An Alternative Way towards Preparation of Hydrophobically Associating Polyacrylamide:Chemical Post-Modification

Hydrophobically associating polyacrylamides (HAPAMs) are derivatives from polyacrylamides by incorporating a small amount of hydrophobic moieties along the water-soluble mainchain. They are now becoming a class of promising candidates as thickeners or rheology modifiers in the formulations where rheology is necessary to be regulated, such as tertiary oil recovery, drilling fluids, hydraulic fracturing and coatings. Due to association of hydrophobes in nano-domains, their aqueous solutions exhibit very interesting rheological properties and better stability against salts than the unmodified precursor, polyacrylamide.Generally, there are two synthetic routes to introduce hydrophobic portion onto water-soluble polymer chains; i.e., direct copolymerization of hydrophobic and hydrophilic monomers, and post-polymerization functionalization[1]. In the case of HAPAM polymers, a commonly accepted method is micellar copolymerization in which an appropriate surfactant is employed to solubilize both monomers. However, it is widely reported[2] that the obtained polymers via micellar polymerization are characterized by: (i) blocky distribution of the hydrophobes; (ii) compositional inhomogeneity and (iii) strong dependence of solution properties on the block length.In this work, the alternative process, i.e., chemical post-modification, is employed to synthesize HAPAM polymers by direct N-alkylation of parent polyacrylamide (Figure 1) in dimethyl sulfoxide[3,4].PAM HAPAMFig. 1 Schematic route to prepare HAPAM by direct N-alkylation of PAMIt is found that the final incorporation of hydrophobic groups is in good agreement with the feed ratio[4], in contrast with that from micellar copolymerization which always brings about composition drift. Furthermore, unique rheological responses to shear rate, salt, temperature are also evidenced[5].     

Dissolution of cellulose in aqueous NaOH/urea solution: role of urea

Urea can improve the solubility and stability of cellulose in aqueous alkali solution, while its role has not come to a conclusion. To reveal the role of urea in solution, NMR was introduced to investigate the interaction between urea and the other components in solution. Results from chemical shifts and longitudinal relaxation times show that: (1) urea has no strong direct interaction with cellulose as well as NaOH; (2) urea does not have much influence on the structural dynamics of water. Urea may play its role through van der Waals force. It may accumulate on the cellulose hydrophobic region to prevent dissolved cellulose molecules from re-gathering. The driving force for the self-assembly of cellulose and urea molecules might be hydrophobic interaction. In the process of cellulose dissolution, OH^? breaks the hydrogen bonds, Na⁺ hydrations stabilize the hydrophilic hydroxyl groups and urea stabilizes the hydrophobic part of cellulose.     

Apparent and Transfer Molar Volumes for Aqueous Solution Containing Polyethylene Glycols and Amino Acid Ionic Liquids at 298.15 K

The densities of 1-n-butyl-3-methylimidazolium ([Bmim]) based amino acid ionic liquids (AAILs) prepared from glycine [Gly], alanine [Ala], and valine [Val], namely [Bmim][Gly], [Bmim][Ala] and [Bmim][Val], in aqueous?~?0.2 mol·kg^?1 polyethylene glycol (PEG400, PEG600 or PEG1000) and PEG400 solutions containing?~?(0.0946, 0.1891 and 0.3820) mol·kg^?1 of [Bmim][Gly], have been determined at 298.15 K. The experimental densities were used to evaluate the apparent molar volumes in the mixed solvent system and further used to obtain transfer molar volumes of AAILs for their transfer from water to aqueous PEG solutions and of PEG400 for its transfer from water to aqueous solutions containing (0.0946, 0.1891 and 0.3820) mol·kg^?1 of [Bmim][Gly]. The transfer molar volumes of AAILs and of PEG400 are found to be negative. The effects of alkyl chain-length variation on the anion of AAILs as well as the chain-length of PEG on transfer molar volumes are investigated and discussed in terms of hydrophobic–hydrophilic, hydrophobic–hydrophobic, and ion–hydrophobic interactions.     

Amphiphilic copolymers of some aromatic vinyl compounds and an electrolytic monomer as potential media for photosensitized electron transfer: Fluorescence quenching by an amphiphilic electron acceptor in aqueous media

Amphiphilic polymers were prepared by the copolymerization of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and aromatic vinyl compounds such as 9-vinylphenanthrene (VPh) and 1-vinylpyrene (VPy) with the expectation that they would serve as potential media for photosensitized electron transfer reactions. AMPS strongly solubilizes the hydrophobic segments into water; i.e., poly(AMPS-co-VPh) with VPh mole fraction (f_Ph) up to about 0.60 and poly(AMPS-co-VPy) with VPy mole fraction (f_Py) up to about 0.35 were found to be soluble in water. Poly(AMPS-co-VPh) in aqueous solution, as compared with that in DMF solution, showed a broad fluorescence spectrum with significant tailing in the longer-wavelength region along with a decrease in the intensity of the structured, monomer fluorescence band. These phenomena seem to imply the presence of an excimerlike interaction of phenanthryl groups in an aqueous solution through which the fluorescence from excited VPh units may be partly self-quenched. A considerable enhancement of the fluorescence from sodium 8-anilino-1-naphthalenesulfonate (ANS) caused by hydrophobic interaction of the probe with poly(AMPS-co-VPh) in aqueous solution indicated that these copolymers assume micellar structures. The fluorescence of these copolymers in aqueous solutions was quenched by bis(2-hydroxyethyl)terephthalate (BHET), an amphiphilic quencher, far more effectively than by fumaric acid, a hydrophilic quencher. This tendency is particularly strong for the copolymers with higher content of hydrophobic units. The second-order rate constants for the quenching of poly(AMPS-co-VPh) (f_Ph = 0.58) by BHET were found to be ca. 3 × 10¹⁰ and 1.5 × 10⁹ M^?1 s^?1 in aqueous and in DMF solution, respectively. The larger value in an aqueous solution is presumably due to an increase of the effective concentration of the amphiphilic quencher around the VPh sequences of the copolymer resulting from hydrophobic interaction.     

疏水蛋白吸附碳纳米管的分子动力学模拟

采用分子动力学方法模拟了水溶液中Ⅱ型疏水蛋白HFBI在单壁碳纳米管(SWNTs)表面的吸附过程, 考察了3种不同的HFBI初始取向, 并计算了结合自由能, 从累计240 ns的模拟轨迹中得到了不同的吸附结构. 结果表明, 当HFBI完全通过疏水面与SWNTs作用时, 其结合自由能最有利吸附, 且吸附最稳定. 另外, 由于HFBI含有4个二硫键, 因此吸附过程几乎不改变其二级结构.     

PROPERTIES OF DILUTE SOLUTIONS OF HYDROPHOBICALLY MODIFIED POLY (4-VINYL PYRIDINE) (POLYSOAPS)

Hydrophobically modified poly(4-vinyl pyridines) by alkyl bromides arekinds of polysoap similar to the surfactant. Properties of dilute solutions were studiedthrough the viscosity measurements in pure water and NaCl solutions. In aqueous solu-tions of polysoaps hydrophobic interaction can be attributed to aggregation of hydrophobicgroups of the polysoap main chains. The hydrophobic groups of polysoap can aggregate toform hydrophobic microdomains (micelles) in aqueous solution. This is a compact confor-mation. The formation of such microdomains is a process of dynamic equilibrium.     

Micellar Copolymerization:the State of the Arts

Over the past two decades, hydrophobically modified water-soluble polymers (HMWSPs),particularly hydrophobically associating polyacrylamides (HAPAMs), have attracted increased interest owing to their practical and fundamental importance[1]. This system usually consists of a hydrophilic backbone with a small proportion (generally less than 2 mol %) of hydrophobic pendent groups. When dissolved in aqueous solutions, the apolar moieties tend to exclude water and are held together by intra- and intermolecular hydrophobic associations. This leads to a transitional network structure that induces a substantial increase in solution viscosity. Such viscosity-building ability is further elevated upon adding salt or increasing temperature due to the enhanced polarity.Additionally, the dynamic associating junctions can be ruptured upon high shear stress, but re-formed when the force ceases. All these unique properties enable HAPAMs attractiveness to various industrial uses in which the control of fluid theology is required.However, it is a great challenge to synthesize HAPAMs since acrylamide and hydrophobic comonomers are mutually incompatible. After attempts using heterogeneous, inverse emulsion,microemulsion, and precipitation copolymerization processes, the commonly accepted method is micellar free radical copolymerization in which an appropriate surfactant is used to solubilize the hydrophobic comonomer[2].In this paper, the sate of the arts for micellar copolymerization is comprehensively reviewed:1. the mechanism of micellar copolymerization;2. parameters affecting micellar copolymerization, including:(1) nature and level of hydrophobic comonomer;(2) nature and content of surfactant used;(3) initiator and temperature.3. structural characteristics of HAPAMs prepared via micellar copolymerization;4. properties of HAPAMs prepared via micellar copolymerization:(1) dilute solution properties;(2) semi-dilute solution properties.5. applications of micellar copolymerization.In short, the main goal of this review is trying to establish an interrelationship among .synthetic method, structure and properties, so as to give a guideline for "tailoring" the polymer structure to satisfy different specific end use.     

Expected and unexpected results from combined beta-hairpin design elements

A model beta-hairpin dodecapeptide [EFGWVpGKWTIK] was designed by including a favorable D-ProGly Type II' beta-turn sequence and a Trp-zip interaction, while also incorporating a beta-strand unfavorable glycine residue in the N-terminal strand. This peptide is highly folded and monomeric in aqueous solution as determined by combined analysis with circular dichroism and 1H NMR spectroscopy. A peptide representing the folded conformation of the model beta-hairpin [cyclic(EFGWVpGKWTIKpG)] and a linear peptide representing the unfolded conformation [EFGWVPGKWTIK] yield unexpected relative deviations between the CD and 1H NMR spectroscopic results that are attributed to variations in the packing interactions of the aromatic side chains. Mutational analysis of the model beta-hairpin indicates that the Trp-zip interaction favors folding and stability relative to an alternate hydrophobic cluster between Trp and Tyr residues [EFGYVpGKWTIK]. The significance of select diagonal interactions in the model beta-hairpin was tested by rearranging the cross-strand hydrophobic interactions to provide a folded peptide [EWFGIpGKTYWK] displaying evidence of an unusual backbone conformation at the hydrophobic cluster. This unusual conformation does not appear to be a result of the glycine residue in the beta-strand, as replacement with a serine results in a peptide [EWFSIpGKTYWK] with a similar and seemingly characteristic CD spectrum. However, an alternate arrangement of hydrophobic residues with a Trp-zip interaction in a similar position to the parent beta-hairpin [EGFWVpGKWITK] results in a folded beta-hairpin conformation. The differences between side chain packing of these peptides precludes meaningful thermodynamic analysis and illustrates the caution necessary when interpreting beta-hairpin folding thermodynamics that are driven, at least in part, by aromatic cross strand interactions.     

Detection of hydrophobic microdomains in anionic polyelectrolytes with tris-(4,7-diphenyl-1,10-phenanthroline)3Cr(III)

This paper describes the changes in the luminescent properties of the tris-(4,7-diphenyl-1,10-phenanthroline)(3)Cr(III), [Cr(dip)(3)](3+) complex in an aqueous solution of three polyelectrolytes containing cyclohexyl, phenyl or 1-naphthyl groups in the side chain. When the polyelectrolytes form hydrophobic microdomains the luminescence of [Cr(dip)(3)](3+) is affected. The luminescence increases in the presence of cyclohexyl groups in the side chains, but decreases in the presence of phenyl and naphthyl groups (in that order). This fact can be explained in terms of a reductive quenching mechanism between the complex and the aromatic groups. Indeed, experiments performed with the complex and the alcohols corresponding to the functional groups, i.e., cyclohexanol, phenol, and naphthol, also show the same behavior, confirming the interaction with the functional groups and not other components of the polyelectrolyte. The luminescent properties of the [Cr(dip)(3)](3+) complex allow the detection of hydrophobic microdomains arising from the host-guest interaction. Moreover, the complex is able to distinguish between a nonaromatic hydrophobic microdomain and an aromatic one.     

Well-resolved, new water morphologies obtained by modification of the hydrophilic/hydrophobic character and shapes of the supporting layers

The planar coordination complexes [Ag2L2] (L = ophen or obpy, where Hophen = 1H-[1,10]phenanthrolin-2-one and Hobpy = 1H-[2,2']bipyridinyl-6-one) contain both hydrophilic and hydrophobic functional groups. Crystallization of these structurally related complexes in an aqueous medium gives [Ag2(ophen)2]2 x 6H2O (1), [Ag2(obpy)2]3 x 18H2O (2), and [Ag2(obpy)2]2 x 14.5H2O (3). Novel water morphologies are observed in these crystalline hydrates, similar 2D metal-organic layers in which the planar [Ag2L2] complexes as building blocks are stacked alternately together and provide hydrogen bond acceptor sites (O atoms) for anchoring 1D water chains or 2D water layers on their surfaces. In the wavelike metal-organic framework of 1, the wide hydrophobic region renders the formation of 1D water tapes containing four- and six-membered water rings, whereas more complex 2D water layers are sandwiched in 2 and 3, owing to the fact that the surfaces of the supporting layers are more hydrophilic than 1.     

Fluorescence Studied of Dissociation Constant of Cyclodextrin With Phenol Derivatives

α-and β-cyclodextrins consisting of six and seven glucose residues respectively, have lipophilic cavities with different inner diameters. They form host-guest inclusion complexes with hydrophobic organic and organometallic guest molecules in aqueous solution. These host-guest complexes have proved to be excellent model systems for studying the nature of noncovalent bonding forces in aqueous media. They have provided valuable insights into the hydrophobic effect and London dispersion forces and are good model for understanding the specificity of enzyme substrate interactions [1] Evidence for the formation of inclusion complexes have been provided from calovimetric titration [2] NMR[33], circular dichroism[4], U V[1] and fluorescence spectra[5] and conductometric method[6] etc. H ere we report a new fluorimetric method for a study on the reaction of the host-guest inclusion complexes of cyclodextrin with phenols. Dissociation constants (Kd) of the inclusion complexes of some phenols with α-β-cyclodextrin are estimated based on the variation of the fluorescent intensity and modified Harad' equations.     

One-dimensional Chain Dibenzo-18-crown-6 Complex: [K(DB18-C-6)]2[Pt(SCN)6]·2H2O·C2H4Cl2 Assembled by Cation-π Interactions

Introduction Cation-πinteractions have come to be appreciated as an important noncovalent binding force in many areas of modern chemistry, such as material design, molecular biology, host-guest and supramolecular chemistry[1-3].     

Communication: Direct observation of a hydrophobic bond in loop closure of a capped (-OCH2CH2-)n oligomer in water

The small r variation of the probability density P(r) for end-to-end separations of a -CH(2)CH(3) capped (-OCH(2)CH(2)-)(n) oligomer in water is computed to be closely similar to the CH(4)···CH(4) potential of mean force under the same circumstances. Since the aqueous solution CH(4)···CH(4) potential of mean force is the natural physical definition of a primitive hydrophobic bond, the present result identifies an experimentally accessible circumstance for direct observation of a hydrophobic bond which has not been observed previously because of the low solubility of CH(4) in water. The physical picture is that the soluble chain molecules carry the capping groups into aqueous solution, and permits them to find one another with reasonable frequency. Comparison with the corresponding results without the solvent shows that hydration of the solute oxygen atoms swells the chain molecule globule. This supports the view that the chain molecule globule might have a secondary effect on the hydrophobic interaction that is of first interest here. The volume of the chain molecule globule is important for comparing the probabilities with and without solvent because it characterizes the local concentration of capping groups. Study of other capping groups to enable x-ray and neutron diffraction measurements of P(r) is discussed.     

Influence of alkyl substitution on the gas-phase stability of 1-adamantyl cation and on the solvent effects in the solvolysis of 1-bromoadamantane

1-Adamantyl cations having three methyl groups or one, two, or three isopropyl groups on the 3-, 5-, and 7-positions were found by FT ICR to be more stable than the 1-adamantyl cation and that the stability increases with the number of isopropyl group. The relative stabilities calculated by PM3 were in good agreement with the experimental results. In contrast, the sequence of the rates for the solvolysis in nonaqueous solvents are 3,5,7-(Me)(3)-1-AdBr < 1-bromoadamantane (1-AdBr) < 3,5,7-(n-Pr)(3)-1-AdBr < 3,5,7-(i-Pr)(3)-1-AdBr. The rates of solvolysis of 3,5,7-(i-Pr)(3)-1-AdBr and 3,5,7-(n-Pr)(3)-1-AdBr relative to 1-AdBr at 25 degrees C are 15 and 3.8 in EtOH, respectively, but markedly decreases with the increase in the amount of added water, reaching 0.84 and 0.15, respectively, in 60% EtOH. Reflecting these effects of water, the Grunwald-Winstein (GW) relationship for 3,5,7-(i-Pr)(3)-1-AdBr and 3,5,7-(n-Pr)(3)-1-AdBr against Y(Br) is linear for nonaqueous alcohols (EtOH, MeOH, TFE-EtOH, TFE, 97% HFIP), but marked downward deviations are observed for aqueous organic solvents, in particular, aqueous ethanol and aqueous acetone. The effect of the alkyl substituents to diminish relative solvolytic reactivity in EtOH-H(2)O mixtures may be ascribed to a blend of steric hindrance to Betarphinsted base-type hydration to the beta-hydrogens and hydrophobic interaction of the alkyl groups with ethanol to make the primary solvation shell less ionizing. The introduction of one nonyl group to the 3-position showed much smaller deviations in the GW relationship than the case of 3,5,7-(n-Pr)(3)-1-AdBr. The markedly decelerated solvolysis of alkylated 1-bromoadamantanes in aqueous organic solvents is a kinetic version of anomalously diminished dissociation of alkylbenzoic acids in aqueous ethanol and aqueous tert-butyl alcohol that was demonstrated by Wepster and co-workers a decade ago and ascribed to hydrophobic effects.     

Noncovalent water-based materials: robust yet adaptive

The adaptive properties of noncovalent materials allow easy processing, facile recycling, self-healing, and stimuli responsiveness. However, the poor robustness of noncovalent systems has hampered their use in real-life applications. In this Concept Article we discuss the possibility of creating robust noncovalent arrays by utilizing strong hydrophobic interactions. We describe examples from our work on aqueous assemblies based on aromatic amphiphiles with extended hydrophobic cores. These arrays exhibit fascinating properties, including robustness, multiple stimuli-responsiveness, and pathway-dependent self-assembly. We have shown that this can lead to functional materials (filtration membranes) rivaling covalent systems. We anticipate that water-based noncovalent materials have the potential to replace or complement conventional polymer materials in various fields, and to promote novel applications that require the combination of robustness and adaptivity.     

Association Complexes of Calix[6]arenes with Amino Acids Explained by Energy-Partitioning Methods

Intermolecular complexes with calixarenes are intriguing because of multiple possibilities of noncovalent binding for both polar and nonpolar molecules, including docking in the calixarene cavity. In this contribution calix[6]arenes interacting with amino acids are studied with an additional aim to show that tools such as symmetry-adapted perturbation theory (SAPT), functional-group SAPT (F-SAPT), and systematic molecular fragmentation (SMF) methods may provide explanations for different numbers of noncovalent bonds and of their varying strength for various calixarene conformers and guest molecules. The partitioning of the interaction energy provides an easy way to identify hydrogen bonds, including those with unconventional hydrogen acceptors, as well as other noncovalent bonds, and to find repulsive destabilizing interactions between functional groups. Various other features can be explained by energy partitioning, such as the red shift of an IR stretching frequency for some hydroxy groups, which arises from their attraction to the phenyl ring of calixarene. Pairs of hydrogen bonds and other noncovalent bonds of similar magnitude found by F-SAPT explain an increase in the stability of both inclusion and outer complexes.     

A competitive particle nucleation mechanism in the polymerization of homogenized styrene emulsions

The effects of concentrations of surfactant (sodium lauryl sulfate [SLS]) and initiator (sodium persulfate [SPS]) on the polymerization of homogenized styrene emulsions, stabilized by SLS/lauryl methacrylate (LMA) or SLS/stearyl methacrylate (SMA), were studied. The rate of polymerization increases with increasing [SLS] or [SPS]. In addition to monomer droplet nucleation, the formation of particle nuclei in the aqueous phase (homogeneous nucleation) plays a crucial role in the polymerization kinetics. In comparison with the LMA containing polymerization system, monomer droplet nucleation becomes more important when the more hydrophobic SMA was used as the costabilizer. Furthermore, the degree of homogeneous nucleation increases with increasing [SPS].     

Pyridine analogues of the antimetastatic Ru(III) complex NAMI-A targeting non-covalent interactions with albumin

A series of pyridine-based derivatives of the antimetastatic Ru(III) complex imidazolium [trans-RuCl(4)(1H-imidazole)(DMSO-S)] (NAMI-A) have been synthesized along with their sodium-ion compensated analogues. These compounds have been characterized by X-ray crystallography, electron paramagnetic resonance (EPR), NMR, and electrochemistry, with the goal of probing their noncovalent interactions with human serum albumin (hsA). EPR studies show that the choice of imidazolium ligands and compensating ions does not strongly influence the rates of ligand exchange processes in aqueous buffer solutions. By contrast, the rate of formation and persistence of interactions of the complexes with hsA is found to be strongly dependent on the properties of the axial ligands. The stability of noncovalent binding is shown to correlate with the anticipated ability of the various pyridine ligands to interact with the hydrophobic binding domains of hsA. These interactions prevent the oligomerization of the complexes in solution and limit the rate of covalent binding to albumin amino acid side chains. Electrochemical studies demonstrate relatively high reduction potentials for these complexes, leading to the formation of Ru(II) species in aqueous solutions containing biological reducing agents, such as ascorbate. However, EPR measurements indicate that while noncovalent interactions with hsA do not prevent reduction, covalent binding produces persistent mononuclear Ru(III) species under these conditions.     

Surfactant solutions and porous substrates: spreading and imbibition

In Section 1, spreading of small liquid drops over thin dry porous layers is investigated from both theoretical and experimental points of view [V.M. Starov, S.R. Kosvintsev, V.D. Sobolev, M.G. Velarde, S.A. Zhdanov, J. Colloid Interface Sci. 252 (2002) 397]. Drop motion over a porous layer is caused by an interplay of two processes: (a) the spreading of the drop over already saturated parts of the porous layer, which results in an expanding of the drop base, and (b) the imbibition of the liquid from the drop into the porous substrate, which results in a shrinkage of the drop base and an expanding of the wetted region inside the porous layer. As a result of these two competing processes, the radius of the drop goes through a maximum value over time. A system of two differential equations has been derived to describe the evolution with time of radii of both the drop base and the wetted region inside the porous layer. This system includes two parameters, one accounts for the effective lubrication coefficient of the liquid over the wetted porous substrate, and the other is a combination of permeability and effective capillary pressure inside the porous layer. Two additional experiments were used for an independent determination of these two parameters. The system of differential equations does not include any fitting parameter after these two parameters are determined. Experiments were carried out on the spreading of silicone oil drops over various dry microfiltration membranes (permeable in both normal and tangential directions). The time evolution of the radii of both the drop base and the wetted region inside the porous layer were monitored. All experimental data fell on two universal curves if appropriate scales are used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer on dimensionless time. The predicted theoretical relationships are two universal curves accounting quite satisfactory for the experimental data. According to theory predictions [1]: (i) the dynamic contact angle dependence on the same dimensionless time as before should be a universal function, and (ii) the dynamic contact angle should change rapidly over an initial short stage of spreading and should remain a constant value over the duration of the rest of the spreading process. The constancy of the contact angle on this stage has nothing to do with hysteresis of the contact angle: there is no hysteresis in the system under investigation. These conclusions again are in good agreement with experimental observations [V.M. Starov, S.R. Kosvintsev, V.D. Sobolev, M.G. Velarde, S.A. Zhdanov, J. Colloid Interface Sci. 252 (2002) 397]. In Section 2, experimental investigations are reviewed on the spreading of small drops of aqueous SDS solutions over dry thin porous substrates (nitrocellulose membranes) in the case of partial wetting [S. Zhdanov, V. Starov, V. Sobolev, M. Velarde, Spreading of aqueous SDS solutions over nitrocellulose membranes. J. Colloid Interface Sci. 264 (2003) 481-489]. The time evolution was monitored of the radii of both the drop base and the wetted area inside the porous substrate. The total duration of the spreading process was subdivided into three stages-the first stage: the drop base expands until the maximum value of the drop base is reached; the contact angle rapidly decreases during this stage; the second stage: the radius of the drop base remains constant and the contact angle decreases linearly with time; the third stage: the drop base shrinks and the contact angle remains constant. The wetted area inside the porous substrate expends during the whole spreading process. Appropriate scales were used with a plot of the dimensionless radii of the drop base, of the wetted area inside the porous substrate, and the dynamic contact angle on the dimensionless time. Experimental data showed [S. Zhdanov, V. Starov, V. Sobolev, M. Velarde, Spreading of aqueous SDS solutions over nitrocellulose membranes. J. Colloid Interface Sci. 264 (2003) 481-489]: the overall time of the spreading of drops of SDS solution over dry thin porous substrates decreases with the increase of surfactant concentration; the difference between advancing and hydrodynamic receding contact angles decreases with the surfactant concentration increase; the constancy of the contact angle during the third stage of spreading has nothing to do with the hysteresis of contact angle, but determined by the hydrodynamic reasons. It is shown using independent spreading experiments of the same drops on nonporous nitrocellulose substrate that the static receding contact angle is equal to zero, which supports the conclusion on the hydrodynamic nature of the hydrodynamic receding contact angle on porous substrates. In Section 3, a theory is developed to describe a spontaneous imbibition of surfactant solutions into hydrophobic capillaries, which takes into account the micelle disintegration and the concentration decreasing close to the moving meniscus as a result of adsorption, as well as the surface diffusion of surfactant molecules [N.V. Churaev, G.A. Martynov, V.M. Starov, Z.M. Zorin, Colloid Polym. Sci. 259 (1981) 747]. The theory predictions are in good agreement with the experimental investigations on the spontaneous imbibition of the nonionic aqueous surfactant solution, Syntamide-5, into hydrophobized quartz capillaries. A theory of the spontaneous capillary rise of surfactant solutions in hydrophobic capillaries is presented, which connects the experimental observations with the adsorption of surfactant molecules in front of the moving meniscus on the bare hydrophobic interface [V.J. Starov, Colloid Interface Sci. 270 (2003)]. In Section 4, capillary imbibition of aqueous surfactant solutions into dry porous substrates is investigated from both theoretical and experimental points of view in the case of partial wetting [V. Straov, S. Zhdanov, M. Velarde, J. Colloid Interface Sci. 273 (2004) 589]. Cylindrical capillaries are used as a model of porous media for theoretical treatment of the problem. It is shown that if an averaged pore size of the porous medium is below a critical value, then the permeability of the porous medium is not influenced by the presence of surfactants at any concentration: the imbibition front moves exactly in the same way as in the case of the imbibition of the pure water. The critical radius is determined by the adsorption of the surfactant molecules on the inner surface of the pores. If an averaged pore size is bigger than the critical value, then the permeability increases with surfactant concentration. These theoretical conclusions are in agreement with experimental observations. In Section 5, the spreading of surfactant solutions over hydrophobic surfaces is considered from both theoretical and experimental points of view [V.M. Starov, S.R. Kosvintsev, M.G. Velarde, J. Colloid Interface Sci. 227 (2000) 185]. Water droplets do not wet a virgin solid hydrophobic substrate. It is shown that the transfer of surfactant molecules from the water droplet onto the hydrophobic surface changes the wetting characteristics in front of the drop on the three-phase contact line. The surfactant molecules increase the solid-vapor interfacial tension and hydrophilise the initially hydrophobic solid substrate just in front of the spreading drop. This process causes water drops to spread over time. The time of evolution of the spreading of a water droplet is predicted and compared with experimental observations. The assumption that surfactant transfer from the drop surface onto the solid hydrophobic substrate controls the rate of spreading is confirmed by experimental observations. In Section 6, the process of the spontaneous spreading of a droplet of a polar liquid over solid substrate is analyzed in the case when amphiphilic molecules (or their amphiphilic fragments) of the substrate surface layer are capable of overturning, resulting in a partial hydrophilisation of the surface [V.M. Starov, V.M. Rudoy, V.I. Ivanov, Colloid J. (Russian Academy of Sciences English Transaction) 61 (3) (1999) 374]. Such a situation may take place, for example, during contact of an aqueous droplet with the surface of a polymer whose macromolecules have hydrophilic side groups capable of rotating around the backbone and during the wetting of polymers containing surface-active additives or Langmuir-Blodgett films composed of amphiphilic molecules. It was shown that droplet spreading is possible only if the lateral interaction between neighbouring amphiphilic molecules (or groups) takes place. This interaction results in the tangential transfer of "the overturning state" to some distance in front of the advancing three-phase contact line making it partially hydrophilic. The quantitative theory describing the kinetics of droplet spreading is developed with allowance for this mechanism of self-organization of the surface layer of a substrate in the contact with a droplet.     

Ionic liquid induced changes in the properties of aqueous zwitterionic surfactant solution

Modifying properties of aqueous surfactant solutions by addition of external additives is an important area of research. Unusual properties of ionic liquids (ILs) make them ideal candidates for this purpose. Changes in important physicochemical properties of aqueous zwitterionic N-dodecyl- N, N-dimethyl-3-ammonio-1-propanesulfonate (SB-12) surfactant solution upon addition of hydrophilic IL 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF 4], are reported. Dynamic light scattering results indicate a dramatic reduction in the average micellar size in the presence of [bmim][BF 4]; micellar (or micelle-like) aggregation in the presence of as high as 30 wt % [bmim][BF 4] is confirmed. Responses from fluorescence probes are used to obtain critical micelle concentration (cmc), aggregation number ( N agg), and dipolarity and microfluidity of the micellar pseudophase of aqueous SB-12 in the presence of [bmim][BF 4]. In general, increasing the amount of [bmim][BF 4] to 30 wt % results in decrease in N agg and increase in cmc. Increase in the dipolarity and the microfluidity of the probe cybotactic region within the micellar pseudophase is observed on increasing [bmim][BF 4] concentration in the solution. It is attributed to increased water penetration into the micellar pseudophase as [bmim][BF 4] is added to aqueous SB-12. It is proposed that IL [bmim][BF 4] behaves similar to an electrolyte and/or a cosurfactant when present at low concentrations and as a polar cosolvent when present at high concentrations. Electrostatic attraction between cation of IL and anion of zwitterion, and anion of IL and cation of zwitterion at low concentrations of [bmim][BF 4] is evoked to explain the observed changes. Presence of IL as cosolvent appears to reduce the efficiency of micellization process by reducing the hydrophobic effect.