Harald Høiland

Partial molal volumes and isentropic partial molal compressibilities of surface-active agents in aqueous solution

Abstract The densities and the isentropic coefficients of compressibility of sodium-, potassium-, and tetramethylammonium laurate and of sodium dodecyl sulfate in aqueous solution have been measured. The apparent molal volumes and the isentropic apparent molal compressibilities have been evaluated at concentrations above and below the critical micelle concentration (CMC). The change in partial molal volumes and compressibilities in the formation of micelles has been evaluated from these measurements. These quantities have also been determined from conductance measurements. These two independent methods of measurements gave consistent values. The volume change and the compressibility change of micellization were positive and more positive the smaller the ionic radius of the counterion. The isentropic apparent molal compressibilities above the CMC are large and positive and indicate that the interior of the micelles resembles liquid hydrocarbons.

Partial molal volumes and compressibilities of the homologous series of sodium alkylcarboxylates, R6COONaR13COONa, in aqueous solution

Abstract The changes in partial molal volume (Δ V m ) and compressibility (Δ κ m ) in the formation of micelles have been determined at 25°C for the homologous series of sodium-alkylcarboxylates, R 6 COONaR 13 COONa. Δ V m has been determined by density measurements and by conductance measurements at different pressures. Δκ m has been evaluated from the conductance measurements and from ultrasound measurements. For each quantity the two independent methods of measurements gave consistent values. Δ V m and Δ κ m both increase with increasing chain length of the surfactant, but not in a linear manner. An analysis of the quantity Δ V m shows that the difference between successive values of Δ V m in the homologous series does not represent the group partial molal volume of the CH 2 group added to the micelle. The fraction of counterions associated with the micelles has been determined by EMF measurements, using a membrane electrode. This quantity was found to increase with increasing chain length.

Solubilization of alcohols and alkanes in aqueous solution of sodium dodecyl sulfate

Abstract The distribution coefficient of some alcohols and alkanes between water and micelles at 298.15 K have been calculated from solubility measurements in aqueous solutions of sodium dodecyl sulfate (NaDDS) with and without added salt. The total solubilities of alcohols and alkanes in aqueous solutions of NaDDS increase with the solubility in the aqueous medium. Simultaneously the distribution coefficients decrease. Addition of salt leads to a decrease in the total solubilities and a corresponding increase in the distribution coefficients. The results suggest that the solubility of the alcohol or alkane in the aqueous phase as well as the micellar aggregation number play a significant part in the solubilization process. The localization of the solubilizate in the micelles have been briefly discussed, and it is suggested that the alcohols are solubilized both at the micellar surface and in the micellar interior.

Partial molal volumes, expansibilities, and compressibilities for aqueous alcohol solutions between 5°C and 40°C

Density and ultrasound measurements have been carried out for aqueous solutions of primary and secondary alcohols and α,ω-diols at 5, 10, 20, and 40°C. The density data have been used to calculate partial molal volumes and expansibilities at infinite dilution. The ultrasound data have been used to calculate isentropic partial molal compressibilities which have been converted to isothermal values by using the expansibility data and literature data on partial molal heat capacities. The difference between the isentropic and isothermal partial molal compressibilities is practically zero at 5°C, and increases only slightly with temperature. Group partial molal compressibilities have been evaluated. The hydrophobic group compressibilities increase with temperature while the hydrophilic group compressibilities remain constant or decrease with temperature.

Aggregative processes in aqueous solutions of isomeric 2-butoxyethanols at 298.15 K

Ultrasound speed measurements across the entire composition range of aqueous mixtures of both isobutoxyethanol (iC4E1) and tert-butoxyethanol (tC4E1) have been made at 298.15 K with a sonic solution monitor that employs a “pulse–echo–overlap” technique. In addition, densities of aqueous mixtures of tC4E1 were determined using a vibrating tube densimeter. These new data were complemented with literature values for densities of aqueous mixtures of tC4E1 and for densities and sound speeds of aqueous mixtures of n-butoxyethanol (nC4E1). In all cases, density values were converted to molar volumes, Vm, and excess molar volumes, VmE. Estimates of the isentropic molar compression, KS,m [= −(∂Vm/∂p)S], and of its excess counterpart, KS,mE, were obtained from the combination of the ultrasound speeds and density values. Data reduction procedures were used to generate consistent sets of values for thermodynamic properties of isomeric amphiphiles with increasing degree of alkyl branching. The graphs for the composition dependence of excess partial molar volumes and isentropic compressions of water show enhanced visual impact. These graphs are used for presenting evidence for identifying the prevailing patterns of molecular aggregation. Segmented-composition models, including a version onto which a mass action component has been grafted, are employed, together with a simplified pseudo-phase model, to analyse the various excess molar quantities. The experimental evidence thus obtained is used to relate the effect of chain branching with the degree of self-aggregation of amphiphiles in aqueous solution. An unexpectedly low self-aggregation among iC4E1 molecules is found and discussed in terms of vicinity to the lower critical solution temperature.

Thermodynamic properties of peptide solutions. Part 11. Partial molar isentropic pressure coefficients in aqueous solutions of some tripeptides that model protein side-chains.

The partial molar isentropic pressure coefficients at infinite dilution K infinity s,2 (K infinity s,2 = -(delta V infinity 2/delta p)s, where V infinity 2 is the partial molar volume at infinite dilution) have been determined for nine tripeptides in aqueous solution at 25 degrees C. The tripeptides are all of sequence glycyl-X-glycine, where X is an amino acid. These K infinity s,2 results, in conjunction with those of glycylglycylglycine, were used to estimate the amino acid side-chain contributions to K infinity s,2 of peptides. These side-chain contributions are critically compared with previous estimates based on K infinity s,2 data for the amino acids. The variation in the side-chain contributions derived using the peptide K infinity s,2 results has been rationalized in terms of likely peptide-solvent interactions.

Thermodynamic properties of peptide solutions: 7. Partial molar isentropic pressure coefficients of some dipeptides in aqueous solution

The partial molar isentropic pressure coefficients at infinite dilution, KS,2o, have been determined for a number of dipeptides in aqueous solution at 25°C. For a series of dipeptides of sequence gly-X, where X is an amino acid with a neutral side chain, the KS,2o values are all more negative than that for diglycine. The results are discussed in terms of the hydration of the side chains. There are significant differences in the KS,2o values for sequence isomeric dipeptides. These differences can be rationalized in terms of the mutual interactions between the side chain and the ionic end groups in the dipeptides. Possible relationships between KS,2o and V2o, the partial molar volume at infinite dilution, were investigated. For the dipeptides of sequence gly-X there is an interesting linear relationship between KS,2o/V2o and V2o.

Volumetric properties of binary mixtures of water with methoxy(ethoxy)nethanols

Density and ultrasonic speed measurements have been made, respectively, by means of a densimeter operating in a flow mode and by a sonic solution monitor using a ‘sing-around’ technique, for aqueous mixtures of 2-(2-methoxyethoxy)ethanol and 2-[2-methoxy(ethoxy)2]ethanol, at 298.15 K, across the entire composition range. The density values were readily converted to molar volumes, V, and excess molar volumes, VE. Estimates of the isentropic molar quantity, Ks, equal to –(∂V/∂p)s, and of its excess counterpart, KES, have been obtained from the density values, in combination with the ultrasonic speed values. The magnitude of VE increases with the introduction of oxyethylene groups into the molecule of amphiphile. The composition dependence of KES shows a similar increase, but one that is limited to the water-rich region. Data reduction procedures have been used to calculate several related quantities leading to graphs with enhanced visual impact. Our results have been compared with those reported earlier for methanol–water, and 2-methoxyethanol–water. Graphical analyses of the data sets are presented in an effort to find a plausible rationale for the observed trends in terms of prevailing patterns of molecular aggregation, while the various excess quantities considered are analysed using a segmented-composition model.

Self-Association of Medium-Chain Alcohols in n -Decane Solutions

Self-association of medium-chain alcohols in n-decane solutions has been studied by infrared absorption of the fundamental OH stretching vibration. The alcohols investigated were 1-propanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 1-butanol, 1-pentanol, and 1-hexanol. Infrared spectra were acquired for varying alcohol molalities, the highest concentration being 0.2 mol/kg. The spectra for each alcohol were collected in a data matrix. The bilinear multicomponent data were successfully resolved into spectra and concentration profiles by a multivariate method. The result indicates that monomers dominate the spectral variance in the low-molality region, while cyclic oligomers dominate in the upper concentration range. It further indicates that minor amounts of open-chain aggregates may be present. The monomer and cyclic tetramer appear to be the dominant species, while the amount of open-chain aggregates was negligible even in the low-molality region. The equilibrium constants for the monomer–tetramer association reactions (K1–4) were calculated by a least-squares method. The calculated values for the equilibrium constants, based on the molality, range from 138 to 106 for the linear alcohol molecules. The result shows that 1-butanol, 1-pentanol, and 1-hexanol have similar constants, while 1-propanol displays a markedly higher value. The equilibrium constants obtained for 2-methyl-1-propanol and 2-methyl-2-propanol were 77 and 39, respectively. The considerably lower values for the branched alcohol molecules indicate that steric interaction between the chain prevents self-association into larger aggregates.

Thermodynamic properties of peptide solutions 8. Isentropic pressure coefficients (∂ V2,/∂p)s of the apparent molar volume V2, for each of the aqueous solutes: diglycine, triglycine, and tetraglycine

The partial molar isentropic pressure coefficients at infinite dilution k∞S, 2 have been redetermined for the peptides diglycine, triglycine, and tetraglycine in aqueous solutions at the temperature 298.15 K. The results are compared with those reported in the literature. For the series of oligoglycines, (gly)n with n = 1 to n = 4, k∞S, 2 is not a linear function of n. This result is contrary to that reported in the literature.