Hydrophobicity and counterion effects on the binding of ionic surfactants to uncharged polymeric hydrogels

Gel swelling experiments have been used to study the binding of ionic surfactants to a series of nonionic alkylacrylamide hydrogels of increasing hydrophobicity. The binding of hexadecyl trimethylammonium (C16TA+) to uncharged gels is sensitive to both the hydrophobicity of the gel and the counterion to the surfactant. There is a minimum hydrophobicity threshold below which binding of the surfactant does not occur, and this is influenced by the counterion to the surfactant. The surfactant concentration at the onset of binding, the critical association concentration (cac), decreases with increasing gel hydrophobicity. The maximum swelling of the gel (at intermediate network hydrophobicity) increases in the order of the Hofmeister series of anions, bromide (Br-) < chloride (Cl-) < acetate (Ac-). At higher gel hydrophobicity, differences in swelling are no longer observed on changing the counterion. A minimum hydrophobicity threshold was also found for the binding of the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl-di(ethylene oxide)-sulfate (SD-(EO)2-S). Differences in the swelling behavior with network hydrophobicity are explained in terms of the degree of saturation of the gel with surfactant at the cmc.     

Interactions between cationic starch and anionic surfactants

The surface tensions and the phase equilibria of dilute aqueous cationic starch (CS)/surfactant systems were investigated. The degree of substitution of the CS varied from 0.014 to 0.772. The surfactants investigated were sodium dodecyl sulphate (SDS), potassium octanoate (KOct), potassium dodecanoate (KDod) and sodium oleate (NaOl). The concentrations of CS were 0.001, 0.01 and 0.1 w%.Critical association concentrations (cac) occur at surfactant concentrations well below the critical micelle concentrations of the surfactants, except for KOct, KDod and NaOl at the lowest CS concentrations investigated (0.001 w%). The surface tensions of CS/surfactant solutions decrease strongly already below the cac. This is attributed to the formation of surface active associates by ion condensation. Associative phase separation of gels formed by CS and surfactant takes place at extremely low concentrations when the surfactant/polymer charge ratio is somewhat larger than 1. The gel is higly viscous and contains 40–60% water, depending on the concentration of electrolyte, the surfactant hydrocarbon chain length and the nature of the polar head of the surfactant.The concentration at which the phase separation occurs decreases with increasing surfactant chain length and the concentration of simple electrolyte, factors that promote micelle formation. This indicates that the gels are formed by association of CS to surfactant micelles. When surfactant well in excess of charge equivalence is added, the gels dissolve because the CS/surfactant complexes acquire a high charge.     

Wormlike micelles in poly(oxyethylene) surfactant solution: Growth control through hydrophilic-group size variation

Rheological responses of colloidal gels formed from fumed silica suspensions in aqueous KOH solution at pH 11 by the addition of cationic surfactants, such as dodecyltrimethylammonium chloride (C12 TAC) and hexadodecyltrimethylammonium chloride (C16 TAC) have been investigated as functions of silica and surfactant concentrations. Stable and aggregated fumed silica suspensions with negative charges cause gelling by adding the cationic surfactants through electrical neutralization of their micelles. The resulting critical strain and storage modulus of the gelled silica suspension increase with an increase in the surfactant concentration, irrespective of the cationic surfactant. This means that the higher the surfactant concentration is, the more effective the electrical neutralization interaction through the micelle of the cationic surfactant is. Moreover, the resulting gels can be classified into the strong-link gel and the weak-link one in the presence of C12 TAC and C16 TAC, respectively, from a comparison of the silica volume fraction dependences of critical strain and storage modulus with the fractal gel model.     

Effect of polymer charge density and ionic strength on the formation of complexes between sodium arylamido-2-methyl-1-propane-sulfonate-co-acrylamide gels and cetylpyridinium chloride

The effects of sodium chloride on the composition and structure of polyelectrolyte gel-surfactant complexes (PSCs) formed by the sodium salt of acrylamide-2-methyl-1-propane-sulfonic acid-co-acrylamide gels and cetylpyridinium chloride have been studied. At a low ionic strength of the solution, the composition of all the complexes is close to stoichiometric by charge. In the presence of 0.3 M sodium chloride, the composition of the complexes formed by the gel with 99 mol % charged groups is close to stoichiometric, while for the gel with 33 mol % charged monomer units, a nonstoichiometric complex with a high excess of the surfactant is formed. Further decrease of the charge density up to 10 mol % leads to partial or complete dissociation of the PSCs. The study of PSCs by the method of small-angle X-ray scattering (SAXS) shows that the complexes formed by the gels with high and intermediate charge densities are highly ordered. The decrease of the charge density of the swollen networks at first leads to a change in symmetry of the ordered domains in the PSCs and then to their disordering. The formation of nonstoichiometric PSCs at a high enough concentration of salt is explained by the effect of fitting, when the packing of the surfactant and polymer components in the PSCs is improved due to the inclusion of extra surfactant molecules together with their counterions in the ordered domains.     

Surface complexation of DNA with a cationic surfactant

We have studied the surface complexation of DNA with a cationic surfactant (DTAB) using a combination of methods: dynamic surface tension, ellipsometry and Brewster angle microscopy. Below the surfactant critical aggregation concentration (cac), complexation occurs only at the surface, and the results are consistent with neutralization of the surfactant charges by the free polymer ions. Above the cac, surfactant starts to bind cooperatively to DNA in the bulk, and adsorption of the preformed hydrophobic surfactant DNA aggregate is now possible, leading to thick surface layers. At still higher concentrations of surfactant (still below saturation of binding in the bulk), there is decrease in adsorption due to competition with bulk aggregates. Finally, as surfactant concentration is increased still further, bulk aggregates become less soluble and large amounts are adsorbed, forming a surface layer, which is solid-like and brittle.     

Phase behavior of aqueous polyion-surfactant ion complex salts: effects of polyion charge density

The effect of varying the fraction of charged monomer units of the polyion in aqueous polyion-oppositely charged surfactant complex salts has been investigated. The complex salts used were based on cetyltrimethylammonium (C16TA+) with three different polymeric counterions: poly(acrylate) (PA-) or poly(acrylate) copolymerized with either dimethylacrylamide (PA-/DAM) or N-isopropylamide (PA-/NIPAM). The charge density of the polyion was varied by either adding poly(acrylic) acid (PAA) to the C16TAPA complex salt (annealed charges) or by varying the fraction of uncharged units in the C16TAPA/DAM or C16TAPA/NIPAM complex salts (quenched charges). The formed phases were studied visually between crossed polarizers and by small angle X-ray scattering (SAXS). Both types of complex salts (annealed and quenched) formed hexagonal phases at high fractions of charged monomers and low water contents. Upon increasing the water content, a cubic phase of the Pm3n space group was found. Upon further addition of water, a miscibility gap with the cubic phase in equilibrium with pure water was found. Decreasing the fraction of charged monomers in the annealed complex salt resulted in an increase of the curvature of the surfactant aggregates. Only at very low (<0.05) fractions of charged monomers did the packing of the surfactant aggregates lose long-range order, and eventually, the miscibility gap disappeared. For the quenched complex salts, the changes upon decreasing the fraction of charged monomers in the polyion were similar, but the loss of long-range order occurred at much higher fractions of charged monomers. The average surfactant aggregation number in the surfactant aggregates, which was similar for the annealed and quenched systems, decreased when the fraction of charged monomers was decreased.     

Interactions between covalently crosslinked ethyl(hydroxyethyl)cellulose and SDS

The equilibrium swelling of chemically crosslinked gels based on ethyl(hydroxyethyl)cellulose (EHEC) in aqueous solutions of sodium dodecyl sulphate (SDS) was studied as a function of the SDS concentration at various temperatures and salt concentrations. Comparisons were made with gels based on poly-N-isopropylacrylamide (p-NIPA). Both polymers are known to form complexes with SDS above a critical association concentration (cac) of the surfactant, and both display a lower critical solution temperature (LCST) in water. For both types of gels, an increase in the equilibrium gel volume was seen with increasing SDS concentration above the cac, up to a maximum value when the SDS concentration in the external solution reached the critical micelle concentration (cmc). Above the cmc, the equilibrium gel volume decreased slowly with increasing SDS concentration. A volume collapse of the EHEC gels was observed in a temperature interval around the LCST of EHEC in solution. Above the cac, the collapse transition moved monotonically towards higher temperatures with added SDS. At lower SDS concentrations, however, the opposite trend was found. The swelling of the gel was less in the presence of salt and SDS, and a pronounced minimum in swelling appeared with added SDS when the salt concentration was sufficiently high (ca. 10 mmoles/l). Under these salt conditions, the LCST of the linear EHEC also passes through a deep minimum (below room temperature) on addition of SDS.     

Co-adsorption of carboxymethyl-cellulose and cationic surfactants at the air-water interface

We investigated the interaction between an anionic polyelectrolyte (carboxymethylcellulose) and cationic surfactants (DTAB, TTAB, and CTAB) at the air/water interface, using surface tension, ellipsometry, and Brewster angle microscopy techniques. At low surfactant concentration, a synergistic phenomenon is observed due to the co-adsorption of polyelectrolyte/surfactant complexes at the interface, which decreases the surface tension. When the surfactant critical aggregation concentration (cac) is reached, the adsorption saturates and the thickness of the adsorbed monolayer remains constant until another characteristic surfactant concentration, C0, is reached, at which all the polymer charges are bound to surfactant in bulk. Above C0, the absorbed monolayer becomes much thicker, suggesting adsorption of bulk aggregates, which have become more hydrophobic due to charge neutralization.     

Salt effect on the complex formation between cationic gemini surfactant and anionic polyelectrolyte in aqueous solution

Salt effect on the interaction of anionic polyelectrolyte sodium carboxymethylcellulose (NaCMC) with cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)6N(CH3)2C12H25]Br2 (C12C6C12Br2) has been investigated using turbidimetric titration, steady-state fluorescence, and mobility measurement. It is found that the critical aggregation concentration(cac) for C12C6C12Br2/NaCMC complexes depends little on addition of sodium bromide (NaBr). However, in the presence of nonionic surfactant Triton X-100 (TX100), the critical ionic surfactant mole fraction for the onset of complex formation (Yc) increases markedly with increasing NaBr concentration. These salt effects are supposed as the overall result from competition between the increase of interaction and the screening of interaction. The increase of interaction is referred to as the effect that the larger micelle with higher surface charge density induced by salt has a stronger interaction with oppositely charged polyelectrolyte. The screening of interaction is referred to as the salt screening of electrostatic attraction between the polymer chain and the surfactant. For complex formation between C12C6C12Br2 and NaCMC, the increase of interaction probably compensates the screening of interaction, leading to constant cac values at different salt concentrations. For complex formation between the C12C6C12Br2/TX100 mixed micelle and NaCMC, the screening of interaction probably plays a dominant role, leading to higher suppression of electrostatic binding of micelles to polyelectrolyte.     

Interactions of polyelectrolyte brushes with oppositely charged surfactants

Using Brownian dynamics simulations, we study the effect of the charge ratio, the surfactant length, and the grafting density on the conformational behavior of the complex formed by the polyelectrolyte brush with oppositely charged surfactants. In our simulations, the polyelectrolyte chains and surfactants are represented by a coarse-grained bead-spring model, and the solvent is treated implicitly. It is found that varying the charge ratio induces different morphologies of surfactant aggregates adsorbed onto the brush. At high charge ratios, the density profiles of surfactant monomers indicate that surfactant aggregates exhibit a layer-by-layer arrangement. The surfactant length has a strong effect on the adsorption behavior of surfactants. The lengthening of surfactant leads to a collapsed brush configuration, but a reswelling of the brush with further increasing the surfactant length is observed. The collapse of the brush is attributed to the enhancement of surfactants binding to polyelectrolyte chains. The reswelling is due to an increase in the volume of adsorbed surfactant aggregates. At the largest grafting density investigated, enhanced excluded volume interactions limit the adsorption of surfactant within the polyelectrolyte brush. We also find that end monomers in polyelectrolyte chains exhibit a bimodal distribution in cases of large surfactant lengths and high charge ratios.     

Responsive polymer gels: double-stranded versus single-stranded DNA

Cross-linking of polyelectrolytes such as DNA gives gels that are osmotically highly swollen but contract upon addition of electrolytes and, in particular, upon association of oppositely charged cosolutes with the polyelectrolyte chain. The deswelling behavior of cross-linked DNA gels thus reflects the DNA-cosolute interactions and provides a basis for the development of responsive DNA formulations. Gels of both single- and double-stranded DNA have interesting applications, and a comparison between them provides the basis for understanding mechanisms. Denaturation of cross-linked ds-DNA gels was induced by heating them above the melting temperature and then cooling. This process, studied by fluorescence using ethidium bromide, appeared to be reversible when a heating/cooling cycle was performed. The swelling behavior upon addition of different cosolutes, such as metal ions, polyamines, charged proteins, and surfactants, was investigated for different DNA gel samples, including long and short ds-DNA and long and short ss-DNA. The DNA molecular weight was found to have only a slight effect on the deswelling curves, whereas conformation exhibited a pronounced effect. In general, single-stranded DNA gels exhibited a larger collapse in the presence of cations than did double-stranded DNA. This difference was more pronounced with surfactants than with the other cosolutes investigated. The difference between double- and single-stranded DNA was attributed to differences in linear charge density, chain flexibility, and hydrophobicity. For surfactants with different chain lengths, the swelling behavior displayed by ss-DNA can be interpreted in terms of an interplay between hydrophobic and electrostatic interactions, the latter being influenced by polymer flexibility. Increasing hydrophobicity of the network leads to a decreased critical aggregation concentration (cac) for the surfactant/gel complex, as a result of the strengthened hydrophobic attractive force between the surfactant and the gel chain. The swelling of DNA gels appears to be reversible and to be independent of DNA conformation. Surfactant-induced deswelling of DNA gels under some conditions appears to be quite homogeneous, whereas under other conditions, there is a separation into a collapsed region in the outer parts of the gel sample and an inside swollen part. Such "skin" formation is quite different for ss- and ds-DNA, with ss-DNA giving more pronounced skin formation over a wider range of binding ratio, beta. For example, no macroscopic separation into collapsed and swollen regions was observed at intermediate degrees of binding for ds-DNA gels, whereas a dense surfactant-rich surface phase (skin) was found to coexist with a swollen core network for ss-DNA gels with beta>0.5. One explanation for this difference is the large deformation energy required for the compression of the very stiff ds-DNA chains.     

Polyelectrolyte gel transitions: experimental aspects of charge inhomogeneity in the swelling and segmental attractions in the shrinking

This paper aims to provide a systematic discussion based on our experimental results both previously published and unpublished, to promote better understanding of volume-phase transitions in polyelectrolyte gels. Special attention was paid to the distribution of network charges as well as to the attractive interaction among polymer segments. From looking at how these effects appear in the swelling curves, an exploration of the nature of polyelectrolyte gel transitions was attempted. Two sorts of polyelectrolyte gels, temperature-responsive ionic gels based on N-isopropylacrylamide (NIPA) and cationic poly(ethyleneimine) (PEI) gels, were mainly employed with various modifications. The charge inhomogeneity within the gel phase was created by surfactant binding, immobilized enzyme reaction and physical entrapment of polyions. The attractive interactions holding the gel in a collapsed state were studied in comparison with phase separations of the corresponding linear polyelectrolyte. The main conclusions are summarized as follows: (i) The charge inhomogeneity exhibits a large influence on the volume transition in ionic gels. (ii) Hydrogen bonding and hydrophobic association, other than electrostatic attraction, can be considered to play an important role in the segmental association. (iii) Stably associated segments via one or more of these attractive interactions causes a large hysteresis in the swelling process, in which the repulsive interaction among the fixed charges on the network is dominant as shown in the Katchalsky's model. (iv) A distribution of "neutral but hydrophilic" moieties (e.g., ion pair or salt-linkage formed between the opposite charged groups) within the gel shows a marked effect on the temperature-induced volume collapse, the aspect of which is similar to that observed in the gels with a charge inhomogeneity.     

含时密度泛函理论研究低带隙的中性和带电的交替共聚芴的光学特性

用含时的密度泛函（TD—DFT）方法研究了低带隙的中性和带电的交替共聚芴Green 1）,该化合物是由烷染取代芴和（1,2,5-噻吩基-3,4-硫重氮基）喹喔啉噻吩（T—TDQ—T）单元交替重复组成,对他们的激发态特性用二维（2D）和三维（3D）实空间分析方法做了进一步分析．对于中性的Green 1,分别得到其带隙、键能、激子结合能和核驰豫能．用3D跃迁密度方法对中性和带电的Green 1的跃迁偶极矩进行比较可显示出跃迁偶极矩的取向和强度;用3D电荷差异密度方法显示出激发后的中性和带电的Green 1电荷重新分布和比较,用2D实空间分析方法（跃迁密度矩阵）来研究中性和带电的Green 1处于激发态时的电子空穴相干性．中性Green 1的激发态特性分别用TD—DFT和ZINDO两种方法进行了计算,比较得出电子-电子相互作用（在TD—DFT中）对激发态性质的重要影响．     

Hydrogel composites of neutral and slightly charged poly (acrylamide) gels with incorporated bentonite. Interaction with salt,linear polyelectrolytes and ionic surfactants

The swelling behavior in the solutions of sodium chloride, linear polyelectrolytes and ionic surfactants of the composites based on clay mineral bentonite (BENT) embedded in neutral and slightly charged poly(acrylamide) (PAAm) gels is studied. Negatively charged flat clay particles incorporated into polymer gel adsorb oppositely charged surfactant and linear polyelectrolyte and attract the charged chains of cationic polymer matrix. The results of SAXS study manifest the formation of lamella structure of the cationic surfactant adsorbed by the clay plates. The gels loaded with the clay show a strong response to changes in the nature and the composition of the ionic environment.     

Studies on the interaction of bacterial capsular polysaccharide- Klebsiella K16 with cationic and mixed surfactants

The spectral studies of cationic dyes, pinacyanol chloride (PCYN) and acridine orange (AO) with capsular polysaccharide Klebsiella K16 (PK16) biopolymer in micellar media reveal many interesting phenomena. Intensity of the metachromatic band (μ) at 490 nm decreases gradually on addition of cationic single surfactant to the biopolymer PK16–dye system of P/D = 30, whereas the intensity of α and β bands reach to the value of original pure dye. As a result, the cationic surfactant destroys the metachromatic compound and forms a new complex with biopolymer PK16 by freeing the dye molecule. Enhancement of fluorescence intensity of AO-PK16 system with cationic surfactant is another evidence for the binding between the biopolymer and the surfactant. Interaction between the biopolymer and mixed surfactant has also been studied. Finally, the binding ability of cationic surfactants with or without non ionic surfactant, the idea of the critical aggregation concentration (cac) of the surfactant, mole fraction and the charge density of mixed surfactant for binding with PK16 and also the site of interaction have been pointed out.     

Effect of additives on swelling of covalent DNA gels

The volumetric response of polymer gels on cosolute addition depends on the interaction of the polymer with the cosolute and can be used as a simple and sensitive way of elucidating these interactions. Here we report on DNA networks, prepared by crosslinking double-stranded DNA with ethylene glycol diglycidyl ether (EGDE); these have been investigated with respect to their swelling in aqueous solution containing different additives, such as metal ions, polyamines, charged proteins, and surfactants. The deswelling on addition of metal ions occurs at lower concentrations with increasing valency of the counterion. The collapse of the gels in the presence of trivalent ions seems to follow the same kind of mechanism as the interaction in solution, but addition of these ions leads to DNA denaturation and formation of single-stranded DNA. Striking features were found in the deswelling of DNA gels by chitosan, spermine, spermidine, lysozyme, poly-l-lysine and poly-l-arginine. Chitosan is the most efficient cosolute of those investigated with respect to DNA gel collapse. The effect of the cationic surfactant tail length on the volume phase transition of DNA gels was studied as a function of surfactant concentration. Cationic surfactants effectively collapsed the gel from the critical aggregation concentration (cac), decreasing with increasing length of the hydrophobic tail. In several cases, the deswelling as a function of cosolute concentration shows a pronounced two-step behavior, which is interpreted in terms of a combination of DNA chain condensation and general osmotic deswelling. The studies included investigations on the state of the DNA chain after deswelling, on the reversibility of the deswelling as well as on the kinetics. With the exception for the trivalent lanthanide ions, it appears that the DNA chain always retains a double-helix conformation; with these metal ions, single-stranded DNA is found. The deswelling appears to be reversible as exemplified by addition of anionic surfactant subsequent to gel collapsed by cationic surfactant and addition of sodium bromide to gels collapsed by a polycation. An investigation of the kinetics shows that an increase in the surfactant tail length gives a pronouncedly slower deswelling kinetics.     

Surfactant polymer interactions between strongly interacting cationic surfactants and anionic polyelectrolytes from conductivity and turbidity measurements

The interactions between oppositely charged surfactant/polymer mixtures have been studied using conductivity and turbidity measurements. The dependence of aggregation phenomenon on the chain length and head group modifications of conventional cationic surfactants, i.e., hexadecyl- (HTAB), tetradecyl- (TTAB), and dodecyltrimethylammonium bromides (DTAB) and dimeric cationic surfactants, i.e., decyl- (DeDGB) and dodecyldimethylgemini bromides (DDGB), is investigated. It was observed that cationic surfactants induce cooperative binding with anionic polyelectrolytes at critical aggregation concentration (cac). The cac values are considerably lower than the critical micelle concentration (cmc) values for the same surfactant. After the complete complexation, free micelles are formed at the apparent critical micelle concentration (acmc), which is slightly higher in aqueous polyelectrolyte than in pure water. Among the conventional and dimeric cationic surfactants, DTAB and DeDGB, respectively, have been found to have least interactions with oppositely charged polyelectrolytes.     

The structures of complexes between polyethylene imine and sodium dodecyl sulfate in D2O: a scattering study

The association between a highly branched polyelectrolyte with ionizable groups, polyethylene imine (PEI), and an anionic surfactant, sodium dodecyl sulfate (SDS), has been investigated at two pH values, using small-angle neutron and light scattering. The scattering data allow us to obtain a detailed picture of the association structures formed. Small-angle neutron scattering (SANS) measurements in solutions containing highly charged PEI at low pH and low SDS concentrations indicate the presence of disklike aggregates. The aggregates change to a more complex three-dimensional structure with increasing surfactant concentration. One pronounced feature in the scattering curves is the presence of a Bragg-like peak at high q-values observed at a surfactant concentration of 4.2 mM and above. This scattering feature is attributed to the formation of a common well-ordered PEI/SDS structure, in analogue to what has been reported for other polyelectrolyte-surfactant systems. Precipitation occurred at the charge neutralization point, and X-ray diffraction measurements on the precipitate confirmed the existence of an ordered structure within the PEI/SDS aggregates, which was identified as a lamellar internal organization. Polyethylene imine has a low charge density in alkaline solutions. At pH 10.1 and under conditions where the surfactant was contrast matched, the SANS scattering curves showed only small changes with increasing surfactant concentration. This suggests that the polymer acts as a template onto which the surfactant molecules aggregate. Data from both static light scattering and SANS recorded under conditions where SDS and to a lower degree PEI contribute to the scattering were found to be consistent with a structure of stacked elliptic bilayers. These structures increased in size and became more compact as the surfactant concentration was increased up to the charge neutralization point.     

Binding of dodecyltrimethylammonium bromide to pH-responsive nanocolloids containing cross-linked methacrylic acid-ethyl acrylate copolymers

The binding of dodecyltrimethylammonium bromide (DoTab) to cross-linked methacrylic acid-ethyl acrylate (MAA-EA) copolymers with various MAA/EA molar ratios at different degrees of neutralization (alpha) was quantitatively studied using isothermal titration calorimetry, dynamic light scattering, surfactant selective electrode, and electrophoresis techniques. The surfactant binds to the polymers at all degrees of neutralization, but via different mechanisms. When alpha is sufficiently high, the binding is primarily electrostatic interaction between the surfactant and ionized polymer chains, which is reinforced by the micellization of electrostatically bound surfactant molecules. The saturation takes place at charge ratio ([DoTa(+)]/[ approximately COO(-)]) close to 1, indicating that the binding is a one-to-one charge neutralization between the cationic surfactant headgroups and anionic carboxylate sites of the polymers. When alpha is low, the binding of DoTab to the unneutralized polymers is driven by the hydrophobic interaction. The onset of hydrophobic binding takes place at DoTab concentration as low as 0.01 mM in 0.05 wt % polymer solution, where the saturation occurs at C(DoTab) approximately 0.19 mM and the amount of bound surfactant is approximately 0.09 mmol of DoTab/(g of polymer) at saturation concentration. The binding results in the formation of the polymer-surfactant complex. For the polymer with low MAA/EA molar ratio, the complex coagulates at a higher DoTab concentration that leads to phase separation; however, for polymers with high MAA/EA molar ratio, the complex remains dispersed and the mixture is stable even at high DoTab concentration.     

Influence of interchain interactions on the electronic properties of neutral and charged oligodiacetylenes carrying bulk substituents

We discuss the effect of the interchain interactions on the electronic properties of a "dimer", named (CHD(4))(2), built from two molecules of a fully carbazolyl-substituted oligodiacetylene containing four repeating units. Each carbazolyl moiety is connected to its respective backbone through a methylene spacer, and the excitations of the neutral and doubly positively charged dimer are obtained using the CIS (configuration interaction including singles) and CEO (collective electronic oscillator) models. The separation distance between the backbones is fixed to a value that could possibly imply a very weak, if any, interchain interaction between the oligomers. In the charged dimer, where we have not been able to perform CEO calculations, it can be expected from previous results that the CIS method will behave satisfactorily. Contrary to the neutral case and surprisingly enough, the simulated photoinduced absorption (PA) spectra indicate a substituent-induced strong interaction between the oligomers, due to the very large amount of excess charge on the distorted oligomer which is spread over its carbazolyl moieties. Broadening of the lowest energy band due to the latter interaction improves the agreement between the theoretical and the experimental PA spectrum for the corresponding polymer. The through-space charge transfer directly connecting the charged with the neutral molecule could supply an efficient channel for charge transport in substituted polydiacetylenes.