Effects of the Method of Preparation and Dispersion Media on the Optical Properties and Particle Sizes of Aqueous Dispersions of a Double-Chain Cationic Surfactant.

As inferred from visual observations and turbidity measurements, the average radius of the unilamellar vesicles formed in water from the cationic double-chain surfactant didodecyldimethylammonium bromide (DDAB) varies with the method of preparation, being ∼24 nm after sonication (SS method) and ∼74 nm after extrusion/ultrafiltration (SE method). The radii were larger when the vesicles were produced in 10 mM NaBr, ∼65 nm for the SS method and ∼280 nm for the SE method. The specific turbidity, or turbidity per unit path length divided by the surfactant weight fraction, w, of these vesicular dispersions increased with decreasing w until a constant value was reached at w*, which depends on the preparation method and the dispersion medium. The constant specific turbidities are indicative of single and independent scattering and were used to estimate vesicle radii by solving the specific turbidity equations derived for the Rayleigh-Debye-Gans (RDG) regime. Two turbidity equations were used, one accounting for absorbance errors due to some scattered light reaching the detector and another with no correction. Estimates of the average distances between the vesicles and their corresponding Debye lengths were obtained for evaluating the importance of intervesicle electrostatic interactions, which could lead to dependent scattering at higher weight fractions.

Rayleigh and Rayleigh-Debye-Gans light scattering intensities and spetroturbidimetry of dispersions of unilamellar vesicles and multilamellar liposomes.

HYPOTHESIS: Since the volume fraction of the surfactant bilayer(s), of thickness db, in a vesicle and liposome is smaller than one, the dependences of the Rayleigh (R) scattering intensity and turbidity on the particle radius a are weaker than those for a homogeneous sphere, which are proportional to a3. The dependences of the Rayleigh-Debye-Gans (RDG) scattering intensity and turbidity on a are also weaker. Work done: The dependences of the effective relative refractive index on a, db, and dw (water layer thickness) were derived. The specific Rayleigh ratio [Formula: see text] and the specific turbidity τ** for single and independent scattering were derived analytically for R and RDG scattering. Spectroturbidimetry data at 25°C for a cationic double-chain surfactant, didodecyldimethylammonium bromide (DDAB) were compared to the turbidity predictions. FINDINGS: For R scattering, [Formula: see text] and τ** are proportional to a2db for vesicles, and to a3dbdw+db for liposomes. For RDG and particle radii 20-1000 nm, τ** is proportional to an, where n is 2 to 0.4 for vesicles and 2 to 1.1 for liposomes. Turbidity data for DDAB vesicles are consistent with the RDG predictions, which are also used to estimate the vesicles' sizes. RDG applies to liposomes < 800 nm and to much larger sizes for vesicles.

Accurate Determination of the Equilibrium Surface Tension Values with Area Perturbation Tests.

We demonstrate two robust protocols for determining the equilibrium surface tension (EST) values with area perturbation tests. The EST values should be indirectly determined from the dynamic surface tension (DST) values when surface tension (ST) values are at steady-state and stable against perturbations. The emerging bubble method (EBM) and the spinning bubble method (SBM) were chosen, because, with these methods, it is simple to introduce area perturbations while continuing dynamic tension measurements. Abrupt expansion or compression of an air bubble was used as a source of area perturbation for the EBM. For the SBM, changes in the rotation frequency of the sample solution were used to produce area perturbations. A Triton X-100 aqueous solution of a fixed concentration above its critical micelle concentration (CMC) was used as a model surfactant solution. The determined EST value of the model air/water interface from the EBM was 31.5 ± 0.1 mN·m-1 and that from the SBM was 30.8 ± 0.2 mN·m-1. The two protocols described in the article provide robust criteria for establishing the EST values.

Effects of Light Dispersed Particles on the Stability of Dense Suspended Particles Against Sedimentation.

A novel method in which vesicular dispersions of the double-chain cationic surfactant DDAB (didodecyldimethylammonium bromide) stabilize suspensions of high density titania particles was recently presented (Yang, Y.-J; Corti, D.S.; Franses, E. I. Langmuir 2015, 31, 8802-8808). At high enough DDAB concentration, the vesicles form a close-packed structure, providing strong resistance to the sedimentation of the titania particles, while the dispersions remain highly shear-thinning with moderate limiting viscosities. Here, to elucidate the key factors of the mechanism by which vesicles or other nonsettling particles stabilize high density particles against sedimentation, we use Brownian dynamics simulations (BDS) to examine the sedimentation behavior of mixtures of "dense particles" that settle rapidly on their own and "light particles" that represent nonsettling "rigid vesicles". BDS confirm that for large enough values of the volume fraction ϕ2 of the light particles, the dense particles should remain suspended. The rheological behavior of the mixtures is also computed with BDS. The observed shear-thinning behavior of the light particle dispersion suggests that the suspensions of the dense particles are still flowable at high shear stresses. Furthermore, the local viscosity of light particles around the dense particles significantly increases with increasing ϕ2, particularly when the same gravitational force applied in the BDS is exerted on a dense particle. The arrangement of light particles around the moving dense particles is an important factor in determining the stability of the dense particles against sedimentation. The BDS results indicate that dispersions of nonsettling particles provide a general method for the stabilization against sedimentation of high density particles.

A New "Quasi-Dynamic" Method for Determining the Hamaker Constant of Solids Using an Atomic Force Microscope.

In order to minimize the effects of surface roughness and deformation, a new method for estimating the Hamaker constant, A, of solids using the approach-to-contact regime of an atomic force microscope (AFM) is presented. First, a previous "jump-into-contact" quasi-static method for determining A from AFM measurements is analyzed and then extended to include various AFM tip-surface force models of interest. Then, to test the efficacy of the "jump-into-contact" method, a dynamic model of the AFM tip motion is developed. For finite AFM cantilever-surface approach speeds, a true "jump" point, or limit of stability, is found not to appear, and the quasi-static model fails to represent the dynamic tip behavior at close tip-surface separations. Hence, a new "quasi-dynamic" method for estimating A is proposed that uses the dynamically well-defined deflection at which the tip and surface first come into contact, dc, instead of the dynamically ill-defined "jump" point. With the new method, an apparent Hamaker constant, Aapp, is calculated from dc and a corresponding quasi-static-based equation. Since Aapp depends on the cantilever's approach speed, vc, and the AFM's sampling resolution, δ, a double extrapolation procedure is used to determine Aapp in the quasi-static (vc → 0) and continuous sampling (δ → 0) limits, thereby recovering the "true" value of A. The accuracy of the new method is validated using simulated AFM data. To enable the experimental implementation of this method, a new dimensionless parameter τ is introduced to guide cantilever selection and the AFM operating conditions. The value of τ quantifies how close a given cantilever is to its quasi-static limit for a chosen cantilever-surface approach speed. For sufficiently small values of τ (i.e., a cantilever that effectively behaves "quasi-statically"), simulated data indicate that Aapp will be within ∼3% or less of the inputted value of the Hamaker constant. This implies that Hamaker constants can be reliably estimated using a single measurement taken with an appropriately chosen cantilever and a slow, yet practical, approach speed (with no extrapolation required). This result is confirmed by the very good agreement found between the experimental AFM results obtained using this new method and previously reported predictions of A for amorphous silica, polystyrene, and α-Al2O3 substrates obtained using the Lifshitz method.

Non-ideal diffusion effects, short-range ordering, and unsteady-state effects strongly influence Brownian aggregation rates in concentrated dispersions of interacting spheres.

Brownian aggregation rates are determined for concentrated dispersions of interacting particles with Brownian dynamics (BD) simulations and various theoretical models. Using simulation results as benchmarks, the predictions of the classical Fuchs-Smoluchowski (FS) model are shown to be quite inaccurate for concentrated dispersions. A new aggregation model is presented which provides significantly improved predictions. This model is developed on the basis of the fundamental measure theory (FMT) which is a rigorous "liquid-state" dynamic density-functional theory (DDFT) approach. It provides a major improvement of the FS model by considering short-range ordering, non-ideal diffusion, and unsteady-state effects. These were recently shown by the authors to play important roles in Brownian aggregation of hard spheres at high concentrations. Two types of interparticle interaction potentials are examined, the purely attractive van der Waals potential and the DLVO potential which includes van der Waals attraction and electrostatic double layer repulsion. For dispersions of particles with purely attractive interactions, the FS model underpredicts the aggregation rates by up to 1000 fold. In the presence of strong interparticle repulsive forces, its predictions are in fair agreement with the BD simulation results for dilute systems with particle volume fractions ϕ < < 0.1. In contrast, the predictions of the new FM-DDFT based model compare favorably with the BD simulation results, in both cases, up to ϕ = 0.3. A new quantitative measure for colloidal dispersion stability, different from the classical FS stability ratio, is proposed on the basis of aggregation half-times. Hence, a better mechanistic understanding of Brownian aggregation is obtained for concentrated dispersions of particles with either attractive or repulsive interactions, or both.

Use of Close-Packed Vesicular Dispersions to Stabilize Colloidal Particle Dispersions against Sedimentation.

For many applications of colloidal dispersions, the particles must be suspended for a long time. This is often accomplished by preventing agglomeration, which generates aggregates of increasing size. Nevertheless, many colloidal dispersions of dense particles may settle even without agglomeration. Preventing sedimentation without significantly increasing the bulk dispersion viscosity is difficult and has received little attention in the literature. However, settling can be drastically reduced through the novel use of close-packed vesicular dispersions at high enough concentrations, which are non-Newtonian shear-thinning fluids. Such dispersions have much higher viscosities at the low shear stresses "felt" by sedimenting colloidal particles than at the high shear stresses relevant to bulk dispersion flow. In a practical example, dense TiO2 nanoparticles which normally would settle rapidly can remain suspended for at least 6 months without any observable sedimentation when they are introduced into a close-packed vesicular dispersion, while the dispersion retains its flowability. Cryo-TEM images reveal that the vesicles in these dispersions are tightly close-packed. Dynamic light scattering and electrophoretic mobility data also confirm that the vesicles in such dispersions have very low mobilities.

Effect of sodium dodecylsulfate monomers and micelles on the stability of aqueous dispersions of titanium dioxide pigment nanoparticles against agglomeration and sedimentation.

HYPOTHESIS: As more sodium dodecylsulfate (SDS) monomers adsorb at the water/titanium dioxide (TiO2) nanoparticles interface, the particles become more stable against agglomeration and sediment more slowly. SDS micelles are not expected to adsorb on the particles and affect the stability against agglomeration or sedimentation. Since micelles are smaller than the 300 nm TiO2 nanoparticles studied, they may introduce depletion forces which may affect the dispersion stability. EXPERIMENTS AND MODELS: Sedimentation times were measured in water and in 100 mM NaCl for SDS concentrations from 0.1 to 200 mM. Adsorption densities of SDS and zeta potentials of particles were measured. Dynamic light scattering was used to measure average diameters of particles or particle agglomerates. Modeling of sedimentation/diffusion was done to predict sedimentation times of particles. Modeling of agglomeration rates was done to help predict sedimentation rates of clusters. FINDINGS: At SDS concentrations close to or above the cmc, up to 60 mM in water or 115 mM in 100 mM NaCl, the nanoparticles sediment most slowly without any agglomeration. At higher micelle concentration, SDS micelle depletion forces are very strong, causing fast flocculation, without coagulation. Then sedimentation occurs much faster. The effective micelle depletant size includes about 4 Debye lengths of the charged micelles or particles.

Insights into chromatographic enantiomeric separation of allenes on cellulose carbamate stationary phase.

Allenes are cumulenes with three contiguous carbons linked together through double bonds. 1,3-disubstituted allenes are not superimposable on their mirror image; as a consequence they are chiral. Chiral allenes are increasingly important in organic synthesis due to their interesting reactivity. Because of their applications in the field of asymmetric catalysis and in the pharmaceutical industry their optical purity is always a parameter which needs to be determined. In this article, we report the enantiomeric separation of hexa-3,4-diene-3-ylbenzene, an aromatic allene, on a cellulose carbamate (Chiralcel OD-3) stationary phase, using heptane as the mobile phase. Spectroscopic studies using infrared (IR) and vibrational circular dichroism revealed that, in the presence of heptane, the stationary phase undergoes a conformational change due to intermolecular H-bonding between the CO and NH of the neighboring polymer chains. Van't Hoff plots for the retention factor, k, showed that the retention of the two enantiomers is dominated by the enthalpy, while the plot for the selectivity, α, is entropy driven. This suggests that the enantioselectivity is a result of inclusion of the enantiomers in the cavities of the chrial stationary phase. VCD spectra, along with density functional theory calculation (DFT) of the interaction between each enantiomer and the chiral stationary phase, supported the chromatographic elution order findings.

Nonideal diffusion effects and short-range ordering lead to higher aggregation rates in concentrated hard-sphere dispersions.

Brownian aggregation in concentrated hard-sphere dispersions is studied using models and Brownian dynamics (BD) simulations. Two new theoretical models are presented and compared to several existing approaches and BD simulation results, which serve as benchmarks. The first new model is an improvement over an existing local density approximation (LDA)-based model. The other is based on the more rigorous Fundamental measure theory (FMT) applied to the "liquid-state" dynamic density-functional theory (DDFT). Both models provide significant improvements over the classical Smoluchowski model. The predictions of the new FM-DDFT-based model for aggregation kinetics are in excellent agreement with BD simulation results for dispersions with initial particle volume fractions, ϕ, up to 0.35 (close to the hard-sphere freezing transition at ϕ = 0.494). In contrast to previous approaches, the nonideal particle diffusion effects and the initial and time-dependent short-range ordering in concentrated dispersions due to entropic packing effects are explicitly considered here, in addition to the unsteady-state effects. The greater accuracy of the FM-DDFT-based model compared to that of the LDA-based models indicates that nonlocal contributions to particle diffusion (only accounted for in the former) play important roles in aggregation. At high concentrations, the FM-DDFT-based model predicts aggregation half-times and gelation times that are up to 2 orders of magnitude shorter than those of the Smoluchowski model. Moreover, the FM-DDFT-based model predicts asymmetric cluster-cluster aggregation rate constants, at least for short times. Overall, a rigorous mechanistic understanding of the enhancement of aggregation kinetics in concentrated dispersions is provided.

Effect of alcohol aggregation on the retention factors of chiral solutes with an amylose-based sorbent: modeling and implications for the adsorption mechanism.

Various displacement models in the literature have been widely used for understanding the adsorption mechanisms of solutes in various chromatography systems. The models were used for describing the often-observed linear plots of the logarithms of the retention factor versus the logarithms of the polar modifier concentration CI(0). The slopes of such a plot was inferred to be equal to the number of the displaced modifier molecules upon adsorption of one solute molecule, and were generally found to be greater than 1. In this study, the retention factors of four structurally related chiral solutes, ethyl lactate (EL), methyl mandelate (MM), benzoin (B), and pantolactone (PL), were measured for the amylose tris[(S)-α-methylbenzylcarbamate] sorbent, or AS, as a function of the concentration of isopropanol (IPA) in n-hexane. With increasing IPA concentration CI(0), the slopes increase from less than 1, at a concentration range from 0.13 to 1.3M, to slightly more than 1 at higher concentrations. Such slopes cannot be explained by the conventional retention models. It was found previously for monovalent solutes that such slopes can only be explained when the aggregation of the mobile phase modifier, isopropyl alcohol, was accounted for. A new retention model is presented here, accounting for alcohol aggregation, multivalent solute adsorption, multivalent solute-alcohol complexation, alcohol adsorption, and solute intra hydrogen-bonding, which occur in these four solutes. The slope is found to be controlled by three key dimensionless groups, the fraction of the sorbent binding sites covered by IPA, the fraction of the solute molecules in complex form, and the fraction of the IPA molecules in aggregate form. The limiting slope at a very high IPA concentration is equal to the value of (x+y)/n, where x is the number of the solute-sorbent binding sites and y is the number of the alcohol molecules in the solute-alcohol complex, and n is the alcohol aggregation number. The model was tested with the HPLC data of two sets of chiral solutes, one set of new data presented here and of one set of literature data by Gyimesi-Forrás et al. (2009), for which there is no known intramolecular H-bonding. For the first set of solutes, the values of the equilibrium constants for intramolecular hydrogen bonding were calculated from our previous IR data. The value of the parameter y was fixed on the basis of the number of the solute functional groups, IR data, and the results of DFT and MD simulations. The retention factors in pure hexane (k0) were found experimentally for EL, MM, and B; for PL they were estimated from the data. Then the values of x and the complexation equilibrium constants were estimated. The model fits fairly well our new data, and less well the more-limited literature data, for which the k0 values were unavailable, and the retention factors were obtained over a narrow range of IPA concentrations. For EL and PL, results of infrared spectroscopy, density functional theory, and molecular dynamics simulations indicated strong solute-IPA complexation, and multiple solute-sorbent binding sites, which are consistent with the fitting results. Hence, the new model has been shown to be more reliable than the previous models for estimating the numbers of the potential binding sites of multivalent solutes.

Chiral recognition mechanism of acyloin-containing chiral solutes by amylose tris[(S)-α-methylbenzylcarbamate].

Although polysaccharide sorbents have been widely used for chiral separations, the recognition mechanisms have not been fully elucidated. In this study, we focus on one important commercial sorbent, amylose tris[(S)-α-methylbenzylcarbamate] (AS) sorbent. Four solutes containing acyloin, O═C-C-OH, which has a hydroxyl group in the α-position of a carbonyl group, were studied: ethyl lactate (EL), methyl mandelate (MM), benzoin (B), and pantolactone (PL). The observed retention factors (kR and kS) and enantioselectivities (α = kR/ kS) were determined in n-hexane and in hexane-isopropanol (IPA) solutions. Infrared (IR) spectroscopy and density functional theory (DFT) simulations of the interactions of these solutes with the side chains of the polymer led to a general hypothesis for the chiral recognition mechanism for these solutes: A strong H-bond forms as the primary (or "leading") nonenantioselective interaction (or "anchor" point) between the solute OH group of each enantiomer and the sorbent C═O group. A weaker H-bond forms preferably for the R enantiomer between the solute C═O groups and the sorbent NH groups. The S enantiomer is prevented from forming such a bond for steric restrictions. A third interaction might involve the O groups of the phenyl groups of the solutes. IR spectroscopy shows evidence of an intramolecular H-bond for all four solutes. The retention factors were found to increase with increasing strength of the intermolecular H-bond and with decreasing strength of the intramolecular H-bond. The enantioselectivities were found to correlate with the molecular rigidity or flexibility, as determined from the distribution of the torsion angles of the acyloin group. The enantioselectivity was higher for the more rigid molecules. Simulations of left-handed AS with 200 n-hexane molecules indicated no effect of hexane on the H-bonds in AS. Monte Carlo (MC) and molecular dynamics (MD) "docking" simulations of AS with these solutes revealed certain chiral cavities that can lead to chiral discrimination. The results support the proposed mechanism.

Retention models and interaction mechanisms of acetone and other carbonyl-containing molecules with amylose tris[(S)-α-methylbenzylcarbamate] sorbent.

The stoichiometric displacement models developed in the literature have been widely used for understanding the adsorption mechanisms of solutes in various chromatography systems. The models were used to explain the linear plots of the logarithms of the solute retention factor versus the molar concentration of a competitive modifier in an inert solvent. The slope of the linear plot was inferred to be the total number of modifier molecules displaced from the sorbent and from the solute-modifier complex upon adsorption of a solute molecule. The slopes reported in the literature were generally greater than 1. In this study, we determined the retention factors of five monovalent solutes, acetone, cyclo hexanone, benzaldehyde, phenylacetaldehyde, and hydrocinnamaldehyde, on a derivatized polysaccharide sorbent, amylose tris[(S)-α-methylbenzylcarbamate], or AS, as a function of the concentration of a polar modifier isopropanol (IPA) in n-hexane (an inert solvent). Each solute has one CO functional group, which can form an H-bond with a sorbent NH group and the OH group of IPA. The slopes, from 0.25 to 0.45, of the log-log plots are less than 1, which cannot be explained by the literature displacement models. The results of Infrared Spectroscopy and Density Functional Theory simulations show clear evidence of acetone-IPA complexation and IPA aggregation with average aggregation number n=3. A new thermodynamic retention model is developed to take into account IPA aggregation, IPA-solute complexation, and competitive adsorption. Dimensionless group analysis indicates that aggregation of IPA can lead to slopes B below 1, even at high IPA concentrations. The model parameters (IPA aggregation number and equilibrium constants) are estimated from the retention factors at different IPA concentrations. The retention model and the parameters are further validated with dynamic chromatography simulations. The results show that the aggregation leads to a significant reduction in the IPA monomer concentration, which affects the IPA-sorbent binding and the IPA-solute complexation. As a result, the slope of the log-log plot at a high IPA concentration approaches 1/n without complexation, or 2/n with complexation. The variations of B between the five achiral solutes can be due to different strengths of solute-IPA complexation. Hence, the complexation and aggregation of the polar modifier in the mobile phase must be accounted for in the retention models used in the interpretation of the retention factors and the adsorption mechanisms.

New models and predictions for Brownian coagulation of non-interacting spheres.

The classical steady-state Smoluchowski model for Brownian coagulation is evaluated using Brownian Dynamics Simulations (BDS) as a benchmark. The predictions of this approach compare favorably with the results of BDS only in the dilute limit, that is, for volume fractions of φ≤5×10(-4). From the solution of the more general unsteady-state diffusion equation, a new model for coagulation is developed. The resulting coagulation rate constant is time-dependent and approaches the steady-state limit only at large times. Moreover, in contrast to the Smoluchowski model, this rate constant depends on the particle size, with the transient effects becoming more significant at larger sizes. The predictions of the unsteady-state model agree well with the BDS results up to volume fractions of about φ=0.1, at which the aggregation half-time predicted by the Smoluchowski model is five times that of the BDS. A new procedure to extract the aggregation rate constant from simulation results based on this model is presented. The choice of the rate constant kernel used in the population balance equations for complete aggregation is also evaluated. Extension of the new model to a variable rate constant kernel leads to increased accuracy of the predictions, especially for φ≤5×10(-3). This size-dependence of the rate constant kernel affects particularly the predictions for initially polydisperse sphere systems. In addition, the model is extended to account in a novel way for both short-range viscous two-particle interactions and long-range many-particle Hydrodynamic Interactions (HI). Predictions including HI agree best with the BDS results. The new models presented here offer accurate and computationally less-intensive predictions of the coagulation dynamics while also accounting for hydrodynamic coupling.

Infrared spectroscopy and molecular simulations of a polymeric sorbent and its enantioselective interactions with benzoin enantiomers.

Retention factors, k(R) and k(S), and enantioselectivities, S ≡ k(R)/k(S), of amylose tris[(S)-α-methylbenzylcarbamate] (AS) sorbent for benzoin (B) enantiomers were measured for various isopropyl alcohol (IPA)/n-hexane compositions of the high-performance liquid chromatography (HPLC) mobile phase. Novel data for pure n-hexane show that k(R) = 106, k(S) = 49.6, and S = 2.13. With some IPA from 0.5 to 10 vol %, with S = 1.8-1.4, the retention factors were smaller. Infrared spectra showed evidence of substantial hydrogen bonding (H-bonding) interactions in the pure polymer phase and additional H-bonding interactions between AS and benzoin. Density functional theory (DFT) was used to model the chain-chain and chain-benzoin H-bonding and other interactions. DFT was also used to predict fairly well the IR wavenumber shifts caused by the H-bonds. DFT simulations of IR bands of NH and C═O allowed for the first time the predictions of relative intensities and relative populations of H-bonding strengths. Molecular dynamics (MD) simulations were used to model a single 12-mer polymer chain. MD simulations predicted the existence of various potentially enantioselective cavities, two of which are sufficiently large to accommodate a benzoin molecule. Then "docking" studies of benzoin in AS with MD, Monte Carlo (MC), and MC/MD simulations were done to probe the AS-B interactions. The observed enantioselectivities are predicted to be primarily due to two H-bonds, of the kind AS CO···HO (R)-benzoin and AS NH···OC (R)-benzoin, and two π-π (phenyl-phenyl) interactions for (R)-benzoin and one H-bond, of type AS CO···HO (S)-benzoin, and one π-π interaction for (S)-benzoin. The MC/MD predictions are consistent with the HPLC and IR results.

Effect of Triton X-100 on the stability of aqueous dispersions of copper phthalocyanine pigment nanoparticles.

The effect of Triton X-100 on the colloidal dispersion stability of CuPc-U (unsulfonated and hydrophobic) and CuPc-S (surface sulfonated and hydrophilic) particles in aqueous solutions (water and NaNO(3)) was investigated at 25 °C. Its adsorption density was determined from surfactant concentrations analyzed by an HPLC method with a UV detector. The experimental dispersion stability ratios of the particles were determined from dynamic light scattering (DLS) data, with the Rayleigh-Debye-Gans (RDG) light scattering theory. The adsorption densities of Triton X-100 on both the CuPc-U and CuPc-S increase with increasing concentration of surfactant up to the critical micelle concentration (cmc), and then reach a plateau. The maximum adsorption density Γ(m) is higher for the CuPc-U (d(h)=160 nm) than that for the CuPc-S (d(h)=90 nm). The hydrophobic chains are inferred to be adsorbed onto the surfaces, and the hydrophilic ethylene oxide chains are in a coil conformation. The W(app)-values for the CuPc-U dispersions are affected mainly by the surfactant fractional surface coverage θ. Adding NaNO(3) has no significant effect on the dispersion stability. The stabilization mechanism for the CuPc-U is inferred to be primarily steric, as expected. The stability ratios for the CuPc-S in solutions with NaNO(3) are higher than those for CuPc-U, and decrease with increasing concentration of NaNO(3), indicating that the stabilization is affected by the screening of electrostatic repulsive forces. The zeta potential is not a good predictor of the electrostatic stabilization, pointing to the need for new and improved theories.