Romain Bordes

Role of an amide bond for self-assembly of surfactants

Self-assembly in solution and adsorption at the air-water interface and at solid surfaces were investigated for two amino-acid-based surfactants with conductimetry, NMR, tensiometry, quartz crystal microbalance with monitoring of the dissipation (QCM-D), and surface plasmon resonance (SPR). The surfactants studied were sodium N-lauroylglycinate and sodium N-lauroylsarcosinate, differing only in a methyl group on the amide nitrogen for the sarcosinate. Thus, the glycinate but not the sarcosinate surfactant is capable of forming intermolecular hydrogen bonds via the amide group. It was found that the amide bond, N-methylated or not, gave a substantial contribution to the hydrophilicity of the amphiphile. The ability to form intermolecular hydrogen bonds led to tighter packing at the air-water interface and at a hydrophobic surface. It also increased the tendency for precipitation as an acid-soap pair on addition of acid. Adsorption of the surfactants at a gold surface was also investigated and gave unexpected results. The sarcosine-based surfactant seemed to give bilayer adsorption, while the glycine derivative adsorbed as a monolayer.

Adsorption of cationic gemini surfactants at solid surfaces studied by QCM-D and SPR: effect of the rigidity of the spacer.

Two small series of cationic gemini surfactants with dodecyl tails have been synthesized and evaluated with respect to self-assembly in bulk water and at different solid surfaces. The first series contained a flexible alkane spacer and is denoted 12-n-12, with n = 2, 4, and 6. The second series had a phenylene group connected to the quaternary nitrogens in either the meta or para position and the surfactants are referred to as 12-m-Φ-12 and 12-p-Φ-12, respectively. The phenylene group is a rigid linker unit. The critical micelle concentration (cmc) was determined both by tensiometry and by conductometry, and the packing density of the surfactants at the air-water interface was calculated from the Gibbs equation. The cmc values for the geminis with a rigid spacer, 12-m-Φ-12 and 12-p-Φ-12, were of the same order of magnitude as for 12-4-12, which is the flexible surfactant that most closely matches the phenylene-based surfactants with respect to hydrophobicity, measured as log P, and distance between the positively charged nitrogen atoms. The adsorption of flexible and rigid surfactants was investigated on gold, silicon dioxide (silica), gold made hydrophobic by the self-assembly of hexadecanethiol, and gold made hydrophilic by the self-assembly of 16-hydroxyhexadecanethiol. On all of the surfaces, there was a reverse relationship between the adsorbed amount at the cmc and the length of the spacer (i.e., 12-2-12 gave the highest and 12-6-12 gave the lowest amount of adsorbed material). The adsorption pattern was similar for all of the surfactants when recorded at 25 °C. Thus, one can conclude that a rigid spacer does not render the self-assembly of a gemini surfactant difficult, neither in bulk water nor at solid surfaces. However, on one of the surfaces-untreated gold-the adsorbed amount of the geminis with a rigid spacer at 40 °C was approximately twice the values obtained at 25 °C. This is interpreted as the formation of an interdigitated bilayer at 25 °C and a regular bilayer without interpenetration of the alkyl chains at 40 °C.

Amino acid-based surfactants – do they deserve more attention?

The 20 standard amino acids (together with a few more that are not used in the biosynthesis of proteins) constitute a versatile tool box for synthesis of surfactants. Anionic, cationic and zwitterionic amphiphiles can be prepared and surfactants with several functional groups can be obtained by the proper choice of starting amino acid. This review gives examples of procedures used for preparation and discusses important physicochemical properties of the amphiphiles and how these can be taken advantage of for various applications. Micelles with a chiral surface can be obtained by self-assembly of enantiomerically pure surfactants and such supramolecular chirality can be utilized for asymmetric organic synthesis and for preparation of mesoporous materials with chiral pores. Surfactants based on amino acids with two carboxyl groups are effective chelating agents and can be used as collectors in mineral ore flotation. A surfactant based on cysteine readily oxidizes into the corresponding cystine compound, which can be regarded as a gemini surfactant. The facile and reversible cysteine-cystine transformation has been taken advantage of in the design of a switchable surfactant. A very attractive aspect of surfactants based on amino acids is that the polar head-group is entirely natural and that the linkage to the hydrophobic tail, which is often an ester or an amide bond, is easily cleaved. The rate of degradation can be tailored by the structure of the amphiphile. The ester linkage in betaine ester surfactants is particularly susceptible to alkaline hydrolysis and this surfactant type can be used as a biocide with short-lived action. This paper is not intended as a full review on the topic. Instead it highlights concepts that are unique to amino acid-based surfactants and that we believe can have practical implications.

Counterion specificity of surfactants based on dicarboxylic amino acids

The behavior in solution of a series of amino acid-based surfactants having two carboxyl groups separated by a spacer of one, two, or three carbon atoms has been investigated. All three surfactants precipitated on addition of acid, but the aspartate surfactant (with a two-carbon spacer) was considerably more resistant to precipitation than the aminomalonate surfactant (one-carbon spacer) and the glutamate surfactant (three-carbon spacer). The interactions with the monovalent counterions lithium, sodium, and potassium were investigated by conductivity. It was found that lithium ions bound the strongest and potassium ions the weakest to the surfactant micelles. These results were interpreted using the hard and soft acid-base theory. Comparing the three surfactants with respect to binding of one specific counterion, sodium, showed that the aminomalonate surfactant, which has the shortest spacer, bound sodium ions the strongest and the glutamate surfactant, which has the longest spacer, had the lowest affinity for the counterion. Also that could be explained by the hard and soft acid-base concept. The glutamate surfactant was found to be considerably more resistant to calcium ions than the two other surfactants. This was attributed to this surfactant forming an intermolecular complex with the calcium ion at the air-water interface while the aminomalonate and the aspartate surfactants, with shorter distance between the carboxylate groups could form six- and seven-membered intramolecular calcium complexes.

Separation of Bulk Effects and Bound Mass during Adsorption of Surfactants Probed by Quartz Crystal Microbalance with Dissipation: Insight into Data Interpretation

The assessment of adsorbed surfactant mass by quartz crystal microbalance with dissipation (QCM-D) monitoring is often complicated due to large bulk responses, particularly for surfactants with high critical micelle concentration (CMC). We present in this work means to interpret QCM-D data that enables the response from the bulk contribution to be separated from the response originating from adsorbed mass. Adsorption of two surfactants, Triton X100 and C12AspNa2 with low and high CMCs, respectively, at the gold-liquid interface surface has been evaluated. Two different approaches to quantify the bulk response are compared. The first approach involves the use of a nonadsorbing surface (silica), yielding a calibration curve for the concentration dependent bulk response. The second method is based on the fact that the overtone-dependent QCM-D response that originates from changes in the bulk differs from that induced by the adsorbed layer of the surfactants. Under the reasonable assumption that the bulk solution and the adsorbed surfactants can be treated as a Newtonian liquid and an acoustically rigid film, it is demonstrated that the bulk contribution can be quantified without control measurements involving inert surfaces. An excellent agreement between the two methods is reported.

Probe diffusion in phase-separated bicontinuous biopolymer gels.

Probe diffusion was determined in phase separated bicontinuous gels prepared by acid-induced gelation of the whey protein isolate-gellan gum system. The topological characterization of the phase-separated gel systems is achieved by confocal microscopy and the diffusion measurements are performed using pulsed field gradient (PFG) NMR and fluorescence recovery after photo-bleaching (FRAP). These two techniques gave complementary information about the mass transport at different time- and length scales, PFG NMR provided global diffusion rates in the gel systems, while FRAP enabled the measurements of diffusion in different phases of the phase-separated gels. The results revealed that the phase-separated gel with the largest characteristic wavelength had the fastest diffusion coefficient, while the gel with smaller microstructures had a slower probe diffusion rate. By using the diffusion data obtained by FRAP and the structural data from confocal microscopy, modelling through the lattice-Boltzmann framework was carried out to simulate the global diffusion and verify the validity of the experimental measurements. With this approach it was found that discrepancies between the two experimental techniques can be rationalized in terms of probe distribution between the different phases of the system. The combination of different techniques allowed the determination of diffusion in a phase-separated biopolymer gel and gave a clearer picture of this complex system. We also illustrate the difficulties that can arise if precautions are not taken to understand the system-probe interactions.

Flotation selectivity of novel alkyl dicarboxylate reagents for apatite-calcite separation

The investigation aims to demonstrate the conceptual thoughts behind developing mineral specific reagents for use in flotation of calcium containing ores. For this purpose, a series of dicarboxylate-based surfactants with varying distance between the carboxylate groups (one, two or three methylene groups) was synthesized. A surfactant with the same alkyl chain length but with only one carboxylate group was also synthesized and evaluated. The adsorption behavior of these new reagents on pure apatite and pure calcite surfaces was studied using Hallimond tube flotation, FTIR and ζ potential measurements. The relation between the adsorption behavior of a given surfactant at a specific mineral surface and its molecular structure over a range of concentrations and pH values, as well as the region of maximum recovery, was established. It was found that one of the reagents, with a specific distance between the carboxylate groups, was much more selective for a particular mineral surface than the other homologues. For example, out of the four compounds synthesized, only the one where the carboxylate groups were separated by a single methylene group floated apatite but not calcite, whereas calcite was efficiently floated with the monocarboxylic reagent, but not with the other reagents synthesized. This selective adsorption of a given surfactant to a particular mineral surface relative to other mineral surfaces as evidenced in the flotation studies was substantiated by ζ potential and infra-red spectroscopy data.

Nanostructured fluids from degradable nonionic surfactants for the cleaning of works of art from polymer contaminants

Nanostructured fluids containing anionic surfactants are among the best performing systems for the cleaning of works of art. Though efficient, their application may result in the formation of a precipitate, due to the combination with divalent cations that might leach out from the artifact. We propose here two new aqueous formulations based on nonionic surfactants, which are non-toxic, readily biodegradable and insensitive to the presence of divalent ions. The cleaning properties of water-nonionic surfactant-2-butanone (MEK) were assessed both on model surfaces and on a XIII century fresco that could not be cleaned using conventional methods. Structural information on nanofluids has been gathered by means of small-angle neutron scattering, dynamic light scattering and nuclear magnetic resonance with diffusion monitoring. Beside the above-mentioned advantages, these formulations turned out to be considerably more efficient in the removal of polymer coatings than those based on anionic surfactants. Our results indicate that the cleaning process most likely consists of two steps: initially, the polymer film is swollen by the MEK dissolved in the continuous domain of the nanofluid; in the second stage, surfactant aggregates come into play by promoting the removal of the polymer film with a detergency-like mechanism. The efficiency can be tuned by the composition and nature of amphiphiles and is promoted by working as close as possible to the cloud point of the formulation, where the second step proceeds at maximum rate.

Micellization of true amphoteric surfactants

The physical chemical behavior of a series of N-alkyl amino acid-based surfactants has been investigated. The series comprises four different types of amino acids as polar headgroups: glycine, aminomalonic acid, aspartic acid and glutamic acid, and for each type three homologues were synthesized: the octyl, decyl and dodecyl derivative. Aminomalonic acid, aspartic acid and glutamic acid are dicarboxylic amino acids with one, two and three methylene groups as spacer between the carboxylic groups, respectively. Compared with the more common N-acyl surfactants based on the same amino acids, many of the N-alkyl derivatives exhibited relatively high Krafft temperatures. The N-alkyl derivatives also had considerably lower critical micelle concentrations (CMCs) and they gave low values of surface tension at the CMC. The length of the spacer between the two carboxylic groups did not much influence the micellization. Some of the surfactants, in particular the lower homologues of N-alkylglycinate surfactants, gave unusually low surface tension values. The low values are most likely due to formation of a mixed monolayer at the surface, comprising of alternating anionic N-alkylglycinate and cationic N-protonated-N-alkylglycine. In a plot of conductivity vs. surfactant concentration there was no kink on the curve around the CMC, as determined by tensiometry. The absence of such a kink is in accordance with the view that self-assembly of the N-alkyl amino acid-based surfactants involves formation of mixed micelles consisting of alternating N-alkyl amino acid anion and N-protonated-N-alkyl amino acid also in the bulk solution. The protonation of the N-alkyl amino acid anion, which generates hydroxyl ions, is driven by the energetically favorable formation of mixed micelles consisting of anionic and cationic amphiphiles.

Formation and relaxation kinetics of starch–particle complexes

The formation and relaxation kinetics of starch-particle complexes were investigated in this study. The combination of cationic nanoparticles in suspension and anionic starch in solution gave rise to aggregate formation which was studied by dynamic light scattering, revealing the initial adsorption of the starch molecules on the particle surface. By examining the stability ratio, W, it was found that even in the most destabilized state, i.e. at charge neutralization, the starch chains had induced steric stabilization to the system. At higher particle and starch concentrations relaxation of the aggregates could be seen, as monitored by a decrease in turbidity with time. This relaxation was evaluated by fitting the data to the Kohlrausch-Williams-Watts function. It was found that irrespective of the starch to particle charge ratio the relaxation time was similar. Moreover, a molecular weight dependence on the relaxation time was found, as well as a more pronounced initial aggregated state for the higher molecular weight starch. This initial aggregate state could be due to bridging flocculation. With time, as the starch chains have relaxed into a final conformation on the particle surface, bridging will be less important and is gradually replaced by patches that will cause patchwise flocculation. After an equilibration time no molecular weight dependence on aggregation could be seen, which confirms the patchwise flocculation mechanism.