New Polymer Tensiometers: Measuring Matric Pressures Down to the Wilting Point

Tensiometers are commonly used for measuring soil water matric pressures. Unfortunately, the water-filled reservoir of conventional tensiometers limits their applicability to soil water matric pressures above approximately –0.085 MPa. Tensiometers filled with a polymer solution instead of water are able to measure a larger range of soil water matric pressures. We designed and constructed six prototype polymer tensiometers (previously called osmotic tensiometers) consisting of a wide-range pressure transducer with a temperature sensor, a stainless steel casing, and a ceramic plate with a membrane preventing polymer leakage. A polymer chamber (0.1–2.2 cm3) was located between the pressure transducer and the plate. We tested the polymer tensiometers for long-term operation, the effects of temperature, response times, and performance in a repacked sandy loam under laboratory conditions. Several months of continuous operation caused a gradual drop in the osmotic pressure, for which we developed a suitable correction. The osmotic potential of polymer solutions is temperature dependent, and requires calibration before installation. The response times to sudden and gradual changes in ambient temperature were found to be affected by polymer chamber height and polymer type. Practically useful response times (<0.2 d) are feasible, particularly for chambers shorter than 0.20 cm. We demonstrated the ability of the instrument to measure the range of soil water pressures in which plant roots are able to take up water (from 0 to –1.6 MPa), to regain pressure without user interference and to function properly for time periods of up to 1 yr

Surfactant adsorption to soil components and soils

<p>Soils are complex and widely varying mixtures of organic matter and inorganic materials; adsorption of surfactants to soils is therefore related to the soil composition. We first discuss the properties of surfactants, including the critical micelle concentration (CMC) and surfactant adsorption on water/air interfaces, the latter gives an impression of surfactant adsorption to a hydrophobic surface and illustrates the importance of the CMC for the adsorption process. Then attention is paid to the most important types of soil particles: humic and fulvic acids, silica, metal oxides and layered aluminosilicates. Information is provided on their structure, surface properties and primary (proton) charge characteristics, which are all important for surfactant binding. Subsequently, the adsorption of different types of surfactants on these individual soil components is discussed in detail, based on mainly experimental results and considering the specific (chemical) and electrostatic interactions, with hydrophobic attraction as an important component of the specific interactions. Adsorption models that can describe the features semi-quantitatively are briefly discussed. In the last part of the paper some trends of surfactant adsorption on soils are briefly discussed together with some complications that may occur and finally the consequences of surfactant adsorption for soil colloidal stability and permeability are considered. When we seek to understand the fate of surfactants in soil and aqueous environments, the hydrophobicity and charge density of the soil or soil particles, must be considered together with the structure, hydrophobicity and charge of the surfactants, because these factors affect the adsorption. The pH and ionic strength are important parameters with respect to the charge density of the particles. As surfactant adsorption influences soil structure and permeability, insight in surfactant adsorption to soil particles is useful for good soil management.</p

Interaction between lysozyme and humic acid in layer-by-layer assemblies:Effects of pH and ionic strength

The interaction between protein and soluble organic matter is studied through layer-by-layer assembly of lysozyme (LSZ) and purified Aldrich humic acid (PAHA) at a solid surface (2-D) and in solution (3-D). By bringing a silica surface in alternating contact with solutions of LSZ and PAHA a layer-by-layer LSZ-PAHA assembly is formed. At pH 5 the negative charge density of PAHA is about 3 times that of the positive LSZ; the layers of LSZ and PAHA are stable and the adsorbed amounts decrease with increasing ionic strength. The mass ratios PAHA/LSZ in the layers depend on the ionic strength; K+ incorporation is relatively large (similar to 25%) when PAHA is the outer layer of the assembly. At pH 6 and 8, and moderate ionic strength (0-100 mmol L-1 KCl) the assembly is accompanied by partial solubilization of positive LSZ by the much more negative PAHA followed by desorption of the complex. The solubilization increases with increasing pH, and decreases with increasing KCl concentration. At 400 mmol L-1 KCl the electrostatic interactions are so well screened that the assembly is no longer accompanied by layer erosion. Assembly of PAHA and LSZ in solution is also investigated at pH 5 and 5 mmol L-1 KCl. The PAHA/LSZ mass ratio at the iso-electric point of the assembly depends on the order of the addition. When LSZ is added to the negative assembly K+ is incorporated in the complex, but when PAHA is added to the positive assembly PAHA and LSZ neutralize each other. (C) 2014 Elsevier Inc. All rights reserved