Dissolution rate studies of cholesterol monohydrate in bile acid-lecithin solutions using the rotatingdisk method

Prediction of In Vitro Drug Dissolution into Fed-state Biorelevant Media: Contributions of Solubility Enhancement and Relatively Low Colloid Diffusivity

A model was previously derived to predict in vitro dissolution of drug into surfactant solution and showed good predictability for pharmaceutical surfactants, where surfactant-mediated enhanced drug dissolution was several fold less than enhanced solubility (about 3-fold or less) due to drug-loaded micelles exhibiting slower diffusivity than free drug. The present objective was to quantitatively assess the contributions of biorelevant media-mediated solubility and diffusivity on enhanced drug dissolution in FeSSGF and FeSSIF-V2. Three poorly water soluble drugs were subjected to dissolution into FeSSGF and FeSSIF-V2, as well as their corresponding "surfactant-free" media. Solubility and laser diffraction analysis of drug in FeSSGF and dynamic light-scattering studies (DLS) of drug in FeSSIF-V2 were conducted. Results showed drug-saturated FeSSGF globules and FeSSIF-V2 mixed micelles were large and slow diffusing (diffusivities of about 1×10^-9 and 7×10^-8 cm²/s, respectively), compared to free drug (about 7×10^-6 cm²/s) and drug-bound micelles from pharmaceutical surfactants (about 0.5-1×10^-6 cm²/s). Of the three drugs, griseofulvin exhibited the greatest biorelevant media-enhanced solubility and dissolution (652-fold and 6.23-fold respectively in FeSSGF, and 190-fold and 12.7-fold respectively in FeSSIF-V2), but slow colloid diffusivity markedly attenuated large solubility benefits, particularly in FeSSGF.

Aqueous solubility study of salts of benzylamine derivatives and p-substituted benzoic acid derivatives using X-ray crystallographic analysis

Twenty two p-substituted benzoic acid derivates were used to prepare salts of N-methylbenzylamine (II) and N,N-dimethylbenzylamine (III), respectively. Only five salts of (II) and two salts of (III) were obtained in a crystalline state. The solubility of these salts was orders of magnitude higher than those reported for the corresponding salts of benzylamine (I). Thermal analysis indicated that the increased solubility was caused by reduced crystal lattice energy, which was most likely due to the reduced number of strong hydrogen bonds of the salt of (II) and (III). X-ray crystallographic analysis of p-hydroxybenzoic acid salt of (I), (II) and (III) suggested that the reduced number of hydrogen bonds caused the apparent higher solubility. Further analyses of seven salts of (I) were performed. It was not possible to identify any relationship between the number of hydrogen bonds and the corresponding solubility of the salts.

Correlation of aqueous solubility of salts of benzylamine with experimentally and theoretically derived parameters. A multivariate data analysis approach

Twenty two salts of benzylamine and p-substituted benzoic acids were prepared and characterized. The p-substituent was varied with regard to electronic, hydrophobic, and steric effects as well as hydrogen bonding potential. A multivariate data analysis was used to describe the relationship between the aqueous solubility of the salts and experimentally determined physicochemical parameters and theoretically derived molecular descriptors. The model, based on all descriptors, gave R(2)=0.86 and Q(2)=0.72. The most significant descriptors exhibiting VIP (variance of importance) values above 1.0 were intrinsic dissolution rate, intrinsic solubility of the unionized acids (S(0)), Hansch's hydrophobic parameter, Charton's steric parameter and molecular weight (MW). Statistically good models for predicting solubility of a selected test set were obtained by using simple models consisting of a few descriptors only: (i) Charton, Hansch and MW (R(2)=0.73; Q(2)=0.70), and (ii) Charton and S(0) (R(2)=0.74; Q(2)=0.72).

Quantitative relationship between solubility, initial dissolution rate and heat of solution of chiral drugs

PURPOSE

The aim of this study was to clarify the quantitative relationship between solubility, initial dissolution rate and heat of solution of racemic compound and its enantiomers.

METHODS

Propranolol, propranolol HCl, tyrosine, and tryptophan were used as typical chiral drugs. The heat of solution of chiral drug was measured by an isothermal microcalorimeter and the heat of fusion was measured by a DSC. The free energy difference for the dissolution of drug was calculated from the solubility and initial dissolution rate data.

RESULTS

The free energy difference and enthalpy difference of the dissolution between the racemic compound and enantiomer of propranolol, propranolol hydrochloride, tyrosine, and tryptophan were obtained by the solubility, initial dissolution rate and heat of solution data. A good linearity was observed in the free energy difference and the enthalpy difference for the dissolution of them, except for propranolol HCl data. By considering the dissociation in solution, the data of propranolol HCl followed the regression line.

CONCLUSIONS

The free energy difference of the dissolution was linearly dependent on the enthalpy difference for the racemic compound and its enantiomers. The results fit the theoretical equation. It could be possible to estimate the solubility of chiral insoluble drug from the thermal data.

Physiochemical and physiological mechanisms for the effects of food on drug absorption: the role of lipids and pH

Drugs are absorbed after oral administration as a consequence of a complex array of interactions between the drug, its formulation, and the gastrointestinal (GI) tract. The presence of food within the GI tract impacts significantly on transit profiles, pH, and its solubilization capacity. Consequently, food would be expected to affect the absorption of co-administered drugs when their physicochemical properties are sensitive to these changes. The physicochemical basis by which ingested food/lipids induce changes in the GI tract and influence drug absorption are reviewed. The process of lipid digestion is briefly reviewed and considered in the context of the absorption of poorly water-soluble drugs. The effect of food on GI pH is reviewed in terms of location (stomach, upper and lower small intestine) and the temporal relationship between pH and drug absorption. Case studies are presented in which postprandial changes in bioavailability are rationalized in terms of the sensitivity of the physicochemical properties of the administered drug to the altered GI environment.

Programmable drug delivery from an erodible association polymer system

An erodible association polymer system based on blends of cellulose acetate phthalate (CAP) and Pluronic F127, a block copolymer of poly(ethylene oxide) and poly(propylene oxide), has been investigated for its applicability to rate-programmed drug delivery. The compatibility and thermal properties were characterized by DSC and FTIR. Results from the thermal analysis indicate that the blends are compatible above 50% CAP, as revealed by a single composition-dependent glass transition temperature (Tg). The existence of molecular association through intermolecular hydrogen bonding between the carboxylic acid and the ether oxygen groups is supported by the observation of an upward shift in the IR carbonyl stretching frequency at increasing Pluronic F127 concentrations. Using theophylline as a model drug, the in vitro polymer erosion and drug release characteristics of the present polymer system were evaluated at different buffer pH's on a rotating-disk apparatus. The results show that the rates of both polymer erosion and drug release increase with the Pluronic F127 concentration in the blend. Further, at pH 4, the polymer erosion is minimal and the theophylline release appears to be governed mainly by diffusion through the polymer matrix. In contrast, at pH 7.4, the theophylline release is controlled primarily by the polymer surface erosion. To demonstrate the unique approach to programmed drug release based on the concept of nonuniform initial drug distribution, pulsatile patterns of drug release have been achieved successfully from the present surface-erodible polymer system using a multilaminate sample design with alternating drug-loaded layers.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of the dissolution rate controlling process for isomeric amides in alkane solvents

The mechanisms that control the dissolution rates of chemical compounds in liquids have long been of interest to pharmaceutical scientists. Generally, control of the dissolution rate can be classified as being by interfacial reaction rate or by the rate of mass transport. Little work has been done in the area of sparingly soluble compounds dissolving in nonpolar organic solvents. In this study the dissolution of three isomers of methylacetanilide was investigated in three nonpolar organic solvents (hexane, heptane, and cyclohexane). The dissolution apparatus used a flat plate into which the nondisintegrating tablet could be placed so that dissolution occurred only from one face of the tablet. Agitation was provided by a four-bladed stirrer whose outer edge was 2 cm from the tablet surface. Dissolution data were collected only for concentrations less than 5% of the saturation solubility of the given compound in the given solvent. All dissolution profiles were linear. Dissolution rates were obtained from the slopes of these plots. Plots of In (dissolution rate) versus In (stirring speed) were also linear and yielded slopes that were close to the value of 0.50 predicted by the convective diffusion model employed.

The effect of enzymatic reaction on dissolution rate: theoretical analysis and experimental test

The dissolution behavior of N-acetylphenylalanine ethyl ester (1) and N-benzoyltyrosine ethyl ester (2) from a rotating disk into aqueous solutions containing the enzyme alpha-chymotrypsin was investigated. The effect of the bulk enzymatic reaction on the dissolution rates is modeled using the continuity equation where the reaction term is considered a constant throughout the reaction zone. Dimensional analysis on the continuity equation defines the important parameter R* = KcatE0h2/(CsD) which is the ratio of the diffusion time to the reaction time. This parameter correctly predicted the fact that the enzymatic reaction had only a slight impact on the dissolution of the highly soluble 1 while the effect on the less soluble 2 was large. Also predicted by R* is the dissolution dependence on the catalytic rate constant. The variation of this rate constant with pH is consistent with the dependence on pH found for the dissolution rate of 2. It is further demonstrated that the decrease in dissolution rate with solubility can be significantly reduced when the dissolving compound is an enzyme substrate. For the two compounds used in this study the dissolution rate decreased with the square root of solubility, as predicted by the theoretical analysis in the presence of enzyme. Other experiments included the variation of the enzyme concentration and the rotational speed of the spinning disk. All experiments were designed to show how R* could correctly predict the relative importance of the convective, diffusive, and reactive processes.

Program for evaluating drug dissolution kinetics in preformulation

The in vitro dissolution kinetics of various drugs were studied at different experimental conditions using both a centrically rotating disc method and a modified excentrically rotating disc method. The combination of these two methods provides a preformulation program that includes many important pharmaceutical and biopharmaceutical parameters related to drug dissolution. The influence of the hydrodynamic conditions on the dissolution rate was demonstrated and the intrinsic dissolution tendency was obtained. Further, the acid dissociation constant, diffusion coefficient and enthalpy of dissolution can be calculated from data obtained from the rotating disc experiments. The relationship between dissolution rate and aqueous solubility of each drug tested is also considered.

Cholesterol monohydrate dissolution rate studies in aqueous micellar sodium chenodeoxycholate solutions

The dissolution rate of cholesterol monohydrate in various concentrations of sodium chenodeoxycholate (1) was significantly influenced by the addition of strong electrolytes. The mass transfer resistances decreased with increasing electrolyte concentrations and attained an asymptotic minimum value predicted and experimentally established for the convective/diffusion-controlled situation. Reduction of the interfacial barrier to dissolution was many times more sensitive to Mg2+ than to Na+ at equimolar concentrations. Cholesterol monohydrate solubilities increased nonlinearly with increasing 1 in 0.01 M phosphate buffer at pH 8.0 and was not influenced by the presence of strong electrolytes. Measured diffusion coefficients gave supporting evidence that the effective micellar size remained the same within the various experimental systems up to 116 mM chenodeoxycholate. The experimental findings indicated that the interfacial barrier is electrostatic in character. They are consistent with the phenomenon of diffusion of negatively charged micelles toward a negatively charged cholesterol monohydrate surface and the subsequent collision complex transfer of cholesterol molecules at the crystal surface. The results and mechanistic interpretations are also in accord with the previous model studies on cholesterol monohydrate dissolution in the presence of mixed micelles composed of nonionic and ionic surfactants.

In-vitro enhancement of cholesterol dissolution by commonly used drugs

A rotating disc apparatus was used to study the dissolution of cholesterol in sodium cholate solutions and ox bile. Drugs with structures that render them capable of lowering interfacial resistance were tested and shown to increase cholesterol dissolution rates in both systems. In sodium cholate solutions, loperamide (3 X 10(-4)M) increased the rate of dissolution by over six times, and a similar effect was observed with amitriptyline (3 X 10(-3)M), diphenhydramine (3 X 10(-3)M), dicyclomine (3 X 10(-3)M) and propantheline (3 X 10(-3)M). These drugs are as effective as benzalkonium chloride at these concentrations. Amitriptyline, propantheline, dicyclomine and diphenhydramine also increased cholesterol dissolution rates into ox bile. If these drugs are excreted into human bile in sufficient quantities and in an active form they may be able to enhance the speed of cholesterol gallstone dissolution therapy.

Evaluation of lincomycin as a cholesterol gallstone dissolution rate accelerator

These studies were undertaken to test the hypothesis that interfacial resistance may be an important rate-limiting factor in cholesterol gallstone dissolution. The addition of lincomycin hydrochloride to the gallbladder bile of dogs in an in vitro bath system resulted in an acceleration in the rate of dissolution of a compressed cholesterol monohydrate pellet incubating in the bile. However, the constant infusion of lincomycin for 13 d directly into the gallbladders of conscious, unrestrained dogs, which resulted in biliary lincomycin concentrations comparable to that of the in vitro tests, did not alter the dissolution rate of a compressed cholesterol monohydrate pellet which had been surgically placed into the gallbladder. We therefore conclude that the interfacial resistance between the cholesterol monohydrate pellet and the bile may be reduced by the addition of lincomycin to the gallbladder bile which, in the in vitro environment, results in an acceleration in the rate of dissolution of compressed cholesterol pellets. However, the ineffectiveness of lincomycin in accelerating the dissolution of cholesterol pellets in vivo suggests that interfacial resistance is not the only rate-limiting factor in gallstone dissolution. Other factors, such as mixing, may also be critical.

Kinetics and mechanisms of cholesterol gallstone dissolution

The purpose of this paper is to review the past and current in vitro studies aimed at understanding the mechanisms of cholesterol gallstone dissolution in bile. As is pointed out, there has been considerable progress in this area during the past 15 years with regard to the physical chemistry of the dissolution process.

Kinetics and thermodynamics of dissolution of lecithin by bile salts

The kinetics of dissolution of dispersions of egg phosphatidylcholine (lecithin) by bile salts was studied by observing the decrease in turbidity as mixed micelles of lecithin and bile salts were formed. The rate of dissolution of lecithin corresponding to formation of mixed micelles was studied in the presence of dihydroxy bile salts, sodium deoxycholate, sodium chenodeoxycholate, sodium ursodeoxycholate and one trihydroxy bile salt, sodium cholate. The rate of dissolution of lecithin and mixed micelle formation decreased in the order: chenodeoxycholate greater than deoxycholate greater than cholate greater than ursodeoxycholate. Kinetic solvent isotope studies in D2O, along with measurement of enthalpies of mixed micelle formation both in H2O and D2O, suggest that formation and stabilization of mixed micelles are related to "hydrophobicity" as estimated by high performance liquid chromatography retention factors.

Dissolution rate of cholesterol in monooctanoin

Monooctanoin is used clinically for dissolution of common bile duct cholesterol gallstones. A number of factors influencing the dissolution rate of cholesterol monohydrate in this solvent were investigated. Water increased cholesterol dissolution rate in a manner inconsistent with previous solubility measurements. Dissolution rate increased approximately 50% in the presence of 10 to 15% water in monooctanoin. Further studies on viscosity, the effect of polymers and temperature showed that the solvent viscosity had a dominating influence on dissolution rate. This was thought to be caused by the interaction of cholesterol with the solvent in the dissolving surface layer which caused a decrease in the diffusion coefficient (and dissolution rate) of cholesterol. Reducing viscosity and increasing temperature were identified as possible approaches for increasing cholesterol gallstone dissolution rate in monooctanoin.

Importance of viscosity in the dissolution rate of cholesterol in monooctanoin solutions

Several factors affecting the dissolution rate of cholesterol in monooctanoin were investigated. This solvent is used clinically for dissolution of residual cholesterol gallstones in the bile duct after cholecystectomy. The effect of added water on dissolution rate, measured using the static- or rotating-disk methods, was not consistent with the previously measured solubility. The discrepancy was found to be due to the decreasing viscosity of the solvent as water was added. Addition of cholesterol, however, increased the viscosity of monooctanoin. The viscosity effect on dissolution rate was investigated further by addition of polymers (povidone and poloxamer 237) which increased solvent viscosity. Dissolution rate was proportional to viscosity to the -0.4 power with these polymers. An equation was derived which predicts that dissolution rate should be proportional to viscosity to the -2/3 power. The predicted exponent was very close to reported experimental values for benzoic acid, but the dissolution rate/viscosity relationship for cholesterol in aqueous monooctanoin was nonlinear with apparent exponents of -0.65 to -2.3. Although the Arrhenius activation energies for viscosity (3.79 kcal/mol) and dissolution rate constant (3.66 kcal/mol) were almost equal for benzoic acid, a nonlinear relationship was again observed for cholesterol in aqueous monooctanoin with approximate Ea values of 5.6-10 kcal/mol. The strong influence of viscosity on dissolution rate in this system is attributed to the viscosity-increasing effect of cholesterol in the diffusion layer. The increased viscosity at higher cholesterol concentrations reduces the diffusion coefficient of cholesterol and causes the dissolution rate to be slower even though solubility may have been higher.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of benzalkonium chloride on the dissolution rate behavior of several solid-phase preparations of cholesterol in bile acid solutions

Cholesterol dissolution rate accelerators, such as benzalkonium chloride, function by reducing the interfacial barrier that exists between the negatively-charged bile acid micelle and the negatively-charged cholesterol surface. It has been proposed that this reaction is accomplished by the binding of the positively-charged accelerator to the negatively-charged micelle. An earlier report showed that different solid preparations of cholesterol give different dissolution rates under the same conditions and these differences can be primarily accounted for by variations in the interfacial transport constant (P). By using the rotating disk dissolution apparatus and the Levich theory it has been possible to study the dissolution behavior of different cholesterol solid phases as a function of the benzalkonium chloride concentration. It was shown that the ratios of P values for the different phases are relatively constant over the range of the accelerator concentrations. This suggests that the accelerators act primarily on the micelle to enhance dissolution rate.

Dissolution kinetics of carboxylic acids I: effect of pH under unbuffered conditions

The dissolution behavior of benzoic acid, 2-naphthoic acid, and indomethacin from rotating compressed disks into aqueous solutions of constant ionic strength (mu = 0.5 with potassium chloride) at 25 degrees was investigated. The pH of the bulk aqueous medium was maintained during dissolution by means of a pH-stat apparatus. A model for the initial steady-state dissolution rate of a monoprotic carboxylic acid was derived from Fick's second law of diffusion. This model assumed that diffusion-controlled mass transport and simple, instantaneously established reaction equilibria existed across a postulated diffusion layer. Using previously determined intrinsic solubilities, pKa values, and diffusion coefficients, the model was found to predict the dissolution rates of these acids accurately as a function of the bulk solution pH. Hydroxide ion and water were the only reactive base species present in the bulk solution. The concentration profiles of all of the species across the diffusion layer were generated for a given bulk pH. Furthermore, the model generated values for the pH profile within the microclimate of the diffusion layer and the pH at the solid-solution boundary.

Adsorption of lecithin by cholesterol

Egg lecithin was adsorbed significantly by cholesterol monohydrate crystals. Adsorption data obtained at initial concentrations of less than 1.1 mM lecithin fitted the Langmuir equation. The calculated adsorption capacity suggested formation of a lecithin bilayer or a mixed bilayer of lecithin and cholesterol. The amount of lecithin adsorbed was highly dependent on the cholate concentration in the incubation medium. Minimal adsorption was observed at approximately 5 mM cholate. The presence of quaternary ammonium slats and dioctyl sodium sulfosuccinate caused desorption. The finding of an adsorptive layer supported the existence of an interfacial barrier than controls cholesterol dissolution.

Rotating-disk method for determining cutaneous metabolism

Since design and evaluation of topical dosage forms should account for both skin permeation and cutaneous metabolism, an in vitro system was formulated to determine the metabolic component of viable guinea pig skin utilizing the diffusion layer property of the rotating disk. The drug investigated was vidarabine, an antiviral agent, which was rapidly metabolized to 9-beta-D-arabinofuranosylhypoxanthine. The aqueous diffusion coefficient of the drug was determined by the capillary cell method. The rotating-disk system was standardized using benzoic acid. The dorsal skin of a guinea pig was removed after shaving, and the epidermal section was excised by a keratome. After the section was mounted on the stainless steel disk with a tissue adhesive, the preparation was immersed in a 10-ml beaker containing 5 ml of drug solution at 37 degrees. At suitable intervals, samples were withdrawn, separated by TLC, and assayed by liquid scintillation. The enzyme rate constant was 1.54 x 10(-1) sec-1.

Dissolution kinetics of cholesterol in simulated bile II: influence of simulated bile composition

Normal human gallbladder bile and gallbladder bile of patients undergoing chenodeoxycholic acid therapy were simulated by using appropriate combinations of taurine and glycine conjugates of cholic, chenodeoxycholic, and deoxycholic acids. Also, the total bile acid concentration and the total bile acid to lecithin ratio were varied over physiological ranges. Dissolution rates of cholesterol monohydrate pellets (model gallstone) in these solutions were 90--99% interfacially controlled. Even under conditions favorable for dissolution, i.e., high bile acid concentration and high bile acid to lecithin ratio, the interfacial resistances were extremely large. These results are of the same order of magnitude as those found in the limited studies with actual gallbladder bile and suggest that the bile acids, lecithin, and the electrolytes are the primary determinants of the interfacial resistance for cholesterol dissolution. Furthermore, the kinetics of dissolution were always much faster with the chenodeoxycholic acid-rich compositions than with the corresponding normal compositions. This finding suggests, therefore, that in addition to desaturating bile with respect to cholesterol, the feeding of chenodeoxycholic acid further facilitates cholesterol gallstone dissolution by reducing the interfacial resistance of the process.

The dissolution mechanism in a system undergoing complexation: salicylamide in caffeine solution

The dissolution rate of compressed salicylamide discs has been measured in water and in caffeine solutions of increasing concentration at 15, 25, 37 and 45 degrees in an apparatus rotating at 48 rev min-1 or more. Dissolution rate profiles showed breaks indicative of a shift in the mechanism of dissolution from interfacial towards transport control. The shifts occurred at higher caffeine concentrations on increasing the agitation rate or temperature. The dependencies of dissolution rates on agitation rates typified the intermediate type of dissolution and Arrhenius plots indicated that interfacial and transport processes participated in salicylamide dissolution.

Dissolution kinetics of cholesterol in simulated bile I: Influence of bile acid type and concentration, bile acid-lecithin ration, and added electrolyte

A physical model approach was utilized to investigate cholesterol monohydrate dissolution kinetics in simulated bile. The static pellet method and the Berthoud theory were employed to assess the contributions of the diffusion-convection mass transfer resistance and those of the interfacial resistance to the overall kinetics. For almost all situations studied, the interfacial resistance was the dominant rate-determining factor. The effects of four bile acids and their concentrations, the bile acid-lecithin ratio, and the added electrolytes and their concentrations on the interfacial resistance were examined. The results were correlated with those obtained with human bile samples, and the indications were that the kinetics of cholesterol dissolution in bile may be explainable on the basis of the principal bile acids, lecithin, and the electrolytes in the bile.

Dissolution rates of model gallstones in human and animal biles and importance of interfacial resistance

Cholesterol monohydrate dissolution kinetics in human gallbladder bile were studied to determine the magnitudes of the in vitro dissolution rates, the rate resistances in human gallbladder bile, and the extent that the interfacial resistance is the rate-determining factor. Dissolution rate studies also were conducted using human duodenal bile and animal bile for comparison. The dissolution rate resistance, R, ranged from 10(4) sec/cm for chicken bile to 10(4)-10(6) sec/cm for human bile. Interfacial resistance was the rate-determining factor for essentially all results. Where chemical composition data were obtained, the R values for the human bile samples were consistent with predictions made from the simulated bile studies. In two human gallbladder specimens having low bile acid-lecithin molar ratios (i.e., 2.9 and 2.3), very high R values of 1.9 X 10(5) and 4.1 X 10(5) sec/cm were found. These values were in good agreement with the findings in the simulated bile studies and suggest that stone dissolution in patients with low bile acid-lecithin ratios may proceed very slowly, even when the bile is highly undersaturated with respect to cholesterol.

Cholesterol gallstone dissolution rate accelerators I: Exploratory investigations

Various compounds that might function as cholesterol gallstone dissolution accelerators were studied. The dissolution rates of cholesterol monohydrate pellets in synthetic bile (116 mM sodium cholate-32 mM lecithin) containing the agent at various concentration levels were determined. In the absence of any dissolution rate accelerator, the dissolution kinetics for cholesterol previously were found to be interfacial resistance controlled, and the rates were around 20 times less than the diffusion-controlled rates in the present experiments. Primary, secondary, and tertiary amines and quaternary ammonium compounds were effective accelerators. When the alkyl chain lengths were long enough and/or when the agent concentrations were high enough, the dissolution rates generally approached diffusion-controlled rates. Steroidal amines generally had good activity. Anionic and nonionic surfactants had little or negative activity.