Free Monomer Concentration Research Articles

Equilibrium state model for surfactants in oils: Colloidal assembly and adsorption

An equilibrium state model addressing the aggregation and adsorption of colloidal assemblies in apolar solvents (oils) via monomer exchange is presented. The model is based on the previously reported step-wise aggregation response of fatty acids and monoglycerides in bio-oils, and captures surface crowding via scaled particle theory (SPT). The sensitivity of key observables - mean aggregation number, adsorbed surfactant amount, and free monomer concentration - to model parameters is demonstrated. Fits to molecular modelling based aggregation and adsorption data of oleic acid and monoolein reveal that the model accurately reproduces chemically specific aggregate exponential distributions in both bulk and surface phases, even outside of its parameterization conditions. A biased state model, where the initial bulk aggregation step (dimer formation) differs from other steps results in a notable improvement in accuracy. Fits to various phospholipid adsorption isotherms demonstrate the applicability of the model to isotherm type experimental data. The fits reveal either monolayer or aggregate like adsorption structures, depending on surfactant head group charge. The presented model provides an easily accessible, computationally feasible means to estimate colloidal assembly and adsorption in oil environments, and enables assessment of surfactant aggregation propensity and adsorption energetics.

The solubility product extends the buffering concept to heterotypic biomolecular condensates.

Biomolecular condensates are formed by liquid-liquid phase separation (LLPS) of multivalent molecules. LLPS from a single ("homotypic") constituent is governed by buffering: above a threshold, free monomer concentration is clamped, with all added molecules entering the condensed phase. However, both experiment and theory demonstrate that buffering fails for the concentration dependence of multicomponent ("heterotypic") LLPS. Using network-free stochastic modeling, we demonstrate that LLPS can be described by the solubility product constant (Ksp): the product of free monomer concentrations, accounting for the ideal stoichiometries governed by the valencies, displays a threshold above which additional monomers are funneled into large clusters; this reduces to simple buffering for homotypic systems. The Ksp regulates the composition of the dilute phase for a wide range of valencies and stoichiometries. The role of Ksp is further supported by coarse-grained spatial particle simulations. Thus, the solubility product offers a general formulation for the concentration dependence of LLPS.

Broaden reflection bandwidth of chiral nematic liquid crystals by chiral compound-loaded thermally responsive microcapsules

ABSTRACT Controllable broadband reflection of cholesteric liquid crystal (N*-LC) materials has sparked enormous interests due to its significance in areas such as infrared shielding glass films and LC displays. In this study, we report a novel method to broaden the bandwidth of the chiral nematic liquid crystal (N*-LC) by a thermally responsive microcapsule (TRMC) loaded with chiral compound. Diffusion of chiral compound loaded in the microcapsules triggered by heating resulted in non-uniform pitch distribution, which were locked by UV-radical polymerisation. The results showed that the reflection bandwidth could be broadened by adjusting mass of the microcapsules, heating time, UV intensity, concentration of UV-polymerisable free radical monomer and polymerisation temperature. The reflection bandwidth was broadened from 240 nm to 374 nm by optimising the above factors.

Mass-Balance Models for Scrutinizing Supramolecular (Co)polymerizations in Thermodynamic Equilibrium.

ConspectusRecent years have witnessed increasing attention on supramolecular polymerization, i.e., the formation of one-dimensional aggregates in which the monomeric units bind together via reversible and usually highly directional non-covalent interactions. Because of the presence of these reversible interactions, such as hydrogen bonding, π–π interactions, or metal coordination, supramolecular polymers exhibit numerous desirable properties ranging from high thermoresponsiveness to self-healing and great capacity for processability and recycling. These properties relate to intriguing experimentally observed nonlinear effects such as the monomer-dependent presence of a critical temperature for aggregation and a solvent- and temperature-tunable aggregate morphology. For coassemblies this is complemented with monomer-ratio- and monomer-compatibility-dependent internal order as well as majority-rules-type chiral amplification. However, the dynamic nature of the (co)polymers and the intricate interplay of many interactions make these effects difficult to rationalize without theoretical models.This Account presents recent advances in the development and use of equilibrium models for supramolecular copolymerization based on mass balances, mainly developed by our group. The basic idea of these models is that we describe a supramolecular (co)polymerization by a set of independent equilibrium reactions, like monomer associations and dissociations, and that in thermodynamic equilibrium the concentrations of the reactants and products in each reaction are coupled via the equilibrium constant of that reaction. Recursion then allows the concentration of each possible aggregate to be written as a function of the free monomer concentrations. Because a monomer should be present either as a free monomer or in one of the aggregates, a set of n equations can be formed with the n free monomer concentrations as the only unknowns. This set of mass-balance equations can then be solved numerically, yielding the free monomer concentrations, from which the complete system can be reconstituted.By a step-by-step extension of the model for the aggregation of a single monomer type to include the formation of multiple aggregate types and the coassembly of multiple monomer types, we can capture increasingly complex supramolecular (co)polymerizations. In each step we illustrate how the extended model explains in detail another of the experimentally observed nonlinear effects, with the common denominator that small differences in association energies are intricately amplified at the supramolecular level. We finally arrive at our latest and most general approach to modeling (cooperative) supramolecular (co)polymerization, which encompasses all of our earlier models and shows great promise to help rationalize also future systems featuring ever-increasing complexity.

Counterintuitive consequences of competitive pathways in supramolecular polymerizations

Competition between supramolecular polymers for the same monomers can lead to strong responses in these systems. Using numerical simulations, we show that the sharpness and asymmetry of the transition from one thermodynamically stable polymer to another upon changing temperature is determined by the most unstable rather than the most stable supramolecular polymer. The width and asymmetry of the transitions are shown to depend on the relative and absolute nucleation penalties of the competing supramolecular polymerizations.

Regression Analysis for Nucleation-Elongation Model of Supramolecular Assembly: How To Determine Nucleus Size.

Nucleation-elongation is known to give satisfactory descriptions of many supramolecular polymerization systems in thermal equilibrium. Its key feature is the necessity to form a "nucleus" consisting of a certain number of monomer units before being able to grow into a longer polymer chain. The size of the nucleus has significant implications for the understanding of the supramolecular polymerization mechanism. Here we investigate how experiments can give information on the nucleus size by regression analysis of various types of measurements. The measurements of free monomer concentrations, diffusion coefficients, and calorimetric response as functions of concentration or temperature are considered. The nucleation-elongation model with a general value for the nucleus size is used to provide mathematical expressions for these experimental observables. Numerical experiments are performed where experimental errors are simulated by computer-generated random numbers, and it is investigated whether least-squares fitting analyses can give the correct values of the nucleus size in the presence of experimental errors. It is recommended that the calorimetric measurements such as differential scanning calorimetry (DSC) or isothermal titration calorimetry (ITC) be performed under various conditions to correctly determine the nucleus size experimentally.

What is Responsible for Atypical Dependence of the Rate of Amyloid Formation on Protein Concentration: Fibril-Catalyzed Initiation of New Fibrils or Competition with Oligomers?

An abnormal dependence of the rate of amyloid formation on protein concentration has been recently observed by Meisl et al. for Aβ40 peptides associated with Alzheimer's disease. To explain this effect, Meisl et al. proposed a novel mechanism of fibril growth: the fibril-catalyzed initiation of fibril formation. In this paper we offer an alternative explanation of the observed anomalous kinetics: formation of metastable oligomers competing with fibril formation by decreasing the concentration of the fibril-forming free monomers. Here we show that the oligomer sizes resulting from the anomalous dependence of the fibril growth rate on protein concentration are close to the sizes of oligomers observed by electron microscopy.

Anomalous Kinetics of Amyloidogenesis Suggest a Competition between Oligomers and Fibrils

Meisl et al. have recently observed an anomalous dependence of the amyloid formation rate on the protein concentration. A novel mechanism of fibril growth has been proposed by Meisl et al. to explain the abnormality; it consists in the fibril-catalyzed initiation of fibril formation with saturation of catalytic sites at high concentrations of substrates. Our article describes an alternative explanation of the anomalous kinetics, assuming that the formation of metastable oligomers competes with fibril formation by decreasing the concentration of free monomers. Oligomers are indeed observed in the course of amyloid formation, but are usually considered as seeds of amyloid fibrils rather as their competitors. However, the oligomers visually detectable by electron microscopy were shown to be close in size to those that can be derived from the anomalous dependence of the amyloid growth rate on the protein concentration, given that the anomaly results from competition between oligomer formation and amyloidogenesis.

Anomalous Kinetics of Amyloidogenesis Suggest a Competition between Oligomers and Fibrils

Meisl et al. have recently observed an anomalous dependence of the amyloid formation rate on the protein concentration. A novel mechanism of fibril growth has been proposed by Meisl et al. to explain the abnormality; it consists in the fibril-catalyzed initiation of fibril formation with saturation of catalytic sites at high concentrations of substrates. Our article describes an alternative explanation of the anomalous kinetics, assuming that the formation of metastable oligomers competes with fibril formation by decreasing the concentration of free monomers. Oligomers are indeed observed in the course of amyloid formation, but are usually considered as seeds of amyloid fibrils rather as their competitors. However, the oligomers visually detectable by electron microscopy were shown to be close in size to those that can be derived from the anomalous dependence of the amyloid growth rate on the protein concentration, given that the anomaly results from competition between oligomer formation and amyloidogenesis.

The Effect of Hydrogen Bonding on Radical Semi-Batch Copolymerization of Butyl Acrylate and 2-Hydroxyethyl Acrylate.

The radical copolymerization of butyl acrylate (BA) and 2-hydroxyethyl acrylate (HEA) was investigated under batch and semi-batch operations, with a focus on the influence of hydrogen-bonding on acrylate backbiting. The effect of hydrogen bonding on HEA to BA relative incorporation rates during copolymerization, previously seen in low-conversion kinetic studies, was also observed under high-conversion semi-batch conditions. However, overall reaction rates (as indicated by free monomer concentrations), polymer molar masses, and branching levels did not vary as copolymer HEA content was increased from 0 to 40 wt % in the semi-batch system. In contrast, introduction of a H-bonding solvent, n-pentanol, led to an observable decrease in branching levels, and branching levels were also reduced in batch (co)polymerizations with HEA. These differences can be attributed to the low levels of unreacted HEA in the starved-feed semi-batch system.

Hierarchical Fullerene Assembly: Seeded Coprecipitation and Electric Field Directed Assembly.

Hierarchical C60 colloidal films are assembled from nanoscale to macroscale. Fullerene molecular crystals are grown via seeded cosolvent precipitation with mixed solvent [tetrahydronaphthalene (THN)/trimethylpyridine (TMP)] and antisolvent 2-propanol. The fullerene solutions are aged under illumination, which due to the presence of TMP reduces the free monomer concentration through fullerene aggregation into nanoparticles. The nanoparticles seed the growth of monodisperse fullerene colloids on injection into the antisolvent. Diverse colloidal morphologies are prepared as a function of injection volume and fullerene solution concentration. The high fullerene solubility of THN enables C60 colloids to be prepared in quantities sufficient for assembly (5 × 108 ). Electric fields are applied to colloidal C60 platelets confined to two dimensions. The particles assemble under dipolar forces, dielectrophoretic forces, and electrohydrodynamic flows. Frequency-dependent phase transitions occur at the critical Maxwell-Wagner crossover frequency, where the effective polarizability of the particles in the medium is substantially reduced. Structures form as a function of field strength, frequency, and confinement including hexagonal, oblique, string fluid, coexistent hexagonal-rhombic, and tetratic.

A Minimalistic Kinetic Model for Amyloid Self-Assembly

Amyloid aggregation is a phenomenon in which proteins/peptides self-assemble into highly ordered structures that are associated with diseases (e.g. Alzheimer's and Parkinson's) as well as several native biological functions. A wide range of proteins/peptides can form these structures while displaying remarkable phenomenological similarities, both structural and kinetic. Thus, there is a need for a unifying framework that would enable a better understanding of the phenomenon of amyloid formation. In the current work, we develop a coarse-grained model for amyloid formation that accounts for experimentally known phenomenon like polymerization, depolymerization (under different polymer-states) and the conformational transition of monomers during aggregation. The model is analytically solvable, and can be simulated for timescales and lengthscales that are biologically relevant. Our mathematical model and kinetic Monte carlo simulations shed light on the vital signatures of amyloid growth, including a plausible explanation for the concentration-independence of fibril growth velocities at large free-monomer concentrations. Interestingly, our simulations also reveal the relationship between the heterogeneity in filament lengths (variances) and the rate of conformational transition. The length fluctuation data suggest that an increased rate of conformational transition not only alters the extent of fluctuation in filament lengths, but also the timescales at which the heterogeneity is maximum. Using our analytical expressions for aggregation and beta-sheet growth velocities, we predict a multi-phasic behavior of proteins/peptides in response to varying kinetic parameters. Our model also captures other interesting aspects of amyloid growth including the potential of growing fibrils to generate mechanical forces and the experimentally observed phenomenon of intermittent nature of filament growth. Overall, the study could significantly augment our fundamental understanding of the complex phenomenon of amyloid aggregation.

Dynamics of actin monomers assembled into long filaments

We investigate the dynamics of actin monomers that are assembled into long filaments via the particle-based Brownian dynamics simulations. In order to study the dynamics of long filaments containing up to several hundred protomers, a coarse-grained model for actin polymerization involving several simplifications is used. In order to overcome the large separation of time scales between the diffusive motion of the free monomers and the relatively slow polymerized and depolymerized processes at the two ends of the filaments, all polymerized and depolymerized rates are rescaled by a dimensionless parameter. Actin protomers within a filament generally possess three nucleotide states corresponding to a bound adenosine triphosphate (ATP), adenosine diphosphate with inorganic phosphate (ADP. Pi), and ADP molecules in the presence of ATP hydrolysis. Here in this paper, single nucleotide state and two nucleotide states of actin protomers are described by the simplified theoretical model, giving the dependence of the growth rate on actin concentration. The simplest case where all protomers are identical, is provided by the assembly of ADP-actins. In the simulations, the growth rate is found to increase linearly with free monomer concentration, which agrees quantitatively with in vitro experimental result. These surprised phenomena observed in the experiments, such as treadmilling processes and length diffusion of actin filaments at the steady state, are presented in detail by Brownian dynamics simulations. For free actin concentrations close to the critical concentration, cT ccr, T, the filaments undergo treadmilling, that is, they grow at the barbed end and shrink at the pointed end, leading to the directed translational motion of the filament. In the absence of ATP hydrolysis, the functional dependence of a length diffusion constant on ADP-actin monomer concentration implies that a length diffusion constant is found to increase linearly with ADP-actin monomer concentration. With the coupling of ATP hydrolysis, a peak of the filament length diffusion as a function of ATP-actin monomer concentration is observed i. e. , the length diffusion coefficient is peaked near to 35 mon2/s below the critical concentration and recovers to the expected estimate of 1 mon2/s above the critical concentration. These obtained results are well consistent with the experimental results and stochastic theoretical analysis. Furthermore, several other quantities and relations that are difficult to study experimentally but provide nontrivial crosschecks on the consistency of our simulations, are investigated in the particle-based simulations. The particle-based simulations developed in our studies would easily extend to study a variety of more complex systems, such as the assembly process of other dynamic cytoskeletons

The N-terminal region of amyloid β controls the aggregation rate and fibril stability at low pH through a gain of function mechanism.

Alzheimer's disease is linked to a pathological polymerization of the endogenous amyloid β-peptide (Aβ) that ultimately forms amyloid plaques within the human brain. We used surface plasmon resonance (SPR) to measure the kinetic properties of Aβ fibril formation under different conditions during the polymerization process. For all polymerization processes, a critical concentration of free monomers, as defined by the dissociation equilibrium constant (K(D)), is required for the buildup of the polymer, for example, amyloid fibrils. At concentrations below the K(D), polymerization cannot occur. However, the K(D) for Aβ has previously been shown to be several orders of magnitude higher than the concentrations found in the cerebrospinal and interstitial fluids of the human brain, and the mechanism by which Aβ amyloid forms in vivo has been a matter of debate. Using SPR, we found that the K(D) of Aβ dramatically decreases as a result of lowering the pH. Importantly, this effect enables Aβ to polymerize within a picomolar concentration range that is close to the physiological Aβ concentration within the human brain. The stabilizing effect is dynamic, fully reversible, and notably pronounced within the pH range found within the endosomal and lysosomal pathways. Through sequential truncation, we show that the N-terminal region of Aβ contributes to the enhanced fibrillar stability due to a gain of function mechanism at low pH. Our results present a possible route for amyloid formation at very low Aβ concentrations and raise the question of whether amyloid formation in vivo is restricted to a low pH environment. These results have general implications for the development of therapeutic interventions.

Alternatives to the use of synthetic organic coagulant aids in drinking water treatment: improvements in the application of the crude extract of Moringa oleifera seed

Drinking water treatment is a process based on multiple stages that has a main objective to provide water safe enough to be consumed by humans. Coagulation–flocculation is used to remove colloidal and suspended solids. This process improves the performance of subsequent stages (as sedimentation or filtration) as well as the water quality with a desired end-use. For many years, inorganic and organic synthetic polyelectrolytes have been used in coagulation–flocculation processes. However, its use has been deeply studied recently to determine the potential impact of residual concentration of these substances on human health and the environment. Strict regulations limit the concentration of free residual monomer after the addition of polyacrylamide (PAM) in drinking water treatment and study the effect of interaction of the residues with disinfection products. Therefore, in the last years there has been a resurgence of interest to use natural materials with the same performance that synthetic, but with lower hazard for the environment and humans. This work studies the use of the flocculant extracted from Moringa oleifera seed, in combination with polyaluminum chloride (PAC). The performance is compared with the combination PAC–PAM in terms of coagulant activity and physical–chemical quality of the water treated. Jar test was carried out using two types of natural water (with presence of bentonites) and different combinations of coagulant and flocculants. Results show that coagulant activity of PAC–Moringa combination is comparable with the results obtained with PAC–PAM, reducing initial turbidity up to 90% in all the tests. With regard to physical–chemical quality of the treated water, PAC–Moringa produces values under the drinking water quality standards for all the parameters analyzed. It is remarkable that the decrease of 50% in the trihalomethanes formation potential rate shown for PAC–Moringa combination, observed when treating natural water with presence of bentonites. Therefore, the results obtain in this work encourage the use of Moringa oleifera extract as a natural, low cost, effective, and low-toxicity alternative to the use of synthetic organic polyelectrolytes as polyacrylamide for drinking water treatment.

Molecular Dynamics Simulation of SDS and CTAB Micellization and Prediction of Partition Equilibria with COSMOmic

Molecular dynamics (MD) simulations of the self-assembly of different ionic surfactants have been performed in order to obtain representative micellar structures. Subsequently, these structures were used to predict the partition behavior of various solutes in these micelles with COSMOmic, an extension of COSMO-RS. This paper includes multiple self-assembled micelles of SDS (sodium dodecyl sulfate, anionic surfactant) and CTAB (cetyltrimethylammoniumbromide, cationic surfactant) at different concentrations. Micellar size, density profiles, and shape (eccentricity) have been investigated. However, the size strongly depends on the functional definition of a micelle. For this reason, we present a method based on the free monomer concentration in aqueous solution as an optimization criterion for the micelle definition. The combination of MD with COSMOmic has the benefit of combining detailed atomistic information from MD with fast calculations of COSMOmic. For the first time the influence of micelle structure on pratition equilibria, predicted with COSMOmic, were investigated. In case of SDS more than 4600 and for CTAB more than 800 single micelles have been studied. The predictions of the partition coefficients with COSMOmic are in good agreement with experimental data. Additionally, the most favorable locations of selected molecules in the micelles as well as probable energy barriers are determined even for complex solutes such as toluene, propanolol, ephedrine, acetone, phenol, lidocaine, syringic acid, coumarin, isovanillin, ferulic acid, and vanillic acid. This method can therefore be applied as a potential screening tool for solutes (e.g., drugs) to find the optimal solute-surfactant combination.

Controlling the Fluorescent Properties of Aqueous CdTe Nanocrystals by Modulating the Growth Rate

Although high quality aqueous CdTe and CdTeS alloyed quantum dots (QDs) have been synthesized in recent years, the relationship between the fluorescent properties and the growth kinetics has not been well documented yet. In this paper, 3-mercaptopropionic acid stabilized aqueous CdTe nanocrystals (NCs) are generally prepared with an improvement of the "traditional" synthetic approach, where the preparation of the red emission NCs is usually time-consuming due to the slow growth rate. The investigation on the kinetic and thermodynamic growth process shows that the growth can be effectively accelerated by decreasing the ligand concentration from 0.06 to 0.01 mol/L or elevating the growth temperature from 120 to 240 degrees C. In contrast to previous results, the quantum yield (QY) of the CdTe NCs is heightened to 45% only by increasing properly the growth rate. Our experiments depict that high growth rate favours high concentration of free monomers and thus decreases the number of the surface Te atoms of the NCs, leading to the enhancement of the photoluminescence (PL) QY.

Role of polymer–surfactant interactions in foams: Effects of pH and surfactant head group for cationic polyvinylamine and anionic surfactants

In a previous study (Langmuir, 28 (2012) 4996) we showed that the foamability of mixed solutions of the cationic polymer polyvinylamine (PVAm) and the anionic surfactant sodium dodecyl sulphate (SDS) is strongly reduced, whereas the stability of the formed foams is strongly enhanced, as compared to the solutions of SDS alone. Here we study in more detail the foaming properties of mixed solutions of PVAm with anionic surfactants. The effect of surfactant head group is studied by comparing SDS with another anionic surfactant (sodium dodecyl oxyethylene sulphate, SDP1S) which contains an additional ethoxy fragment in the charged head-group. For changing the electrostatic polymer–surfactant interactions, we varied pH between 6 and 10, thus crossing the polymer pKa ≈ 8.6. The foam tests showed that the foamability of all mixed solutions is strongly reduced in the entire range of pH values studied. The negative effect of PVAm on solution foamability is highest at low pH, where the polymer charge density is the highest. Model experiments revealed that the reduced foamability is due to prolonged lag-time for formation of mixed adsorption layer on bubble surfaces. Surprisingly, we found that the stability of SDP1S foams is also reduced strongly by PVAm at pH 6 (contrary to SDS and to conventional understanding). The obtained results indicate that the ethoxy group in SDP1S enhances the surfactant association with the polymer molecules, thus decreasing the concentration of free surfactant monomers, necessary to adsorb on the solution surface for foam stabilization during foaming and immediately after it. These results clearly demonstrate that excessively strong polymer–surfactant interactions could be a problem in the formation and stabilization of foams from mixed solutions. Moreover, the effect of cationic polymers on the foaming properties of anionic surfactants could depend significantly on the specific head group of the surfactant.

Self-assembly of coarse-grained ionic surfactants accelerated by graphics processing units

Due to the relatively long time scales inherent to ionic surfactant self-assembly (>μs), an aggressive computational approach is needed to obtain converged data on micellar solutions. This work presents a study of micellization using a coarse-grained (CG) model of aqueous ionic surfactants in replicated molecular dynamics (MD) simulations run on graphics processing unit hardware. The performance of our implementation of the CG model with electrostatics into the HOOMD-Blue GPU-accelerated MD software package is comparable to that of a modest sized cluster running a highly optimized parallel CPU code. From 0.36 ms of cumulative trajectory data, we are able to predict equilibrium thermodynamic and morphological properties of ionic surfactant micellar solutions. Estimating the critical micelle concentrations (CMC) from the free monomer (ρ1) and premicellar concentrations obtained from simulations of sodium hexyl sulfate (S6S, CMC of 460 ± 6 mM) at high (1 M) concentration, a value in good agreement with experimental results is obtained; however, the same method applied to simulations of sodium nonyl sulfate (S9S, ρ1 of 2.4 ± 0.01 mM) and sodium dodecyl sulfate (SDS, ρ1 of 0.02 ± 0.01 mM) at the same total concentration systematically underestimates the CMCs. An alternative method for calculating the CMC is presented, where the free monomer concentration computed from high concentration CG-MD data is used as the input to a simple theoretical model which can be used to extrapolate to a more accurate prediction of the CMC. Better agreement between the empirical and predicted CMC is obtained from this theory for S9S (28.7 ± 0.3 mM) and SDS (3.32 ± 0.04 mM), though the CMC for S6S is slightly underestimated (304 ± 3 mM). We also present statistically converged morphological data, including aggregation number distributions and the principal components of the gyration tensor. This data suggest a transition from spherical micelles to rod-like at a specific aggregation number, which increases with increasing hydrocarbon length.

Solvation of Sodium Octanoate Micelles in Concentrated Urea Solution Studied by Means of Molecular Dynamics Simulations

The effects of urea on self-assembling remains a challenging topic on surface chemistry, and computational modeling may have a role on the unraveling of the molecular mechanisms underlying these effects. Bearing that in mind, we performed a set of molecular dynamics simulations to assess the effects of urea on the self-assembling properties of sodium octanoate, an anionic surfactant, as compared to the aggregation of the same surfactant in pure water as the solvent. The concentration of free monomers increased 3-fold in the presence of urea, in agreement with the accepted view that urea should increase monomer solubility. Regarding the size distribution of micellar aggregates, the urea solution favored smaller micelles and a narrower distribution. Preferential solvation by either water or urea changed along the surfactant molecules, from urea-rich shells around apolar atoms at the end of the hydrophobic tails to nearly no urea at the polar headgroups. This solvation profile is consistent with two different hypotheses from the literature: on one hand, urea molecules interact directly with apolar atoms from the hydrophobic tails, acting as a surfactant, and on the other hand the presence of urea molecules increases the hydration of polar sites. Another important observation regards the solvent structure, which exhibits a complex composition profile around both water and urea molecules. Although the solvent structure was appreciably different in each case, the free energy calculations for the dissociation of a pair of octanoate molecules pointed to a purely enthalpic free energy loss in urea solution, a finding that does not lend support to the third hypothesis that is often claimed as accounting for the urea effects, namely, that urea disrupts water structure and that this structural change decreases the hydrophobic effect due to an entropy change. The presence of urea had no significant effect on the molecular structure of the surfactant molecules, although it caused chain dynamics to become slower. The overall picture arising from the molecular-scale data extracted from our computational models is somewhat different from the traditional views about the structural and dynamical features of self-assembled surfactant systems, pointing out the need for more studies on other self-organized systems using a realistic model system as a way to achieve a more detailed picture.