Small-angle X-ray scattering, light scattering, and NMR study of PEO-PPO-PEO triblock copolymer/cationic surfactant complexes in aqueous solution

The formation of triblock copolymer/surfactant complexes upon mixing a nonionic Pluronic polymer (PEO-PPO-PEO) with a cationic surfactant, hexadecyltrimethylammonium chloride (CTAC), has been studied in dilute aqueous solutions using small-angle X-ray scattering, static and dynamic light scattering, and self-diffusion NMR. The studied copolymer (denoted P123, EO(20)PO(68)EO(20)) forms micelles with a radius of 10 nm and a molecular weight of 7.5 x 10(5), composed of a hydrophobic PPO-rich core of radius 4 nm and a water swollen PEO corona. The P123/CTAC system has been investigated between 1 and 5 wt % P123 and with varying surfactant concentration up to approximately 170 mM CTAC (or a molar ratio n(CTAC)/n(P123) = 19.3). When CTAC is mixed with micellar P123 solutions, two different types of complexes are observed at various CTAC concentrations. At low molar ratios (>/=0.5) a "P123 micelle-CTAC" complex is obtained as the CTAC monomers associate noncooperatively with the P123 micelle, forming a spherical complex. Here, an increased interaction between the complexes with increasing CTAC concentration is observed. The interaction has been investigated by determining the structure factor obtained by using the generalized indirect Fourier transformation (GIFT) method. The interaction between the P123 micelle-CTAC complexes was modeled using the Percus-Yevick closure. For the low molar ratios a small decrease in the apparent molecular weight of the complex was obtained, whereas the major effect was the increase in electrostatic repulsion between the complexes. Between molar ratios 1.9 and 9 two coexisting complexes were found, one P123 micelle-CTAC complex and one "CTAC-P123" complex. The latter one consists of one or a few P123 unimers and a few CTAC monomers. As the CTAC concentration increases above a molar ratio of 9, the P123 micelles are broken up and only the CTAC-P123 complex that is slightly smaller than a CTAC micelle exists. The interaction between the P123/CTAC complexes was modeled with the hypernetted-chain closure using a Yukawa type potential in the GIFT analysis, due to the stronger electrostatic repulsion.     

Gelation of aqueous pectin solutions: a dynamic light scattering study

We report the dynamic light scattering study of the gelation of aqueous solutions of the biopolymer, pectin, induced by the addition of calcium chloride. The time correlation function data are analyzed under the framework of the coupling model. As the solution enters the semidilute regime where gelation sets in, the relaxation process shows a stretched exponential behavior. The stretching exponent decreases and the characteristic time of the stretched exponential diverges as the system evolves to a gel. Aqueous pectin solutions in the presence of 0.1 M NaCl show similar behavior. Thus, the molecular relaxation modes of pectin solutions can be well described by the coupling model.     

Dynamic and static light scattering study on the sol-gel transition of resorcinol-formaldehyde aqueous solution

Structure formation during the sol-gel transition of resorcinol-formaldehyde (RF) solutions was traced by dynamic light scattering (DLS) and static light scattering (SLS) techniques. The decay time spectra obtained by DLS revealed that both the growth rates of colloidal particles formed during the early stage of the sol-gel transition and the time required for the colloidal particles to form a firm network structure could be related to the ratio of catalyst to water (C/W) of the starting RF solution. SLS results indicated that the molecular weight of colloidal particles increased with the progress of the sol-gel transition, the rate of which was also affected by the value of C/W. The mesoporosity of RF aerogels, which were prepared by drying RF hydrogels with supercritical carbon dioxide, was confirmed to depend on the size of colloidal particles estimated from the decay time spectrum collected at the last stage of the sol-gel transition.     

Characterization of superlighting polymer-DNA aggregates: a fluorescence and light scattering study

The massive amplification of fluorescence signal observed upon hybridization of as few as five DNA molecules into self-assembled structures formed between a cationic polymer and DNA oligonucleotides is investigated. These superlighting polymer-DNA aggregates were studied by fluorescence spectroscopy, static and dynamic light scattering, and zeta potential measurements in order to characterize the aggregation behavior and to understand the processes involved during DNA detection. Multi-angle laser light scattering was also used to obtain the weight-average aggregate mass (AM), the aggregation number (Nagg), the radius of gyration (Rg), and the dissymmetry ratio (z). These results have been used, together with TEM imaging, to propose a suitable physical model for the aggregates.     

Temperature-induced association and gelation of aqueous solutions of pectin. A dynamic light scattering study

Dynamic light scattering (DLS) measurements were carried out on aqueous solutions of low-methoxyl pectin at different temperatures and polymer concentration. Low temperature and increased polymer concentration promote the formation of multichain aggregates. The time correlation data obtained from the DLS experiments revealed, for all polymer solutions, the existence of two relaxation modes, one single exponential at short times followed by a stretched exponential at longer times. In the semidilute regime, a temperature reduction induced enhanced chain associations in the solutions with high values of the slow relaxation time and a strong wave vector dependence of the slow mode. These features could be rationalized in the framework of the coupling model of Ngai. At low temperatures (10 °C), gelation occurs in the semidilute regime and a transparent gel is formed. In this state, the profile of the correlation function changes and nonergodic signs are observed. The conjecture is that the association complexes and the gel network are stabilized through intermolecular hydrogen bonds, which are broken-up at higher temperatures. The hydrogen-bonded structures are formed in a process where the polymer chains have been “zipped” together in a cooperative manner.     

Inclusion complexes between beta-cyclodextrin and a Gemini surfactant in aqueous solution: an NMR study

(1)H NMR spectra, diffusion-ordered NMR (DOSY), and 2D rotating-frame Overhauser enhancement spectroscopy (ROESY) experiments for aqueous solutions at 298 K containing the gemini surfactant, bis (dodecyl dimethylammonium)diethyl ether dibromide (12-EO(1)-12), in the absence and presence of beta-cyclodextrin (beta-CD) were used to characterize the surfactant and to determine the effects of the complexation in the micellization. For the binary system, the critical micelle concentration (cmc), the aggregation number, the stepwise micellization constant, and the size of the monomer have been obtained by studying the dependence of the chemical shifts and the self-diffusion coefficients with the concentration of surfactant. For the ternary system, the analysis of the (1)H NMR spectra and the self-diffusion coefficients reveal the formation of complexes of 1:1 and 2:1 stoichiometry (beta-CD:gemini), with a calculated stability constant for the second binding step higher than that of the first. The values of the hydrodynamic radii of the complexes were obtained from the calculated diffusion coefficients. The presence of beta-CD modifies the cmc in an extension that indicates mainly the formation of a 2:1 complex. The analysis of the chemical shifts of the surfactant indicates the nonparticipation of the complexes into the micelles. ROE enhancements depend substantially on the amount of the macrocycle added and therefore on the stoichiometry; at low concentrations of beta-CD, one of the hydrocarbon chains binds favorably with the cavity whereas the other interacts with the outer face. By contrast, at higher concentrations of beta-CD, the two hydrocarbon tails are included in two different macrocycles.     

Analysis of aggregates of human immunoglobulin G using size-exclusion chromatography,static and dynamic light scattering

Large aggregates (Mr: 10(6)-10(7) g/mol) of human immunoglobulins are present in extremely small concentrations in IgG preparations (<0.1%). Traces of large protein aggregates cannot be determined by conventional size-exclusion chromatography (SEC) using UV detection due to limitations in sensitivity. The conventional analysis of IgG by SEC is limited to dimers and oligomers. Using light scattering it is possible to determine significant differences concerning the aggregate composition and the extent of protein aggregation in samples of different process steps. Two different pilot preparations were analyzed by SEC with UV and static light scattering detection and compared to dynamic light scattering in the batch mode. The change of large aggregates could be monitored and data were corroborated by dynamic light scattering.     

Viscosity of aqueous DNA solutions determined using dynamic light scattering

Viscosity is a key parameter for characterising the behaviour of liquids and their flow. It is, however, difficult to measure precisely, reproducibly and accurately for aqueous solutions on a micro-litre volume scale, which is what is usually needed for biological samples. We report the development of a new method for measuring dynamic viscosity by measuring dynamic light scattering (DLS) data for a range of particles of well-defined size. Most applications of DLS involve determining particle size for samples of known viscosity. We inverted the usual protocol and endeavoured to determine viscosity for samples of known particle size. Viscosity measurements for water and aqueous solutions of calf thymus DNA made using DLS were compared with those from a U-tube viscometer. The styrene particles, frequently used as particle size standards, gave unsatisfactory results for our DNA samples as did C-6 derivatized silica and positively charged amino polystyrene microspheres. However, negatively charged carboxylate polystyrene microspheres particles readily gave accurate viscosity measurements over a range of temperatures (0-100 °C). The sample volume required depends on the cuvette used to measure DLS, but can be performed with samples sizes ranging from 40 to 3000 μL. The sample can then be recovered for subsequent experiments. The DLS method is simple to perform at different temperatures and provides data of accuracy significantly above that of a U-tube viscometer. Our results also indicate a way forward to account accurately for solution viscosity in the normal applications of DLS to particle size determination by including the appropriate non-interacting particles as an internal standard.     

1H NMR study of self-association of actinomycin D in an aqueous solution

This paper presents the results of a proton magnetic resonance study (500 MHz) of self-association of actinomycin D (AMD) antibiotic in an aqueous solution. The equailibrium constants and thermo-dynamic parameters (ΔH, ΔS) of molecular association as well as the limiting values of proton chemical shifts of associate molecules were determined from the concentration and temperature dependences of¹H NMR chemical shifts of AMD. The results were analyzed using dimeric and infinite-dimensional cooperative models of molecular self-association. The value of the cooperativity parameter indicates that AMD self-association is anticooperative, i.e., formation of aggregates larger than dimers is energetically unfavorable. The values of induced proton chemical shifts were used to determine the most probable mutual orientation of chromophores in AMD structure. Sevastopol State Technical University. Berkbeck College, London University. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 1, pp. 81–88, January–February, 1995. Translated by L. Smolina     

Sodium: Dibenzo-30-crown-10 aggregates in solution: Na-23 NMR study

(n + 1):n sodium:dibenzo-30-crown-10 complexes are formed in nitromethane solutions. A model based on the coexistence in solution of 1:1, 2:1 and 3:2 complexes can account for the observed Na-23 chemical shifts, transverse and longitudinal relaxation rates. Logarithms of the equilibrium constants of formation at (25±1)°C are (2.1±0.3) and (2.5±0.3) for the 2:1 and 3:2 complexes respectively. The characteristic Na-23 relaxation rates and chemical shifts of the aggregated complexes are compatible with a structure of ion pairs linked by the crown ether.     

Ternary complexes between cationic GdIII chelates and anionic metabolites in aqueous solution: an NMR relaxometric study

The (1)H and (17)O NMR relaxometric properties of two cationic complexes formed by Gd(III) with a macrocyclic heptadentate triamide ligand, L(1), and its Nmethylated analogue, L(2), have been investigated in aqueous media as a function of pH, temperature and magnetic field strength. The complexes possess two water molecules in their inner coordination sphere for which the rate of exchange has been found to be sensibly faster for the Nmethylated derivative and explained in terms of electronic effects (decrease of the charge density at the metal center) and perturbation of the network of hydrogen-bonded water molecules in the outer hydration sphere. The proton relaxivity shows a marked dependence from pH and decreases of about six units in the pH range 6.5 to 9.0. This has been accounted for by the displacement of the two water molecules by dissolved carbonate which acts as a chelating anion. The formation of ternary complexes with lactate, malonate, citrate, acetate, fluoride and hydrogenphosphate has been monitored by (1)H NMR relaxometric titrations at 20 MHz and pH 6.3 and the value of the affinity constant, K, and of the relaxivity of the adducts could be obtained. Lactate, malonate and citrate interact strongly with the complexes (log K > or =3.7) and coordinate in a bidendate mode by displacing both water molecules. Larger affinity constants have been measured for GdL(2). Acetate, fluoride and hydrogenphosphate form monoaqua ternary complexes which were investigated in detail with regard to their relaxometric properties. The NMR dispersion (NMRD) profiles indicate a large contribution to the relaxivity of the adducts from water molecules belonging to the second hydration shell of the complexes and hydrogen-bonded to the anion. A VT (17)O NMR study has shown a marked increase of the rate of water exchange upon binding which is explained by coordination of the anion in an equatorial site, thus leaving the water molecule in an apical position, more accessible for interactions with the solvent molecules of the second hydration shell which facilitate the exchange process.     

Hydrodynamic radius of polyethylene glycol in solution obtained by dynamic light scattering

In order to compare the size characterizations in poly(ethylene glycol) (PEG) obtained by dynamic light scattering (DLS) and small angle neutron scattering (SANS), DLS experiments were performed in various PEG solutions to ascertain the hydrodynamic radius. Data from the experiments were analyzed by using a method to eliminate effects of PEG aggregation on dynamic correlation functions. The results of the analysis were then compared to the radii of gyration reported from SANS experiments. The relation between the hydrodynamic radius, obtained by DLS, and the radius of gyration, obtained by SANS, in PEG in solution was found to be in agreement with a previously obtained relation for PEG, where the radius of gyration was found by static light scattering.     

Dynamic light scattering study of cetyltrimethylammonium bromide aqueous solutions

Cetyltrimethylammonium bromide (CTAB) aqueous solutions are studied by dynamic light scattering method in a wide concentration range covering the first and second critical micelle concentrations (CMC₁ and CMC₂, respectively). Nonmonotonic and ambiguous behavior of diffusion coefficients D with an increase in concentrations above CMC1 is revealed. An increase in the D values in the first decade of CTAB concentration above CMC1 agrees with known published data for aqueous solutions of ionic surfactants. It is shown that an increase in the ionic strength of solution with the addition of KBr leads to a decrease in the positive slope of the dependence of diffusion coefficients on CTAB concentration up to zero at 0.05 M KBr. Two relaxation processes corresponding to large and small D values are simultaneously observed in micellar solutions, beginning with 0.03 M CTAB concentration. The data obtained are compared with published data, as well as with the results of viscosity measurements. The performed analysis indicates that the observed relaxation processes are explained by the coexistence of spherical and nonspherical micelles. It is established that micelles acquire a cylindrical shape at CTAB concentrations ranging from 0.2 to 0.25 M. Hydrodynamic radii and lengths of micelles are calculated.     

Characterization of riboflavin-photosensitized changes in aqueous solutions of alginate by dynamic light scattering

The effect of irradiation, in the wavelength range of 310-800 nm, on aqueous solutions (pH = 7.4) of alginate in the presence of the photosensitizer riboflavin (RF) has been investigated with the aid of dynamic light scattering (DLS). Under aerobic conditions light irradiation of RF causes scission of the polymer chains which affects the polymer dynamics. The time correlation data obtained from DLS experiments showed at all conditions the existence of two relaxation modes: one single exponential at short times, followed by a stretched exponential at longer times. The slow relaxation time revealed, over the whole considered concentration range, lower values for the alginate/RF system, whereas no effect of photochemical degradation was observed for the fast relaxation time in the semidilute regime. The results suggest that the photochemically induced fragmentation of alginate affects the slow relaxation mode, associated with disengagement relaxation of individual chains or cluster relaxation, in a similar way as the zero-shear viscosity. These findings provide detailed insight into the dynamics of the polymer matrix, and this knowledge can be useful in the context of controlled-release delivery of drugs. The chemical units of alginate (M = mannuronic acid and G = guluronic acid).     

The solubilisation behaviour of some dichloroalkanes in aqueous solutions of PEO-PPO-PEO triblock copolymers: a dynamic light scattering, fluorescence spectroscopy, and SANS study

The aggregation behaviour of PEO-PPO-PEO triblock copolymers in water and in water + chlorinated additive mixtures was studied by means of fluorescence spectroscopy, dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The copolymers were chosen such as to investigate the effects of molecular architecture (L35 and 10R5) and molecular weight by keeping constant the hydrophilic/hydrophobic balance (F88 and F108). 1,2-Dichloroethane was used as a prototype of water basins contaminants. The hydrodynamic radius of the block copolymer aggregates (R(h,M)) and the intensity ratio of pyrene of the first and the third vibrational band (I(1)/I(3)) were determined as a function of temperature (10-45 degrees C) and concentration. The copolymer architecture essentially does not affect R(h,M) in the entire range of temperature and concentration investigated. At a given temperature, increasing macromolecular size leads to a decrease of R(h,M). With rising temperature R(h,M) also decreases. According to the DLS results, the I(1)/I(3) change with temperature clearly detects the aggregation only for F88 and F108. The presence of 1,2-dichloroethane, at concentrations close to its solubility in water, does not lead to changes in the distribution of hydrodynamic radii for L35 and 10R5. Larger quantities of additive induce the formation of quite polydisperse mixed aggregates for L35 and of networks for 10R5. In the case of F88 and F108, low concentrations of additive lead to formation of mixed aggregates with smaller R(h,M). The SANS results corroborate the DLS and fluorescence findings proving enhancement of the copolymer aggregation through the presence of 1,2-dichloroethane. The DLS findings combined with those from the fluorescence spectroscopy provide some insight into the site of solubilisation of the additive in the aggregates.     

Analysis of the interactions between human serum albumin/amphiphilic penicillin in different aqueous media: an isothermal titration calorimetry and dynamic light scattering study

The complexation process of the amphiphilic penicillins sodium cloxacillin and sodium dicloxacillin with the protein human serum albumin (HSA) in aqueous buffered solutions of pH 4.5 and 7.4 at 25 °C was investigated through isothermal titration calorimetry (ITC) and dynamic light scattering. ITC experiments were carried out in the very dilute regime and showed that although hydrophobic interactions are the leading forces for complexation, electrostatic interactions also play an important role. The possibility of the formation of hydrogen bonds is also deduced from experimental data. The thermodynamic quantities of the binding mechanism, i.e, the enthalpy, , entropy, , Gibbs energy, , binding constant, and the number of binding sites, n_i, were obtained. The binding was saturable and is characterised by Langmuir adsorption isotherms. From ITC data and following a theoretical model, the number of bound and free penicillin molecules was calculated. From Scatchard plots, and n_i were obtained and compared with those from ITC data. The interaction potential between the HSA–penicillin complexes and their stability were determined at pH 7.4 from the dependence of the diffusion coefficients on protein concentration by application of the DLVO colloidal stability theory. The results indicate decreasing stability of the colloidal dispersion of the drug–protein complexes with increase in the concentration of added drug.     

Study of aqueous solutions of fullerenol-d by the dynamic light scattering method

Dynamic light scattering method was used to determine the average size of fullerenol-d associates and the dependence of the electrokinetic ζ-potential on the concentration of aqueous solutions of fullerenol-d.     

Core-corona structure of cubic silsesquioxane-poly(ethylene oxide) in aqueous solution: fluorescence, light scattering, and TEM studies

Well-defined amphiphilic cubic silsesquioxane-poly(ethylene oxide) (CSSQ-PEO) was prepared from octakis (dimethylsiloxy)octasilsesquioxane (Q8M8(H)) and allyl-PEO through a hydrosilylation reaction. The structure of CSSQ-PEO was characterized by nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography (GPC). The amphiphilic properties and aggregation process of CSSQ-PEO in aqueous solution were investigated by fluorescence, dynamic and static light scattering (DLS and SLS), and transmission electron microscopy (TEM). The critical aggregation concentration (CAC) determined by fluorescence measurements was found to be 0.28 mg/mL. Combinations of DLS, SLS, and TEM studies showed the existence of core-corona micelle with hydrophobic CSSQ as the core and hydrophilic PEO as the corona in aqueous solution. The observation of two size distribution peaks from DLS measurements revealed the coexistence of small amounts of unassociated unimolecular micelles (approximately 10% of the scattered intensity) together with micellar aggregates when the CSSQ-PEO concentration was < or = 2 mg/mL. The hydrodynamic radii (R(h)) of unassociated unimolecular micelle and micellar aggregates were found to be 26 and 79 nm, respectively. A large R(g)/R(h) ratio (1.46) and the extremely small value of average chain density (4 x 10(-4) g/cm3) indicate the small hydrophobic CSSQ core was surrounded by the extended PEO coronae. The aggregation number (N(agg)) of CSSQ-PEO in aqueous solution was found to be 38 +/- 2 from SLS and 31-40 from TEM, respectively. The long PEO segments act as a spacer between the spherical aggregates, which facilitate the formation of a network-like structure at high concentration.     

Characterization of sodium dodecylsulphate and dodecylphosphocholine mixed micelles through NMR and dynamic light scattering

The complexity of biological membranes leads to the use of extremely simplified models in biophysical investigations of membrane‐bound proteins and peptides. Liposomes are probably the most widely used membrane models due, especially, to their versatility in terms of electric charge and size. However, liquid‐state NMR suffers the lack of such a model, because even the smallest liposomes slowly tumble in solution, resulting in a dramatic signals broadening. Micelles are typically used as good substitutes, with sodium dodecylsulphate (SDS) and dodecylphosphocholine (DPC) being the most widely employed surfactants. However, they are always used separately to mimic prokaryotic and eukaryotic membranes, respectively, and accurate investigations as a function of surface charge cannot be performed. In this work, the critical micelle concentration (CMC) of binary mixtures with different SDS/DPC ratios has been determined by following the chemical shift variation of selected ¹H and ³¹P NMR signals as a function of total surfactant concentration. The regular solution theory and the Motomura's formalism have been applied to characterize the micellization both in water and in phosphate buffer saline, and results were compared with those obtained directly from the experimental NMR chemical shift. The ζ‐potential and size distribution of the mixed micelles have been estimated with dynamic light scattering measurements. Results showed that SDS and DPC are synergic and can be used together to prepare mixed micelles with different negative/zwitterionic surfactants molar ratio. Copyright © 2013 John Wiley & Sons, Ltd.