Molecular dynamics simulations of interaction between sub-bituminous coal and water

The high moisture content of sub-bituminous coal is associated with the interactions between coal and water. Because of complex composition and structure, the graphite surface modified by hydroxyl, carboxyl and carbonyl groups was used to represent the surface model of sub-bituminous coal according to XPS results. Density profiles for oxygen atoms and hydrogen atoms indicate that the coal surface properties affect the structural and dynamic characteristics of the interfacial water molecules. The interfacial water exhibits much more ordering than bulk water. The results of radial distribution functions, mean square displacement and local self-diffusion coefficient for water molecule related to three oxygen moieties confirmed that the water molecules prefer to absorb with carboxylic groups, and adsorption of water molecules at the hydroxy and carbonyl is similar.     

Molecular dynamics simulations and contact angle of surfactant at the coal–water interface

Wettability of nonylphenol ethoxylate with four ethylene oxide groups (NP-4) on a subbituminous coal was carried out. As the concentration of NP-4 gradually increases, the contact angle firstly increases and then decreases with maximum contact angle at about critical micelle concentration (CMC) of NP-4. The monolayer adsorption behaviour of NP-4 on the model surface of Hatcher subbituminous coal was investigated by means of molecular dynamics simulations. The surfactant molecules could be detected at the water–coal interface. The water molecules are repelled and stronger hydrophobicity of the coal is obtained in the presence of NP-4, which are consistent with contact angle results at low concentration. The aggregated structure of the surfactant molecules on the coal surface in terms of head group and tail group density profiles along the perpendicular direction shows that the ethoxylate groups of the surfactant are attached at the solid surfaces. The negative interaction energy between NP-4 and the subbituminous coal surface calculated suggests that adsorption process is spontaneous. The self-diffusion coefficients results indicate that the presence of NP-4 causes higher water mobility meaning improving the hydrophobicity of low-rank coal, which is consistent with the experimental results of contact angle.     

Effects of nonionic surfactants on the solubilization and mineralization of phenanthrene in soil-water systems

The solubilization and mineralization of (14)C-phenanthrene in soil-water systems was examined with several commercially available surface-active agents, viz., an alkyl ethoxylate C(12)E(4); two alkylphenol ethoxylate surfactants: C(8)PE(9.5) and C(9)PE(10.5); two sorbitan ethoxylate surfactants: the sorbitan monolaurate (Tween 20) and the sorbitan monooleate (Tween 80); two pairs of nonionic ethoxylate surfactant mixtures: C(12)E(4)/C(12)E(23) at a 1:1 ratio, and C(12-15)E(3)/C(12-15)E(9) at a 1:3 ratio; and two surfactants possessing relatively high critical micelle concentration (CMC) values and low aggregation numbers: CHAPS and octyglucoside. Surface tension experiments were performed to evaluate surfactant sorption onto soil and the surfactant doses required to attain the CMC in the soil-water systems. Surfactant solubilization of (14)C-phenanthrene commenced with the onset of micellization. The addition of surface-active agents was observed not to be beneficial to the microbial mineralization of phenanthrene in the soil-water systems and, for supra-CMC surfactant doses, phenanthrene mineralization was completely inhibited for all the surfactants tested. A comparison of solubilization, surface tension, and mineralization data confirms that the inhibitory effect on microbial degradation of phenanthrene is related to the CMC of the surfactant in the presence of soil. Additional tests demonstrated the recovery of mineralization upon dilution of surfactant concentration to sub-CMC levels, and a relatively high exit rate for phenanthrene from micelles. These tests suggest that the inhibitory effect is probably related to a reversible physiological surfactant micelle-bacteria interaction, possibly through partial complexing or release of membrane material with disrupting membrane lamellar structure. This study indicates that nonionic surfactant solubilization of sorbed hydrophobic organic compounds from soil may not be beneficial for the concomitant enhancement of soil bioremediation. Additional work is needed to address physicochemical processes for bioavailability enhancement, and effects of solubilizing agents on microorganisms for remediation and treatment of hydrophobic organic compounds and nonaqueous phase liquids. (c) 1992 John Wiley & Sons Inc.     

Exploring the effect of oxygen-containing functional groups on the water-holding capacity of lignite

Graphene oxide with different degrees of oxidation was prepared and selected as a model compound of lignite to study quantitatively, using both experiment and theoretical calculation methods, the effect on water-holding capacity of oxygen-containing functional groups. The experimental results showed that graphite can be oxidized, and forms epoxy groups most easily, followed by hydroxyl and carboxyl groups. The prepared graphene oxide forms a membrane-state as a single layer structure, with an irregular surface. The water-holding capacity of lignite increased with the content of oxygen-containing functional groups. The influence on the configuration of water molecule clusters and binding energy of water molecules of different oxygen-containing functional groups was calculated by density functional theory. The calculation results indicated that the configuration of water molecule clusters was totally changed by oxygen-containing functional groups. The order of binding energy produced by oxygen-containing functional groups and water molecules was as follows: carboxyl > edge phenol hydroxyl >epoxy group. Finally, it can be concluded that the potential to form more hydrogen bonds is the key factor influencing the interaction energy between model compounds and water molecules.     

Degradation of polycyclic aromatic hydrocarbons in the presence of synthetic surfactants.

The biodegradation of polycyclic aromatic hydrocarbons (PAH) often is limited by low water solubility and dissolution rate. Nonionic surfactants and sodium dodecyl sulfate increased the concentration of PAH in the water phase because of solubilization. The degradation of PAH was inhibited by sodium dodecyl sulfate because this surfactant was preferred as a growth substrate. Growth of mixed cultures with phenanthrene and fluoranthene solubilized by a nonionic surfactant prior to inoculation was exponential, indicating a high bioavailability of the solubilized hydrocarbons. Nonionic surfactants of the alkylethoxylate type and the alkylphenolethoxylate type with an average ethoxylate chain length of 9 to 12 monomers were toxic to a PAH-degrading Mycobacterium sp. and to several PAH-degrading mixed cultures. Toxicity of the surfactants decreased with increasing hydrophilicity, i.e., with increasing ethoxylate chain length. Nontoxic surfactants enhanced the degradation of fluorene, phenanthrene, anthracene, fluoranthene, and pyrene.     

The mechanism of restructuring of surfactant monolayer on mica surface in aqueous solution: molecular dynamics simulation

Molecular dynamics simulation was employed to investigate the restructuring process of CTAB monolayer at mica/water interface. The reversing process of CTAB monolayer was exploited by diffusion of water molecules, reversing of CTAB molecules with time evolution and restructuring of the surfactant monolayer. The results showed that bromide ions around surfactant head groups diffused into bulk water readily due to the electrostatic repulsion caused by negatively charged mica surface. Meanwhile, because of the electrostatic attraction between water molecules and mica surface, part of water molecules can penetrate the surfactant monolayer to form water channel which bridges bulk water and mica surface. The monolayer structure was disturbed by diffusion of bromide ions and formation of water channel. Few of the head groups of surfactants tended to reverse and enter into aqueous solution. The number of reversed surfactant molecules increased with time evolution. The monolayer restructured into bilayer structure gradually. Finally, a cylindrical aggregate was obtained.     

Protein extration from an aqueous phase into a reversed micellar phase: Effect of water content and reversed micellar composition

In the system composed of the cationic surfactant TOMAC (10 mM), the nonionic (co)surfactant Rewopal HV5 (2 mM), and octanol (0.1% v/v) in isooctane, reversed micelles are formed upon contact with an aqueous phase containing 50 mM ethylene diamine. alpha-Amylase can be transferred from the aqueous phase into reversed micelles in the pH range 9.5 to 10.5 and re-extracted into a second aqueous phase of different composition. The size of the reversed micelles (as reflected in the water content of the organic phase) can be varied by changes in percentage of octanol, type of counterion in the aqueous phase, or in the number of ethoxylate head groups of the nonionic surfactant. An increase in size results in transfer at lower pH values. Experiments in which the charge density in the reversed micellar interface was changed by incorporation of charged derivatives of the nonionic surfactant, without influencing the water content, revealed that an increased charge density facilitated transfer, resulting in a broader transfer profile. Replacement of TOMAC by other quaternary ammonium surfactants differing in number and length of tails revealed that, of the 14 surfactants tested, only 2 gave appreciable amounts of transfer. The amount of transfer is related to the dynamics of phase separation of the surfactants: those giving a poor phase separation inactivate the enzyme. This inactivation is caused by electrostatic interactions between the charged surfactant head groups and charged groups on the enzyme. Electrostatic interactions are the first step of transfer, and can result in either incorporation in a reversed micelle, or, if reversed micelle formation is slow, in enzyme inactivation. (c) 1995 John Wiley & Sons, Inc.     

Antibody adsorption on the surface of water studied by neutron reflection

Surface and interfacial adsorption of antibody molecules could cause structural unfolding and desorbed molecules could trigger solution aggregation, resulting in the compromise of physical stability. Although antibody adsorption is important and its relevance to many mechanistic processes has been proposed, few techniques can offer direct structural information about antibody adsorption under different conditions. The main aim of this study was to demonstrate the power of neutron reflection to unravel the amount and structural conformation of the adsorbed antibody layers at the air/water interface with and without surfactant, using a monoclonal antibody ‘COE-3′ as the model. By selecting isotopic contrasts from different ratios of H₂O and D₂O, the adsorbed amount, thickness and extent of the immersion of the antibody layer could be determined unambiguously. Upon mixing with the commonly-used non-ionic surfactant Polysorbate 80 (Tween 80), the surfactant in the mixed layer could be distinguished from antibody by using both hydrogenated and deuterated surfactants. Neutron reflection measurements from the co-adsorbed layers in null reflecting water revealed that, although the surfactant started to remove antibody from the surface at 1/100 critical micelle concentration (CMC) of the surfactant, complete removal was not achieved until above 1/10 CMC. The neutron study also revealed that antibody molecules retained their globular structure when either adsorbed by themselves or co-adsorbed with the surfactant under the conditions studied.     

Biodegradation of naphthalene in aqueous nonionic surfactant systems.

The principal objective of this study was to quantify the bioavailability of micelle-solubilized naphthalene to naphthalene-degrading microorganisms comprising a mixed population isolated from contaminated waste and soils. Two nonionic surfactants were used, an alkylethoxylate, Brij 30 (C12E4), and an alkylphenol ethoxylate, Triton X-100 (C8PE9.5). Batch experiments were used to evaluate the effects of aqueous, micellized nonionic surfactants on the microbial mineralization of naphthalene and salicylic acid, an intermediate compound formed in the pathway of microbial degradation of naphthalene. The extent of solubilization and biodegradation under aerobic conditions was monitored by radiotracer and spectrophotometric techniques. Experimental results showed that surfactant concentrations above the critical micelle concentration were not toxic to the naphthalene-degrading bacteria and that the presence of surfactant micelles did not inhibit mineralization of naphthalene. Naphthalene solubilized by micelles of Brij 30 or Triton X-100 in liquid media was bioavailable and degradable by the mixed culture of bacteria.     

Effect of a commercial alcohol ethoxylate surfactant (C<Subscript>11-15</Subscript>E<Subscript>7</Subscript>) on

Biodegradation of poorly soluble polycyclic aromatic hydrocarbons (PAHs) has been a challenge in bioremediation. In recent years, surfactant-enhanced bioremediation of PAH contaminants has attracted great attention in research. In this study, biodegradation of phenanthrene as a model PAHs solubilized in saline micellar solutions of a biodegradable commercial alcohol ethoxylate nonionic surfactant was investigated. The critical micelle concentration (CMC) of the surfactant and its solubilization capacity for phenanthrene were examined in an artificial saline water medium, and a type of marine bacteria, Neptunomonas naphthovorans, was studied for the biodegradation of phenanthrene solubilized in the surfactant micellar solutions of the saline medium. It is found that the solubility of phenanthrene in the surfactant micellar solutions increased linearly with the surfactant concentrations, but, at a fixed phenanthrene concentration, the biodegradability of phenanthrene in the micellar solutions decreased with the increase of the surfactant concentrations. This was attributed to the reduced bioavailability of phenanthrene, due to its increased solubilization extent in the micellar phase and possibly lowered mass transfer rate from the micellar phase into the aqueous phase or into the bacterial cells. In addition, an inhibitory effect of the surfactant on the bacterial growth at high surfactant concentrations may also play a role. It is concluded that the surfactant largely enhanced the solubilization of phenanthrene in the saline water medium, but excess existence of the surfactant in the medium should be minimized or avoided for the biodegradation of phenanthrene by Neptunomonas naphthovorans.     

Structure of the dysprosium-glycocholate complex in submicellar aqueous solution: paramagnetic mapping by proton nuclear magnetic resonance spectroscopy. An approximation for the intrinsic "bound" relaxation rates in the case of nondilute paramagnetic systems

A paramagnetic NMR study of the structure of the calcium-glycocholate complex in submicellar solution, utilizing dysprosium as an isomorphous lanthanide replacement of calcium, is presented. The dysprosium-induced relaxation rate (1/T1) enhancements of certain glycocholate protons have been used to estimate internuclear distances between these protons and the metal ion. An approximation to calculate the intrinsic relaxation rate (1/T1) enhancements for a nondilute paramagnetic solution is given in the Appendix. From these data, and analysis based on conformation averaging and minimum energy conformations, a molecular model of the dysprosium-glycocholate complex in submicellar aqueous solution has been constructed. In this model the metal ion has a unidentate, first-sphere interaction with the proximal oxygen atom of the glycine carboxyl. The metal ion has second-sphere interactions with the peptide bond carbonyl oxygen (3.6 A) and the distal carboxyl oxygen (4.4 A). The metal ion to hydroxyl oxygen distances (8.4-12.4 A) are not compatible with any metal ion to hydroxyl coordination. The side chain appears to exist in one predominant conformation. All six oxygen atoms of glycocholate, the peptide bond carbonyl, the carboxyl group, and the hydroxyl groups are on the alpha face of the bile salt molecule. On the basis of these features we conclude that in the submicellar state the solution structure of the dysprosium-glycocholate complex displays a metal ion enhanced segregation of polar versus nonpolar groups to the two separate faces of the molecule, which may result in a facilitated hydrophobic interaction of different complex units.     

Screening of the structural,topological, and electronic properties of the functionalized Graphene nanosheets as potential Tegafur anticancer drug carriers using DFT method

In the present work, we apply comprehensive theoretical calculations in order to study Tegafur drug adsorption on the nanostructured functionalized Graphene with hydroxyl, epoxide, carbonyl, and carboxyl groups in the water environment. The physical nature of Tegafur adsorption offers advantages in terms of easy desorption of anticancer molecule with no structural or electronic change of the adsorbed drug. By functionalization of Graphene nanosheet with a carbonyl group, a considerable increase on the binding energy between Tegafur drug and the nanosheet is noted. Diminish in energy gap with the adsorption of Tegafur drug on the functionalized nanosheets shows that the reactivity of functionalized complexes increases upon loading of the drug molecule. Besides, the adsorption process yields an increase of the polarity which causes the possibility of the solubility and dispersion of the considered complexes enhances. This result is indicative the suitability of the nanomaterials toward Tegafur drug delivery within the biological environments. The high solvation energy of Tegafur anticancer drug adsorbed functionalized Graphene models enforced their applicability as nanocarriers in the living system. These results are extremely relevant that the chemical modi?cation of Graphene nanosheet using covalent functionalization scheme is an effectual approach for loading and delivery of Tegafur drug molecule within biological systems.     

Disintegration by surfactants of egg yolk phosphatidylcholine vesicles stabilized with carboxymethylchitin

Disintegration by surfactants of egg yolk phosphatidylcholine vesicles stabilized with carboxymethylchitin was investigated by measuring the amount released of a marker dye from the vesicles. In solutions of pH around 7, anionic and nonionic surfactants caused vesicle disintegration at very low concentrations, while cationic surfactants produced a breakdown of the vesicles at rather high concentrations. Increase in the alkyl chain-length of surfactant molecules brought about decrease in the surfactant concentration at which vesicle disintegration starts. As the length of the polyoxyethylene chain in nonionic surfactant molecules increased, the tendency of vesicle disintegration to occur decreased. Both anionic and cationic surfactants gave clear solutions above their critical micelle concentrations when they acted on the phospholipid vesicles, whereas nonionic surfactants left ghost cell-like debris consisting of carboxymethylchitin molecules in their micellar solutions. The effect of pH on vesicle disintegration was notable for ionic surfactants but not for nonionic surfactants. Thus, anionic surfactants increased the degree of disintegration as pH increased, while cationic surfactants produced an identical vesicle disintegration curve below pH 8 above which the curve started to shift toward the lower concentration region of the agents. These findings were explained in terms of surfactant penetration into phospholipid bilayers and solubilization of phospholipid molecules by surfactant micelles.     

Coaggregate of amphiphilic zinc chlorins with synthetic surfactants in an aqueous medium to an artificial supramolecular light-harvesting system

Aqueous assemblies of zinc chlorins possessing a nonionic (oligo)oxyethylene, a cationic quaternary ammonium or an anionic sulfonate group were prepared in the presence of a synthetic surfactant. The nonionic zinc chlorin formed aggregates when admixed with a nonionic surfactant such as Triton X-100 to give a highly ordered oligomeric J-aggregate similarly as natural bacteriochlorophyll-c or d does in a chlorosome. In addition, the coassemblies of the cationic zinc chlorin with an anionic surfactant and of the anionic zinc chlorin with a cationic surfactant gave large oligomers of these chlorophyllous pigments. The structures of hydrophilic groups in both the zinc chlorin and surfactant molecules controlled their aqueous coassemblies.     

Surfactant structure effects in protein separations using nonionic microemulsions

In this article, the extraction of cytochrome c utilizing various nonionic surfactant microemulsions has been tested to determine the effect of surfactant structure on protein partitioning. Surfactants tested include a linear alcohol ethoxylate (Neodol 91-2.5), two alkyl phenol ethoxylates (lgepal CO-520, Trycol 6985), and a series of alkyl sorbitan esters that are either ethoxylated (Tweens) or un-ethoxylated (Spans). Initial attempts to extract hemoglobin into Neodol 91-2.5 Winsor II microemulsions (oil-continuous) appeared successful based on heme estimation. Careful analysis showed that the hemoglobin had dissociated prior to extraction and that only the heme was extracted with false positive results. In fact, Neodol 91-2.5 microemulsions were unable to extract a variety of proteins with differing biophysical properties. Among all the other nonionic surfactant microemulsions tested only those made using sorbitan esters extracted significant amounts of cytochrome c. The partition coefficients achieved in this study are more than an order of magnitude higher than that seen previously in the literature for comparable sorbitan systems. However, this partition coefficient is extremely sensitive to ionic strength. At an ionic strength as low as 0.001 M, the partition coefficient is reduced to that seen in previous studies. We have found that protein partitioning in sorbitan ester microemulsions is not a function of water content. In addition, extraction is not a function of either alkyl chain length, or polyethylene oxide molecular weight. Hence, the sorbitan group appears to have an important role in extraction, possibly through a weak electrostatic protein-surfactant interaction. (c) 1995 John Wiley & Sons, Inc.     

Analysis of protein main-chain solvation as a function of secondary structure

We have analysed the hydration of main-chain carbonyl and amide groups in 24 high-resolution well-refined protein structures as a function of the secondary structure in which these polar groups occur. We find that main-chain atoms in beta-sheets are as hydrated as those in alpha-helices, with most interactions involving "free" amide and carbonyl groups that do not participate in secondary structure hydrogen bonds. The distributions of water molecules around these non-bonded carbonyl groups reflect specific steric interactions due to the local secondary structure. Approximately 20% and 4%, respectively of bonded carbonyl and amide groups interact with solvent. These include interactions with carbonyl groups on the exposed faces of alpha-helices that have been correlated previously with bending of the helix. Water molecules interacting with alpha-helices occur mainly at the amino and carbonyl termini of the helices, in which case the solvent sites maintain the hydrogen bonding by bridging between residues i and i-3 or i-4 at the amino terminus and between i and i+3 or i+4 at the carbonyl terminus. We also see a number of solvent-mediated Ncap and Ccap interactions. The water molecules interacting with beta-sheets occur mainly at the edges, in which case they extend the sheet structure, or at the ends of strands, in which case they extend the beta-ladder. In summary, the solvent networks appear to extend the hydrogen-bonding structure of the secondary structures. In beta-turns, which usually occur at the surface of a protein, exposed amide and carbonyl groups are often hydrated, especially close to glycine residues. Occasionally water molecules form a bridge between residues i and i+3 in the turn and this may provide extra stabilization.     

Cholesterol effects on the phosphatidylcholine bilayer polar region: a molecular simulation study

A molecular dynamics (MD) simulation of a fully hydrated, liquid-crystalline dimyristoylphosphatidylcholine (DMPC)-Chol bilayer membrane containing approximately 22 mol% Chol was carried out for 4.3 ns. The bilayer reached thermal equilibrium after 2.3 ns of MD simulation. A 2.0-ns trajectory generated during 2.3-4.3 ns of MD simulation was used for analyses to determine the effects of Chol on the membrane/water interfacial region. In this region, 70% of Chol molecules are linked to DMPC molecules via short-distance interactions, where the Chol hydroxyl group (OH-Chol) is 1) charge paired to methyl groups of the DMPC choline moiety ( approximately 34%), via the hydroxyl oxygen atom (Och); 2) water bridged to carbonyl ( approximately 19%) and nonester phosphate ( approximately 14%) oxygen atoms, via both Och and the hydroxyl hydrogen atom (Hch); and 3) directly hydrogen (H) bonded to carbonyl ( approximately 11%) and nonester phosphate ( approximately 5%) oxygen atoms, via Hch ( approximately 17% of DMPC-Chol links are multiple). DMPC's gamma-chain carbonyl oxygen atom is involved in 44% of water bridges and 51% of direct H bonds formed between DMPC and Chol. On average, a Chol molecule forms 0.9 links with DMPC molecules, while a DMPC molecule forms 2.2 and 0.3 links with DMPC and Chol molecules, respectively. OH-Chol makes hydrogen bonds with 1.1 water molecules, preferentially via Hch. The average number of water molecules H bonded to the DMPC headgroup is increased by 7% in the presence of Chol. These results indicate that inclusion of Chol decreases interlipid links and increases hydration in the polar region of the membrane.     

Aqueous micellar systems in membrane protein crystallization. Partial miscibility of a nonionic surfactant in the presence of salt or polyethylene glycol

The effects of NaCl and polyethylene glycol (PEG) 4000 on the lower consolution boundary (LCB) of a nonionic surfactant (C8E5) were studied and compared. Micellar systems were NaCl and PEG 4000 are present are often used in membrane protein crystallization. While sodium chloride shifts the surfactant LCB to lower temperatures without a significant change in the shape of the boundary, PEG produces a large solubility change strongly depending on the surfactant concentration. The salt effect is explained by a reduced interaction of the micellar oligooxyethylene chains with the water and the PEG effect by an unfavourable configurational interaction between the C8E5 micelles and PEG molecules.     

Behavior of cholesterol and its effect on head group and chain conformations in lipid bilayers: a molecular dynamics study.

Cholesterol molecules were put into a computer-modeled hydrated bilayer of dimyristoyl phosphatidyl choline molecules, and molecular dynamics simulations were run to characterize the effect of this important molecule on membrane structure and dynamics. The effect was judged by observing differences in order parameters, tilt angles, and the fraction of gauche bonds along the hydrocarbon chains between lipids adjacent to cholesterol molecules and comparing them with those further away. It was observed that cholesterol causes an increase in the fraction of trans dihedrals and motional ordering of chains close to the rigid steroid ring system with a decrease in the kink population. The hydrogen-bonding interactions between cholesterol and lipid molecules were determined from radial distribution calculations and showed the cholesterol hydroxyl groups either solvated by water, or forming hydrogen bond contacts with the oxygens of lipid carbonyl and phosphate groups. The dynamics and conformation of the cholesterol molecules were investigated and it was seen that they had a smaller tilt with respect to the bilayer normal than the lipid chains and furthermore that the hydrocarbon tail of the cholesterol was conformationally flexible.     

Investigation on the mechanism of crystallization of soluble protein in the presence of nonionic surfactant

The mechanism of crystallization of soluble, globular protein (lysozyme) in the presence of nonionic surfactant C8E4 (tetraoxyethylene glycol monooctyl ether) was examined using both static and dynamic light scattering. The interprotein interaction was found to be attractive in solution conditions that yielded crystals and repulsive in the noncrystallizing solution conditions. The validity of the second virial coefficient as a criterion for predicting protein crystallization could be established even in the presence of nonionic surfactants. Our experiments indicate that the origin of the change in interactions can be attributed to the adsorption of nonionic surfactant monomers on soluble proteins, which is generally assumed to be the case with only membrane proteins. This adsorption screens the hydrophobic attractive force and enhances the hydration and electrostatic repulsive forces between protein molecules. Thus at low surfactant concentration, the effective protein-protein interaction remains repulsive. Large surfactant concentrations promote protein crystallization, possibly due to the attractive depletion force caused by the intervening free surfactant micelles.