The Adsorption of Gemini and Conventional Surfactants onto Some Soil Solids and the Removal of 2-Naphthol by the Soil Surfaces

The adsorption of two cationic gemini surfactants, [C(n)H(2n+1) N(+)(CH(3))(2)-CH(2)CH(2)](2).2Br(-), where n=12 and 14, on limestone, sand, and clay (Na-montmorillonite) from their aqueous solution in double-distilled water and the effect of this adsorption on the removal of 2-naphthol have been studied. Compared to those of conventional cationic surfactants with similar single hydrophilic and hydrophobic groups (C(n)H(2n+1)N(+)(CH(3))(3).Br(-), where n=12 and 14), the molar adsorptions of the gemini and the conventional surfactants on Na-montmorillonite are almost identical and very close to their cation exchange capacities. On sand and limestone, the molar adsorption of the cationic gemini surfactants is much larger than that of their corresponding conventional surfactants. Adsorption studies of the pollutants onto the three kinds of solids treated by either the gemini or the conventional surfactants show that the former are both more efficient and more effective at removing 2-naphthol from the aqueous phase. On all three soil solids, the addition of KBr increases the efficiency of the adsorption of both types of cationics and for most cases increases also the maximum amount adsorbed, but decreases slightly the efficiency of removal of 2-naphthol. On limestone, the anionic gemini adsorbs with one hydrophilic group oriented toward the Ca(2+) sites on the surface and its second hydrophilic group oriented toward the aqueous phase. The conventional anionic surfactant forms a double layer. The gemini anionic is more efficient and more effective than the conventional anionic in the removal of 2-naphathol from the aqueous phase. Both anionic conventional and gemini surfactants have no adsorption on sand. The adsorption mechanisms for all the surfactants on the three soil solid surfaces are discussed. Copyright 2001 Academic Press.     

Microcalorimetric study on the interaction of dissymmetric gemini surfactants with DNA

The interaction of a series of dissymmetric gemini surfactants, [C(m)H(2m+1)(CH(3))(2)N(CH(2))(6)N(CH(3))(2)C(n)H(2n+1)]Br(2) (designated as C(m)C(6)C(n)Br(2), with constant m+n=24, and m=12, 14, 16, and 18) with DNA in 10 mM NaCl solution has been investigated by isothermal titration microcalorimetry (ITC). The curves for titration of the surfactants into DNA solution show noticeable differences from those into 10 mM NaCl solution without DNA. It is attributed to the interaction between DNA and surfactants. The critical aggregation concentration (CAC), the saturation concentration (C(2)), and the thermodynamic parameters for the aggregation and interaction processes were obtained from the calorimetric titration curves. The results show that the dissymmetry degree (m/n) has a marked effect on the interaction of the C(m)C(6)C(n)Br(2) surfactants with DNA. The CAC and C(2) tend to become smaller with increased m/n. The enthalpy change (DeltaH(agg)) and the Gibbs free energy change (DeltaG(agg)) for aggregation become more negative down the series, indicating that the hydrophobic interaction between the hydrophobic chains of the surfactant molecules increases and the aggregation process is more spontaneous with increased m/n. The entropy changes of aggregation (DeltaS(agg)) are all positive and TDeltaS(agg) is much larger than |DeltaH(agg)|, revealing that the aggregation process is mainly entropy-driven. However, the calculated Gibbs free energy (DeltaG(DS)) for the interaction between the gemini surfactants and DNA becomes less negative with increased m/n, which reveals that the interaction between the gemini surfactants and DNA tends to be weaker with increased m/n. This is induced by the disruption of the chain-chain hydrophobic interaction between the surfactant molecules at higher m/n, where the entropy change DeltaS(DS) for the interaction process tends to be an unfavorable factor. In addition, the DNA concentration also has a remarkable influence on the interaction.     

KOH catalysed preparation of activated carbon aerogels for dye adsorption

Surfactants prevent the irreversible aggregation of partially refolded proteins, and they are also known to assist in protein refolding. A novel approach to protein refolding that utilizes a pair of low molecular weight folding assistants, a detergent and cyclodextrin, was proposed by Rozema and Gellman (D. Rozema, S.H. Gellman, J. Am. Chem. Soc. 117 (1995) 2373). We report the refolding of bovine serum albumin (BSA) assisted by these artificial chaperones, utilizing gemini surfactants for the first time. A combination of cationic gemini surfactants, bis(cetyldimethylammonium)pentane dibromide (C(16)H(33)(CH(3))(2)N(+)-(CH(2))(5)-N(+)(CH(3))(2)C(16)H(33)·2Br(-) designated as G5 and bis(cetyldimethylammonium)hexane dibromide (C(16)H(33)(CH(3))(2)N(+)-(CH(2))(6)-N(+)(CH(3))(2)C(16)H(33)·2Br(-) designated as G6 and cyclodextrins, was used to refold guanidinium chloride (GdCl) denatured BSA in the artificial chaperone assisted two step method. The single chain cationic surfactant cetyltrimethylammonium bromide (CTAB) was used for comparative studies. The studies were carried out in an aqueous medium at pH 7.0 using circular dichroism, dynamic light scattering and ANS binding studies. The denatured BSA was found to get refolded by very small concentrations of gemini surfactant at which the single chain counterpart was found to be ineffective. Different from the single chain surfactant, the gemini surfactants exhibit much stronger electrostatic and hydrophobic interactions with the protein and are thus effective at much lower concentrations. Based on the present study it is expected that gemini surfactants may prove useful in the protein refolding operations and may thus be effectively employed to circumvent the problem of misfolding and aggregation.     

Surface properties, aggregation behavior and micellization thermodynamics of a class of gemini surfactants with ethyl ammonium headgroups

Cationic gemini surfactant homologues alkanediyl-α,ω-bis(dodecyldiethylammonium bromide), [C(12)H(25)(CH(3)CH(2))(2)N(CH(2))(S)N(CH(2)CH(3))(2)C(12)H(25)]Br(2) (where S=2, 4, 6, 8, 10, 12, 16, 20), referred to as C(12)C(S)C(12)(Et) were synthesized systematically. This paper focused on various properties of the above gemini surfactants in order to give a full understanding of this series of surfactants. The following points are covered: (1) surface properties, which include (i) effect of the spacer carbon number on the general properties and (ii) the effect of added NaBr on the general surface properties; (2) aggregation behavior in bulk solution, including (i) morphologies of above gemini surfactants classed as having short spacers, middle-length spacers and long spacers and (ii) superior vesicle stability against high NaBr concentration for the long spacer gemini surfactants; (3) thermodynamic properties during micellization and the effect of spacer carbon number on them; and (4) perspectives for the further use and application of these compounds.     

Effect of hydrocarbon parts of the polar headgroup on surfactant aggregates in gemini and bola surfactant solutions

Cationic gemini surfactant homologues alkanediyl-alpha,omega-bis(dodecyldiethylammonium) bromide, [C12H25(CH3CH2)2N(CH2)SN(CH2CH3)2C12H25]Br2, where S = 4, 6, 8, 10, or 12, referred to as C12CSC12(Et), and cationic bolaamphiphiles BPHEAB (biphenyl-4,4'-bis(oxyhexamethylenetriethylammonium) bromide), PHEAB (phenyl-4,4'- bis(oxyhexamethylenetriethylammonium) bromide) were synthesized, and their aggregation behaviors in aqueous solution were studied and compared by means of dynamic light scattering, fluorescence entrapment, and transmission electron microscopy. Spherical vesicles were found in the aqueous solutions of these gemini and bola surfactants, which can be attributed to the increase of the hydrocarbon parts of the polar headgroup of the surfactants. In combination with the result of the other gemini with headgroup of propyl group, the increase of the hydrophobic parts of the surfactant polar headgroup will be beneficial to enhance the aggregation capability of the gemini and bola surfactants. Both of the vesicles formed in the gemini and bola systems showed good stabilities with time and temperature, but different stability with salt due to the different membrane conformations of surfactant molecules in the vesicles.     

Self-assembly of cationic surfactants that contain thioether groups in the hydrophobic tails

Self-assembly in aqueous solutions of cationic surfactants that carry thioether groups in their hydrophobic tails has been investigated. Of particular interest was the identification of possible changes in the aggregate structure due to the presence of sulfur atoms. Solutions of four different compounds [CH(3)CH(2)S(CH(2))(10)N(CH(3))(3)(+)Br(-) (2-10), CH(3)(CH(2))(5)S(CH(2))(6)N(CH(3))(3)(+)Br(-) (6-6), CH(3)(CH(2))(7)S(CH(2))(6)N(CH(3))(3)(+)Br(-) (8-6), and CH(3)(CH(2))(7)S(CH(2))(8)N(CH(3))(3)(+)Br(-) (8-8)] were characterized by (1)H NMR, (13)C NMR, NMR diffusometry, and conductivity measurements. In addition to investigating aqueous solutions containing each of the thioethers present as the sole solute, mixtures of 2-10 or 6-6 with dodecyltrimethylammonium bromide (DTAB) were studied. The addition of a sulfide group to the hydrophobic tail causes an increase in the critical micelle concentration but has a limited effect on the aggregate structure. Micelles are formed at a well-defined concentration for all of the investigated surfactants and surfactant mixtures. However, a comparison of the behavior of concentrated solutions of 8-8 to that of solutions of hexadecyltrimethylammonium bromide (CTAB) of similar concentrations suggests that the presence of a sulfur atom decreases the tendency for micellar growth. This may be a consequence of a slightly higher preference for the micellar surface of a sulfur atom as compared to that of a methylene group in a similar position, an idea that is also supported by results for the surfactant mixtures.     

Role of added counterions in the micellar growth of bisquaternary ammonium halide surfactant (14-s-14): 1H NMR and viscometric studies

The change in the morphology of a series of dicationic gemini surfactants C(14)H(29)(CH(3))(2)N(+)-(CH(2))(s)-N(+)(CH(3))(2)C(14)H(29), 2Br(-) (14-s-14; s=4-6) on their interaction with inorganic (KBr, KNO(3), KSCN) and organic salts (NaBenz, NaSal) have been thoroughly investigated by means of (1)H NMR spectral analysis and the results are well supported by viscosity measurements. The presence of salt counterions results in structural transition (spherical to nonspherical) of gemini micelles in aqueous solution. With an increase in salt concentration all the three gemini surfactants showed changes in their aggregate morphology. This change is dependent on the nature and size of the added counterion. The effect of inorganic counterions on the micellar growth is observed to follow the Hofmeister series (Br(-) < NO(3)(-) < SCN(-)). The roles of organic counterions are discussed on the basis of probable solubilization sites of the substrate molecule in the gemini micelles, showing more growth in case of Sal(-) than Benz(-). The results are confirmed in terms of the obtained values of chemical shift (δ), line width at half height (lw), and relative viscosity (η(r)). Also, the growth of micelles was most pronounced for the gemini surfactant with the shortest spacer (s=4). This was attributed to the unique molecular structure of gemini surfactant micelles having flexible polymethylene spacer chain linking the twin polar headgroups.     

Effect of the spacer length on the association and adsorption behavior of dissymmetric gemini surfactants

A series of dissymmetric gemini surfactants with the general formula [C12H25(CH3)2N(CH2)sN(CH3)2C14H29]Br2 designed as 12-s-14, where s=2, 6, and 10, were synthesized and their physicochemical properties investigated. The effect of spacer length on Krafft temperature, adsorption at the air/solution interface, and association in aqueous solution was studied by tensiometry, conductometry, and cryo-transmission electron microscopy. The Krafft temperature was found to increase linearly with spacer length. In the submicellar concentration range the dissymmetric 12-s-14 surfactants display ion pairing and premicellar association. Adsorption at air/solution interfaces and micellization in aqueous solution are similar to the behavior of their symmetric counterparts and depend strongly on spacer length.     

Effect of surfactant structure on the stability of carbon nanotubes in aqueous solution

The suspending behaviors of multiple-wall carbon nanotubes (MWNTs), including pristine MWNTs (p-MWNTs) and acid-mixture-treated MWNTs (MWNTCOOH), stabilized by cationic single-chain surfactant, dodecyltrimethylammonium bromide (DTAB), and cationic gemini surfactant hexyl-alpha,beta-bis(dodecyldimethylammonium bromide) (C 12C 6C 12Br 2) were studied systematically. The surfactant structure influences the suspendability of MWNTs dramatically as well as the surfactant adsorption behavior on the nanotubes. Although both the surfactants can disperse the MWNTs effectively, they actually show different stabilizing ability. DTAB is not capable of stabilizing these two MWNTs below critical micelle concentration (CMC). However, C 12C 6C 12Br 2 can suspend both the nanotubes effectively even well below its CMC. Moreover, the adsorption of these two surfactants reaches equilibrium at twice the CMC with the original MWNT concentration of 2 mg/mL, 2 mM for C 12C 6C 12Br 2, and 30 mM for DTAB. After the adsorption equilibrium, the maximum amounts of the two suspended MWNTs in C 12C 6C 12Br 2 solution are about twice as much as those in DTAB solution. The strong hydrophobic interaction among the C 12C 6C 12Br 2 molecules and between the C 12C 6C 12Br 2 molecules and the nanotubes as well as the high charge capacity of C 12C 6C 12Br 2 lead to its much stronger adsorption ability on the MWNTs and result in its superior stabilizing ability for the MWNTs in aqueous phase. The gemini surfactant provides a possibility to effectively stabilize the MWNTs in aqueous solutions even at very low surfactant concentration well below its CMC.     

Investigation of complexes formed by interaction of cationic gemini surfactants with deoxyribonucleic acid

Cationic gemini surfactants, N,N-bis(dimethylalkyl)-alpha,omega-alkanediammonium dibromide [C(m)H(2m+1)(CH(3))(2)N(+)(CH(2))(s)N(+)(CH(3))(2)C(m)H(2m+1) x 2 Br(-), or m-s-m], have proven to be effective synthetic vectors for gene delivery (transfection). Complexes (lipoplexes) of gemini compounds, where m = 12, s = 3, 12 and m = 18 : 1(oleyl), s = 2, 3, 6, with DNA have been investigated using isothermal titration calorimetry (ITC), dynamic light scattering (DLS), zeta potential, atomic force microscopy (AFM) and circular dichroism (CD) techniques. The results show that lipoplex properties depend on the structural properties of the gemini surfactants, the presence of the helper lipid dioleoylphosphatidylethanolamine (DOPE), and the titration sequence. ITC data show that the interaction between DNA and gemini surfactants is endothermic and the observed enthalpy vs. charge ratio profile depends upon the titration sequence. Isoelectric points (IP) of lipoplex formation were estimated from the zeta potential measurements and show good agreement with the reaction endpoints (RP) obtained from ITC. DLS data indicate that DNA is condensed in the lipoplex. AFM images suggest that the lipoplex morphology changes from isolated globular-like aggregated particles to larger-size aggregates with great diversity in morphology. This change is further accentuated by the presence of DOPE in the lipoplexes. The results are interpreted in terms of some current models of lipoplex formation.     

Synthesis and properties of thioether spacer containing gemini imidazolium surfactants

Twelve new gemini imidazolium surfactants have been synthesized, having dodecyl, tetradecyl, hexadecyl, and octadecyl chain lengths and three different spacers (i.e., -S-(CH(2))(n)-S-), where n = 2, 3, and 4 and their surface properties have been evaluated by surface tension and conductivity methods. The thermal degradation of these new gemini surfactants was determined by thermogravimetric analysis (TGA). These surfactants have low cmc values as compared to other categories of gemini cationic surfactants and exhibit peculiarities at sufficiently low concentration because they were able to form premicellar aggregates over a wide range of concentration below their cmc values. The DNA binding affinity of these gemini surfactants determined by agarose gel electrophoresis and ethidium bromide exclusion experiments established their strong interaction with DNA, thereby protecting it against enzymatic degradation.     

Hydrophilicity of Polar and Apolar Domains of Amphiphiles

The hydrophilicity of polar and apolar domains of various amphiphiles was systematically estimated for their homologues and analogues by measuring the molar adiabatic compressibility of an aqueous solution at infinite dilution. The homologues of protic alkyl H(CH(2))(n)-, perfluoroalkyl F(CF(2))(n)-, and alkylphenyl H(CH(2))(n)(C(6)H(5))- groups (n=0-10) were chosen to represent apolar hydrophobic domains. The polar hydrophilic domains tested were -SO(4)Na, -SO(3)Na, -COONH(4), -N(CH(3))(3)Br, N(C(m)H(2m+1))(4)Br (m=1-5), and -NH(CH(2))(n)SO(3) (n=3, 4) groups. Also tested were the tetraphenyl ionic compounds (C(6)H(5))(4)MX (M=B/X=Na, M=P/X=Cl, M=As/X=Cl) to study the effect of the ionic sign of the core atom across the tetraphenyl apolar shell, the polyethylene glycols H(OCH(2)CH(2))(m)OH (m=1-4) to study the role of apolar -CH(2)- units in the hydrophilic oxyethylene group, and the zwitterionic dimethylaminoalkylsulfonate (CH(3))(2)NH(CH(2))(n)SO(3) homologues to study the effect of intramolecular salt formation on the hydrophilicity of the zwitterion. The adiabatic compressibility of the solution was calculated from measurement of the sound velocity and density of solutions. The introduction of laboratory automation and the numerical control of the system improved the accuracies and efficiencies of the measurements a great deal. The range of the temperature scan was 0-40 degrees C with an effective accuracy of +/-0.001 degrees C and the concentration was automatically scanned down to far below the cmc of the surfactant. The hydrophilicity of various polar and apolar substances was estimated as the decrease of molar adiabatic compressibility of the aqueous solution with increased concentration of their homologues and analogues. The hydrophobic hydration of nonpolar substances was found to be very small at room temperature and was barely detected above 40 degrees C; however, it became large as the temperature was lowered and attained a maximum at 0 degrees C. The cationic charge of quaternary ammonium N(+)(C(n)H(2n+1))(4) was found to enhance the hydrophobic hydration of methylene groups located at a distance of 4 to 6 ? from the core nitrogen atom, while the terminal negative charge of the anionic surfactant R-SO(4)(-), R-SO(3)(-), or R-COO(-) was found to decrease the hydrophobic hydration of -CH(2)- units within the same range. The hydrophilicity of quaternary ammonium and the tetraphenyl ions should be synergistically given by both hydrophobic and ionic hydrations. The hydrophilicity of the perfluoromethylene unit -CF(2)- was found to have a value comparable to that of the protic methylene unit -CH(2)-. The hydrophobic hydration seems to offer a good measure of the hydrophilicity of apolar substances; however, it does not necessarily represent the "hydrophobicity" of the apolar segment when the "surface activity" of the amphiphile is concerned. Copyright 2000 Academic Press.     

Synthesis and dilute aqueous solution properties of ester functionalized cationic gemini surfactants having different ethylene oxide units as spacer

New series of ester functionalized quaternary ammonium gemini surfactants having different ethylene oxide units as spacer have been synthesized and investigated for their aggregation behavior and thermodynamic properties of micellization by surface tension, conductivity, and fluorescence methods. The critical micelle concentration (cmc) of these gemini surfactants increases with the increase in the length of polar hydrophilic ethylene oxide spacer. The micellization process has been found to be entropy-driven and dependent on both the tendency of the hydrophobic group of the surfactants to transfer from aqueous environment to interior of micelle as well as the rearrangement of flexible ester-linked ethylene oxide units (hydrophilic spacer) into aqueous phase. The polar ester functional groups and pairs of nonbonding electrons on oxygen atom of ethylene oxide spacer form hydrogen bonding with water molecules enhancing their solubility in aqueous system.     

Aggregation properties of supralong-chain surfactants with double or triple quaternary ammonium head groups

Double or triple quaternary ammonium head groups were designed to improve the solubility of supralong alkyl chain surfactants. In the surfactant head group, quaternary ammonium groups are connected by an ethylene spacer. Micellar shapes of divalent surfactants [C(n)H(2n)(+1)N(+)(CH(3))(2)-(CH(2))(2)-N(+)(CH(3))(3) 2Br(-): C(n)-2Am (n=18, 20, and 22)] and trivalent surfactants [C(n)H(2n)(+1)N(+)(CH(3))(2)-(CH(2))(2)-N(+)(CH(3))(2)-(CH(2))(2)-N(+)(CH(3))(3) 3Br(-): C(n)-3Am (n=18, 20, and 22)] were studied in aqueous solutions by means of dynamic light scattering (DLS) and transmission electron microscopy (TEM). Changes in the surfactant concentration have a small influence on the apparent hydrodynamic radii (r(h)) of the molecular aggregates in both surfactant series. Average values of r(h) of aggregates are 60-90 nm for C(n)-2Am (n=18, 20, and 22) and 2-40 nm for C(n)-3Am (n=18, 20, and 22). TEM micrographs showed that aggregates of C(n)-2Am (n=18, 20, and 22) typically formed rod-like micelles. In contrast, trivalent surfactants of C(n)-3Am (n=18, 20, and 22) formed spherical (C(18)-3Am) or ellipsoidal micelles (C(20)-3Am and C(22)-3Am). Moreover, the degree of micellar counterion binding for these surfactants was determined by using a bromide ion-selective electrode, which indicated relatively high values (0.8-0.9) for C(n)-2Am (n=18, 20, and 22) and more common values (0.5-0.8) for C(n)-3Am (n=18, 20, and 22). The size of the aggregates is closely related to the degree of counterion binding.     

Specific ion pairing and interfacial hydration as controlling factors in gemini micelle morphology. Chemical trapping studies

Results from chemical trapping experiments in micellar solutions containing 1.5-5 mM aqueous solutions of three didodecyl dicationic dibromide gemini surfactants with different methylene spacer lengths (12-n-12 2Br where n = 2-4 CH(2) groups) gave quantitative estimates of the molarities of interfacial bromide (Br(m)) and water (H(2)O(m)), the fractions of free and paired headgroups and counterions, and the net headgroup charge. These results are one of the most detailed compositional studies of an association colloid interface to date. Br(m) increases and H(2)O(m) decreases as n decreases and the two cationic charges are closer together. The 12-2-12 2Br gemini (the only one of the three geminis known to form threadlike micelles) shows a marked increase in Br(m) (from 2.3 to 3.6 M) and a decrease in H(2)O(m) (from 35 to 17 M) at the exceptionally low surfactant concentration in the vicinity of the previously reported sphere-to-rod transition or second cmc concentration. Rod formation occurs because of an increase in headgroup-counterion association and dehydration at the micelle surface that depend on both the free energies of hydration and specific ion interactions and surfactant and counterion concentrations. These and other recent chemical trapping results support a new model for the balance of forces controlling morphological transitions of association colloids. The hydrophobic effect drives the formation of headgroup-counterion pairs, which have a lower demand for water of hydration. Release of water permits tighter packing and formation of cylindrical aggregates.     

Polymerizable cationic gemini surfactant

A polymerizable cationic gemini surfactant, [CH(2)=C(CH(3))COO(CH(2))(11)N(+)CH(3))(2)CH(2)](2).2Br(-), 1 has been synthesized and its basic interfacial properties were investigated (in water and in the presence of 0.05 M NaBr). For comparison, the properties of monomeric surfactant corresponding to 1, CH(2)=C(CH(3))COO(CH(2))(11)N(+)(CH(3))(3).Br(-), 2, were also investigated. Parameters studied include cmc (critical micelle concentration), C(20) (required to reduce the surface tension of the solvent by 20 mN/m), gamma(cmc) (the surface tension at the cmc), Gamma(cmc) (the maximum surface excess concentration at the air/water interface), A(min) (the minimum area per surfactant molecule at the air/water interface), and cmc/C(20) ratio (a measure of the tendency to form micelles relative to adsorb at the air/water interface). For the polymerizable gemini surfactant, 1, the methacryloxy groups at the terminal of each hydrophobic group in a molecule have no contact with the air/water interface in the monolayer, whereas for the corresponding monomeric surfactant, 2, the methacryloxy group contacts at the interface forming a looped configuration like a bolaamphiphile. Polymerized micelles of the gemini surfactant are fairly small monodisperse and spherical particles with a mean diameter of 3 nm.     

Comparative studies on interactions of bovine serum albumin with cationic gemini and single-chain surfactants

The interactions of bovine serum albumin (BSA) with cationic gemini surfactants alkanediyl-alpha,omega-bis(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)(S)N(CH3)2C12H25]Br2 (designated as C12C(S)C12Br2, S = 3, 6, and 12) and single-chain surfactant dodecyltrimethylammonium bromide (DTAB) have been studied with isothermal titration microcalorimetry, turbidity, fluorescence spectroscopy, and circular dichroism at pH 7.0. Comparing with DTAB, C12C(S)C12Br2 have much stronger binding ability with BSA to induce the denaturation of BSA at very low molar ratio of C12C(S)C12Br2/BSA, and C12C(S)C12Br2 have a much stronger tendency to form insoluble complexes with BSA. The binding of C12C(S)C12Br2 to BSA generates larger endothermic peaks. The first endothermic peak is much stronger than that of the second endothermic peak. The double charges and strong hydrophobicity of the gemini surfactants are the main reasons for these observations. In addition, the spectra results show that the binding of DTAB to BSA only promotes BSA unfolding and aggregation, whereas the secondary structure of BSA is possibly stabilized by a small amount of C12C(S)C12Br2 , even if the small amount of binding C12C(S)C12Br2 could induce the loss of the tertiary structure of BSA. This result may be related to the double tails of gemini surfactants, which may generate the hydrophobic linkages between the nonpolar residues of BSA.     

Nanoscale aggregate structures of trisiloxane surfactants at the solid-liquid interface

The self-associating structures at the solid-liquid interface of three nonionic trisiloxane surfactants ((CH3)3SiO)2Si(CH3)(CH2)3(OCH2CH2)n OH (n = 6, 8, and 12), or BEn, are studied as a function of substrate properties by atomic force microscopy (AFM) imaging and force measurement. These trisiloxane surfactants are known as superwetters, which promote rapid spreading of dilute aqueous solutions on low-energy surfaces. This study also attempts to relate the BEn surface aggregate structures at the solid-liquid interface to their superwetting behavior. Four substrates are used in the study: muscovite mica, highly oriented pyrolytic graphite, and oxidized silicon wafer with and without a full monolayer of self-assembled n-octadecyltrichlorosilane (OTS). The concentration of BEn is fixed at 2 times the critical aggregation concentration (CAC). The BEn surfactants are only weakly attracted to hydrophilic surfaces, more on oxidized silicon than on mica. All three form ordinary planar monolayers on HOPG and OTS-covered oxidized silicon. The significance of surfactant adsorption on the AFM tip is investigated by comparing the force curves obtained by tips with and without thiol modification. The surface aggregate structures of the BEn surfactants correlate with their bulk structures and do not exhibit anomalous adsorption behavior. The adsorption behavior of the BEn superwetters is similar to that of the CmEn surfactants. Thus, our results confirm previous work showing that superwetting shares its main features with other classes of surfactants.     

A thermodynamic study to evidence the alpha,omega-dichloroalkane/ block copolymer mixed aggregates formation: effect of the copolymer architecture

The thermodynamics of alpha,omega-dichloroalkanes in aqueous solutions of (ethylene oxide)(11)(propylene oxide)(16)(ethylene oxide)(11) (L35) and (propylene oxide)(8)(ethylene oxide)(23)(propylene oxide)(8) (10R5) was determined at 298 and 305 K. Modeling the experimental data allowed to calculate the standard free energy (DeltaG(D)(o)/w) and the volume (DeltaV(D)/w) for the additive-copolymer mixed aggregates formation per additive molecule. DeltaG(D)(o)/w for Cl(2)CH(2) and Cl(2)(CH(2))(2) evidenced that the process is controlled by the forces exercising between the chlorine atoms and the OH groups of the copolymer micelles protruded into the aqueous phase. Cl(2)(CH(2))(3) experiences both the hydrophilic and hydrophobic domains into the aggregates. The hydrophobic interactions are more significant in 10R5 whereas the hydrophilic ones are more significant in L35. Temperature increase does not influence DeltaG(D)(o)/w in 10R5, whereas, it does influence DeltaG(D)(o)/w in L35, enhancing the ability of the aggregate to extract the chlorinated compounds from the aqueous phase. The DeltaV(D)/w values are consistent with the free energy results. These insights agree with those predicted by the Flory liquid lattice theory. The calculations extended to several alpha,omega-dichloroalkanes showed that Cl(2)CH(2) and Cl(2)(CH(2))(2) prefer poly(ethylene oxide) (PEO), Cl(2)(CH(2))(3) exhibits the same affinity for both PEO and poly(propylene oxide) (PPO), whereas the more hydrophobic additives show a preference for PPO. The copolymer architecture plays a relevant role in the alpha,omega-dichloroalkane solubilization into the polymeric aggregates.     

Analogs of Grubbs' second generation catalyst with hydrophilic phosphine ligands: phase transfer activation of ring closing alkene metathesis

Analogs of Grubbs' second generation catalyst with hydrophilic phosphine ligands are synthesized, and those with Cy(2)PCH(2)CH(2)N(CH(3))(3)(+) Cl(-) and Cy(2)PCHCH(2)CH(2)N(CH(3))(2)(+)CH(2)CH(2) Cl(-) give much faster ring closing metatheses under CH(2)Cl(2)/aqueous or CH(2)Cl(2)/aqueous HCl biphasic as opposed to CH(2)Cl(2) monophasic conditions. This is attributed to rapid phase transfer of the dissociated ligand to the aqueous phase, where under acidic conditions it is protonated.