Jin-Xin Xiao

Phase behavior and protein partitioning in aqueous two-phase systems of cationic--anionic surfactant mixtures.

Cationic-anionic surfactant mixtures can form aqueous two-phase systems. Such aqueous surfactant two-phase systems (ASTP systems) can be used for separation and purification of biomaterials. In this work we investigated the phase behavior and the partitioning of BSA and lysozyme in the ASTP system formed by mixtures of dodecyltriethylammonium bromide and sodium dodecylsulfate (SDS). The pseudo ternary phase diagram of these mixtures at low total surfactant concentrations contains two narrow two-phase regions, which represent two kinds of different ASTP systems formed when cationic and anionic surfactants are in excess, respectively (called ASTP-C and ASTP-A). The phase separation is associative, one phase is surfactant-rich, and the other phase is surfactant-depleted. Mechanisms behind the phase behavior are discussed. The phase behavior, especially phase separation time and phase volume ratio, is strongly influenced by total concentration and molar ratio of mixed surfactants. The effect of molar ratio is strong, which enables one to get desired phase systems also at very low total concentration by tuning the molar ratio of the surfactants. It was shown that the marked differences of surfactant concentration between the phases makes proteins distribute with different partitioning coefficients. The charges on the micellar surface, which can be adjusted by tuning the molar ratio of cationic surfactants to anionic surfactants, enhance the selectivity of protein partitioning by electrostatic effects. At pH 7.1, in the ASTP-C systems, negatively charged BSA is concentrated in the surfactant-rich phase and positively charged lysozyme in the surfactant-depleted phase, while in ASTP-A systems, a totally opposite partitioning was observed. It was shown that lysozyme could retain activity in ASTP systems.

Demicellization of a Mixture of Cationic−Anionic Hydrogenated/Fluorinated Surfactants through Selective Inclusion by α- and β-Cyclodextrin

The interactions between alpha- and beta-cyclodextrin (alpha-/beta-CD) and an equimolar mixture of octyltriethylammonium bromide (OTEAB) and sodium perfluorooctanoate (SPFO) were studied by 1H and 19F NMR, surface tension, conductivity, and dynamic light scattering. It was shown that beta-CD could destroy the mixed micelles of OTEAB-SPFO by selective inclusion of SPFO. As beta-CD was added, the system was observed to undergo a process like this: beta-CD preferentially included SPFO to form 1:1 beta-CD/SPFO complexes. As the inclusion of SPFO was almost saturated, the mixed micelles broke and all OTEAB was released and exposed to aqueous surroundings. Then 1:1 beta-CD/OTEAB and 2:1 beta-CD/SPFO complexes significantly formed simultaneously. Contrary to beta-CD, alpha-CD exhibited selective inclusion to OTEAB and only weak association with SPFO. alpha-CD could also destroy the mixed micelles of OTEAB-SPFO; however, the demicellization ability of alpha-CD is much smaller than that of beta-CD. These conclusions were also well supported by the calculations of binding constants and DeltaG degrees . Different from the complexes of CD/conventional surfactants, the complexes of beta-CD/SPFO or alpha-CD/OTEAB formed by selective inclusion of CD in the mixed cationic-anionic surfactants may have contributed to the surface activity of the aqueous mixtures. The complexes of alpha-CD/OTEAB showed much more significant contribution to the surface activity than that of the complexes of beta-CD/SPFO.

Unusual location of the pyrene probe solubilized in the micellar solutions of tetraalkylammonium perfluorooctanoates

The aqueous solutions of tetraalkylammonium perfluorooctanoates (C7F15COON(CnH2n+1)4, n = 1, 2, 3, 4, abbr. TMAPFO, TEAPFO, TPAPFO, TBAPFO) were studied by steady-state and time-resolved fluorescence with pyrene as a probe, and the ammonium perfluorooctanoate (APFO) was also studied as a comparison. Unusual response of pyrene fluorescence was observed. The intensity ratio of the first and third vibronic peaks of pyrene (I1/I3) greatly varied with the counterions for these fluorinated surfactants after their critical micelle concentration (cmc): for TEAPFO, the I1/I3 abnormally increased to a value higher than that of water, while for TBAPFO the I1/I3 value exhibited a small extent of decease after cmc. However, no significant change of I1/I3 was observed during the micellization of TMAPFO and TPAPFO. The lifetime of pyrene fluorescence (τ0) among the systems of tetraalkylammonium perfluorooctanoates (TAAPFOs) was observed to be increased with the hydrophobicity of counterions. Moreover, the time-resolved fluorescence decay curves of pyrene for all micellar solutions exhibited single exponential decay, suggesting a single environment for pyrene, i.e. each surfactant had a single location of pyrene solubilized in its micelle. The performances of the pyrene probe in concentrated solutions of neat ammonium/tetraalkylammonium chlorides (NH4Cl/TAACls) were also investigated and an unusually high I1/I3 could be found for tetraethylammonium chloride. It reached a conclusion that pyrene located in the counterion layer of the TAAPFO micelles, which could be ascribed to the hydrophobic interaction and cation–π interaction between pyrene and tetraalkylammonium ions (TAA+) and the immiscibility between pyrene and the fluorocarbon core.

Protein–surfactant interaction: Differences between fluorinated and hydrogenated surfactants

The interactions of proteins with fluorinated/hydrogenated surfactants were investigated by circular dichroism and turbidity measurement. Pairs of fluorinated and hydrogenated surfactants with similar critical micelle concentrations (cmc), including sodium perfluorooctanoate/sodium decylsulfate and lithium perfluorononanoate/sodium dodecylsulfate were compared in view of their interactions with proteins including BSA, lysozyme, beta-lactoglobulin and ubiquitin. It was found that fluorinated surfactants exhibited stronger interactions with proteins than hydrogenated ones, which, however, depended on the structures of both proteins and surfactant molecules. If the proteins are very stable, or the surfactant-protein interactions are very strong, such differences between the two kinds of surfactants might be indistinguishable.

Interaction between β‐Cyclodextrin and Mixed Cationic‐Anionic Surfactants (2): Aggregation Behavior

The interactions between β‐cyclodextrin (β‐CD) and the mixtures of cationic‐anionic surfactants in the aqueous solution were investigated by surface tension, rheology, and dynamic light scattering measurements. It was shown that the key‐lock interactions between β‐CD and mixed cationic‐anionic surfactants were stronger than the electrostatic/hydrophobic interactions between cationic and anionic surfactants. The inclusion of β‐CD to surfactants could destroy the ion‐pair and aggregates of cationic‐anionic surfactants, and even inhibited the precipitation of the mixed cationic‐anionic surfactants. Furthermore, the inclusion of β‐CD to surfactants could also destroy the hydrogen bond between β‐CD molecules, inducing the disassociation of the aggregation formed by β‐CD themselves.

Dielectric Spectroscopy Investigation on the Interaction of Poly(diallyldimethylammonium chloride) with Sodium Decyl Sulfate in Aqueous Solution

The interaction between poly(diallyldimethylammonium chloride) (PDADMAC) and ionic surfactant sodium decyl sulfate (C(10)SO(4)Na) in aqueous solution was investigated by means of dielectric relaxation spectroscopy. To better understand the interaction, the dielectric behaviors of PDADMAC solution and C(10)SO(4)Na solution were also separately studied. For PDADMAC solution, two relaxations were observed, which were attributed to the polarization of loosely bound counterions along the directions parallel and perpendicular to the PDADMAC chain. For C(10)SO(4)Na solution, dielectric relaxation was observed at submicellar concentrations, which is ascribed to the counterion diffusion around premicelles. For the aqueous solutions of a PDADMAC/C(10)SO(4)Na mixture with different C(10)SO(4)Na concentrations, three surfactant concentration regions characterized by different dielectric behaviors were observed. The dielectric behavior in different regions was discussed through comparing it with that of PDADMAC solution and C(10)SO(4)Na solution. The possible interaction pattern and microstructure of the PDADMAC/C(10)SO(4)Na complex were proposed on the basis of the dielectric behavior.

Interaction between β‐Cyclodextrin and Mixed Cationic‐Anionic Surfactants (1): Thermodynamic Study

The interactions between β‐cyclodextrin (β‐CD) and the mixtures of cationic‐anionic surfactants in aqueous solution were investigated by surface tension and 1H NMR measurements. It was shown that the critical micelle concentration (cmc) increased linearly with the increase of β‐CD concentration. Furthermore, β‐CD formed 1∶1 inclusion complex with both cationic and anionic surfactants in the mixed surfactant systems, and no significant selective inclusion was observed. The thermodynamic parameters of the inclusion process of β‐CD to mixed cationic‐anionic surfactants were calculated by a numerical method based on the surface tension measurements, and it was found that the inclusion process was both enthalpy and entropy favorable.

Anomalous effects of concentrated salts on the equimolarly mixed cationic–anionic surfactant mixtures

Effects of alkali metal halides (NaX, X = F−, Cl−, Br−, I−; KX, X = Cl−, Br−, I−; and CsCl) on the morphology of the aggregates of decyltriethylammonium bromide and sodium decylsulfonate (C10NE–C10SO3) mixtures with equimolar ratios were studied by static/dynamic light scattering and viscosity measurements. The average hydrodynamic radii of the aggregates, the apparent aggregation number, and the relative viscosity as functions of both the concentrations of surfactants and added salts were investigated, respectively. It showed that the addition of a small amount of NaX (e.g., 0.1 M) had only small effects on the C10NE–C10SO3 vesicles. However, striking anion specificity, which differed anomalously with the anions of salts varying from F− to I−, was observed when large amounts of NaX (e.g., 1 M) were added. The size of the C10NE–C10SO3 aggregates was normally increased and then a liquid–liquid phase separation occurred with the addition of NaF; meanwhile, the addition of NaCl had little effect on the C10NE–C10SO3 aggregates under experimental conditions; however, the size of the C10NE–C10SO3 aggregates was abnormally decreased when high concentrations of NaBr or NaI (up to 2 M) were added, implying a vesicle-to-micelle transition which might be due to a decrease in the intramicellar attraction between oppositely charged headgroups by electrical shielding, and the mixtures still remained homogeneous. A cation specificity of added salts was also observed with the increase of the ionic radii from Na+ to Cs+. However, it was noted that the influence of increasing the ionic radii of cations on the aggregate size was not as pronounced as that of anions in the presence of high concentrations of salts. The aggregates of C10NE–C10SO3 became a little smaller when concentrated alkali metal halides with bigger cations were added into this homogeneous equimolar cationic–anionic surfactant mixture. Both the observed cation and anion specificities were consistent with the Hofmeister series.

Aggregation behavior of N-alkyl perfluorooctanesulfonamides in dimethyl sulfoxide solution.

N-alkyl perfluorooctanesulfonamides (C8F17SO2NHCnH2n+1, FC8-HCn, n = 2, 4, 6, 8) were shown to form aggregates in dimethyl sulfoxide (DMSO). Surface tension results revealed that the dissolution of FC8-HCn reduced the surface tension of DMSO in a manner analogous to common surfactants in aqueous solutions. Maximum surface excess amount (Gamma(max)) and minimum surface area per molecule (Amin) at the air-liquid interface were estimated. Gamma(max) decreases and Amin increases with an increase of the hydrocarbon chain length of FC8-HCn. Steady-state fluorescence and NMR measurements demonstrated that both fluorocarbon and hydrocarbon chains of FC8-HCn molecules were incorporated inside the aggregates. UV-vis spectroscopy confirmed the formation of aggregates and determined the critical micelle concentration (cmc) of FC8-HC6 and FC8-HC8 solutions. The thermodynamic parameters DeltaG(0)(agg), DeltaH(0)(agg), and DeltaS(0)(agg) for the aggregate formation of FC8-HCn in DMSO derived from the temperature dependence of the cmc revealed that the aggregate formation is an enthalpy-driven process, which was further confirmed by isothermal titration calorimetry (ITC) measurements. Moreover, the absolute values of DeltaG(0)(agg) and DeltaH(0)(agg) increase with an increase of the hydrocarbon chain length of FC8-HCn at 298 K.

Phase Behavior of the Mixed Systems of α-, β-Cyclodextrin and Cationic-Anionic Surfactants

The phase behavior of mixtures of α-, β-cyclodextrin (α-, β-CD) and equimolarly mixed CnH2n+1N(C2H5)3Br-CnH2n+1SO3,4Na (CnNE-CnSO3,4Na, n = 8, 10, 12) was studied. CDs could not only destroy the precipitates of cationic-anionic surfactants by forming 1:1 CD/surfactant complexes, but also induce precipitation by forming 2:1 CD/surfactant complexes, depending on the concentration, CD types, and chain lengths of surfactants. Precipitation of 2:1 complexes was ascribed to the synergism of electrostatic attractions and intermolecular hydrogen bonding. For the initially turbid C12NE-C12SO4Na, it turned clear and then formed precipitates upon continuous addition of β-CD, but the addition of α-CD only induced precipitation. It showed that α-CD and the chain length of C12 were more favored than β-CD and C10, respectively, to form precipitates of CD/surfactant complexes. XRD showed that the precipitates exhibited channel-type structure. The increase of temperature could induce the dissolution of precipitates, due to the enhancement of solubility instead of decomplexation of CD/surfactant complexes. The addition of salts could dissolve the precipitates owing to the shielding effect of salts on the interactions between opposite-charged headgroups. The sequence of their ability that NaI > KBr > NaBr > NaCl was consistent with Hofmeister series.