Surfactants in Agriculture: Sorptivity and Water Imbibition Study

French Associates Institute for Agriculture and Biotechnology of Drylands Thuc Nguyen Thi and Gilboa Arye (nguyenthithuclik20@gmail.com), (aryeg@bgu.ac.il) Sorptivity and water imbibition into air-dry surfactant-containing soil 05 May 2020

Introduction Surfactant introduction to agricultural soils is commonly investigated in conjunction with amendment of water repellent soils (WRS); (hydrophobic). Surfactant can aid to the wetting of initially WRS by reducing the contact-angle formed at the solid\liquid\air interface and consequently increasing its infiltration capacity. Surfactant introduction to initially wettable soils (hydrophilic) reported to improve uniformity of soil moisture distribution and crop yields. The role of surfactant on wettability of initially wettable soil, in particular, under wetting and drying cycles received minor attention. Objective The main objective of this study was to quantify the sorptivity and water imbibition of air-dry, initially wettable sand subjected to wetting and drying cycles with: (i) anionic (SDS), (ii) cationic (CTAB) and (iii) nonionic (TX-100) surfactants.

Hypothesis Non-ionic Anionic Cationic SDS TX-100 CTAB + X ++ Hydrophobic interaction (Mainly with SOM) Hydrogen bonding Weekly adsorbed Easily desorbed Van der Waals interaction Hydrogen bonding Weekly adsorbed Easily desorbed Electrostatic interaction Strongly adsorbed Not-easily desorbed Low to moderate reduction in soil-solution surface-tension Induce high hydrophobicity Significate reduction in soil-solution surface-tension Induce low to moderate hydrophobicity

Material and Methods SDS Surfactant TX-100 CTAB Surfactants surface tension measurements by Wilhelmy plate method (WPM) C14H22O(C2H4O)n (n = 9-10) 647 Non-ionic 0.22 Molecular formula NaC12H25SO4 C19H42BrN Molecular mass Category CMC (mM, 25oC) 288 364 Anionic 8.2 Cationic 0.92 Capillary rise method (CRM) Wilhelmy plate method (WPM) Quartz sand Saturated by capillary rise at C/CMC= 0-3 0-5 cycles Oven drying 65 C/24hr Force balance Contact angle * No reference liquid (e.g. ethanol) required Water imbibition Sorptivity Water/Ethanol Contact angle CRM and WPM

Results surface tension at the liquid-air interface 80 The critical micelle concentration (CMC) calculated from the intersection between the linear part of reduction in surface tension and the point from which no further or low reduction can be observed (indicated as vertical dashed lines) 70 Surface tension (mN/m) 60 50 Surfactants efficiency : 40 1. TX-100: CMC=0.25 mM, L=32.6 mN/m 2. CTAB: CMC=1.0 mM, L=36.1 mN/m 3. SDS: CMC=8.0 mM, L=28.7 mN/m TX-100 CTAB SDS 30 TX 0.25mM CMC CTAB 1 mM CMC SDS 8 mM CMC 20 -6 -5 -4 -3 -2 -1 Log C (mM)

Results CRM vs. WPM contact angle 80 80 100 The contact angle obtained by the CRM vs. WPM suggest that significant hydrophobicity induced only by CTAB 80 60 60 Contact Angle Contact Angle Contact Angle 60 40 40 SDS CPM-CA CTAB CPM-CA 40 CTAB CPR-CA SDS CPR-CA TX-100 CPR-CA TX-100 CPM-CA Reduced imbibition rate for TX100 and SDS may be affected by low surface tension and may misinterpreted as a higher contact angle when using the CRM Cycle 1 Cycle 3 Cycle 5 Cycle 1 Cycle 3 Cycle 5 20 Cycle 1 Cycle 3 Cycle 5 20 20 0 0 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 C/CMC C/CMC C/CMC 80 80 100 TX-100 WPM-CA SDS WPM-CA 80 Cycle 1 Cycle 3 Cycle 5 Cycle 1 Cycle 3 Cycle 5 60 60 60 Contact Angle Contact Angle Contact Angle 40 40 40 CTAB WPM-CA 20 20 20 Cycle 1 Cycle 3 Cycle 5 0 0 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 C/CMC C/CMC C/CMC

Results sorptivity 0.4 0.4 0.4 CTAB treated sand - Cycle 1 0.3 0.3 0.3 Sorptivity (mL s-1/2) Sorptivity (mL s-1/2) Sorptivity (mL s-1/2) Experimental data ST scaling data ST*Cos scaling data Control 0.2 0.2 0.2 ?? ?? ????0 ????? ?0 ?? ?0= ?? TX100 treated sand- Cycle 1 0.1 SDS-treated sand - Cycle 1 0.1 0.1 Experimental data ST scaling data Control Experimental data ST scaling Control 0.0 0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 C/CMC C/CMC ratios C/CMC ratios ?0= 0.328 0.0363 mL s-1/2 0.4 0.4 0.4 CTAB treated sand - Cycle 3 Scaling Swonly by surface-tension: TX-100 adequate predication of S0 SDS adequate predication of S0 for cycle-5 CTAB inadequate predication of S0 0.3 0.3 0.3 Sorptivity (mL s-1/2) Sorptivity (mL s-1/2) Sorptivity (mL s-1/2) 0.2 0.2 0.2 Experimental data ST scaling data ST*Cos scaling data Control TX100-treated sand - Cycle 3 SDS-treated sand - Cycle 3 0.1 0.1 0.1 Experimental data ST scaling data Control Experimental data ST scaling data Control 0.0 0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 C/CMC ratios C/CMC ratios C/CMC ratios Scaling Swby surface-tension and contact angle: CTAB approaching Swdata, yet, inadequate 0.4 0.4 0.4 CTAB-treated sand - Cycle 5 0.3 0.3 0.3 Sorptivity (mL s-1/2) Sorptivity (mL s-1/2) Sorptivity (mL s-1/2) 0.2 0.2 0.2 Experimental data ST scaling data ST*Cos scaling data Control TX-treated sands - Cycle 5 0.1 SDS-treated sand - Cycle 5 0.1 0.1 Experimental data ST scaling data Control Experimental data ST scaling data Control 0.0 0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 C/CMC ratios C/CMC ratios C/CMC ratios

Conclusion Surfactant adsorption to initially wettable soil may reduced water capillary absorption. Cationic surfactant (CTAB) may reduce unsaturated water flow-rate due to induced hydrophobicity. Anionic (SDS) and non-ionic (TX-100) surfactants may reduce unsaturated water flow-rate due to surfactant detachment into the soil solution, which in turn, reduce the surface tension at the water/air interface.