Mixing behavior of the biosurfactant, rhamnolipid, with a conventional anionic surfactant, sodium dodecyl benzene sulfonate

The use of small angle neutron scattering, SANS, neutron reflectivity, NR, and surface tension to study the mixing properties of the biosurfactant rhamnolipid with a conventional anionic surfactant, sodium dodecyl 6-benzene sulfonate, LAS, is reported. The monorhamnose rhamnolipid, R1, mixes close to ideally with LAS at the air-water interface, whereas for mixtures of LAS with the dirhamnose rhamnolipid, R2, the LAS strongly partitions to the air-water interface relative to R2, probably because of the steric hindrance of the larger R2 headgroup. These trends in the binary mixtures are also reflected in the ternary R1/R2/LAS mixtures. However, for these ternary mixtures, there is also a pronounced synergy in the total adsorption, which reaches a maximum for a LAS/rhamnolipid mole ratio of about 0.6 and a R1/R2 mol ratio of about 0.5, an effect which is not observed in the binary mixtures. In solution, the R1/LAS mixtures form relatively small globular micelles, L(1), at low surfactant concentrations (<20 mM), more planar structures (lamellar, L(α), unilamellar/multilamellar vesicles, ulv/mlv) are formed at higher surfactant concentrations for R1 and LAS rich compositions, and a large mixed phase (L(α)/L(1) and L(1)/L(α)) region forms at intermediate surfactant compositions. In contrast, for the R2/LAS mixtures, the higher preferred curvature of R2 dominates the phase behavior. The predominant microstructure is in the form of small globular micelles, except for solution compositions rich in LAS (>80 mol % LAS) where more planar structures are formed. For the ternary mixtures, there is an evolution in the resulting phase behavior from one dominated by L(1) (R2 rich) to one dominated by planar structures, L(α), (R1, LAS rich), and which strongly depends upon the LAS/rhamnolipid and R1/R2 mole ratio.

Adsorption of nonionic and mixed nonionic/cationic surfactants onto hydrophilic and hydrophobic cellulose thin films

The adsorption of the nonionic surfactant hexaethylene monododecyl ether, C(12)E(6), and the mixed nonionic/cationic surfactants C(12)E(6) and hexadecyl trimethyl ammonium bromide, C(16)TAB, onto the hydrophilic and hydrophobic surfaces of thin cellulose films, formed by Langmuir-Blodgett, L-B, deposition, have been studied by neutron reflectivity. For the surfactant mixtures, considerable nonideal mixing is observed at both hydrophobic and hydrophilic surfaces. The results demonstrate that the C(12)E(6), C(12)E(6)/C(16)TAB mixture and solvent have a greater penetration into the cellulose film upon adsorption, compared to that observed in previous studies of C(16)TAB adsorbed onto cellulose, due to the presence of the nonionic surfactant. From the range of measurements made, it is concluded that both the presence of the nonionic surfactant and the nature of the cellulose films are both contributing factors to this increased penetration and swelling of the cellulose film.

Interplay between the surface adsorption and solution-phase behavior in dialkyl chain cationic-nonionic surfactant mixtures

Neutron reflectivity, NR, and surface tension have been used to study the adsorption at the air-solution interface of mixtures of the dialkyl chain cationic surfactant dihexadecyl dimethyl ammonium bromide (DHDAB) and the nonionic surfactants monododecyl triethylene glycol (C12E3), monododecyl hexaethylene glycol (C12E6), and monododecyl dodecaethylene glycol (C12E12). The adsorption behavior of the surfactant mixtures with solution composition shows a marked departure from ideal mixing that is not consistent with current theories of nonideal mixing. For all three binary surfactant mixtures there is a critical composition below which the surface is totally dominated by the cationic surfactant. The onset of nonionic surfactant adsorption (expressed as a mole fraction of the nonionic surfactant) increases in composition as the ethylene oxide chain length of the nonionic cosurfactant increases from E3 to E12. Furthermore, the variation in the adsorption is strongly correlated with the variation in the phase behavior of the solution that is in equilibrium with the surface. The adsorbed amounts of DHDAB and the nonionic cosurfactants have been used to estimate the monomer concentration that is in equilibrium with the surface and are shown to be in reasonable qualitative agreement with the variation in the mixed critical aggregation concentration (cac).

Effects of surface pressure on the structure of distearoylphosphatidylcholine monolayers formed at the air/water interface

The structure of the monolayer formed at an air/water interface by the phospholipid distearoylphosphatidylcholine (DSPC) has been determined as a function of the monolayer surface pressure (pi) using Brewster angle microscopy and neutron reflectivity. The microscopy studies demonstrate that the DSPC molecules form an extremely homogeneous monolayer on the water surface with no evidence of any domain formation. The neutron reflectivity measurements provide information on the thickness of the DSPC alkyl chains, head groups, and associated solvent distributions, along with the separations between these distributions and the interfacial area per molecule. Partial structure factor analyses of the reflectivity data show that the area occupied by each DSPC molecule decreases from 49 A2 at pi = 20 mN/m to 44 A2 at pi = 50 mN/m. There are concomitant increases in the widths of the lipids' alkyl chains and headgroup distributions (modeled as Gaussians), with the former rising from 18 A (at pi = 20 mN/m) to 20 A (at pi = 50 mN/m) and the latter rising from 14 A (at pi = 20 mN/m) to 18 A (at pi = 50 mN/m). The compression of the monolayer is also shown to give rise to an increased surface roughness, the principal component of which is found to be the thermal roughness caused by capillary waves. At all surface pressures studied (covering the range from 20 to 50 mN/m), the alkyl chains and head groups of the DSPC are found to have roughly the same orientations, with the alkyl chains tilted with respect to the surface normal by about 34 degrees and the head groups lying parallel to the interface normal, projecting vertically down into the aqueous subphase. Given the various trends noted on how the structure of the DSPC monolayer changes as a function of pi, we extrapolate to consider the structure of the monolayer immediately before its collapse.

Structure of partially fluorinated surfactant monolayers at the air-water interface

Partially fluorinated cationic surfactants of the form C(n)F(2n+1)C(m)H(2m)N(CH3)Br have been prepared, and their behavior at the air-water interface has been studied using surface tension measurements and neutron reflectometry. The degree of fluorination has been varied while keeping the overall chain lengths similar. The results are compared with those previously obtained for C16H33N(CH3)Br (C16TAB). The structural studies show a decrease in molecular orientation with increasing fluorination. The mean tilt away from the surface normal varies from 55 degrees for C16TAB to 25 degrees for C8F17C6H12N(CH3)Br. The interfacial layer roughness is observed to be lower than that expected for a pure fluorocarbon surfactant.

The overflowing cylinder sixty years on

The overflowing cylinder (OFC) was developed at Kodak sixty years ago and remains an elegant, precise and versatile tool for studying the dynamic surface properties of surfactant solutions today. The principles and design of the OFC are introduced and the use of the OFC to study adsorption kinetics and Marangoni effects is explained. Examples are provided from the study of pure surfactant solutions, mixed surfactants and polymer-surfactant complexes.

Comparison of the coadsorption of benzyl alcohol and phenyl ethanol with the cationic surfactant, hexadecyl trimethyl ammonium bromide, at the air-water interface

A comparison of the coadsorption of benzyl alcohol and phenyl ethanol with the cationic surfactant, hexadecyl trimethyl ammonium bromide, C16TAB, at the air-water interface is made using the specular reflection of neutrons. The phenyl ethanol is more surface active than the benzyl alcohol, and competes more effectively with the C16TAB for the interface. The structure of the C16TAB component in the mixed monolayer is compared with the structure of the pure C16TAB monolayer at an equivalent area per molecule. The addition of the aromatic alcohol subtly alters the conformation of the C16TAB and draws it closer to the aqueous subphase. The center of the alcohol distribution is located in the interface adjacent to the C6 group of the C16TAB alkyl chain closest to the headgroup. Compared to the benzyl alcohol, the more hydrophobic phenyl ethanol is slightly farther away from the headgroup, and has a greater impact on the conformation of the alkyl chain of the C16TAB.

Adsorption of aromatic counterions at the surfactant/water interface: a neutron reflectivity study of hydroxybenzoate and chlorobenzoate counterions at the hexadecyl trimethylammonium surfactant/water interface

Specular neutron reflectivity has been used to investigate the adsorption of the aromatic counterions hydroxybenzoate and chlorobenzoate at the hexadecyl trimethylammonium bromide surfactant monolayer/water interface. The degree of counterion binding and the location of the counterions at the interface are shown to depend on the isomeric form of the counterion. For hydroxybenzoate, the para-substituted counterion is located within the headgroup region of the surfactant monolayer, and there is of order one counterion for every two surfactant ions. For the ortho-substituted counterion, the degree of counterion binding is higher. There is of order 0.85 counterions for each surfactant ion, and the counterion is located within the hydrophobic region of the monolayer, some 5 A from the center of the headgroup distribution. Similar results were found for the chlorobenzoate counterion, but in that case it was the para-substituted counterion that was more tightly bound and located within the hydrophobic region of the surfactant monolayer. The results for the ortho-substituted hydroxybenzoate and for the para-substituted chlorobenzoate are consistent with those previously reported for the para-tosylate. The results are discussed in the context of the ability of the specific aromatic counterion isomer to promote massive micellar growth, and the results shed light on that mechanism.

Adsorption of Nonionic Mixtures at the Air–Water Interface: Effects of Temperature and Electrolyte

Specular neutron reflection has been used to investigate the effects of temperature and added electrolyte on the adsorption of nonionic surfactants and nonionic surfactant mixtures at the air-water interface. For the alkyl poly-oxyethylene oxide nonionic surfactants, C(n)EO(m), the adsorption at the air-water interface is independent of temperature for surfactants with shorter ethylene oxide groups, whereas there is an increasing tendency for increased adsorption with temperature for surfactants with longer ethylene oxide groups. The addition of "salting in" (sodium thiocyanate, NaSCN) and "salting out" (sodium chloride, NaCl, sodium sulphate, Na2SO4) electrolyte results in reduced and enhanced adsorption, respectively, for C12EO8, whereas both types of electrolyte result in enhanced adsorption for C12EO12. The addition of electrolyte does not substantially alter the temperature dependence of the adsorption of the pure monolayers. For the nonionic mixtures of C12EO3/C12EO8 increasing temperature results in a surface richer in the least surface-active component, C12EO8. For the same nonionic mixture, the addition of "salting in" and "salting out" electrolyte results in an reduced and increased adsorption, respectively. The addition of "salting in" electrolyte results in a surface more rich in C12EO3, whereas for the addition of both "salting in" and "salting out" electrolyte the surface composition is essentially unaltered.

Surfactant layers at the air/water interface: structure and composition

The use of neutron reflectometry to study the structure and composition of surfactant layers adsorbed at the air/water interface is reviewed. A critical assessment of the results from this new technique is made by comparing them with the information available from all other techniques capable of investigating this interface.