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Cleavable surfactants, i.e., surfactants in which a weak bond has deliberately been inserted into the molecule, are of interest when remaining surface-active material at a surface can cause problems. Ester and amide bonds are established as week linkages in surfactants but these generate an acid when they hydrolyze. For some applications, acidic degradation products are unwanted. Surfactants with a carbonate bond between the polar headgroup and the hydrophobic tail are of interest for such purposes. In this article, we compare the physical–chemical properties of nonionic carbonate surfactants with those of the corresponding ester and amide surfactants. The half-lives of the different cleavable surfactants are also compared and it was found that a carbonate bond is slightly more stable to alkaline hydrolysis than an ester bond when present in otherwise identical structures. A nonionic Gemini surfactant with a carbonate bond in the spacer, which on hydrolysis decomposes into two identical single-tailed nonionic amphiphiles, is also presented. The hydrolysis kinetics for this surfactant was studied in some detail and it was found that it degrades much faster at low temperature than at higher temperature. This anti-Arrhenius type of hydrolysis kinetics is proposed to be due to the reverse solubility versus temperature behavior of ethoxylated surfactants.

The effects of self-assembly on the hydrolysis kinetics of surfactants that contain ester bonds are discussed. A number of examples on how reaction rates and apparent reaction orders can be modulated by changes in the conditions, including an instance of apparent zero-order kinetics, are presented. Furthermore, it is shown that the examples on reaction kinetics display parallels and connections to important physicochemical aspects of surfactant aggregates, namely, the “reservoir function” of micelles and the fact that the headgroup region of micelles constitutes an aqueous environment largely distinct from the bulk solution. The examples presented can be used in teaching organic as well as physical chemistry. The text is written with the intention to be largely self-contained, in order to make it accessible for readers with different background experience.

Water-dispersible organosilica nanoparticles were synthesized using microemulsions and micellar solutions. Octadecyltrimethoxysilane (OTMS) was used as the silica source resulting in particles having a hydrophobic interior with the ability to host oil. The diameters of the formed particles could be varied between 60 and 200 nm, depending on the amount of added oil and OTMS. The size of the particles was determined using dynamic light scattering and transmission electron microscopy. Exchange-coupled diffusion nuclear magnetic resonance experiments were performed to study the exchange rates between the particles and the surrounding media. Triethylamine and tributylamine were used as probe molecules, and it was shown that they had longer mean residence times in the particles compared with in the bulk. Moreover, it was found that the mean residence time of the probe molecules increased significantly when the particles contained oil. The results also showed that the mean residence time of tributylamine was longer than that of triethylamine. Furthermore, by the use of UV-vis spectrophotometry, it was shown that the particles were able to take up benzophenone from water solutions.

A Heck-type reaction, comprising a carbon–carbon coupling between an arylboronic acid and styrene, has been performed using a water soluble rhodium catalyst entrapped in the water-filled pores of mesoporous silica particles. The catalyst-loaded inorganic particles were dispersed in a non-polar medium, either a solution of the reactants in an aromatic solvent (toluene or p-xylene) or a solvent-free mixture of the reactants using a large excess of styrene. The reaction occurred at the oil–water interface, i.e., at the silica pore openings. The yield obtained was higher than that obtained in the conventional liquid–liquid two-phase system. The major advantage with having the catalyst “heterogenized” by entrapment into the siliceous material is the simplicity of the work-up process. The porous particles, containing the catalyst, are simply removed by filtration after completed reaction.

A homolog pure nonionic surfactant, tetra(ethylene glycol) mono-n-octaneamide, was synthesized. The surfactant was characterized by determining the critical micelle concentration, cloud point, and biodegradation. Hydrolysis catalyzed by an acid, an alkali, a peroxide, and enzymes [peptidase from porcine, amidase from Pseudomonas aeruginosa, and two lipases, Mucor miehei lipase (MML) and Candida antarctica lipase B (CALB)] was investigated using 1H nuclear magnetic resonance. The surfactant was stable toward the acid, alkali, and peroxide. When subjected to peptidase and CALB, the amide was cleaved, although at a low rate. No reaction was obtained when using the amidase or MML. The biodegradation test resulted in more than 60% degradation after 28 d. Based on these results, the amide surfactant can be considered chemically stable, yet highly biodegradable, which generally is an ideal combination of properties for a surfactant.

Small unilamellar vesicles (approximately 100 nm in diameter) form spontaneously in aqueous mixtures of histidine and sodium dodecyl benzenesulfonate. By manipulating pH, a gradual transition from micelles to vesicles to bilayers to precipitate is observed. The self-assembly of vesicles occurs over a wide range of compositions when the solution pH is lower than 6.0, the pKa of the imidazole moiety on the histidine molecule. This phenomenon is likely the result of attractive interactions between the negatively charged benzenesulfonate headgroups and the positively charged imidazole group in the amino acid. Similar results are obtained when imidazole salt itself is used.

Base-catalyzed hydrolysis, biodegradation, and enzyme-catalyzed hydrolysis of a series of four monoesters of tetra(ethylene glycol) have been investigated. The surfactants varied in substitution on the α-carbon of the acyl chain, from no substitution, to 2-methyl, to 2-ethyl, and on to 2,2-dimethyl. All surfactants were based on C8-acids except the methyl-substituted, which was based on a C7-acid. The hydrolysis was investigated using 1H nuclear magnetic resonance. The surfactants showed a pronounced difference in stability with respect to type of substitution in the vicinity of the ester bond. In alka-line hydrolysis the most significant difference in reactivity lay between the surfactant with an ethyl group and the surfactant with a methyl group in the α-position of the acyl chain. However, in the biodegradation studies these surfactants broke down at almost exactly the same rate as the nonsubstituted surfactant. In the biodegradation test, the disubstituted surfactant deviated considerably. Two lipases, from Mucor miehei (MML) and Candida antarctica B (CALB), were used in the enzyme-catalyzed hydrolysis. The surfactant with no substitution was found to hydrolyze much faster than the other surfactants, and the hydrolytic activity of MML, but not CALB, increased in the presence of surfactant micelles.

A series of four homologous pure nonionic surfactants, all monoesters of tetra(ethylene glycol), were synthesized. The ester surfactants varied in the degree of substitution on the α-carbon of the acyl chain, from no substitution to 2-methyl, to 2-ethyl, and on to 2,2-dimethyl. All surfactants were based on C8-acids except the 2-methyl-substituted, which was based on a C7-acid. The ester surfactants were characterized by critical micelle concentration (CMC) and cloud point. Base-catalyzed hydrolysis was investigated by using 1H NMR and tensiometry. The surfactants showed a pronounced difference in hydrolytic reactivity; the nonsubstituted surfactant was 90 times more reactive than the disubstituted, and the reactivity of the methyl-substituted surfactant was 14 times more reactive than the ethyl-substituted. Hydrolysis studies above the CMC revealed that the ester bond of the aggregated surfactant is protected from attack by hydroxide ions; thus, only surfactants in monomeric form are being cleaved.

Using a water-in-oil microemulsion system, silica nanoparticles containing superparamagnetic iron oxide (SPIO) crystals have been prepared and characterized. With this method, the loading of iron oxide crystals, the thickness of the silica shells, and the overall particle sizes are tunable. Moving from low to high water concentration, within the microemulsion region, resulted in a gradual shift from larger particles, ca. 100 nm and fully loaded with SPIOs, to smaller particles, ca. 30 nm containing only one or a few SPIOs. By varying the amount of silica precursor, the thickness of the silica shell was altered. Field dependent magnetization measurements showed the magnetic properties of the SPIOs were preserved after the synthesis.

Optical tweezers were used to study the interaction and attachment of human bone cells to various types of medical implant materials. Ideally, the implant should facilitate cell attachment and promote migration of the progenitor cells in order to decrease the healing time. It is therefore of interest, in a controlled manner, to be able to monitor the cell adhesion process. Results from such studies would help foresee the clinical outcome of integrating medical implants. The interactions between two primary cell culture models, human gingival fibroblasts and bone forming human osteoblast cells, and three different implant materials, glass, titanium, and hydroxyapatite, were studied. A novel type of optical tweezers, which has a newly designed quadrant detector and a powerful 3 W laser was constructed and force calibrated using two different methods: one method in which the stiffness of the optical trap was obtained by monitoring the phase lag between the trap and the moved object when imposing a forced oscillation on the trapped object and another method in which the maximum trapping force was derived from the critical velocity at which the object escapes the trap. Polystyrene beads as well as cells were utilized for the calibrations. This is the first time that cells have been used directly for these types of force calibrations and, hence, direct measurements of forces exerted on cells can be performed, thus avoiding the difficulties often encountered when translating the results obtained from cell measurements to the calibrations obtained with reference materials. This more straightforward approach represents an advantage in comparison to established methods.

The consequences of including amide bonds into the structure of short-chain nonionic surfactants have been studied. Of particular interest were the possible effects of the hydrogen bonding ability of the amide group on the micellar shape. The aggregate structure and hydration of two different amide-containing surfactants, C7H15CO-NH-(CH2CH2O)(4)H and C7H15CO-(NH-C3H6-CO)(2)N(CH3)(2), were investigated using NMR diffusometry (pulsed gradient spin echo NMR) as the main technique. Data from experiments on the surfactants, the hydrophobic probe molecule hexamethyldisilane (HMDS), and water were interpreted to gain information about the solution structures, and the results were compared to those on a previously studied alcohol ethoxylate surfactant of similar size, C8E4. Both of the amide-containing surfactants form small micelles within the whole investigated concentration range. At the critical micelle concentration, the aggregates are most probably spherical, and with increasing surfactant concentration there are indications of either a minor aggregate growth or agglomeration of the micelles. In addition, it was found that the presence of amide groups in the surfactant inhibits the intermicellar transport of HMDS, which occurs in the C8E4 system. From measurements on water diffusion in the three surfactant systems, it could be concluded that the surfactant hydration is higher when amide bonds are present.

Mixtures of hydrophobically modified hydroxyethyl cellulose (HMHEC) and alkali-sensitive cleavable betaine ester surfactants have been studied by viscometry, 1H NMR, absorbance measurements, and birefringence determinations. Before the hydrolysis, the surfactants behaved as conventional nondegradable surfactants in terms of the effect on the viscosity of increasing surfactant concentration. As the surfactants were hydrolyzed, systems with time-dependent viscosity were obtained. The viscosity either decreased monotonically or went through a maximum as a function of time, depending on the initial surfactant concentration. Different surfactant chain lengths gave rise to different viscosity profiles. The rate of hydrolysis, and thus the time-dependency of the surfactant concentration, could be controlled by changing the pH of the solution.

Previous studies have shown that the alkaline hydrolysis of cleavable ester surfactants is strongly affected by aggregation. The alkaline hydrolysis of the cationic species decyl betainate (DB) is strongly enhanced by micellization, whereas the nonionic species tetra(ethylene glycol)mono-n-octanoate (TEO) is virtually protected when residing in aggregates. In the present work, mixtures of DB and TEO were studied at concentrations above the critical micelle concentration, and the rate of hydrolysis of each surfactant in the presence of the other was assessed. The micellar interaction parameter (beta) was determined from the critical micelle concentrations of various mixtures of the two surfactants. The result (beta = -2.4) indicates a moderate net attraction. The hydrolysis of the surfactants was monitored using 1H NMR. It was shown that the hydrolysis of DB exhibits the main characteristics of the pseudophase ion-exchange model and that the reaction rate decreases with an increasing molar ratio of TEO. There are indications that the hydrolysis rate parallels the expected total counterion binding to the mixed micelles. The hydrolysis of TEO was not affected by the presence of DB. However, complementary experiments showed that it is possible to accelerate or retard the hydrolysis of TEO by coaggregation with stable cationic or anionic surfactants, respectively.

A linear and a branched nonionic cleavable surfactants containing a carbonate bond have been prepared from tetra(ethylene glycol) and an alkylchloroformate. The stability of these carbonate surfactants was determined by investigating their hydrolysis and biodegradability characteristics. The hydrolysis was catalyzed by alkali or enzymes (esterase from porcine liver and lipases from Mucor miehei and Candida antarctica B) and was monitored using 1H NMR. It was found that the stability toward alkali was higher for a carbonate surfactant than for a corresponding surfactant with an ester as weak bond. Biodegradation tests resulted in more than 60% degradation after 28 days for both carbonate surfactants. Physicochemical properties, such as critical micelle concentration (CMC), cloud point, area per molecule, and surface tension at the CMC, were determined and compared to those obtained from similar surfactants containing ester, amide, or ether bonds. It was found that the carbonate linkage is hydrophobic and that the oxycarbonyl part of the carbonate group is equivalent, in a formal sense, to an extra methylene group in the alkyl chain of the surfactant.

Unexpected colloidal assemblies form in aqueous mixtures of sodium dodecylbenzenesulfonate (SDBS) with the following imidazoline compounds: 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (V-44, which is a commonly used free-radical initiator), 2,2'-tetramethylenedi-2-imidazoline (TMI), and the main recombination product (RP) from the decomposition of V-44. All of these imidazoline compounds act as hydrotropes. As the molar ratio of imidazoline to SDBS increases, a gradual transition from micelles to vesicles to bilayers to precipitate is observed. V-44 decomposes slowly at 25 degrees C, and the phase diagrams of V-44/SDBS and RP/SDBS are similar. The vesicular region observed in mixtures of TMI/SDBS is larger in composition than that of V-44/SDBS and RP/SDBS mixtures. At equimolar compositions of SDBS and RP, a novel colloidal structure with multiple closely packed bilayers is observed. In these mixtures, small unilamellar vesicles (80 nm in diameter) form spontaneously, although with time they coexist with a small amount of precipitate and their size increases steadily. The self-assembly of vesicles occurs over a wide range of compositions when the solution pH is lower than the pK(a) of the imidazoline moiety. Quasi-elastic light scattering, cryogenic transmission electron microscopy, nuclear magnetic resonance, and small-angle neutron scattering were used to determine the characteristic length scales and properties of the assemblies.