Brij-35 with dodecyltrimethylammonium bromide on the distribution in the water/octane system and adsorption at the liquid/liquid interface. The composition of mixed adsorption layers was determined and interaction parameters between molecules were calculated according to the Rosen model.