Werner Ulbricht

The aggregation behavior of poly-(oxyethylene)-poly-(oxypropylene)-poly-(oxyethylene)-block-copolymers in aqueous solution

Aqueous solutions of blockcopolymers, consisting of a polyoxypropyleneblock (POP) with a polyoxyethylene-block (POE) at each side, were studied using surface and interfacial tension measurements, static and dynamic light scattering and smallangle neutron scattering techniques, electric birefringence, rheological and DSC-measurements. The compounds were commercial samples and had an approximate average composition EO20PO70EO20, EO18PO58EO18, and EO106PO69EO106. All three compounds formed micelles above a critical concentration. The size of the micellar core is determined by the length of the hydrophobic poly-propyleneoxide block. The transfer energy of a propyleneoxide unit from the aqueous to the micellar phase is about 0.3 kT at room temperature and hence a quarter of the corresponding value for a CH2-group.The aggregation number of the micelles increases strongly with increasing temperature while the hydrodynamic radius remains constant in first approximation. The smallangle neutron scattering (SANS) data show at higher concentrations a strong correlation peak. Both the SANS- and the light-scattering data can be interpreted on the basis of the theory of hard sphere particles.Solutions with a volume fraction beyond about 0.2 gellifie when the temperature is raised above a characteristic value that is at the lowest concentrations slightly above room temperature, shift to lower values with increasing concentrations. Below this gelation temperature DSC-measurements show a phase transition with enthalpies between 40J/g and 80J/g, which is probably due to the dehydration of the PO-groups; this transition can also be observed at low concentrations where no gelation takes place. The position of the correlation peak of the SANS-data is not affected by the gel formation. Some samples, however, show clear evidence of long-range order and seem therefore to consist of cubic liquid crystalline phases. The shear moduli of the gels can qualitatively be understood on the basis of hard sphere models.

Transition of Rodlike to Globular Micelles by the Solubilization of Additives

The solubilization of hydrocarbons in surfactant solutions containing rodlike micelles was studied using static and dynamic light scattering techniques. It was found that these systems have a rather high solubilization capacity for aliphatic hydrocarbons; this solubilization leads to a shrinkage of the rods,and above a characteristic concentration of the hydrocarbon to a rod to sphere transition. The spheres can then accommodate more hydrocarbon and grow to micro-emulsion droplets. On the other hand the solubilization capacity of rodlike micelles for aromatic hydrocarbons is much smaller and these hydrocarbons even stabilize the rods. A model is proposed which can explain the experimental data on the basis of simple geometrical considerations and the packing parameter of the surfactants.

Effect of alcohol on the properties of micellar systems: II. Chemical relaxation studies of the dynamics of mixed alcohol + surfactant micelles

Abstract Chemical relaxation methods (T-jump, shock tube, and ultrasonic absorption) have been used to study the dynamics of micellar solutions of long chain surfactants (tetradecyl and hexadecyltrimethylammonium bromides (TTAB and CTAB, respectively) and tetradecylpyridinium bromide) and medium chain length alcohols (butanol to hexanol). These systems have been found to be characterized by three relaxation processes. The two fast ones correspond to the exchange of alcohol molecules and surfactants between mixed alcohol + surfactant micelles and surrounding solution; the slow one is associated with the mixed micelle formation-dissolution process. The most extensive results have been obtained with the TTAB-pentanol system and have been interpreted on the basis of Aniansson theory for the dynamics of mixed micelles, and of the data obtained in Part I in this series. The results indicate (1) that the reaction of association of alcohol molecules to TTAB micelles is diffusion controlled and that the residence time of pentanol in these micelles is of about 5 × 10 −8 sec; (2) the rate constant for the dissociation of a surfactant from a mixed micelle and the relaxation time for the surfactant exchange between micelles and surrounding solution are decreased, upon addition of alcohol; and (3) the micellar structures become very labile in the presence of alcohol. This labilization may reflect a shift of the minimum of the micelle size distribution function toward smaller aggregation numbers.

Chemical relaxation and equilibrium studies of aqueous solutions of laurylsulfate micelles in the presence of divalent metal ions

Abstract Equilibrium methods (conductivity, activity and density) as well as chemical relaxation methods (ultrasonic absorption, T-jump, p-jump and shock-tube) have been used to investigate aqueous micellar solutions of several divalent metal laurylsulfates (MLS 2 ) and of sodium laurylsulfate (NaLS) in the presence of divalent metal ions. The conductivity and activity data clearly show that divalent metal counterions are more bound to laurylsulfate (LS) micelles than Na + ions. The density data, however, indicate that the binding of M 2+ to LS micelles gives rise to a very small volume change and, therefore, that this binding does not proceed beyond the stage of an outer-outer sphere complex formation between counterions and micelle ionic head groups. The kinetic measurements show the existence of two relaxation processes which, as for NaLS, appear to be due to the exchange of LS ions between the micelles proper (fast process; relaxation time τ 1 ) and to the micelle formation-dissolution equilibrium (slow process; relaxation time τ 2 ). The kinetic results have been interpreted in terms of Aniansson and Wall [ J. Phys. Chem. 78, 1024 (1974); 79 , 857 (1975)] theory. The results concerning the fast process indicate that as for detergents with monovalent counterions the association of LS ions to a micelle proper is close to diffusion controlled. The LS micelles formed in the presence of divalent counterions are less polydisperse than with monovalent counterions. The rate constant for the dissociation of one LS ion from a micelle proper is smaller in the presence of Ni 2+ or Co 2+ ions than with Na + . This observation has been related to the stronger association of M 2+ relatively to Na + , to LS micelles. Most of the results relative to the slow relaxation process have been explained in terms of an association of counterions to the micelle nucleus and/or a decrease of aggregation number of the micelle nucleus upon increasing detergent concentration or addition of divalent metal chlorides (MCl 2 ). The data concerning the change of τ 2 of NaLS solutions upon addition of MCI 2 or mixing to MLS 2 have been qualitatively interpreted in terms of these two effects in addition to the change of cmc which prevails at low concentration of added MCl 2 or MLS 2 .

Investigations on a detergent system with rodlike micelles

Conductivity, kinetic, static and dynamic light scattering, electric birefringence and rheological measurements were carried out on aqueous solutions of Tetradecylpyridinium-n-Heptanesulfonate (C14PyC7SO3) up to high concentrations. In dilute solutions between the critical micelle concentration (cmc) and another characteristic concentration (ct) spherical micelles were detected whose radii were independent of detergent concentration and equal to the length of a detergent molecule; the aggregation numbern of these micelles of about 100 monomers per micelle was also in agreement with the existence of normal spherical micelles of aC14-detergent.Above the concentrationct, the spherical micelles were found to grow to rodlike aggregates whose short axis was still independent of concentration and equal to the length of a monomer, while the lengthsL of the rods increased with increasing detergent concentration. When the lengthsL of the rods became comparable with the mean distancea between them, the starting interaction between the rods slowed down their growth. In this concentration range of overlapping rods, the data could be evaluated with a recently developed theory by Doi and Edwards for stiff rods. The rods reached finally a maximum length of about 500 å and decreased again in size upon further increase of concentration when the overlap ratioL/a reached a value of about 1,5.

Zur Kinetik der Mizellbildung von Alkylpyridiniumhalogeniden

ZusammenfassungIn der vorliegenden Arbeit werden Ergebnisse von kinetischen Messungen an Alkylpyridiniumhalogeniden geschildert und diskutiert. Oberhalb der kritischen Mizellbildungskonzentration (cmc) konnten dabei an den meisten Systemen zwei Relaxationszeiten beobachtet werden, deren Konzentrations-, Temperaturund Gegenionenkonzentrationsabhängigkeit untersucht wurden.Nach einer vor kurzem entwickelten Theorie vonG. Aniansson et al. ist dabei die schnelle Relaxationszeit τ1 charakteristisch für die Verschiebung der Verteilungskurve im Aggregationsraum, wobei die Gesamtkonzentration der Mizellen konstant bleibt. Aus dieser Relaxationszeit kann die Lebensdauer eines Monomeren in der Mizelle sowie seine Einbau geschwindigkeit erhalten werden. Aus der Temperaturabhängigkeit von τ1 läßt sich die Reaktionsenthalpie für den Einbau eines Monomeren in die Mizelle gewinnen. Darüber hinaus kann bei bekannter Aggregationszahl die Verteilungsbreite erhalten werden. Im theoretischen Teil wird ein einfacher, aber vorher nicht bekannter Weg für die Herleitung der Relaxationsgleichung vorgestellt. Die langsame Relaxationszeit τ2 ist wesentlich bedingt durch die Konzentration der Oligomeren im Verteilungsminimum, die als Mizellkeime angesehen werden können. Die Konzentration dieser Keime läßt sich aus den Messungen gewinnen, und aus der Temperaturabhängigkeit von τ2 erhält man die Reaktionsenthalpie für die Keimbildung. Die relativ komplizierte Abhängigkeit der beiden Relaxationszeiten von den Parametern Totalkonzentration, Temperatur und Gegenionenkonzentration läßt sich quantitativ mit den beiden Relaxationsgleichungen beschreiben und verstehen. Die Untersuchungen führen zu dem überraschenden Ergebnis, daß die Reaktionsenthalpie für die Anlagerung eines MonomerenδH1 an ein bereits vorhandenes Aggregat von der Größe dieses Aggregats abhängt. Mit wachsender Assoziationszahl variiertΔH1 von positiven zu negativen Werten.SummaryKinetic measurements on alkylpyridiniumhalides are presented and the data are interpreted on the basis of the theory recently developed byG. Aniansson et äl.For most of the studied systems two relaxation times have been observed. Their dependence on surfactant concentration, temperature and concentration of counterions have been investigated.The fast relaxation time τ is characteristic for the shift of the distribution curve in the aggregation space. The total concentration of micelles remains constant during this process. The lifetime of a monomer in the micelle and its rate of insertion can be obtained from this relaxation time. The temperature dependence of τ1 gives the heat of reaction for the insertion of a monomer into the micelle. Furthermore, if the aggregation number of the micelle is known, the distribution width can be calculated too.In the theoretical section, a simple, but previously unknown method for the derivation of the relaxation equation is given.The slow relaxation time τ2 is essentially influenced by the concentration of oligomers at the distribution minimum, which can be regarded as nuclei for micelles. The concentration of these nuclei can be obtained from the measurements, and from the temperature dependence of τ2 the heat of reaction for the formation of the nuclei can be calculated. The relatively complicated dependence of the two relaxation times on the parameters total concentration, temperature and concentration of counterions can be quantitatively described and understood with the two relaxation equations. The investigations led to the surprising result, that the heat of reaction for the association of a monomer to an already existing aggregate depends on the size of this aggregate. With increasing aggregation numberΔH1 varies from positive to negative values.

Nonionic micelles in mixed water-glycerol solvent

Abstract The aggregation properties of the nonionic amphiphile C 12 E 8 in water-glycerol mixtures are investigated at various glycerol volume fractions X by surface tension, light scattering, refractometry, and viscosimetry measurements. In the range 0 X X , and the aggregation number m decreases with X . For X > 0.4, the CMC grows considerably, and the Krafft temperature becomes larger than room temperature. The minimum of the cloud point curve decreases from 74°C at X = 0 to 43°C at X = 0.6.

Kinetic investigations at the cloud point of nonionic surfactants

Abstract Kinetic relaxation measurements using the T-jump technique, electric birefringence, and light-scattering measurements were carried out with aqueous solutions of the neutral detergents C10H21-(OC2H4)4OH, C12H25(OC2H4)4OH, C8H17(OC2H4)4OH, C9H19Ph(OC2H4)5OH, C9H19Ph(OC2H4)8OH, and Triton X-100 (Ph stands for the phenylen group) above and below the cloud point Tc. Two relaxation processes could be detected in the clear solutions below Tc. These processes could be associated with the change of the aggregation number and concentration of micelles according to the theory of Aniansson and Wall. The C12E4 system shows electric birefringence below Tc from which it can be concluded that the micelles are nonspherical. In the turbid solutions above Tc up to six relaxation times could be observed by recording the turbidity as a function of time. These processes could be associated with the normal micellar effects in clear solutions, with the transport of matter to the droplets of the dispersed new phase that is formed above Tc and with the equilibration of the droplets, respectively. The aggregation number of the micelles and the dimensions of the droplets are calculated from the relaxation times. These values agree fairly well with the values obtained from light-scattering measurements.

SANS-measurements on micellar solutions of perfluoro-detergents

Small-angle-neutron-scattering-(SANS)-measurements were carried out with soulutions of Tetramethylam-moniumperfluoroctanesulfonate (TMAFOS) and Diethylammoniumperfluorononanoate (DEAFN) in pure D2O and in mixtures of D2O and H2O. For the TMAFOS-solutions the experimentally observed scattering functions, i.e. the coherent scattering intensity as a function of the scattering angles, could be fitted very well for large scattering angles with the theoretically calculated scattering function for rodlike particles and the dimensions of the rods could be evaluated from these fits. The radius of the rods was independent of the detergent concentration and equal to the double length of a detergent molecule. For small scattering angles the scattering function showed a maximum which was due to nearest neighbour order between the aggregates resulting from the intermicellar interaction. From this maximum a mean distance between the aggregates and hence the lengths L of the rods could be calculated.For the DEAFN-solutions the observed scattering function showed no maximum what clearly indicates the absence of nearest neighbour order between the aggregates. The experimentally observed scattering functions could not be fitted on the basis of rodlike aggregates, but agreed rather well with the theoretically calculated scattering functions for disclike aggregates and also for vesicle-like double layers. The dimensions for the discs could be evaluated from the fits.

Alkanediyl-α,ω-bis(dimethylalkylammonium bromide) surfactants: Part 8. Pressure-jump study of the kinetics of micellar equilibria in aqueous solutions of alkanediyl-α,ω-bis(dimethyldodecylammonium bromide) surfactants

Abstract The pressure-jump relaxation technique has been used for a study of the kinetics of micellar equilibria in aqueous solutions of cationic surfactant dimers of the alkanediyl-α-ω-bis(dodecyldimethylammonium bromide) type with alkanediyl=(CH2)2, (CH2)3, (CH2)4, (CH2)6, and (CH2)8. Owing to experimental constraints the measurements were restricted to low concentrations, close to the surfactant critical micelle concentration. As for conventional (monomeric) surfactants, two relaxation processes have been evidenced in favorable situations. The fast relaxation process is associated with the surfactant exchange between micelles and bulk phase. The slow relaxation process is associated with the micelle formation–breakup. As for conventional surfactants the micelle formation–breakup proceeds via stepwise entry–exit of one surfactant at a time into–from its micelles. At the difference of conventional surfactants, the rate constant for the entry of a surfactant dimer into its micelles is slower than for a diffusion-controlled process. Also the residence time of a surfactant dimer in its micelles and the lifetime of surfactant dimer micelles are much longer than for the corresponding conventional surfactants.