Anne Marit Blokhus

The effect of crude oil acid fractions on wettability as studied by interfacial tension and contact angles

Abstract Naphthenic acids have been extracted from three North Sea crude oils with different acid numbers. The acid fractions were diluted with toluene to their original crude oil concentrations, and also further diluted and compared at equal concentrations (milligram acid/gram oil). The toluene phases were then equilibrated with an aqueous phase, and the interfacial tension (IFT) and contact angles were measured as a function of equilibrium pH and acid concentrations. The aqueous phase used was 0.5 M NaCl(aq), and the contact angle experiments were performed on silicate glass cover slips. The results show a correspondence between the decrease in interfacial tension as a function of pH and the acid number of the original crude oils. Generally, low contact angles were observed at high and low pH values, whereas angles up to 47° were observed in the intermediate pH range. The contact angles did not show a direct correspondence with the acid numbers but gave the same trend as the corresponding whole crude oils (presented in a previous work). Observations from both interfacial tension and contact angle experiments emphasize the importance of acid structures, or acid types, present in the fractions compared to the acid concentrations. Acid characterization by gas chromatography with a mass sensitive detector (GC-MS) showed significant differences in the molecular structures for the acids dominating each of the three fractions. Fractions enriched in alkyl acids, phenols and cyclic acids were found to have impact on the wetting properties of the silicate surface, whereas fractions enriched in more complicated aromatic ring structures with high degree of carboxylic compounds did not affect the contact angle. FT-IR spectra of the acid extracts confirmed the results from the GC-MS analysis and showed correspondence to the acid numbers of the three crude oils.

Effect of polar organic components on wettability as studied by adsorption and contact angles

Abstract Adsorption of polar organic components onto the rock surface is one of the mechanisms that is believed to cause wetting alteration of a reservoir rock. Polar compounds in crude oil that are believed to be responsible for surface interactions and wetting properties include carboxylic and phenolic acids, organic bases and metal complexes. Known compounds, representative of these naturally occurring polar organic compounds in crude oil, have been chosen for adsorption and contact angle studies. Contact angles and adsorption isotherms in solid–oil–brine model systems have been examined as a function of component concentration in either water or oil and of salinity and pH of the water phase. The systems investigated consist of isooctane oil phase and water solutions of NaCl and CaCl2 as the water phase. Silicate glass and α-alumina were used as solid phases. The polar compounds were 1-naphtoic acid, 5-indanol, quinoline, vanadyl–octaethyl–porphyrin (VO OEP) and dihydrogen–octaethyl–porphyrin (H2 OEP). The compounds chosen represent functionalities that are found in polar crude oil fractions, such as asphaltenes, but still can be considered well-defined model substances. The components were adsorbed from the oil phase as well as from the water phase, both for contact angle and adsorption isotherm experiments. The results demonstrate the difference in adsorption behaviour between compounds with different functional groups of varying polarity and acidity. The importance of solid–solute interactions compared to solvent–solute interactions is demonstrated.

Solubilization of heptanols and α,ω-alkanediols in aqueous solutions of sodium dodecyl sulfate

Abstract The distribution coefficients of 1- to 4-heptanol and 1,8-octanediol, 1,9-nonanediol, and 1,10-decanediol between micelles and water have been determined from solubility measurements at 298.15 K. The surfactant was sodium dodecyl sulfate. The data show a negative correlation between the distribution coefficient and the solubility of the alcohol or diol in water: the lower the solubility in pure water, the higher the distribution coefficient. The total solubilities of the alcohols and diols increase with the sodium dodecyl sulfate (NaDDS) content, but for 1-heptanol a maximum is reached at a NaDDS content of about 0.04 m , suggesting a change in the solubilization mechanism. The localization of the solubilizate in the micelles has also been discussed. The data suggest that 3- and 4-heptanol and the diols are solubilized at the micellar surface only, while 2-heptanol and especially 1-heptanol are solubilized both at the surface and in the interior of the micelles.

Surfactants and cosurfactants in lamellar liquid crystals and adsorbed on solid surfaces: I. The model system sodium dodecyl sulfate/butanol or sodium dodecyl sulfate/benzyl alcohol and α-alumina

Abstract Phase diagrams of the model systems sodium dodecyl sulfate (SDS)/1-butanol/water and SDS/benzyl alcohol/water have been determined. In both model systems we have found a lamellar lyotropic liquid crystalline phase D with regular molecular packing of the surfactant/cosurfactant. The model systems have also been used for adsorption studies on solid A1 2 O 3 in 0.3 M NaCl. At the adsorption plateau SDS is found to cover an area of 0.40 nm 2 , 1-butanol an area of 0.30 nm 2 , and benzyl alcohol an area of 0.24 nm 2 . Corresponding values in the lamellar D phase (without NaCl) are 0.40 (average value), 0.14, and 0.20 nm 2 . The good agreement between the area requirements of SDS under these conditions reveals a close connection of an ion-condensation model for the lamellar phase and the surface charge distribution on the solid alumina surface. The discrepancy in the areas for the butanol is attributed to differing water solubilities, which affect the structuring in the monolayer at the solid surface more than in the bilayer in the D phase and/or longitudinal translation of the alcohol molecules in molecular bilayers. For long-chain alcohols a value of around 0.20 nm 2 is obtained in both cases.

Solubilization of 1-hexanol in hexadecyltrimethylammonium bromide, sodium dodecyl sulfate, and sodium decanoate

The solubility of hexanol has been measured in aqueous solutions of hexadecyltrimethylammonium bromide (HTAB), sodium dodecyl sulfate (NaDDS), and sodium decanoate (NaC10). In the HTAB and NaDDS solutions the hexanol solubility decreased abruptly above a surfactant content of about 0.05 mole kg−1, suggesting a structural change from spherical to anisodiametric micelles. The total solubility of hexanol shows that it must be solubilized both in the palisade layer and in the micellar interior of these spherical micelles. After the structural change has taken place less hexanol is solubilized so that all can be accomodated in the palisade layer. No decrease in solubility has been observed for the NaC10 system, and the total hexanol solubility is so small that all can be accommodated in the palisade at all NaC10 concentrations. ΔHo and ΔSo of solubilization for spherical micelles have been calculated from the temperature dependence of the distribution coefficient.

Surfactant and cosurfactant packing in the lyotropic liquid crystalline phase D and adsorbed on a solid surface

Abstract Competitive adsorption of sodium dodecyl sulfate (SDS) and 1-butanol (C4OH) onto solid Al2O3 from aqueous solution of 0.3 M NaCl has been studied. Itis found that the affinity of C4OH toward the solid surfaceis significantly lower than that of SDS. A bulk concentration of 1 10 renders a surface concentration of 1 1 . A method of separating the area requirements for the components is proposed in analogy to the geometries in the lamellar lyotropic liquid crystalline phase D. Calculation gives 0.39 nm2 for SDS and 0.30 nm2 for C4OH on alumina. The last value is higher than the comparable value from D phases, which may indicate a more horizontal orientation of the adsorbed molecule at the solid surface. These conclusions are, however, based on a complete coverage of the solid surface by the adsorbed species.

Effects of Organic Acids and Bases, and Oil Composition on Wettability

Correlations between acid and base numbers, crude oil composition and wettability for crude oil/water/quartz systems have been investigated. Twelve North Sea crude oils were characterised with respect to asphaltene content and composition of saturates, resins and aromatics by the use of thin liquid chromatography. Acid and base numbers were measured for each crude oil. The acid numbers varied from 0.02 to 2.8, while the base numbers were in the range of 0.3 to 1.6. One of the crude oils was separated into asphaltenes and deasphaltened oil, measuring acid and base number for both fractions. Based on different acid and base contents three of the oils were chosen for adhesion tests and contact angle experiments on quartz surfaces as a function of pH. These crude oils were also investigated with respect to interfacial activity. The amount of acids and bases was found to be proportional to the amount of polars (NSO and asphaltenes). The acid and base numbers were correlated, meaning that a high acid number usually corresponded to a high base number. For the fractionated oil the deasphaltened oil was found to contain approximately equal amounts of acids and bases as the original crude oil. The results from contact angle measurements show more water wet surface at higher acid number, and more oil wet behaviour at higher base number. The interfacial tension of the crude oil/brine interface increased with increasing base/acid ratio.

Surfactant and cosurfactants in lamellar liquid crystals and adsorbed on solid surfaces: II. Influence of electrolyte on the model system sodium dodecyl sulphate/benzyl alcohol and α-alumina

Abstract The adsorption of sodium dodecyl sulphate (SDS) and benzyl alcohol (BzOH) onto solid Al2O3 from aqueous solutions has been studied at varying contents on NaCl. A fixed amount of SDS at the saturation adsorption level of SDS (5 × 10−2 M) was chosen. The adsorption of both SDS and BzOH was found to increase with increasing salinity. At high electrolyte content a considerable amount of alcohol is located in the adsorbed phase. Calculations of the individual area requirements of the SDS and BzOH molecules in the adsorbed state was undertaken and values of 0.40 and 0.24 nm2 were obtained for the SDS and BzOH, respectively. Parallels are drawn to the packing of the studied surfactant and alcohol in the lamellar lyotropic crystalline D phase in the same model system without electrolyte.

Counterion association to aqueous sodium dodecyl sulfate micelles in the presence of organic additives

Abstract The effect of polar, nonpolar, and mixed polar-nonpolar additives on the counterion association, the structure, and the size of aqueous sodium dodecyl sulfate micelles has been examined by measuring the conductivity, the viscosity, and the electromotive force at 298.15 K. Octane does not affect the degree of counterion association, but the mobility of the micelles decreases. This is mainly due to an increased micellar volume. Small quantities of 1-butanol and 1-hexanol decrease the degree of counterion association. At higher hexanol molalities larger micellar aggregates, rod- or disk-like micelles are formed. Long-chained α,ω-alkanediols, 1,9-nonanediol and 1,10-decanediol, also reduce the degree of counterion association. The shorter diols apparently affect the micellar system mainly through their effect on the solvent. The effect of the mixed additives is intermediate to that of pure components. Changes in the aggregation number, the micellar radius, and the number of associated counterions have been estimated from the conductivity and electromotive force measurements. Addition of octane does not affect the degree of associated counterions, but the micellar aggregation number increases. Addition of hexanol lowers both the micellar aggregation number and the fraction of associated counterions.

Surfactants and cosurfactants in lamellar liquid crystals and adsorbed on solid surfaces: III. The model system sodium dodecanoate/benzyl alcohol and α-alumina

Abstract The competitive adsorption of sodium dodecanoate and benzyl alcohol from aqueous solution onto solid α-alumina has been determined at 35°C without and with 0.05 M NaCl. The adsorption isotherm of the surfactant is found to pass through two plateau states under both bulk conditions. In electrolyte-free solutions the first adsorption plateau is in good agreement with monolayer packing of the surfactant (0.41 nm 2 ), while the saturation value agrees with bilayer packing. Without added electrolyte, benzyl alcohol is found to occupy about 0.18 nm 2 in the surfactant surface bilayers. In the presence of NaCl, a denser packing of both the surfactant and the alcohol is obtained. A parallel between molecular organization in lyotropic liquid crystalline D phases with a lamellar geometry and the adsorbed phase gives a good agreement.