Autophobing on Liquid Subphases Driven by the Interfacial Transport of Amphiphilic Molecules

Abstract

We investigated the phenomenon of incomplete wetting of a high energy liquid subphase by drops of pure amphiphilic molecules as well as drops of amphiphile solutions which are immiscible with the subphase. We show that amphiphiles escape across the contact line of the drop, move on the subphase/vapor interface and form a submonolayer or full monolayer external to the drop. If this monolayer is sufficiently dense, it can reduce the surface tension of the subphase, raise the contact angle of the drop and prevent the drop from fully wetting the subphase. This phenomenon is called autophobing and has been extensively studied on solid substrates. For the liquid subphase studied here, we measure the surface tensions of the three relevant interfaces before and after the drop is deposited. The measured surface tension external to the drop shows that amphiphiles can move across the contact line and form a monolayer outside of the drop. In some cases, at equilibrium, the monolayer is in a sufficiently packed state to create the nonwetting condition. In other cases, at equilibrium the monolayer density is insufficient to lower the surface tension enough to achieve the nonwetting condition. Unlike on solid substrates where the formation of the monolayer external to the drop is kinetically hindered, the amphiphiles can move rapidly across the liquid subphase by Marangoni driven surface transport and local equilbirum is achieved. However, because the amphiphile inventory and subphase area are limited, the achievement of autophobing on a liquid subphase depends not only on the instrinsic subphase/amphiphile interaction but also on the total amphiphile inventory and area of the liquid subphase.

Gravity driven current during the coalescence of two sessile drops

“…52 Surface tension is measured using a Wilhelmy pin connected to a balance on a MicroTroughX Langmuir trough. 53 The Pt pin is washed following a standard cleaning protocol and burned with a Bunsen burner to remove residual organics, after which the Pt pin is placed back on the Langmuir trough balance and the balance is zeroed in the air environment. The apparatus is calibrated using the known PDMS surface tension (19.8 mN/m) to obtain the calibration constant and again checked for consistency by measuring the surface tension of DI water.…”

Effect of chemical structure of organics on pore wetting

“…Several studies have been reported in literatures on the autophobing phenomena of amphiphilic organic compounds spreading on a high-energy hydrophilic surface (Afsar-Siddiqui et al, 2003a, b, 2004Craster and Matar, 2007;Kumar et al, 2003;Sharma et al, 2012).ͺ EZ &ͺϭϴͺ EZ &ͺϭϵ Most of the amphiphlic organics are the key and fundamental components of nonionic surfactants, which are tailed with hydrophobic groups (apolar fatty alkyl chains) but headed with hydrophilic functional groups, such as -OH, -NH 2 and -COOH. However, the effects of the functional groups and alkyl groups of these amphiphilic organic compounds on the pore wetting of high-energy hydrophilic surface have not been systematically investigated.…”

“…The surface tensions of liquids were determined by pendant drop experiments (First Ten Angstroms). The large contact angles of long-chain amphiphilic organic compounds in a glass pore might be caused by the adsorbed oriented amphiphile layers on the high-energy surface, which is called autophobing (Afsar-Siddiqui et al, 2003a, b, 2004Atkin and Warr, 2007;Craster and Matar, 2007;Garoff, 1995, 1996;Kumar et al, 2003;Novotny and Marmur, 1991;Qu et al, 2002;Sharma et al, 2012;Souda, 2012). Several organics with low surface tensions have been reported that they cannot spread completely on high energy solid surfaces under their own saturated vapor (Fox et al, 1955;Hare and Zisman, ϭϱ 1955;Zisman, 1964).…”

Transport of a partially wetted particle at the liquid/vapor interface under the influence of an externally imposed surfactant generated Marangoni stress

“…These surface tension gradients cause the drop to spread convectively on the liquid subphase. As soon as the surfactant drop touches the liquid subphase, surfactant molecules escape ahead of the contact line and spread across the bare subphase/vapor interface [12,14,15]. The leading edge of this spreading monolayer is referred to as the surfactant front and is identifiable by a liquid ridge [16,17].…”